Asymmetric [C + NC + CC] coupling entry to the naphthyridinomycin natural product family: Formal total synthesis of cyanocycline A and bioxalomycin β2

Article abstract of DOI:10.1021/jo200553g

A full account of our [C + NC + CC] coupling approach to the naphthyridinomycin family of natural products is presented, culminating in formal total syntheses of cyanocycline A and bioxalomycin β2. The key complexity-building reaction in the synthesis involves the Ag ^I-catalyzed endo-selective [C + NC + CC] coupling of aldehyde 7, (S)-glycyl sultam 8, and methyl acrylate (9) to provide the highly functionalized pyrrolidine 6, which was carried forward to an advanced intermediate (compound 33) in Fukuyama's synthesis of cyanocycline A. Since cyanocycline A has been converted to bioxalomycin β2, this constitutes a formal synthesis of the latter natural product as well. The multicomponent reaction-based strategy reduces the number of steps previously needed to assemble these complex molecular targets by one-third. This work highlights the utility of the asymmetric [C + NC + CC] coupling reaction in the context of a complex pyrrolidine-containing target and provides an illustrative guide for its application to other synthesis problems. The synthesis also fueled collaborative biological and biochemical research that identified a unique small molecule inhibitor of cell migration (compound 30).

Full text of DOI:10.1021/jo200553g

ARTICLE
pubs.acs.org/joc
Asymmetric [C þ NC þ CC] Coupling Entry to the Naphthyridinomycin
Natural Product Family: Formal Total Synthesis of Cyanocycline A and
Bioxalomycin β2
::
Philip Garner,*,§ H. Umit Kaniskan,† Charles M. Keyari,‡ and Laksiri Weerasinghe‡
†Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078, United States
‡Department of Chemistry, Washington State University, Pullman, Washington 99164-4630, United States
S Supporting Information
b
ABSTRACT:
A full account of our [C þ NC þ CC] coupling approach to the naphthyridinomycin family of natural products is presented,
culminating in formal total syntheses of cyanocycline A and bioxalomycin β2. The key complexity-building reaction in the synthesis
involves the AgI-catalyzed endo-selective [C þ NC þ CC] coupling of aldehyde 7, (S)-glycyl sultam 8, and methyl acrylate (9) to
provide the highly functionalized pyrrolidine 6, which was carried forward to an advanced intermediate (compound 33) in
Fukuyama’s synthesis of cyanocycline A. Since cyanocycline A has been converted to bioxalomycin β2, this constitutes a formal
synthesis of the latter natural product as well. The multicomponent reaction-based strategy reduces the number of steps previously
needed to assemble these complex molecular targets by one-third. This work highlights the utility of the asymmetric [C þ NC þ
CC] coupling reaction in the context of a complex pyrrolidine-containing target and provides an illustrative guide for its application
to other synthesis problems. The synthesis also fueled collaborative biological and biochemical research that identified a unique
small molecule inhibitor of cell migration (compound 30).
’ INTRODUCTION
being based on NMR studies alone. Considering the history of
naphthyridinomycin, dnacin B1 may actually possess a bioxalo-
mycin-like oxazolidine G-ring (Mþ ꢀ H2O parent in MS).
The cyanocyclines,1ꢀ3 SF-1739HP,4 dnacins (Actinosynnema
pretiosum C-14482),5ꢀ7 bioxalomycins (Streptomyces viridostati-
cus and S. lusitanus),8,9 and aclidinomycins (Streptomyces halstedi
KB012)10 comprise the naphthyridinomycin family of tetrahy-
droisoquinoline antibiotics.11 As a group, these natural products
share a number of structural features (principally, their ABCD
ring skeleta) with the simpler quinocarcin family of tetrahydro-
isoquinolines.11 Although naphthyridinomycin was the first
member of this family to be characterized (by X-ray crystallo-
graphy),12,13 Ellestad and co-workers subsequently showed that
this compound is actually an artifact formed by hydrolysis of the
bioxalomycin G-ring.9 The structure of bioxalomycin β2 was
established by its chemical conversion to cyanocycline A (treat-
ment with KCN, pH 8.0), whose structure had been unambiguously
assigned by X-ray crystallography4 and total synthesis (see
below). The conversion of cyanocycline A to bioxalomycin
β2 (by treatment with AgNO3) was also reported in this paper.
The structures of the dnacins and aclidinomycins are less secure,
Members of the naphthyridinomycin family of tetrahydroiso-
quinoline natural products exhibit varying degrees of antibacterial
and anticancer activities.11 The cytotoxic biological activities of
these compounds have long been attributed to their interaction
with DNA (based on in vitro experiments and computational
modeling), but there is growing evidence that proteins may be
biological targets as well. For example, the dnacins have been found
to inhibit the dual specificity phosphatase cdc25B,14 a protein
involved in regulation of the cell division cycle and whose over-
expression is connected with cancer cell proliferation. DX-52-1, a
derivative of quinocarcin, binds strongly to the ERM protein radixin
and inhibits cell migration, a process essential to cancer metastasis
and tumor progression.15 As a result of its potency for a specific
Received: March 12, 2011
Published: May 31, 2011
r
2011 American Chemical Society
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from L-glutamic acid 5-methyl ester).20 While these total syntheses
were ground breaking in their own right, they do not represent
especially efficient and general synthetic routes to the naphthyr-
idinomycin family. In addition to these successful efforts, notable
synthetic excursions targeting the naphthyridinomycin family have
also been made by the groups of Danishefsky,21 Williams,22 Myers,23
Fukuyama,24 as well as our own.25 In this Article, we detail our
asymmetric [C þ NC þ CC] coupling approach to the naphthyr-
idinomycin family, culminating in an efficient formal total synthesis
of cyanocycline A and bioxalomycin β2.26 The approach described
herein provides a viable framework for synthesis of the entire
naphthyridinomycin family as well as structurally related analogues.
’ RESULTS AND DISCUSSION
Retrosynthetic Analysis. Our retrosynthetic logic is shown in
Scheme 1. The first disconnection (ring C) involves the applica-
tion of a Strecker transform to cyanocycline A (1) to give HCN
(2) and the amino aldehyde 3. Drawing from Fukuyama’s
racemic synthesis of 1,19a we fully expected that trapping of the
intermediate iminium species by cyanide would proceed from
the less-hindered convex face. Fukuyama’s synthesis also pro-
vided good precedent for the second disconnection (ring B) via
a PictetꢀSpengler transform on 3 to give the aldehyde 4
and aminophenol 5. We reasoned that this reaction would be
tolerant of changes in substituents R (urethane to methyl) and X
(methoxy to sulfamoyl). Disconnection of the E-ring lactam
leads to the highly functionalized pyrrolidine 6. With the end
game in place, access to this key intermediate would essentially
solve the cyanocycline A synthesis problem. On paper it appears
that such a pyrrolidine might be readily prepared using existing
[3 þ 2] cycloaddition protocols. However, it soon became
apparent that the existing dipolar cycloaddition methodology27
was wholly inadequate for our purposes.28 This state of affairs led
us on a methodological journey that resulted in our development
of the asymmetric [C þ NC þ CC] coupling reaction,29 the endo
version of which30 was perfectly suited to ours needs. Application
of this [C þ NC þ CC] transform effectively deconstructs
the target’s D-ring to give the [C þ NC þ CC] coupling
protein target, the antimigratory effect of DX-52-1 is observed at
subtoxic doses. These observations support the notion that other
members of the naphthyridinomycin and quinocarcin families may
also inhibit therapeutically relevant proteins and could therefore be
used to develop small molecule drugs. However, the limited
availability of these complex natural products and, consequently,
analogues thereof has severely hindered biological studies on the
naphthyridinomycin family. It was in this context that set out to
devise an efficient synthetic strategy that could be applied to both
natural and unnatural members of the naphthyridinomycin family.
The densely functionalized polycyclic structures associated with
the naphthyridinomycin family present a significant synthetic
challenge. Cyanocycline A and bioxalomycin β2 are the only
members of the naphthyridinomycin family to be synthesized to
date.11,16 In 1986, Evans reported the first total synthesis of racemic
cyanocycline A (30 steps from cyclopentadiene),17 and an asym-
metric synthesis of (þ)-cyanocycline A was subsequently described
in a dissertation (35 steps from 2,6-dimethoxytoluene).18 Fukuyama
disclosed his group’s synthesis of racemic cyanocycline A in 1987
(32 steps from tert-butyl azidoformate).19 In 1992, he reported
the asymmetric synthesis of (þ)-naphthyridinomycin (29 steps
Scheme 1. Retrosynthetic Analysis
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Scheme 2. Synthesis of Aldehyde 7
Figure 1. HPLC analysis of the crude Mosher esters prepared from alcohol 16.
components, R,β-diaminoaldehyde 7, 1(S)-glycylsultam 8,31 and
methyl acrylate (9). Based on our earlier work, we were confident
that the required pyrrolidine stereochemistry would predomi-
nate in this multicomponent reaction. Aldehyde 7 was to be
prepared via asymmetric homologation32 of the readily available
serinal derivative 10.33
C13b and C13c stereocenters34 along with a precursor to the
target’s A-ring, is depicted in Scheme 2. The sequence began with
the stereoselective addition of a Grignard reagent, prepared from
bromide 11,35 to the known serinal-derived nitrone 12 at ꢀ50 °C
to yield a single crystalline hydroxylamine 13 in 71% yield. The
outcome of this Grignard addition was expected on the basis of
the work of Merino,32 who had performed an analogous addition
of phenylmagnesium bromide to 12 and established the product
Synthesis of Aldehyde 7. Asymmetric synthesis of the
starting aldehyde 7, a compound that incorporates the target’s
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Scheme 3. Cornerstone [C þ NC þ CC] Coupling Reaction
stereochemistry by a combination of chiroptical and chemical
methods. The high syn-selectivity can be rationalized by a
substrate-controlled addition of the Grignard reagent to nitrone
12 via an A-strain-induced pre-TS conformation. Our stereo-
chemical assignment would be corroborated by correlation of 13
with cyanocycline A. Interestingly, while compounds that con-
tain the N-Boc 2,2-dimethyl-1,3-oxazolidine moiety generally
exist as dynamic mixtures of conformers in solution, 13 appears
to be locked in a very stable conformation (its 1H NMR spectrum
shows a single set of well-resolved signals at rt). Zinc-mediated
reduction of hydroxylamine 13 proceeded cleanly to give the
secondary amine 14, which was immediately treated with benzyl
chloroformate to provide the urethane 15 in 85% overall yield.
Mildly acidic methanolysis of the acetonide afforded the alcohol
16 in 71% isolated yield. This compound was shown to be >98%
enantiomerically pure through a separate Mosher ester study
(Figure 1). Finally, DessꢀMartin oxidation of 16 gave the
desired aldehyde 7 in 85% yield.
Asymmetric [C þ NC þ CC] Coupling Reaction. With the
aldehyde 7 in hand, we were ready for the cornerstone asym-
metric [C þ NC þ CC] coupling reaction. When the standard
endo-selective asymmetric [C þ NC þ CC] coupling reaction
conditions were applied to 7 (1 equiv aldehyde, 1.1 equiv
glycylsultam, 3 equiv dipolarophile, and 5 mol % AgOAc), a
mixture of pyrrolidines was produced in 38% yield along with
what appeared to be a mixture of imidazolines and oxazolidines.36
Formation of these byproducts is indicative of ineffective trap-
ping of the sterically congested azomethine ylide by a relatively
poor dipolarophile. This problem was solved by employing
methyl acrylate (9) as the solvent and increasing the amount
of catalyst to 10 mol %. The optimized reaction conditions
involved combining aldehyde 7 and (1S)-glycylsultam 8 in methyl
acrylate with 10 mol % AgOAc at room temperature to give a
white solid, mp 79ꢀ81 °C, isolated in 73% yield after flash
chromatography (Scheme 3). On the basis of our understanding
of the asymmetric [C þ NC þ CC] coupling process,29 we
anticipated that the [3 þ 2] cycloaddition would proceed via a
pre-TS ensemble such as 20. The diastereofacial selectivity in this
model would be determined by a ylide conformation that places
the N-acyl sultam CO and SO2 dipoles anti to each other while
still maintaining the usual imine-carbonyl chelation by Ag(I). In
this model, the coordinated acrylate dipolarophile approaches
the ylide from the least hindered endo-si face opposite the pro-R
sulfoxide moiety. The high-resolution mass spectrum of this solid
was consistent with the molecular formula of 6. Although this
material was homogeneous by normal phase TLC and had a
relatively sharp melting point, its 1H NMR spectrum could not be
satisfactorily resolved into a single set of signals, even when the
sample was heated to 110 °C in DMSO-d6. We initially attributed
this anomalous NMR behavior to the presence of one or more
bonds with relatively high rotation barriers. (This, in fact, turns
out to be the case for other compounds that were encountered
downstream in the synthesis.) However, subsequent analysis of
this material by careful reverse-phase HPLC revealed it to consist
of a 4:1 mixture of diastereoisomeric cycloadducts 6 and 19.
The major cycloadduct diastereomer 6 was initially assigned as
the desired product of endo-si addition on the basis of our chiral
sultam-directed [3 þ 2] cycloaddition pre-TS model (see en-
semble 20). This stereochemical assignment was ultimately
confirmed by the successful correlation of 6 with the natural
product cyanocycline A, whose structure had been unambi-
guously determined by X-ray crystallography. We hypothesized
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Scheme 4. Completion of the Cyanocycline A Synthesis
that the minor diastereomer was most likely the result of (1) endo-re
addition of 9 to the (E,E)-azomethine ylide, (2) the exo-si addi-
tion product, or (3) the product of endo-si addition to the
R-epimerized azomethine ylide. This issue was clarified in the
following manner. The cycloadduct mixture (6 þ 19) was sub-
jected to Sm(OTf)3-mediated methanolysis,37 removing the chiral
sultam moiety and producing a pair of diastereomeric diesters 21
and 22 in good yield. To determine the structure of 22 unambigu-
ously, it was decided to prepare an authentic sample of the endo-Re
product for comparison. Accordingly, an asymmetric [C þ NC þ
CC] coupling reaction was performed with aldehyde 7, the
antipodal 1(R)-glycylsultam ent-8 and methyl acrylate (9). We
have previously shown that absolute stereocontrol in the asym-
metric [C þ NC þ CC] reaction is governed by the camphorsul-
tam auxiliary.30 This reaction produced a cycloadduct 23 in good
yield, which was diastereomeric to 6. Methanolysis of 23 produced
a compound that was shown to be identical to the diester 22 in the
previously prepared (21 þ 22) mixture by careful HPLC and NMR
analyses (see Supporting Information). These experiments unam-
biguously established the structure of the minor cycloadduct 19.
Thus, it appears that we have a stereochemically mismatched [3 þ 2]
cycloaddition between ylide 20 (derived from 7 and 8) and
methyl acrylate (9) versus a matched [3 þ 2] cycloaddition (via the
diastereomeric ylide derived from 7 and ent-8). In the former case,
the ability of the chiral sultam to dominate the stereochemical
outcome of this reaction (auxiliary control) was opposed by the
inherent directing effect of the CHNHBoc stereocenter (substrate
control). Fortunately, the unresolved mixture of 6 and 19 could be
used directly in the next reaction, obviating the need for their
separation at this stage. The stereocontrolled assembly of cyclo-
adduct 6, which possesses the target’s A and D rings as well as 5 of
its 8 stereocenters, essentially solves the cyanocycline A synthesis
problem in line with the strategy outlined in Scheme 1.
The synthetic end game (Scheme 4) began with Pd-catalyzed
hydrogenolysis of the 6 þ 19 mixture to give the pure δ-lactam
24 in an isolated yield of 57% (71% based on 6). In contrast to its
precursor 6, compound 24 was readily purified by flash chroma-
tography. This multistep transformation involves removal of the
O-benzyl, N-benzyl, and N-Cbz protecting groups, followed by
intramolecular acylation of the free amine by the methyl ester to
install the target’s E-ring.38 The spontaneous lactamization
also confirmed the endo-cycloadduct stereochemistry at this
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Figure 2. Expanded 600 MHz COSY (top) and NOESY (bottom) spectra of compound 30 showing key correlations.
stage since an exo-cycloadduct would resist formation of the
corresponding trans-fused bicycle. To preclude potential inter-
ference with the upcoming PictetꢀSpengler process, the pyrro-
lidine amine was converted to its N-Cbz derivative 25 in 85%
yield. The urethane moiety also served as a convenient precursor
to the target’s N-Me group (vide infra). Removal of the N-Boc
group released the phenolic amine 26, providing the necessary
synthon for the PictetꢀSpengler reaction that would install ring
B. In the event, the reaction of 26 with benzyloxyacetaldehyde in
the presence of acetic acid plus molecular sieves produced the
tetrahydroisoquinoline 27 in 86% overall yield. The 9(R) ster-
eochemistry in 27 was tentatively assigned on the basis of
Fukuyama’s precedent19 and eventually confirmed by correlation
with cyanocycline A. Reprotection of the free phenol produced
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compound 28 in 81% yield.39 The reduction of 28 with LiAlH4 not
only released the chiral auxiliary but also converted the urethane
moiety to the required N-methyl group,40 producing the primary
alcohol 29 in 61% yield. In this way, two desired functional group
transformations were achieved in a single step. Following the
strategy of Fukuyama,19 the C-ring was assembled using a two-step
sequence. First, Swern oxidation of 29 produced a mixture of
hemiaminals that was immediately subjected to the action of
TMSCN in the presence of anhydrous ZnCl2 to deliver aminonitrile
30 in 45% yield. The latter reaction presumably involves the
stereoselective addition of cyanide to the less hindered convex face
of an intermediate iminium ion. Due to its rigid structure, com-
pound 30 exhibited a particularly well-resolved 1H NMR spectrum
that allowed extensive structural analysis via COSY and NOESY
experiments (Figure 2). In particular, the observation of NOESY
cross-peaks between H7 and H5R as well as H7 and H90pro-S
supported our configuration assignments for C9 and C7. The
oxazolidine ring was then introduced using methodology developed
by Pelletier as follows.41 The lactam moiety of 30 was converted to
the corresponding thiolactam with Lawesson’s reagent, and this
compound was reduced with Raney Ni to give the stable imine 31 in
63% yield.42 This imine reacted smoothly with hot ethylene oxide in
MeOH to afford the desired 3a(R) oxazolidine 32 in 58% yield.
Finally, 32 was treated with BCl3 to remove the benzyl ether
protecting groups and give diol 33. This compound corresponded
to an advanced intermediate that Fukuyama had converted to
cyanocycline A,19 thus completing a formal synthesis of the natural
product. Since cyanocycline A is convertible to bioxalomycin β2
through the agency of Ag(I),9 the attainment of 33 also constitutes
a formal synthesis of this natural product as well.
atmosphere of argon. Air-sensitive liquids were transferred via syringe
or cannula through rubber septa. Reagent grade solvents were used for
extraction and flash chromatography. Anhydrous solvents were prepared
as follows: THF was distilled from Na/benzophenone under argon;
dichloromethane (CH2Cl2) and benzene were distilled from CaH2
under argon. Diisopropylethylamine (DIEA) and triethylamine (TEA)
were distilled from CaH2 under argon. All other reagents and solvents
were purchased from commercial sources were used directly without
further purification. The progress of reactions was monitored by
analytical thin layer chromatography (TLC, silica gel F-254 plates).
TLC plates were visualized first with UV illumination (254 nm) followed
by charring using either ninhydrin stain (0.3% ninhydrin (w/v) in 97:3
EtOH/AcOH) or a modification of Hanessian’s stain (10 g ammonium
molybdate ((NH4)6Mo7O24 4H2O) and 5 g cerium sulfate (Ce(SO4)2)
3
in 1 L 10% aq H2SO4). Flash column chromatography was performed on
flash grade (230ꢀ400 mesh) silica gel. The solvent compositions
reported for all chromatographic separations are on a volume/volume
(v/v) basis. High performance liquid chromatography (HPLC) was
carried out using an X-Bridge C18 (3 ꢁ 250 mm column) for analytical
separations and X-Bridge prep C18 (19 ꢁ 150 mm) column for
semipreparative separations. Melting points are uncorrected. Optical
rotations were recorded at room temperature at the sodium D line
(589 nm). 1H NMR spectra were recorded at 300, 400, 500, or 600 MHz
and are reported in parts per million (ppm) on the δ scale relative to
tetramethylsilane (TMS) as an internal standard (δ 0.00). 13C NMR
spectra were recorded at 75.5, 125.7, 150.8, or 100 MHz and are reported
in parts per million (ppm) on the δ scale relative to CDCl3 as an internal
standard (δ 77.00). Unless indicated otherwise, NMR spectra were
acquired at ambient temperature. High resolution mass spectrometry
(HRMS) was performed using either FAB or ESI techniques.
Aromatic Bromide 11. To a suspension of NaH (3.1 g, 78 mmol,
60% dispersion in mineral oil) in DMF (70 mL) was added a solution of
5-bromo-2,4-dimethoxy-3-methylphenol35 (16.0 g, 64.8 mmol) in DMF
(70 mL) at 0 °C under argon via a dropping funnel over 15 min. After 1 h
of stirring at this temperature, benzyl bromide (9.24 mL, 13.3 g, 77.8
mmol) was added, and the reaction was stirred for 3 h, when TLC
analysis showed the reaction to be complete. The mixture was diluted
with water (150 mL) and extracted with Et2O (3 ꢁ 100 mL). The
combined organic layers were dried over MgSO4, filtered, and concen-
trated under reduced pressure. Purification by flash column chromatog-
raphy (25:1 then 15:1 hexanes/EtOAc) gave 11 as a yellow oil (20.5 g,
94%). Compound 11 is a yellow oil that forms clear colorless crystals on
standing at rt. Rf 0.51 (9:1 hexanes/EtOAc); mp 45ꢀ47 °C; 1H NMR
(500 MHz, CDCl3) δ 7.43 (d, J = 7.2 Hz, 2H), 7.39 (t, J = 7.4 Hz, 2H),
7.33 (t, J = 7.1 Hz, 1H), 7.00 (s, 1H), 5.04 (s, 2H), 3.82 (s, 3H), 3.75
(s, 3H), 2.25 (s, 3H); 13C NMR (125.7 MHz, CDCl3) δ 149.8, 148.8,
147.9, 136.6, 128.6, 128.0, 127.3, 127.2, 115.5, 110.6, 71.2, 60.4, 60.4,
10.1; HRMS (FAB) m/z calcd for C16H17BrO3 [Mþ] 336.0361, found
336.0369.
Nitrone 12. To a well-stirred solution of N-Boc-D-serinal acetonide
1033 (12.5 g, 54.5 mmol) in CH2Cl2 (500 mL) were added anhydrous
magnesium sulfate (9.84 g, 81.8 mmol) and N-benzylhydroxylamine
(6.71 g, 54.5 mmol) sequentially, and the resulting mixture was stirred at
rt for 4 h. The reaction was filtered, and the filtrate concentrated to give
the crude product. Purification by flash chromatography on silica gel
(3:1 EtOAc/hexanes) gave the pure nitrone 12 as a colorless oil (12.95 g,
71%). [R]D þ48.8 (c 1.8, CHCl3), [lit.32 for ent-12. [R]D ꢀ46.6 (c 1.8,
CHCl3)]; 1H NMR (500 MHz, CDCl3, 55 °C) δ 7.43ꢀ7.35 (m 5H),
6.74 (br s, 1H), 4.93 (br s, 1H), 4.87 (s, 2H), 4.18 (dd, J = 9.2, 6.8 Hz,
1H), 4.06 (dd, J = 9.4, 2.4 Hz, 1H), 1.56 (s, 3H), 1.50 (s, 3H), 1.37 (s,
9H); 13C NMR (125.7 MHz, CDCl3, 55 °C) δ 139.8, 132.7, 129.1,
128.9, 94.3, 80.4, 69.1, 66.4, 55.2, 28.3, 26.5, 23.4.
’ CONCLUSION
Our formal synthesis of cyanocycline A was accomplished in
22 linear steps from 2,6-dimethoxytoluene (19 steps from the
readily available serinal 10). This represents the most efficient
synthesis of cyanocycline A to date and illustrates the value of the
asymmetric [C þ NC þ CC] coupling reaction in the context of
a complex synthetic problem. With appropriate modification, this
same [C þ NC þ CC] coupling-based synthetic strategy may be
used to access both natural and unnatural members of the
naphthyridinomycin family. This [C þ NC þ CC] synthesis
of cyanocycline A also enabled biological and biochemical studies
on the naphthyridinomycin family that led to some significant
discoveries. As part of our collaboration with Gabriel Fenteany’s
group at the University of Connecticut, advanced intermediate
30 (which we have designated as HUK-921) was found to inhibit
cell migration, a process involved in cancer metastasis and tumor
progression.43 Although HUK-921 turned out to be a less potent
inhibitor of cell migration than DX-52-1,15 it exerted its inhibitory
effect in a different manner via selective binding to galectin-3,
revealing a potential therapeutic target that had not previously
been implicated in cell migration. This was the first example of a
non-carbohydrate small molecule inhibitor of this protein, illus-
trating the untapped potential of the naphthyridinomycin family as
a source of small molecule probes of protein-mediated transduc-
tion as well as novel drug leads for the treatment of cancer.
’ EXPERIMENTAL SECTION
General Experimental Methods. All moisture-sensitive reac-
tions were performed in flame-dried glassware under an inert, dry
Hydroxylamine 13. A solution of aromatic bromide 11 (17.9 g,
53.1 mmol) in 30 mL of dry THF was added to a stirred suspension of
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Mg turnings (1.29 g, 53.1 mmol) in 30 mL THF. A crystal of I2 was
added, and the mixture was heated to reflux. Grignard formation was
slow, so 1,2-dibromoethane (100 μL) was added, and the mixture was
heated under reflux for ∼1 h at which point almost all of the Mg had
reacted. The cooled Grignard mixture was then transferred via a cannula
to a stirred solution of nitrone 12 (11.8 g, 35.4 mmol) in 30 mL of dry
THF at ꢀ50 °C. The reaction was stirred at this temperature for 5 h
when TLC showed that almost all of the nitrone was consumed. The
mixture was partitioned between saturated NH4Cl solution (80 mL) and
Et2O (2 ꢁ 100 mL). The combined organic layers were dried over
MgSO4, filtered, and concentrated. The crude product was purified
by flash chromatography (9:1 hexanes/EtOAc) to yield hydroxylamine
13 as a white foam (14.9 g, 71%). Rf 0.39 (9:1 hexanes/EtOAc);
[R]D ꢀ35.0 (c 0.84, CHCl3); 1H NMR (300 MHz, CDCl3) δ
7.48ꢀ7.07 (11H), 5.10 (d, J = 11.6 Hz, 1H), 4.98 (d, J = 11.6 Hz,
1H), 4.54 (dd, J = 10.8, 5.0 Hz, 1H), 4.23 (d, J = 10.8 Hz, 1H), 3.87 (s,
3H), 3.85 (d, J = 10.8 Hz, 1H), 3.83 (d, J = 9.1 Hz, 1H), 3.76 (dd, J = 9.1,
5.2 Hz, 1H), 3.65 (s, 3H), 3.49 (d, J = 10.9 Hz, 1H), 3.45 (d, J = 5.3 Hz,
1H), 2.27 (s, 3H), 1.64 (s, 3H), 1.59 (s, 9H), 1.56 (s, 3H); 13C NMR
(150.8 MHz, CDCl3) δ 154.8, 152.5, 148.2, 148.1, 139.9, 137.5, 128.7,
128.5, 127.9, 127.8, 126.6, 125.0, 124.3, 113.1, 94.4, 81.3, 70.9, 65.9, 63.6,
61.0, 60.5, 60.1, 58.7, 28.8, 28.1, 25.1, 10.3; HRMS (FAB) m/z calcd for
C34H45N2O7 [MHþ] 593.3221, found 593.3234.
Acetonide 15. To a solution of hydroxylamine 13 (14.8 g, 25.0
mmol) in EtOH (270 mL) were added saturated aq NH4Cl (270 mL)
and zinc dust (32.6 g, 500 mmol). The mixture was heated under reflux
overnight (bath temperature ∼90 °C), when TLC analysis showed the
reaction to be complete. The reaction mixture was cooled to room
temperature and extracted with EtOAc (3 ꢁ 100 mL). The combined
extracts were dried over MgSO4, filtered, and concentrated under
reduced pressure to give crude 14. The resulting white foam was
dissolved in dioxane (450 mL) and cooled to 0 °C. To this solution
were added CbzCl (4.9 mL, 27 mmol) and 7% aq NaHCO3 (80 mL),
and the mixture was stirred for 4 h, when TLC showed the reaction to be
complete. The mixture was partitioned between water (450 mL) and
EtOAc (3 ꢁ 300 mL). The combined organic phases were dried over
MgSO4, filtered, and concentrated. The crude product was purified by
flash chromatography (17:3 hexanes/EtOAc) to give 15 as a yellow oil
(15.1 g, 85%). Rf 0.23 (85:15 hexanes/EtOAc); [R]D þ48.8 (c 1.8,
CHCl3); 1H NMR (600 MHz, DMSO-d6, 90 °C) δ 7.45ꢀ7.16 (10H),
7.00ꢀ6.87 (3H), 6.75 (br s, 1H), 6.66 (s, 2H), 5.75 (d, J = 10.2 Hz, 1H),
5.10 (br s, 1H), 5.00 (d, J = 12.4 Hz, 2H), 4.91ꢀ4.81 (m, 2H), 4.69 (d,
J = 17.0 Hz, 1H), 4.27 (d, J = 16.9 Hz, 1H), 3.84 (dd, J = 9.4, 5.6 Hz, 1H),
3.55 (s, 3H), 3.50 (s, 3H), 3.32 (d, J = 9.4 Hz, 1H), 1.99 (s, 3H), 1.57 (s,
3H), 1.47 (s, 12H); 13C NMR (150.8 MHz, DMSO-d6, 90 °C) δ 156.8,
153.0, 153.0, 148.9, 148.1, 139.7, 138.2, 137.6, 128.9, 128.7, 128.3, 128.1,
128.0, 127.6, 127.0, 126.2, 125.8, 125.5, 113.7, 94.9, 80.1, 71.7, 67.2, 66.1,
60.9, 60.2, 56.8, 55.5, 48.5, 28.8, 27.6, 25.0 10.0; HRMS (FAB) m/z calcd
for C42H50N2O8 [Mþ] 710.3567, found 710.3557.
1H), 3.24 (br s, 1H), 2.01 (s, 2H), 1.40 (s, 9H); 13C NMR (125.7 MHz,
DMSO-d6, 90 °C) δ 161.4, 157.2, 155.9, 152.9, 148.8, 148.0, 139.5,
138.1, 129.0, 128.8, 128.3, 128.2, 128.1, 128.0, 127.8, 127.2, 126.4, 125.5,
125.3, 113.5, 78.7, 71.6, 67.3, 61.9, 61.0, 60.4, 53.9, 48.3, 45.3, 28.9, 9.8;
HRMS (ESI) m/z (%) for C39H47N2O8 [MHþ] calcd 671.3327, found
671.3343.
(R)-Mosher Ester 17. A solution of (R)-(þ)-Mosher acid (14 mg,
0.060 mmol) in dry CH2Cl2 (1 mL) was combined with amino alcohol
16 (13 mg, 0.020 mmol), DCC (12 mg, 0.060 mmol), and DMAP
(13 mg, 0.060 mmol). This mixture was stirred at room temperature
for 18ꢀ20 h when the reaction was judged to be complete by TLC. The
reaction mixture was filtered through a cotton plug to remove N,N0-
dicyclohexylurea and roughly purified by flash chromatography (7:3
hexanes/EtOAc) to give 17 as a yellow oil (24 mg, 122%). Care was
taken to combine all fractions that contained Mosher ester. Rf 0.56 (7:3
hexanes/EtOAc); HPLC: C18 (3 ꢁ 250 mm) XBridge column,
gradient: 95% to 5% H2O in CH3CN over 30 min, flow rate
0.5 mL/min, UV detection at 254 nm, tR: 40.5 min, identified by
coinjection with 18; 1H NMR (600 MHz, DMSO-d6, 90 °C) δ 7.6ꢀ7.2
(15H), 7.1ꢀ6.9 (4H), 6.7 (s, 2H), 6.0ꢀ6.05 (br s, 1H), 5.6 (d, J=
10.8 Hz, 1H), 5.1ꢀ5.09 (2H), 5.0 (d, J = 12.6 Hz, 1H), 4.92 (d, J = 12.6
Hz, 1H), 4.69 (m, 1H), 4.52 (d, J = 17 Hz, 1H), 4.32 (dd, J = 11.4, 3.6
Hz, 1H), 4.2 (d, J = 17 Hz, 1H), 4.12 (dd, J = 11.4, 4.0 Hz, 1H), 3.65 (s,
3H), 3.42 (s, 3H), 3.48 (s, 3H), 2.0 (s, 3H), 1.4 (s, 9H); 13C NMR
(150.8 MHz, DMSO-d6, 90 °C) δ 166.5, 152.8, 149.3, 148.3, 139.2,
137.4, 132.3, 130.3, 129.04, 128.8, 128.3, 127.9, 127.2, 126.6, 124.4,
113.5, 71.7, 67.5, 66.9, 66.8, 61.1, 60.4, 55.8, 55.5, 54.8, 48.5, 48.3, 35.1,
33.9, 28.8, 24.5, 10.1; 19F NMR (300 MHz, DMSO-d6, 22 °C) δ 3.95
(br s), 3.93 (br s); HRMS (ESI) m/z calcd for C49H53F3N2O10Na
[MNaþ] 909.3545, found 909.3537.
(S)-Mosher Ester 18. A solution of (S)-(þ)-Mosher acid (14 mg,
0.060 mmol) in dry CH2Cl2 (1 mL) was combined with amino alcohol
16 (13 mg, 0.020 mmol), DCC (12 mg, 0.060 mmol), and DMAP (13
mg, 0.060 mmol). The mixture was stirred at room temperature for
18ꢀ20 h when the reaction was judged to be complete by TLC. The
reaction mixture was filtered through a cotton plug to remove N,N0-
dicyclohexylurea and roughly purified by flash chromatography (7:3
hexanes/EtOAc) to give 18 as a yellow oil (25 mg, 125%). Care was
taken to combine all fractions that contained Mosher ester. Rf 0.58 (7:3
hexanes/EtOAc); HPLC: C18 (3 ꢁ 250 mm) XBridge column,
gradient: 95% to 5% H2O in CH3CN over 45 min, flow rate
0.5 mL/min, UV detection at 214 nm, tR: 33.7 min, identified by
coinjection with 17; 1H NMR (600 MHz, DMSO-d6, 90 °C) δ 7.50 ꢀ
7.19 (15H), 7.04 (br s, 1H), 7.02 ꢀ 6.96 (m, 3H), 6.73 (m, 2H), 5.62
(d, J = 10.9 Hz, 1H), 5.12 (d, J = 11.8 Hz, 1H), 5.08 (d, J = 12.5 Hz,
1H), 4.98 (d, J = 12.1 Hz, 1H), 4.91 (d, J = 12.0 Hz, 1H), 4.70 (s, 1H),
4.51 (d, J = 16.6 Hz, 1H), 4.33 ꢀ 4.16 (m, 3H), 3.65 (s, 3H), 3.48 (s,
3H), 3.45 (s, 3H), 2.02 (s, 3H), 1.38 (s, 9H); 13C NMR (150.8 MHz,
DMSO-d6, 90 °C) δ 166.5, 152.8, 149.3, 148.4, 137.4, 132.4, 129.0,
128.8, 128.2, 127.9, 127.2, 126.6, 113.4, 66.8, 61.1, 60.4, 54.8, 48.4,
33.9, 28.8, 25.0, 10.1; 19F NMR (300 MHz, DMSO-d6, 22 °C) δ 3.80
(br s), 3.72 (br s); HRMS (ESI) m/z calcd for C49H53F3N2O10Na
[MNaþ] 909.3545, found 909.3537.
Alcohol 16. To a solution of acetonide 15 (15.9 g, 22.4 mmol) in
MeOH (580 mL) was added catalytic amount of p-TsOH H2O (318
3
mg, 1.84 mmol). The reaction was stirred at room temperature for 48 h
but was still incomplete according to TLC. The MeOH was evaporated
under reduced pressure, and the residue was partitioned between
saturated aq NaHCO3 (300 mL) and CH2Cl2 (2 ꢁ 200 mL). The
combined organic layers were dried over MgSO4 and concentrated to a
yellow oil. The crude product was purified by flash chromatography to
give 16 as a white foam (8.8 g, 71%, 80% based on recovered starting
material). Mp 48ꢀ49 °C; Rf 0.33 (3:2 hexanes/EtOAc); [R]D ꢀ38.1 (c
0.1 CHCl3); 1H NMR (600 MHz, DMSO-d6, 90 °C) δ 7.42ꢀ7.20 (m,
10H), 7.20ꢀ6.94 (br s, 2H), 6.94ꢀ6.88 (br s, 2H), 6.76ꢀ6.68 (br s,
2H), 5.78 (d, J = 10.3 Hz, 1H), 5.55 (br s, 1H), 5.11 (br s, 2H), 4.97 (d,
J = 12.1 Hz, 1H), 4.90 (d, J = 12.1 Hz, 1H), 4.53 (d, J = 16.9 Hz,1H), 4.32
(br s, 2H), 4.19 (d, J = 16.9 Hz,1H), 3.62 (s, 3H), 3.52 (s, 3H), 3.53 (br s,
Aldehyde 7. To a stirred mixture of alcohol 16 (8.80 g, 13.1 mmol)
in CH2Cl2 (70 mL) at room temperature was added DessꢀMartin
periodinane (8.35 g, 19.7 mmol). The reaction mixture was stirred at
room temperature for 1 h, when TLC analysis showed the reaction to be
complete. The reaction mixture was washed with a 1:1 mixture (v/v) of
1 N Na2S2O3 and saturated aq NaHCO3 (250 mL), water (100 mL), and
brine (100 mL). The organic layer was dried over MgSO4, filtered, and
concentrated. Flash chromatography (7:3 hexanes/EtOAc) gave 7 as a
yellow oil (7.42 g, 85%). Rf 0.48 (7:3 hexanes/EtOAc); [R]D ꢀ28.8
1
(c 1.82, CHCl3); H NMR (500 MHz, CDCl3, complex mixture of
rotomers, ONLY diagnostic signals are identified) δ 9.41 (br s, CHO),
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7.52ꢀ6.74 (16H), 5.91 (NH), 5.53 (d, J = 8.7 Hz), 4.48 (d, J = 16.1
Hz), 4.09 (d, J = 15.6 Hz), 3.79 (s, OMe), 3.58 (s, OMe), 2.14 (s, Me),
1.33 (s, t-Bu); 13C NMR (150.8 MHz, CDCl3) δ 198.7, 170.9, 155.5,
148.2, 136.3, 128.8, 128.7, 128.3, 128.2, 128.0, 127.6, 127.3, 112.3,
79.8, 71.3, 67.9, 64.9, 55.5, 50.3, 30.9, 28.7, 21.4, 19.3, 14.5, 13.9, 10.2;
HRMS (FAB) m/z calcd for C39H45N2O8 [MHþ] 669.3170, found
669.3189.
showed the reaction to be complete. The mixture was partitioned
between saturated NH4Cl solution (20 mL) and CH2Cl2 (3 ꢁ
20 mL). The combined organic layers were dried over MgSO4, filtered,
and concentrated. The crude product was purified by flash chromatog-
raphy (3:2 hexanes/EtOAc) to give 23 as a white solid (0.27 g, 77%
yield). Mp 79ꢀ81 °C; Rf 0.35 (2:1 hexanes/EtOAc); [R]D þ60.6 (c 1.8,
CHCl3); HPLC: C18 (3 ꢁ 250 mm) XBridge column, gradient, 80% to
20% H2O in CH3CN over 32 min, flow rate 0.5 mL/min, UV detection
at 254 nm, tR: 17.7 min; 1H NMR (600 MHz, DMSO-d6, 90 °C, mixture
of rotamers, diagnostic signals are identified) δ 7.44 (m, 2H), 7.33 (m,
7H), 6.97ꢀ6.87 (4H), 6.62 (s, 2H), 5.78 (d, J = 10.7 Hz, 1H), 5.12 (d,
J = 10.9 Hz, 1H), 5.02 (dd, J = 12.0 Hz, 2H), 4.48 (m, 1H), 4.37 (t, J = 8.6
Hz, 1H), 4.06ꢀ3.78 (m, 1H), 3.78ꢀ3.68 (m, 4H), 3.67ꢀ3.60 (m, 6H),
3.53 (d, 2H), 3.50ꢀ3.40 (m, 1H), 3.23 (m, 1H), 2.39 (d, J = 6.9 Hz, 1H),
2.26ꢀ2.15 (m, 1H), 2.10ꢀ1.94 (5H), 1.94ꢀ1.76 (3H), 1.42 (s, 9H),
1.14 (s, 3H), 0.98 (s, 3H); 13C NMR (150.7 MHz, DMSO-d6, 90 °C) δ
171.8, 157.2, 153.5, 148.1, 138.0, 129.2, 128.9, 128.7, 128.3, 128.1, 127.6,
71.6, 67.1, 65.5, 60.4, 53.3, 52.3, 48.7, 48.1, 45.2, 41.3, 41.2, 40.9, 40.7,
40.5, 40.3, 38.6, 32.8, 29.1, 28.9, 26.7, 21.1, 20.2, 10.0; HRMS (ESI) m/z
for C55H68N4O12S [MHþ] calcd 1009.4627, found 1009.4601.
Methyl Ester 22. To a stirring solution of 23 (65 mg, 0.06 mmol) in
dry MeOH (0.5 mL) was added Sm(OTf)3 (40 mg, 0.069 mmol). The
reaction mixture was purged with argon and stirred at room temperature
for 1 h when TLC analysis showed the reaction to be complete. The
reaction mixture was concentrated, diluted with 15 mL of EtOAc, and
washed with saturated NaCl solution (15 mL) and saturated NaHCO3
solution (15 mL). The organic phase was dried over MgSO4, filtered,
and concentrated. Flash chromatography (7:3 hexanes/EtOAc) af-
forded 22 as a colorless oil (0.176 g, 71%). Rf 0.35 (1:1 EtOAc/hexanes);
HPLC: C18 (3 ꢁ 250 mm) XBridge column, gradient: 95% to 5% H2O
in CH3CN over 45 min, flow rate 0.5 mL/min, UV detection at 214 nm,
tR: 31.5 min, identified by coinjection with (21 þ 22); 1H NMR (600
MHz, CDCl3, complex mixture of rotamers, ONLY diagnostic signals
are identified) δ 7.55 - 6.59 (16H), 5.91 (d, J = 10.8 Hz), 5.82 (d, J = 10.3
Hz, 1H), 5.40 (d, J = 12.6 Hz,), 5.20 (dd, J = 12.3, 23.0 Hz 1H), 5.06 (d,
J = 13.9 Hz), 5.01 (d, J = 13.9 Hz), 4.88 (d, J = 11.9 Hz), 4.80 (d, J = 16.2
Hz), 4.61 (d, J = 17.0 Hz), 4.59 ꢀ 4.50 (m, 1H), 4.16 (d, J = 16.1 Hz),
4.03 (d, J = 16.3 Hz), 3.84 (t, J = 8.9 Hz 1H), 3.80 (s, OMe), 3.77 (s,
OMe), 3.69 (s, OMe), 3.59 (s, OMe), 3.47 (dd, J = 8.3, 5.7 Hz, 1H), 3.01
(m, 1H), 2.39 (m, 1H), 2.25 (m, 1H), 2.08 (s, aromatic Me), 2.01 (s,
aromatic Me), 1.48 (s, Boc), 1.42 (s, Boc); 13C NMR (150.8 MHz,
CDCl3) δ 174.8, 174.6, 171.9, 171.7, 157.5, 157.0, 156.0, 155.9, 148.7,
148.2, 139.1, 138.9, 137.3, 136.8, 128.8, 128.7, 128.6, 128.4, 128.1, 128.0,
127.9, 127.8, 127.7, 127.6, 127.5, 127.4, 126.9, 79.3, 78.4, 71.1, 67.5, 61.0,
60.5, 59.5, 59.4, 52.6, 51.2, 47.3, 47.0, 32.4, 32.3, 29.9, 28.7, 28.6, 9.9, 9.8;
HRMS (ESI) m/z calcd for C46H55N3O11 [MHþ] 826. 3915, found
826.3916.
Cycloadducts 6 þ 19. Aldehyde 7 (7.40 g, 11.1 mmol), (1S)-
glycylsultam 8 (6.03 g, 22.1 mmol), and AgOAc (190 mg, 1.1 mmol)
were combined in methyl acrylate (40.0 mL, 444 mmol), and the
reaction mixture was stirred at room temperature for 2.5 h when TLC
analysis showed the reaction to be complete. The mixture was parti-
tioned between saturated aq NH4Cl (100 mL) and CH2Cl2 (3 ꢁ
100 mL). The combined organic layers were dried over MgSO4, filtered,
and concentrated to yield a yellow oil. The crude product was purified by
flash chromatography (3:2 EtOAc/hexanes) to yield an inseparable 4:1
mixture of 6 þ 19 as a white solid (8.14 g, 73%). Mp 79ꢀ81 °C; Rf 0.35
(2:1 hexanes/EtOAc); [R]D ꢀ62.6 (c 1.73, CHCl3); HPLC: C18 (3 ꢁ
250 mm) XBridge column, gradient: 80% to 20% H2O in CH3CN over
32 min, flow rate 0.5 mL/min, UV detection at 254 nm, tR: 18.8 min (19)
and 20.7 min (6) in a ratio of 1:4; 1H NMR (500 MHz, C6D6, 70 °C,
complex mixture of diastereomers and rotamers, diagnostic signals are
identified) δ 7.48 (d, J = 7.2 Hz, 2H), 7.31 (s, 1H), 7.24 (t, J = 7.4 Hz,
2H), 7.18ꢀ6.88 (aromatic protons, 11H), 6.27 (br s), 5.99 (d, J = 8.1
Hz), 5.14 (d, J = 12.1 Hz), 5.07 (d, J = 12.1 Hz), 4.94 (br s, 1H), 4.67 (br
s), 4.45ꢀ4.41 (m), 4.46 (br t, J = 3.0 Hz, 1H), 4.37 (d, J = 16.0 Hz, 1H),
4.34 (d, J = 16.0 Hz, 1H), 3.72 (br s, 3H), 3.69 (s, 3H), 3.62 (br s, 3H),
3.49 (s, 1H), 3.18 (1H), 2.80 (m þ d, J = 13.3 Hz, 2H), 2.71 (d, J = 13.9
Hz, 1H), 2.42 (br s, 1H), 2.16 (m, 1H), 2.12 (s, 3H), 1.96 (d, J = 14.2 Hz,
1H), 1.76 (dd, J = 13.8, 7.8 Hz, 1H), 1.41 (s, 9H), 1.35ꢀ1.28 (m, 2H),
1.14 (br t, J = 12.0 Hz, 1H), 0.94 (s, 3H), 0.76 (br t, J = 11.5 Hz, 1H),
0.65 (br t, J = 11.3 Hz, 1H), 0.43 (s, 3H); 13C NMR (125.7 MHz, C6D6,
70 °C) δ 174.8, 171.3, 157.8, 155.9, 151.8, 147.6, 138.0, 137.2, 136.4,
128.4, 128.3, 127.8, 127.6, 127.5, 127.4, 127.0, 126.2, 125.3, 111.6, 78.9,
70.6, 67.3, 64.6, 61.0, 60.3, 60.1, 52.9, 52.1, 48.6, 48.5, 47.8, 44.5, 44.4,
38.1, 36.6, 32.6, 28.3, 26.5, 20.8, 20.6, 19.8, 9.8 HRMS (ESI) m/z (%) for
C55H68N4O12S [MHþ] calcd 1009.4627, found 1009.4601.
Methyl Esters 21 þ 22. To a solution of 6 þ 19 (200 mg, 0.198
mmol) in dry MeOH (2.0 mL) was added Sm(OTf)3 (118 mg, 0.198
mmol). The turbid yellow reaction mixture was purged with argon and
stirred for 1 h at room temperature. The mixture was partitioned
between saturated NH4Cl (30 mL) and EtOAc (3 ꢁ 30 mL). The
combined organic layers were dried over MgSO4 and concentrated. The
crude product was purified by flash chromatography (7:3 hexanes/
EtOAc) to afford an inseparable mixture of 21 þ 22 as a yellow oil (97
mg, 62%). Rf 0.41 (1:1 EtOAc/hexanes); HPLC: C18 (3 ꢁ 250 mm)
XBridge column, gradient, 95% to 5% H2O in CH3CN over 45 min, flow
rate 0.5 mL/min, UV detection at 214 nm, tR: 31.5 min (22) and 32.3
Bicyclic Lactam 24. To a stirred solution of cycloadducts 6 þ19
(5.30 g, 5.25 mmol) in MeOH (500 mL) was added PdꢀC (1.3 g)
portion-wise under argon. The reaction flask was then purged with H2
gas and stirred at rt under hydrogen (balloon) for 60 h when TLC
analysis showed the reaction to be complete. The mixture was filtered
through a pad of Celite 545 and concentrated. Purification of this residue
by flash chromatography yielded 24 as a white solid (2.0 g, 57%). Mp
180ꢀ182 °C; Rf 0.37 (19:1 CH2Cl2/MeOH); [R]D ꢀ78.1 (c 1.56,
CHCl3); 1H NMR (500 MHz, CDCl3, mixture of rotamers, diagnostic
signals are identified) δ 6.81 (s, 1H), 5.85 (s, 1H), 5.74 (d, J = 10.0 Hz,
1H), 4.98 (s, 1H), 4.41 (t, J = 6.8 Hz, 2H), 4.10 (t, J = 8.3 Hz 1H), 3.89
(dd, J = 7.0, 5.1 Hz, 1H), 3.79ꢀ3.68 (m, 1H), 3.76 (s, 3H), 3.74 (s, 3H),
3.53ꢀ3.42 (m, 2H), 3.06ꢀ2.95 (m, 1H), 2.66ꢀ2.56 (m, 1H), 2.40
(dt, J = 13.0, 6.3 Hz, 1H), 2.23 (s, 3H), 2.05 (dd, J = 13.8, 7.9 Hz, 1H),
1.91 (br s, 3H), 1.43 (t, J = 9.8 Hz, 1H), 1.33 (s, 9H), 1.23 (s, 3H), 1.10
(s, 2H), 0.98 (s, 3H); 13C NMR (125.7 MHz, CDCl3) δ 173.4, 155.9,
149.50, 145.9, 145.6, 125.7, 124.6, 111.2, 79.5, 65.1, 61.3, 60.6, 60.4, 58.5,
1
min (21) in a ratio of 1:3); H NMR (600 MHz, CDCl3, complex
mixture of rotomers, ONLY diagnostic signals are identified) δ 3.80 (s,
minor OMe), 3.77 (s, minor OMe), 3.76 (s, major OMe), 3.72 (s, major
OMe), 3.70 (s, major OMe), 3.69 (s, minor OMe), 3.67 (s, major OMe),
3.63 (s, major OMe), 3.60 (s, major OMe), 3.59 (s, minor OMe), 2.12
(s, major Me), 2.08 (s, minor Me), 2.05 (s, major Me), 2.01 (s, minor
Me), 1.46 (s, minor Boc), 1.41 (s, minor Boc), 1.32 (s, major Boc); 13C
NMR (150.8 MHz, DMSO-d6, 90 °C) δ 175.7, 172.3, 155.9, 153.8,
149.4, 148.4, 139.9, 129.4, 129.2, 128.4, 128.7, 128.6, 127.9, 127.6, 126.6,
126.1, 125.4, 113.8, 78.8, 71.9, 61.4, 60.7, 59.9, 59.3, 55.6, 52.5, 48.4,
32.9, 29.3, 10.4; HRMS (ESI) m/z calcd for C46H55N3O11 [MHþ] 826.
3915, found 826.3916.
Cycloadduct 23. Aldehyde 7 (0.30 g, 0.35 mmol), (1R)-glycylsul-
tam ent-8 (0.19 g, 0.69 mmol) and AgOAc (9 mg, 10 mol %) were
combined in methyl acrylate (9, 2.1 mL, 240 mmol), and the reaction
mixture was stirred at room temperature for 4 h when TLC analysis
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53.8, 52.9, 51.6, 51.2, 48.7, 47.8, 44.5, 43.4, 37.9, 33.9, 32.6, 29.2, 28.1,
27.9, 26.5, 20.7, 19.9, 9.9; HRMS (ESI) m/z for C32H47N4O9S [MHþ]
calcd 663.3058, found 663.3088.
73.4, 67.7, 65.0, 61.7, 61.6, 60.0, 59.8, 59.3, 59.2, 52.8, 52.7, 52.4, 52.2,
50.4, 49.5, 48.7, 48.6, 47.7, 47.6, 47.3 47.2, 44.5, 44.4, 41.1, 40.2, 37.8,
36.6, 32.6, 26.8, 21.0, 20.8, 19.8, 9.8; HRMS (FAB) m/z for
C44H53N4O10S [MHþ] calcd 829.3482, found 829.3480.
A small amount of byproduct iv was also isolated as a white solid
(8%). Rf 0.31 (19:1 CH2Cl2/MeOH); mp 140ꢀ143 °C; [R]D ꢀ9.6 (c
0.9, CHCl3); 1H NMR (600 MHz, DMSO-d6, 90 °C) δ 8.36 (s, 1H),
6.68 (s, 1H), 5.63 (br s, 1H), 4.78 (s, 1H), 3.76 (s, 4H), 3.67 (s, 3H),
3.63 (s, 1H), 3.58 (s, 3H), 3.41 (br s, 1H), 3.35 (m, 1H), 2.36 (br t, J =
11.7 Hz, 1H), 2.10 (s, 3H), 1.22 (br s, 9H); 13C NMR (125.7 MHz,
CDCl3) δ 178.7, 171.2, 154.6, 149.1, 145.6, 145.3, 127.7, 124.6, 111.5,
110.3, 79.2, 77.2, 77.0, 76.7, 64.0, 62.3, 61.2, 60.4, 53.4, 52.4, 50.8, 50.6,
48.2, 29.0, 28.1, 27.7, 10.1; HRMS (ESI) m/z for C23H34N3O8 [MH]þ
calcd 480.2340, found 480.2382.
Compound 28. To a stirred solution of the phenol 27 (1.80 g, 2.17
mmol) and K2CO3 (0.90 g, 3.3 mmol) in DMF (38 mL) was added BnBr
(0.39 mL, 3.3 mmol), and the resulting mixture was stirred at 50 °C for
3 h. The reaction mixture was partitioned between water (100 mL) and
EtOAc (3 ꢁ 100 mL). The organic layers were combined, dried over
MgSO4, filtered, and concentrated under reduced pressure. Flash
chromatography of this residue yielded pure 28 (1.62 g, 81%, 92%
based on recovered starting material). Rf 0.32 (3:1 EtOAc/hexanes);
1
[R]D ꢀ32.4 (c 2.30, CHCl3); H NMR (500 MHz, CDCl3, complex
mixture of conformers, ONLY diagnostic signals are reported) δ
7.46ꢀ7.15 (15H), 5.29 (s, NH), 5.27 (s, NH), 5.03 (d, J = 11.3 Hz),
4.81 (d, J = 11.6 Hz), 3.79 (s, major OMe), 3.75 (s, minor OMe), 3.74 (s,
major OMe), 2.54 (q, J = 11.8 Hz), 2.20 (s, aromatic Me), 1.02 (s, minor
auxiliary Me), 0.96 (s, major auxiliary Me), 0.90 (s, minor auxiliary Me),
0.87 (s, major auxiliary Me); 13C NMR (125.7 MHz, CDCl3) δ 170.9,
170.6, 170.2, 154.5, 154.3, 152.8, 152.7, 152.5, 145.8, 145.8, 139.3, 139.2,
137.5, 137.4, 136.3, 136.2, 128.8, 128.5, 128.4, 128.4, 128.2, 128.1, 127.9,
127.9, 126.8, 126.8, 124.0, 123.9, 123.9, 77.2, 76.1, 75.9, 74.4, 72.5, 67.6,
67.5, 65.1, 64.9, 61.5, 61.4, 60.4, 60.1, 60.0, 59.5, 59.4, 53.4, 52.8, 52.7,
50.8, 49.9, 48.7, 48.6, 47.9, 47.9, 47.6, 47.5, 44.4, 44.3, 41.3, 40.3, 37.8,
37.7, 34.8, 33.8, 32.6, 32.5, 29.7, 26.2, 20.9, 20.7, 19.8, 19.7, 9.8; HRMS
(FAB) m/z for C51H59N4O10S [MHþ] calcd 919.3946, found 919.3964.
Tetracyclic Alcohol 29. To a stirred mixture of LiAlH4 (159 mg,
4.20 mmol) in dry THF (7 mL) was added a solution of 28 (1.54 g, 1.68
mmol) in dry THF (10 mL) at 0 °C, and the reaction mixture was
allowed to stir for 1 h. The mixture was diluted with aq saturated NH4Cl
(50 mL) and extracted with CH2Cl2 (3 ꢁ 50 mL). The combined
organic layers were dried over MgSO4, filtered, and concentrated under
reduced pressure to give a yellowish residue. Flash chromatography
(95:5 CH2Cl2/MeOH) furnished pure 29 as yellow solid (619 mg,
61%). Mp 69ꢀ71 °C; Rf = 0.31 (9:1 CH2Cl2/MeOH þ 1% AcOH);
[R]D þ35.6 (c 1.47, CHCl3); 1H NMR (500 MHz, CDCl3) δ
7.43ꢀ7.22 (m, 10H), 5.21 (s, 1H), 5.05 (d, J = 10.9 Hz, 1H), 4.84 (d,
J = 10.9 Hz, 1H), 4.52 (s, 1H), 4.49 (d, J = 11.9 Hz, 1H), 4.45 (d, J = 11.9
Hz, 1H), 4.43 (d, J = 5.8 Hz, 1H), 4.23 (dd, J = 8.3, 2.2 Hz, 1H), 3.82 (s,
3H), 3.77 (s, 3H), 3.70 (dd, J = 11.3, 4.0 Hz, 1H), 3.62 (dd, J = 11.3, 2.7
Hz, 1H), 3.32 (t, J = 8.4 Hz, 1H), 3.15 (dd, J = 10.7, 4.1 Hz, 1H),
3.11ꢀ3.12 (m, 1H), 2.96ꢀ2.88 (m, 1H), 2.75ꢀ2.66 (m, 1H), 2.40 (s,
3H), 2.31ꢀ2.24 (m, 2H), 2.22 (s, 3H); 13C NMR (125.7 MHz, CDCl3)
δ 174, 152.8, 152.4, 145.9, 138.7, 137.4, 128.4, 128.3, 127.9, 127.6, 127.3,
124.9, 124.1, 75.3, 74.4, 73.3, 66.6, 66.2, 62.1, 61.7, 60.4, 53.8, 49.9, 48.2,
39.6, 39.5, 31.8, 9.7; HRMS (FAB) m/z for C34H42N3O6 [MHþ] calcd
588.3074, found 588. 3089.
Pentacyclic Nitrile 30. To a cooled solution (ꢀ60 °C) of oxalyl
chloride (77 μL, 0.88 mmol) in dry CH2Cl2 (2 mL) was added dry
DMSO (85 μL, 1.20 mmol) via a syringe, and the resulting mixture was
stirred for about 5 min. The alcohol 29 (235 mg, 0.407 mmol) in
CH2Cl2 (3 mL) was added slowly over 2 min and stirred for 1 h at this
temperature. Triethylamine (281 μL, 2.00 mmol) was added slowly, and
the reaction was stirred at ꢀ60 °C for an additional 15 min at which
point it was allowed to warm to room temperature. The reaction mixture
was diluted with CH2Cl2 (50 mL), washed with brine (50 mL), dried
over MgSO4, and concentrated under reduced pressure. The residue was
dissolved in 4 mL of CH2Cl2, and to this solution was added a 0.5 N
solution of ZnCl2 in THF (1.0 mL, 0.5 mmol) and TMSCN (100 μL, 0.8
mmol) sequentially. The resulting mixture was stirred at room tempera-
ture for 2 h. The reaction mixture was partitioned between water (20 mL)
and CH2Cl2 (3 ꢁ 40 mL). The combined organic phases were dried over
MgSO4, filtered, and concentrated under reduced pressure. Flash chroma-
tography (95:5 CH2Cl2/MeOH) gave pure 30 (107 mg, 45%). Rf = 0.4
Cbz-Protected Pyrrolidine 25. To a stirred solution of the
bicyclic lactam 24 (2.00 g, 3.02 mmol) in dry THF (250 mL) at 0 °C
was added diisopropylethylamine (0.69 mL, 3.9 mmol) followed by
CbzCl (0.45 mL, 3.2 mmol). The mixture was stirred for 2 h at 0 °C
when TLC analysis showed the reaction to be complete. Saturated
NH4Cl solution (100 mL) was added, and the mixture was extracted
with CH2Cl2 (3 ꢁ 100 mL). The combined organic layers were
dried over MgSO4, filtered, and concentrated under reduced pressure
to yield a white solid. Purification by flash chromatography gave 25 as a
white solid (2.04 g, 85%). Mp 203ꢀ205 °C; Rf 0.59 (19:1 CH2CL2/
1
MeOH); [R]D ꢀ74.6 (c 1.54, CHCl3); H NMR (500 MHz, CDCl3,
complex mixture of conformers, ONLY diagnostic signals are reported)
δ 7.45ꢀ7.00 (6H), 6.85 (s, NH), 6.83 (s, NH), 3.80 (s, minor OMe),
3.78 (s, minor OMe), 3.76 (s, major OMe), 3.71 (s, major OMe), 2.24
(s, major aromatic Me), 2.17 (s, minor aromatic Me), 1.33 (s, major Boc
Me), 1.29 (s, minor Boc Me), 1.11 (s, major auxiliary Me), 0.99 (s, major
auxiliary Me), 0.98 (s, minor auxiliary Me), 0.96 (s, minor auxiliary Me);
13C NMR (125 MHz, CDCl3) δ 172.0, 170.9, 155.6, 154.0, 149.4, 145.7,
136.2, 129.0, 128.3, 128.2, 128.1, 127.8, 127.7, 126.0, 124.3, 111.5, 110.7,
78.6, 70.6, 67.7, 65.0, 61.3, 60.8, 60.6, 58.8, 58.6, 57.0, 53.4, 52.9, 52.1,
50.8, 50.6, 49.1, 47.8, 44.5, 42.4, 41.2, 37.6, 32.8, 32.7, 29.5, 28.2, 28.1,
27.8, 26.5, 20.8, 20.6, 20.0, 10.1, 9.8; HRMS (FAB) m/z for
C40H53N4O11S [MHþ] calcd 797.3426, found 797.3433.
PictetꢀSpengler Product 27. To a stirred mixture of the bicyclic
lactam 25 (2.0 g, 2.5 mmol) in CH2Cl2 (68 mL) was added TFA
(3.87 mL, 50.2 mmol) at 0 °C. The reaction was allowed to warm to
room temperature over 5 h when TLC analysis showed the reaction to
be complete. The solvent was removed under reduced pressure, and
diethyl ether was added to the residue to precipitate the TFA salt of 26.
The salt was collected and dissolved in CH2Cl2 (12 mL). Acetic acid
(158 μL) and 4 Å MS were added sequentially, and the resulting mixture
was degassed via three freezeꢀpumpꢀthaw cycles with argon. A
solution of benzyloxyacetaldehyde (415 mg, 2.51 mmol, freshly pre-
pared by DessꢀMartin oxidation of 2-benzyloxyethanol) in CH2Cl2
(6 mL) was added in portions over 2 h at room temperature when TLC
analysis showed the reaction to be complete. The mixture was filtered to
remove the molecular sieves and then partitioned between saturated aq
NaHCO3 (100 mL) and CH2Cl2 (2 ꢁ 100 mL). The combined organic
layers were dried over MgSO4, filtered, and concentrated under reduced
pressure to give a yellow oil. The residue was purified by flash
chromatography (3:1 EtOAc/hexanes) to afford compound 27 (1.76
g, 86%) as a yellow solid. Rf 0.22 (3:1 EtOAc/hexanes); [R]D ꢀ17.5 (c
0.96, CHCl3); 1H NMR (500 MHz, CDCl3, complex mixture of
conformers, ONLY diagnostic signals are reported) δ 8.23 (s, NH),
8.01 (s, NH), 5.43 (br s), 5.26 (d, J = 12.5 Hz), 5.13 (d, J = 12.1 Hz), 5.08
(d, J = 12.2 Hz), 5.05 (d, J = 12.1 Hz), 3.79 (s, OMe), 3.70 (s, OMe),
3.44 (d, J = 13.9 Hz,), 3.32 (d, J = 13.7 Hz), 2.20 (s, aromatic Me), 1.07
(s, minor auxiliary Me), 1.01 (s, minor auxiliary Me), 0.96 (s, major
auxiliary Me), 0.93 (s, major auxiliary Me); 13C NMR (100 MHz,
CDCl3) δ 171.3, 170.9, 154.4, 154.0, 149.5, 144.3, 136.1, 136.0, 128.8,
128.5, 128.4, 128.2, 128.0, 127.9, 127.8, 127.7, 127.6, 123.5, 122.7, 122.6,
5292
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The Journal of Organic Chemistry
ARTICLE
(95:52 CH2Cl2/MeOH); [R]D = þ48.5 (c 0.69, CHCl3); 1H NMR (500
MHz, CDCl3) δ 7.51 (d, J = 7.1 Hz, 2H), 7.39 (t, J = 7.3 Hz, 2H),
7.36ꢀ7.23 (4H), 7.17 (d, J = 6.8 Hz, 2H), 5.18 (s, 1H), 5.11 (d, J = 10.7 Hz,
1H), 4.94 (d, J = 10.7 Hz, 1H), 4.63 (br s, 1H), 4.61 (br s, 1H), 4.51 (d, J =
7.6 Hz, 1H), 4.32 (s, 2H), 3.84 (s, 3H), 3.76ꢀ3.72 (m, 1H), 3.74 (s, 3H),
3.44 (t, J = 8.9 Hz, 1H), 3.39 (m, 1H), 3.29 (br s, 1H), 3.19ꢀ 3.10 (m, 2H),
2.65ꢀ2.52 (m, 1H), 2.39 (br s, 3H), 2.22 (s, 3H), 2.08 (dd, J = 13.2, 4.4 Hz,
1H); 13C NMR (75.5 MHz, CDCl3) δ 174.2, 152.6, 152.1, 145.3, 137.9,
136.9, 128.5, 128.3, 128.2, 128.2, 127.4, 127.4, 126.0, 124.9, 123.1, 118.0,
78.2, 74.9, 72.8, 63.9, 62.4, 61.9, 60.2, 57.9, 55.8, 53.3, 47.8, 41.4, 39.3, 30.5,
9.7; HRMS (FAB) m/z for C35H39N4O5 (MHþ) calcd 595.2915, found
595.2921.
Pentacyclic Imine 31. To a flask charged with Lawesson’s reagent
(0.11 g, 0.26 mmol) was added 30 (0.11 g, 0.18 mmol) in benzene
(5 mL). This mixture was heated under reflux for 1 h. After cooling to
room temperature, the mixture was partitioned between EtOAc and a
dilute aq solution of NaHCO3. The organic layer was dried over MgSO4,
filtered, and concentrated to a yellow residue. The residue was dissolved
in acetone (4 mL) and Raney nickel (washed three times with acetone)
was added. After stirring for 30 min, the reaction mixture was filtered
through a short plug of Celite and concentrated. Flash chromatography
(3:1 EtOAc/hexanes) yielded pure 31 as a yellow solid (63%). Mp
86ꢀ89 °C; Rf 0.36 (3:1 EtOAc/hexanes); [R]D þ27.5 (c 0.6, CHCl3);
1H NMR (500 MHz, CDCl3) δ 8.00 (t, J = 2.6 Hz, 1H), 7.54ꢀ7.50
(2H), 7.4ꢀ7.27 (5H), 7.20ꢀ7.15 (3H), 5.13 (d, J = 10.9 Hz, 1H), 4.95
(d, J = 10.7 Hz, 1H), 4.64ꢀ4.62 (m, 1H), 4.56 (d, J = 3.5 Hz, 1H), 4.47
(d, J = 8.6 Hz, 1H), 4.35 (d, J = 11.4 Hz, 1H), 4.18 (d, J = 11.9 Hz, 1H),
3.85 (s, 3H), 3.80 (s, 3H), 3.70 (dd, J = 9.5, 1.7 Hz, 1H), 3.37 (dd, J = 9.5,
8.0 Hz, 1H), 3.34ꢀ3.28 (m, 1H), 3.19ꢀ3.13 (m, 1H), 3.02 (t, J = 2.1 Hz,
1H), 2.84ꢀ2.77 (m, 1H), 2.38 (s, 3H), 2.37ꢀ2.32 (m, 1H), 2.24 (s, 3H),
1.66 (dd, J = 12.8, 5.3 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 174.8,
151.9, 151.6, 145.1, 138.1, 137.3, 128.5, 128.4, 128.3, 128.0, 127.5, 127.4,
125.7, 124.8, 118.3, 78.7, 74.8, 72.6, 62.8, 62.4, 61.8, 60.3, 57.8, 56.0,
55.6, 51.8, 41.3, 37.1, 29.7, 28.8, 9.4; HRMS (ESI) m/z for C35H39N4O4
[MH]þ calcd 579.2971, found 579. 2959.
3.45ꢀ3.42 (m, 1H), 3.40ꢀ3.38 (m, 1H), 3.24ꢀ3.22 (m, 1H), 3.04 (d, J =
2.92, Hz, 1H), 2.99ꢀ2.87 (m, 2H), 2.34 (s, 3H), 2.26 (dt, J= 12.5, 6.63 Hz,
1H), 2.24 (s, 3H), 1.80 (dd, J = 12.8, 6.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3) δ 148.6, 145.7, 142.5, 126.1, 122.3, 119.9, 117.8, 93.1, 62.6, 61.8,
61.7, 61.2, 60.8, 60.1, 57.3, 54.7, 54.0, 49.7, 49.5, 41.4, 35.3, 29.0, 9.9;
HRMS (FAB) m/z (%) for C23H31N4O5 [MHþ] calcd 443.2289, found
443.2299. (A tabulated comparison of our1H NMR data with that reported
by Fukuyama and Li19 is provided in the Supporting Information.)
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR data for
S
b
compounds 11, 12, 13, 14, 15, 16, 7, 17, 18, (6 þ 19), (21 þ
22), 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, and 33; HPLC data
for (6 þ 19), (21 þ 22), 22, and 23. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: ppg@wsu.edu.
Present Addresses
§Washington State University.
’ ACKNOWLEDGMENT
This research was supported by a grant from the National
Science Foundation (CHE-0553313) and the Washington State
University College of Sciences.
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1H), 7.41ꢀ7.12 (7H), 7.16ꢀ7.13 (m, 2H), 5.14 (d, J = 10.5 Hz, 1H), 4.91
(d, J = 10.5 Hz, 1H), 4.71 (s, 1H), 4.58 (d, J = 3.7 Hz, 1H), 4.56 (dd, J = 8.6,
1.58 Hz, 1H), 4.37 (d, J = 11.9 Hz, 1H), 4.21 (d, J = 11.9 Hz, 1H), 4.03 (t,
J = 8.9 Hz, 1H), 3.85 (s, 3H), 3.84ꢀ3.79 (m, 1H), 3.61 (s, 3H), 3.63ꢀ3.56
(3H), 3.35ꢀ3.30 (m, 1H), 3.23ꢀ3.21 (m, 1H), 3.16ꢀ3.13 (m, 1H), 2.92
(d, J = 2.7 Hz, 1H), 2.91ꢀ2.78 (2H), 2.37 (s, 3H), 2.26 (dt, J = 12.7, 6.5 Hz,
1H), 2.21 (s, 3H), 1.85 (dd, J = 12.6, 6.5 Hz, 1H); 13C NMR (100 MHz) δ
151.6, 151.5, 145.5, 138.4, 137.4, 128.6, 128.5, 128.2, 128.1, 127.6, 127.4,
127.2, 127.1, 123.7, 119.0, 94.2, 78.2, 74.9, 72.4, 62.8, 61.9, 61.6, 60.9, 60.2,
58.2, 55.3, 55.2, 50.7, 50.6, 41.4, 35.6, 29.7, 9.7; HRMS (FAB) m/z (%) for
C37H43N4O5 [MHþ] calcd 623.3228, found 623.3215.
Fukuyama’s Alcohol 33. To a ꢀ78 °C solution of 32 (13 mg,
0.021 mmol) in dry CH2Cl2 (1 mL) was slowly added BCl3 (140 μL,
0.13 mmol, 1.0 M solution in CH2Cl2). The resulting mixture was
allowed to stir at this temperature for 2 h. The mixture was diluted with
CH2Cl2 (10 mL) and poured into a dilute solution of NaHCO3 (10 mL).
The aqueous phase was extracted with CH2Cl2 (3 ꢁ 10 mL), and the
combined organic layers were dried over Na2SO4, filtered, and concentrated.
The residue was purified by preparative TLC (4:1 EtOAc/hexanes) to yield
33 (4.8 mg, 52%). Rf 0.16 (4:1 EtOAc/hexanes); [R]D ꢀ9.0 (c 0.48,
CHCl3); 1H NMR (400 MHz, CDCl3) δ 6.34 (br s, 1H), 5.74 (s, 1H), 4.80
(s, 1H), 4.39 (s, 1H), 3.97 (d, J = 3.32 Hz, 1H), 3.96ꢀ3.90 (m, 1H),
3.88ꢀ3.87 (m, 2H), 3.81 (s, 3H), 3.75ꢀ3.71 (m, 2H), 3.62 (s, 3H),
5293
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The Journal of Organic Chemistry
ARTICLE
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(35) Bromide 11 was prepared starting from commercially available
2,6-dimethoxytoluene in 5 steps (68% overall yield) via (1) Friedelꢀ
Crafts acylation with propionyl chloride, (2) pTsOH-catalyzed
BayerꢀVilliger oxidation with mCPBA, (3) aromatic bromination with
NBS, (4) saponification of the propionate with KOH, and (5) benzyla-
tion of the phenol with benzyl bromide. For detailed descriptions of the
first four steps, see: Marques, M. M.; Pichlmair, M. M. B.; Martin, H. J.;
M. B. J. Org. Chem. 1997, 62, 493.
(26) Preliminary communication: Kaniskan, H. U.; Garner, P. J. Am.
Chem. Soc. 2007, 129, 15460.
::
Mulzer, J. Synthesis 2002, 2766.
::
(36) Kaniskan, H. U. A Formal Total Synthesis of Bioxalomycin Beta
2. Ph.D. Dissertation, Case Western Reserve University, 2007.
(37) Evans, D. A.; Scheidt, K. A.; Downey, C. W. Org. Lett. 2001,
3, 3009. This reagent, originally used for the methanolysis of hindered
Evans’ N-acyloxazolidinone auxiliaries, is to be recommended for
removal of the camphorsultam auxiliary as well. Furthermore, when
the cycloadduct mixture 19 þ 20 was treated with the more conven-
tional Mg(OMe)2/MeOH reagent, the desired methanolysis was ac-
companied by nucleophilic acyl substitution of the Boc by the
pyrrolidine to give a bicyclic urea.
(27) Recent reviews of asymmetric azomethine ylide cycloadditions:
(a) Husinec, S.; Savic, V. Tetrahedron: Asymmetry 2005, 16, 2047.
(b) Pandey, G.; Banjeree, P.; Gadre, S. R. Chem. Rev. 2006, 106, 4484.
(c) Pellissier, H. Tetrahedron 2007, 63, 3235. (d) Stanley, L. M.; Sibi,
M. P. Chem. Rev. 2008, 108, 2887.
(28) The obvious choice for constructing a pyrrolidine such as iii
would involve the [3 þ 2] cycloaddition of an azomethine ylide ii as
exemplified below for the prototypical Grigg reaction (or its asymmetric
catalytic variants). However, prior to our methodological work in this
area, it was not generally possible to apply this chemistry to azomethine
ylides such as ii derived from aliphatic aldimines i capable of tautomer-
ization (R = C(sp3)-H). Only when this problem was solved could the
critical issue of controlling the pyrrolidine stereochemistry in the
presence of the chiral R group (see structure 6) be dealt with.
(38) When this reaction was performed on a larger scale, a small
amount of the product iv resulting from cyclization onto the N-acyl
sultam was also isolated. The absolute stereochemistry about the
pyrrolidine ring was not established.
(29) For an account describing our group’s development of the
asymmetric [C þ NC þ CC] coupling reaction, see: Garner, P.;
::
Kaniskan, H. U. Curr. Org. Synth. 2010, 7, 348. The N-acyl camphor-
sultam plays a critical role in this embodiment of an in situ azomethine
ylide generation/cycloaddition by both facilitating the underlying multi-
component reaction cascade and influencing the stereochemical out-
(39) Conducting this reaction at 50 °C (rather than the originally
reported 60 °C) minimized the formation of a byproduct resulting from
N-benzylation of the tetrahydroisoquinoline ring of 28.
(40) (a) Pallavicini, M.; Moroni, B.; Bolchi, C.; Cilia, A.; Clementi,
F.; Fumagalli, L.; Gotti, C.; Meneghetti, F.; Riganti, L.; Vistoli, G.; Valoti,
E. Bioorg. Med. Chem. Lett. 2006, 16, 5610. (b) McManus, H. A.;
Fleming, M. J.; Lautens, M. Angew. Chem., Int. Ed. 2007, 46, 433.
(41) Pelletier, S. W.; Nowacki, J.; Mody, N. V. Synth. Commun. 1979,
9, 201.
come of the [3 þ 2] cycloaddition in a predictable manner.
::
(30) Garner, P.; Kaniskan, H. U.; Hu, J.; Youngs, W. J.; Panzner, M.
Org. Lett. 2006, 8, 3647.
(31) For a very effective and scalable preparation of 8 via the
Delꢀepine reaction, see: Isleyen, A.; Gonsky, C.; Ronald, R. C.; Garner,
P. Synthesis 2009, 1261.
(32) Merino, P.; Lanaspa, A.; Merchan, F. L.; Tejero, T. Tetrahedron:
Asymmetry 1998, 9, 629.
(42) Imine 31 is unusually resistant to tautomerization that would
lead to a reactive enamine because its R-proton is forced to be
orthogonal to the CdN π-bond.
(43) Kahsai, A. W.; Cui, J.; Kaniskan, H. U.; Garner, P. P.; Fenteany,
::
G. J. Biol. Chem. 2008, 383, 24534.
(33) Serinal 10 was conveniently prepared using a modification
of Dondoni’s procedure: Dondoni, A.; Perrone, D. Org. Synth. 2000,
77, 64. In our procedure the final oxidation was performed with the
DessꢀMartin reagent: Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991,
113, 7277.
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dx.doi.org/10.1021/jo200553g |J. Org. Chem. 2011, 76, 5283–5294