Enantioselective total syntheses of plectosphaeroic acids B and C

Article abstract of DOI:10.1021/jo4015479

Evolution of the synthetic strategy that culminated in the first total syntheses of the structurally unique plectosphaeroic acids B (2) and C (3) is described. The successful enantioselective route to (+)-2 and (+)-3 proceeds in 6 and 11 steps from the kn

Full text of DOI:10.1021/jo4015479

Article
pubs.acs.org/joc
Enantioselective Total Syntheses of Plectosphaeroic Acids B and C
Salman Y. Jabri and Larry E. Overman*
Department of Chemistry, 1102 Natural Sciences II, University of California, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: Evolution of the synthetic strategy that
culminated in the first total syntheses of the structurally
unique plectosphaeroic acids B (2) and C (3) is described.
The successful enantioselective route to (+)-2 and (+)-3
proceeds in 6 and 11 steps from the known hexahydro-2H-
pyrazinopyrrolo[2,3-b]indole-1,4-dione 39, which in turn is
available in enantiomerically pure form by chemical synthesis.
The central challenge in this synthesis endeavor was uniting
the hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-1,4-dione and
cinnabarinic acid fragments of these marine alkaloids. Critical
for achieving this successful C−N bond formation was the use
of an iodocinnabarinic acid diester in which the amino group
was masked with two Boc substituents, a Cu(I) carboxylate complex and the weak base KOAc. The highly congested C−N bond
generated in this coupling, in conjunction with the delicate nature of the densely functionalized coupling partners, provided a
striking testament to the power of modern copper-mediated amination methods. Two approaches, one stereoselective, for
introducing the methylthio substituents of (+)-plectosphaeroic acid B were developed. The epitrisulfide ring of
(+)-plectosphaeroic acid C was formed by ring expansion of an epidisulfide precursor.
tryptophan units, a structural feature shared by other, less
modified, tryptophan alkaloids such as quadrigemine C6 and
psychotetramine.7 A number of ETP alkaloids and their
methylthio congeners (e.g., 6−14) share a hexahydro-2H-
pyrazinopyrrolo[2,3-b]indole-1,4-dione fragment 5 that is
homologous to the northern fragment of 1−3.8 The southern
cinnabarinic acid (4) fragment, particularly its 2-amino-
phenoxazin-3-one core, is also familiar in natural products,
some of which show potential cancer chemotherapeutic
applications.9
The plectosphaeroic acids (1−3) were isolated from a
program to discover inhibitors of the enzyme indoleamine 2,3-
dioxygenase (IDO) from marine sources.4 Inhibition of IDO
has been identified as a potential new approach to cancer
therapy in which the immune system would be activated to
combat solid tumors.10 The plectosphaeroic acids (1−3) and
cinnabarinic acid (4) were reported to be equipotent (IC50 = 2
μM) against human recombinant IDO, whereas T988 A (10),
which was coisolated with 1−3, was found to be inactive.11
Considering that some portion of the southern fragment was
responsible for IDO inhibition, we became intrigued by the
further biological evaluation and the considerable synthetic
challenges12−14 associated with the synthesis of these molecules
possessing two antitumor pharmacophores.15
INTRODUCTION
■
Epipolythiodioxopiperazines (ETPs) and their methylthio
congeners constitute a group of structurally complex and
biologically active fungal alkaloids that are characterized
structurally by a transannular polysulfide unit joining carbons
3 and 6 of the 2,5-dioxopiperazine ring or thioethers appended
at these carbons (Figure 1).1 A spectrum of biological
properties has been reported for this family of natural products,
including antimicrobial, antiviral, antifungal, and anticancer
activities.2 Recent attention has focused on the chemo-
therapeutic potential of ETP molecules, as several studies
indicate in vivo selectivity and cytotoxicity toward several
cancer variants.3 These potent activities are generally attributed
to the labile S−S σ-bonds of the epidi- or epipolysulfide
functionalities.1
The plectosphaeroic acids A−C (1−3) were reported in
2009 by Mauk, Andersen, and co-workers from the marine
fungus Plectosphaerella cucumerina, which was collected in
Barkley Sound, British Columbia, Canada.4 The constitution
and relative configuration of these structurally unique alkaloids
were elucidated largely by NMR analysis, whereas the absolute
configuration of 1−3 was assigned by comparison of circular
dichroism (CD) data with values reported for a related group of
ETP toxins, the leptosins (e.g., 9).5 The plectosphaeroic acids
A−C (1−3) represent a new family of fungal alkaloids that
comprise a hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-1,4-
dione moiety 5 containing sulfur functionality and a
cinnabarinic acid portion 4. As both fragments likely arise
biosynthetically from tryptophan, the plectosphaeroic acids
contain a linear arrangement of four highly modified
Herein, we describe the development of the first total
syntheses of (+)-plectosphaeroic acids B (2) and C (3).16 The
successful approach features a late-stage copper-mediated C−N
Received: July 16, 2013
© XXXX American Chemical Society
A
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Scheme 1. Retrosynthetic Analysis of Plectosphaeroic Acids
A−C
Figure 1. Structures of plectosphaeroic acids A−C and related ETP
and methylthio congeners containing the common fragment 5.
cross-coupling reaction in addition to the stereoselective
incorporation of the methylthio or bridging trisulfide
substituents into the dioxopiperazine subunits of 2 and 3. In
vitro cytotoxicity of plectosphaeroic acids B (2) and C (3) and
their dimethyl esters against human prostate and melanoma
cancer cell lines is described and compared to that of related
structures.
RESULTS AND DISCUSSION
■
Synthesis Plan. Our synthetic planning was guided by
several considerations (Scheme 1). The well-recognized
sensitivity of epipolythiodioxopiperazines to strong bases and
nucleophiles,1,14 and the similar sensitivity of hexahydro-2H-
pyrazinopyrrolo[2,3-b]indole-1,4-diones that contain a hydrox-
yl substituent adjacent to the quaternary carbon stereocenter,17
places limitations on the reactions that could be employed.
Certainly we wanted to defer the sulfidation step until a late
stage of the synthesis. Precedent from Movassaghi’s,13
Sodeoka’s,18 and our19 groups suggested that the installation
of methylthio or epidithio substituents by thiol trapping of N-
acyliminium ions derived from intermediate 15 should be
possible and likely stereoselective. However, forming the
bridging trisulfide functionality of plectosphaeroic acid C (3)
by a short sequence was less certain.20 Attractive to us was the
potential of ring-expanding the likely more accessible
epidisulfide congener 16, a transformation first described by
Taylor and co-workers in 1968 for elaborating naturally
obtained sporisdesmin A (21) to sporidesmin E (22) (eq 1).21
Foreseen as a central challenge in these syntheses would be
uniting the hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-1,4-
dione and cinnabarinic acid fragments. To maximize con-
vergency, we hoped to forge the key C−N bond as late as
possible, for example, between the indoline nitrogen of the
pyrazinopyrroloindole fragment 17 and a halogenated congener
18 of cinnabarinic acid. In spite of the remarkable progress
recorded in recent years in forging aryl−nitrogen bonds,22 the
hindered nature of the required union and the structural
complexity of the coupling partners 17 and 18 were without
precedent.23,24 Hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-
1,4-dione intermediate 17 was envisaged originating from the
B
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bis(tert-butoxycarbonyl) derivative of the natural product
(+)-gliocladin C (19),8c,f,12,25 following a sequence our group
described recently for preparing gliocladine C (6), (+)-T988 C
(7), and (+)-gliocladin A (11).2c,12 We anticipated that
biomimetic, oxidative dimerization of 6-halo-3-hydroxyanthra-
nilic acids 20 would provide access to potential electrophilic
coupling partners 18.26
Scheme 2. Investigating Palladium(0)- and Copper(I)-
Promoted C−N Cross-Coupling Reactions
Initial Development of the C−N Cross-Coupling in
Model Systems. As an opening phase of this inquiry, we
explored the feasibility of joining the two polycyclic fragments
of the plectosphaeroic acids. At the time, there were no reports
of N-arylating structurally complex cyclotryptamines. Thus, we
decided to examine initially such conversions with simplified
aryl fragments in which a 2-halobenzoic acid would substitute
for the cinnabarinic acid electrophile. However, the steric
environment and nucleophilicity of the indoline nucleophile of
the hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-1,4-dione
would not be easily modeled. As a result, we chose to access
an appropriate amine for our preliminary cross-coupling studies
from the bis(tert-butoxycarbonyl) derivative 23 of (+)-gliocla-
din C, an intermediate that is available on multigram scale by
chemical synthesis.2c,12
Salient results from our efforts to deprotect the indoline
nitrogen of 23 are summarized in Table 1. Under thermal
Table 1. Selective Deprotection of the Bis(tert-
butoxycarbonyl) Derivative 23 of Gliocladin C
resulted in rapid consumption of 25. However, N-arylation
products were not detected from this reaction, which returned
ring-fragmentation product 27 in low yield. As the α-oxoimide
carbonyl group of trioxopiperazines is quite electrophilic, a
property we exploit later in our studies, we reasoned that the
tert-butoxide anion generated in situ was incompatible with the
trioxopiperazine fragment of 25. Consistent with this result,
NaO-t-Bu in either THF or toluene decomposed 25, forming
27, within hours at 50 °C in the absence of the palladium
catalyst.28 Other strong bases such as LHMDS also consumed
25 at 50 °C, whereas 25 was stable to Cs2CO3 in toluene at 85
°C. As a result, our further studies of palladium-catalyzed
coupling focused on the use of weak inorganic bases (e.g.,
Cs2CO3, K2CO3 and K3PO4) in aprotic solvents such as
toluene, THF, and 1,4-dioxane. Nevertheless, all attempts to
forge the desired C−N bond between 25 and ethyl 2-
bromobenzoate (26a) or methyl 2-iodobenzoate (26b) using
catalytic or stoichiometric amounts of palladium(0) and various
Buchwald biarylmonophosphine ligands such as RuPhos,
XPhos, and SPhos,22f,g as well as rac-BINAP and tri-o-
tolylphosphine, were unproductive.
entry
conditions
175 °C (neat), 5 min
140 °C (neat), 11 min
yield (%)
a
1
2
24/19 (1:1)
24/19, 83 (3:1)
a,b
b
b
3
4
Sc(OTf)3 (0.15 equiv), MeCN, 4 h, rt
25, 76
(i) 175 °C (neat), 15 min; (ii) Boc2O (1.0 equiv), 25, 95
DMAP, THF
a
b
1
Relative ratio as determined by H NMR. Yield after chromato-
graphic purification.
conditions, the Boc substituent on the indole nitrogen atom
was more labile (entries 1 and 2). Lewis acids were more
successful, with catalytic amounts of Sc(OTf)3 selectively
removing the indoline Boc group to give 25 in 76% yield (entry
3). However, this reaction required careful monitoring to
prevent cleavage of the second Boc substituent and proved
somewhat unreliable as the reaction was scaled. Thus, an
alternative two-step (one-pot) process involving thermal
deprotection of both nitrogen atoms of 23, followed by
selective reprotection of the indole nitrogen, was utilized to
generate large quantities of the cross-coupling precursor 25
(entry 4).
With indoline 25 in hand, we commenced C−N cross-
coupling studies by investigating palladium catalysts (Scheme
2).27 Attempting to join 25 with ethyl 2-bromobenzoate (26a)
using 10 mol % of Buchwald’s palladacyclic precatalyst ligated
with RuPhos22f,g and excess Cs2CO3 in t-BuOH at 85 °C
Copper(I) salts proved more successful.22a,b,d,e After an
extensive examination of reaction parameters, it was found that
coupling of pyrazinopyrroloindole 25 with 2 equiv of methyl 2-
iodobenzoate in the presence of 4 equiv of either CuI, CuBr, or
CuCl and 5 equiv of Cs2CO3 in toluene at 110 °C for 24 h
promoted C−N bond formation to give 28b as the major
product (Scheme 2). Using CuCl, the coupled product 28b was
isolated in 85% yield. Reducing the amount of the copper salt
lowered the reaction efficiency, as did replacing Cs2CO3 with
K2CO3 or K3PO4. In a brief attempt to develop a catalytic
transformation, it was found that the combination of 0.5 equiv
of CuI and 1,3-bis(diphenylphosphino)propane (dppp) also
furnished 28b in 85% yield. Having successfully formed the C−
N bond in this model system with copper(I), we shifted to the
C
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next phase of our studies, introduction of more complex and
delicate cinnabarinic acid electrophiles.
oxidation with benzoquinone.32 Simple filtration delivered 2-
amino-8-iodophenoxazin-3-one diester 31 in 56% yield from
acid 29. By carefully controlling the amount of Boc2O used,
slow addition of 1.2 equiv or addition of 2−3 equiv, the 2-
amino group of 31 was mono- or diprotected to give 32 or 33
in useful yields.33
Our efforts to join the two fragments of the plectosphaeroic
acids began by attempting to form a C−N bond between the
tert-butoxycarbonyl derivative 25 of gliocladin C and iodide 32
(Table 2). However, C−N bond formation was not observed
using any of the copper(I) conditions that we had identified
previously (e.g., excess CuCl, Cs2CO3, PhMe, 110 °C). In
addition, minor modifications of these conditions also were not
fruitful.34
C−N Cross-Coupling with Cinnabarinic Acid Deriva-
tives and Initial Attempts To Elaborate the Coupled
Products. Our cross-coupling approach required an efficient
route for the preparation of cinnabarinic acid derivatives having
a halide (or pseudohalide) incorporated at C8 (e.g., 31)
(Scheme 3). As noted previously, such intermediates should be
Scheme 3. Preparation of the 8-Iodophenoxazinone
Coupling Partners
A search for an alternate copper reagent,22a,b,d,e identified
copper(I) thiophene-2-carboxylate (CuTC)35,36 as particularly
effective (Table 2). When 25 was allowed to react with 2 equiv
of 32, 4 equiv of CuTC, and excess Cs2CO3 in toluene at 110
°C, coupled product 34 was formed, albeit in low yield (entry
1). The conversion to 34 was improved upon switching to
K2CO3 as base (entry 2), whereas a change from toluene to
dioxane proved detrimental (data not shown). In contrast to
our previous observations, substantial hydrodehalogenation and
minor amounts of homocoupling of iodide 32 were observed
when using CuTC; a result suggesting that this copper
complex, unlike other complexes investigated to this point,
was more efficient in oxidative addition to the C−I σ-bond of
32. A potential explanation for the efficacy of CuTC is its
increased solubility in nonpolar solvents.37,38 A breakthrough
was made when the electrophile was switched to iodide 33 in
which the 2-amino group of the iodophenoxazine was doubly
protected. Although the reaction of 25 with 1.5 equiv of 33 in
the presence of 3 equiv of CuTC and excess K2CO3 in toluene
at 110 °C was inefficient (entry 3),39 carrying out this reaction
at 90 °C gave 35 in 48% yield (entry 4). Further reduction of
the reaction temperature proved detrimental (entry 5).
available by oxidative dimerization of 6-halo-3-hydroxyanthra-
nilic acids and derivatives (e.g., 29).26,29,30 This transformation
could be performed with 3-hydroxy-6-iodoanthranilic acid
(29);31 however, we found it more convenient to esterify 29
and then immediately expose the intermediate ester 30 to
Table 2. Optimizing the CuTC-Mediated Cross-Coupling Reaction
a
entry
iodide
25/iodide
CuTC (equiv)
base
temp (°C)
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
32
32
33
33
33
33
33
33
33
1.0:2.0
1.0:2.0
1.0:1.5
1.0:1.5
1.0:1.5
1.0:1.5
1.0:1.5
1.0:2.3
1.0:3.0
4
4
3
3
3
3
6
3
3
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
110
110
110
90
24
24
34, <5
34, 10−20
b
24
35, <5
24
35, 48
35, 19
35, 57
35, 38
35, 67
35, 67
80
120
96
90
90
24
90
36
90
24
a
b
Yields of 34 or 35 after purification by preparatory TLC. Substantial amounts of 32 were observed at this temperature.
D
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However, additional optimization (entries 6−9) improved the
conversion to 35 to 67% yield.40
undesired, β diols 38a and 38b with high stereoselectivity (see
the table in Scheme 4).42 The relative configurations of the syn
diol products were assigned from the diagnostic H NOE’s
1
We were encouraged by the successful union of hexahydro-
2H-pyrazinopyrrolo[2,3-b]indole-1,4-dione 25 and iodide 33 to
increase the complexity of the amine nucleophile. We chose a
measured approach, first by examining whether a potentially
delicate N,O-acetal functionality at C3 would prove problem-
atic. To arrive at a coupling partner of this type,
methylmagnesium chloride was added chemoselectively to the
α-oxoimide carbonyl group of 25 followed by silylation of the
resulting alcohol products to give epimers 36a and 36b (1.2:1.0
dr) in 70% yield over two steps (Scheme 4). Submitting these
observed for their C11 methine hydrogens;12 additionally, the
minor isomer obtained from dihydroxylation of 37b was
transformed into bis(methylthio)ether 45b (vide infra). We
were forced to turn to investigate the crucial fragment coupling
with more complex hexahydro-2H-pyrazinopyrrolo[2,3-b]-
indole-1,4-dione intermediates already containing oxygen
functionality properly installed at C11 and C11a, a strategy
that proved ultimately to be successful.
Total Synthesis of (+)-Plectosphaeroic Acid B (2).
From the outset of this project, we envisaged the
pyrazinopyrroloindole fragment 5 of (+)-plectosphaeroic acid
B (2) originating from intermediates prepared by our group en
route to (+)-gliocladine C (5) and congeners.2c,12 During the
synthesis of 5, the bis(tert-butoxycarbonyl) derivative 23 of
(+)-gliocladin C was transformed in four efficient steps to
products 39a and 39b shown in Scheme 5. For the synthesis of
Scheme 4. Increasing Complexity in the C−N Cross-
Coupling and Unexpected Facial Selectivity During
Dihydroxylation of the C11,C11a-Double Bond
Scheme 5. Deprotection of the Indoline Nitrogen of 39a and
39b
2, we needed to remove the Boc group from the indoline
nitrogen atom of these intermediates, a task anticipated to be
complicated by the various acid-labile functionalities that were
present.
C3-N,O-acetals to the C−N cross-coupling conditions
identified in Table 2 showed that N-arylation was efficient
with both epimers to give the coupled products 37 in 68−72%
yields.
Buoyed by this success, we turned to advance toward the
plectosphaeroic acids by dihydroxylation of the alkene double
bond of coupled products 37a and 37b. In our previous studies
with congeners of 37 bearing a Boc protecting group on the
indoline nitrogen, we had observed high α-facial selectivity in
dihydroxylation of the C11−C11a π bond.2c,12 To our surprise,
oxidation of both 37a and 37b with OsO4/4-methylmorpholine
N-oxide (NMO), AD-mix α or AD-mix β41 afforded the
Salient results from our efforts to unmask the indoline
nitrogen atom of 39a and 39b are summarized in Scheme 5. As
we were concerned about the sensitivity of these molecules to
acid, we initially examined the two-step (single-pot) procedure
developed earlier in our studies (entry 1). This sequence
provided products 40a and 40b, whose relative and absolute
configuration was confirmed by single-crystal X-ray analysis, in
60−80% yield.43 In both products, the configuration of the
C11a acetate was inverted during the thermolytic Boc-removal
step, consistent with higher thermodynamic stability of a β-
C11a-epimer in this ring system. Although this sequence was
E
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Table 3. C−N Cross-Coupling with Late-Stage Hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-1,4-dione Intermediates
a
b
c
c
entry
indoline
40a
indoline/iodide
Cu(I) (equiv)
base
yield of 42 (%)
yield of 43 (%)
1
2
3
4
1:2−3
1:3
CuTC (3−6)
CuTC (6)
CuTC (6)
K2CO3
KOAc
CsOAc
KOAc
10−30
40
5−20
<5
d
40a
40a
1:3
<5
nd
40a or 40b
1:3
CuOAc (6)
51−58
a
b
c
d
All reactions conducted at 0.05 M. Excess base (4−8 equiv) was used. Yields after purification by preparatory TLC. nd = not determined.
best conducted on small scale (multiple vessels containing 40
mg of 39a or 39b were run in parallel), it allowed us to secure
sufficient quantities of 40a and 40b to move forward in the
synthesis. Success was registered also in the chemoselective
deprotection of 39a under acidic conditions. Exposure of this
epimer to 1.5 equiv of trifluoromethanesulfonic (triflic) acid in
CH2Cl2 at −40 °C for 2−3 h furnished 40a in 70−80% yield,
also with concomitant inversion of the C11a stereocenter.44
This transformation required careful monitoring, as prolonged
reaction times or an increase in temperature resulted in
expulsion of the silyloxy group at C3 to give substantial
amounts of α-methylene intermediate 41. Under identical
conditions, epimer 39b was converted exclusively to elimi-
nation product 41 (82% yield). Attempts to prevent the
formation of 41 from siloxy epimer 39b by lowering the
reaction temperature, reducing the amount of acid or using silyl
triflates (e.g., TMSOTf and TBSOTf) were uniformly
unsuccessful.
With convenient access to intermediates 40a and 40b in
hand, the essential C−N cross-coupling reaction of these more
intricate substrates was examined (Table 3). We began by
exploring the union of stereoisomer 40a and cinnabarinic
iodide 33. The CuTC/K2CO3 conditions that we had
previously found to be optimal with less elaborate intermediates
containing a C11,C11a double bond resulted in inefficient
conversion to coupled product 42a (entry 1). A major
byproduct formed in this reaction was the C11a thiophene-2-
carboxylate analog 43 of the starting diacetate 40a. As
activation of the angular N,O-acetal appeared unavoidable, to
minimize the formation of 43, KOAc was substituted for
K2CO3. This change decreased the formation of 43, giving
coupled product 42a in 40% yield (entry 2). It was eventually
found that exposure of 40a or 40b to 3 equiv of iodide 33, 6
equiv of CuOAc and excess KOAc in toluene at 90 °C delivered
42a and 42b in 51−58% yield (entry 4).45
Two methods were developed for introducing the methylthio
substituents of (+)-plectosphaeroic acid B (2) (Scheme 6). In
the most direct approach, reaction of either N,O-acetal epimer
of dioxopiperazines 42a or 42b with a large excess of both BF3·
OEt2 and methanethiol at −78 °C with slow warming to room
temperature gave an identical 1.3:1.0 separable mixture of
bis(methylthio)ethers 44b and 44a in high yield (79% from
42a, 92% from 42b). The relative configuration of these
products was secured from 1H NOE experiments.46 An
identical mixture of epimers was formed when a pure sample
of either stereoisomereric product was resubmitted to the
reaction conditions, establishing that the thermodynamic ratio
of C3 methylthio epimers is produced under these conditions.
In addition, this equilibration allowed the undesired C3 α-
methylthio epimer 44a to be recycled. Alternatively, in the
sequence we currently prefer, reaction of N,O-acetals 42 with
excess H2S and BF3·OEt2, followed by alkylation of the
resulting thiols with MeI afforded 44b as a single stereoisomer
in 80−88% yield.47 Bis(methylthio)ether 44b was prepared on
useful scales by both of the procedures summarized in Scheme
6.
The difference in stereochemical outcome of these two
sulfidation procedures is striking. Because bis(methylthio)-
ethers 44b and 44a were established to equilibrate under the
acidic conditions of the sulfidation step, a similar thermody-
namic equilibration would be expected to occur during the BF3·
OEt2-promoted reaction of 42 with H2S to form the dithiol
congeners of 44a and 44b. Thus, one potential explanation is a
considerably higher thermodynamic stability of the cis-dithiol
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carbon C3 substituent (Figure 1). We chose to initially address
the first obstacle by developing a method to elaborate
intermediates prepared en route to (+)-plectosphaeroic acid
B to an analogous product containing a trisulfide fragment, 13-
deoxyplectosphaeroic acid C (45).
Scheme 6. Completion of the Total Synthesis of
Plectosphaeroic Acid B (2)
The ring-expanding sequence that we eventually imple-
mented to incorporate the epitrisulfide functionality originated
from several observations made during our efforts to introduce
the methylthio groups of plectosphaeroic acid B (2).
Specifically, we had observed the formation of epidisulfide 46
(isolated in up to 80% yield) upon exposing N,O-acetals 42 to
excess H2S and BF3·OEt2 and subsequently to excess K2CO3 in
acetone in the absence of MeI (Scheme 7A). In addition, minor
quantities of 47, the trisulfide congener of 46, were also formed
1
from this sequence (10−20% by H NMR analysis). When no
attempt was made to remove the last traces of H2S from the
crude dithiol intermediate, these products were formed within
30 min in the second step.53 Consistent with our expectation
intermediates. Such a trend was seen in a computational study
of cis-dithiol and cis-bis(methylthio) isomers in closely related
model compounds.48 Alternatively, and in our view more likely,
a mixture of dithiol epimers equilibrates49 during the
methylation step, with the observed cis stereoselectivity
potentially resulting from intramolecular thio coordination of
the intermediate potassium thiolate nucleophile.
Scheme 7. (A) Epipolysulfide Formation and (B) Initial
Ring-Expansion Studies
All that remained to finish the total synthesis of
plectosphaeroic acid B (2) was cleavage of the three ester
groups of bis(methylthio) intermediate 44b. However, the
sensitivity of the cinnabarinic acid fragment was worrisome.50
Methanolysis of the acetyl group at C11 was achieved by
exposure of 44b to excess La(OTf)3 and 1 equiv of DMAP in
methanol at 50 °C.51 In the absence of DMAP, the
deprotection step was sluggish, and the resulting product was
not stable to prolonged exposure to the reaction conditions.
Attempts to promote concomitant hydrolysis of the methyl
esters of the phenoxazinone fragment by addition of small
amounts of water to this reaction led to decomposition. As a
result, the methyl esters were cleaved by classical demethylation
with LiI in pyridine at 90 °C.52 In this way, plectosphaeroic acid
B (2) was obtained in 65% yield over two steps after
purification by HPLC. The optical rotation of synthetic 2,
23
[α]D +228 (c 0.08, MeOH), was higher than the value
reported for the natural sample, [α]23D +69.8 (c 0.27, MeOH);
however, all other spectroscopic data, including CD spectra,
compared well.
Total Synthesis of (+)-Plectosphaeroic Acid C (3). As a
second total synthesis objective in this area, we selected
(+)-plectosphaeroic acid C (3) whose synthesis would present
two additional challenges: introduction of the bridging trisulfide
functionality and incorporation of oxygenation on the one-
G
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that trisulfide 47 was formed by ring-expansion of epidisulfide
precursor 46,21,54 exposure of a pure sample of 46 to 1 equiv of
Na2S in acetone at room temperature provided a 4−5:1 mixture
of epitrisulfide 47 and epitetrasulfide 48 products in combined
yields of up to 75% (Scheme 7B).55,56
Scheme 9. Elaboration of exo-Methylene 41 to Intermediates
51 and 52
A streamlined process for installing the epitrisulfide
functionality and confirmation that the alcohol and carboxylic
acid protecting groups could be removed in the presence of an
epitrisulfide was achieved in our successful elaboration of N,O-
acetals 42 to 13-deoxyplectosphaeroic acid C (45) (Scheme 8).
Scheme 8. Epitrisulfide Formation and Preparation of 13-
Deoxyplectosphaeroic Acid C (45)
After BF3·OEt2-mediated reaction of 42 with H2S, the crude
dithiol products were dissolved in acetone, followed by
sequential additions of K2CO3 and Na2S. In this way,
epitrisulfide 47 was isolated in 64% yield from the epimer
mixture 42. To our delight, the two-step sequence for cleaving
the ester groups developed en route to (+)-plectosphaeroic acid
B (2) was not adversely effected by the presence of the
epitrisulfide functionality, allowing 13-deoxyplectosphaeroic
acid C (45) to be prepared in 55% yield from epitrisulfide
precursor 47.
We turned to the synthesis of (+)-plectosphaeroic acid C (3)
itself. In order to install a hydroxyl group at C13, we envisaged
oxidizing the exomethylene double bond of intermediate 41, a
product that could be formed in high yield by the reaction of
intermediates 39a and 39b with triflic acid (Scheme 5).57
Oxidation of 41 by reaction with catalytic amounts of OsO4 in
the presence of NMO provided one major product, diol
diacetate 50 (Scheme 9). That the acetyl group had been
transferred from the angular C11a hydroxyl group of
intermediate 49 was apparent from the NMR signals of the
methylene group (1H NMR δ 4.42 (d) and 4.29 (d); 13C NMR
δ 65.4). Although minor amounts of 49, in which the C11a
acetyl group had not been transferred could be obtained, it was
more convenient to promote complete transfer of the acetyl
group by stirring the crude reaction product with SiO2 in ethyl
acetate prior to chromatographic purification to yield 50.
Silylation of the tertiary alcohols of 50 furnished pyrazino-
pyrroloindole 51, an 8:1 mixture of separable epimers, in 75%
overall yield from 41; these isomers undoubtedly result from
inconsequential partial epimerization at C3 during the silylation
step.49
KOAc in PhMe at 90 °C to give coupled products 52-major
and 52-minor in 72% and 74% yields, respectively. The
increase in efficiency in this C−N coupling step relative to the
corresponding N-arylation steps in the synthesis of 2 (50−58%
yield) is attributable to the enhanced stability of the C3- and
C11a-N,O-acetals of 51 and 52. Specifically, N-acyliminium ion
formation at C3 and C11a would be much less favorable in
these intermediates wherein the leaving group is a silyl ether
rather than an acetoxy substituent.
To no surprise, this diminished reactivity of the C3- and
C11a-N,O-acetals complicated the sulfidation step. For
example, treating 52-major with (i) H2S and BF3·OEt2 in
CH2Cl2 or MeNO2 at −78 °C to rt, (ii) H2S and Sc(OTf)3 in
MeCN at −78 °C to rt, or (iii) Na2S in a 1:1 mixture of TFA/
MeNO2 at 0 °C to rt were not successful. In these reactions,
either sulfur was not incorporated, or after prolonged exposure
(i.e., >12 h) decomposition was observed. For this reason, it
was decided to convert the oxygen substituents at C3 and C11a
to alternate leaving groups. Removal of both silyl groups of 52-
major and 52-minor upon reaction with TBAF and AcOH in
THF gave diols 53-major and 53-minor in >90% yields.58
By incorporating better leaving groups at C3 and C11a, the
total synthesis of (+)-plectosphaeroic acid C (3) was
successfully completed (Scheme 10).12,18 After standard
acetylation of a mixture of epimeric diols 53 arising from the
fragment-coupling and desilylation steps, the 5:1 mixture of
tetraacetates 54 was exposed to H2S and BF3·OEt2 in CH2Cl2
Joining of the two fragments of (+)-plectosphaeroic acid C
(3) was accomplished by individually treating epimers 51 with
3 equiv of iodide 33 and 6 equiv of CuOAc in the presence of
H
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cell lines, DU145 (human prostate) and A2058 (melanoma),
Scheme 10. Completion of the Total Synthesis of
Plectosphaeroic Acid C
were determined (Figure 2).63 It was found that 2, 3, and
at −78 °C to room temperature. After removing residual H2S
and stirring the crude dithiol products with SiO2 in ethyl
acetate, epidisulfide 55 and epitrisulfide 56 were produced in an
1
85:15 ratio (by H NMR analysis). Subsequent ring-expansion
of this mixture by reaction with Na2S gave a 2.5:1 mixture of
epitrisulfide 56 and epitetrasulfide 57, respectively. On larger
scales (30−60 mg), it was more convenient to process the
crude dithiol intermediates without rigorous removal of residual
H2S to yield a 1:6:2 mixture of epidi-, epitri-, and epitetrasulfide
products 55−57.59 As the central S−S σ-bond of tetrasulfides is
considerably weaker than the corresponding bonds of disulfides
or trisulfides,60 we were able to selectively reduce the
epitetrasulfide component of these mixtures by exposing the
reaction product to 1 equiv of triphenylphosphine (relative to
the amount of 57) in CH2Cl2 at room temperature.61 Ensuing
cleavage of the remaining ester groups then provided
(+)-plectosphaeroic acid C (3) in 33% overall yield from
tetraacetate 54. In the final, dealkylative transformation to
liberate the carboxylic acids of 3, switching to the non-
coordinating solvent toluene significantly increased the reaction
rate,62 which proved necessary as 3, unlike its C3 methyl
congener 45, was not stable to prolonged exposure to the
Figure 2. Activity against prostate (DU145) and melanoma (A2508)
cancer cell lines.
cinnabarinic acid (4)64 were inactive within the detection limits
of the assay (>5 μM) (entry 1). However, the dimethyl ester
analog 58 of plectosphaeroic acid C exhibited comparable
potency to that of T988 C (7) against the two cancer cell lines
(entries 2 and 3).65 Of note, the epidisulfide analogue 46 of
plectosphaeroic acid B and the structurally simpler ETP 59
having an identical hexahydro-2H-pyrazinopyrrolo[2,3-b]-
indole-1,4-dione moiety show similar activities (entries 4 and
5). Cinnabarinic acid dimethyl ester (60), a moderately active
inhibitor of IDO,11 was inactive (entry 6). In no instance were
the corresponding bis(methylthio) congeners active against
these cell lines (data not shown). Taken together, these data
indicate that (i) only the presence of the epipolysulfide
functionality is required for in vitro cytotoxicity activity against
the DU145 and A2058 cell lines and (ii) the presence of the
carboxylic acid substituents in plectosphaeroic acid C (3) is
detrimental to its cell-based cytotoxicity.
reaction conditions. The optical rotation of synthetic 3, [α]23
D
+494 (c 0.06, MeOH), was substantially higher than the value
reported for the natural sample, [α]23 +136 (c 0.17, MeOH);
D
however, the congruence of all other spectroscopic data,
including CD spectra, left little doubt that natural (+)-plectos-
phaeroic acid C (3) had been synthesized.
Preliminary Biological Evaluation. Having synthesized
(+)-plectosphaeroic acids B (2) and C (3), their activities and
those of selected related molecules against two invasive cancer
I
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the vial sequentially (under a N2 atmosphere). After 1 h (in some
cases, additional Boc2O was added to drive the reaction to
completion), the reaction mixture was poured into a mixture of
EtOAc (20 mL) and saturated aqueous NH4Cl (10 mL). The layers
were separated, and the aqueous layer was extracted with EtOAc (2 ×
10 mL). The combined organic layers were washed with brine (10
mL), dried over Na2SO4, and concentrated under reduced pressure.
The crude residue was purified by silica gel chromatography (1:3
EtOAc/hexanes) to afford the title compound (+)-25 (118 mg, 95%)
as a tan foam: [α]D +148, [α]577 +144, [α]546 +139 (c = 0.2,
CONCLUSION
■
The first total syntheses of plectosphaeroic acids B (2) and C
(3) are reported, syntheses that confirm the unique structure
and absolute configuration of the plectosphaeroic acids.4 The
synthetic sequence to (+)-2 and (+)-3 proceeded in 6 and 11
steps from known hexahydro-2H-pyrazinopyrrolo[2,3-b]indole-
1,4-diones 39a and 39b, which in turn are available in
enantiomerically pure form in 14 steps from indole and
isatin.12 Our approach to these natural products featured the
late-stage merger of the cinnabarinic acid and hexahydro-2H-
pyrazinopyrrolo[2,3-b]indole-1,4-dione fragments by a copper-
mediated process. The highly congested C−N bond generated
in this coupling, in conjunction with the delicate nature of the
densely functionalized precursors, provide striking testament to
the power of modern copper-mediated amination method-
s.22a,b,d,e The sequence developed in this investigation for
stereoselectively introducing the methylthio substituents of 2
and the ring-expanding procedure for fashioning the bridging
trisulfide of 3 were essential to simplifying the synthesis of
these polyfunctional natural products. We anticipate that the
late-stage ring-expansion procedure employed in these
syntheses should be of utility for synthesizing tri- and
tetrasulfide congeners of other epipolythiodiketopiperazines.
24
24
24
CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.15 (d, J = 8.3 Hz, 1H),
7.60 (s, 1H), 7.33 (d, J = 6.9 Hz, 1H), 7.30 (dt, J = 8.3, 1.0 Hz, 1H),
7.19 (d, J = 7.2 Hz, 1H), 7.15 (dt, J = 7.7, 1.2 Hz, 1H), 7.09 (dt, J =
8.0, 0.9 Hz, 1H), 7.01 (s, 1H), 6.88 (d, J = 7.9 Hz, 1H), 6.75−6.70
(comp, 2H), 6.28 (d, J = 2.6 Hz, 1H), 3.25 (s, 3H), 1.67 (s, 9H); 13C
NMR (125 MHz, acetone-d6) δ 158.6 (C), 158.0 (C), 150.7 (C),
150.3 (C), 150.1 (C), 137.9 (C), 133.9 (C), 130.2 (CH), 130.1 (C),
129.0 (C), 125.7 (CH), 125.4 (CH), 125.3 (CH), 124.3 (CH), 123.7
(CH), 122.0 (C), 120.9 (CH), 120.0 (CH), 116.3 (CH), 110.9 (CH),
85.0 (C), 84.0 (CH), 60.6 (C), 28.3 (CH3), 27.2 (CH3); IR (film)
3365, 3102, 3064, 2979, 1736, 1686, 1606, 1452; TLC Rf 0.32 (1:1
EtOAc/hexanes), 0.41 (2:98 MeOH/CH2Cl2, plate was eluted twice);
HRMS (ESI) m/z calcd for C27H24N4O5Na (M + Na)+ 507.1644,
found 507.1643.
tert-Butyl 3-((3aS,8aR)-2-(Methylcarbamoyl)-8,8a-
dihydropyrrolo[2,3-b]indol-3a(3H)-yl)-1H-indole-1-carboxylate
(27). Indoline 25 (12 mg, 25 μmol), Buchwald’s palladacyclic
precatalyst ligated with RuPhos66 (1.0 mg, 1.3 μmol), RuPhos (0.5
mg, 1.0 μmol), and Cs2CO3 (16 mg, 50 μmol) were added to a 1-dram
vial (in a nitrogen-filled glovebox). After t-BuOH (100 μL) was added
to this mixture, the vial was sealed (with a top, which contained a
Teflon-lined septum) and brought outside of the glovebox. Ethyl 2-
bromobenzoate (26a) (3.3 μL, 21 μmol) was injected into the reaction
vessel, and then the reaction mixture was heated at 85 °C (preheated
oil bath) for 12 h, at which point the reaction mixture was cooled to rt
and EtOAc (400 μL) was added. The mixture was filtered through a
short pad of diatomaceous earth, and the filtrate was concentrated in
vacuo. The crude residue was purified by preparatory thin layer
chromatography (1:4 EtOAc/hexanes) to afford the title compound
27 (2 mg, 19%, with no detectable quantity of the desired coupled
EXPERIMENTAL SECTION
■
N6-(tert-Butoxycarbonyl) derivative of gliocladin C (24). The
bis(tert-butoxycarbonyl) derivative 23 of gliocladin C12 (3 mg, 5.1
μmol) was heated in a sealed vial (neat) at 140 °C (preheated oil bath)
for 11 min. The reaction mixture was allowed to cool to rt, and then
the crude product was directly purified by preparatory thin-layer
chromatography (1:1 EtOAc/hexanes) to afford the title compound
24 as a yellow solid (2 mg, 83%; formation of minor quantities of
1
gliocladin C (19) also observed, 3:1 24/19): H NMR (500 MHz,
acetone-d6) δ 10.42 (br s, 1H), 7.88 (d, J = 8.1 Hz, 1H), 7.46 (d, J =
8.1 Hz), 7.40 (d, J = 7.4 Hz), 7.36 (t, J = 7.6 Hz), 7.33 (d, J = 8.2 Hz),
7.15−7.11 (comp, 3H), 7.02 (s, 1H), 6.94 (t, J = 7.4 Hz), 6.83 (s, 1H),
3.23 (s, 3H), 1.54 (s, 9H); 13C NMR (125 MHz, acetone-d6) δ 158.6
(C), 157.9 (C), 152.7 (C), 150.8 (C), 142.8 (C), 138.6 (C), 135.0
(C), 133.9 (C), 129.7 (CH), 126.3 (C), 125.64 (CH), 125.57 (CH),
125.1 (CH), 124.5 (CH), 123.1 (CH), 120.4 (CH), 119.9 (CH),
118.3 (CH), 115.3 (C), 112.9 (CH), 85.4 (CH), 82.9 (C), 59.2 (C),
28.2 (CH3), 27.2 (CH3); IR (film) 3331, 3105, 3058, 2977, 2930,
1714, 1683, 1599, 1478; TLC Rf 0.17 (1:1 EtOAc/hexanes); HRMS
(ESI) m/z calcd for C27H24N4O5Na (M + Na)+ 507.1644, found
507.1623.
N1′-(tert-Butoxycarbonyl) Derivative of Gliocladin C ((+)-25).
Method A. A solution of scandium triflate (1.6 mg, 3.3 μmol) and
MeCN (50 μL) was added dropwise to a solution of the bis(tert-
butoxycarbonyl) derivative 23 of gliocladin C (19 mg, 33 μmol) and
MeCN (0.33 mL) cooled to 0 °C. The reaction mixture was allowed
to warm to rt. After 3 h, the reaction mixture was poured into a
mixture of ethyl acetate (EtOAc) (10 mL) and saturated aqueous
NaHCO3 (5 mL). The layers were separated, and the aqueous layer
was further extracted with EtOAc (2 × 5 mL). The combined organic
layers were washed sequentially with H2O (10 mL) and brine (10
mL), dried over Na2SO4, and concentrated under reduced pressure.
The crude product was purified by silica gel chromatography (1:9
EtOAc/hexanes → 3:7 EtOAc/hexanes) to afford the title compound
(+)-25 as a yellow solid (12 mg, 76%).
1
product 28a): H NMR (500 MHz, CDCl3) δ 8.13 (br d, J = 6.3 Hz,
1H), 7.42 (d, J = 7.9 Hz, 1H), 7.30 (s, 1H), 7.29 (t, J = 7.6 Hz, 1H),
7.19 (d, J = 7.3, 1H), 7.18 (t, J = 7.3 Hz, 1H), 7.15−7.11 (comp, 2H),
6.82 (t, J = 7.4, 1H), 6.76 (d, J = 7.8, 1H), 6.02 (s, 1H), 4.67 (s, 1H),
3.85 (d, J = 18.6 Hz, 1H), 3.70 (d, J = 18.6 Hz, 1H), 2.90 (d, J = 5.1
Hz, 1H), 1.63 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 170.5 (C),
162.7 (C), 149.8 (C), 147.5 (C), 150.1 (C), 132.7 (C), 129.3 (CH),
128.6 (C), 125.3 (CH), 124.8 (CH), 123.4 (CH), 123.0 (C), 122.9
(CH), 120.5 (CH), 119.9 (CH), 115.9 (CH), 111.0 (CH), 96.6 (CH),
84.1 (C), 54.9 (C), 48.4 (CH2), 28.4 (CH3), 26.2 (CH3); HRMS
(ESI) m/z calcd for C25H26N4O3Na (M + Na)+ 453.1903, found
453.1908.
tert-Butyl 3-((5aR,10bS)-6-(2-(Methoxycarbonyl)phenyl)-2-
methyl-1,3,4-trioxo-1,2,3,4,5a,6-hexahydro-10bH-pyrazino-
[1′,2′:1,5]pyrrolo[2,3-b]indol-10b-yl)-1H-indole-1-carboxylate
((+)-28b). Indoline 25 (10 mg, 21 μmol), CuCl (8.0 mg, 81 μmol) [or
0.5 equiv of CuI/1,3-bis(diphenylphosphino)propane (dppp)], and
Cs2CO3 (34 mg, 105 μmol) were added to a 1-dram vial (in a
nitrogen-filled glovebox). After PhMe (400 μL, degassed via freeze−
pump−thaw cycles prior to use) and methyl 2-iodobenzoate (26b)
(64 μL, 42 μmol, 10% v/v solution in toluene) were added to this
mixture, the vial was sealed (with a Teflon-lined cap) and brought
outside of the glovebox. The heterogeneous mixture was stirred at 110
°C (preheated oil bath) for 24 h, at which point the reaction mixture
was cooled to rt and EtOAc (400 μL) was added. The mixture was
filtered through a short pad of diatomaceous earth, then the filtrate was
concentrated in vacuo. The crude residue was purified sequentially by
chromatography on silica gel (1:9 EtOAc/hexanes →3:2 EtOAc/
hexanes) then preparatory thin layer chromatography (to separate the
product from residual, copolar 25) (1.5% MeOH/CH2Cl2) to afford
Method B. The bis(tert-butoxycarbonyl) derivative 23 of gliocladin
C (150 mg, 260 μmol, concentrated in vacuo from a THF solution
into a vial) was heated (neat) under a slight vacuum (160 mmHg) at
180 °C (preheated oil bath) for 20 min (reaction times can vary
slightly; removal of both Boc groups of 23 were monitored by TLC).
The reaction mixture was allowed to cool to rt, and then THF (4 mL),
di-tert-butyl dicarbonate (Boc2O) (56 mg, 260 μmol), and N,N-
dimethyl-4-aminopyridine (DMAP) (6.0 mg, 4.9 μmol) were added to
J
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the title compound (+)-28b (11 mg, 85%) as an orange solid: [α]D
solution of 3-hydroxy-6-iodo-2-nitrobenzoic acid (S3) (7.3 g, 24
mmol) and THF (200 mL) was added a solution of Na2S2O4 (41 g,
240 mmol) and water (100 mL). The reaction mixture was stirred
vigorously at 50 °C for 3−4 h and allowed to cool to rt, and EtOAc
(300 mL) was added. The layers were separated, and the aqueous layer
was extracted with EtOAc (2 × 200 mL). The combined organic layers
were washed with brine (50 mL), dried over Na2SO4, and
concentrated under reduced pressure to afford the title compound
24
1
+168, [α]577 +170, [α]546 +174 (c = 0.9, CH2Cl2); H NMR (500
24
24
MHz, 353 K, DMSO-d6) δ 8.11 (d, J = 8.4 Hz, 1H), 7.84 (d, J = 7.0
Hz, 1H), 7.64 (t, J = 7.1 Hz, 1H), 7.56 (s, 1H), 7.50 (d, J = 7.3 Hz,
1H), 7.47 (t, J = 7.7 Hz, 1H), 7.36 (t, J = 8.2 Hz, 1H), 7.32 (d, J = 7.4
Hz, 1H), 7.23 (d, J = 7.2 Hz, 1H), 7.18 (t, J = 7.9 Hz, 1H), 7.08 (dt, J
= 7.9, 1.1 Hz, 1H), 7.03 (s, 1H), 6.74 (t, J = 7.4 Hz, 1H), 6.72 (s, 1H),
6.15 (d, J = 7.9 Hz, 1H), 3.44 (s, 3H), 3.17 (s, 3H), 1.66 (s, 9H); 13C
NMR (125 MHz, 353 K, DMSO-d6) δ 165.8 (C), 156.71 (C), 156.66
(C), 149.8 (C), 148.7 (C), 148.6 (C), 139.2 (C), 135.3 (C), 132.9
(CH), 132.0 (C), 130.3 (2 × CH), 128.7 (CH), 127.7 (CH), 127.4
(C), 127.2 (CH), 124.34 (CH), 124.32 (2 × CH), 124.1 (CH), 122.5
(CH), 119.9 (CH), 119.5 (C), 118.5 (CH), 114.7 (CH), 106.7 (CH),
86.6 (CH), 84.1 (C), 58.4 (C), 51.22 (CH3), 27.4 (CH3), 26.3 (CH3);
IR (film) 3101, 3054, 2979, 2950, 1731, 1715, 1687, 1598, 1452; TLC
Rf 0.35 (1:1 EtOAc/hexanes), 0.50 (2:98 MeOH/CH2Cl2, plate was
eluted twice); HRMS (ESI) m/z calcd for C35H30N4O7Na (M + Na)+
641.2012, found 641.1995.
6-Iodo-3-methoxy-2-nitrobenzoic Acid (S2). This procedure is
a modification of a procedure reported by Fairfax and Yang.29 To a
suspension of 2-methoxy-3-nitrobenzoic acid (S1) (10.0 g, 50.7 mmol)
in concentrated H2SO4 (150 mL) were added Ag2SO4 (20.6 g, 65.9
mmol) and iodine (13.5 g, 53.5 mmol). The heterogeneous mixture
was shielded from light (by covering the reaction vessel with
aluminum foil) and stirred vigorously at rt for 48 h, at which point
the reaction mixture was cooled in an ice−water bath and water (200
mL, ice−water mixture) was slowly added to the reaction mixture.
Caution: exotherm or rapid increase in temperature observed over the
course of the addition. The resulting precipitate was separated from the
mother liquor by filtration and then dissolved in acetone (200−300
mL). The insoluble material was removed by filtration. The mother
liquor was dried over Na2SO4 and concentrated under reduced
pressure to afford the title compound S2 (12 g, 73%) as a yellow solid
in sufficient purity to use in the next step: 1H NMR (500 MHz,
DMSO-d6) δ 8.07 (d, J = 8.9 Hz, 1H), 7.24 (d, J = 9.0 Hz, 1H), 3.91
(s, 3H); 13C NMR (125 MHz, DMSO-d6) δ 165.8 (C), 150.6 (C),
143.0 (CH), 138.7 (C), 134.1 (C), 117.1 (CH), 81.6 (C), 57.2 (CH3);
IR (film) 3074, ∼3000 (broad), 2947, 1713; HRMS (ESI) m/z calcd
for C8H6INO5Na (M + Na)+ 345.9189, found 345.9188; mp 178−181
°C.
1
29 (4.6 g, 70%) in sufficient purity to use in the next step: H NMR
(500 MHz, DMSO-d6) δ 9.82 (s, 1H), 7.80 (br s, 2H), 6.93 (d, J = 8.2
Hz, 1H), 6.48 (d, J = 8.2 Hz, 1H); 13C NMR (125 MHz, DMSO-d6) δ
169.0 (C), 144.8 (C), 137.5 (C), 126.8 (CH), 121.4 (C), 116.4 (CH),
80.7 (C); IR (KBr) 3423, 3221, 2921, 1603; HRMS (ESI) m/z calcd
for C7H7INO3 (M + H)+ 279.9471, found 279.9479; mp 180−182 °C
dec.
2-Amino-8-iodo-3-oxo-3H-phenoxazine-1,9-dicarboxylic
Acid (S5). To a solution of 3-hydroxy-4-iodoanthranilic acid 29 (50
mg, 0.18 mmol) in MeOH (4 mL) was added 1,4-benzoquinone (33
mg, 0.30 mmol, freshly recrystallized from petroleum ether).11,32 The
reaction mixture was maintained at 50 °C for 3 h (the reaction was
shielded from light by covering the reaction vessel with aluminum
foil), followed by placement in a freezer (−20 °C) for 12 h. The
resulting suspension was concentrated under reduced pressure (to
approximately 1/2 volume) and then filtered to separate the mother
liquor from the precipitate, which was subsequently washed with
EtOAc/hexane (10−20 mL, 1:1) to afford the title compound S5 (25
mg, 66%) as an orange-red solid with no further purification: 1H NMR
(500 MHz, DMSO-d6) δ 9.72 (s, 1H), 8.92 (s, 1H), 7.92 (d, J = 8.7
Hz, 1H), 7.40 (d, J = 8.6 Hz, 1H), 6.62 (s, 1H); 13C NMR (125 MHz,
DMSO-d6) δ 178.0 (C), 168.7 (C), 167.9 (C), 152.7 (C), 150.9 (C),
148.1 (C), 142.1 (C), 138.6 (CH), 137.7 (C), 127.7 (C), 118.3 (CH),
105.5 (CH), 92.4 (C), 87.0 (C); IR (KBr) 3402, 3282, 3082, 2917,
1721, 1645, 1578; HRMS (ESI) m/z calcd for C14H6IN2O6 (M − H)−
424.9271, found 424.9274.
Dimethyl 2-Amino-8-iodo-3-oxo-3H-phenoxazine-1,9-dicar-
boxylate (31). To a suspension of 3-hydroxy-4-iodoanthranilic acid
29 (4.4 g, 16 mmol) in a cosolvent mixture of PhMe and MeOH (160,
50 mL, respectively) was slowly added trimethylsilyldiazomethane
(TMSCHN2) (8.8 mL, 18 mmol, 2.0 M solution in hexane) with
stirring. Caution: evolution of N2(g) over the course of the addition. After
30 min, the resulting solution was poured into a mixture of EtOAc (50
mL) and water (25 mL). The layers were separated, and the aqueous
layer was extracted with EtOAc (2 × 20 mL). The combined organic
layers were washed with brine (10 mL), dried over Na2SO4, and
concentrated under reduced pressure to afford methyl 3-hydroxy-4-
iodoanthranilate (30), which was used immediately in the following
transformation: TLC Rf 0.45 (3:7 EtOAc/hexanes). To a solution of 2-
aminophenol 30 (16 mmol) in MeOH (160 mL) was added 1,4-
benzoquinone (3.0 g, 27 mmol, freshly recrystallized from petroleum
ether).11,32 The reaction mixture was maintained at 50 °C for 3 h (the
reaction was shielded from light by covering the reaction vessel with
aluminum foil), followed by placement in a freezer (−20 °C) for 12 h.
The resulting suspension was concentrated under reduced pressure (to
approximately 1/2 volume), then filtered to separate the mother liquor
from the precipitate, which was subsequently washed with EtOAc/
hexane (100 mL, 1:1) to afford the title compound 31 (2.0 g, 56%) as
3-Hydroxy-6-iodo-2-nitrobenzoic Acid (S3). This procedure is
a modification of a procedure reported by Fuji and co-workers.67 To a
solution of 6-iodo-3-methoxy-2-nitrobenzoic acid (S2) (14.5 g, 44.9
mmol), Et2S (40 mL), and CH2Cl2 (400 mL) was added AlCl3 (21.0 g,
157 mmol) in a portionwise fashion. The reaction mixture was stirred
at 40 °C for 48 h (the reaction progress was monitored by 1H NMR of
aliquots sampled during the course of the reaction) and then allowed
to cool to rt and concentrated under reduced pressure (to
approximately 1/3 volume). The crude residue was poured into a
mixture of EtOAc (600 mL) and aqueous solution of HCl (0.125 M,
450 mL). The layers were separated, and the aqueous layer was
extracted with EtOAc (2 × 200 mL). The combined organic layers
were washed with brine (200 mL), dried over Na2SO4 and
concentrated under reduced pressure. The crude product was
crystallized (in some cases, precipitated) from CH2Cl2 (100 mL) in
order to remove minor amounts of the hydrodehalogenated
byproduct, 3-hydroxy-2-nitrobenzoic acid (S4) (CAS no. 602-00-6)
1
an orange-red solid of sufficient purity to use in the next step: H
1
NMR (500 MHz, DMSO-d6) δ 8.00 (br s, 2H), 7.87 (d, J = 7.4 Hz,
1H), 7.39 (d, J = 7.4 Hz, 1H), 6.49 (s, 1H), 3.92 (s, 3H), 3.80 (s, 3H);
13C NMR (125 MHz, DMSO-d6) δ 178.4 (C), 167.2 (C), 167.1 (C),
148.9 (C), 148.4 (C), 146.2 (C), 141.3 (C), 138.6 (C), 138.0 (CH),
131.6 (C), 118.4 (CH), 104.3 (CH), 99.0 (C), 86.6 (C), 52.7 (CH3),
51.6 (CH3); IR (film) 3446, 3326, 3075, 2948, 2922, 2854, 1730,
1672, 1641, 1575; TLC Rf 0.36 (3:7 EtOAc/hexanes), 0.52 (3:7
EtOAc/hexanes, plate was eluted twice); HRMS (ESI) m/z calcd for
C16H11IN2O6Na (M + Na)+ 476.9560, found 476.9573; mp 233−235
°C.
(1:10, S4/S3 by H NMR). After removal of the mother liquor by
filtration, the product was redissolved in Et2O to remove residual,
purple-colored insoluble material, which was filtered away. The filtrate
was concentrated under reduced pressure to afford the title compound
1
S3 (6.10 g, second crop: 2.10 g; combined 59%) as a tan solid: H
NMR (500 MHz, CD3OD) δ 7.89 (d, J = 8.8 Hz, 1H), 6.90 (d, J = 8.8
Hz, 1H); 13C NMR (125 MHz, CD3OD) δ 168.7 (C), 152.5 (C),
145.0 (CH), 138.6 (C), 137.2 (C), 122.4 (CH), 79.7 (C); IR (KBr)
3411, 3088, ∼3000 (broad), 2898, 2654, 2590, 2526, 1701, 1600,
1442; HRMS (ESI) m/z calcd for C7H3INO5 (M − H)− 307.9056,
found 307.9051; mp 174−176 °C dec.
Dimethyl 2-((tert-Butoxycarbonyl)amino)-8-iodo-3-oxo-3H-
phenoxazine-1,9-dicarboxylate (32). To a solution of 2-amino-
phenoxazinone 31 (300 mg, 0.66 mmol) in THF (13 mL) were added
3-Hydroxy-6-iodoanthranilic Acid (29). This procedure is a
modification of a procedure reported by Fairfax and Yang.29 To a
K
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
sequentially Boc2O (170 mg, 0.79 mmol) and DMAP (16 mg, 0.13
mmol). After 2 h, the reaction mixture was poured into a mixture of
EtOAc (60 mL) and saturated aqueous NH4Cl (30 mL). The layers
were separated, and the aqueous layer was extracted with EtOAc (2 ×
30 mL). The combined organic layers were washed with brine (30
mL), dried over Na2SO4, and concentrated under reduced pressure.
The crude residue was purified by silica gel chromatography (1:3
EtOAc/hexanes) to afford the title compound 32 (200 mg, 54%; the
remainder of mass balance consisted of a copolar mixture of starting
material 31 and the bis(tert-butoxycarbonyl) derivative 33) as an
1714, 1692, 1642; TLC Rf 0.41 (4:6 EtOAc/hexanes); HRMS (ESI)
m/z calcd for C53H50N6O15Na (M + Na)+ 1033.3232, found
1033.3240.
tert-Butyl 3-((5aR,10bS)-3-Hydroxy-2,3-dimethyl-1,4-dioxo-
1,2,3,4,5a,6-hexahydro-10bH-pyrazino[1′,2′:1,5]pyrrolo[2,3-b]-
indol-10b-yl)-1H-indole-1-carboxylate (S6). This procedure is a
modification of a procedure reported by Overman and co-workers.12 A
THF (11 mL) solution of indoline 25 (260 mg, 0.54 mmol) was
cooled to −78 °C, and methylmagnesium chloride (540 μL, 1.6 mmol,
2.9 M solution in THF) was added slowly. The reaction mixture was
maintained at −78 °C for 3 h, and AcOH (120 μL, 2.0 mmol) was
added. The cooling bath was removed, and the reaction mixture was
allowed to warm to rt. The solvent was removed under reduced
pressure, and H2O (15 mL) was added to the concentrate. The
aqueous phase was extracted with EtOAc (3 × 15 mL), and the
combined organic layers were washed with brine (15 mL), dried over
Na2SO4, and then concentrated under reduced pressure. The crude
residue was purified by chromatography on silica gel (1:3 EtOAc/
hexanes →2:3 EtOAc/hexanes) to afford the title compound S6 (190
mg, 71%, 3:2 mixture of alcohol epimers) as an amorphous colorless
solid: 1H NMR (500 MHz, CDCl3) δ 8.12 (d, J = 6.4 Hz, 0.6H), 8.08
(d, J = 6.4 Hz, 0.4H), 7.49 (s, 0.6H), 7.46 (s, 0.4H), 7.27 (m, 0.4H),
7.21 (t, J = 7.7 Hz, 0.4H), 7.14 (t, J = 7.5 Hz, 0.4H), 7.08−7.00 comp,
3.8H), 6.94 (d, J = 7.5 Hz, 1H), 6.77−6.72 (comp, 1H), 6.67 (d, J =
7.9 Hz, 0.6H), 6.67 (d, J = 7.8 Hz, 0.4H), 6.59 (s, 1H), 6.57 (s, 1H),
6.20 (d, J = 2.6 Hz, 0.6H), 6.12 (app s, 0.4H), 5.42 (app s, 0.4H), 5.38
(app s, 0.6H), 4.29 (s, 0.6H), 4.25 (s, 0.4H), 3.10 (s, 1.8H), 3.09 (s,
1.2H), 1.73 (s, 3H), 1.68−1.66 (comp, 12H); 13C NMR (125 MHz,
CDCl3) δ 165.0 (C), 164.5 (C), 155.7 (2 × C), 149.8 (2 × C), 148.0
(C), 147.9 (C), 136.4 (2 × C), 132.5 (C), 132.2 (C), 129.7 (C),
129.42 (CH), 129.38 (CH), 129.2 (C), 128.1 (2 × C), 125.1 (CH),
124.9 (CH), 124.8 (CH), 124.7 (CH), 123.3 (2 × CH), 123.1 (CH),
123.0 (CH), 121.8 (CH), 121.5 (C), 121.2 (CH), 121.1 (C), 120.2
(CH), 119.9 (CH), 119.8 (CH), 115.8 (CH), 115.7 (CH), 110.0
(CH), 109.8 (CH), 85.3 (C), 84.9 (C) 84.6 (C), 84.5 (C), 83.2 (CH),
83.1 (CH), 59.8 (C), 59.5 (C), 28.44 (CH3), 28.42 (CH3), 27.5
(CH3), 27.3 (CH3), 26.5 (CH3), 26.1 (CH3); IR (film) 3339, 2979,
1734, 1675, 1642, 1453; TLC Rf 0.15 (3:7 EtOAc/hexanes); HRMS
(ESI) m/z calcd for C28H28N4O5Na (M + Na)+ 523.1957, found
523.1953.
1
orange/red solid: H NMR (500 MHz, CDCl3) δ 8.01 (s, 1H), 7.88
(d, J = 8.7 Hz, 1H), 7.18 (d, J = 8.7 Hz, 1H), 6.48 (s, 1H), 4.03 (s,
3H), 3.92 (s, 3H), 1.50 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 179.0
(C), 166.8 (C), 164.4 (C), 151.1 (C), 148.1 (C), 147.4 (C), 142.6
(C), 141.3 (CH), 141.0 (C), 135.0 (C), 131.9 (C), 118.4 (CH), 116.9
(C), 105.2 (CH), 86.4 (C), 83.0 (C), 53.3 (CH3), 52.5 (CH3), 28.2
(CH3); IR (film) 3330, 2980, 2950, 2917, 2849, 1737, 1631, 1509,
1492; TLC Rf 0.64 (3:7 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C21H19IN2O8Na (M + Na)+ 577.0084, found 577.0086.
Dimethyl 2-(Bis(tert-butoxycarbonyl)amino)-8-iodo-3-oxo-
3H-phenoxazine-1,9-dicarboxylate (33). To a THF (88 mL)
solution of 2-aminophenoxazinone 31 (2.0 g, 4.4 mmol) were
sequentially added Boc2O (2.3 g, 11 mmol) and DMAP (50 mg,
0.10 mmol). After 2 h (in some cases, additional Boc2O was added to
drive the reaction to completion), the reaction mixture was poured
into a mixture of EtOAc (300 mL) and saturated aqueous NH4Cl (200
mL). The layers were separated, and the aqueous layer was extracted
with EtOAc (2 × 100 mL). The combined organic layers were washed
with brine (100 mL), dried over Na2SO4, and concentrated under
reduced pressure. The crude residue was purified by silica gel
chromatography (1:3 EtOAc/hexanes) to afford the title compound
33 (2.2 g, 76%) as a red solid: 1H NMR (500 MHz, CDCl3) δ 8.01 (s,
1H), 7.92 (d, J = 8.7 Hz, 1H), 7.15 (d, J = 8.7 Hz, 1H), 6.42 (s, 1H),
3.99 (s, 3H), 3.91 (s, 3H), 1.41 (s, 18H); 13C NMR (125 MHz,
CDCl3) δ 179.9 (C), 166.5 (C), 162.6 (C), 149.2 (C), 148.2 (C),
147.5 (C), 143.5 (C), 142.8 (CH), 141.4 (C), 138.5 (C), 133.8 (C),
131.3 (C), 118.7 (CH), 107.2 (CH), 86.2 (C), 84.2 (C), 53.3 (CH3),
53.0 (CH3), 27.9 (CH3); IR (film) 2980, 2953, 1801, 1745, 1640;
TLC Rf 0.48 (3:7 EtOAc/hexanes, plate was eluted twice); HRMS
(ESI) m/z calcd for C26H27IN2O10Na (M + Na)+ 677.0608, found
677.0611.
tert-Butyl 3-((5aR,10bS)-3-((tert-Butyldimethylsilyl)oxy)-2,3-
dimethyl-1,4-dioxo-1,2,3,4,5a,6-hexahydro-10bH-pyrazino-
[1′,2′:1,5]pyrrolo[2,3-b]indol-10b-yl)-1H-indole-1-carboxylate
(36). This procedure is a modification of a procedure reported by
Overman and co-workers.12 To a solution of alcohols S6 (190 mg,
0.38 mmol), DMAP (46 mg, 0.38 mmol), and triethylamine (0.53 mL,
3.8 mmol) in DMF (4.0 mL) maintained at 0 °C was added TBSOTf
(0.52 mL, 2.3 mmol) slowly. The cold bath was removed, and the
reaction mixture was maintained at rt for 12 h, whereupon EtOAc (20
mL) was added followed by saturated aqueous NH4Cl (20 mL). The
mixture was transferred to a separatory funnel, the layers were
separated, and the aqueous phase was further extracted with EtOAc (2
× 15 mL). The combined organic layers were washed with brine (15
mL), dried over Na2SO4, and concentrated under reduced pressure.
The crude residue was purified by chromatography on silica gel (1:3
EtOAc/hexanes →2:3 EtOAc/hexanes) to afford the title compounds
36 as colorless solids (125 mg of the major epimer 36a, 105 mg of the
minor epimer 36b; 98% overall yield, 1.2:1.0 ratio of C3-epimers).
Cross-Coupled Product ((−)-35). The components of this
reaction were combined in a screw-top vial inside a N2-filled glovebox.
The reaction vial was sealed with a Teflon-lined cap, brought outside
the glovebox, and heated in an aluminum block for the period of time
indicated. The reaction vessel was charged with indoline 25 (10 mg, 21
μmol), phenoxazinone iodide 33 (30 mg, 46 μmol), copper(I)
thiophene-2-carboxylate (CuTC)36 (12 mg, 62 μmol), and K2CO3 (11
mg, 83 μmol), and then PhMe (200 μL) was added by syringe. The
heterogeneous mixture was stirred at 90 °C for 36 h. After being
allowed to cool to rt, the reaction mixture was passed directly through
a short plug of silica gel (eluting with EtOAc) to remove the metal
salts, then concentrated under reduced pressure. The crude residue
was purified by chromatography on silica gel (1:19 EtOAc/hexanes →
1:3 EtOAc/hexanes) to afford the title compound (−)-35 (14 mg,
67%) as a red solid: [α]D24 −68.7, [α]57724 −72.2 (c = 0.075, CH2Cl2);
1H NMR (500 MHz, CDCl3) δ 8.18 (d, J = 7.0 Hz, 1H), 7.68 (d, J =
7.3 Hz, 1H), 7.6−7.5 (comp, 2H), 7.35 (t, J = 7.6 Hz, 1H), 7.15 (t, J =
7.6 Hz, 1H), 7.12 (t, J = 7.3 Hz, 1H), 7.05 (d, J = 4.1 Hz, 1H), 6.96 (d,
J = 7.6, 1H), 6.83 (s, 1H), 6.78 (m, 1H), 6.75 (s, 1H), 6.45 (s, 1H),
6.37 (d, J = 7.0 Hz, 1H), 3.84 (s, 3H), 3.63 (s, 3H), 3.41 (s, 3H), 1.70
(s, 9H), 1.43 (s, 18H); 13C NMR (125 MHz, CDCl3) δ 180.1 (C),
165.2 (C), 162.6 (C), 157.2 (C), 157.0 (C), 150.4 (C), 149.6 (C),
149.1 (C), 148.7 (C), 148.5 (CH), 147.3 (C), 143.0 (CH), 138.3 (C),
136.5 (C), 136.3 (C), 135.8 (C), 133.8 (C), 133.5 (CH), 131.6 (C),
131.0 (C), 129.8 (CH), 127.8 (C), 126.9 (C), 126.3 (CH), 125.4
(CH), 124.9 (CH), 124.1 (CH), 123.4 (CH), 120.8 (CH), 119.5 (2 ×
CH), 119.1 (C), 116.2 (CH), 108.2 (CH), 106.9 (CH), 88.0 (CH),
85.0 (C), 84.2 (C), 59.3 (C), 52.9 (2 × CH3), 28.4 (CH3), 27.9
(CH3), 27.7 (CH3); IR (film) 3056, 2980, 2952, 2933, 1804, 1741,
Data for the major, C3α-OTBS epimer, (+)-36a: [α]D24 +187, [α]577
24
+195, [α]54624 +220, [α]43524 +352 (c = 0.54, CH2Cl2); 1H NMR (500
MHz, CDCl3) δ 8.14 (d, J = 7.2 Hz, 1H), 7.46 (s, 1H), 7.29 (t, J = 7.2
Hz, 1H), 7.15−7.05 (comp, 3H), 7.02 (d, J = 7.4 Hz, 1H), 6.76 (d, J =
7.9 Hz, 1H), 6.74 (t, J = 7.4 Hz, 1H), 6.57 (s, 1H), 6.16 (s, 1H), 5.35
(s, 1H), 3.07 (s, 3H), 1.74 (s, 3H), 1.67 (s, 9H), 0.86 (s, 9H), 0.10 (s
3H), 0.01 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 163.0 (C), 156.0
(C), 149.7 (C), 148.1 (C), 136.4 (C), 132.6 (C), 129.6 (C), 129.3
(CH), 128.2 (C), 125.0 (CH), 124.7 (CH), 123.5 (CH), 123.0 (CH),
121.3 (C), 120.9 (CH), 120.1 (2 × CH), 115.7 (CH), 109.9 (CH),
87.1 (C), 84.3 (C) 83.4 (CH), 59.5 (C), 28.4 (CH3), 27.8 (CH3), 26.5
(CH3), 25.8 (CH3), 18.4 (C), −3.6 (CH3), −3.9 (CH3); IR (film)
L
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The Journal of Organic Chemistry
Article
3356, 3054, 2953, 2931, 1736, 1682, 1649, 1607, 1453; TLC Rf 0.38
(3:7 EtOAc/hexanes); HRMS (ESI) m/z calcd for C34H42N4O5SiNa
(M + Na)+ 637.2822, found 637.2820. Data for minor, C3β-OTBS
epimer, (+)-36b: [α]D +118, [α]577 +121, [α]546 +134, [α]435
(−)-38b and S7 (8 mg, 77%, 7:1 dr). The analogous reaction of
(−)-37b (10 mg, 8.8 μmol) using OsO4 (22 mg, 2.1 μmol, 2.5 wt % in
t-BuOH) and NMO (2 mg, 17 μmol) in acetone/H2O (4:1, 2 mL)
provided isomers (−)-38b and S7 (5 mg, 49%, reaction was quenched
prior to full consumption of starting material, 5:1 dr) as red solids.
Data for the minor isomer S7:70 1H NMR (500 MHz, acetone-d6) δ
8.55 (s 1H), 8.18 (d, J = 8.0 Hz, 1H), 8.00 (d, J = 8.5 Hz, 1H), 7.77 (d,
J = 8.7 Hz, 1H), 7.28 (t, J = 7.5 Hz, 1H), 7.13 (t, J = 7.8 Hz, 1H), 7.10
(s, 1H), 7.09 (t, 1H), 6.94 (d, J = 7.3 Hz, 1H), 6.77 (s, 1H), 6.70 (t, J
= 7.4 Hz, 1H), 6.53 (s, 1H), 6.30 (d, J = 7.9 Hz, 1H), 6.00 (s, 1H),
5.52 (d, J = 5.7 Hz, 1H), 4.74 (s, 1H), 3.90 (s, 3H), 3.64 (s, 3H), 3.02
(s, 3H), 1.70 (s, 12H), 1.40 (s, 18H), 0.78 (s, 9H), 0.07 (s, 3H), −0.47
(s, 3H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 171.2 (C),
166.5 (C), 165.9 (C), 163.7 (C), 150.6 (C), 149.9 (C), 149.5 (C),
149.1 (C), 147.5 (C), 143.5 (C), 138.83 (C), 138.75 (C), 138.0 (C),
136.0 (C), 134.8 (C), 134.0 (C), 133.1 (CH), 131.6 (C), 131.2 (C),
129.5 (CH), 129.1 (CH), 124.7 (CH), 124.4 (CH), 123.4 (CH),
121.1 (CH), 120.7 (CH), 119.8 (CH), 116.9 (C), 116.4 (CH), 109.9
(CH), 106.9 (CH), 88.3 (C), 86.5 (CH), 85.5 (C), 84.6 (C), 83.7
(C), 82.4 (CH), 57.7 (C), 53.2 (CH3), 52.7 (CH3), 28.4 (CH3), 28.0
(CH3), 27.4 (CH3), 26.3 (2 × CH3), 19.4 (C), −2.46 (CH3), −3.39
(CH3); TLC Rf 0.07 (2:3 EtOAc/hexanes); LRMS (ESI) m/z calcd
24
24
24
24
+155 (c = 0.54, CH2Cl2); 1H NMR (500 MHz, CDCl3) δ 8.13 (d, J =
6.5 Hz, 1H), 7.46 (s, 1H), 7.27 (t, J = 7.1 Hz, 1H), 7.15−7.05 comp,
3H), 6.74 (d, J = 7.4, 1H), 6.74 (d, J = 7.7 Hz, 1H), 6.73 (t, J = 7.6 Hz,
1H), 6.59 (s, 1H), 6.21 (s, 1H), 5.38 (s, 1H), 3.07 (s, 3H), 1.73 (s,
3H), 1.67 (s, 9H), 0.86 (s, 9H), 0.12 (s 3H), 0.01 (s, 3H); 13C NMR
(125 MHz, CDCl3) δ 163.1 (C), 155.6 (C), 149.7 (C), 148.0 (C),
136.4 (C), 132.5 (C), 129.7 (C), 129.3 (CH), 128.2 (C), 125.0 (CH),
124.7 (CH), 123.4 (CH), 123.0 (CH), 121.3 (C), 121.0 (CH), 120.0
(CH), 119.9 (CH), 115.7 (CH), 109.7 (CH), 87.1 (C), 84.4 (C) 83.4
(CH), 59.3 (C), 28.4 (CH3), 27.9 (CH3), 27.1 (CH3), 25.8 (CH3),
18.5 (C), −3.5 (CH3), −3.8 (CH3); IR (film) 3365, 3064, 2979, 1736,
1686, 1606, 1452; TLC Rf 0.44 (3:7 EtOAc/hexanes); HRMS (ESI)
m/z calcd for C34H42N4O5SiNa (M + Na)+ 637.2822, found 637.2825.
Cross-Coupled Product (−)-37b (C3β-OTBS epimer). The
components of this reaction were combined in a screw-top vial inside a
N2-filled glovebox. The reaction vial was sealed with a Teflon-lined
cap, then brought outside the glovebox and heated in an aluminum
block for the period of time indicated. The reaction vessel was charged
with indoline 36b (40 mg, 65 μmol), phenoxazinone iodide 33 (130
mg, 200 μmol), copper(I) thiophene-2-carboxylate (CuTC)36 (63 mg,
330 μmol), and K2CO3 (50 mg, 360 μmol), and then PhMe (2.6 mL)
was added by syringe. The heterogeneous mixture was stirred at 90 °C
for 48 h. After being allowed to cool to rt, the reaction mixture was
passed directly through a short plug of silica gel (eluting with EtOAc)
to remove the metal salts and then concentrated under reduced
pressure. The crude residue was purified by chromatography on silica
gel (1:19 EtOAc/hexanes →1:3 EtOAc/hexanes) to afford the title
for C60H70N6O17SiNa (M + Na)+ 1197.4, found 1197.5. Data for the
1
major isomer (−)-38b: [α]D −47.6 (c = 0.03, CH2Cl2); H NMR
24
(500 MHz, acetone-d6) δ 8.17 (d, J = 8.5 Hz, 1H), 7.85 (d, J = 8.8 Hz,
1H), 7.70 (s, 1H), 7.60 (d, J = 8.8 Hz, 1H), 7.46 (d, J = 7.3 Hz, 1H),
7.41 (d, J = 8.0 Hz, 1H), 7.33 (t, J = 7.6 Hz, 1H), 7.16 (t, J = 7.6 Hz,
1H), 7.10 (t, J = 7.7 Hz, 1H), 6.76 (d, J = 7.5 Hz, 1H), 6.50 (s, 1H),
6.41 (s, 1H), 6.34 (d, J = 7.5 Hz, 1H), 5.18 (br s, 1H), 5.11 (app s,
1H), 4.72 (br s, 1H), 3.88 (s, 3H), 3.73 (s, 3H), 2.98 (s, 3H), 1.67 (s,
9H), 1.65 (s, 3H), 1.39 (s, 18H), 0.89 (s, 9H), 0.15 (s, 3H), −0.01 (s,
3H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 168.5 (C), 166.7
(C), 166.2 (C), 163.7 (C), 150.3 (C), 150.2 (C), 149.8 (C), 149.4
(C), 147.6 (C), 143.6 (C), 138.9 (C), 137.2 (C), 136.6 (C), 136.5
(CH), 135.7 (C), 134.9 (C), 131.8 (C), 129.5 (CH), 129.4 (C), 129.3
(C), 128.7 (CH), 125.4 (CH), 125.1 (CH), 123.8 (CH), 123.0 (C),
121.3 (CH), 119.7 (CH), 119.0 (CH), 116.2 (CH), 108.3 (CH),
106.8 (CH), 86.9 (C), 84.9 (C), 84.5 (C), 83.7 (C), 79.9 (2 × CH),
63.3 (CH3), 55.6 (C), 52.5 (CH3), 28.3 (CH3), 28.0 (CH3), 27.9
(CH3), 26.1 (CH3), 25.0 (CH3), 18.9 (C), −2.12 (CH3), −2.96
(CH3); IR (film) 3457, 2978, 2953, 2932, 2857, 1806, 1740, 1699,
1643, 1371; TLC Rf 0.20 (2:3 EtOAc/hexanes); HRMS (ESI) m/z
calcd for C60H70N6O17SiNa (M + Na)+ 1197.4464, found 1197.4486.
compound 37b (51 mg, 68%) as a red solid: [α]D −150 (c = 0.05,
24
MeOH); 37b was observed as a 9:1 mixture of atropisomers by NMR
at 298 K (subsequent data is provided for the major isomer only); 1H
NMR (500 MHz, acetone-d6) δ 8.20 (d, J = 8.3 Hz, 1H), 7.98 (br d,
1H), 7.80−7.70 (comp, 2H), 7.36 (m, 1H), 7.20−7.10 (comp, 4H),
6.77 (t, J = 6.9 Hz, 1H), 6.67 (s, 1H), 6.56 (s, 1H), 6.52 (s, 1H), 6.38
(d, J = 7.9 Hz, 1H), 3.89 (s, 1H), 3.58 (br s, 3H), 3.09 (s, 1H), 1.68 (s,
9H), 1.55 (s, 3H), 1.39 (s, 18H), 0.83 (s, 9H), −0.08 (s, 3H), −0.35
(s, 3H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 165.7 (C),
165.4 (C), 163.6 (2 × C), 155.9 (C), 150.3 (C), 150.0 (C), 149.8 (C),
149.6 (C), 147.6 (C), 143.8 (C), 138.9 (C), 137.3 (C), 136.5 (C),
134.8 (CH), 133.4 (C), 131.7 (C), 129.7 (CH), 129.0 (C), 125.66
(CH), 125.59 (CH), 125.4 (C), 123.9 (CH), 121.1 (C), 121.0 (CH),
120.9 (CH), 120.8 (CH), 119.9 (CH), 116.5 (CH), 109.0 (CH),
106.9 (CH), 88.7 (CH), 88.0 (C), 85.1 (C), 83.7 (C), 59.9 (C), 53.2
(CH3), 52.7 (CH3), 28.3 (CH3), 28.0 (CH3), 27.6 (CH3), 27.4 (CH3),
26.3 (CH3), 19.3 (C), −2.7 (CH3), −3.4 (CH3);68 IR (film) 2977,
2953, 2917, 2850, 1806, 1743, 1701, 1643; TLC Rf 0.35 (3:7 EtOAc/
hexanes, plate was eluted twice); HRMS (ESI) m/z calcd for
C60H68N6O15SiNa (M + Na)+ 1163.4409, found 1163.4402.
1
Note, for comparison, the H NMR of (−)-38b was also obtained in
1
CDCl3: H NMR (500 MHz, CDCl3) δ 8.14 (br d, J = 5.7 Hz, 1H),
7.73 (d, J = 8.8 Hz, 1H), 7.60 (s, 1H), 7.51 (br d, 1H), 7.36 (br d,
1H), 7.29 (t, J = 7.6 Hz, 1H), 7.18 (t, J = 7.9 Hz, 1H), 7.18−7.12
(comp, 2H), 6.83 (t, J = 7.4 Hz, 1H), 6.43 (s, 1H), 6.40 (br s, 1H),
4.96 (s, 1H), 3.87 (s, 3H), 3.81 (br s, 1H), 3.67 (br s, 1H), 3.17 (s,
3H), 3.01 (s, 3H), 1.67 (s, 9H), 1.65 (s, 3H), 1.41 (s, 18H), 0.88 (s,
9H), 0.14 (s, 3H), 0.04 (s, 3H).
Sharpless Dihydroxylation Product (−)-38b (C3β-OTBS
Epimer). These procedures are modifications of procedures reported
by Overman and co-workers.12 A flask was charged with AD-mix-α
(150 mg),41 (−)-37b (15 mg, 13 μmol), methane sulfonamide (6.3
mg, 66 μmol), and (DHQ)2PHAL (1.3 mg, 1.6 μmol), and then t-
BuOH/H2O/acetone (3:2:1, 3 mL) was added followed by additional
K2OsO4·H2O (1.2 mg, 3.3 μmol).69 The resulting heterogeneous
mixture was stirred vigorously at rt for 4 h, the reaction was cooled to
0 °C, and solid Na2SO3 (450 mg) was added. The cold bath was
removed, and the mixture was stirred for 1 h at rt. Water (5 mL) was
added to the reaction mixture, which was transferred to a separatory
funnel, and then the aqueous phase was extracted with EtOAc (3 × 8
mL). The combined organic layers were washed with brine (10 mL),
dried over Na2SO4, and concentrated under reduced pressure. The
crude residue was purified by chromatography on silica gel (1:10
EtOAc/hexanes →2:3 EtOAc/hexanes) to afford the title compound
(−)-38b (10 mg, 65%, >10:1 dr) as a red solid. The corresponding
reaction of (−)-37b (10 mg, 8.8 μmol) using AD-mix-β (with
additional K2OsO4·H2O and (DHQD)2PHAL) provided isomers
Sharpless Dihydroxylation Product (+)-38a (C3α-OTBS
Epimer). Following a similar procedure as described for the formation
of (−)-37b, indoline (+)-36a (40 mg, 65 μmol) and phenoxazinone
iodide 33 (130 mg, 200 μmol) were converted to intermediate 37a
(53 mg, 72%) as a red solid: TLC Rf 0.25 (3:7 EtOAc/hexanes, plate
was eluted twice), 37a is copolar with the hydrodehalogenated
byproduct of 33 and the yield is based on the corresponding ratio
1
obtained upon H NMR analysis. Analytically pure samples of 37a
were not obtained for characterization purposes. Following a similar
procedure as described for formation of 38b, dihydroxylation of 37a
(15 mg, 13 μmol) using AD-mix-α provided the title compound
(+)-38a (15 mg, 97%, >10:1 dr) as a red solid. The corresponding
reaction of 37a (10 mg, 8.8 μmol) using AD-mix-β (with additional
K2OsO4·H2O and (DHQD)2PHAL) provided (+)-38a (8 mg, 77%,
>10:1 dr). The analogous reaction of 37a (10 mg, 8.8 μmol) using
OsO4 (22 mg, 2.1 μmol, 2.5 wt % in t-BuOH) and NMO (2 mg, 17
μmol) in acetone/H2O (4:1, 2 mL) provided (+)-38a (8 mg, 77%,
>10:1 dr). Subsequent data is provided for the major isomer only:
[α]D24 +61.8 (c = 0.05, CH2Cl2); 1H NMR (500 MHz, CDCl3) δ 8.14
M
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(br d, J = 7.2 Hz, 1H), 7.72 (d, J = 8.8 Hz, 1H), 7.69 (s, 1H), 7.42 (d, J
= 8.7 Hz, 1H), 7.39 (t, J = 7.9 Hz, 1H), 7.34−7.32 (comp, 2H), 7.17
(app t, J = 7.1 Hz, 1H), 7.15 (app t, J = 6.9 Hz, 1H), 6.81 (app t, J =
7.4 Hz, 1H), 6.45 (comp, 1H), 6.44 (s, 1H), 6.43 (s, 1H), 5.01 (d, J =
2.5 Hz, 1H), 3.98 (br s, 1H), 3.88 (s, 3H), 3.73 (s, 1H), 3.46 (s, 3H),
2.96 (s, 3H), 1.68 (s, 9H), 1.53 (s, 3H), 1.41 (s, 18H), 0.80 (s, 9H),
−0.06 (s, 3H), −0.19 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 180.0
(C), 167.4 (C), 166.2 (C), 165.4 (C), 162.8 (C), 149.8 (C), 149.4
(C), 149.2 (C), 148.5 (C), 146.7 (C), 142.7 (C), 138.3 (C), 137.0
(C), 136.3 (C), 136.0 (C), 134.8 (CH), 134.0 (C), 131.3 (C), 130.2
(CH), 128.2 (C), 127.9 (CH), 127.8 (C), 125.0 (CH), 123.7 (CH),
123.4 (CH), 121.1 (C), 120.9 (CH), 120.4 (CH), 117.3 (CH), 115.6
(CH), 109.3 (CH), 106.8 (CH), 85.9 (C), 84.4 (C), 84.0 (C), 83.5
(C), 82.7 (CH), 78.7 (CH), 54.4 (C), 53.0 (CH3), 52.7 (CH3), 28.5
(CH3), 28.4 (CH3), 27.9 (CH3), 27.0 (CH3), 26.1 (CH3), 19.0 (C),
−2.91 (CH3), −3.24 (CH3); IR (film) 3443, 2979, 2951, 2931, 2855,
1805, 1741, 1643; TLC Rf 0.16 (2:3 EtOAc/hexanes); HRMS (ESI)
m/z calcd for C60H70N6O17SiNa (M + Na)+ 1197.4464, found
1197.4436.
(3R,5aR,10bR,11S,11aS)-10b-(1-(tert-Butoxycarbonyl)-1H-
indol-3-yl)-3-((tert-butyldimethylsilyl)oxy)-2,3-dimethyl-1,4-
dioxo-1,2,3,4,5a,6,10b,11-octahydro-11aH-pyrazino[1′,2′:1,5]-
pyrrolo[2,3-b]indole-11,11a-diyl Diacetate ((+)-40a). Method A.
To a solution of intermediate 39a12 (40 mg, 48 μmol) in CH2Cl2
maintained at −78 °C was added TfOH (84 μL, 48 μmol, 5% solution
in CH2Cl2). The reaction mixture was warmed to −40 °C and then
maintained at −40 °C for 4 h (note: reaction times may vary; close
monitoring was required to prevent further reaction), whereupon it
was quenched by the addition of pyridine (10 μL). After 5 min, the
mixture was poured into a mixture of EtOAc (20 mL) and saturated
aqueous NaHCO3 (10 mL). The layers were separated, and the
aqueous layer was further extracted with EtOAc (10 mL). The
combined organic layers were washed sequentially with H2O (10 mL)
and brine (10 mL), dried over Na2SO4, and concentrated under
reduced pressure. The crude product was purified by silica gel
chromatography (1:9 EtOAc/hexanes →1:3 EtOAc/hexanes) to afford
the title compound (+)-40a (28 mg, 80%, in some of these reactions
enamide 41 was also isolated as a minor byproduct).
(+)-40a was confirmed by single crystal X-ray diffraction of an
isopropanol solvate, mp 150−153 °C, formed by slow evaporation of
the title compound from isopropanol.43 Data for bis(tert-butoxycar-
bonyl) derivative (−)-S8a: [α]D −3.2, [α]577 −3.5, [α]546 −2.8,
24
24
24
[α]43524 −16.0 (c 0.16, CH2Cl2); 1H NMR (500 MHz, CDCl3) δ 8.11
(br d, J = 6.6 Hz, 1H), 7.89 (d, J = 7.9 Hz, 1H), 7.70 (d, J = 7.4 Hz,
1H), 7.57 (d, J = 7.9 Hz, 1H), 7.40 (s, 1H), 7.34 (s, 1H), 7.32−7.28
(comp, 3H), 7.25 (t, J = 7.6 Hz, 1H), 7.19 (t, J = 7.4 Hz, 1H), 6.11 (s,
1H), 3.03 (s, 3H), 1.90 (s, 3H), 1.61 (s, 9H), 1.49 (s, 9H), 1.43 (2s,
6H), 0.98 (s, 9H), 0.43 (s 3H), 0.40 (s, 3H); 13C NMR (125 MHz,
CDCl3) δ 168.3 (C), 168.0 (C), 166.9 (C), 158.9 (C), 151.9 (C),
149.4 (C), 141.9 (C), 136.4 (C), 131.7 (C), 129.3 (CH), 128.0 (C),
127.5 (CH), 125.6 (CH), 124.7 (CH), 123.4 (CH), 122.9 (CH),
121.5 (CH), 116.8 (C), 115.8 (CH), 115.5 (CH), 92.0 (C), 87.1 (C),
84.4 (C), 83.4 (CH), 82.8 (C), 80.1 (CH), 56.9 (C), 28.5 (CH3), 28.2
(CH3), 27.9 (CH3), 26.1 (CH3), 24.8 (CH3), 20.5 (CH3), 20.3 (CH3),
18.9 (C), −2.38 (CH3), −2.42 (CH3); IR (film) 2953, 2929, 2853,
1766, 1740, 1714, 1682, 1484, 1455, 1371; TLC Rf 0.35 (1:3 EtOAc/
hexanes); HRMS (ESI) m/z calcd for C43H56N4O11SiNa (M + Na)+
855.3613, found 855.3622.
(3S,5aR,10bR,11S,11aS)-10b-(1-(tert-Butoxycarbonyl)-1H-
indol-3-yl)-3-((tert-butyldimethylsilyl)oxy)-2,3-dimethyl-1,4-
dioxo-1,2,3,4,5a,6,10b,11-octahydro-11aH-pyrazino[1′,2′:1,5]-
pyrrolo[2,3-b]indole-11,11a-diyl Diacetate ((+)-40b). Following
a similar procedure as described in method B for formation of (+)-40a,
intermediate 39b12 (200 mg, 0.26 mmol) was converted to the title
compound (+)-40b (110 mg, 63%; in some of these reactions the
bis(tert-butoxycarbonyl) derivative (−)-S8b was also isolated as a
minor byproduct also as a colorless foam), which was isolated as a
colorless foam: [α]D +50.7, [α]577 +52.4, [α]546 +60.5, [α]435
24
24
24
24
1
+112, [α]405 +144 (c 0.10, CH2Cl2); H NMR (500 MHz, acetone-
24
d6) δ 8.16 (d, J = 8.3 Hz, 1H), 7.56 (d, J = 7.4 Hz, 1H), 7.52 (d, J = 8.0
Hz, 1H), 7.48 (s, 1H), 7.34 (t, J = 7.8 Hz, 1H), 7.26 (t, J = 7.6 Hz,
1H), 7.18 (t, J = 7.6 Hz, 1H), 6.93 (t, J = 7.4 Hz, 1H), 6.77 (d, J = 7.8
Hz, 1H), 6.52 (s, 1H), 6.49 (d, J = 2.8 Hz, 1H), 6.11 (s, 1H), 3.03 (s,
3H), 1.97 (s, 3H), 1.63 (s, 9H), 1.44 (s, 3H), 1.41 (s, 3H), 0.93 (s,
9H), 0.34 (s 3H), 0.16 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ
168.7 (C), 168.5 (C), 167.7 (C), 160.2 (C), 150.0 (C), 149.9 (C),
137.1 (C), 130.2 (C), 130.0 (CH), 129.2 (C), 126.8 (CH), 125.7
(CH), 125.4 (CH), 123.3 (CH), 121.9 (CH), 119.6 (CH), 119.2 (C),
116.4 (CH), 110.6 (CH), 91.2 (C), 86.9 (C) 85.0 (C), 84.1 (CH),
82.2 (CH), 59.6 (C), 29.4 (CH3), 28.2 (CH3), 28.0 (CH3), 26.3
(CH3), 20.5 (CH3), 20.2 (CH3), 19.2 (C), −2.9 (CH3), −3.6 (CH3);
IR (film) 3376, 2954, 2917, 2850, 1766, 1740, 1682, 1609, 1371, 1215;
TLC Rf 0.25 (3:7 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C38H48N4O9SiNa (M + Na)+ 755.3088, found 755.3098. Data for
bis(tert-butoxycarbonyl) derivative (−)-S8b: [α]D −20.5, [α]577
Method B. In multiple sealed vials, intermediate 39a (5 × 40 mg,
0.048 mmol) was maintained (neat) at 175 °C in a preheated oil bath
for 40−50 min. The reaction vessels were allowed to cool to rt, then
THF (0.48 mL), DMAP (1 mg) and Boc2O (10 mg, 0.048 mmol, 0.1
mL of a premade 0.5 M THF solution) were added sequentially to the
reaction mixtures. After 1 h, the reaction mixtures were combined and
poured into a mixture of EtOAc (20 mL) and saturated aqueous
NH4Cl (10 mL). The layers were separated and the aqueous layer was
further extracted with EtOAc (10 mL). The combined organic layers
were washed successively with H2O (10 mL) and brine (10 mL), dried
over Na2SO4 and concentrated under reduced pressure. The crude
product was purified by silica gel chromatography (1:9 EtOAc/
hexanes →1:3 EtOAc/hexanes) to afford the title compound (+)-40a
(125 mg, 71%; in some of these reactions the bis(tert-butoxycarbonyl)
derivative (−)-S8a was also isolated as a minor byproduct also as a
colorless foam)71 as a colorless foam: [α]D +93.8, [α]577 +95.0,
24
24
−31.4, [α]54624 −37.8, [α]43524 −82.9 (c 0.18, CH2Cl2); 1H NMR (500
MHz, CDCl3) δ 8.12 (br d, J = 6.4 Hz, 1H), 7.72 (br d, 1H), 7.71 (d, J
= 7.6 Hz, 1H), 7.40 (s, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.32 (t, J = 8.0
Hz, 1H), 7.29 (t, J = 7.5 Hz, 1H), 7.27 (s, 1H), 7.22−7.18 (comp,
2H), 6.11 (s, 1H), 3.04 (s, 3H), 1.94 (s, 3H), 1.61 (s, 9H), 1.55 (s,
9H), 1.45 (s, 3H), 1.33 (s, 3H), 0.92 (s, 9H), 0.33 (s 3H), 0.14 (s,
3H); 13C NMR (125 MHz, CDCl3) δ 167.8 (C), 167.3 (C), 165.4
(C), 159.6 (C), 152.0 (C), 149.4 (C), 142.1 (C), 136.3 (C), 132.0
(C), 129.3 (CH), 128.0 (C), 127.1 (CH), 125.7 (CH), 124.7 (CH),
123.7 (CH), 122.7 (CH), 120.9 (CH), 116.7 (C), 116.4 (CH), 115.7
(CH), 91.1 (C), 85.9 (C), 84.5 (C), 83.9 (CH), 83.1 (C), 81.8 (CH),
56.9 (C), 28.5 (CH3), 28.4 (CH3), 28.3 (CH3), 28.1 (CH3), 25.9
(CH3), 20.5 (CH3), 20.3 (CH3), 18.6 (C), −3.2 (CH3), −4.1 (CH3);
IR (film) 2954, 2917, 2850, 1768, 1737, 1716, 1685, 1476, 1455, 1370;
TLC Rf 0.35 (1:3 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C43H56N4O11SiNa (M + Na)+ 855.3613, found 855.3600.
24
24
1
[α]546 +100, [α]435 +178 (c 0.14, CH2Cl2); H NMR (500 MHz,
24
24
CDCl3) δ 8.10 (br d, J = 5.5 Hz, 1H), 7.61 (d, J = 7.5 Hz, 1H), 7.59
(d, J = 8.0 Hz, 1H), 7.40 (s, 1H), 7.27 (t, J = 7.8 Hz, 1H), 7.19 (t, J =
7.8 Hz, 1H), 7.13 (t, J = 7.7 Hz, 1H), 6.92 (t, J = 7.4 Hz, 1H), 6.64 (d,
J = 7.8 Hz, 1H), 6.52 (s, 1H), 6.26 (s, 1H), 4.86 (s, 1H), 3.02 (s, 3H),
1.90 (s, 3H), 1.61 (s, 9H), 1.45 (s, 3H), 1.37 (s, 3H), 0.98 (s, 9H),
0.37 (s 3H), 0.33 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 169.3 (C),
168.13 (C), 168.08 (C), 159.1 (C), 149.5 (C), 147.2 (C), 136.4 (C),
129.5 (C), 129.3 (CH), 128.4 (C), 126.7 (CH), 125.6 (CH), 124.6
(CH), 122.6 (CH), 121.4 (CH), 119.9 (CH), 117.4 (C), 115.6 (CH),
109.7 (CH), 90.6 (C), 87.2 (C) 84.3 (C), 82.4 (CH), 80.9 (CH), 58.5
(C), 28.3 (CH3), 28.0 (CH3), 25.9 (CH3), 24.0 (CH3), 20.5 (CH3),
20.4 (CH3), 18.8 (C), −2.4 (CH3), −3.1 (CH3); IR (film) 3372, 2954,
2930, 2856, 1741, 1682, 1609, 1370, 1216; TLC Rf 0.28 (1:3 EtOAc/
hexanes); HRMS (ESI) m/z calcd for C38H48N4O9SiNa (M + Na)+
755.3088, found 755.3083. The relative and absolute configuration of
(5aR,10bR,11S,11aS)-10b-(1-(tert-Butoxycarbonyl)-1H-indol-
3-yl)-2-methyl-3-methylene-1,4-dioxo-1,2,3,4,5a,6,10b,11-oc-
tahydro-11aH-pyrazino[1′,2′:1,5]pyrrolo[2,3-b]indole-11,11a-
diyl Diacetate ((+)-41). To a solution of intermediate 39b (100 mg,
0.12 mmol) in CH2Cl2 (2.5 mL) maintained at −78 °C was added
TfOH (16 μL, 0.18 mmol, 5% solution in CH2Cl2). The reaction
mixture was warmed to −40 °C, maintained at −40 °C for 4 h (note:
N
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
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reaction times may vary, monitoring required to prevent further
cleavage of the remaining Boc group), and then quenched by the
addition of pyridine (20 μL). After 5 min, the mixture was poured into
a mixture of EtOAc (20 mL) and saturated aqueous NaHCO3 (10
mL). The layers were separated, and the aqueous layer was further
extracted with EtOAc (10 mL). The combined organic layers were
washed successively with H2O (10 mL) and brine (10 mL), dried over
Na2SO4, and concentrated under reduced pressure. A second batch of
39b (220 mg, 0.26 mmol) was processed in a similar fashion. The
crude product from both reactions were combined and purified by
silica gel chromatography (1:9 EtOAc/hexanes →1:3 EtOAc/hexanes)
to afford the title compound (+)-41 (185 mg, 82%) as a colorless
solid: [α]D +36.0, [α]577 +46.1, [α]546 +44.9, [α]435 +87.9 (c
116.4 (CH, major), 114.3 (CH, minor), 108.8 (CH, major), 106.8
(CH, major), 106.7 (CH, minor), 92.0 (C, major), 91.5 (C, minor),
88.5 (CH, minor), 88.2 (C, minor), 87.9 (C, major), 86.7 (CH,
major), 85.34 (C, minor), 85.25 (C, major), 83.74 (CH, minor), 83.70
(C, major), 83.4 (CH, major), 59.6 (C, minor), 58.5 (C, major), 53.3
(CH3, minor), 53.2 (CH3, minor), 53.1 (CH3, major), 51.8 (CH3,
major), 28.3 (CH3, major), 28.2 (CH3, major), 27.93 (CH3, minor),
27.92 (CH3, major), 26.35 (CH3, minor), 26.32 (CH3, major), 24.83
(CH3, minor), 24.80 (CH3, major), 20.5 (CH3, major), 20.3 (CH3,
major), 20.1 (CH3, minor), 19.30 (C, minor), 19.28 (C, major), −2.0
(CH3, minor), −2.3 (CH3, major), −2.4 (CH3, minor), −2.7 (CH3,
major);73 IR (film) 2954, 2918, 2850, 1807, 1744, 1683, 1643, 1578,
1369; TLC Rf 0.10 (3:7 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C64H74N6O19SiNa (M + Na)+ 1281.4675, found 1281.4697. During
preliminary cross-coupling experiments, allowing 40a to react with
varying amounts of 33, CuTC, and K2CO3 in PhMe at 90 °C resulted
in the formation of 43 as a tan solid, in minor quantities. Data for
thiophene-2-carboxylate adduct (+)-43: [α]D +29.8, [α]577 +38.3,
24
24
24
24
0.13, CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.15 (d, J = 8.4 Hz,
1H), 7.60 (d, J = 8.7 Hz, 1H), 7.59 (d, J = 8.0 Hz, 1H), 7.48 (s, 1H),
7.33 (t, J = 8.2 Hz, 1H), 7.24 (t, J = 7.3 Hz, 1H), 7.19 (t, J = 7.8 Hz,
1H), 6.94 (t, J = 7.5 Hz, 1H), 6.79 (d, J = 7.8 Hz, 1H), 6.51 (s, 1H),
6.38 (s, 1H), 6.19 (s, 1H), 5.83 (s, 1H), 5.21 (s 1H), 3.23 (s, 3H), 1.64
(s, 9H), 1.43 (s, 6H); 13C NMR (125 MHz, acetone-d6) δ 169.9 (C),
168.7 (C), 160.6 (C), 159.0 (C), 150.0 (C), 149.8 (C), 139.1 (C),
137.1 (C), 130.1 (CH), 129.3 (C), 126.8 (CH), 125.9 (CH), 125.5
(CH), 123.5 (CH), 122.2 (CH), 119.7 (CH), 119.0 (C), 116.3 (CH),
110.8 (CH), 103.8 (CH2), 91.9 (C), 85.0 (C), 83.9 (CH), 81.7 (CH),
58.9 (C), 28.2 (CH3), 20.30 (CH3), 20.28 (CH3);72 IR (film) 3374,
3055, 2979, 2917, 2849, 1740, 1695, 1615, 1371; TLC Rf 0.28 (3:7
EtOAc/hexanes); HRMS (ESI) m/z calcd for C32H32N4O8Na (M +
Na)+ 623.2118, found 623.2111.
24
24
1
[α]546 +44.0, [α]435 +93.8 (c 0.10, CH2Cl2); H NMR (500 MHz,
24
24
acetone-d6) δ 8.16 (d, J = 7.8 Hz, 1H), 7.81 (d, J = 4.8 Hz, 1H), 7.72
(d, J = 8.1 Hz, 1H), 7.71 (d, J = 6.8 Hz, 1H), 7.49 (s, 1H), 7.36 (t, J =
7.8 Hz, 1H), 7.28 (t, J = 7.6 Hz, 1H), 7.06 (t, J = 7.6 Hz, 1H), 7.01 (t,
J = 7.3 Hz, 1H), 6.98 (t, J = 4.3 Hz, 1H), 6.62 (d, J =1.5 Hz, 1H), 6.56
(s, 1H), 6.49 (s, 1H), 6.48 (d, J = 7.8 Hz, 1H), 6.37 (s, 1H), 3.05 (s,
3H), 1.99 (s, 3H), 1.64 (s, 9H), 1.50 (s, 3H), 1.01 (s, 9H), 0.42 (s
3H), 0.32 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 167.9 (C),
167.8 (C), 160.3 (C), 159.0 (C), 149.3 (C), 149.0 (C), 136.5 (C),
135.4 (CH), 135.0 (CH), 131.8 (C), 129.7 (CH), 129.6 (C), 128.5
(C), 128.2 (CH), 126.0 (CH), 125.1 (CH), 124.8 (CH), 122.8 (CH),
121.6 (CH), 119.03 (C), 119.00 (CH), 115.7 (CH), 110.3 (CH), 91.8
(C), 87.3 (C) 84.4 (C), 82.9 (CH), 81.6 (CH), 59.0 (C), 27.65
(CH3), 27.57 (CH3), 25.7 (CH3), 23.9 (CH3), 19.8 (CH3), 18.7 (C),
−2.9 (CH3), −3.4 (CH3); IR (film) 3376, 2954, 2918, 2851, 1765,
1736, 1710, 1683, 1608, 1371, 1218; TLC Rf 0.56 (3:7 EtOAc/
hexanes, plate was eluted twice); HRMS (ESI) m/z calcd for
C41H48N4O9SSiNa (M + Na)+ 823.2809, found 823.2798.
Cross-Coupled Product ((+)-42a). The components of this
reaction were combined in a screw-top vial inside a nitrogen-filled
glovebox. The reaction vial was sealed with a Teflon-lined cap, then
brought outside the glovebox and heated in an aluminum block for the
period of time indicated. The reaction vessel was charged with indoline
40a (10 mg, 14 μmol), phenoxazinone iodide 33 (27 mg, 42 μmol),
copper(I) acetate (CuOAc) (10 mg, 82 μmol), and KOAc (3.0 mg, 31
μmol), and then PhMe (200 μL) was added by syringe. The reaction
mixture was maintained at 90 °C for 8 h. After being allowed to cool to
rt, the reaction mixture was passed directly through a short plug of
silica gel (eluting with EtOAc) to remove the metal salts, then
concentrated under reduced pressure. The crude residue was purified
by chromatography on silica gel (1:19 EtOAc/hexanes →1:3 EtOAc/
hexanes) to afford the title compound (+)-42a (10 mg, 58%) as a red
solid: [α]D24 +43.9 (c 0.063, MeOH). Compound 42a was observed as
a 3:1 mixture of atropisomers by NMR at 298 K: 1H NMR (500 MHz,
acetone-d6) δ 8.21 (d, J = 8.3 Hz, 1.33H), 7.76 (d, J = 7.5 Hz, 1.33H),
7.67 (d, J = 8.8 Hz, 1H), 7.62 (d, J = 8.0 Hz, 1H), 7.56 (s, 1H), 7.53
(s, 0.33H), 7.51 (d, J = 8.1 Hz, 0.33H), 7.41−7.38 (comp, 1.66H),
7.31 (d, J = 8.8 Hz, 1H), 7.30−7.19 (comp, 3H), 7.08 (t, J = 7.4 Hz,
1H), 6.92 (d, J = 8.8 Hz, 0.33H), 6.89 (s, 1H), 6.72 (s, 0.33H), 6.69
(d, J = 8.0 Hz, 0.33H), 6.48 (s, 1H), 6.44 (s, 0.33H), 6.31 (d, J = 7.9
Hz, 1H), 6.20 (s, 1H), 6.11 (s, 0.33H), 3.94 (s, 1H), 3.89 (s, 1H), 3.79
(s, 3H), 3.04 (s, 1H), 3.02 (s, 3H), 2.88 (s, 3H), 2.00 (s, 1H), 1.89 (s,
3H), 1.65 (s, 12H), 1.52 (s, 4H), 1.39 (s, 3H), 1.39 (s, 6H), 1.37 (2s,
19H), 1.01 (s, 1H), 0.98 (s, 9H), 0.40 (s, 1H), 0.35 (s, 1H), 0.32 (s
3H), 0.30 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C,
major), 170.0 (C, major), 169.6 (C, minor), 168.5 (C, major), 168.3
(C, minor), 168.2 (C, major), 167.4 (C, minor), 166.4 (C, minor),
165.4 (C, major), 163.73 (C, minor), 163.65 (C, major), 159.3 (C,
minor), 159.2 (C, major), 151.0 (C, minor), 150.3 (C, minor), 150.2
(C, major), 150.0 (C, major), 149.8 (C, major), 148.8 (C, major),
148.4 (C, minor), 148.2 (C, major), 144.2 (C, major), 144.0 (C,
minor), 140.3 (C, minor), 139.0 (C, major), 137.9 (C, minor), 137.33
(C, minor), 137.27 (C, major), 137.0 (C, major), 136.1 (CH, major),
134.83 (CH, minor), 134.75 (C, major), 134.71 (C, major), 131.84
(C, major), 131.79 (C, minor), 131.3 (C, minor), 130.22 (C, major),
130.19 (CH, minor), 130.1 (CH, major), 129.1 (C, major), 128.8 (C,
minor), 128.6 (CH, minor), 128.1 (CH, major), 126.1 (CH, major),
125.8 (CH, minor), 125.6 (C, minor), 125.5 (CH, major), 123.6 (CH,
major), 123.5 (CH, minor), 122.5 (CH, minor), 122.3 (CH, major),
122.1 (CH, minor), 120.5 (CH, major), 120.4 (CH, minor), 119.4
(CH, major), 118.5 (C, minor), 118.1 (C, major), 116.5 (CH, minor),
Cross-Coupled Product ((+)-42b). Following a similar procedure
as described for formation of 42a, intermediate (+)-40b (17 mg, 23
μmol) was converted to the title compound (+)-42b (15 mg, 51%),
1
which was isolated as a red solid: [α]D +28.0 (c 0.053, MeOH); H
24
NMR (500 MHz, acetone-d6) δ 8.22 (d, J = 8.3 Hz, 1H), 7.74 (d, J =
7.5 Hz, 1H), 7.59 (d, J = 8.8 Hz, 1H), 7.56 (s, 1H), 7.41−7.38 (comp,
3H), 7.29 (t, J = 7.6 Hz, 1H), 7.23 (t, J = 7.7 Hz, 1H), 7.09 (t, J = 7.7
Hz, 1H), 6.79 (s, 1H), 6.48 (s, 1H), 6.34 (d, J = 7.6 Hz, 1H), 6.11 (s,
1H), 3.78 (s, 3H), 3.04 (s, 3H), 2.84 (s, 3H), 1.96 (s, 3H), 1.65 (s,
9H), 1.59 (s, 3H), 1.40 (2s, 21H), 0.98 (s, 9H), 0.24 (s 3H), 0.09 (s,
3H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 169.4 (C), 168.3
(C), 167.8 (C), 165.3 (C), 163.6 (C), 159.8 (C), 150.1 (C), 150.0
(C), 149.7 (C), 149.0 (C), 148.1 (C), 144.2 (C), 139.0 (C), 137.22
(C), 137.17 (CH), 136.7 (CH), 134.79 (C), 134.76 (C), 131.9 (C),
130.1 (CH), 129.8 (C), 129.1 (C), 127.9 (CH), 126.1 (CH), 125.6
(CH), 123.3 (CH), 121.8 (CH), 120.6 (CH), 119.4 (CH), 118.3 (C),
116.5 (CH), 108.9 (CH), 106.9 (CH), 91.6 (C), 88.0 (CH), 87.1 (C),
85.3 (C), 84.3 (CH), 83.4 (C), 59.0 (C), 53.1 (CH3), 51.7 (CH3),
29.4 (CH3),74 28.2 (CH3), 28.1 (CH3), 27.9 (CH3), 26.2 (CH3), 20.6
(CH3), 20.1 (CH3), 19.2 (C), −2.8 (CH3), −3.1 (CH3); IR (film)
2979, 2952, 2929, 2851, 1807, 1768, 1747, 1687, 1577, 1370; TLC Rf
0.24 (4:6 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C64H74N6O19SiNa (M + Na)+ 1281.4675, found 1281.4680.
Bis(methylthio)ether (44). Method A. Hydrogen sulfide (bp −60
°C, ca. 200 μL) was condensed at −78 °C in a thick-walled, glass
pressure tube fitted with a rubber septum. A solution of (+)-42a (5.0
mg, 4.0 μmol) in CH2Cl2 (400 μL) and BF3·OEt2 (10 μL, 80 μmol)
were injected sequentially into the reaction vessel maintained at −78
°C. The rubber septum was replaced by a Teflon screw cap, which was
used to seal the vessel. The cold bath was removed and the reaction
mixture was allowed to warm to rt behind a blast shield. After 1 h, the
reaction mixture was cooled to −78 °C, and the Teflon cap was
replaced by the rubber septum, which was equipped with a needle
vented to base (KOH/isopropanol) and bleach traps (attached in
O
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
series). The cooling bath was removed and the resulting brown
suspension was allowed to warm up to rt. Upon evolution of the
majority of hydrogen sulfide gas, an argon-filled balloon was attached
(by needle) to fully purge the reaction mixture, which was
subsequently poured into a mixture of EtOAc (2 mL) and saturated
aqueous NH4Cl (2 mL). The layers were separated and the aqueous
layer was extracted with EtOAc (2 mL). The combined organic layers
were washed with brine (4 mL), dried over Na2SO4 and filtered. To
this solution was immediately added MeI (20 μL, 200 μmol) and
K2CO3 (14 mg, 100 μmol). Performing the S-alkylation step prior to any
further manipulations prevented competitive oxidation of the dithiol
intermediate to the undesired epidisulfide congener. The reaction mixture
was stirred for 12 h at rt and then poured into a mixture of EtOAc (10
mL) and saturated aqueous NH4Cl (10 mL). The layers were
separated, and the aqueous layer was extracted with EtOAc (2 × 5
mL). The combined organic layers were washed with brine (10 mL),
dried over Na2SO4 and concentrated under reduced pressure. The
crude residue was purified by preparatory thin layer chromatography
(6/4 EtOAc/hexanes) to afford the title compound (−)-44b (3.0 mg,
88%) as a single diastereomer (by 1H NMR analysis). In
corresponding experiments with N,O-acetal intermediate (+)-42b
(10 mg, 8.0 μmol), the title compound (−)-44b (5.5 mg, 80%) was
obtained in similar efficiency and diastereoselectivity as a red solid.
Method B. Methanethiol (bp 6 °C, ca. 200 μL) was condensed at
−78 °C in a thick-walled, glass pressure tube fitted with a rubber
septum. A solution of (+)-42a (23 mg, 18 μmol) in CH2Cl2 (400 μL)
and BF3·OEt2 (30 μL, 240 μmol) was injected sequentially into the
reaction vessel maintained at −78 °C. The rubber septum was replaced
by a Teflon screw cap, which was used to seal the vessel. The cold bath
was removed, and the reaction mixture was allowed to warm to rt
behind a blast shield. After 1 h, the reaction mixture was cooled back
down to −78 °C, and the Teflon cap was replaced by the rubber
septum, which was equipped with a needle vented to base (KOH/
isopropanol) and bleach traps (attached in series). The cooling bath
was removed and the resulting brown suspension was allowed to warm
to rt. Upon evolution of the majority of methanethiol gas, an argon-
filled balloon was attached (by needle) to fully purge the reaction
mixture, which was subsequently poured into a mixture of EtOAc (10
mL) and saturated aqueous NaHCO3 (10 mL). The layers were
separated, and the aqueous layer was extracted with EtOAc (2 × 10
mL). The combined organic layers were washed with brine (10 mL),
dried over Na2SO4, and then concentrated under reduced pressure.
This reaction was conducted three times consecutively, and then the
crude residues were combined for purification by preparatory thin
layer chromatography (6/4 EtOAc/hexanes) to afford the title
compound (−)-44b (21 mg, 45%) and (−)-44a (16 mg, 34%) as
red solids. In corresponding experiments with (+)-42b (30 mg, 24
μmol), the title compound (−)-44b and (−)-44a were obtained in
similar efficiency (19 mg, 92% overall) and diastereoselectivity (1.3:1
dr, respectively) as a red solid. Data for cis-bis(methylthio)ether
(−)-44b: [α]D −142, [α]577 −168, [α]546 −217 (c 0.050,
(s, 1H), 7.73 (d, J = 7.3 Hz, 1H), 7.67 (d, J = 8.7 Hz, 1H), 7.62 (d, J =
8.7 Hz, 1H), 7.54 (d, J = 8.0 Hz, 1H), 7.45 (d, J = 8.2 Hz, 1H), 7.2−
7.1 (comp, 3H), 7.07 (s, 1H), 7.04 (t, J = 7.5 Hz, 1H), 6.90 (t, J = 7.4
Hz, 1H), 6.44 (s, 1H), 6.30 (s, 1H), 6.16 (d, J = 7.8 Hz, 1H), 3.73 (s,
3H), 3.08 (s, 3H), 2.69 (s, 3H), 2.13 (s, 3H), 2.05 (s, 3H), 1.85 (s,
3H), 1.29 (s, 3H). Data for trans-bis(methylthio)ether (−)-44a: [α]D
24
−103, [α]577 −110, [α]546 −136 (c 0.11, CH2Cl2); 44a was
24
24
observed as a 4:1 mixture of atropisomers by NMR at 298 K
(subsequent data is provided for the major isomer only); H NMR
1
(500 MHz, acetone-d6) δ 10.39 (s, 1H), 7.73 (d, J = 7.3 Hz, 1H), 7.61
(d, J = 8.0 Hz, 1H), 7.60 (d, J = 8.6 Hz, 1H), 7.53 (d, J = 8.8 Hz, 1H),
7.46 (d, J = 8.1 Hz, 1H), 7.2−7.1 (comp, 3H), 7.05 (t, J = 7.8 Hz, 1H),
7.04 (s, 1H), 6.92 (t, J = 7.5 Hz, 1H), 6.45 (s, 1H), 6.23 (s, 1H), 6.19
(d, J = 7.9 Hz, 1H), 3.73 (s, 3H), 3.11 (s, 3H), 2.62 (s, 3H), 2.16 (s,
3H), 2.12 (s, 3H), 1.74 (s, 3H), 1.29 (s, 3H); 13C NMR (125 MHz,
acetone-d6) δ 179.0 (C), 169.3 (C), 168.6 (C), 166.2 (C), 165.1 (C),
163.6 (C), 151.3 (C), 151.2 (C), 150.4 (C), 146.8 (C), 142.1 (C),
138.7 (C), 136.4 (C), 135.1 (C), 132.6 (C), 132.1 (CH), 130.9 (C),
130.1 (CH), 127.2 (CH), 127.1 (C), 125.1 (CH), 122.5 (CH), 121.9
(CH), 119.8 (CH), 119.3 (CH), 118.2 (CH), 112.98 (CH), 112.96
(C), 108.1 (CH), 105.3 (CH), 87.3 (CH), 81.6 (CH), 74.5 (C), 73.8
(C), 58.6 (C), 51.7 (CH3), 51.2 (CH3), 30.2 (CH3), 25.2 (CH3), 20.3
(CH3), 16.1 (CH3), 13.2 (CH3);75 IR (film) 3389, 3307, 2947, 2917,
2849, 1747, 1667, 1581, 1377; TLC Rf 0.28 (1:1 EtOAc/hexanes, plate
was eluted twice); HRMS (ESI) m/z calcd for C43H38N6O10S2Na (M
+ Na)+ 885.1989, found 885.1960.
(+)-Plectosphaeroic Acid B (2).4 To a solution of cis-
bis(methylthio)ether (−)-44b (8.0 mg, 9.3 μmol) in MeOH (0.10
mL) were added lanthanum(III) trifluoromethanesulfonate (56 mg, 93
μmol) and DMAP (1.4 mg, 11 μmol). The reaction mixture was
maintained at 45 °C. After 5 h (the consumption of the starting
material was monitored by TLC, heating times varied in some
instances), the reaction mixture was allowed to cool to rt and then
poured into a mixture of EtOAc (10 mL) and saturated aqueous
NH4Cl (10 mL). The layers were separated, and the aqueous layer was
extracted with EtOAc (2 × 10 mL). The combined organic layers were
washed with brine (10 mL), dried over Na2SO4, and concentrated
under reduced pressure. The crude residue was purified by preparatory
thin-layer chromatography (7:3 EtOAc/hexanes) to afford the
deacetylated, C11-secondary alcohol intermediate (6.0 mg, contami-
nated with a closely eluting byproduct arising from the in situ
hydrolysis of a methyl ester group) as a red solid. This solid was
dissolved in pyridine (0.10 mL), and LiI (50 mg, 370 μmol) was added
to the solution. The reaction mixture was stirred for 12 h at 90 °C
(formation of product was observed by RP-18 TLC), allowed to cool
to rt, and poured into a mixture of EtOAc (5 mL) and 1 N HCl (2
mL). The layers were separated, and the organic layers was washed
with 1 N HCl (2 × 2 mL). The combined aqueous layers were
extracted with EtOAc (4 mL), the combined organic layers were dried
over Na2SO4 and concentrated under reduced pressure. The residue
was purified by reverse-phase HPLC (step-gradient eluting 20%, 40%,
60%, 65%, 70%, 75%, 90% MeOH/H2O + 0.1% TFA) to afford
plectosphaeroic acid B (2) (4.8 mg, 65% over two steps, eluting at 75%
24
24
24
1
CH2Cl2); H NMR (500 MHz, CDCl3) δ 8.21 (s, 1H), 7.71−7.67
(comp, 2H), 7.45 (d, J = 8.0 Hz, 1H), 7.36 (d, J = 8.8 Hz, 1H), 7.31
(d, J = 8.2 Hz, 1H), 7.26 (s, 1H), 7.16−7.13 (comp, 1H), 7.10 (s, 1H),
7.15 (t, J = 7.6 Hz, 1H), 7.02 (d, J = 2.5 Hz, 1H), 6.89 (t, J = 7.4 Hz,
1H), 6.45 (s, 1H), 6.29 (s, 1H), 6.22 (d, J = 7.8 Hz, 1H), 3.75 (s, 3H),
3.10 (s, 3H), 2.65 (s, 3H), 2.16 (s, 3H), 2.00 (s, 3H), 1.87 (s, 3H),
1.32 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 178.5 (C), 169.1 (C),
168.9 (C), 166.1 (C), 163.8 (C), 163.5 (C), 150.8 (C), 150.2 (C),
149.4 (C), 145.9 (C), 141.1 (C), 137.3 (C), 135.6 (C), 134.7 (C),
132.0 (C), 131.4 (CH), 129.7 (C), 129.6 (CH), 126.1 (C), 126.0
(CH), 124.2 (CH), 122.3 (CH), 121.1 (CH), 120.1 (CH), 118.8
(CH), 117.4 (CH), 112.9 (C), 111.8 (CH), 107.7 (CH), 105.0 (CH),
98.2 (C), 86.1 (CH), 81.2 (CH), 72.6 (C), 67.2 (C), 57.5 (C), 51.6
(CH3), 51.3 (CH3), 29.2 (CH3), 23.7 (CH3), 20.5 (CH3), 16.4 (CH3),
14.6 (CH3); IR (film) 3412, 3308, 2947, 2918, 2849, 1745, 1668,
1581, 1374; TLC Rf 0.38 (1:1 EtOAc/hexanes, plate was eluted twice);
HRMS (ESI) m/z calcd for C43H38N6O10S21Na (M + Na)+ 885.1989,
found 885.1968. Note, for comparison, the H NMR of (−)-44b was
also obtained in acetone-d6: 1H NMR (500 MHz, acetone-d6) δ 10.37
MeOH/H2O + 0.1% TFA) as a red solid: [α]D +228 (c 0.08,
24
MeOH), compare with reported value [α]D +69.8 (c 0.27, MeOH)
24
1
of the natural sample; H NMR (500 MHz, DMSO-d6) δ 10.85 (s,
1H), 9.72 (s, 1H), 8.91 (s, 1H), 8.06 (d, J = 7.6 Hz, 1H), 7.72 (d, J =
8.8 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H), 7.57 (d, J = 7.6 Hz, 1H), 7.36
(comp, 2H), 7.09 (m, 1H), 7.06 (m, 1H), 6.90 (t, J = 7.6 Hz, 1H),
6.74 (s, 1H), 6.68 (s, 1H), 6.56 (t, J = 7.4 Hz, 1H), 5.98 (d, J = 7.9 Hz,
1H), 5.73 (br s, 1H), 5.44 (br s, 1H), 2.93 (s, 3H), 2.14 (s, 3H), 1.88
(s, 3H), 1.59 (s, 3H); 13C NMR (125 MHz, DMSO-d6) δ 178.1 (C),
168.9 (C), 167.2 (C), 163.6 (C), 163.5 (C), 152.6 (C), 151.0 (C),
149.1 (C), 147.6 (C), 141.2 (C), 137.1 (C), 135.0 (C), 133.4 (C),
133.3 (C), 130.7 (CH), 127.9 (CH), 127.4 (C), 125.3 (C), 122.5
(CH), 122.4 (CH), 121.5 (CH), 120.9 (CH), 118.6 (CH), 118.0
(CH), 117.8 (CH), 115.9 (C), 111.4 (CH), 106.7 (CH), 105.5 (CH),
92.4 (C), 86.1 (C), 78.6 (CH), 74.1 (C), 66.3 (C), 58.6 (C), 28.5
(CH3), 22.6 (CH3), 15.7 (CH3), 13.9 (CH3); RP-18 TLC Rf 0.57 (4:1
P
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
MeOH/H2O); HRMS (ESI) m/z calcd for C39H31N6O9S2 (M − H)−
791.1594, found 791.1610.
(attached in series). The cooling bath was removed and the resulting
brown suspension was allowed to warm up to rt. Upon evolution of
the majority of hydrogen sulfide gas, an argon-filled balloon was
attached (by needle) to fully purge the reaction mixture, which was
subsequently poured into a mixture of EtOAc (2 mL) and saturated
aqueous NH4Cl (2 mL). The layers were separated and the aqueous
layer was extracted with EtOAc (2 mL). The combined organic layers
were washed with brine (4 mL), dried over Na2SO4, and concentrated
under reduced pressure. The residue was dissolved in acetone (1 mL)
and K2CO3 (10 mg, 72 μmol) was added. The resulting heterogeneous
mixture was stirred for 1 h, at which point Na2S (0.7 mg, 9.0 μmol)
was added. The reaction mixture was stirred for 12 h at rt, then poured
into a mixture of EtOAc (10 mL) and saturated aqueous NH4Cl (10
mL). The layers were separated and the aqueous layer was extracted
with EtOAc (2 × 5 mL). The combined organic layers were washed
with brine (10 mL), dried over Na2SO4 and concentrated under
reduced pressure. The crude residue was purified by preparatory thin
layer chromatography (6:4 EtOAc/hexanes) to afford the title
compound (+)-47 (5.0 mg, 64%) as a red solid.76
C3-epi-Plectosphaeroic acid B ((+)-S9). Following a procedure
similar to that described for formation of plectosphaeroic acid B (2),
(−)-44a (18 mg, 19 μmol) was converted to the title compound
(+)-S9 (8.0 mg, 48%), which was isolated as a red solid: [α]D +236
24
(c 0.062, MeOH); S9 was observed as an 8:1 mixture of atropisomers
by NMR at 298 K (subsequent data is provided for the major isomer
only); 1H NMR (500 MHz, DMSO-d6) δ 10.87 (s, 1H), 9.72 (s, 1H),
8.91 (s, 1H), 8.00 (d, J = 7.7 Hz, 1H), 7.72 (d, J = 8.8 Hz, 1H), 7.6−
7.5 (comp, 2H), 7.35 (d, J = 8.1 Hz, 1H), 7.31 (s, 1H), 7.1−7.0
(comp, 2H), 6.93 (t, J = 7.7 Hz, 1H), 6.69 (s, 1H), 6.68 (s, 1H), 6.60
(t, J = 7.3 Hz, 1H), 6.10 (br s, 1H), 6.01 (d, J = 7.9 Hz, 1H), 5.41 (s,
1H), 2.97 (s, 3H), 2.10 (s, 3H), 1.71 (s, 3H), 1.53 (s, 3H); 13C NMR
(125 MHz, DMSO-d6) δ 178.1 (C), 168.9 (C), 167.1 (C), 164.7 (C),
162.8 (C), 152.6 (C), 150.9 (C), 149.3 (C), 147.6 (C), 141.2 (C),
137.1 (C), 135.2 (C), 133.2 (2 × C), 130.6 (CH), 128.7 (CH), 127.6
(C), 125.3 (C), 122.9 (CH), 122.4 (CH), 121.4 (CH), 120.9 (CH),
118.6 (CH), 118.3 (CH), 117.7 (CH), 115.8 (C), 111.4 (CH), 106.9
(CH), 105.5 (CH), 92.4 (C), 86.7 (CH), 78.7 (CH), 75.4 (C), 73.3
(C), 58.7 (C), 29.4 (CH3), 25.1 (CH3), 15.3 (CH3), 12.3 (CH3); IR
(film) 3374, 3270, 3062, 2919, 2850, 1681, 1588, 1487, 1385; RP-18
TLC Rf 0.53 (4:1 MeOH/H2O); HRMS (ESI) m/z calcd for
C39H31N6O9S2 (M − H)− 791.1594, found 791.1595.
Method B. To a solution of disulfide (−)-46 (5.0 mg, 6.0 μmol) was
added Na2S (0.50 mg, 6.4 μmol). The reaction mixture was stirred for
1 h at rt, then poured into a mixture of EtOAc (20 mL) and water (10
mL). The layers were separated, and the aqueous layer was extracted
with EtOAc (2 × 20 mL). The combined organic layers were washed
with brine (10 mL), dried over Na2SO4, and concentrated under
reduced pressure. An identical reaction was conducted on 9 mg of
(−)-46. The crude residue from both reactions was combined for
purification by preparatory thin layer chromatography (3:7 EtOAc/
hexanes) to afford the title compound (+)-47 and 48 (11 mg, 75%; 5:1
ratio). Data for epitrisulfide (+)-47: [α]D +61.3, [α]577 +64.9,
Epidisulfidedioxopiperazine ((−)-46). A similar procedure for
sulfidation as described for formation of 44 using H2S and BF3·OEt2
was followed, except the organic layer upon quenching of this step was
concentrated under reduced pressure and the resulting crude mixture
was redissolved in acetone (0.5 mL) and K2CO3 (14 mg, 100 μmol)
was added. The mixture was subsequently processed in a similar
fashion. Using this procedure, intermediate 42a and 42b (5−10 mg/
run) were converted to epidisulfide (−)-46 containing minor amounts
of trisulfide (+)-47 (3−6 mg, 70−80%, 5−10:1 ratio of products). An
alternative procedure for the S−S bond-forming step involved adding
SiO2 (100 mg; instead of K2CO3) to the crude mixture obtained from
sulfidating 26 mg of 42 dissolved in acetone (0.5 mL). The resulting
heterogeneous mixture was vigorously stirred for 24 h (reaction times
varied) and filtered, and the filtrate was concentrated under reduced
pressure. The residue was purified by preparatory thin layer
chromatography (7:3 EtOAc/hexanes) to afford (−)-46 and (+)-47
(12 mg, 70%, 7:1 ratio) as a red solid: [α]D24 −23.8, [α]57724 −33.2 (c
24
24
1
[α]546 +72.4 (c 0.12, MeOH); H NMR (500 MHz, acetone-d6) δ
24
10.38 (br s, 1H), 8.19 (d, J = 8.7 Hz, 1H), 7.70 (d, J = 8.1 Hz, 1H),
7.68 (d, J = 7.8 Hz, 1H), 7.63 (d, J = 8.8 Hz, 1H), 7.47 (d, J = 8.1 Hz,
1H), 7.27 (t, J = 7.7 Hz, 1H), 7.16 (t, J = 7.5 Hz, 1H), 7.11 (t, J = 7.5
Hz, 1H), 7.04 (s, 1H), 6.99 (t, J = 7.5 Hz, 1H), 6.95 (s, 1H), 6.47 (s,
1H), 6.43 (d, J = 8.1 Hz, 1H), 6.33 (s, 1H), 3.74 (s, 3H), 3.21 (s, 3H),
2.66 (s, 3H), 1.74 (s, 3H), 1.47 (s, 3H); 13C NMR (125 MHz,
acetone-d6) δ 179.0 (C), 169.2 (C), 168.5 (C), 166.8 (C), 166.4 (C),
164.1 (C), 152.4 (C), 151.1 (C), 150.4 (C), 146.8 (C), 142.1 (C),
138.8 (C), 135.4 (C), 135.2 (C), 132.7 (CH), 132.4 (C), 131.3 (CH),
127.7 (C), 127.2 (2 × CH), 127.0 (C), 122.6 (CH), 122.0 (CH),
120.3 (CH), 120.1 (CH), 117.9 (CH), 113.0 (CH), 111.3 (C), 109.5
(CH), 105.3 (CH), 99.1 (C), 85.9 (CH), 85.7 (C), 84.1 (CH), 73.2
(C), 58.4 (C), 51.8 (CH3), 51.4 (CH3), 29.6 (CH3), 21.4 (CH3), 20.7
(CH3);77 IR (film) 3410, 3313, 3056, 2997, 2948, 2917, 2849, 1738,
1686, 1579, 1488; TLC Rf 0.40 (4:6 EtOAc/hexanes, plate was eluted
twice; compare with Rf 0.37 for 48); HRMS (ESI) m/z calcd for
C41H32N6O10S3Na (M + Na)+ 887.1240, found 887.1238. Only minor
quantities of tetrasulfide 48 suitable for mass spectrometric analysis
could be obtained in pure form: HRMS (ESI) m/z calcd for
C41H32N6O10S4Na (M + Na)+ 919.0961, found 919.0952).
1
0.12, MeOH); H NMR (500 MHz, acetone-d6) δ 10.42 (br s, 1H),
7.80 (d, J = 7.8 Hz, 1H), 7.71 (d, J = 7.3 Hz, 1H), 7.64 (m, 1H), 7.51
(d, J = 9.0 Hz, 1H), 7.49 (d, J = 8.2 Hz, 1H), 7.23 (t, J = 7.5 Hz, 1H),
7.19 (t, J = 7.3 Hz, 1H), 7.15 (t, J = 7.2 Hz, 1H), 7.05 (d, J = 2.1 Hz,
1H), 7.02 (t, J = 7.6 Hz, 1H), 7.01 (s, 1H), 6.44 (s, 1H), 6.42 (br d, J =
7.2 Hz, 1H), 6.36 (s, 1H), 3.74 (s, 3H), 3.01 (s, 3H), 2.60 (br s, 3H),
1.91 (s, 3H), 1.47 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 178.9
(C), 169.2 (C), 169.0 (C), 166.3 (C), 164.1 (C), 162.8 (C), 151.1
(C), 150.4 (C), 149.4 (C), 146.7 (C), 141.7 (C), 138.8 (C), 134.4
(C), 132.8 (CH), 132.3 (C), 130.7 (C), 130.5 (CH), 127.5 (CH),
127.4 (C), 126.1 (CH), 125.9 (CH), 122.7 (CH), 122.3 (CH), 120.6
(CH), 120.1 (CH), 117.6 (CH), 113.0 (CH), 111.7 (C), 109.1 (CH),
105.2 (CH), 99.1 (C), 86.8 (CH), 80.6 (CH), 77.2 (C), 76.2 (C), 60.6
(C), 51.8 (CH3), 51.4 (CH3), 27.6 (CH3), 20.4 (CH3), 18.0 (CH3);
IR (film) 3411, 3323, 3055, 2950, 2917, 2849, 1737, 1703, 1578, 1489;
TLC Rf 0.33 (4:6 EtOAc/hexanes, plate was eluted twice); HRMS
(ESI) m/z calcd for C41H32N6O10S2Na (M + Na)+ 855.1519, found
855.1542.
Epitrisulfidedioxopiperazine ((+)-47). Method A. Hydrogen
sulfide (bp −60 °C, ca. 400 μL) was condensed at −78 °C in a thick-
walled, glass pressure tube fitted with a rubber septum. A solution of
42 (12 mg, 9.5 μmol) in CH2Cl2 (400 μL) and BF3·OEt2 (20 μL, 160
μmol) were injected sequentially into the reaction vessel maintained at
−78 °C. The rubber septum was replaced by the corresponding Teflon
screw cap, which was used to seal the vessel. The cold bath was
removed and the reaction mixture was allowed to warm to rt behind a
blast shield. After 1 h, the reaction mixture was cooled to −78 °C, the
Teflon cap was replaced by the rubber septum, which was equipped
with a needle vented to base (KOH/isopropanol) and bleach traps
C13-Deoxyplectosphaeroic Acid C ((+)-45). Following a similar
procedure as described for formation of 2, intermediate (+)-47 (5 mg,
5.8 μmol) was converted to acid (+)-45 (2.6 mg, 55%) as a red solid:
1
[α]D +416, (c 0.06, MeOH); H NMR (500 MHz, DMSO-d6) δ
24
10.85 (s, 1H), 9.72 (s, 1H), 8.90 (s, 1H), 8.01 (d, J = 8.7 Hz, 1H), 7.97
(d, J = 8.9 Hz, 1H), 7.74 (d, J = 8.8 Hz, 1H), 7.60 (d, J = 7.6 Hz, 1H),
7.38−7.35 (comp, 2H), 7.11−7.04 (comp, 3H), 6.71 (t, J = 7.5 Hz,
1H), 6.68 (s, 1H), 6.64 (s, 1H), 6.33 (d, J = 7.0 Hz, 1H), 6.28 (d, J =
8.0 Hz, 1H), 5.53 (d, J = 6.9 Hz, 1H), 3.06 (s, 3H), 1.55 (s, 3H); 13C
NMR (125 MHz, DMSO-d6) δ 178.1 (C), 168.9 (C), 167.5 (C), 166.3
(C), 163.3 (C), 152.6 (C), 150.9 (C), 150.5 (C), 147.6 (C), 141.2
(C), 137.1 (C), 135.3 (C), 132.1 (C), 131.3 (CH), 129.8 (C), 129.3
(CH), 127.3 (C), 125.1 (C), 125.0 (CH), 122.6 (C), 121.1 (CH),
121.0 (CH), 119.5 (CH), 118.8 (CH), 117.3 (CH), 113.7 (C), 111.5
(CH), 108.7 (CH), 105.5 (CH), 92.5 (C), 86.8 (C), 85.7 (CH), 81.8
(CH), 71.9 (C), 58.4 (C), 27.8 (CH3), 20.7 (CH3); IR (film) 3363,
3243, 2954, 2916, 2849, 1681, 1584, 1481; RP-18 TLC Rf 0.36 (7:3
MeOH/H2O); HRMS (ESI) m/z calcd for C37H25N6O9S3Na2 (M −
H + 2Na)+ 839.0641, found 839.0637.
Q
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
tert-Butyl 3-((3R,5aR,10bR,11S,11aS)-11-Acetoxy-3-(acetoxy-
m e t h y l ) - 3 , 1 1 a - d i h y d r o x y - 2 - m e t h y l - 1 , 4 - d i o x o -
1,2,3,4,5a,6,11,11a-octahydro-10bH-pyrazino[1′,2′:1,5]-
pyrrolo[2,3-b]indol-10b-yl)-1H-indole-1-carboxylate ((+)-50).
To a solution of enamide (+)-41 (70 mg, 0.12 mmol), NMO (28
mg, 0.24 mmol), and a cosolvent mixture of acetone and H2O (1.2 and
0.3 mL, respectively) was added OsO4 (60 μL, 6 μmol, 2.5 wt %
solution in t-BuOH). The reaction mixture was stirred for 2 h at rt and
then poured into a mixture of EtOAc (10 mL) and 10% aqueous
sodium thiosulfate. The layers were separated and the aqueous layer
was extracted with EtOAc (2 × 5 mL). The combined organic layers
were washed sequentially with H2O (10 mL) and brine (10 mL), dried
over Na2SO4 ,and concentrated under reduced pressure. The crude
residue was redissolved in EtOAc (1 mL) and SiO2 (100 mg) was
added. The resulting suspension was stirred for 1 h at rt (reaction
times can vary; the reaction was monitored for consumption of the
minor product, 49 by TLC: Rf 0.12, 7:3 EtOAc/hexanes), and filtered,
and the filtrate was concentrated under reduced pressure. The residue
was purified by silica gel chromatography (3:7 → 7:3 EtOAc/hexanes)
to afford the title compound (+)-50 (58 mg, 78%) as a colorless solid:
[α]D +140, [α]577 +145, [α]546 +162, [α]435 +330 (c 0.48,
(CH3), −4.2 (CH3); IR (film) 3380, 2954, 2929, 2856, 1756, 1739,
1694, 1678, 1454, 1371; TLC Rf 0.61 (3:7 EtOAc/hexanes); HRMS
(ESI) m/z calcd for C44H62N4O10Si2Na (M + Na)+ 885.3902, found
885.3907. Data for the minor epimer, (+)-51-minor: [α]D +57.7,
24
[α]577 +58.3, [α]546 +69.1, [α]435 +140.0, [α]405 +190 (c 0.34,
24
24
24
24
CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.14 (d, J = 8.0 Hz, 1H),
7.59 (d, J = 7.9 Hz, 1H), 7.53 (d, J = 7.4 Hz, 1H), 7.40 (s, 1H), 7.33 (t,
J = 7.7 Hz, 1H), 7.25 (t, J = 7.6 Hz, 1H), 7.10 (t, J = 7.6 Hz, 1H), 6.86
(t, J = 7.4 Hz, 1H), 6.71 (d, J = 7.8 Hz, 1H), 6.55 (br s, 1H), 6.53 (d, J
= 3.2 Hz, 1H), 6.16 (s, 1H), 4.72 (d, J = 10.9 Hz, 1H), 4.50 (d, J =
10.9 Hz, 1H), 3.02 (s, 3H), 2.15 (s, 3H), 1.62 (s, 9H), 1.38 (s, 3H),
0.94 (s, 9H), 0.48 (s, 9H), 0.43 (s, 3H), 0.21 (s, 3H), 0.14 (s, 3H),
0.01 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 170.2 (C), 168.8
(C), 164.2 (C), 160.1 (C), 150.2 (C), 149.9 (C), 137.0 (C), 130.3
(CH), 129.9 (C), 129.6 (C), 127.0 (CH), 125.8 (CH), 125.3 (CH),
123.4 (CH), 122.3 (CH), 120.7 (C), 119.4 (CH), 116.2 (CH), 110.9
(CH), 91.8 (C), 86.8 (C), 84.9 (C, CH), 81.5 (CH), 65.7 (CH2), 59.1
(C), 28.2 (CH3), 28.1 (CH3), 26.40 (CH3), 26.36 (CH3), 21.1 (CH3),
20.7 (CH3), 19.2 (C), 18.7 (C), −2.4 (CH3), −3.1 (CH3), −3.6
(CH3), −3.7 (CH3); IR (film) 3380, 2954, 2928, 2856, 1754, 1740,
1681, 1454, 1372; TLC Rf 0.47 (3:7 EtOAc/hexanes); HRMS (ESI)
m/z calcd for C44H62N4O10Si2Na (M + Na)+ 885.3902, found
885.3905.
24
24
24
24
CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.13 (d, J = 8.2 Hz, 1H),
7.75 (d, J = 7.9 Hz, 1H), 7.46 (s, 1H), 7.40 (d, 1H), 7.32 (t, J = 7.4 Hz,
1H), 7.23 (t, J = 7.4 Hz, 1H), 7.10 (t, J = 7.5 Hz, 1H), 6.80 (t, J = 7.4
Hz, 1H), 6.72 (d, J = 7.7 Hz, 1H), 6.47 (s, 1H), 6.25 (s, 1H), 6.20 (s,
1H), 5.50 (s, 1H), 4.42 (d, J = 11.3 Hz, 1H), 4.29 (d, J = 11.3 Hz, 1H),
2.99 (s, 3H), 1.90 (s, 3H), 1.63 (s, 9H), 1.45 (s, 3H); 13C NMR (125
MHz, acetone-d6) δ 170.6 (C), 168.8 (C), 166.7 (C), 165.3 (C), 150.0
(C), 149.9 (C), 137.0 (C), 130.1 (C), 129.8 (CH), 129.5 (C),126.1
(CH), 125.6 (CH), 125.3 (CH), 123.3 (CH), 122.6 (CH), 119.9 (C),
119.2 (CH), 116.1 (CH), 110.6 (CH), 90.6 (C), 84.8 (C), 84.7 (CH),
83.8 (CH), 80.8 (CH), 65.4 (CH2), 58.5 (C), 28.2 (CH3), 27.6
(CH3), 20.9 (CH3), 20.7 (CH3); IR (film) 3365, 2979, 2917, 2849,
1740, 1650, 1453, 1370; TLC Rf 0.27 (7:3 EtOAc/hexanes); HRMS
(ESI) m/z calcd for C32H34N4O10Na (M + Na)+ 657.2173, found
657.2175.
Cross-Coupled Product ((+)-52-major). Following a similar
procedure as described for formation of 42a, intermediate (+)-51-
major (3 runs in parallel: 50 mg, 58 μmol per run) and phenoxazinone
iodide 33 (114 mg, 170 μmol per run) were converted to (+)-52-
major (168 mg combined, 72%) as an orange solid: [α]D +26.2 (c
24
1
0.052, MeOH); H NMR (500 MHz, acetone-d6) δ 8.19 (d, J = 8.2
Hz, 1H), 7.70 (d, J = 7.4 Hz, 1H), 7.71 (d, J = 7.9 Hz, 1H), 7.66−7.62
(comp, 3H), 7.50 (s, 1H), 7.37 (t, J = 7.6 Hz, 1H), 7.25 (t, J = 7.6 Hz,
1H), 7.14 (t, J = 7.6 Hz, 1H), 7.00 (t, J = 7.3 Hz, 1H), 6.99 (s, 1H),
6.49 (s, 1H), 6.31 (s, 1H), 6.30 (d, 1H), 4.42 (d, J = 11.4 Hz, 1H),
4.34 (d, J = 11.3 Hz, 1H), 3.83 (s, 3H), 3.12 (s, 3H), 3.09 (s, 3H), 2.15
(s, 3H), 1.65 (s, 9H), 1.38 (s, 18H), 1.35 (s, 3H), 0.93 (s, 3H), 0.55 (s,
3H), 0.25 (s, 3H), 0.21 (s, 3H), 0.18 (s, 3H), 0.07 (s, 3H); 13C NMR
(125 MHz, acetone-d6) δ 180.6 (C), 170.2 (C), 168.4 (C), 165.60
(C), 165.55 (C), 163.6 (C), 163.5 (C), 150.0 (C), 149.7 (C), 149.3
(C), 147.9 (C), 144.1 (C), 139.0 (C), 137.2 (C), 137.0 (C), 136.2
(CH), 135.8 (C), 134.8 (C), 131.9 (C), 130.7 (C), 129.6 (CH), 129.4
(C), 127.2 (CH), 126.0 (CH), 125.4 (CH), 123.1 (CH), 122.7 (CH),
120.9 (CH) 119.8 (C), 118.9 (CH), 116.3 (CH), 109.9 (CH), 106.9
(CH), 93.9 (C), 86.5 (CH), 86.2 (C), 85.1 (C), 83.9 (CH), 83.7 (C),
65.4 (CH2), 58.1 (C), 53.1 (CH3), 52.1 (CH3), 28.2 (CH3), 27.9
(CH3), 26.6 (CH3), 26.4 (CH3), 21.1 (CH3), 20.5 (CH3), 19.9 (C),
18.8 (C), −2.4 (CH3), −2.8 (CH3), −3.7 (CH3), −3.9 (CH3);78 IR
(film) 2956, 2929, 2856, 1807, 1746, 1712, 1679, 1461, 1371; TLC Rf
0.45 (3:7 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C70H88N6O20Si2Na (M + Na)+ 1411.5490, found 1411.5460.
tert-Butyl 3-((5aR,10bR,11S,11aS)-11-Acetoxy-3-(acetoxy-
methyl)-3,11a-bis((tert-butyldimethylsilyl)oxy)-2-methyl-1,4-
dioxo-1,2,3,4,5a,6,11,11a-octahydro-10bH-pyrazino[1′,2′:1,5]-
pyrrolo[2,3-b]indol-10b-yl)-1H-indole-1-carboxylate (51). To a
solution of 50 (200 mg, 0.32 mmol), DMAP (38 mg, 0.31 mmol), and
triethylamine (0.41 mL, 3.2 mmol) in DMF (3.2 mL) maintained at 0
°C was added TBSOTf (0.360 mL, 1.6 mmol) slowly. The cold bath
was removed, and the reaction mixture was maintained at rt for 12 h,
whereupon EtOAc (30 mL) was added followed by saturated aqueous
NH4Cl (20 mL). The mixture was transferred to a separatory funnel,
the layers were separated, and the aqueous phase was further extracted
with EtOAc (2 × 15 mL). The combined organic extracts were washed
sequentially with 10% aqueous LiCl (2 × 15 mL) and brine (15 mL),
dried over Na2SO4, and then concentrated under reduced pressure.
The crude residue was purified by chromatography on silica gel (1:10
EtOAc/hexanes →1:3 EtOAc/hexanes) to afford the title compounds
51 as colorless solids (230 mg of the major epimer 51-major, 30 mg of
the minor epimer 51-minor; 96% overall yield, 8:1 ratio of C3-
Cross-Coupled Product (52-minor). Following a similar
procedure as described for formation of 42a, intermediate (+)-51-
minor (30 mg, 35 μmol) and phenoxazinone iodide 33 (68 mg, 100
μmol per run) were converted to 52-minor (36 mg, 74%) as an orange
1
solid: H NMR (500 MHz, acetone-d6) δ 8.20 (d, J = 8.2 Hz, 1H),
epimers). Data for the major epimer, (+)-51-major: [α]D +126.8,
7.70−7.64 (comp, 3H), 7.53 (d, J = 7.8 Hz, 1H), 7.43 (s, 1H), 7.37 (t,
J = 7.7 Hz, 1H), 7.27 (t, J = 7.3 Hz, 1H), 7.15 (t, J = 7.6 Hz, 1H), 7.03
(t, J = 7.3 Hz, 1H), 6.89 (s, 1H), 6.48 (s, 1H), 6.29 (d, J = 7.9 Hz,
1H), 6.21 (s, 1H), 4.64 (d, J = 10.8 Hz, 1H), 4.55 (d, J = 10.8 Hz, 1H),
3.78 (s, 3H), 3.03 (s, 3H), 2.78 (s, 3H), 2.19 (s, 3H), 1.64 (s, 9H),
1.36 (s, 18H), 1.35 (s, 3H), 0.96 (s, 3H), 0.57 (s, 3H), 0.28 (s, 3H),
0.15 (s, 3H), 0.15 (s, 3H), 0.07 (s, 3H); 13C NMR (125 MHz,
acetone-d6) δ 180.6 (C), 170.1 (C), 168.6 (C), 165.3 (C), 164.32 (C),
164.25 (C), 163.6 (C), 150.0 (C), 149.7 (C), 149.5 (C), 147.9 (C),
143.9 (C), 139.0 (C), 137.3 (C), 136.4 (C), 135.7 (CH), 134.8 (C),
133.5 (C), 132.1 (C), 132.0 (C), 130.5 (CH), 129.8 (C), 129.7 (C),
128.1 (CH), 126.1 (CH), 125.4 (CH), 123.5 (CH), 122.3 (CH),
120.6 (CH) 119.6 (C), 119.1 (CH), 116.3 (CH), 109.3 (CH), 106.8
(CH), 92.6 (C), 87.31 (C), 87.29 (CH), 85.12 (C), 85.05 (CH), 83.7
(C), 65.4 (CH2), 58.3 (C), 53.1 (CH3), 51.6 (CH3), 28.2 (CH3), 27.9
(CH3), 26.6 (CH3), 26.3 (CH3), 21.2 (CH3), 20.6 (CH3), 19.3 (C),
24
[α]577 +131, [α]546 +152, [α]435 +293, [α]405 +396 (c 0.34,
24
24
24
24
CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.13 (d, J = 8.1 Hz, 1H),
7.67 (d, J = 8.0 Hz, 1H), 7.49 (d, J = 7.5 Hz, 1H), 7.41 (s, 1H), 7.32 (t,
J = 7.7 Hz, 1H), 7.21 (t, J = 7.7 Hz, 1H), 7.08 (t, J = 7.5 Hz, 1H), 6.83
(t, J = 7.5 Hz, 1H), 6.71 (d, J = 7.8 Hz, 1H), 6.51 (d, J = 2.2 Hz, 1H),
6.48 (br s, 1H), 6.25 (s, 1H), 4.45 (d, J = 11.5 Hz, 1H), 4.33 (d, J =
11.5 Hz, 1H), 3.14 (s, 3H), 2.11 (s, 3H), 1.62 (s, 9H), 1.41 (s, 3H),
1.00 (s, 9H), 0.44 (s, 9H), 0.40 (s, 3H), 0.28 (s, 3H), 0.20 (s, 3H),
0.01 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 170.3 (C), 168.7
(C), 165.5 (C), 163.5 (C), 150.0 (2 × C), 137.0 (C), 130.5 (CH),
129.6 (C), 129.4 (C), 126.1 (CH), 125.7 (CH), 125.3 (CH), 123.1
(CH), 122.5 (CH), 120.8 (C), 119.9 (CH), 116.2 (CH), 111.2 (CH),
92.5 (C), 86.7 (C), 84.9 (C), 83.6 (CH), 80.9 (CH), 69.0 (CH2), 59.0
(C), 30.1 (CH3), 28.2 (CH3), 26.7 (CH3), 26.2 (CH3), 20.9 (CH3),
20.7 (CH3), 19.9 (C), 18.6 (C), −2.4 (CH3), −2.8 (CH3), −3.7
R
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
18.8 (C), −2.0 (CH3), −2.2 (CH3), −3.3 (CH3), −3.6 (CH3); IR
(film) 2952, 2929, 2856, 1806, 1747, 1712; TLC Rf 0.20 (3:7 EtOAc/
hexanes, copolar with iodide 32); HRMS (ESI) m/z calcd for
C70H88N6O20Si2Na (M + Na)+ 1411.5490, found 1411.5490.
148.7 (C), 148.0 (C), 144.1 (C), 139.0 (C), 137.2 (2 × C), 135.4
(CH), 134.83 (C), 134.77 (C), 131.8 (C), 130.8 (C), 130.3 (CH),
129.4 (C), 128.0 (CH), 126.3 (CH), 125.5 (CH), 124.0 (CH), 123.4
(CH), 120.8 (CH) 119.3 (CH), 117.7 (C), 115.9 (CH), 109.1 (CH),
106.9 (CH), 92.8 (C), 87.6 (CH), 86.7 (C), 85.2 (C), 83.7 (C), 83.0
(CH), 64.6 (CH2), 57.7 (C), 53.1 (CH3), 51.8 (CH3), 29.4 (CH3),
28.2 (CH3), 27.9 (CH3), 20.9 (CH3), 20.7 (CH3), 20.4 (CH3), 20.3
(CH3); IR (film) 2979, 2917, 2849, 1806, 1747, 1694, 1644, 1369;
TLC Rf 0.18 (1:1 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C3,C11a-Dihydroxydioxopiperazine ((−)-53-major). Tetra-n-
butylammonium fluoride (TBAF) (85 μL, 85 μmol, 1.0 M in THF)
was added to a solution of intermediate (+)-52-major (56 mg, 40
μmol), acetic acid (AcOH) (7.0 μL, 12 μmol), and THF (0.40 mL) at
0 °C. The reaction mixture was maintained at 0 °C for 30 min then
poured into a mixture of EtOAc (10 mL) and saturated aqueous
NH4Cl (5 mL). The layers were separated and the aqueous layer was
further extracted with EtOAc (2 × 5 mL). The combined organic
layers were washed successively with saturated aqueous NH4Cl (2 × 5
mL) and brine (5 mL), dried over Na2SO4, and then concentrated
under reduced pressure. The crude product was purified by silica gel
chromatography (7:3 EtOAc/hexanes) to afford the title compound
C62H64N6O22Na (M + Na)+ 1267.3971, found 1267.3987. Data for
1
(+)-54-minor: [α]D +50.5 (c 0.043, CH2Cl2); H NMR (500 MHz,
24
acetone-d6) δ 8.23 (d, J = 8.3 Hz, 1H), 7.78 (d, J = 8.8 Hz, 1H), 7.73
(d, J = 7.4 Hz, 1H), 7.64 (d, J = 7.9 Hz, 1H), 7.58 (d, J = 8.8 Hz, 1H),
7.55 (s, 1H), 7.40 (t, J = 7.8 Hz, 1H), 7.32 (t, J = 7.6 Hz, 1H), 7.24 (t,
J = 7.7 Hz, 1H), 7.07 (t, J = 7.5 Hz, 1H), 6.98 (s, 1H), 6.59 (s, 1H),
6.46 (s, 1H), 6.39 (d, J = 7.9 Hz, 1H), 4.75 (d, J = 11.4 Hz, 1H), 4.59
(d, J = 11.4 Hz, 1H), 3.80 (s, 3H), 3.05 (s, 3H), 2.87 (s, 3H), 2.20 (s,
3H), 2.12 (s, 3H), 1.65 (s, 9H), 1.47 (s, 3H), 1.46 (s, 3H), 1.37 (s,
18H); 13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 170.2 (C),
168.7 (C), 168.5 (C), 167.4 (C), 165.6 (C), 163.7 (C), 162.1 (C),
162.0 (C), 150.1 (C), 150.0 (C), 149.7 (C), 149.2 (C), 147.7 (C),
144.0 (C), 138.9 (C), 137.5 (CH), 137.3 (C), 136.6 (C), 135.1 (C),
134.8 (C), 131.6 (C), 130.3 (CH), 130.0 (C), 129.4 (C), 128.0 (CH),
126.3 (CH), 125.6 (CH), 123.6 (CH), 122.1 (CH), 120.5 (CH) 118.9
(CH), 117.9 (C), 116.5 (CH), 108.8 (CH), 106.8 (CH), 91.7 (C),
87.20 (CH), 87.17 (C), 85.3 (C), 83.7 (C), 83.2 (CH), 64.2 (CH2),
57.8 (C), 53.1 (CH3), 51.8 (CH3), 29.2 (CH3), 28.2 (CH3), 27.9
(CH3), 21.1 (CH3), 21.0 (CH3), 20.6 (CH3), 20.6 (CH3); IR (film)
2979, 2917, 2848, 1806, 1747, 1703, 1644, 1370; TLC Rf 0.11 (1:1
EtOAc/hexanes); HRMS (ESI) m/z calcd for C62H64N6O22Na (M +
Na)+ 1267.3971, found 1267.3956.
(−)-53-major as a red solid (45 mg, 96%): [α]D −94.4 (c 0.050,
24
CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ 8.19 (d, J = 8.2 Hz, 1H),
7.83 (d, J = 8.8 Hz, 1H), 7.71 (d, J = 7.9 Hz, 1H), 7.64−7.60 (comp,
2H), 7.57 (s, 1H), 7.37 (t, J = 7.7 Hz, 1H), 7.27 (t, J = 7.4 Hz, 1H),
7.14 (t, J = 7.6 Hz, 1H), 7.00 (s, 1H), 6.93 (t, J = 7.3 Hz, 1H), 6.49 (s,
1H), 6.32 (s, 1H), 6.29 (d, J = 7.8 Hz, 1H), 6.17 (s, 1H), 6.01 (s, 1H),
4.48 (d, J = 11.4 Hz, 1H), 3.87 (s, 3H), 3.86 (d, 1H), 3.24 (s, 3H),
2.94 (s, 3H), 2.08 (s, 3H), 1.67 (s, 9H), 1.40 (s, 3H), 1.39 (s, 18H);
13C NMR (125 MHz, acetone-d6) δ 180.6 (C), 170.3 (C), 168.5 (C),
166.8 (C), 166.0 (C), 165.3 (C), 163.8 (C), 150.2 (C), 150.1 (C),
150.0 (C), 149.7 (C), 147.7 (C), 144.0 (C), 138.9 (C), 137.4 (C),
137.2 (C), 136.4 (CH), 136.1 (C), 134.8 (C), 131.6 (C), 130.9 (C),
130.0 (CH), 129.3 (C), 126.7 (CH), 125.9 (CH), 125.4 (CH), 123.4
(CH), 122.7 (CH), 120.3 (CH) 119.1 (C), 119.0 (CH), 116.2 (CH),
109.1 (CH), 106.8 (CH), 91.6 (C), 86.1 (CH), 85.1 (C), 84.6 (C),
84.3 (CH), 83.7 (C), 64.7 (CH2), 57.7 (C), 53.1 (CH3), 52.3 (CH3),
28.2 (CH3), 27.9 (CH3), 27.5 (CH3), 21.0 (CH3), 20.5 (CH3); IR
(film) 3360, 2981, 2952, 2933, 2918, 2849, 1806, 1747, 1644, 1371;
TLC Rf 0.17 (7:3 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C58H60N6O20Na (M + Na)+ 1183.3760, found 1183.3765.
Dimethyl 8-((4S,6aR,11bR,12S,12aS)-12-Acetoxy-4-(acetoxy-
methyl)-11b-(1H-indol-3-yl)-14-methyl-5,13-dioxo-4,5,11b,12-
tetrahydro-4,12a-(epiminomethano)[1,2,3,5]trithiazepino-
[5′,4′:1,5]pyrrolo[2,3-b]indol-7(6aH)-yl)-2-amino-3-oxo-3H-
phenoxazine-1,9-dicarboxylate ((+)-56). Method A. Hydrogen
sulfide (bp −60 °C, ca. 0.20 mL) was condensed at −78 °C in a thick-
walled, glass pressure tube fitted with a rubber septum. A solution of
54 (3 runs in series: 30 mg, 50 mg, 60 mg; 5:1 mixture of epimers) in
CH2Cl2 (0.60 mL) and BF3·OEt2 (30 μL, 0.24 mmol) were injected
sequentially into the reaction vessel maintained at −78 °C. The rubber
septum was replaced by a Teflon screw cap, which was used to seal the
vessel. The cold bath was removed and the reaction mixture was
allowed to warm to rt behind a blast shield. After 1 h, the reaction
mixture was cooled to −78 °C, the Teflon cap was replaced by the
rubber septum, which was equipped with a needle vented to base
(KOH/isopropanol) and bleach traps (attached in series). The cooling
bath was removed and the resulting brown suspension was allowed to
warm up to rt. Upon evolution of the majority of hydrogen sulfide gas,
an argon-filled balloon was attached (by needle) to fully purge the
reaction mixture, which was subsequently poured into a mixture of
EtOAc (2 mL) and saturated aqueous NH4Cl (2 mL). The layers were
separated and the aqueous layer was extracted with EtOAc (2 mL).
The combined organic layers were washed with brine (4 mL), dried
over Na2SO4, and concentrated under reduced pressure (residual
volatile components were not rigorously removed). The crude residue
was immediately dissolved in acetone (1 mL) and 400 wt % of SiO2
(120, 150, 180 mg, respectively) was added. The resulting
heterogeneous mixture was stirred vigorously for 12−48 h (reaction
times were found to be variable and required monitoring for
completion; see Table S4 (Supporting Information) for more details)
and then filtered, and the filtrate was concentrated under reduced
pressure. The residue from all three reactions was purified by silica gel
chromatography (3:7 EtOAc/hexanes) to afford a mixture of ETP
products, disulfide 55, trisulfide 56 and tetrasulfide 57 (50 mg, 48%,
Acetylated Intermediate (54). Tetra-n-butylammonium fluoride
(TBAF) (310 μL, 0.31 mmol, 1.0 M in THF) was added to a solution
of intermediate epimers 52 (227 mg, 5:1 ratio of epimers, 0.16 mmol),
acetic acid (AcOH) (25 μL, 0.40 mmol) and THF (0.40 mL) at 0 °C.
The reaction mixture was maintained at 0 °C for 30 min then poured
into a mixture of EtOAc (10 mL) and saturated aqueous NH4Cl (5
mL). The layers were separated, and the aqueous layer was further
extracted with EtOAc (2 × 5 mL). The combined organic layers were
washed successively with saturated aqueous NH4Cl (2 × 5 mL) and
brine (5 mL), dried over Na2SO4, and concentrated under reduced
pressure. The diol intermediates 53 were of sufficient purity to be
taken forward without further purification. To a solution of this
intermediate in CH2Cl2 (7.3 mL) was added DMAP (142 mg, 1.2
mmol) and acetic anhydride (50 μL, 0.52 mmol). The resulting
mixture was maintained for 12 h at rt, then poured into a mixture of
EtOAc (30 mL) and saturated aqueous NH4Cl (20 mL). The layers
were separated, and the aqueous layer was extracted with EtOAc (2 ×
15 mL). The combined organic layers were washed successively with
H2O (20 mL) and brine (20 mL), dried over Na2SO4, then
concentrated under reduced pressure. The crude residue was purified
by chromatography on silica gel (1:5 EtOAc/hexanes →6:4 EtOAc/
hexanes) to afford the title compounds 54 (175 mg, 86% over two
steps, 5:1 ratio of epimers) as a deep red solid: Analytical samples for
both epimers were obtained. Data for (−)-54-major: [α]D −45.8,
24
[α]57724 −48.6 (c 0.058, CH2Cl2); 1H NMR (500 MHz, acetone-d6) δ
8.17 (d, J = 8.3 Hz, 1H), 8.10 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 7.3 Hz,
1H), 7.62 (d, J = 8.8 Hz, 1H), 7.48 (s, 1H), 7.42 (d, J = 8.8 Hz, 1H),
7.37 (t, J = 7.7 Hz, 1H), 7.28−7.22 (comp, 2H), 7.11 (t, J = 7.4 Hz,
1H), 6.98 (s, 1H), 6.49 (s, 1H), 6.43 (s, 1H), 6.94 (d, J = 7.9 Hz, 1H),
4.86 (d, J = 11.9 Hz, 1H), 4.55 (d, J = 11.9 Hz, 1H), 3.81 (s, 3H), 3.00
(s, 3H), 2.80 (s, 3H), 2.23 (s, 3H), 1.65 (s, 9H), 1.50 (s, 3H), 1.47 (s,
3H), 1.39 (s, 3H), 1.37 (s, 18H); 13C NMR (125 MHz, acetone-d6) δ
180.6 (C), 170.2 (C), 169.5 (C), 169.4 (C), 168.8 (C), 165.5 (C),
163.7 (C), 163.3 (C), 160.6 (C), 150.1 (C), 150.0 (C), 149.7 (C),
1
1:6:2 ratio of products, determined by H NMR) as an orange solid.
Method B. The procedure described above was followed for
processing 54 (10 mg, 8.0 μmol), except upon quenching the
sulfidation step, the crude residue was dissolved in EtOAc (2 mL) and
concentrated under reduced pressure three additional cycles prior to
S
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
the addition of EtOAc (1 mL) and SiO2 (40 mg). Upon purification,
an inseparable mixture of ETP products, disulfide 55 and trisulfide 56
(7 mg, 7:1 ratio of products, determined by H NMR), as an orange
3H), 3.82 (dd, J = 12.5, 7.6 Hz, 1H), 3.37 (s, 3H), 3.24 (s, 3H), 1.96
(s, 3H), 1.47 (s, 3H); 13C NMR (125 MHz, acetone-d6) δ 179.0 (C),
169.2 (C), 168.4 (C), 167.5 (C), 164.1 (C), 151.9 (C), 151.1 (C),
150.3 (C), 146.7 (C), 141.9 (C), 138.6 (C), 136.3 (C), 135.0 (C),
132.5 (CH), 132.4 (C), 130.6 (CH), 130.2 (C), 127.4 (C), 126.7
(CH), 125.0 (CH), 122.5 (CH), 122.4 (CH), 120.5 (CH), 120.0
(CH), 117.9 (CH), 114.3 (C), 112.5 (CH), 109.9 (CH), 105.3 (CH),
99.2 (C), 87.6 (C), 87.1 (CH), 84.4 (CH), 76.6 (C), 61.9 (CH2), 59.5
(C), 52.4 (CH3), 51.9 (CH3), 27.8 (CH3); IR (film) 3390, 3323, 2951,
2918, 2849, 1738, 1681, 1582; TLC Rf 0.41 (7:3 EtOAc/hexanes, plate
was eluted twice); HRMS (ESI) m/z calcd for C39H30N6O10S3Na (M
+ Na)+ 861.1083, found 861.1087.
1
solid was obtained. To a solution of this mixture of ETPs 55 and 56 (7
mg, 7.8 μmol) in acetone (1 mL) was added Na2S (0.60 mg, 7.6
μmol). The resulting heterogeneous mixture was stirred for 0.5 h at rt
and then poured into a mixture of EtOAc (2 mL) and saturated
aqueous NH4Cl (2 mL). The combined organic layers were washed
with brine (4 mL), dried over Na2SO4, and then concentrated under
reduced pressure to afford a mixture of ETP products, trisulfide 56 and
79
1
tetrasulfide 57 (2.5:1 ratio of products, determined by H NMR).
The mixture was dissolved in CH2Cl2 (0.5 mL) then Ph3P (40 μL,
0.05 M solution in CH2Cl2) was added. After 0.5 h, the mixture was
eluted through a short column of silica gel (1:2 EtOAc/hexanes, then
2:1 EtOAc/hexanes) to afford the title compound (+)-56 (5.0 mg,
67%) as a red solid: [α]D +47.3, [α]577 +49.5, [α]546 +53.9 (c
(+)-Plectosphaeroic Acid C (3). To a solution of ETP (+)-58
(5.0 mg, 6.0 μmol) in PhMe (0.10 mL) was added LiI (3 mg, 22
μmol). The reaction mixture was stirred for 1 h at 90 °C (formation of
product was observed by RP-18 TLC), a second portion of LiI (3 mg,
22 μmol) was added, and the reaction mixture was heated for 1 h at 90
°C. This sequence was repeated two more times (total: 12 mg of LiI, 4
h at 90 °C) before the reaction was allowed to cool to rt and poured
into a mixture of EtOAc (5 mL) and 0.1% aqueous TFA (2 mL). The
layers were separated, the aqueous layers were extracted with EtOAc
(4 mL), and then the combined organic layers were dried over
Na2SO4, filtered, and concentrated under reduced pressure. This
reaction was conducted in series on 1, 2, 5, 5 mg scales (processed
identically), and then the crude residue from all four reactions was
purified by reverse-phase HPLC (step-gradient eluting 20%, 40%, 50%,
60%, 70%, 95% MeCN/H2O + 0.1% TFA) to afford plectosphaeroic
acid C (3) (9 mg, 64%, eluting at 60% MeCN/H2O + 0.1% TFA) as a
24
24
24
1
0.06, CH2Cl2); H NMR (500 MHz, acetone-d6) δ 10.41 (br s, 1H),
7.96 (d, J = 8.7 Hz, 1H), 7.70−7.67 (comp, 2H), 7.63 (d, J = 8.7 Hz,
1H), 7.47 (d, J = 8.1 Hz, 1H), 7.27 (t, J = 7.7 Hz, 1H), 7.18 (t, J = 7.3
Hz, 1H), 7.11 (t, J = 7.2 Hz, 1H), 7.07 (d, J = 2.5 Hz, 1H), 7.00 (d, J =
7.4 Hz, 1H), 6.94 (s, 1H), 6.46 (s, 1H), 6.43 (d, J = 8.0 Hz, 1H), 6.36
(s, 1H), 4.65 (d, J = 12.3 Hz, 1H), 4.36 (d, J = 12.3 Hz, 1H), 3.73 (s,
3H), 3.27 (s, 3H), 2.68 (s, 3H), 1.96 (s, 3H), 1.47 (s, 3H); 13C NMR
(125 MHz, acetone-d6) δ 179.0 (C), 169.9 (C), 169.2 (C), 168.5 (C),
166.7 (C), 166.4 (C), 162.1 (C), 152.3 (C), 151.1 (C), 150.4 (C),
146.8 (C), 142.1 (C), 138.8 (C), 135.3 (C), 135.0 (C), 132.8 (CH),
132.4 (C), 131.4 (CH), 127.5 (C), 127.2 (CH), 127.04 (CH), 126.95
(C), 122.7 (CH), 121.8 (CH), 120.4 (CH), 120.2 (CH), 118.0 (CH),
113.0 (CH), 111.3 (C), 109.4 (CH), 105.3 (CH), 99.1 (C), 86.0
(CH), 85.1 (C), 84.0 (CH), 74.9 (C), 61.6 (CH2), 58.6 (C), 51.8
(CH3), 51.4 (CH3), 28.5 (CH3), 20.6 (CH3), 20.6 (CH3); IR (film)
3394, 3322, 2917, 2849, 1746, 1694, 1576, 1486; TLC Rf 0.25 (4:6
EtOAc/hexanes); HRMS (ESI) m/z calcd for C43H34N6O12S3Na (M +
Na)+ 945.1295, found 945.1281. Data for epidisulfidedioxopiperazine
red solid: [α]D +494 (c 0.06, MeOH), compare with reported value
24
1
[α]D +136 (c 0.17, MeOH) of the natural sample; H NMR (500
24
MHz, DMSO-d6) δ 10.86 (br s, 1H), 9.72 (br s, 1H), 8.92 (br s, 1H),
8.02 (d, J = 7.7 Hz, 1H), 7.95 (d, J = 8.6 Hz, 1H), 7.74 (d, J = 8.8 Hz,
1H), 7.61 (d, J = 7.5 Hz, 1H), 7.37 (m, 1H), 7.36 (m, 1H), 7.10 (t, J =
7.0 Hz, 1H), 7.06 (comp, 2H), 6.72 (t, J = 7.4 Hz, 1H), 6.70 (s, 1H),
6.68 (s, 1H), 6.45 (br s, 1H), 6.32 (d, J = 7.8 Hz, 1H), 5.54 (s, 1H),
5.48 (br s, 1H), 3.78 (m, 1H), 3.61 (m, 1H), 3.11 (s, 3H); 13C NMR
(125 MHz, acetone-d6) δ 178.1 (C), 168.9 (C), 167.5 (C), 166.8 (C),
162.1 (C), 152.7 (C), 150.8 (C), 150.3 (C), 141.1 (C), 137.1 (C),
135.5 (C), 132.0 (C), 131.2 (CH), 129.8 (C), 129.3 (CH), 127.4 (C),
125.1 (C, CH), 122.6 (CH), 121.1 (2 × CH), 119.5 (CH), 118.8
(CH), 117.3 (CH), 113.8 (C), 111.5 (CH), 108.7 (CH), 105.6 (CH),
92.5 (C), 86.3 (C), 85.7 (C), 81.7 (CH), 75.6 (C), 59.5 (CH2), 58.5
(C), 27.3 (CH3); RP-18 TLC Rf 0.50 (1:1 MeCN/H2O); HRMS
(ESI) m/z calcd for C37H25N6O10S3 (M − H)− 809.0794, found
809.0779.
1
55: H NMR (500 MHz, acetone-d6) δ 10.40 (br s, 1H), 7.81 (d, J =
7.7 Hz, 1H), 7.71 (d, J = 7.4 Hz, 1H), 7.64 (m, 1H), 7.69−7.62
(comp, 2H), 7.24 (t, J = 7.8 Hz, 1H), 7.21−7.16 (comp, 2H), 7.08−
7.04 (comp, 2H), 7.02 (s, 1H), 6.44 (d, 1H), 6.43 (s, 2H), 6.39 (s,
1H), 4.82 (d, J = 12.7 Hz, 1H), 4.77 (d, J = 12.7 Hz, 1H), 3.71 (s, 3H),
3.10 (s, 3H), 2.60 (br s, 3H), 2.11 (s, 3H), 1.50 (s, 3H); 13C NMR
(125 MHz, acetone-d6) δ 178.9 (C), 170.1 (C), 169.1 (C), 169.0 (C),
166.2 (C), 163.8 (C), 161.1 (C), 151.0 (C), 150.4 (C), 149.3 (C),
146.7 (C), 141.7 (C), 138.8 (C), 138.7 (C), 132.8 (CH), 132.3 (C),
131.4 (C), 130.6 (CH), 127.6 (CH), 127.42 (C), 127.38 (C), 126.0
(CH), 122.7 (CH), 122.3 (CH), 120.7 (CH), 120.2 (CH), 117.6
(CH), 113.0 (CH), 111.5 (C), 109.4 (CH), 105.2 (CH), 99.0 (C),
86.9 (CH), 80.3 (CH), 77.3 (C), 77.1 (C), 60.9 (CH2), 60.8 (C), 51.8
(CH3), 51.5 (CH3), 28.5 (CH3), 20.6 (CH3), 20.5 (CH3); TLC Rf
0.25 (4:6 EtOAc/hexanes); HRMS (ESI) m/z calcd for
C43H34N6O12S2Na (M + Na)+ 913.1574, found 913.1587.
Cinnabarinic Acid (4).11,32b,c This procedure is a modification of
a procedure reported by Manthey and co-workers.32b To a solution of
3-hydroxyanthranilic acid (S10) (350 mg, 2.3 mmol) and EtOH (200
mL) was added 1,4-benzoquhinone (420 mg, 3.3 mmol, freshly
recrystallized). The reaction mixture was maintained at 40 °C for 12 h
and then cooled to rt, and the desired product (250 mg, 72%) was
filtered away from the mother liquor as an orange/red solid in
sufficient purity. The spectroscopic data obtained for the product
matched previously reported data:32c 1H NMR (500 MHz, DMSO-d6)
δ 9.72 (br s 1H), 8.78 (br s, 1H), 7.94 (d, J = 7.5 Hz, 1H), 7.94 (d, J =
7.6 Hz, 1H), 7.60 (d, J = 8.0 Hz, 1H), 6.60 (s, 1H).
Dimethyl 2-Amino-9-iodophenoxazin-3-one 1,10-dicarbox-
ylate (60).81 To a solution of 3-hydroxyanthranilic acid methyl ester
(S11)82 (280 mg, 1.7 mmol) and EtOH (56 mL) was added 1,4-
benzoquinone (310 mg, 2.8 mmol, freshly recrystallized). The reaction
mixture was maintained at 40 °C for 24 h and then cooled to rt and
concentrated in vacuo. The crude residue was suspended in ethyl
acetate/hexanes (1:1, 60 mL), and the desired product (160 mg, 56%)
was filtered away from the mother liquor as an orange/red solid in
sufficient purity. The spectroscopic data obtained for the product
matched previously reported data:11 1H NMR (500 MHz, DMSO-d6)
δ 7.8 (br s, 2H), 7.65 (d, J = 6.1 Hz, 1H), 7.59−7.54 (m, 2H), 6.49 (s,
1H), 3.89 (s, 3H), 3.83 (s, 3H).
Plectosphaeroic Acid C Dimethyl Ester ((+)-58). To a solution
of ETP (+)-56 (11 mg, 11 μmol) in MeOH (0.30 mL) were added
lanthanum(III) trifluoromethanesulfonate (66 mg, 110 μmol) and
DMAP (1.6 mg, 12 μmol). The reaction mixture was maintained at 45
°C. After 5 h (the consumption of the starting material was monitored
by TLC), the reaction mixture was allowed to cool to rt and then
poured into a mixture of EtOAc (10 mL) and saturated aqueous
NH4Cl (10 mL). The layers were separated, and the aqueous layer was
extracted with EtOAc (2 × 10 mL). The combined organic layers were
washed with brine (10 mL), dried over Na2SO4 and concentrated
under reduced pressure. The crude residue was purified by preparatory
thin layer chromatography (7:3 EtOAc/hexanes) to afford inter-
mediate (+)-58 (7.0 mg, 76%) as a red solid:80 [α]D24 +181.4, [α]577
24
+190.0, [α]546 +203.6 (c 0.07, CH2Cl2); 1H NMR (500 MHz,
24
acetone-d6) δ 10.25 (br s, 1H), 8.02 (d, J = 8.7 Hz, 1H), 7.97 (d, J =
8.1 Hz, 1H), 7.66 (d, J = 7.3 Hz, 1H), 7.62 (d, J = 8.8 Hz, 1H), 7.41
(d, J = 8.0 Hz, 1H), 7.32 (s, 1H), 7.18−7.11 (comp, 2H), 7.07 (t, J =
7.6 Hz, 1H), 6.84 (d, J = 7.3 Hz, 1H), 6.78 (s, 1H), 6.47 (s, 1H), 6.45
(d, J = 7.7 Hz, 1H), 5.52 (d, J = 4.8 Hz, 1H), 5.17 (d, J = 4.8 Hz, 1H),
4.25 (app t, J = 7.0 Hz, 1H), 3.99 (dd, J = 12.5, 6.4 Hz, 1H), 3.86 (s,
T
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(7) For structure determination and total synthesis, see: Foo, K.;
Newhouse, T.; Mori, I.; Takayama, H.; Baran, P. S. Angew. Chem., Int.
Ed. 2011, 50, 2716−2719.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra of new compounds, CD
(8) (a) Takahashi, C.; Numata, A.; Ito, Y.; Matsumura, E.; Araki, H.;
Iwaki, H.; Kushida, K. J. Chem. Soc., Perkin Trans. 1 1994, 1859−1864.
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(11) In a recent report, the IC50 of cinnabarinic acid (4) was reported
to be 0.46 μM, compared with 2 μM reported earlier;4 see: Pasceri, R.;
Siegel, D.; Ross, D.; Moody, C. J. J. Med. Chem. 2013, 56, 3310−3317.
(12) For total syntheses of (+)-gliocladin C (19) and (+)-gliocladine
C (6), see: DeLorbe, J. E.; Jabri, S. Y.; Mennen, S. M.; Overman, L. E.;
Zhang, F.-L. J. Am. Chem. Soc. 2011, 133, 6549−6552. For total
syntheses of (+)-T988 C (7), bionectin A (8), leptosin D (9), and
gliocladin A (11), see ref 2c.
spectra for 2 and 3, CIF for 40a and details of the X-ray
analysis, complete author listing for ref 3b, and additional
experimental and computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: leoverma@uci.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support was provided by the National Institute of
General Medical Sciences of NIH (R01GM-30859). S.Y.J.
thanks Eli Lilly and Co. and Bristol-Myers Squibb Co. for
graduate fellowships. Computational studies were performed
on hardware purchased with funding from CRIF (CHE-
0840513). NMR and mass spectra were determined using
instruments purchased with the assistance of NSF and NIH
shared instrumentation grants. We thank Dr. Joseph Ziller and
Dr. John Greaves, Department of Chemistry, UC Irvine, for
their assistance with X-ray and mass spectrometric analyses and
Dr. Nathan Crawford, Department of Chemistry, UC Irvine, for
helpful discussions regarding computational studies. In
addition, we thank Dr. David Horne and Dr. Sangkil Nam of
the City of Hope Developmental Cancer Therapeutics Program
for in vitro anticancer testing, which was supported by grants
from the A. Gary Anderson Family Foundation and NIH
P30CA-33572.
(13) For the total synthesis (+)-gliocladin B (14), see: (a) Boyer, N.;
Movassaghi, M. Chem. Sci. 2012, 3, 1798−1803. For the recent total
syntheses of bionectin A (8) and gliocladin A (11), see: (b) Coste, A.;
Adams, T. C.; Movassaghi, M. Chem. Sci. 2013, 4, 3191−3197.
(14) For a review of the total syntheses of ETPs, see ref 1c. For
recent syntheses, see: (a) Nicolaou, K. C.; Totokotsopoulos, S.;
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(38) A parallel study investigating palladium(0) for this C−N cross-
coupling was also conducted. Although hydrodehalogenation and
homocoupling of 32 was observed (using 0.1−1.0 equiv of Pd2dba3
and RuPhos, Cs2CO3/K2CO3, toluene/dioxane), no formation of 34
was detected in any of these reactions.
(39) One Boc-protecting group of 33 was cleaved under these
conditions, and a significant amount of the mono(tert-butoxycarbonyl)
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of 39a or formation of multiple byproducts.
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reaction. In addition, at the outset of our work, bimolecular arylation
of the indoline nitrogen of a cyclotryptamine to our knowledge had
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(47) Signals attributable to the undesired C3 α-methylthio epimer
1
44a were not apparent in the H NMR of the crude mixture of this
́ ́
Wolanski, M.; Peczynska−Czoch, W.; Mlochowski, J. Synth. Commun.
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2007, 37, 1779−1789 and references therein..
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compounds (see the Supporting Information) is calculated (B3-LYP/
def2-TZVP) to be 2.5 (or 2.0; when zero-point energies are
incorporated) kcal/mol more stable than its C3 epimer, whereas this
difference is 0.9 (or 0.8; when zero-point energies are incorporated)
kcal/mol in the bis(methylthio)ether series; however, this difference is
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molar ratios from 2:1 to 1:6 25/32 were examined) or adding
stoichiometric quantities of copper and ligands (e.g., CuI and dppp or
bpy) were unsuccessful. Switching to amine bases (e.g., Et3N,
pyridine) and polar aprotic solvents (e.g., THF, dioxane, DMA,
DMF or NMP) commonly used in copper-promoted C−N cross-
couplings were either ineffective or led to decomposition of 25. In all
reactions, iodide 32 was recovered largely intact with little to no
hydrodehalogenation or homocoupling being detected.
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Scheme 6 immediately after aqueous quenching of the sufidation
(without concentrating the organic layer under reduced pressure) to
minimize competitive oxidation of the dithiol intermediate to
epidisulfide 46.
(54) For related ring-expansions of ETPs, see: (a) Murdock, K. C. J.
Med. Chem. 1974, 17, 827−835. (b) Ottenheijm, H. C. J.; Herscheid, J.
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2749. For CuTC in a related C−N cross-coupling, see: (b) Li, G.;
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carboxylate (CAS No. 68986-76-5) was important. We used reagent
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Seguin-Swartz, G.; Abrams, A. R. Phytochemistry 1990, 29, 777−782.
(55) Resubmitting this mixture of ETPs to these conditions did not
increase conversion to 48. Allowing either 47 or 48 individually to
react with 1 equiv of Na2S returned this mixture of tri- and tetrasulfide
V
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ETPs. Addition of an excess of Na2S resulted in decomposition. These
are heterogeneous reactions, and it is difficult to draw conclusions
from this study. However, a possible explanation for these results is
that upon formation of epitrisulfude 47, a second molecule of Na2S
reopens the polysulfide ring. The corresponding mixture of ETPs
would be obtained from competing ring-closing mechanisms:
nucleophilic ring closure (with concomitant expulsion of one sulfur
atom) to return 47 or formation of the tetrasulfide bridge of 48.
(56) We also examined the use of Na2S4 for this transformation.
Exposing disulfide 46 to 1 equiv of Na2S4 in acetone gave a 1.4:1
mixture of ETPs 47 and 48. Resubmitting this product mixture to
these conditions brought about no change. In none of these
experiments were products containing more than four sulfur atoms
detected by MS analysis.
(57) On larger scales (0.1−0.3 g), it was most convenient to expose a
mixture of 39a,b to TfOH in CH2Cl2 at −25 °C to promote complete
expulsion of the oxygen substituent at C3. However, a portion of the
Boc group on the indole nitrogen atom was cleaved under these
conditions and could be selectively protected by subsequent treatment
of the crude mixture with a corresponding amount of Boc2O in the
presence of DMAP in THF prior to isolation of 41 in 67% yield.
(58) Attempts to sulfidate the major diol product 53 were
unsuccessful. Endeavoring to activate the alcohol groups of 53-
major in the presence of H2S in MeCN with Hf(OTf)3, a Lewis acid
with high oxophilicity but reduced thiophilicity that was utilized by
Boyer and Movassaghi in their total synthesis of gliocladin B (14),13a
failed to yield the desired dithiol product.
(74) In the 13C NMR spectrum of 42b, this carbon coincided with
the acetone-d6 (solvent) peaks and was assigned by DEPT and
HMQC experiments.
(75) In the 13C NMR spectrum of 44a, a quaternary carbon was not
detected or coincided with another carbon resonance.
(76) A corresponding experiment (42a: 4 mg, 3.2 μmol) was
conducted wherein Na2S4 (0.6 mg, 3.4 μmol) was used instead of
Na2S. A mixture of (+)-47 and 48 (64%, 1.4:1 ratio of products) was
obtained upon purification.
(77) In the 13C NMR spectrum of 47, overlapping carbons were
assigned by HMQC, DEPT, and HMBC experiments.
(78) In the 13C NMR spectrum of 52-major, one quaternary carbon
was not detected or coincided with another carbon resonance.
(79) The addition of small amounts of Na2S proved challenging. In
some cases, adjusting the amount of Ph3P added in the subsequent
step to selectively decompose the corresponding tetrasulfide ETP 57
was employed.
(80) At this step, the resulting di-, tri-, and tetrasulfide ETP products
could be efficiently separated by preparatory thin-layer chromatog-
raphy: Rf 0.38, 0.41, 0.29, respectively (7:3 EtOAc/hexanes, plate was
eluted twice).
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Fielder, D. A.; Collins, F. W. J. Nat. Prod. 1995, 58, 456−458.
(59) This sequence required careful monitoring, as reaction times
were variable; for additional details, see the Supporting Information.
(60) Pickering, T. L.; Saunders, K. J.; Tobolsky, A. V. J. Am. Chem.
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(61) See refs 8a,d, 54b, and: (a) Hino, T.; Sato, T. Chem. Pharm. Bull.
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ETPs, see: (b) Herscheid, J. D. M.; Tijhuis, M. W.; Noordik, J. H.;
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(63) Cell viability was determined by Dr. Sangkil Nam at the
Beckman Research Institute, City of Hope Comprehensive Cancer
Center, using a MTS metabolic activity assay as described by the
supplier (Promega); see: Nam, S.; Williams, A.; Vultur, A.; List, A.;
Bhalla, K.; Smith, D.; Lee, F. Y.; Jove, R. Mol. Cancer Ther. 2007, 6,
1400−1405.
(64) A synthetic sample of cinnabarinic acid (4) and the dimethyl
ester 60 were prepared in a fashion similar to that described for
preparation of 31. See the Experimental Section for details.
(65) The synthesis and in vitro cytotoxicity activity of T988 C (7),
59 and related ETPs against DU145 and A2025 were previously
reported and used in the present context for comparison. See ref 2c.
(66) The Pd/RuPhos precatalyst (CAS No. 1028206-60-1) is
commercially available.
(67) Node, M.; Kawabata, T.; Ohta, K.; Fujimoto, M.; Fujita, E.; Fuji,
K. J. Org. Chem. 1984, 49, 3641−3643.
(68) In the 13C NMR spectrum of (−)-37b, two quaternary carbons
were not detected or coincided with another carbon resonance.
(69) Additional K2OsO4·H2O and (DHQ)2PHAL were needed in
order to consume all starting material.
(70) The relative configuration of S7 was confirmed chemically by
elaboration to 44b in two steps: (i) acetylation of the alcohols and (ii)
sulfidation by the two-step procedure (vide infra).
(71) This two-step sequence gave yields ranging from 60−80% yields
on various scales (5−100 mg of 39a).
(72) In the 13C spectrum of 41, one tertiary carbon coincided with
the solvent peak (acetone) and was assigned by an HMQC
experiment.
(73) In the 13C NMR spectrum of 42a, multiple carbons of the minor
atropisomer were not detected or coincided with other carbon
resonances.
W
dx.doi.org/10.1021/jo4015479 | J. Org. Chem. XXXX, XXX, XXX−XXX