Total synthesis of (+)-asperazine

Article abstract of DOI:10.1016/j.tet.2007.05.127

The first total synthesis of the structurally novel cyclotryptophan alkaloid asperazine is reported. The central step in the synthetic sequence is a diastereoselective intramolecular Heck reaction in which the substituent controlling stereoselection is ex

Full text of DOI:10.1016/j.tet.2007.05.127

Tetrahedron 63 (2007) 8499–8513
Total synthesis of (D)-asperazine
y
*
Steven P. Govek and Larry E. Overman
Department of Chemistry, 1102 Natural Sciences II, University of California, Irvine, CA 92697-2025, USA
Received 29 March 2007; revised 10 May 2007; accepted 11 May 2007
Available online 6 June 2007
Dedicated to Professor Hisashi Yamamoto in honor of his many pioneering contributions to organic chemistry and receipt of the 2006 Tetrahedron Prize
Abstract—The first total synthesis of the structurally novel cyclotryptophan alkaloid asperazine is reported. The central step in the synthetic
sequence is a diastereoselective intramolecular Heck reaction in which the substituent controlling stereoselection is external to the ring being
formed. This synthesis confirmed the structure of (+)-asperazine (1) proposed by Crews and co-workers and provided material for additional
biological studies. The in vitro cytotoxicity originally reported for the marine isolate was not confirmed with synthetic (+)-asperazine.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
are cis.6 The relative configuration at C2, C3, and C11 was
ascertained by 1H NMR NOE experiments. Finally, the con-
figurational relationship between the hexacyclic moiety and
the pendant diketopiperazine was assumed based on the like-
lihood that (S)-tryptophan was the biogenetic precursor of
both tryptophan fragments.
In 1997, Crews and co-workers reported the isolation of
asperazine (1) from a saltwater culture of the fungus Asper-
gillus niger obtained from a Caribbean Hyrtios sponge
(Fig. 1).1 Asperazine was reported to display an unusual pro-
file of cytotoxicity. Whereas no activity was seen in antibac-
terial (Bacillus subtilis) or antifungal (Candida albicans)
assays, significant differential in vitro cytotoxicity was
observed against human leukemia.1,2 Unfortunately, further
biological studies were not possible as a result of the small
amount of asperazine isolated,1 and the inability to regrow
asperazine-producing A. niger cultures.3
The unavailability of asperazine from natural sources, its
potential selective antileukemic activity, and its uncommon
structure led us to pursue its total synthesis. In 2001, we re-
ported the first, and to date only, total synthesis of asperazine
(1).7 This synthesis confirmed the structure of asperazine
proposed by Crews and co-workers and provided additional
material for biological studies. In this paper, we report full
details of this total synthesis and the experiments that led
to the successful synthesis strategy. Moreover, we report
that synthetic asperazine, unlike the original natural isolate,
shows no significant antileukemic activity in the Corbett–
Valeriote soft agar disk diffusion assay.2
The structure of asperazine was assigned by a combination
of NMR experiments, mass spectrometry, and chemical deg-
radation. Initial NMR investigations coupled with mass
spectra indicated that asperazine contained two tryptophan
and two phenylalanine units, as such showing structural sim-
ilarity to WIN 64821 (2)4 and ditryptophenaline.5 However,
unlike these alkaloids, asperazine was not C2-symmetric.
Further NMR analysis demonstrated that the aromatic peri
carbon of one indole unit was attached to the quaternary benz-
ylic carbon of a cyclotryptophan fragment (the C3–C24
linkage of 1). Hydrolysis of asperazine provided (R)-phenyl-
alanine, leading to the assignment of the R configuration
at C15 and C37. The relative configuration of C11 and
C15 followed from the absence of a five-bond coupling
between these hydrogens, which is often seen in proline-
containing diketopiperazines when comparable hydrogens
* Corresponding author. Tel.: +1 949 824 7156; fax: +1 949 824 3866;
e-mail: leoverma@uci.edu
Present address: Kalypsys, Inc., 10420 Wateridge Circle, San Diego, CA
y
92121, USA.
Figure 1. Structures of asperazine and WIN 64821.
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.127

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S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
1.1. Synthesis plan
would control diastereoselection. Such an approach for con-
structing the pivotal C3–C24 s-bond had several appealing
features. Foremost, it would allow us to explore the ability
of a stereocenter external to the ring being formed to regulate
stereoselection in a Heck cyclization. Although the use of
stereogenic centers in the chain connecting the reacting part-
ners has been exploited often in the stereocontrolled con-
struction of rings by intramolecular Heck reactions,12 few
studies have examined the influence of stereocenters exter-
nal to the nascent ring.13 A second appeal of this strategy
was the anticipated ease with which the configuration of
the newly formed C3 quaternary-carbon stereocenter could
be ascertained. Because the configuration of the C–C p-
bond and the sp3 stereocenter C3 in Heck product 5 should
be correlated by the suprafacial stereospecificity of the mi-
gratory insertion and b-hydride elimination steps, the abso-
lute configuration at C3 should be ascertained easily from
the geometry of the trisubstituted alkene. Finally, the Heck
cyclization precursor 6 was seen as arising from (S)-trypto-
phan, 2-iodoaniline, and serine. As events transpired, we
learned that the C11 stereocenter of precursor 6 would
need to be of the S absolute configuration, thus deriving
from (R)-serine, to achieve the desired stereocontrol at C3
in the pivotal Heck cyclization. At the outset, this fact was
not known with certainty; thus, the ready availability of
either serine enantiomer was an important consideration
in the genesis of our synthesis plan.
Foremost in a synthetic endeavor targeting asperazine (1) is
the need to construct the quaternary-carbon stereocenter C3,
which unites the two tryptophan-derived fragments.8 Com-
plicating this task, the indole unit is appended at a sterically
congested peri aromatic carbon. Another consideration af-
fecting our planning was the possibility that asperazine
might not have the relative configuration depicted in repre-
sentation 1. We have already noted that there was no exper-
imental evidence for the absolute configuration at C34.
Moreover, it remained a possibility that the hydrolytic deg-
radation of asperazine yielding (R)-phenylalanine had not
cleaved both diketopiperazine units. Thus, we sought to pur-
sue a synthetic approach in which either (R)- or (S)-phenyl-
alanine and (R)- or (S)-tryptophan fragments could be
synthetic inputs.
The strategy we chose to develop is outlined in retrosyn-
thetic format in Scheme 1 for the specific case in which
structure 1 correctly represents the three-dimensional struc-
ture of asperazine. Removing the two (R)-phenylalanine
fragments from 1 provides cis-1,2,3,3a,8,8a-hexahydropyr-
rolo[2,3-b]indole (for simplicity, hereafter referred to as
cis-pyrrolidinoindoline) 3. Further disconnection of the pyr-
rolidine ring gives oxindole 4. It is conceivable that this in-
termediate and its quaternary-carbon stereocenter could be
assembled by a catalytic asymmetric intramolecular Heck
reaction,9 an approach we have employed successfully in
the total synthesis of several cyclotryptamine alkaloids.10
However, we chose to develop an alternate, potentially com-
plimentary approach. A plausible precursor of oxindole 4
would be the dehydro congener 5. At the outset, we enter-
tained the possibilities that hydrogenation of the double
bond of 5 to introduce the C11 stereocenter of 4 might be
controlled by the nearby quaternary stereocenter C3, or if
need be, by a catalyst-controlled hydrogenation.11 We envis-
aged intermediate 5 arising from a diastereoselective intra-
molecular Heck reaction of a,b-unsaturated iodoanilide 6,
in which the serine-derived allylic stereogenic center C11
2. Results and discussion
2.1. Model study for forming the C3 quaternary-carbon
stereocenter
Before initiating our synthetic endeavor, we wished to exam-
ine the feasibility of the proposed diastereoselective Heck
reaction in a simpler system. The synthesis of a model
Heck cyclization substrate, iodoanilide 9, is summarized in
Scheme 2. Since alkene face stereoselection in the pivotal in-
tramolecular Heck cyclization would be controlled by the
Scheme 1. Retrosynthetic disconnection of asperazine.

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8501
substituents at the g-carbon of the a,b-unsaturated anilide
Heck cyclization precursor, we wanted these substituents
to differ considerably in size. As a result, Garner’s aldehyde
7 was selected as the starting material; this commercially
available precursor is easily prepared on a large scale from
(S)-serine.14 Stereoselective Horner–Wadsworth–Emmons
olefination of aldehyde 7 provided (Z)-enoate 8.15 Weinreb
amidation of this intermediate with 2-iodoaniline,16 fol-
lowed by reaction of the secondary amide product with di-
tert-butyldicarbonate gave Heck precursor 9 in 65% overall
yield from aldehyde 7.
We quickly discovered that the desired Heck cyclization
took place efficiently when iodoanilide 9 was exposed to
20 mol % Pd(PPh3)4 and excess 1,2,2,6,6-pentamethylpiper-
idine (PMP) in N,N-dimethylacetamide (DMA) at 80 ꢀC for
6 h. These conditions produced three isomeric oxindole
products 10. After initial resolution of this mixture on silica
gel, and further separation after the Boc-protecting group
had been selectively removed from the oxindole nitrogen
by reaction with K2CO3 in MeOH, oxindole alkene isomers
11, 12, and 13 were isolated, respectively, in 41%, 10%, and
7% overall yields from Heck precursor 9. NMR spectra, in
1
conjunction with the diagnostic H NMR NOE enhance-
ments for isomers 11 and 13 depicted in Scheme 2, readily
established that oxindoles 11 and 13 were E and Z stereoiso-
mers produced in the initial Heck cyclization, whereas 12
was a product of subsequent double bond migration.17
The absolute configuration at the newly formed quaternary-
carbon stereocenter of oxindoles 11–13 was determined as
follows. During a previous investigation in our laboratories,
a convenient HPLC method for measuring the enantiomeric
purity of 1,3-dimethyl-3-(2-hydroxyethyl)oxindole (14) was
developed and the absolute configuration of this 3,3-disub-
stituted oxindole established.18 Thus, oxindoles 11–13
were individually transformed into oxindole 14 by the
four-step sequence summarized in Scheme 2. This study re-
vealed that 11 and 12 were S enantiomers of similar, high
(95–96% ee) enantiomeric purity, whereas oxindole 13 had
the R absolute configuration and a slightly lower enantio-
meric purity (89% ee). Several conclusions could be drawn.
First, as anticipated from mechanistic considerations, the
absolute configuration of the quaternary-carbon stereocenter
and the geometry of the alkene are correlated in such a way
that the configuration of the quaternary stereocenter of the
primary Heck products 11 and 13 can be ascertained from
the configuration of the double bond. This analysis is explic-
itly developed in Scheme 3. Second, oxindole product 12 is
derived almost exclusively from the major Heck product 11.
Thus, diastereoselectivity in the initial Heck cyclization was
an encouraging 9:1, with insertion occurring preferentially
from the a-Re face of a,b-unsaturated amide 9. Third, there
is a slight loss of enantiomeric purity in forming the R Heck
product 13, likely arising from a small amount of this prod-
uct being formed from S oxindole 12 by a second double
bond migration. Finally, if stereoselection in the pivotal
intramolecular Heck reaction in the fully constituted series
Scheme 2. Model study of the proposed diastereoselective intramolecular
Heck reaction.
Scheme 3. The expected relationship of the quaternary-carbon stereocenter and double bond of the oxindole products formed from the Heck cyclization of
alkenyl iodide 9.

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S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
occurs in similar fashion to that observed in this model
series, the Heck cyclization substrate would need to be
assembled from (R)-serine as depicted in Scheme 1.
With vinyl stannane 18 in hand, we turned to the synthesis of
7-iodotryptophan derivative 23 (Scheme 5). Although func-
tionalizing the 7-position of an indole is a difficult task, the
7-position of N-(tert-butoxycarbonyl)indolines is readily
functionalized by ortho lithiation.24 With such a conversion
in mind, (S)-tryptophan methyl ester hydrochloride was
reduced with Et3SiH in trifluoroacetic acid25 and both nitro-
gens were masked with Boc groups to yield indoline 20 as an
inconsequential mixture of epimers. Reduction of this inter-
mediate with NaBH4, followed by reaction of the product
with 2,2-dimethoxypropane (2,2-DMP) and catalytic
p-toluenesulfonic acid gave indoline 21 in high yield.
Upon reaction of this intermediate with 1.4 equiv of s-
BuLi at ꢁ78 ꢀC in the presence of TMEDA, followed by in-
verse quenching of the resulting aryllithium reagent into
a 0 ꢀC ether solution of diiodoethane, the 7-iodo derivative
22 was formed in 73% yield.24 The use of less s-BuLi (1.0
or 1.2 equiv) in the initial lithiation step, for up to 2 h at
ꢁ78 ꢀC, did not result in complete lithiation as confirmed
by deuterium quenching experiments.
2.2. Initial studies in the (S)-serine series
Contemporaneous with the model study described in the pre-
vious section, we initiated the synthesis of a more elaborate
Heck cyclization substrate having a suitably protected
(2-amino-2-carboxyethyl)-1H-indol-7-yl substituent at the
a-carbon of the a,b-unsaturated anilide Heck cyclization
precursor 6 (Scheme 1). Because it was not yet clear at the
time what the absolute configuration of the g stereocenter
would need to be, we chose to use less expensive (S)-serine
in these studies. On the basis of our earlier success in intro-
ducing bulky substituents at the hindered peri carbon of
indoline fragments,10b–d we pursued a strategy in which
a Stille cross-coupling would be a key step.19 The requisite
stannane 18 was assembled from the S enantiomer 7 of
Garner’s aldehyde (Scheme 4).14 Initially, we examined
the elaboration of aldehyde 7 to alkynoate 17 according to
a literature procedure.20 In our hands, this sequence was
not satisfactory as both the formation of the 1,1-dibromo-
alkene intermediate and its conversion to alkynoate 17 were
irreproducible, with enyne 19 being formed as a byproduct.
Following another literature precedent,21 aldehyde 7 was
allowed to react with diazoketophosphonate 15,22 giving
alkyne 16 in 86% yield. The competitive formation of enyne
19 was avoided when the lithium acetylide derivative of
alkyne 16 was quenched in an inverse fashion with methyl
chloroformate, providing alkynoate 17 in 84% yield. Hydro-
stannylation23 of this intermediate was best carried out using
3 mol % of Pd(PPh3)4 and 2 equiv of Bu3SnH in CH2Cl2 at
ꢁ10 ꢀC. These conditions delivered vinyl stannane 18,
a 14:1 mixture of regioisomers, in nearly quantitative yield.
The use of an excess of Bu3SnH was necessitated by the
palladium-catalyzed decomposition of this reagent to give
hexabutylditin and hydrogen gas. In an attempt to avoid
the use of an excess of the tin reagent, alternate catalysts
such as Pd2(dba)3$CHCl3, Pd(PPh3)2Cl2, Pd(dppf)Cl2, and
Rh(PPh3)2Cl were screened; however, all gave inferior
results.
Scheme 5. Preparation of iodotryptophan derivative 23.
Dehydrogenation of iodoindoline 22 to indole analog 23
turned out to be more challenging than anticipated. Reaction
of 22 with DDQ at 70 ꢀC in toluene successfully oxidized the
indoline ring, but also led to partial cleavage of the Boc and
isopropylidene protecting groups. Inclusion of K2CO3 to
buffer the hydroquinone byproduct eliminated these side re-
actions; however, significant amounts of the over-oxidation
product 24 were produced. Switching to quinone reagents
having lower oxidation potentials, such as o- or p-chloranil,
did not improve this reaction. The best solution we found
was to use DDQ and stop the reaction at approximately
50% conversion. Thus, heating a mixture of indoline 22,
DDQ, and K2CO3 in toluene at 70 ꢀC for 12 h provided
a readily separable mixture of iodotryptophan derivative
23 and the starting indoline 22. Iodotryptophan derivative
23 was isolated in 47% yield (91% yield based on consumed
Scheme 4. Preparation of vinyl stannane 18.

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8503
starting material) together with 48% yield of iodoindoline
22, which could be recycled.
To favor productive conformations for the Heck cyclization,
the anilide nitrogen atom needed to be protected. Attempts
to selectively protect this nitrogen resulted in partial func-
tionalization of the indole nitrogen as well. As a result,
Boc and SEM protecting groups were placed on both free
nitrogens to give rise to Heck cyclization precursors 28
and 29. Encouraged by the results in the model series, the
first Heck cyclization was attempted with Boc precursor
28. Unfortunately, no reaction conditions were found to
effect cyclization of this intermediate. For example, heating
of anilide 28 with 40 mol % of Pd(PPh3)4 and excess PMP in
DMA at 80 ꢀC for 72 h resulted only in the formation of
palladium containing adducts.29
The final steps in the construction of Heck cyclization pre-
cursors 28 and 29 are summarized in Scheme 6. Stille
cross-coupling of stannane 18 and iodide 23 to form a,b-
unsaturated ester 25 took place in excellent yield using
trifurylphosphine as the ligand26 and CuI as an additive.27
However, all attempts to elaborate this intermediate to
2-iodoanilide congeners failed. For example, both direct
Weinreb aminolysis of ester 25 with 2-iodoaniline16 and
coupling of the acid derived from 25 with this aniline using
Mukaiyama’s salt,28 procedures we have previously used to
construct related Heck cyclization precursors,18 were unsuc-
cessful. Reasoning that the steric bulk of the Boc substituent
of the indole was the cause, this group was selectively re-
moved by thermolysis of tri-Boc precursor 25 in DMSO at
130 ꢀC. After chromatographic removal of the minor Stille
product derived from the small contaminate of the b-stan-
nane in coupling partner 18, isomerically pure enoate 26
was isolated in 82% yield. Using a slight modification of
Weinreb’s procedure16 in which we added dry K2CO3 to
minimize the cleavage of acid-labile protecting groups, the
condensation of 2-iodoaniline with enoate 26 delivered ani-
lide 27 in 93% yield. NMR analysis showed that this product
was a single alkene stereoisomer, whose Z configuration was
confirmed by 1H NMR NOE experiments.
Much more encouraging were the results obtained with the
SEM substrate 29. A survey of cyclization conditions indi-
cated that Pd0 catalysts containing trifurylphosphine ligands
gave the best results; salient experimental results are
summarized in Table 1. Under the optimum conditions
identified, heating unsaturated iodoanilide 29, 10 mol % of
Pd2(dba)3$CHCl3, 1 equiv of (2-furyl)3P, and excess PMP
in DMA at 90 ꢀC for 8 h led to the formation of a single de-
tectable oxindole product 30, which was isolated in 78%
yield. The structure of this product was readily secured after
the SEM groups were removed from the oxindole and indole
fragments.30 Particularly diagnostic was the strong NOESY
correlation observed between H5 and the C13 methylene
Scheme 6. Heck cyclization and elaboration of product 30 to cis-pyrrolidinoindoline 35.

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S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
Table 1. Heck cyclization of SEM substrate 29
catalysts (Rh/alumina, RhCl(PPh3)3, and Crabtree’s cata-
lyst) and solvents (EtOH, EtOAc, and PhH) were screened.
In all cases, results were inferior to those realized with Pd/
C. To facilitate reduction of the oxindole carbonyl group,
the oxindole nitrogens of hydrogenation products 32 were
selectively protected with Boc groups. At this point, the
C11 epimers of Boc products 33 and 34 could be separated;
however, their relative configuration at C11 could not yet
be determined.
Entry
1
Conditions
Products
19 mol % Pd(PPh3)4, PMP,
DMA, 80 ꢀC, 24 h
35 mol % Pd(PPh3)4, PMP,
DMA, 80 ꢀC, 24 h
16 mol % Pd(PPh3)4, PMP,
MeCN, 80 ꢀC, 18 h
21 mol % Pd(PPh3)4, PMP,
PhMe, 100 ꢀC, 24 h
19 mol % Pd(PPh3)4, K2CO3,
DMA, 80 ꢀC, 18 h
33 mol % Pd(dppf)Cl2, PMP,
DMA, 70–90 ꢀC, 2 d
18 mol % Pd2(dba)3$CHCl3,
Ph3As (144 mol %), PMP, DMA,
100 ꢀC, 2 h
10 mol % Pd2(dba)3$CHCl3,
Ph3P (40 mol %), PMP, DMA,
100 ꢀC, 22 h
27 mol % Pd(OAc)2, Ph3P
(54 mol %), Ag3PO4, DMA,
90 ꢀC, 24 h
25 mol % Pd(OAc)2, Ph3P
(50 mol %), PMP, DMA,
90 ꢀC, 24 h
7.5 mol % Pd2(dba)3$CHCl3,
(2-fur)3P (90 mol %), PMP,
DMA, 80 ꢀC, 30 h
10 mol % Pd2(dba)3$CHCl3,
(2-fur)3P (100 mol %), PMP,
DMA, 90 ꢀC, 8 h
29:30 (1:1)
2
3
4
5
6
7
30 (52%)
Trace reduction
producta
29 and reduction
producta
Reduction
producta only
30 (40%)b
To assign the relative configuration of epimers 33 and 34, as
well as work out chemistry for forming the cis-pyrrolidi-
noindoline moiety, these products were elaborated in an
identical fashion: the oxindole carbonyl group was selec-
tively reduced with NaBH4 at room temperature in ethanol
and then a slight excess of camphorsulfonic acid (CSA)
was added to the crude hemiaminals to promote dehydrative
cyclization. The minor hydrogenation product 34 gave cis-
pyrrolidinoindoline 37 in 37% yield, whereas, the major
diastereomer 33 provided a mixture of the corresponding
cis-pyrrolidinoindoline 35 (17% yield) and 2,3,4,4a,9,9a-
hexahydropyrano[2,3-b]indole 36 (33% yield). In DMSO-
No reaction
30 (56%)
30 (39%)
30 (56%)
30 (67%)
30 (78%)
8
9
10
11
12
1
d6 at 100 ꢀC, the H NMR spectrum of 35 displayed two
NH signals and two OH signals, whereas the spectrum of
hydropyranoindoline 36 showed the presence of three NH
signals and only one OH signal. The relative configuration
at C11 of the epimeric pyrrolidinoindoline products could
now be determined by 1H NMR NOE experiments. The cor-
relation of H5 to the C12 methylene hydrogen that exhibited
a weak NOE to the C11 methine hydrogen was particularly
diagnostic in defining the relative configuration of indolyl
cis-pyrrolidinoindoline 35 (Fig. 2).
a
Deiodo-29.
A 2:1 mixture of isomers.
b
group, which is consistent with 31 being either the (E)-alkyl-
idene isomer or the endocyclic isomer resulting from double
bond migration. The NMR chemical shift of the C12 vinylic
hydrogen of 31 (d 6.33) was more similar to that of the (E)-
alkylidene Heck product 11 (d 6.05) than the chemical shift
of the vinylic hydrogen (C13) of endocyclic isomer 12 (d
5.70), allowing the double bond position of Heck product
31 to be specified.31 Thus, the absolute configuration at the
quaternary-carbon stereocenter C3 of the Heck product 31
is S, opposite to what was the proposed configuration of
this stereocenter of asperazine (1).1
2.3. Total synthesis of (D)-asperazine
The total synthesis of asperazine commenced with the R
enantiomer of Garner’s aldehyde (ent-7) (Scheme 7).14
The seven reactions needed to transform this precursor
into the Heck cyclization substrate 41 were carried out in
the same manner, and with comparable yields, as those uti-
lized to prepare diastereomer 29. The critical intramolecular
Heck reaction of a,b-unsaturated iodoanilide 41 proceeded
in 66% yield at 90 ꢀC in DMA containing 6 equiv of PMP
using the catalyst generated from Pd2(dba)3$CHCl3
(10 mol %) and trifurylphosphine (1 equiv). As in the Heck
cyclization of diastereomer 29, only a single Heck product
42, having the E configuration of the trisubstituted double
bond, was isolated.
Although we now realized that a Heck cyclization precursor
analogous to 29 in which the C11 stereocenter has the S ab-
solute configuration would be required for a total synthesis
of the proposed structure of asperazine (1), we had suffi-
cient supplies of hexacyclic intermediate 31 on hand to
allow us to explore additional steps in the synthesis in
this epimeric series.32 With the Heck reaction realized,
we turned to generate the C11 stereocenter and form the
pyrrolidine ring (Scheme 6). To no surprise, we found
that hydrogenation of the hindered trisubstituted double
bond of Heck product 30 was extremely difficult. For exam-
ple, under forcing conditions (1200 psi H2, 2 equiv of 10%
Pd/C, EtOAc, 23 ꢀC, 56 h) only 30% of the double bond of
30 was reduced.33 Fortunately, analog 31, whose double
bond is more accessible by virtue of lacking a protecting
group on the indole nitrogen, underwent hydrogenation
under less forcing conditions (10% Pd/C, DMF, 800 psi
H2, 23 ꢀC) to give dihydro product 32, an inseparable 4:1
mixture of C11 epimers, in 76% yield. In an attempt to
increase the diastereoselectivity of this reaction, alternate
Figure 2. 1H NMR NOE data for intermediate 35.

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8505
Scheme 7. Diastereoselective Heck cyclization to form oxindole 42.
Our thoughts on the origin of diastereoselection in the Heck
cyclization forming oxindole 42 are the following. Because
of the high degree of steric congestion about the trisubsti-
tuted double bond of precursor 41, we believe that the
Boc-protected oxazoline fragment is likely oriented to place
the smallest substituent at C11, hydrogen, toward palladium
(Fig. 3).34,35 When the aryl palladium fragment approaches
the double bond from the favored direction depicted in
Figure 3, the carbonyl oxygen of the anilide and the N-Boc
substituent at C11 project away from each other. Approach
from the alternate alkene stereoface is disfavored because
it places these groups in close proximity.
SEM groups from 42, catalytic hydrogenation of alkenyl ox-
indole 43 at 1000 psi over Pd/C in DMF at room temperature
provided dihydro product 44 in 95% yield as a 4:1 mixture of
C11 epimers. To avoid the problem encountered earlier of
competitive cyclization of the C12 hydroxyl group to form
a 2,3,4,4a,9,9a-hexahydropyrano[2,3-b]indole product (see
Scheme 6), we chose to mask the primary hydroxyl groups
prior to attempting to form the cis-pyrrolidinoindoline ring
system. Toward this end, intermediate 44 was exposed to
1 M HCl in dioxane at 50 ꢀC to cleave the oxazolidine and
Boc substituents. Without purification, the resulting diamine
diol was allowed to react with a large excess of di-tert-butyl-
dicarbonate and NaOH under phase transfer conditions.36
The C11 epimers were separated on silica gel to provide epi-
mers 45 and 46 in 17% and 59% yields, respectively. Reduc-
tion of major product 46 with lithium triethylborohydride at
ꢁ78/0 ꢀC afforded a mixture of epimeric hemiaminals,
which cyclized upon addition of ethereal HCl.37 Selective
cleavage of the two carbonates with methanolic KOH at
room temperature delivered cis-pyrrolidinoindoline diol 47
in 79% yield from precursor 46.
The elaboration of Heck product 42 to indolyl cis-pyrrolidi-
noindoline 47 is outlined in Scheme 8. After removing the
We turned to the introduction of the two remaining diketopi-
perazine units present in asperazine (Scheme 9). The first
issue was oxidation of the primary alcohol groups of diol
47 to give diacid 48, a conversion that was made challenging
by the presence of the unmasked indole substituent. Many
one- and two-step oxidation procedures (e.g., PDC, Jones’
reagent, ruthenium(VIII) oxidants, TEMPO, KMnO4, AgO,
and Pt/O2) were surveyed. In the end, only one sequence
was found to be efficient: oxidation of diol 47 to the corre-
sponding dialdehyde with DMSO and sulfur trioxide$
pyridine complex,38 followed by sodium chlorite oxidation
of the crude dialdehyde to give diacid 48.39 Coupling of
this intermediate with (R)-phenylalanine methyl ester hydro-
chloride mediated by HATU40 provided dipeptide 49 in 65%
overall yield from diol 47.
All that remained to complete the total synthesis of asper-
azine (1) was to cleave the three Boc-protecting groups of
Figure 3. Model of the insertion step in the intramolecular Heck reaction to
form oxindole 42; most hydrogens are removed for clarity.

8506
S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
Scheme 8. Elaboration of Heck product 42 to cis-pyrrolidinoindoline 47.
intermediate 49 and cyclize, with loss of methanol, to form
the two diketopiperazine rings. To our delight, this series of
five reactions could be accomplished in a single step41 by
heating 49 at 200 ꢀC for 4 h under a stream of argon. This
reaction provided asperazine (1) and two presumed stereo-
isomers42 in a 70:3:2 ratio. Purification of this mixture by
preparative reverse-phase HPLC provided pure asperazine
(1) in 34% yield. However, it was more efficient to carry
out this conversion in two steps: cleavage of the three tert-
butoxycarbonyl protecting groups of dipeptide 49 with
formic acid at room temperature,43,44 followed by heating
the crude deprotected dipeptide in n-butanol at 120 ꢀC for
24 h in the presence of acetic acid.45 After preparative
thin-layer chromatography, asperazine (1), [a]D +95.7 (c
0.2, MeOH), was isolated in 59% yield from this sequence.
product were co-eluted from a C18 reverse-phase HPLC col-
umn (31% MeCN/69% H2O, v/v). The optical rotation of
synthetic asperazine, [a]D +95.7 (c 0.2, MeOH), was higher
than that reported for the natural material, [a]D +52 (c 0.2,
MeOH);1 however, CD spectra were nearly identical (see
Supplementary data).
We considered the possibility that a diastereoisomer of as-
perazine in which C34 and C37 (or C2, C3, C11, and C15)
were epimeric to 1 would not be distinguishable from 1 by
NMR comparisons. However, such a diastereomer should
be discernible by CD analysis. Most revealingly, subtracting
the CD spectra of cyclo-(S)-Trp-(R)-Phe from that of 1 and
adding back that of cyclo-(R)-Trp-(S)-Phe generates a pre-
dicted CD spectrum for the C34, C37 epimer of asperazine
(1) that is readily distinguished from the CD spectrum of 1.
1
Synthetic asperazine showed H NMR spectra in three sol-
vents, 13C NMR spectra in two solvents, and mass spectral
data that compared favorably to those of the natural isolate.1
Moreover, synthetic asperazine and a sample of the natural
Synthetic asperazine was tested by Frederick Valeriote
against L1210 leukemia, Colon38, H116 colon cancer, and
Scheme 9. Final steps in the total synthesis of (+)-asperazine (1).

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8507
H125 lung cancer using the Corbett–Valeriote soft agar disk
4.2. Synthesis of (4S)-4-[2-methoxycarbonyl-2-(tri-
butylstannanyl)vinyl]-2,2-dimethyloxazolidine-3-
carboxylic acid tert-butyl ester (ent-18)
diffusion assay that had been employed in the initial bio-
assays of natural asperazine.1,2 Synthetic asperazine showed
low cytotoxicity against all three cell lines.46 This surprising
result prompted us to repurify synthetic asperazine and
repeat the bioassays; again no significant cytotoxicity
against L1210 leukemia was seen. This finding suggests
that a minor component of the natural isolate is likely re-
sponsible for the observed L1210 cytotoxicity in the initial
biological evaluations of asperazine.1 Because the physical
properties of synthetic and natural asperazine compared
well, we feel it is less likely that the relative and absolute
configuration of natural asperazine differs from that depicted
in structure 1.
A modification of the general procedure of Guibe was
employed.23 A degassed solution of n-Bu3SnH (7.50 mL,
27.9 mmol) and CH2Cl2 (25 mL) was added by syringe
pump (0.5 mL/h) to a degassed solution of ent-1720,52
(4.00 g, 14.1 mmol), (Ph3P)4Pd (500 mg, 0.433 mmol),
and CH2Cl2 (100 mL) at ꢁ10 ꢀC. The solution was concen-
trated for 18 h after the addition was complete. The crude
residue was purified by silica gel chromatography (49:1,
33:1, 24:1, 19:1, 13:1, 9:1 hexanes/EtOAc) to give 7.15 g
(88%) of ent-18, a clear oil, as a 16:1 mixture of re-
1
gioisomers in favor of a-stannane: H NMR (500 MHz,
C6D6, 70 ꢀC) d 6.42 (td, J¼30.4, 7.7 Hz, 1H), 5.24 (br,
1H), 4.13 (dd, J¼9.0, 6.8 Hz, 1H), 3.82 (dd, J¼9.0,
3.1 Hz, 1H), 3.43 (s, 3H), 1.73 (s, 3H), 1.62–1.56 (m, 6H),
1.54 (s, 3H), 1.43 (s, 9H), 1.40–1.31 (m, 6H), 1.06–1.03
(m, 6H), 0.91 (t, J¼7.3 Hz, 9H); 13C NMR (125 MHz,
C6D6, 70 ꢀC) d 170.1, 156.7, 152.1, 135.8, 94.5, 79.5,
69.7, 58.7, 50.9, 29.3, 28.5, 27.5, 27.3, 24.4, 13.7, 10.9; IR
3. Conclusion
The first total synthesis of asperazine (1) was accomplished
in 22 steps from readily available amino acid starting mate-
rials. A pivotal step in the sequence is a diastereoselective
intramolecular Heck reaction (41/42) in which the substit-
uent controlling stereoselection is external to the ring being
formed. The efficiency of this step provides an excellent ex-
ample of the notable utility of intramolecular Heck reactions
for forming highly congested quaternary-carbon stereocen-
ters. This synthesis confirmed the structure of (+)-asperazine
(1) proposed by Crews and co-workers and provided
material for additional biological studies. The in vitro cyto-
toxicity originally reported for the marine isolate was not
confirmed by the repetition of these studies with synthetic
(+)-asperazine.
(neat) 2957, 2929, 2872, 1704, 1604, 1382, 1199 cmꢁ1
;
MS (CI) m/z 518.1932 (518.1933 calcd for C22H40NO5Sn,
Mꢁt-Bu). Anal. Calcd for C26H49NO5Sn: C, 54.37; H,
8.60; N, 2.44. Found: C, 54.60; H, 8.73; N, 2.51. Diagnostic
data for the b-regioisomer of stannane ent-18: a second vi-
nylic hydrogen (d 6.21) was observed as a triplet of doublets
with coupling constants corresponding to an allylic 1H
coupling (3J(1H)¼1.5 Hz) and an allylic 117Sn coupling
(3J(117Sn)¼30.8 Hz).
4.3. Preparation of iodoindole 23
4. Experimental
4.1. General
4.3.1. 3-[(2S)-2-tert-Butoxycarbonylamino-2-methoxy-
carbonylethyl]-2,3-dihydroindole-1-carboxylic acid tert-
butyl ester (20). A modification of the general procedure
of McKenzie was employed.25a Triethylsilane (120 mL,
751 mmol) was added by syringe pump (2.5 mL/min) to a
solution of (S)-TrpOMe$HCl (22 g, 86 mmol) and TFA
(260 mL) at rt. After stirring the mixture at 50 ꢀC for
2 h, the volatiles were removed by distillation (65 ꢀC,
20 mm). The crude residue was dissolved in saturated
aqueous NaHCO3 (200 mL) and extracted with CH2Cl2
(200 mLꢂ6). The combined organic extracts were dried
(MgSO4), filtered, and concentrated to give 22 g of the
known indoline.25b A solution of Boc2O (42.0 mL,
183 mmol) and dioxane (110 mL) was added slowly
(5 mL/min) to a solution of the crude indoline, Na2CO3
(28.0 g, 264 mmol), and H2O (110 mL) at rt. After stirring
for 1.5 h at rt, the cloudy mixture was concentrated, and
the residue was partitioned between EtOAc (200 mL) and
saturated aqueous NaHCO3 (400 mL). The aqueous layer
was extracted with EtOAc (300 mLꢂ3), and the combined
organic extracts were dried (MgSO4), filtered, and concen-
trated. The crude residue was purified by silica gel chroma-
tography (6:1, 3:1, 2:1 hexanes/EtOAc) to give 33.4 g (92%
over two steps) of 20, a clear oil, as a 3:2 mixture of dia-
Air-sensitive reactions were carried out under an atmosphere
of argon. Concentrations were performed under reduced
pressure (ca. 15 mm) with a rotary evaporator, unless other-
wise stated. Tetrahydrofuran (THF), diethyl ether, and di-
chloromethane were degassed with argon and then passed
through two 4ꢂ36 inch columns of anhydrous neutral A-2
alumina (8ꢂ14 mesh; LaRoche Chemicals; activated under
a flow of argon at 350 ꢀC for 3 h) to remove H2O.47 Toluene
was degassed with argon and then passed through one 4ꢂ36-
inch column of Q-5 reactant (Engelhard; activated under
a flow of 5% hydrogen/nitrogen at 250 ꢀC for 3 h) to remove
oxygen then through one 4ꢂ36-inch column of anhydrous
neutral alumina to remove H2O.47 Methanol was distilled
from Mg turnings at atmospheric pressure under a N2 atmo-
sphere. Triethylamine, pyridine, diisopropylethylamine,
diisopropylamine, acetonitrile, benzene (PhH), and methyl
chloroformate were distilled from CaH2 at atmospheric pres-
sure under a N2 atmosphere. N,N-Dimethylacetamide
(DMA), N,N,N0,N0-tetramethylethylenediamine (TMEDA),
and 1,2,2,6,6-pentamethylpiperidine (PMP) were distilled
from CaH2 under reduced pressure (ca. 10 mm).
Pd2(dba)3$CHCl3,48 Pd(PPh3)4,49 and (R)-BINAP50 were
prepared according to established procedures. Molarities
of organolithium reagents were established by titration
with menthol/fluorene.51
1
stereomers: H NMR (500 MHz, DMSO, 100 ꢀC) d 7.62
(t, J¼7.5 Hz, 1H), 7.20 (d, J¼7.4 Hz, 1H), 7.15 (app q,
J¼6.6 Hz, 1H), 6.94 (t, J¼7.4 Hz, 1H), 6.88 (br, 1H, NH),
4.23–4.18 (m, 1H, minor isomer), 4.16–4.11 (m, 1H, major
isomer), 4.06–4.01 (m, 1H), 3.664 (s, 3H), 3.658 (s, 3H,

8508
S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
major isomer), 3.64–3.60 (m, 1H), 3.43–3.35 (m, 1H), 2.15–
2.10 (m, 1H), 1.92–1.84 (m, 1H), 1.531 (s, 9H, major
isomer), 1.529 (s, 9H, minor isomer), 1.43 (s, 9H, minor iso-
mer), 1.42 (s, 9H, major isomer); 13C NMR (125 MHz,
DMSO, 100 ꢀC) d 171.9 (both), 154.7 (both), 151.2 (both),
141.6 (major), 141.5 (minor), 133.8 (major), 133.7 (minor),
127.0 (minor), 126.9 (major), 123.5 (minor), 123.4 (major),
121.5 (both), 113.7 (both), 79.8 (both), 78.0 (both), 53.3
(minor), 52.7 (major), 52.0 (minor), 51.6 (major), 51.0
(both), 36.3 (minor), 36.2 (major), 35.8 (minor), 35.3
(major), 27.6 (bothꢂ2); IR (neat) 3352, 2977, 2932, 1746,
1704, 1602, 1487, 1393, 1171 cmꢁ1; MS (ESI) m/z
443.2162 (443.2158 calcd for C22H32N2NaO6, M+Na).
Na2S2O3 (100 mL) and saturated aqueous NaHCO3
(100 mL). This mixture was extracted with MTBE (200
mLꢂ3), and the combined organic extracts were dried
(MgSO4), filtered, and concentrated. The crude residue
was purified by silica gel chromatography (13:1, 9:1, 6:1,
4:1 hexanes/EtOAc) to give 1.58 g (73%) of 22 as a colorless
foam. Diagnostic data: 1H NMR (400 MHz, CDCl3) d 7.63
(br, 1H), 7.20–7.08 (br, 1H), 6.76 (br, 1H), 4.40–3.50 (br
m, 5H), 3.35–3.05 (br m, 1H), 2.10–1.70 (br m, 2H), 1.70–
1.30 (br m, 24H); IR (neat) 2978, 2935, 1698, 1386,
1166 cmꢁ1; MS (ESI) m/z 581.15 (581.15 calcd for
C24H35IN2NaO5, M+Na).
A mixture of 22 (4.95 g, 8.86 mmol), DDQ (10.0 g,
44.1 mmol), dry K2CO3 (12.5 g, 90.4 mmol), and PhMe
(90 mL) was stirred at 70 ꢀC for 12 h, allowed to cool to
rt, and concentrated. The crude residue was partitioned be-
tween saturated aqueous NaHCO3 (200 mL) and CH2Cl2
(200 mL). The aqueous extract was washed with CH2Cl2
(200 mLꢂ4). The combined organic extracts were dried
(MgSO4), filtered, and concentrated. The crude residue
was purified by silica gel chromatography (13:1, 9:1, 6:1,
4:1 hexanes/EtOAc) to give 2.36 g (48%) of recovered 22
and 2.34 g (47%) of 23 as a colorless foam: [a]2D4 ꢁ38.6,
4.3.2. 3-[(4S)-3-tert-Butoxycarbonyl-2,2-dimethyloxazo-
lidin-4-ylmethyl]-2,3-dihydroindole-1-carboxylic acid
tert-butyl ester (21). Sodium borohydride (490 mg,
13.0 mmol) was added to a stirring mixture of 20 (2.16 g,
5.14 mmol), LiCl (550 mg, 13.0 mmol), and THF (22 mL)
at rt. After 5 min, EtOH (30 mL) was added. The cloudy
mixture was stirred at rt for 5 h and quenched at 0 ꢀC by
careful addition of saturated aqueous NH4Cl (25 mL) and
H2O (75 mL). This mixture was extracted with EtOAc
(100 mLꢂ3), and the combined organic extracts were dried
(MgSO4), filtered, and concentrated to give the correspond-
ing amino alcohol. Diagnostic data: MS (ESI) m/z 415.30
(415.22 calcd for C21H32N2NaO5, M+Na). p-Toluenesul-
fonic acid monohydrate (50 mg, 0.26 mmol) was added to
a solution of this crude product, 2,2-dimethoxypropane
(6.00 mL, 48.8 mmol), and PhH (25 mL) at rt. After 3 h at
rt, the solution was concentrated and partitioned between
MTBE (methyl tert-butyl ether, 100 mL) and saturated
aqueous NaHCO3 (100 mL). The aqueous layer was ex-
tracted with MTBE (100 mLꢂ3), and the combined organic
extracts were dried (MgSO4), filtered, and concentrated. The
crude residue was purified by silica gel chromatography
(13:1, 9:1, 6:1, 4:1 hexanes/EtOAc) to give 2.11 g (95%
over two steps) of 21 as a colorless foam: 1H NMR
(500 MHz, CDCl3) d 7.90–7.30 (br, 1H), 7.23–7.10 (m,
2H), 7.00–6.90 (m, 1H), 4.25–3.55 (br m, 5H), 3.40–3.15
(br m, 1H), 2.25–1.70 (br m, 2H), 1.65–1.35 (br m, 24H);
13C NMR (125 MHz, CDCl3)53 d 152.5, 152.4, 152.2,
151.7, 151.3, 142.5, 134.2, 127.9, 127.3, 124.3, 123.8,
122.3, 122.1, 114.7, 93.9, 93.8, 93.4, 80.5, 80.1, 80.0,
79.9, 67.7, 67.4, 66.5, 56.0, 55.6, 55.2, 54.1, 53.7, 53.2,
39.9, 39.0, 37.1, 36.7, 28.6, 28.4, 27.9, 27.7, 27.0, 26.9,
24.4, 23.1; IR (neat) 2978, 2934, 1698, 1602, 1487,
1392 cmꢁ1; MS (ESI) m/z 455.2531 (455.2522 calcd for
C24H36N2NaO5, M+Na). Anal. Calcd for C24H36N2O5: C,
66.64; H, 8.39; N, 6.48. Found: C, 66.77; H, 8.34; N, 6.48.
24
577
24
546
24
435
24
405
[a]
ꢁ40.2, [a]
ꢁ46.3, [a]
ꢁ86.2, [a]
ꢁ108 (c
1
0.71, CHCl3); H NMR (500 MHz, C6D6, major rotamer)
d 8.11 (d, J¼7.6 Hz, 1H), 7.78 (d, J¼7.5 Hz, 1H), 7.27 (s,
1H), 6.70 (t, J¼7.7 Hz, 1H), 4.16 (br, 1H), 3.58–3.50 (m,
1H), 3.43–3.32 (br m, 2H), 2.68 (br t, J¼12.2 Hz, 1H),
1.60 (s, 3H), 1.44 (s, 9H), 1.42 (s, 9H), 1.36 (s, 3H); 13C
NMR (125 MHz, C6D6, major rotamer) d 152.2, 148.3,
138.6, 134.3, 128.5, 127.9, 126.9, 124.8, 120.2, 117.5,
93.5, 83.6, 79.7, 66.2, 57.7, 28.7, 28.1, 27.9 (2), 24.5; IR
(neat) 2979, 2934, 2876, 1749, 1694, 1391, 1238,
1155 cmꢁ1; MS (CI) m/z 556.1434 (556.1434 calcd for
C24H33IN2O5, M).
4.4. Stille coupling to prepare 38 and deprotection to
form enoate 39
A suspension of Pd2(dba)3$CHCl3 (145 mg, 0.280 mmol
Pd0), (2-furyl)3P (265 mg, 1.14 mmol), and dry NMP
(8 mL) in a base-washed, flame-dried 100 mL Schlenk flask
was stirred for 2 h to furnish a yellow-green homogenous so-
lution. A solution of stannane ent-18 (1.70 g, 2.96 mmol),
iodide 23 (1.04 g, 1.87 mmol), and NMP (12 mL) was
added, and the reaction was degassed by the freeze–pump–
thaw method at ꢁ78 ꢀC three times. Copper(I) iodide
(620 mg, 3.26 mmol) was added, the mixture was stirred
for 19 h at rt, and the reaction was quenched by slow addi-
tion of saturated aqueous KF (10 mL). After stirring for
15 min, the mixture was diluted with MTBE (400 mL) and
washed with water (300 mL) containing concentrated am-
monia solution (15 mL). The aqueous layer was extracted
with MTBE (200 mL). The combined organic extracts
were dried (MgSO4), filtered, concentrated, and the residue
was purified by silica gel chromatography (13:1, 9:1, 6:1, 4:1
hexanes/EtOAc) to give 1.19 g of 38 as a colorless foam.
Diagnostic data: MS (ESI) m/z 736.43 (736.38 calcd for
C38H55N3NaO10, M+Na).
4.3.3. 3-[(4S)-3-tert-Butoxycarbonyl-2,2-dimethyloxazo-
lidin-4-ylmethyl]-7-iodoindole-1-carboxylic acid tert-bu-
tyl ester (23). A modification of the general procedure of
Iwao and Kuraishi was employed.24 A cyclohexane solution
of s-BuLi (5.00 mL, 1.09 M, 5.45 mmol) was added slowly
(internal temperature ꢃꢁ70 ꢀC) to a solution of 21 (1.68 g,
3.88 mmol), TMEDA (1.00 mL, 6.63 mmol), and Et2O
(38 mL). This solution was maintained at ꢁ78 ꢀC for
20 min and then cannulated into a solution of 1,2-diiodo-
ethane (5.50 g, 19.5 mmol) and Et2O (38 mL) at 0 ꢀC. The
reaction was allowed to warm to rt, maintained at rt for
1 h, and poured into a solution of saturated aqueous
A solution of 38 (1.19 g) and DMSO (25 mL) was degassed
(bubble argon through the flask for 30 min, evacuate at 5 mm

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8509
for 15 min, and bubble argon through the flask for 30 min)
and then heated at 130 ꢀC for 6.5 h. After cooling to rt, the
reaction was poured into water (200 mL) and extracted
with Et2O (50 mLꢂ3). The combined organic extracts were
dried (MgSO4), filtered, concentrated, and the residue was
purified by silica gel chromatography (8:1, 6:1, 5:1 hexanes/
EtOAc) to give 939 mg (82%) of 39 as a colorless foam:
C38H49IN4NaO7, M+Na). Anal. Calcd for C38H49IN4O7:
C, 57.00; H, 6.17; N, 7.00. Found: C, 56.69; H, 6.21; N, 6.83.
A solution of amide 40 (4.53 g, 5.65 mmol) and DMF
(35 mL) was added slowly to a stirring mixture of NaH
(1.61 g, 60%, 40.3 mmol) and DMF (40 mL) at 0 ꢀC. After
10 min, trimethylsilylethoxymethyl chloride (3.50 mL,
19.8 mmol) was added slowly. After an additional 20 min
at 0 ꢀC, the reaction was poured into saturated aqueous
LiCl (300 mL) and extracted with MTBE (100 mLꢂ3).
The combined organic extracts were dried (MgSO4), filtered,
and concentrated. The crude residue was purified by silica
gel chromatography (19:1, 13:1, 9:1, 6:1, 4:1, 3:1 hexanes/
EtOAc)54 to give 3.98 g (66%) of 41 as a colorless foam.
In addition, unreacted starting material and mono-SEM-
protected product [MS (ESI) m/z 953.48 (953.34 calcd for
C44H63IN4NaO8Si, M+Na)] were recovered. Resubjection
of these materials to the reaction conditions gave 41 in an
24
24
24
[a]2D4 ꢁ41.7, [a]
ꢁ43.9, [a]
ꢁ52.6, [a]
ꢁ126,
577
546
435
[a] ꢁ166 (c 0.79, CHCl3); 1H NMR (500 MHz, DMSO,
24
405
80 ꢀC) d 10.23 (s, 1H, NH), 7.61 (br d, J¼7.7 Hz, 1H),
7.13 (d, J¼2.3 Hz, 1H), 7.01 (t, J¼7.6 Hz, 1H), 6.92 (dd,
J¼7.2, 0.8 Hz, 1H), 6.28 (d, J¼8.0 Hz, 1H), 5.16 (app td,
J¼7.4, 4.3 Hz, 1H), 4.29 (dd, J¼9.0, 7.0 Hz, 1H), 4.14–
4.10 (m, 1H), 3.98 (br, 1H), 3.80 (dd, J¼7.9, 5.8 Hz, 1H),
3.74 (dd, J¼8.8, 1.6 Hz, 1H), 3.68 (s, 3H), 3.15 (dd,
J¼13.8, 2.8 Hz, 1H), 2.81 (dd, J¼13.8, 10.2 Hz, 1H), 1.53
(s, 6H), 1.50 (s, 3H), 1.48 (s, 9H), 1.44 (s, 9H), 1.43 (s,
13
3H); C NMR (125 MHz, DMSO, 80 ꢀC) d 165.8, 151.2,
26
26
151.0, 146.7, 134.0, 130.2, 127.5, 123.4, 121.7, 121.3,
118.0, 117.9, 110.9, 93.3, 92.8, 79.2, 78.7, 67.6, 65.8,
56.9, 56.1, 51.2, 28.4, 27.8, 27.7, 26.7, 25.9, 24.1, 23.4; IR
(neat) 3317, 2979, 1698, 1667, 1392, 1366 cmꢁ1; MS
(FAB) m/z 613.3363 (613.3363 calcd for C33H47N3O8, M).
Anal. Calcd for C33H47N3O8: C, 64.58; H, 7.72; N, 6.85.
Found: C, 64.44; H, 7.91; N, 6.75.
overall yield of 79%: [a]2D6 +128, [a] +135, [a] +155,
577 546
26
435
26
405
[a]
+302, [a]
+390 (c 0.78, CHCl3); 1H NMR55
(500 MHz, CDCl3) d 8.02, 7.54, 7.41, 7.28, 6.98, 6.82,
6.51, 5.89, 5.82, 5.70, 5.26, 5.03, 4.47, 4.31, 4.16, 4.01,
3.92, 3.83, 3.72, 3.34, 3.24, 2.74, 1.76–1.45, 0.96, 0.78,
0.00, ꢁ0.13; 13C NMR56 (125 MHz, CDCl3) d 169.7,
152.2, 151.9, 151.8, 143.4, 141.7, 139.1, 137.4, 134.6,
134.5, 133.43, 133.35, 130.1, 128.8, 128.0, 127.9, 126.2,
121.1, 120.8, 119.7, 119.5, 118.8, 118.2, 113.6, 98.0, 94.0,
93.7, 93.5, 80.0, 79.92, 79.88, 79.8, 76.0, 68.5, 66.65,
66.57, 66.3, 58.3, 57.1, 29.7, 28.7, 28.5, 28.4, 28.0, 27.9,
27.4, 27.1, 25.0, 24.5, 23.2, 18.0, 17.8, ꢁ1.5; IR (neat)
2978, 2953, 1694, 1644, 1392, 1249 cmꢁ1; MS (ESI) m/z
1083.46 (1083.42 calcd for C50H77IN4NaO9Si2, M+Na).
Anal. Calcd for C50H77IN4O9Si2: C, 56.59; H, 7.31; N,
5.28. Found: C, 56.73; H, 7.41; N, 5.25.
4.5. Elaboration of enoate 39 to Heck cyclization
precursor 41
A modification of the general procedure of Weinreb was
employed.16 A toluene solution of Me3Al (17.0 mL, 2 M,
34.0 mmol) was added slowly to a solution of 2-iodoaniline
(7.51 g, 34.2 mmol) and CH2Cl2 (75 mL) at 0 ꢀC. The solu-
tion was allowed to warm to rt, maintained for 1 h, and then
added slowly to a vigorously stirring mixture of ester 39
(4.16 mg, 6.77 mmol), dried K2CO3 (12.0 g, 87.0 mmol),
and CH2Cl2 (50 mL) at 0 ꢀC. The mixture was allowed to
warm to rt, stirred for 3 h, and quenched by dropwise addi-
tion of a solution of saturated aqueous Na/K tartrate
(12.5 mL) and saturated aqueous NaHCO3 (12.5 mL). After
stirring at rt for 1 h, the mixture was extracted with CH2Cl2
(50 mLꢂ3). The combined organic extracts were dried
(MgSO4), filtered, and concentrated. The crude residue
was purified by silica gel chromatography (9:1, 6:1, 4:1,
3:1 hexanes/EtOAc) to give 5.10 g (94%) of 40 as a colorless
4.6. Heck cyclization to form oxindole 42
A suspension of Pd2(dba)3$CHCl3 (195 mg, 0.377 mmol
Pd0), (2-furyl)3P (440 mg, 1.90 mmol), and dry DMA
(15 mL) in a base-washed, flame-dried 100 mL Schlenk
flask was stirred for 2 h to furnish a yellow-green homoge-
nous solution. Anilide 41 (2.00 g, 1.88 mmol), 1,2,2,6,6-
pentamethylpiperidine (2.00 mL, 11.1 mmol), and DMA
(5 mL) were added, and the reaction was degassed by the
freeze–pump–thaw method at ꢁ78 ꢀC three times. After
10 h at 90 ꢀC, the reaction was allowed to cool to rt, poured
into saturated aqueous LiCl (100 mL), diluted with water
(100 mL), and extracted with MTBE (100 mLꢂ3). The
combined organic extracts were dried (MgSO4), filtered,
concentrated, and the residue was purified by silica gel
chromatography (12:1, 9:1, 6:1, 4:1 hexanes/EtOAc) to
give 1.16 g (66%) of 42 as a colorless foam: [a]2D7 ꢁ120,
24
24
24
foam: [a]2D4 ꢁ40.0, [a] ꢁ43.4, [a] ꢁ51.2, [a] ꢁ115,
577
546
435
[a] ꢁ157 (c 0.70, CHCl3); 1H NMR (400 MHz, DMSO,
24
405
100 ꢀC) d 10.24 (s, 1H), 8.66 (s, 1H), 7.93 (d, J¼8.1 Hz,
1H), 7.77 (dd, J¼7.9, 1.3 Hz, 1H), 7.68 (d, J¼7.3 Hz, 1H),
7.37 (td, J¼7.7, 1.3 Hz, 1H), 7.18 (d, J¼2.2 Hz, 1H),
7.16–7.08 (m, 2H), 6.90 (td, J¼7.6, 1.4 Hz, 1H), 6.15 (d,
J¼8.5 Hz, 1H), 5.34–5.24 (m, 1H), 4.34 (dd, J¼9.0,
2.2 Hz, 1H), 4.18–4.11 (m, 1H), 4.07 (dd, J¼9.1, 4.3 Hz,
1H), 3.82 (dd, J¼8.8, 5.6 Hz, 1H), 3.76 (dd, J¼8.8,
1.8 Hz, 1H), 3.20 (dd, J¼13.9, 3.3 Hz, 1H), 2.84 (dd,
J¼13.9, 10.0 Hz, 1H), 1.56 (s, 3H), 1.532 (s, 3H), 1.527
(s, 3H), 1.49 (s, 9H), 1.48 (s, 9H), 1.45 (s, 3H); 13C NMR
(100 MHz, DMSO, 100 ꢀC) d 164.3, 151.4, 150.9, 138.4,
138.2, 134.0, 133.9, 128.1, 128.0, 126.2, 123.6, 123.4,
121.3, 120.8, 118.4, 118.3, 111.3, 93.2, 92.7, 91.3, 79.4,
78.6, 78.5, 67.7, 65.8, 57.0, 55.9, 28.2, 27.7, 27.6, 26.5,
25.9, 24.3, 23.4; IR (neat) 3333, 2978, 1682, 1583,
1392 cmꢁ1; MS (ESI) m/z 823.2522 (823.2544 calcd for
27
27
546
1
27
27
[a]
ꢁ127, [a]
ꢁ147, [a]
ꢁ305, [a]
ꢁ409
577
435
405
(c 0.75, CHCl3); H NMR (500 MHz, DMSO, 100 ꢀC)
d 7.62 (d, J¼7.4 Hz, 1H), 7.36 (td, J¼7.7, 1.2 Hz, 1H),
7.28 (s, 1H), 7.21 (d, J¼7.0 Hz, 1H), 7.19 (d, J¼7.7 Hz,
1H), 7.10 (td, J¼7.3, 0.9 Hz, 1H), 6.93 (t, J¼7.7 Hz, 1H),
6.76 (br d, J¼7.1 Hz, 1H), 6.41 (t, J¼1.9 Hz, 1H), 5.71
(br, 2H), 5.19 (d, J¼10.9 Hz, 1H), 5.14 (d, J¼10.9 Hz,
1H), 4.16–4.10 (m, 1H), 3.83 (dd, J¼8.2, 6.0 Hz, 1H),
3.77 (dd, J¼12.9, 1.8 Hz, 1H), 3.73 (dd, J¼8.7, 1.9 Hz,
1H), 3.72 (dd, J¼13.5, 1.8 Hz, 1H), 3.62–3.54 (m, 2H),
3.46–3.39 (br m, 1H), 3.34–3.27 (br m, 1H), 3.16 (dd,

8510
S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
J¼14.0, 3.2 Hz, 1H), 2.84 (dd, J¼14.0, 10.0 Hz, 1H), 1.51
(s, 3H), 1.50 (s, 9H), 1.45 (s, 3H), 1.44 (s, 3H), 1.43 (s,
9H), 1.41 (s, 3H), 0.92–0.75 (m, 4H), ꢁ0.04 (s, 9H),
for C38H50N4O7, M). Anal. Calcd for C38H50N4O7: C,
67.63; H, 7.47; N, 8.30. Found: C, 67.48; H, 7.51; N, 8.08.
NMR data for the major epimer (11-S): 1H NMR
(500 MHz, DMSO, 100 ꢀC) d 10.43 (s, 1H), 9.87 (s, 1H),
7.59–7.54 (m, 1H), 7.39 (d, J¼7.4 Hz, 1H), 7.32 (td,
J¼7.7, 1.2 Hz, 1H), 7.17 (d, J¼2.4 Hz, 1H), 7.12 (td,
J¼7.5, 1.0 Hz, 1H), 7.01 (d, J¼7.5 Hz, 1H), 6.97–6.91 (m,
2H), 4.15–4.08 (m, 1H), 3.83 (dd, J¼8.0, 5.9 Hz, 1H),
3.77 (dd, J¼8.7, 1.8 Hz, 1H), 3.73 (dd, J¼8.8, 1.9 Hz,
1H), 3.65–3.54 (m, 2H), 3.14 (dd, J¼14.0, 3.6 Hz, 1H),
2.93 (dd, J¼13.6, 3.2 Hz, 1H), 2.80 (dd, J¼14.1, 4.3 Hz,
1H), 2.61 (d, J¼13.1 Hz, 1H), 1.51 (s, 3H), 1.50 (s, 3H),
1.46 (s, 9H), 1.44 (s, 3H), 1.37 (s, 9H), 1.35 (s, 3H); 13C
NMR (125 MHz, DMSO, 100 ꢀC) d 179.5, 150.9, 150.5,
140.9, 133.5, 129.3, 128.9, 127.9, 126.0, 123.3, 123.0,
121.0, 119.0, 118.0, 117.6, 110.9, 109.7, 92.6, 91.8, 78.8,
78.5, 65.9, 65.1, 56.9, 54.9, 54.0, 37.0, 28.1, 27.6, 27.5,
26.5, 26.4, 23.5, 23.4. NMR data for the minor epimer
(11-R):57 1H NMR (500 MHz, DMSO, 100 ꢀC) d 10.36 (s,
1H), 10.08 (s, 1H), 7.57 (1H), 7.41 (d, J¼7.6 Hz, 1H),
7.32 (1H), 7.17 (1H), 7.12 (1H), 7.02–6.91 (3H), 4.12
(1H), 4.04–3.98 (m, 1H), 3.83 (1H), 3.72 (dd, J¼8.8,
2.0 Hz, 1H), 3.29 (dd, J¼8.8, 5.8 Hz, 1H), 3.15 (1H), 2.99
(d, J¼14.0 Hz, 1H), 2.86–2.77 (2H), 2.49 (1H), 1.47 (s,
9H), 1.45 (s, 3H), 1.43 (s, 9H), 1.42 (s, 3H), 1.39 (s, 3H),
1.33 (s, 3H); 13C NMR (125 MHz, DMSO, 100 ꢀC, diagnos-
tic signals only) d 178.5, 150.7, 141.1, 133.7, 130.3, 128.7,
127.9, 125.8, 123.0, 122.2, 121.0, 119.5, 117.7, 111.0,
109.8, 91.7, 78.9, 65.8, 65.0, 54.8, 54.2, 37.4, 28.0, 26.4,
26.2, 23.4.
13
ꢁ0.07 (s, 9H); C NMR (125 MHz, DMSO, 100 ꢀC)
d 177.1, 150.9, 149.6, 140.6, 136.8, 134.0, 133.6, 131.3,
127.9 (2), 124.5, 123.6, 122.83, 122.75, 118.6, 118.3,
112.3, 109.2, 105.5, 95.1, 92.7, 80.9, 78.7, 78.0, 69.0,
65.7, 65.1, 64.2, 63.6, 56.9, 56,7, 27.7 (2), 27.6, 26.9,
24.7, 24.4, 23.8, 17.1, 17.0, ꢁ2.06, ꢁ2.12; IR (neat) 2953,
1703, 1384, 1366, 1074 cmꢁ1; MS (ESI) m/z 955.5060
(955.5049 calcd for C50H76N4NaO9Si2, M+Na). Anal. Calcd
for C50H76N4O9Si2: C, 64.34; H, 8.21; N, 6.00. Found: C,
64.64; H, 8.27; N, 5.93.
4.7. Elaboration of Heck product 42 to oxindole
intermediate 46
A modification of a procedure reported by Kishi was em-
ployed.30 A solution of 42 (576 mg, 0.617 mmol) and
TBAF (5.0 mL, 1 M in THF, 5.0 mmol) was concentrated
and placed under vacuum (ꢃ0.1 mm) at rt for 65 h. The
resulting residue was partitioned between H2O (200 mL)
and MTBE (100 mL). The aqueous layer was extracted
with MTBE (75 mLꢂ3), and the combined organic extracts
were dried (MgSO4), filtered, and concentrated. A solution
of the residue, Et3N (8 mL), and MeOH (8 mL) was main-
tained at 68 ꢀC for 6 h and then concentrated. The crude
residue was purified by silica gel chromatography (9:1,
6:1, 4:1, 3:1, 2:1 hexanes/EtOAc) to give 352 mg (85%)
26
577
of 43 as a yellow foam: [a]2D6 ꢁ260, [a]
ꢁ273,
26
546
26
435
26
405
[a] ꢁ316, [a] ꢁ642, [a] ꢁ851 (c 0.74, CHCl3);
1H NMR (500 MHz, DMSO, 100 ꢀC) d 10.28 (s, 1H), 9.81
(s, 1H), 7.58 (d, J¼7.8 Hz, 1H), 7.30–7.24 (m, 2H), 7.22
(d, J¼2.3 Hz, 1H), 7.04 (td, J¼7.6, 0.9 Hz, 1H), 6.97 (dd,
J¼8.2, 0.9 Hz, 1H), 6.91 (t, J¼7.7 Hz, 1H), 6.74 (d,
J¼6.8 Hz, 1H), 6.37 (t, J¼1.9 Hz, 1H), 4.15–4.10 (m, 1H),
3.90 (dd, J¼12.8, 1.9 Hz, 1H), 3.82 (dd, J¼8.0, 5.9 Hz,
1H), 3.73 (dd, J¼8.8, 1.9 Hz, 1H), 3.61 (dd, J¼12.8,
2.0 Hz, 1H), 3.18 (dd, J¼14.0, 3.3 Hz, 1H), 2.83 (dd,
J¼14.0, 10.0 Hz, 1H), 1.51 (s, 3H), 1.49 (s, 9H), 1.48 (s,
3H), 1.44 (s, 3H), 1.41 (s, 9H), 1.40 (s, 3H); 13C NMR
(125 MHz, DMSO, 100 ꢀC) d 178.6, 150.9, 149.7, 140.8,
137.4, 133.6, 132.6, 128.4, 127.8, 124.9, 123.6, 123.3,
121.4, 119.1, 117.9, 117.7, 110.7, 109.5, 104.2, 95.0, 92.7,
80.8, 78.6, 65.7, 63.4, 56.9, 56.4, 28.1, 27.7, 27.3, 26.4,
24.8, 24.3, 23.4; IR (neat) 3379, 3260, 2979, 2936, 1694,
A modification of the general procedure of Frechet was
employed.36 A solution of 44 (302 mg, 0.448 mmol), aque-
ous 1 N HCl (3.2 mL), and dioxane (9.6 mL) was heated at
50 ꢀC for 5 h. The reaction was allowed to cool to rt, and
then 1 N NaOH (6 mL), 10 N NaOH (0.4 mL), and Boc2O
(0.50 mL, 2.18 mmol) were added sequentially with vigor-
ous stirring. After 2.5 h at rt, the reaction was poured into
H2O (15 mL) and extracted with EtOAc (50 mLꢂ3) and
then CHCl3 (50 mLꢂ3). The combined organic extracts
were dried (MgSO4), filtered, and concentrated. A mixture
of the residue, CH2Cl2 (10 mL), Boc2O (0.50 mL,
2.18 mmol), n-Bu4HSO4 (15 mg, 0.044 mmol), and 1 N
NaOH (10 mL) was stirred vigorously at rt for 13 h. The
reaction was poured into H2O (25 mL) and extracted with
CHCl3 (50 mLꢂ3). The combined organic extracts were
dried (MgSO4), filtered, and concentrated. The crude residue
was purified by silica gel chromatography (3.5:1, 3:1, 2:1
hexanes/EtOAc) to give 234 mg (59%) of 46 as a colorless
foam, as well as 69 mg (17%) of the C11 epimer, 45 [MS
(ESI) m/z 917.43 (917.45 calcd for C47H66N4NaO13,
1620, 1471, 1385 cmꢁ1
; MS (ESI) m/z 695.3409
(695.3420 calcd for C38H48N4NaO7, M+Na).
A mixture of 43 (400 mg, 0.595 mmol), palladium on carbon
(158 mg, 10 wt %, 0.148 mmol), and DMF (12 mL) was
stirred in a pressure reactor under 1000 psi of hydrogen for
24 h at rt. This mixture was filtered through Celite and the
resulting filter cake was washed with MTBE (300 mL).
The filtrate was washed with saturated aqueous LiCl
(100 mL) and the resulting aqueous layer was extracted
with MTBE (100 mL). The combined organic extracts
were dried (MgSO4), filtered, concentrated, and the residue
was purified by silica gel chromatography (4:1, 3:1 hexanes/
EtOAc) to give 383 mg (95%) of 44, a colorless foam, as an
inseparable 4:1 mixture of epimers. All characterization data
were obtained on this mixture: IR (neat) 3335, 2978, 1698,
1389, 1366 cmꢁ1; MS (CI) m/z 674.3667 (674.3680 calcd
26
26
M+Na)]. Oxindole 46: [a]2D6 ꢁ154, [a] ꢁ163, [a]
577
546
26
435
26
405
ꢁ187, [a]
ꢁ362, [a]
ꢁ466 (c 0.68, CHCl3);
1H NMR (500 MHz, CDCl3) d 10.04 (s, 1H, NH), 7.95
(d, J¼8.0 Hz, 1H, ArH), 7.60 (d, J¼6.9 Hz, 1H), 7.52
(d, J¼7.2 Hz, 1H), 7.43 (t, J¼7.7 Hz, 1H), 7.37 (t,
J¼7.4 Hz, 1H), 7.15 (s, 1H), 6.93 (t, J¼7.7 Hz, 1H),
6.72 (d, J¼7.1 Hz, 1H), 4.83 (br d, J¼6.7 Hz, 1H),
4.20–3.97 (m, 6H, NH), 3.89 (dd, J¼10.9, 3.6 Hz, 1H),
3.12–3.02 (m, 2H), 2.98 (dd, J¼14.4, 8.0 Hz, 1H), 2.46
(dd, J¼12.0, 9.2 Hz, 1H), 1.59 (s, 9H), 1.50 (s, 9H),
1.46 (s, 9H), 1.43 (s, 9H), 1.26 (s, 9H); 13C NMR
(125 MHz, DMSO, 100 ꢀC) d 177.1, 155.3, 154.2, 153.5,

S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
8511
153.2, 148.8, 139.7, 134.6, 129.8, 128.9, 128.5, 126.2,
124.4, 124.2, 121.6, 121.3, 119.4 (2), 116.0, 111.4, 84.6,
82.4, 82.3, 79.4, 79.2, 69.0, 67.2, 56.1, 50.0, 47.1, 36.6,
28.4, 28.2, 28.0, 27.8, 27.7, 27.0; IR (neat) 3390, 2979,
2933, 1788, 1746, 1714, 1607, 1281, 1160 cmꢁ1; MS
(ESI) m/z 917.4523 (917.4524 calcd for C47H66N4NaO13,
M+Na). Anal. Calcd for C47H66N4O13: C, 63.07; H,
7.43; N, 6.26. Found: C, 63.16; H, 7.60; N, 5.90.
added in three equal portions every 45 min to a vigorously
stirring mixture of the crude dialdehyde, NaH2PO4
(105 mg, 0.76 mmol), THF (1 mL), t-BuOH (0.3 mL), 2-
methyl-2-butene (0.3 mL), and H2O (1.0 mL) at rt.39 One
hour after the last addition, the mixture was poured into sat-
urated aqueous NH4Cl (20 mL) and extracted with EtOAc
(25 mLꢂ3). The combined organic extracts were dried
(MgSO4), filtered, and concentrated to give the correspond-
ing crude diacid 48, which was used without purification;
MS (ESI) m/z 729.39 (729.31 calcd for C37H46N4NaO10,
M+Na). O-(7-Azabenzotriazol-1-yl)-N,N,N0,N0-tetramethyl-
uronium hexafluorophosphate40 (HATU, 51 mg, 0.13 mmol)
was added to a solution of the crude diacid 48, (R)-PheOMe$
HCl (21 mg, 0.095 mmol), Et3N (0.050 mL, 0.36 mmol),
and CH2Cl2 (2 mL) at rt. After stirring for 1 h, the mixture
was poured into a solution of saturated aqueous NaHCO3
(20 mL) and 1 N NaOH (0.5 mL) and extracted with EtOAc
(25 mLꢂ2). The combined organic extracts were washed
with a solution of saturated aqueous NH4Cl (20 mL) and
1 N HCl (1 mL). The acidic aqueous layer was extracted
with EtOAc (25 mL), and the combined organic extracts
were dried (MgSO4), filtered, and concentrated. The crude
residue was purified by preparative TLC (26:13:1 CHCl3/
4.8. Closure of the cis-pyrrolidinoindoline ring
to form 47
A modification of a procedure we reported earlier was
employed.37 A THF solution of LiBEt3H (0.30 mL, 1.0 M,
0.30 mmol) was added dropwise to a solution of 46
(59 mg, 0.066 mmol) and THF (4 mL) at ꢁ78 ꢀC. After
allowing the solution to warm to 0 ꢀC, an Et2O solution of
HCl (0.38 mL, 1 M, 0.38 mmol) was added dropwise. The
solution was allowed to warm to rt, maintained for 20 h,
poured into H2O (20 mL), and extracted with CHCl3
(30 mLꢂ3). The combined organic extracts were dried
(MgSO4), filtered, and concentrated to give 64 mg of the
crude cis-pyrrolidinoindoline product: MS (ESI) m/z
901.46 (901.46 calcd for C47H66N4NaO12, M+Na). A solu-
tion of this crude product and degassed methanolic KOH
(5.0 mL, 0.356 M, 1.78 mmol) was maintained at rt for
19 h. The reaction was diluted with CHCl3 (25 mL) and
quenched with aqueous HCl (27 mL, 0.74 M, 2.0 mmol).
The layers were separated and the aqueous layer was
extracted with CHCl3 (30 mLꢂ3). The combined organic
extracts were dried (MgSO4), filtered, and concentrated.
The crude residue was purified by silica gel chromatography
(1:1, 3:5, 1:3 hexanes/EtOAc) to give 35 mg (79% overall)
EtOAc/MeOH) to give 25.5 mg (65% from 47) of dipep-
27
577
tide 49 as a colorless foam: [a]2D7 +20.1, [a]
+20.9,
27
27
27
[a]
+23.4, [a]
+38.4, [a]
+47.6 (c 0.71, CHCl3);
546
435
405
1H NMR (500 MHz, CDCl3) d 8.10 (s, 1H), 7.55 (d,
J¼7.8 Hz, 1H), 7.47 (d, J¼8.0 Hz, 1H), 7.25–7.02 (m,
12H), 6.96–6.76 (m, 5H, NH), 6.74 (s, 1H), 6.21 (d,
J¼8.0 Hz, 1H), 5.00 (br, 1H), 4.79 (dd, J¼8.8, 2.0 Hz,
1H), 4.74 (br, 1H), 4.38 (br, 1H), 4.13 (dd, J¼12.4, 5.8 Hz,
1H), 3.67 (s, 3H), 3.59 (s, 3H), 3.24–3.07 (m, 3H), 3.06–
2.98 (m, 2H), 2.95–2.86 (m, 2H), 2.74 (br, 1H), 1.53 (s,
9H), 1.44 (s, 9H), 1.39 (s, 9H); 13C NMR (125 MHz,
CDCl3) d 171.4, 171.2, 170.7, 170.0, 154.7, 152.4, 141.0,
135.9, 135.5, 133.11, 133.07, 129.3, 129.11, 129.06,
128.8, 128.6, 128.4, 128.3, 127.1, 126.8, 125.5, 124.9,
123.8, 123.5, 120.2, 119.9, 118.7, 116.1, 111.0, 82.54,
82.49, 82.1, 80.1, 62.8, 58.2, 55.0, 53.6, 52.9, 52.13,
52.10, 40.7, 38.2, 37.7, 29.7, 28.3, 28.24, 28.17; IR (neat)
3404, 2977, 2932, 1713, 1514, 1367, 1159 cmꢁ1; MS
(ESI) m/z 1051.47 (1051.48 calcd for C57H68N6NaO12,
M+Na).
1
of 47 as a colorless foam: H NMR (500 MHz, DMSO,
100 ꢀC) d 9.89 (s, 1H, NH), 7.60 (d, J¼8.0 Hz, 1H), 7.51
(d, J¼7.8 Hz, 1H), 7.29–7.22 (m, 2H), 7.11 (d, J¼2.4 Hz,
1H), 7.01 (td, J¼7.5, 0.9 Hz, 1H), 6.89 (t, J¼7.6 Hz,
1H), 6.79 (d, J¼6.8 Hz, 1H), 6.52 (s, 1H), 5.94 (br d, 1H),
4.26–4.00 (br, 1H), 4.23–4.15 (m, 1H), 3.77–3.67 (m, 1H),
3.46–3.33 (m, 3H), 3.10–2.85 (br, 1H), 3.00 (dd, J¼13.5,
8.8 Hz, 1H), 2.92 (dd, J¼14.6, 6.3 Hz, 1H), 2.82–2.74 (m,
2H2), 2.60 (t, J¼9.5 Hz, 1H), 1.44 (s, 9H), 1.38 (s, 9H),
13
1.34 (s, 9H); C NMR (125 MHz, DMSO, 100 ꢀC)
d 154.7, 153.7, 151.5, 140.3, 136.2, 132.2, 129.0, 127.5,
124.2, 123.7, 123.2, 122.5, 118.9, 117.7, 117.6, 116.5,
111.6, 82.5, 80.3, 78.5, 77.0, 62.7, 62.6, 60.1, 57.5, 52.9,
36.8, 27.7, 27.6, 27.4, 26.2; MS (ESI) m/z 701.3555
(701.3527 calcd for C37H50N4NaO8, M+Na).
4.10. (D)-Asperazine (1)
Following the general procedure of Nitecki,43 a solution of
49 (21.7 mg, 0.0211 mmol) and HCO2H (6 mL) was main-
tained at rt for 2 h. The HCO2H was then removed as an
azeotrope with heptane (15 mLꢂ3) to give a crude residue
containing the deprotected dipeptide; MS (ESI) m/z 729.42
(729.34 calcd for C42H45N6O6, M+H). Using a modification
of the general procedure of Suzuki,45 a solution of this
deprotected dipeptide, AcOH (400 mL, 7.0 mmol), and
n-BuOH (9.6 mL) was heated at reflux for 24 h. AcOH
and n-BuOH were then removed in vacuo (0.1 mm) and
the crude residue was purified by preparative TLC (9:1 chlo-
roform/MeOH) to give 8.2 mg (59%) of asperazine (1) as an
amorphous colorless solid. In some repetitions of this exper-
iment, a second purification was needed, so the crude prod-
uct was applied to a C18 reverse-phase HPLC column and
eluted with aqueous acetonitrile (31% CH3CN/69% H2O,
v/v) to provide pure asperazine. Material purified in this
4.9. Preparation of dipeptide 49
Sulfur trioxide$pyridine complex (26 mg, 0.16 mmol) was
added to a solution of diol 47 (25.8 mg, 0.038 mmol),
Et3N (0.10 mL, 0.72 mmol), and DMSO (2 mL) at rt. After
1 h, a second aliquot of SO3$pyr (26 mg, 0.16 mmol) was
added. After an additional hour, the reaction was poured
into a solution of saturated aqueous NH4Cl (15 mL) and sat-
urated aqueous NaHCO3 (15 mL) and extracted with EtOAc
(25 mLꢂ3). The combined organic extracts were dried
(MgSO4), filtered, and concentrated to give the correspond-
ing crude dialdehyde, which was used without purification;
MS (ESI) m/z 697.49 (697.32 calcd for C37H46N4NaO8,
M+Na). Sodium chlorite (45 mg, 80%, 0.40 mmol) was

8512
S. P. Govek, L. E. Overman / Tetrahedron 63 (2007) 8499–8513
1
9. (a) Shibasaki, M.; Vogl, E. M.; Ohshima, T. Comprehensive
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Tran, V. D. J. Am. Chem. Soc. 1990, 112, 6959–6964; (b) For
a comprehensive review of intramolecular Heck reactions,
see: Link, J. T. The Intramolecular Heck Reaction. In
Organic Reactions; Overman, L. E., Ed.; John Wiley and
Sons: New York, NY, 2002; Vol. 60, pp 157–534.
13. Heck reactions in which the stereoinducing element is external
to the ring being formed are uncommon. For two recent exam-
ples, see: (a) Overman, L. E.; Paone, D. V.; Stearns, B. A.
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1454.
fashion showed H NMR spectra (in CDCl3, CD2Cl2, and
CD3CN), 13C NMR spectra (in CDCl3 and CD3CN),58
mass spectral data, and a CD spectrum in MeOH that com-
pared favorably to those of the natural isolate.1 The optical
rotation of synthetic asperazine, [a]D +95.7 (c 0.2, MeOH),
was higher than that reported for the natural material, [a]D
+52 (c 0.2, MeOH).1 Synthetic asperazine and the natural
isolate were co-eluted from a C18 reverse-phase HPLC
column (31% CH3CN/69% H2O, v/v).
Acknowledgements
We thank NIH (HL-25854) for financial support and Glaxo-
Wellcome for fellowship support for S.P.G. We particularly
thank Professor Philip Crews for providing a sample and
copies of unpublished NMR spectra of natural 1 and for sev-
eral valuable discussions, and Professor Frederick Valeriote
for measuring the cytotoxicity of synthetic asperazine. NMR
and mass spectra were determined at UC Irvine with instru-
ments purchased with the assistance of NSF and NIH.
Supplementary data
Copies of 1H and 13C NMR spectra, CD spectra and HPLC
chromatograms for synthetic and natural asperazine can
be found. Supplementary data associated with this article
can be found in the online version, at doi:10.1016/
j.tet.2007.05.127.
14. (a) Garner, P.; Park, J. M. Organic Syntheses, Coll. Vol. 9;
Freeman, J. P., Ed.; John Wiley: New York, NY, 1998;
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52. The procedure employed to prepare this known intermediate is
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53. Because this product is a mixture of epimers and rotamers, all
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34. Diastereoselection in the addition of electrophiles to alkenes
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9:1 hexanes/benzene for pure hexanes in the solvent mixture
prevented this problem. Alternatively, anilide 41 could be
recrystallized from diethyl ether.
1
55. Coalescence of the H NMR spectrum was not observed in
DMSO-d6 even at temperatures as high as 150 ꢀC. As a result,
the 1H NMR in CDCl3 at room temperature is reported; multi-
plicities, coupling constants, and assignments are not reported
because the resonances were broad.
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56. The 13C NMR is also reported in CDCl3 at room temperature.
The number of peaks listed does not correlate to the number
of carbons because of the existence of rotamers at room
temperature.
57. In cases where the multiplicity and/or peak assignment was not
made, the peak was obstructed by the major isomer.
58. Unpublished data for natural asperazine kindly provided by
Professor Phillip Crews.
41. Caballero, D.; Abendano, C.; Menendez, J. C. Tetrahedron:
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