Epi -, Epoxy-, and C2-modified bengamides: Synthesis and biological evaluation

Article abstract of DOI:10.1021/jo4003272

With the objective of investigating the influence of structural modifications of the polyketide chain of the bengamides upon their antitumoral activities, we targeted the preparation of bengamide E analogues with modification of the stereochemistry at C-2 and at C-3, the substituent at the C-2 position, and the presence of oxirane rings. For the synthesis of these analogues, a new synthetic method for asymmetric epoxidation, developed in our laboratories, was employed utilizing the chiral sulfonium salts 22 and 23. In order to access 2-epi-bengamide E from these epoxy amides, a synthetic methodology, developed by Miyashita, allowed an oxirane-ring-opening reaction with a double inversion of the configuration. Alternatively, an aldol reaction provided access to the same analogue in a shorter and more efficient manner. Finally, biological evaluation of all of these bengamide E analogues demonstrated that the polyketide chain is essential for the antitumor activity of these natural products, not being amenable to structural or configurational modifications.

Full text of DOI:10.1021/jo4003272

Article
pubs.acs.org/joc
Epi-, Epoxy‑, and C2-Modified Bengamides: Synthesis and Biological
Evaluation
Francisco Sarabia,* Francisca Martín-Galvez, Cristina García-Ruiz, Antonio Sanchez-Ruiz,†
́
́
and Carlos Vivar-García
́ ́
Department of Organic Chemistry, Faculty of Sciences, University of Malaga, Campus de Teatinos s/n 29071-Malaga, Spain
S
* Supporting Information
ABSTRACT: With the objective of investigating the influence of
structural modifications of the polyketide chain of the bengamides upon
their antitumoral activities, we targeted the preparation of bengamide E
analogues with modification of the stereochemistry at C-2 and at C-3,
the substituent at the C-2 position, and the presence of oxirane rings.
For the synthesis of these analogues, a new synthetic method for
asymmetric epoxidation, developed in our laboratories, was employed
utilizing the chiral sulfonium salts 22 and 23. In order to access 2-epi-
bengamide E from these epoxy amides, a synthetic methodology,
developed by Miyashita, allowed an oxirane-ring-opening reaction with a
double inversion of the configuration. Alternatively, an aldol reaction
provided access to the same analogue in a shorter and more efficient
manner. Finally, biological evaluation of all of these bengamide E
analogues demonstrated that the polyketide chain is essential for the
antitumor activity of these natural products, not being amenable to
structural or configurational modifications.
Curiously, we reported the synthesis of bengamide E′ (17)
before its discovery from natural sources.10 All of these
biological properties displayed by the bengamides, coupled with
their appealing molecular structures, explain the flurry of
activity directed toward their total synthesis11 and analogues
design.12 Among the most prominent analogues synthesized so
far, it is important to highlight compound LAF38912a,b and
other bengamide analogues modified at the caprolactam unit,
which exhibited cytotoxicities in the low nanomolar range and
improved solubilities in water with respect to that displayed by
the natural counterparts.12c,d On the other hand, the ability of
the bengamides of inhibiting methionine aminopeptidase of
mycobacterium tuberculosis has been exploited in the design of
new potential leads for tuberculosis treatment.13
As part of a research program engaged in the development of
new asymmetric methodologies of epoxidation, we recently
designed and synthesized a new class of chiral sulfonium salts,14
for example, 22 and 23, which haven proven to be efficient and
high-yielding tools for the asymmetric synthesis of epoxy
amides, types A and B (Scheme 1A). Encouraged by these
results, we decided to exploit these synthetic tools for the
synthesis of various bioactive compounds.15,16 Thus, on the
basis of our delineated synthetic strategy for bengamides
INTRODUCTION
■
The bengamides (1−21), a family of marine natural products
isolated from sponges of the Jaspidae family (Figure 1),1 have
elicited widespread interest in both biological and chemical
circles due to their prominent antitumor, antihelmintic, and
antibiotic properties.2 Interestingly, the bengamides were found
to bind the methionine aminopeptidases (MetAp1 and
MetAp2),3 enzymes responsible for the cleavage of the N-
terminal initiator methionine residue during protein synthesis.4
A similar mode of action is displayed by the antiangiogenic
agents fumagillin and ovalicin despite their structural differ-
ences.5 As a consequence of the inhibition of these enzymes,
there is a blockade of the cell cycle division of endothelial cells
at the G1 and G2 phases,6 as well as antiangiogenic effect in
epithelial cells.7 Additional biological studies demonstrated that
bengamide A altered the subcellular distribution of the proto-
oncogene c-Src, a substrate of both MetAp, that made it
possible to establish a link between these enzymes and
oncogenes involved in tumor growth.8 More recently, Crews
et al. discovered that the bengamides were capable of inhibiting
the nuclear factor κB (NF-κB).9 This inhibition ability render
bengamides as potential leads for the treatment of diseases
involving inflammation. In conjuction with this valuable finding,
Crews and co-workers isolated bengamide E (15) and two new
congeners of this family, bengamides E′ (17) and F′ (18), from
Myxobacteria virescens in the course of this investigation.
Received: February 12, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Strategy for Bengamides Based on
a
Chiral Sulfur Ylides
Figure 1. Molecular structures of bengamides.
(Scheme 1B), which relied on the construction of an oxirane
ring and subsequent opening to generate the C2/C3
system,17,18 we applied this novel asymmetric epoxidation for
the synthesis of bengamides.10 From epoxy amides 24 and 25,
prepared in good yields and excellent stereoselectivities
according to this new epoxidation methodology, we were able
to prepare bengamide E (15) and a wide array of analogues,
such as 26a−c, via olefin cross metathesis or via palladium-
mediated couplings for the stereoselective installation of the
substituent at the terminal olefinic position (Scheme 1C).
Despite all of the synthetic efforts by us and others directed
toward the bengamides and analogues thereof, little has been
reported regarding the effect of the polyketide chain toward
biological activity. Among the possible modifications of the
polyketide chain of the bengamides, we initially paid attention
to the stereochemistry of the two chiral centers at C2 and at C3
positions. On the other hand, we were intrigued with the
biological role of the methoxyl group at the C2 position, which
apparently is not involved in the coordination with the cobalt
ions present at the enzyme active site, as is the rest of the polyol
system. In addition, inspired by the mode of action of
fumagillin (27), depicted in figure 2, which is characterized
by irreversible inhibition of methionine aminopeptidase via a
nuclepohilic attack of a histidine residue onto the oxirane ring
of the natural product, we considered the introduction of
oxirane rings along the polyketide chain of the bengamides,
a
(A) Synthetic tools, (B) synthetic strategy, (C) synthesized
bengamides.
which could lead to a fumagillin-like interaction with
methionine aminopeptidase (Figure 2).
Consequently, in order to probe the effect of the stereo-
chemistry as well as the biological significance of the methoxyl
group and the presence of oxirane rings along the polyketide
chain on their cytotoxic potency, we targeted the bengamide E
analogues 28−35 and the epoxy derivatives 36−38 as potential
fumagillin-bengamide hybrids (Figure 3). To accomplish this
goal, we decided to extend our synthetic strategy utilizing chiral
sulfonium salts for their preparation.
During these synthetic studies, Zhou et at. reported the
synthesis of the 3,4-bis-epi-bengamide E (39),19 revealing that
these stereochemical changes resulted in a complete loss of
antitumor activity. Previously, Banwell and co-workers
described the synthesis of the enantiomer of bengamide E
(ent-15),20,21 which also resulted in a completely inactive
compound. More recently, during the preparation of the
present manuscript, Li and coworkes have described the
synthesis of the 2-epimer of bengamide E and the desmethyl
B
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Figure 3. Programmed modifications of bengamide E and targeted
analogues.
Figure 2. (A) Mode of action of fumagillin with methionine
aminopeptidases. (B) Epoxy bengamides as new potential fumagillin-
like inhibitors.
derivative 40 (Figure 4), together with a series of truncated
bengamides.22 In this study, Li et al. checked that whereas the
2-epimer and the truncated series of bengamides were
completely inactive against different tumor cell lines, the 2-
hydroxy derivative 40 retained certain cytotoxicity compared
with bengamide E.
RESULTS AND DISCUSSION
■
Synthesis of 2,3-Bis-epi- and 2-epi-Bengamide E. For
the synthesis of the 2,3-bis-epimer of bengamide E, compound
28, we utilized our synthetic strategy for bengamide E by use of
the chiral sulfonium salt 23 prepared from D-methionine. Thus,
starting from alcohol 41,18 its transformation into the aldehyde,
via Swern oxidation,23 was followed by reaction with sulfonium
salt 23, under the same conditions reported by us for
bengamide E. As a result, epoxy amide 42 was obtained in a
reasonable good yield over 2 steps and complete stereo-
selectivity. The synthesis continued with the reduction of epoxy
amide 42 to the corresponding epoxy alcohol 43 by treatment
with lithium triethylborohydride (Super-H)24 and then oxirane-
ring-opening reaction with MeOH in the presence of trimethyl
borate [(MeO)3B] and 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU),25 to provide the corresponding 2-methoxyl opened
Figure 4. Precedents in bengamide analogues modified at the
polyketide chain.
product 44 in 57% yield. Compound 44 was transformed into
the olefin cross metathesis precursor 47 without major
difficulties via the chemistry already described, involving
selective protections and deprotections of the primary and
secondary hydroxyl groups, oxidation, and amide coupling with
caprolactam 46. Having prepared compound 47, we proceeded
with the olefin cross metathesis reaction by treatment with 3-
methyl-1-butene in the presence of the second generation
C
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Hoveyda−Grubbs catalyst 48.26 Unfortunately, this reaction
did not work at all, resulting in only recovered starting material
and no detection of the desired compound 49. Even though the
olefin cross metathesis reaction was proven to be efficient for
installation of the terminal olefinic substituent in the
bengamide derivatives with the correct configuration at the
C-2 and C-3 positions, the failure for other bengamide
precursors, in particular the 2-C-alkyl analogues, as we reported
in our previous article,18 led to uncertainty as in the case of 47.
Consequently, as in previous cases, we sought to install the
terminal isopropyl substituent earlier in the synthesis. As
described in our previous work,18 we efficiently prepared
alcohol 50 via metathesis and then proceeded toward the
synthesis of the targeted 2,3-bis-epimer. The synthetic sequence
leading to the desired compound 49 was carried out without
issue, according to the same synthetic sequence as before for 47
and through compounds 51−54. Finally, the protecting groups
were removed in two steps, consisting of a TBAF treatment to
obtain 55, followed by acidic hydrolysis to afford the targeted
2,3-bis-epimer analogue of bengamide E, compound 28
(Scheme 2).
Scheme 2. Synthesis of 2,3-Bis-epi-Bengamide E (28)
As a comparison, we decided to assess the Sharpless
asymmetric epoxidation27 as an alternative methodology to
obtain epoxy alcohol 52. Toward this aim, the starting alcohol
50 was subjected again to a Swern oxidation, and the resulting
aldehyde was transformed into the α,β-unsaturated ester 56 by
reaction with the in situ ylide prepared from the phosphonium
salt as depicted in Scheme 3.28 The resulting α,β-unsaturated
ester, formed in 48% overall yield from 50, was then treated
with diisobutylaluminum hydride (DIBAL-H) to provide the
allylic alcohol 57 in 86% yield. Sharpless asymmetric
epoxidation of 57 by use of (+)-diethyl L-tartrate [(+)-DET]
afforded the corresponding epoxy alcohol 52 in 61% yield and
in excellent stereoselectivity (Scheme 3). The subsequent
balance of both linear sequences led us to conclude that the
sulfur ylide based methodology was more efficient when
compared to the Sharpless methodology, at least for this case.
The synthesis of the 2-epimer analogue of bengamide E by
means of the epoxide chemistry presents an important
stereochemical problem since a trans epoxide should deliver
an anti opened product. The required syn stereochemistry for
the 2-epimer would require either the generation of a cis
epoxide29 and subsequent oxirane opening or an opening
process from a trans epoxide via a substitution reaction with a
double inversion of configuration, thus resulting in retention of
configuration.30 Since the chemistry of amide-stabilized sulfur
ylides generate in all cases trans epoxides, we focused on the
possibility of undertaking an oxirane-ring-opening process
capable of delivering the syn-opened product. Recently,
Miyashita et al. described the opening of trans-γ,δ-epoxy-α,β-
unsaturated esters (compounds type A) with alkylborates
catalyzed by palladium (0) to yield the corresponding ring-
opened products with syn relative configuration (compounds
type B).31 This stereochemical result can be rationalized
according to Scheme 4.
functionality and stereochemistry, the next step was to remove
the α,β-unsaturated ester appendage. To achieve this trans-
formation, dihydroxylation of 60 by treatment with catalytic
osmium tetroxide33 afforded the corresponding 1,2-diol
product 61. Subsequently, oxidative cleavage of the 1,2-diol
with sodium periodate provided the aldehyde, which was
converted to the acid 62 by treatment with 2,2,6,6-tetramethyl-
1-piperidinyloxy (TEMPO) and (diacetoxyiodo)benzene
(BAIB).34 Coupling of acid 62 with lactam 46 under the
same conditions as before for 45 or 54 resulted in the
formation of the lactam derivative 63. Surprisingly, the
deprotection of the silyl group of 63 by treatment with
TBAF proceeded in a meager 5% yield for 64. Fortunately, the
silylether deprotection by treatment with HF.pyr resulted in a
satisfactory 85% yield for 64. This deprotection allowed a clear
path toward the synthesis of the targeted 2-epi-bengamide E.
To this end, compound 64 was selectively oxidized by the
action of TEMPO/BAIB35 and immediately treated with freshly
prepared methylene triphenylphosphorane to provide the
terminal alkene 65, albeit in a poor 46% overall yield. Finally,
Inspired by this reaction, we proceeded to extend it to our
synthetic endeavor. For this purpose, we prepared compound
59 via oxidation of epoxy alcohol 5832 followed by a Wittig
reaction. Then, 59 was subjected to the action of
trimethylborate in the presence of tetrakis(triphenyl-
phosphine)palladium (0) to obtain, in good yield and as a
only one diastereoisomer, the targeted syn opened product 60.
Having installed the C2/C3 system with the required
D
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Scheme 3. Sharpless Asymmetric Epoxidation for the
Synthesis of Epoxy Alcohol 52
Scheme 5. Synthesis of 2-epi-Bengamide E (29) via Epoxide
Chemistry
Scheme 4. Stereochemical Rationale of the Double Inversion
of Configuration of Substitution Reaction of Unsaturated
trans-Epoxy Esters
Scheme 6. Synthesis of 2-epi-Bengamide E via Aldol
Reaction
olefin cross metathesis under conventional conditions afforded
the corresponding trans disubstituted olefin 66 in a good 75%
yield, which was subjected to a final acetal deprotection step to
furnish 2-epi-bengamide E (29) in 76% yield (Scheme 5).
Taking into account that the construction of the epimer at
C2 of bengamide E through trans-epoxide chemistry required a
synthetic detour that made its synthesis too long for practical
supply of material, we considered an aldol reaction as a shorter
synthetic alternative. To secure the desired absolute and relative
configuration, the Evans methodology36 was selected for this
purpose.
Thus, aldol reaction of the (Z)-boron enolate of
oxazolidinone 67,37 prepared by reaction with dibutylboron
triflate (n-Bu2BOTf) and N,N-diisopropylethylamine (DIPEA),
with the aldehyde obtained from alcohol 50 provided the syn-
aldol product 68 as a single diastereoisomer in an excellent 84%
yield. Silylation of 68, followed by LiBH4 reduction of the
resulting silyl ether 69 provided alcohol 70 in very high yields.
Oxidation of alcohol 70 to the acid, followed by coupling with
46 by the action of (benzotriazol-1-yloxy)tris(dimethylamino)-
phosphonium hexafluorophosphate (BOP), provided the
precursor of 2-epi-bengamide E, compound 71. The depro-
tection steps were carried out without any difficulty to obtain 2-
epi-bengamide E (29) in a straightforward and efficient manner
(Scheme 6).
Synthesis of C2-Modified Bengamides. The synthesis of
the series of C2-modified bengamides (30−35) was achieved
via oxirane-ring-opening reactions of either epoxy alcohol 72
(compounds 30−32) or epoxy amide 73 (compounds 33−35)
as previously described by us elsewhere18 (Scheme 7).
E
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Scheme 7. Synthesis of C2-Modified Bengamides
Scheme 8. Synthesis of Epoxy and Diepoxy Bengamides 37,
38, 84, and 85
Synthesis of Epoxy Bengamides. For the synthesis of the
series of anhydro derivatives of bengamide E, we commenced
with the synthesis of the 2,3-epoxy analogue, compound 36,
which was achieved by acidic treatment of epoxy amide 73,
previously described by us.18 For the preparation of 4,5-epoxy
or 2,3:4,5-diepoxy analogues, a different strategy was required.
Thus, starting from allylic alcohol 74,38 its oxidation to the
corresponding α,β-unsaturated aldehyde, followed by the
reaction with the sulfonium salt 22 under basic conditions,
afforded the corresponding epoxy amide 75 in good overall
yield. Once the first oxirane ring was installed in a
stereoselective fashion, the construction of a second oxirane
group was undertaken via direct reduction of 75 to the epoxy
aldehyde by the action of sodium bis(2-methoxyethoxy)
aluminum hydride (Red-Al),39 followed by a second reaction
with sulfonium salt 22, according to our two-phase method,40
to obtain diepoxy amide 76 in an excellent 85% overall yield
from 75. Reduction of 76 with Super-H yielded the expected
diepoxy alcohol 77 which was considered a key product for the
consecution of both coveted epoxy analogues of bengamide E.
In a first set, diepoxy alcohol 77 was oxidized to the
corresponding diepoxy acid and coupled with lactam 46, as
previously described, to furnish the diepoxy bengamide E
analogue 38. On the other hand, 77 was treated with methanol
in the presence of B(OMe)3 and DBU to yield the
corresponding 2-methoxyl derivative 78 in both chemo- and
regioselective manners. Selective oxidation and coupling with
amino lactam 46 were carried out without problems to obtain
the desired 5,6-epoxy bengamide E 37 (Scheme 8). Given the
positive results obtained in the synthesis of the epoxy and
diepoxy bengamide analogues, we deemed it of interest to
extend this chemistry to the trimethyl silyl derivative 7941
because the presence of this trimethylsilyl moiety can serve as
an isostere for the isopropyl group found in the natural
bengamides, as well as a handle to expand the synthetic
possibilities for further structural modifications at the terminal
olefinic position in virtue of the reactivity of vinylsilanes.42
Thus, proceeding in a similar manner as for 74, 79 was
converted to epoxy and diepoxy bengamides 84 and 85,
through compounds 80−83, in similar, or even better yields
compared with the isopropyl series (Scheme 8).
proliferative potency. The determination of the cytotoxic
properties of all these compounds was performed by measuring
their IC50 values against a panel of different tumor cell lines
using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide (MTT) dye reduction assay.43 This cytotoxicity was
examined in four different cancer cell lines, namely, HL 60
(human promyelocytic leukemia), MDA-MB-231 (human
breast carcinoma), HT1080 (human fibrosarcoma), and
HT29 (human colon adenocarcinoma), and in a primary
culture of nontransformed bovine aorta endothelial (BAE)
cells. Bengamide E and fumagillin were used as controls to
compare the activity of the new synthesized analogues. The
results of this investigation are summarized in Table 1.44
Bengamide E (15) has previously been described to inhibit
the in vitro growth of a tumor cell line at micromolar
concentrations (3.3 μM against MDA-MB-435).2a A more
detailed characterization of its activity and selectivity profile,
however, is missing with the exceptions of the in vitro
antitumor studies carried out by Banwell20 and Li,22 who
reported activity against three tumor cell lines. The
antiproliferative activity obtained for bengamide E in our
biological assays are in agreement with the previously reported
cytotoxic activity, since IC50 values obtained for bengamide E in
the five cell types were in the low micromolar range. On the
Biological Evaluation of Bengamide E and Analogues.
Having prepared the analogues of bengamide E, compounds
28−38, 84, and 85, our next goal in this research was to
evaluate their antitumor properties to determine the influence
of the described structural modifications against the anti-
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Table 1. In Vitro Antitumor Activities (IC50, μM) of Bengamide E, Bengamide E Analogues 28−38 and 85, and Fumagillin (27)
a
against Different Tumor Cell Lines and BAEC
b
tumor cell lines
MDA-MB-
compound
435
A549 HCT116 HUVEC MCF-7 MDA-MB-231
HT29
HT1080
HL60
BAEC
c
d
e
f
bengamide E (15)
bengamide E (15)
bengamide E (15)
bengamide E (15)
3.3
1.9
0.6
0.3
6.71
9.02
3.36
1.64 0.54
100
0.95 0.16 0.29 0.03
89 >100
0.68 0.10 0.28 0.03
81 100
2,3-bis-epi-bengamide E
(28)
2-epi-bengamide E (29)
bengamide Eanalogue 30
bengamide E analogue 31
bengamide E analogue 32
bengamide E analogue 33
bengamide E analogue 34
bengamide E analogue 35
epoxy bengamide 36
100.2 13.3
100
85.2 17.0 93.2 8.1
60.3 15.1 69.7 3.2
100
>100
nd
68
75
>100
>100
>100
>100
>100
>100
nd
nd
>100
nd
nd
>100
89
nd
38.5 4.8
12.5 3.0
100
70.7 12.0 30.2 6.1
25.3 5.2
9.2 0.6
nd
28.9 7.2
4.1 1.3
100
10.6 1.2
79
5.8 0.1
>100
epoxy bengamide 37
100
100
>100
nd
100
diepoxy bengamide 38
100
100
>100
nd
100
TMS epoxy bengamide
85
100
100
>100
72
78
fumagillin
54.3 10.2
38.3 12.5 biphasic
curve
36 7.5
biphasic
curve
a
In vitro cytotoxicities were determined according to the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide dye reduction assay as
detailed in the Experimental Section. The IC50 values were obtained from semilogarithmic dose−response plots as the concentration of compound
b
yielding a 50% of cell survival. nd: not determined MDA-MB-435: human breast carcinoma. A549: non-small cell lung cancer. HCT116: colon
cancer cells. HUVEC: primary human umbilical vein endothelial cells. MCF-7: human breast adenocarcinoma. MDA-MB-231: human breast
carcinoma. HT29: human colon adenocarcinoma. HT1080: human fibrosarcoma. HL60: human promyelocytic leukemia. BAEC: nontransformed
c
d
e
f
bovine aorta endothelial cells. IC50 determined by Crews et al.2a IC50 determined by Banwell et al.20 IC50 determined by Li et al.22 IC50
determined in our laboratories.
other hand, fumagillin (27), a fungal metabolite that potently
inhibits angiogenesis by blocking endothelial cell proliferation
and has advanced into clinical trials for multiple cancers,
showed a biphasic effect on the growth of proliferating
endothelial cells. At lower concentrations there is a first
decrease in cell number, probably due to a cytostatic effect, and
after a plateau covering several orders of concentration, a
second cytotoxic effect is observed. These biphasic dose-
response curves, typical of fumagillin and its derivatives, were
obtained with the tumor cell lines studied, indicating that
fumagillin antiproliferative activity is not endothelial specific,
which is in agreement with previously reported data (Table
1).45
In contrast to fumagillin, bengamide E and its analogues
displayed well-defined cytotoxic activity in tumor cells as well as
in endothelial cells. This is in agreement with previous
observations indicating that inhibition of MetAp2 by the
bengamides does not result in selective inhibition of endothelial
cell proliferation.46 Dose−response curves obtained with
bengamide E and its analogues showed a sharp decrease in
cell survival at concentrations that are around the IC50. Thus, as
these IC50 values reflect, it was quite clear that configurational
changes in the polyketide chain led to a complete loss of
cytotoxic activity, over 2-fold compared with bengamide E.
These data, together with that reported by Zhou regarding the
biological activity of the 3,4-bis-epimer,19 reveal that
modifications of the configuration of the polyketide chain
severely impact the biological activity, demonstrating that the
configuration of the polyketide chain is essential for recognition
and binding to the biological target. Furthermore, it was
observed that the 2-epimer of bengamide E was slightly more
active than its 2,3-bis-epimer, indicating that the methoxyl
group plays an important role in the interaction with the active
site of the enzyme, although probably not to the degree of the
hydroxyl groups at C3, C4, and C5 positions, which seem to be
involved in the coordination with cobalt ions present at the
active site of the enzymes. Anyway, it is possible that the
modification of the configuration at the C2 position leads to a
distortion of the normal conformation of the bengamide
framework, thereby preventing the molecule from adopting the
required shape for binding to the methionine aminopeptidases.
In order to more fully comprehend the role of the methoxyl
group upon the antitumor potency, we resorted to the
biological evaluation of the analogues 30−35. The obtained
cytotoxicities for these compounds clearly showed that the
replacement of the methoxyl group with other functionalities
resulted in a severe effect on activity. Thus, whereas the
replacement of the methoxyl group by a N-methylamino system
(compound 35) led to a loss of activity of 10−20-fold with
respect to bengamide E, the substitution of this group by others
(compounds 30−34) resulted in a total loss of antitumor
activity. In a similar way, the epoxy and diepoxy bengamide E
analogues (36−38 and 85) were completely inactive in the
cytotoxic studies, indicating that the replacement of the 1,2-
hydroxyl systems by an oxirane ring produced a complete lack
of interaction with the active site of the enzymes, not resulting
in a fumagillin-like interaction as we initially surmised.
G
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HPLC, a C8 5 μm column (250 mm × 10.00 nm) was employed with
a flow rate of 4.7 mL/min.
CONCLUSIONS
■
In conclusion, we have described the synthesis of two
stereoisomers of bengamide E, the 2,3-bis-epi- and the 2-epi-
analogues, a collection of C2-modified analogues, and various
epoxy bengamides. Their syntheses were based on our synthetic
methodology of epoxide formation via chiral sulfur ylides.
Whereas this strategy proved to be efficient for the synthesis of
the 2,3-bis-epimer, the C2-modifed analogues, and the epoxy
and diepoxy bengamides, the synthesis of the 2-epimer by
means of the formation of trans epoxides was envisioned to be
more problematic. To overcome this problem, the Miyashita
methodology proved to be a valid and efficient method,
representing the first synthetic application of this strategy in the
synthesis of a bioactive compound. Alternatively, this 2-epimer
was also prepared via aldol reaction. The biological activities of
all of these compounds against a panel of different tumor cell
lines revealed that the stereochemistry at C2 and at C3
positions and the methoxyl group at C2 are essential for
retaining the cytotoxic potency. These biological findings are in
accordance with the proposed interaction of the bengamides
with methionine aminopeptidases in which the hydroxyl groups
at C-3, C-4, and C-5 positions are involved in coordination with
cobalt ions at the active site. Less clear is the importance of the
methoxyl group at the C-2 position. Nonetheless, the lack of
notable activity for either the 2-epi-bengamide E or the other
C2-modified analogues indicates that this methoxyl group plays
an important role in the binding of the compound to the active
site of methionine aminopeptidases. All together, we have
successfully demonstrated the utility and applicability of chiral
sulfonium salts for the synthesis of bengamide analogues
modified at the polyketide chain. In contrast, biological
evaluation indicated that the polyol system is not amenable
to modification due to the strong involvement that the
polyketide chain has in its binding with the active site of the
targeted enzymes. These results provide further support for the
limited tolerance of the bengamide pharmacophore and its
highly specific binding to the enzyme.
Biological Material and Methods. Cell culture media were
purchased from Grand Island (New York, USA) and Walkersville
(Maryland, USA). Fetal bovine serum (FBS) was a product from
Belton (U.K.). Supplements and other chemicals not listed in this
section were obtained from St. Louis (MO, USA). Plastics for cell
culture were supplied by a company from Roskilde (Denmark). Bovine
aortic endothelial (BAE) cells were obtained by collagenase digestion
and maintained in Dulbecco’s modified Eagle’s medium (DMEM)
containing glucose (1 g/L), glutamine (2 mM), penicillin (50 IU/mL),
streptomycin (50 μg/mL), and amphotericin (1.25 μg/mL)
supplemented with 10% FBS. All cancer cell lines used in this study
were obtained from the American Type Culture Collection (ATCC).
Human fibrosarcoma HT1080 cells were maintained in DMEM
containing glucose (4.5 g/L), glutamine (2 mM), penicillin (50 IU/
mL), streptomycin (50 μg/mL), and amphotericin (1.25 μg/mL)
supplemented with 10% FBS. Human colon adenocarcinoma HT29
cells were maintained in McCoy’s 5A medium containing glutamine (2
mM), penicillin (50 IU/mL), streptomycin (50 μg/mL), and
amphotericin (1.25 μg/mL) supplemented with 10% FBS. Human
breast cancer carcinoma MDA-MB-231 and human promyelocytic
leukemia HL60 cells were maintained in RPMI1640 medium
containing glutamine (2 mM), penicillin (50 IU/mL), streptomycin
(50 μg/mL), and amphotericin (1.25 μg/mL) supplemented with 10%
and 20% FBS, respectively.
Epoxy Amide 42. To a solution of sulfonium salt 23 (472 mg, 1.49
mmol, 1.1 equiv) in tBuOH (10 mL) was added a 3.0 M aqueous
NaOH solution (0.49 mL, 1.49 mmol, 1.1 equiv). After 15 min at 25
°C, a solution of crude aldehyde, obtained from alcohol 41 via Swern
oxidation (215 mg, 1.36 mmol, 1.0 equiv), in tBuOH (4 mL) was
added, and the crude reaction mixture was stirred overnight at 25 °C.
After this time, the crude was diluted with EtOAc. The resulting
organic solution was then sequentially washed with water and brine,
dried over anhydrous MgSO4, filtered, and concentrated. The resulting
crude was purified by flash column chromatography (silica gel, 20%
EtOAc in hexanes) to obtain epoxy amide 42 (366 mg, 72% over two
steps) as a yellow foam: Rf = 0.48 (silica gel, 20% EtOAc in hexanes);
1
[α]25D = −22.1 (c 0.2, CH2Cl2); H NMR (400 MHz, CDCl3) δ 1.40
(s, 3 H), 1.44 (s, 3H), 1.52 (s, 3 H), 1.64 (s, 3 H), 1.84−1.92 (m, 1
H), 2.01−2.09 (m, 1 H), 2.10 (s, 3 H), 2.42−2.50 (m, 1 H), 2.53−2.58
(m, 1 H), 3.32 (dd, J = 5.9, 1.9 Hz, 1 H), 3.52−3.54 (m, 1 H), 3.56 (d,
J = 1.9 Hz, 1 H), 3.91 (d, J = 9.2 Hz, 1 H), 4.01 (ddd, J = 9.2, 5.2, 1.3
Hz, 1 H), 4.27 (ddd, J = 10.3, 4.9, 3.4 Hz, 1 H), 4.42 (dd, J = 7.9, 6.9
Hz, 1 H), 5.29 (dt, J = 10.3, 1.0 Hz, 1 H), 5.46 (dt, J = 17.2, 1.1 Hz, 1
H), 5.86 (ddd, J = 17.1, 10.4, 6.7 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ 16.1, 23.3, 26.6, 27.1, 27.3, 31.2, 34.5, 52.1, 56.5, 57.4, 67.3,
80.1, 81.0, 96.3, 110.6, 119.6, 134.6, 163.3; HRMS (ESI-TOF) m/e
372.1852, M + H+ calcd for C18H29NO5S 372.1845.
EXPERIMENTAL SECTION
■
General Techniques. All reactions were carried out under argon
atmosphere with dry, freshly distilled solvents under anhydrous
conditions, unless otherwise noted. Tetrahydrofuran (THF) was
distilled from sodium benzophenone, and methylene chloride
(CH2Cl2) and benzene (PhH) from calcium hydride. Yields refer to
chromatoghraphically and spectroscopically (1H NMR) homogeneous
materials, unless otherwise stated. All solutions used in workup
procedures were saturated unless otherwise noted. All reagents were
purchased at highest commercial quality and used without further
purification unless otherwise stated. All reactions were monitored by
thin-layer chromatography carried out on 0.25 mm silica gel plates
(60F-254) using UV light as visualizing agent and 7% ethanolic
phosphomolybdic acid or p-anisaldehyde solution and heat as
developing agents. Silica gel (60, particle size 0.040−0.063 mm) was
used for flash column chromatography. Preparative thin-layer
chromatography (PTLC) separations were carried out on 0.25, 0.50,
or 1 mm silica gel plates (60F-254). NMR spectra were recorded on a
400 MHz instrument and calibrated using residual undeuterated
solvent as an internal reference. The following abbreviations were used
to explain the multiplicities: s, singlet; d, doublet; t, triplet; q, quartet;
m, multiplet; band, several overlapping signals; b, broad. Optical
rotations were recorded on a polarimeter. High resolution mass
spectra (HRMS) were recorded on an ESI-TOF mass spectrometer in
positive mode. Analytical and preparative HPLC were carried out in a
reversed phase using a reflection index detector. For preparative
Epoxy Alcohol 43. Epoxy amide 42 (70 mg, 0.188 mmol, 1.0
equiv) in THF (2.0 mL) was treated with Super-H (0.50 mL, 0.47
mmol, 2.5 equiv) at 0 °C. After 1 h at this temperature, the reaction
mixture was diluted with Et2O and washed with a saturated aqueous
NH4Cl solution. The aqueous phase was separated and extracted with
Et2O twice, and the combined organic phase was washed with water
and brine, dried over anhydrous MgSO4, and concentrated under
reduced pressure. Purification by flash column chromatography (silica
gel, 40% EtOAc in hexanes) of the obtained crude product provided
epoxy alcohol 43 (24 mg, 63%) as a yellow oil: Rf = 0.38 (silica gel,
1
50% EtOAc in hexanes); [α]25 = −8.5 (c 0.4, CH2Cl2); H NMR
D
(400 MHz, CDCl3) δ 1.42 (s, 3 H), 1.43 (s, 3 H), 3.05−3.17 (m, 2 H),
3.65 (dt, J = 8.3, 4.2 Hz, 1 H), 3.81 (dd, J = 12.4, 4.9 Hz, 1 H), 4.14
(dd, J = 12.4, 2.6 Hz, 1 H), 4.35 (t, J = 7.4 Hz, 1 H), 5.28 (d, J = 10.3
Hz, 1 H), 5.43 (d, J = 17.1 Hz, 1 H), 5.86 (ddd, J = 17.1, 10.3, 6.9 Hz,
1 H); 13C NMR (100 MHz, CDCl3) δ 27.1, 27.2, 54.9, 55.9, 62.9, 79.9,
80.5, 110.3, 119.4, 135.3; HRMS (ESI-TOF) m/e 201.1135, M + H+
calcd for C10H16O4 201.1127.
Diol 44. Epoxy alcohol 43 (24 mg, 0.12 mmol, 1.0 equiv) was
dissolved in a 1:1 mixture of MeOH/B(OMe)3 (3.0 mL), and the
resulting solution was treated with DBU (18 μL, 0.12 mmol, 1.0
H
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equiv) and heated at 70 °C for 12 h. After this time, the reaction
mixture was allowed to reach room temperature, cooled to 0 °C, and
then treated with a saturated aqueous NaHCO3 solution. After the
mixture was stirred for 30 min at 0 °C, EtOAc was added, and both
phases were separated. The aqueous phase was extracted with EtOAc,
and the combined organic extracts were washed with water and brine
and then dried over anhydrous MgSO4, and the solvent was
evaporated under reduced pressure. The resulting crude product was
purified by flash column chromatography (silica gel, 60% EtOAc in
hexanes) to afford diol 44 (16 mg, 57%) as a yellow oil: Rf = 0.21
(silica gel, 60% EtOAc in hexanes); [α]25D = −2.1 (c 1.7, CH2Cl2); 1H
NMR (400 MHz, CDCl3) δ 1.41 (s, 3 H), 1.43 (s, 3 H), 2.60 (bs, 2
H), 3.30−3.34 (m, 1 H), 3.38 (s, 3 H), 3.82 (dd, J = 12.1, 3.5 Hz, 1
H), 3.87 (dd, J = 12.1, 4.3 Hz, 1 H), 3.94 (dd, J = 7.7, 5.6 Hz, 1 H),
3.99 (t, J = 5.9 Hz, 1 H), 4.47 (dd, J = 7.5, 7.3 Hz, 1 H), 5.24−5.26 (m,
1 H), 5.41−5.45 (m, 1 H), 5.91 (ddd, J = 17.2, 10.2, 7.0 Hz, 1 H); 13C
NMR (100 MHz, CDCl3) δ 26.8, 27.1, 58.0, 60.8, 67.5, 78.6, 79.5,
81.6, 109.4, 119.5, 135.0; HRMS (ESI-TOF) m/e 233.1395, M + H+
calcd for C11H20O5 233.1389.
dissolved in DMF (1.0 mL) and treated with DIPEA (24 μL, 0.14
mmol, 2.0 equiv), L-Lys-lactam 46 (17 mg, 0.104 mmol, 1.5 equiv),
and BOP (38 mg, 0.08 mmol, 1.2 equiv) at 25 °C. After being stirred
at this temperature overnight, the crude mixture was diluted with Et2O
and washed with a saturated aqueous NH4Cl solution. The aqueous
phase was washed with Et2O, the combined organic phases were
washed with brine and then dried over anhydrous MgSO4, and the
solvent was evaporated under vacuum. Purification of the obtained
crude product by flash column chromatography (silica gel, 70% EtOAc
in hexanes) provided amide 47 (20 mg, 61% over two steps) as a
yellow foam: Rf = 0.30 (silica gel, 80% EtOAc in hexanes); [α]25
=
D
1
+6.5 (c 0.8, CH2Cl2); H NMR (400 MHz, CDCl3) δ 0.12 (s, 3 H),
0.17 (s, 3 H), 0.89 (s, 9 H), 1.34 (s, 3 H), 1.37 (s, 3 H), 1.41−1.51 (m,
2 H), 1.76−1.87 (m, 2 H), 1.97−2.08 (m, 2 H), 3.23−3.30 (m, 2 H),
3.52 (s, 3 H), 3.82 (d, J = 2.1 Hz, 1 H), 4.03 (dd, J = 7.1, 7.0 Hz, 1 H),
4.27 (dd, J = 7.2, 2.1 Hz, 1 H), 4.43 (ddt, J = 7.0, 5.4, 1.4 Hz, 1 H),
4.53 (ddd, J = 11.2, 6.7, 1.5 Hz, 1 H), 5.17 (dt, J = 10.5, 1.4 Hz, 1 H),
5.40 (dt, J = 17.2, 1.5 Hz, 1 H), 5.95 (ddd, J = 17.2, 10.5, 5.4 Hz, 1 H),
6.03−6.05 (m, 1 H), 7.76 (d, J = 6.7 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ −4.1, −3.7, 18.4, 26.4, 27.4, 27.5, 28.4, 29.3, 31.6, 42.6, 52.2,
60.3, 75.8, 79.8, 80.0, 85.0, 109.4, 116.7, 137.5, 169.1, 175.6; HRMS
(ESI-TOF) m/e 471.2892, M + H+ calcd for C23H42N2O6Si 471.2890.
Epoxy Amide 51. Epoxy amide 51 (725 mg, 67% over two steps)
was prepared from alcohol 50 (534 mg, 2.66 mmol) by a Swern
oxidation, followed by reaction with sulfonium salt 23 (1.01 g, 3.20
mmol, 1.20 equiv) according to the same procedure described above
Alcohol 45. Diol 44 (43 mg, 0.185 mmol, 1.0 equiv) was dissolved
in a 2:1 pyridine/CH2Cl2 (1.75 mL) and cooled to 0 °C. Pivaloyl
chloride (28 μL, 0.22 mmol, 1.2 equiv) was added, the reaction
mixture was stirred at this temperature for 90 min, then the mixture
was diluted with CH2Cl2, the organic phase was washed with a
saturated aqueous NaHCO3 solution and dried over MgSO4, and the
solvent was evaporated under reduced pressure. The resulting crude
was purified by flash column chromatography (silica gel, 20% EtOAc
in hexanes) to afford the corresponding pivaloate ester (33 mg, 67%)
as a yellow oil. To a solution of pivaloate ester (33 mg, 0.104 mmol,
1.0 equiv) in CH2Cl2 (2.0 mL) were added 2,6-lutidine (24 μL, 0.21
mmol, 2.0 equiv) and TBSOTf (36 μL, 0.16 mmol, 1.5 equiv) at 0 °C.
After 1 h at this temperature, the mixture was quenched with MeOH,
diluted with Et2O, and washed with a saturated aqueous NH4Cl
solution. The aqueous phase was extracted with Et2O, the organic
layers were washed with brine and dried over MgSO4, and the solvent
was evaporated under reduced pressure. The crude product was
subjected to purification by flash column chromatography (silica gel,
5% EtOAc in hexanes) to yield the corresponding silyl ether (39 mg,
87%) as a yellow oil. A solution of silyl ether (32 mg, 0.075 mmol, 1.0
equiv) in CH2Cl2 (2.0 mL) was cooled at −78 °C and then treated
with DIBAL (164 μL of a 1.0 M solution in toluene, 0.164 mmol, 2.2
equiv). After 40 min, the reaction was quenched by addition of AcOEt
at −78 °C, and the resulting mixture was treated with a saturated
aqueous Na+/K+ tartrate solution and allowed to reach room
temperature. The resulting mixture was vigorously stirred until a
clear separation of both organic and aqueous phases. The aqueous
phase was then separated, the organic extract was washed with water
and brine and dried (MgSO4), and the solvent was evaporated. The
crude product was purified by flash column chromatography (silica gel,
20% EtOAc in hexanes) to yield alcohol 45 (24 mg, 92%, 50% overall
yield from 44) as a yellow oil: Rf = 0.30 (silica gel, 20% EtOAc in
for the preparation of 42. 51: yellow oil; Rf = 0.52 (silica gel, 20%
1
EtOAc in hexanes); [α]25 = −11.5 (c 0.3, CH2Cl2); H NMR (400
D
MHz, CDCl3) δ 0.96 (d, J = 6.7 Hz, 3 H), 0.97 (d, J = 6.7 Hz, 3 H),
1.30 (s, 3 H), 1.35 (s, 3 H), 1.49 (s, 3 H), 1.59 (s, 3 H), 1.73−1.83 (m,
1 H), 1.95−2.05 (m, 1 H), 2.08 (s, 3 H), 2.28 (do, J = 6.7, 1.3 Hz, 1
H), 2.38−2.48 (m, 1 H), 2.54 (ddd, J = 13.1, 7.9, 5.0 Hz, 1 H), 3.25
(dd, J = 2.1 Hz, 1 H), 3.64 (d, J = 2.0 Hz, 1 H), 3.82 (dd, J = 8.5, 2.2
Hz, 1 H), 3.86 (d, J = 9.2 Hz, 1 H), 3.97 (ddd, J = 9.2, 5.2, 1.3 Hz, 1
H), 4.30 (ddd, J = 8.5, 4.6, 3.3 Hz, 1 H), 4.38 (dd, J = 8.3 Hz, 1 H),
5.37 (ddd, J = 15.4, 8.1, 1.4 Hz, 1 H), 5.87 (dd, J = 15.5, 6.5 Hz, 1 H);
13C NMR (100 MHz, CDCl3) δ 13.8, 22.0, 25.4, 26.1, 26.3, 30.8, 30.9,
52.9, 57.3, 63.2, 78.4, 79.1, 82.6, 84.5, 111.1, 125.9, 142.2, 176.5;
HRMS (ESI-TOF) m/e 414.2321, M + H+ calcd for C21H35NO5S
414.2314.
Epoxy Alcohol 52. Epoxy amide 51 (551 mg, 1.33 mmol, 1.0
equiv) was reduced by treatment with Super-H (3.40 mL, 1.0 M in
THF, 2.5 equiv) according to the procedure described above for 42 to
yield epoxy alcohol 52 (255 mg, 79%) as a yellow oil: Rf = 0.59 (silica
1
gel, 50% EtOAc in hexanes); [α]25 = −18.4 (c 0.3, CH2Cl2); H
D
NMR (400 MHz, CDCl3) δ 0.98 (d, J = 6.9 Hz, 6 H), 1.39 (s, 3 H),
1.40 (s, 3 H), 1.87 (bs, 1 H), 2.27- 2.38 (m, 1 H), 3.12−3.14 (m, 1 H),
3.15−3.18 (m, 1 H), 3.63−3.68 (m, 2 H), 3.94 (ddd, J = 12.7, 4.6, 2.3
Hz, 1 H), 4.29 (t, J = 8.0 Hz, 1 H), 5.38 (dd, J = 15.6, 7.5 Hz, 1 H),
5.82 (dd, J = 15.6, 6.9 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ 21.8,
22.0, 26.5, 27.0, 31.0, 54.5, 56.0, 61.0, 79.0, 80.0, 109.5, 123.0, 144.0;
HRMS (ESI-TOF) m/e 243.1602, M + H+ calcd for C13H22O4
243.1596.
hexanes); [α]25 = +30.4 (c 0.9, CH2Cl2); 1H NMR (400 MHz,
D
CDCl3) δ 0.11 (s, 3 H), 0.12 (s, 3 H), 0.90 (s, 9 H), 1.39 (s, 3 H), 1.44
(s, 3 H), 1.95 (bs, 1 H), 3.35 (q, J = 4.5 Hz, 1 H), 3.40 (s, 3 H), 3.70
(dd, J = 11.9, 4.8 Hz, 1 H), 3.77 (dd, J = 11.9, 4.0 Hz, 1 H), 3.93−3.99
(m, 2 H), 4.42−4.46 (m, 1 H), 5.23 (dt, J = 10.3, 1.3 Hz, 1 H), 5.38
(dt, J = 17.1, 1.3 Hz, 1 H), 5.90 (ddd, J = 17.1, 10.4, 6.6 Hz, 1 H); 13C
NMR (100 MHz, CDCl3) δ −4.1,-3.8, 18.6, 26.4, 27.2, 27.5, 57.9, 60.5,
72.2, 79.0, 81.4, 82.4, 109.3, 118.2, 137.3; HRMS (ESI-TOF) m/e
347.2238, M + H+ calcd for C17H34O5Si 347.2254.
Amide 47. Alcohol 45 (24 mg, 0.07 mmol, 1.0 equiv) was
dissolved in a 1:1 mixture of CH3CN/H2O (2.0 mL), and the resulting
solution was treated with BAIB (136 mg, 0.415 mmol, 6.0 equiv)
followed by TEMPO (6.0 mg, 0.035 mmol, 0.5 equiv) at 25 °C. After
5 h, the crude mixture was diluted with EtOAc and quenched by the
addition of a saturated aqueous Na2S2O3 solution, and after separation
of both layers, the aqueous phase was then extracted with EtOAc. The
organic solution was washed with a saturated aqueous Na2S2O3
solution and then dried over anhydrous MgSO4, and the solvent was
evaporated under reduced pressure. The crude acid (0.07 mmol) was
Diol 53. Diol 53 (129 mg, 55%) was prepared from epoxy alcohol
52 (209 mg, 0.863 mmol) by treatment with MeOH/B(OMe)3 and
DBU according to the same procedure described above for the
preparation of 44. 53: yellow oil; Rf = 0.30 (silica gel, 50% EtOAc in
hexanes); [α]25D = +2.6 (c 0.4, CH2Cl2); 1H NMR (400 MHz, CDCl3)
δ 0.99 (d, J = 6.4 Hz, 3 H), 1.00 (d, J = 6.4 Hz, 3 H), 1.41 (s, 6 H),
2.14 (bs, 2 H), 2.28−2.36 (m, 1 H), 3.26−3.30 (m, 1 H), 3.35 (s, 3
H), 3.81 (dd, J = 12.0, 3.4 Hz, 1 H), 3.86 (dd, J = 12.0, 4.5 Hz, 1 H),
3.91−3.98 (m, 2 H), 4.43 (dd, J = 8.1, 7.5 Hz, 1 H), 5.44 (ddd, J =
15.6, 8.1, 1.6 Hz, 1 H), 5.83 (dd, J = 15.6, 5.9 Hz, 1 H); 13C NMR
(100 MHz, CDCl3) δ 21.8, 21.9, 27.1, 30.7, 56.9, 60.4, 71.1, 78.6, 79.9,
108.6, 124.9, 143.4; HRMS (ESI-TOF) m/e 275.1852, M + H+ calcd
for C14H26O5 275.1858.
Alcohol 54. Alcohol 54 (128 mg, 83% over three steps) was
prepared from diol 53 (113 mg, 0.41 mmol) by sequential treatment
with pivaloyl chloride, TBSOTf, and DIBAL-H according to the same
procedure described above for the preparation of 45. 54: yellow oil; Rf
I
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The Journal of Organic Chemistry
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= 0.59 (silica gel, 30% EtOAc in hexanes); [α]25 = +6.5 (c 0.2,
in CH2Cl2 (10 mL) was added. The reaction mixture was stirred at
−78 °C for 40 min, and then TEA (3.8 mL, 27.30 mmol, 7.5 equiv)
was added at this temperature. After 10 min at −78 °C, the reaction
was allowed to reach room temperature, then diluted with Et2O, and
washed with a saturated aqueous NH4Cl solution. The aqueous phase
was washed with water and brine, dried over anhydrous MgSO4, and
filtered, and the solvent was evaporated under reduced pressure. The
crude aldehyde obtained was used in the next step without purification.
A solution of tributyl(methoxycarbonylmethylene)phosphonium bro-
mide (1.70 g, 4.55 mmol, 1.25 equiv) in CH2Cl2 (5.0 mL) was washed
with a 1.0 M aqueous NaOH solution (2 × 4.6 mL), dried (MgSO4),
and diluted with toluene (4.0 mL). The CH2Cl2 was evaporated, and
the resulting solution was then added to a stirred solution of crude
aldehyde (∼ 3.64 mmol, 1.0 equiv) and benzoic acid (89 mg, 0.73
mmol, 0.2 equiv) in toluene (16 mL) at 90 °C. After 30 min, the
solvent was evaporated, and the residue was purified by flash column
chromatography (silica gel, 10% EtOAc in hexanes) to provide α,β-
unsaturated ester 56 (466 mg, 48% over two steps) as a colorless oil:
D
CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.09 (s, 3 H), 0.10 (s, 3 H),
0.88 (s, 9 H), 0.97 (d, J = 6.4 Hz, 3 H), 0.99 (d, J = 6.4 Hz, 3 H), 1.37
(s, 3 H), 1.40 (s, 3 H), 1.90 (bs, 1 H), 2.26−2.34 (m, 1 H), 3.29−3.33
(m, 1 H), 3.37 (s, 3 H), 3.68 (dd, J = 11.8, 4.3 Hz, 1 H), 3.77 (dd, J =
11.8, 4.3 Hz, 1 H), 3.90 (dd, J = 8.1, 3.8 Hz, 1 H), 3.95 (dd, J = 5.9, 3.8
Hz, 1 H), 4.37 (dd, J = 8.1, 7.5 Hz, 1 H), 5.41 (ddd, J = 15.6, 7.5, 1.1
Hz, 1 H), 5.75 (dd, J = 15.6, 5.9 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ −4.6, −4.2, 18.1, 21.9, 22.0, 26.0, 27.0, 27.1, 30.7, 57.3, 59.9,
71.2, 78.4, 81.5, 81.6, 108.4, 125.4, 142.6; HRMS (ESI-TOF) m/e
389.2718, M + H+ calcd for C20H40O5Si 389.2723.
Amide 49. The oxidation of alcohol 54 (128 mg, 0.33 mmol) and
subsequent coupling with L-Lys-lactam 46 (84 mg, 0.49 mmol, 1.5
equiv) was carried out exactly as described above for 45 to yield amide
49 (112 mg, 66% over two steps) as a white foam: Rf = 0.20 (silica gel,
1
70% EtOAc in hexanes); [α]25 = +9.2 (c 0.3, CH2Cl2); H NMR
D
(400 MHz, CDCl3) δ 0.08 (s, 3 H), 0.12 (s, 3 H), 0.84 (s, 9 H), 0.96
(d, J = 6.9 Hz, 6 H), 1.31 (s, 6 H), 1.34−1.47 (m, 3 H), 1.75−1.82 (m,
1 H), 1.93−2.04 (m, 2 H), 2.22- 2.28 (m, 1 H), 3.18−3.25 (m, 2 H),
3.47 (s, 3 H), 3.79 (d, J = 2.1 Hz, 1 H), 3.95 (dd, J = 6.9, 5.4 Hz, 1 H),
4.20 (dd, J = 6.9, 2.1 Hz, 1 H), 4.36 (dd, J = 6.5, 5.4 Hz, 1 H), 4.47−
4.51 (m, 1 H), 5.43 (ddd, J = 15.6, 6.4, 1.6 Hz, 1 H), 5.71 (dd, J =
15.6, 6.4 Hz, 1 H), 6.37−6.44 (m, 1 H), 7.71 (d, J = 6.9 Hz, 1 H); 13C
NMR (100 MHz, CDCl3) δ −4.6, −4.2, 17.9, 21.9, 22.0, 25.9, 27.0,
27.2, 27.9, 28.8, 30.6, 31.2, 42.1, 51.7, 59.7, 75.2, 79.4, 79.9, 84.5,
108.5, 125.7, 140.8, 168.7, 175.3; HRMS (ESI-TOF) m/e 513.3364, M
+ H+ calcd for C26H48N2O6Si 513.3360.
1
Rf = 0.32 (silica gel, 10% EtOAc in hexanes); H NMR (400 MHz,
CDCl3) δ 1.00 (d, J = 6.7 Hz, 3 H), 1.01 (d, J = 6.7 Hz, 3 H), 1.30 (t, J
= 7.1 Hz, 3 H), 1.43 (s, 3 H), 1.46 (s, 3 H), 2.32 (od, J = 6.7, 1.3 Hz, 1
H), 4.08 (dd, J = 8.2, 8.1 Hz, 1 H), 4.17−4.24 (m, 1 H), 4.20 (q, J =
7.1 Hz, 2 H), 5.38 (ddd, J = 15.5, 7.9, 1.3 Hz, 1 H), 5.81 (dd, J = 15.5,
6.4 Hz, 1 H), 6.11 (dd, J = 15.6, 1.5 Hz, 1 H), 6.84 (dd, J = 15.6, 5.1
Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ 14.2, 21.8, 22.0, 26.7, 27.1,
30.8, 60.6, 79.8, 82.1, 109.5, 122.0, 122.4, 143.1, 144.8, 166.1; HRMS
(ESI-TOF) m/e 269.1736, M + H+ calcd for C15H24O4 269.1753.
Allylic Alcohol 57. A solution of α,β-unsaturated ester 56 (466
mg, 1.74 mmol, 1.0 equiv) in CH2Cl2 (10 mL) was cooled at −78 °C
and then treated with DIBAL-H (4.4 mL of a 1.0 M solution in
toluene, 4.34 mmol, 2.5 equiv). After 40 min, the reaction was
quenched by addition of EtOAc at −78 °C, and the mixture was
allowed to reach room temperature and treated with a saturated
aqueous Na+/K+ tartrate solution. The resulting mixture was
vigorously stirred until a clear separation of both organic and aqueous
phases. The aqueous phase was then separated, the organic extracts
were washed with water and brine and dried (MgSO4), and the solvent
was evaporated under reduced pressure. The crude product was then
purified by flash column chromatography (silica gel, 30% EtOAc in
hexanes) to yield allylic alcohol 57 (338 mg, 86%) as a yellow oil: Rf =
Hydroxy Amide 55. To a solution of silylether amide 49 (52 mg,
0.10 mmol, 1.0 equiv) in THF (3.0 mL) was added TBAF (0.13 mL,
1.0 M in THF, 0.12 mmol, 1.2 equiv) at 25 °C. After 50 min, the
reaction mixture was diluted with Et2O and washed with a saturated
aqueous NH4Cl solution. The aqueous phase was extracted with Et2O,
the combined organic phases were washed with brine and dried over
anhydrous MgSO4, and the solvent was evaporated under reduced
pressure. The crude product was then purified by flash column
chromatography (silica gel, 6% MeOH in CH2Cl2) to obtain alcohol
55 (26 mg, 65%) as a white foam: Rf = 0.53 (silica gel, 8% MeOH in
CH2Cl2); [α]25 = −13.3 (c 0.2, CH2Cl2); 1H NMR (400 MHz,
D
CDCl3) δ 0.97 (d, J = 6.9 Hz, 3 H), 0.98 (d, J = 6.9 Hz, 3 H), 1.38 (s,
6 H), 1.43−1.53 (m, 3 H), 1.75−1.85 (m, 1 H), 1.97−2.05 (m, 2 H),
2.25−2.33 (m, 1 H), 2.48 (bs, 1 H), 3.23−3.31 (m, 2 H), 3.43 (s, 3
H), 3.74 (d, J = 5.4 Hz, 1 H), 3.91 (dd, J = 7.7, 5.4 Hz, 1 H), 3.97 (t, J
= 5.4 Hz, 1 H), 4.47 (t, J = 7.8 Hz, 1 H), 4.51−4.57 (m, 1 H), 5.41
(ddd, J = 15.6, 7.5, 1.1 Hz, 1 H), 5.83 (dd, J = 15.6, 6.4 Hz, 1 H),
6.15−6.19 (bs, 1 H), 7.72 (d, J = 6.7 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ 21.8, 22.0, 26.8, 27.2, 27.9, 28.8, 30.7, 31.3, 42.1, 51.8, 59.1,
72.3, 79.1, 80.0, 82.0, 108.7, 124.8, 143.1, 169.6, 175.1; HRMS (ESI-
TOF) m/e 399.2499, M + H+ calcd for C20H34N2O6 399.2495.
2,3-Bis-epi-bengamide E (28). A solution of alcohol 55 (26 mg,
0.065 mmol, 1.0 equiv) in MeOH (0.5 mL) was treated with a 70%
aqueous AcOH solution (2.0 mL) at 70 °C for 1 h. After this time, the
solvent was removed by evaporation under reduced pressure.
Purification by flash column chromatography (silica gel, 6% MeOH
in CH2Cl2) afforded the 2,3-bis-epimer of bengamide E (28) (14 mg,
82%) as a white foam: Rf = 0.29 (silica gel, 8% MeOH in CH2Cl2);
[α]25D = −24.6 (c 1.0, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.97
(d, J = 6.5 Hz, 3 H), 0.98 (d, J = 6.5 Hz, 3 H), 1.36−1.44 (m, 1 H),
1.49−1.65 (m, 2 H), 1.74−1.86 (m, 2 H), 2.25−2.34 (m, 1 H), 2.78
(bs, 3 H), 3.25−3.27 (m, 2 H), 3.49 (s, 3 H), 3.55−3.57 (m, 1 H),
3.91 (d, J = 3.8 Hz, 1 H), 4.09 (dd, J = 7.0, 3.8 Hz, 1 H), 4.23−4.25
(m, 1 H), 4.55 (dd, J = 10.2, 7.0 Hz, 1 H), 5.53 (ddd, J = 15.6, 6.9, 1.1
Hz, 1 H), 5.75 (dd, J = 15.6, 5.9 Hz, 1 H), 6.15−6.22 (m, 1 H), 7.91
(d, J = 6.9 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ 22.1, 22.2, 27.9,
28.8, 30.8, 31.4, 42.1, 52.0, 59.1, 71.7, 72.8, 74.2, 82.1, 126.0, 140.9,
170.0, 174.9; HRMS (ESI-TOF) m/e 359.2178, M + H+ calcd for
C17H30N2O6 359.2182.
0.20 (silica gel, 30% EtOAc in hexanes); [α]25 = −32.9 (c 0.3,
D
1
CH2Cl2); H NMR (400 MHz, CDCl3) δ 0.99 (d, J = 6.7 Hz, 3 H),
1.00 (d, J = 6.7 Hz, 3 H), 1.43 (s, 3 H), 1.44 (s, 3 H), 1.54 (bs, 1 H),
2.31 (od, J = 6.7, 1.1 Hz, 1 H), 4.03−4.14 (m, 2 H), 4.17 (dd, J = 5.2,
1.6 Hz, 2 H), 5.35 (ddd, J = 15.4, 7.4, 1.3 Hz, 1 H), 5.69 (ddt, J = 15.5,
6.6, 1.6 Hz, 1 H), 5.76 (dd, J = 15.1, 6.5 Hz, 1 H), 5.95 (dtd, J = 15.5,
5.1, 0.7 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ 21.9, 22.2, 26.8,
30.6, 63.2, 80.1, 82.2, 109.3, 119.0, 126.7, 134.5, 135.8; HRMS (ESI-
TOF) m/e 227.1628, M + H+ calcd for C13H22O3 227.1647.
Epoxy Alcohol 52. To a suspension of titanium tetraisopropoxide
(180 μL, 0.59 mmol, 0.4 equiv) and 4 Å molecular sieves (500 mg) in
CH2Cl2 (5.0 mL) was added (+)-DET (110 μL, 0.59 mmol, 0.4 equiv)
at −23 °C. After 15 min at this temperature, a solution of allylic
alcohol 57 (338 mg, 1.49 mmol, 1.0 equiv) in CH2Cl2 (3.0 mL) was
added dropwise, followed by the addition, after 30 min, of tert-butyl
hydroperoxide (TBHP) (980 μL, 5.5 M in decane, 5.38 mmol, 3.9
equiv) at −23 °C. After 8 h at this temperature, the reaction mixture
was quenched by addition of Me2S (0.5 mL) at 0 °C, then the solution
was filtered, and the filtrate was evaporated under reduced pressure.
The crude product was purified by flash column chromatography
(silica gel, 30% EtOAc in hexanes) to obtain epoxy alcohol 52 (222
mg, 61%) whose spectroscopic and physical properties were identical
to those obtained from epoxy amide 51.
α,β-Unsaturated Epoxy Ester 59. A solution of oxalyl chloride
(0.10 mL, 1.18 mmol, 2.0 equiv) in CH2Cl2 (5.0 mL) was cooled to
−78 °C, and DMSO (0.17 mL, 2.36 mmol, 4.0 equiv) was added
dropwise. After 10 min, a solution of alcohol 58 (187 mg, 0.59 mmol,
1.0 equiv) in CH2Cl2 was added. The reaction mixture was stirred at
−78 °C for 40 min, and then TEA (0.49 mL, 3.54 mmol, 6.0 equiv)
was added at this temperature. After 10 min at −78 °C, the reaction
α,β-Unsaturated Ester 56. A solution of oxalyl chloride (0.80 mL,
9.10 mmol, 2.5 equiv) in CH2Cl2 (20.0 mL) was cooled to −78 °C,
and DMSO (1.3 mL, 18.20 mmol, 5.0 equiv) was added dropwise.
After 10 min, a solution of alcohol 50 (729 mg, 3.64 mmol, 1.0 equiv)
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Article
was allowed to reach room temperature, then diluted with Et2O, and
washed with a saturated aqueous NH4Cl solution. The aqueous phase
was washed with water and brine, dried over anhydrous MgSO4, and
filtered, and the solvent evaporated under reduced pressure. The crude
aldehyde obtained was used in the next step without purification. A
solution of tributyl(methoxycarbonylmethylene)phosphonium bro-
mide (275 mg, 0.74 mmol, 1.25 equiv) in CH2Cl2 (5.0 mL) was
washed with a 1.0 M aqueous NaOH solution (twice), dried (MgSO4),
and diluted with toluene. The CH2Cl2 was then evaporated under
vacuum. The resulting solution was then added to a stirred solution of
crude aldehyde (∼0.59 mmol, 1.0 equiv) and benzoic acid (18 mg,
0.15 mmol, 0.25 equiv) in toluene at 95 °C. After 30 min, the solvent
was evaporated under reduced pressure, and the residue was purified
by flash column chromatography (silica gel, 10% → 20% EtOAc in
hexanes) to provide α,β-unsaturated epoxy ester 59 (120 mg, 53%
over two steps) as a yellow oil: Rf = 0.28 (silica gel, 10% EtOAc in
hexanes); 1H NMR (400 MHz, CDCl3) δ 0.08 (s, 6 H), 0.90 (s, 9 H),
1.29 (t, J = 7.2 Hz, 3 H), 1.40 (s, 6 H), 3.06 (dd, J = 3.8, 2.0 Hz, 1 H),
3.53 (dd, J = 7.3, 1.9 Hz, 1 H), 3.72 (dd, J = 10.5, 6.1 Hz, 1 H), 3.85
(dd, J = 10.6, 2.9 Hz, 1 H), 3.99 (dd, J = 7.7, 3.9 Hz, 1 H), 4.05 (ddd, J
= 7.7, 6.1, 3.8 Hz, 1 H), 4.20 (q, J = 7.1 Hz, 2 H), 6.15 (d, J = 15.7, 1
H), 6.67 (dd, J = 15.7, 7.3 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ
−5.40, −5.39, 14.1, 18.3, 25.8, 26.4, 27.0, 53.6, 60.1, 60.6, 63.4, 77.6,
77.9, 110.0, 124.4, 143.6, 165.4; HRMS (ESI-TOF) m/e 387.2212, M
+ H+ calcd for C19H34O6Si 387.2203.
extracted with Et2O, the combined organic layers were washed with a
saturated aqueous NaHCO3 solution and dried (MgSO4), and the
solvent was evaporated under reduced pressure. The crude aldehyde
obtained was used in the next step without purification. The crude
aldehyde (∼ 0.11 mmol, 1.0 equiv) was dissolved in a 1:1 mixture of
CH3CN/H2O (5.0 mL), and the resulting solution was treated with
BAIB (193 mg, 0.60 mmol, 6.0 equiv) followed by TEMPO (4.7 mg,
0.03 mmol, 0.3 equiv) at 25 °C. After 6 h, the crude mixture was
diluted with EtOAc and quenched by the addition of a saturated
aqueous Na2S2O3 solution, and after separation of both layers, the
aqueous phase was then extracted with EtOAc. The combined organic
solution was washed with a saturated aqueous Na2S2O3 solution again
and then dried over anhydrous MgSO4, and the solvent was
evaporated under reduced pressure to obtain crude acid 62, which
was used for the next step without further purification.
Amide 63. Crude acid 62 (∼0.10 mmol, 1.0 equiv) was coupled
with L-Lys-lactam 46 (19 mg, 0.15 mmol, 1.5 equiv) in the same
manner as described above for 47 to yield amide 63 (21 mg, 45% over
three steps) as a yellow oil: Rf = 0.36 (silica gel, EtOAc); [α]25
=
D
−0.26 (c 0.3, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.06 (s, 6 H),
0.90 (s, 9 H), 1.33 (s, 3 H), 1.37 (s, 3 H), 1.40−146 (m, 2 H), 1.77−
1.88 (m, 2 H), 1.97−2.02 (m, 1 H), 2.15−2.18 (m, 1 H), 3.24−3.31
(m, 2 H), 3.50 (s, 3 H), 3.70 (dd, J = 10.5, 6.2 Hz, 1 H), 3.80 (d, J =
6.3 Hz, 1 H), 3.83 (dd, J = 10.5, 4.1 Hz, 1 H), 3.92 (dd, J = 4.8, 1.7 Hz,
1 H), 4.05 (dd, J = 8.1, 1.6 Hz, 1 H), 4.14−4.18 (m, 1 H), 4.55 (ddd, J
= 11.2, 7.0, 1.7 Hz, 1 H), 6.06−6.10 (m, 1 H), 7.86 (d, J = 6.8 Hz, 1
H); 13C NMR (100 MHz, CDCl3) δ −5.5, −5.4, 18.3, 25.9, 26.8, 27.3,
27.9, 28.9, 30.7, 42.0, 51.6, 53.4, 59.1, 63.7, 69.4, 77.9, 81.0, 109.3,
170.1, 175.4; HRMS (ESI-TOF) m/e 475.2842, M + H+ calcd for
C22H42N2O7Si 475.2839.
α,β-Unsaturated-γ-methoxy-δ-hydroxy Ester 60. To a solution
of α,β-unsaturated-γ,δ-epoxy ester 59 (73 mg, 0.19 mmol, 1.0 equiv) in
THF (5.0 mL) were added at 0 °C trimethyl borate (19 μL, 0.25
mmol, 1.3 equiv) and Pd(PPh3)4 (22 mg, 0.019 mmol, 0.1 equiv), and
the mixture was stirred at 0 °C for 30 min. After this time, the reaction
mixture was passed through a silica gel column by the aid of EtOAc,
and the eluate was concentrated in vacuo to obtain a crude product
that was purified by flash column chromatography (silica gel, 20%
EtOAc in hexanes) to afford γ-methoxy-δ-hydroxy ester 60 (58 mg,
73%) as a yellow oil: Rf = 0.30 (silica gel, 20% EtOAc in hexanes);
[α]25D = −10.6 (c 0.2, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.04
(s, 3 H), 0.05 (s, 3 H), 0.87 (s, 9 H), 1.30 (t, J = 7.1 Hz, 3 H), 1.38 (s,
3 H), 1.43 (s, 3 H), 2.71 (d, J = 5.9 Hz, 1 H), 3.36 (s, 3 H), 3.62 (dt, J
= 6.9, 1.8 Hz, 1 H), 3.67 (dd, J = 10.7, 5.9 Hz, 1 H), 3.80 (dd, J = 10.6,
4.1 Hz, 1 H), 3.91 (dt, J = 6.9, 1.0 Hz, 1 H), 3.94 (dd, J = 8.1, 1.9 Hz, 1
H), 4.14−4.20 (m, 1 H), 4.21 (dc, J = 7.1, 1.4 Hz, 2 H), 6.10 (dd, J =
15.8, 1.1 Hz, 1 H), 6.82 (dd, J = 15.8, 7.0 Hz, 1 H); 13C NMR (100
MHz, CDCl3) δ −5.6, −5.5, 14.1, 18.2, 25.8, 26.7, 27.0, 57.4, 60.4,
63.2, 71.6, 76.7, 77.4, 82.3, 109.2, 124.6, 143.9, 165.5; HRMS (ESI-
TOF) m/e 419.2472, M + H+ calcd for C20H38O7Si 419.2465.
Trihydroxy Ester 61. OsO4 (2.5 wt % solution in tBuOH, 66 μL,
0.0065 mmol, 0.05 equiv) was added to a stirred solution of N-
methylmorpholine N-oxide (46 mg, 0.39 mmol, 3.0 equiv) and γ-
methoxy-α,β-unsaturated ester 60 (55 mg, 0.13 mmol, 1.0 equiv) in
THF (5.0 mL). When the reaction was complete (6−8 h), the reaction
mixture was diluted with EtOAc and treated with a saturated aqueous
Na2SO3 solution. The aqueous layer was extracted with EtOAc, the
combined organic layers were dried over MgSO4, and the solvent was
evaporated under reduced pressure to afford the crude compound,
which was purified by flash column chromatography (silica gel, 40%
EtOAc in hexanes) to obtain trihydroxy ester 61 (50 mg, 84%) as a
Diol 64. A solution of silyl ether 63 (20 mg, 0.042 mmol, 1.0 equiv)
in THF (2.0 mL) was treated with HF·pyr (70% solution, 25 μL) at 0
°C. After stirring for 1 h at this temperature, the reaction mixture was
quenched by addition of a saturated aqueous NaHCO3 solution and
diluted with CH2Cl2. After separation of both layers, the aqueous
phase was extracted with CH2Cl2, the combined organic layers were
dried over MgSO4, and the solvent was evaporated under reduced
pressure. The crude product was purified by flash column
chromatography (silica gel, 70% EtOAc in hexanes) to obtain diol
59 (13 mg, 85%) as a yellow oil: Rf = 0.26 (silica gel, 10% MeOH in
CH2Cl2); [α]25 = −15.6 (c 0.1, CH2Cl2); 1H NMR (400 MHz,
D
CDCl3) δ 1.37 (s, 3 H), 1.40 (s, 3 H), 1.80−1.90 (m, 3 H), 1.95−2.10
(m, 2 H), 2.15−2.20 (m, 1 H), 3.23−3.36 (m, 2 H), 3.52 (s, 3 H), 3.66
(dd, J = 12.0, 3.8 Hz, 1 H), 3.81−3.86 (m, 3 H), 4.07 (d, J = 8.5 Hz, 1
H), 4.22 (dd, J = 8.1, 3.9 Hz, 1 H), 4.56 (dd, J = 10.4, 7.1 Hz, 1 H),
6.02 (t, J = 6.8 Hz, 1 H), 7.86 (d, J = 6.2 Hz, 1 H); 13C NMR (100
MHz, CDCl3) δ 26.8, 27.4, 28.0, 29.0, 29.7, 30.8, 42.2, 51.8, 59.3, 61.6,
69.4, 76.2, 81.0, 109.5, 170.1, 175.1; HRMS (ESI-TOF) m/e 361.1958,
M + H+ calcd for C16H28N2O7 361.1975.
Alkene 65. To a solution of diol 64 (10 mg, 0.03 mmol, 1.0 equiv)
in CH2Cl2 (2.0 mL) was added BAIB (29 mg, 0.09 mmol, 3.0 equiv)
and TEMPO (1.4 mg, 0.009 mmol, 0.3 equiv) at 0 °C. After 0.5 h at
this temperature, the crude mixture was diluted with Et2O and
quenched by the addition of a saturated aqueous Na2S2O3 solution,
and after separation of both layers, the aqueous phase was then
extracted with Et2O twice. The organic solution was washed again with
a saturated aqueous Na2S2O3 solution and then dried over anhydrous
MgSO4, and the solvent was evaporated under reduced pressure. The
crude aldehyde (∼0.03 mmol) was dissolved in THF (2.0 mL) and
added dropwise to a freshly prepared solution of methylenetriphenyl-
phosphorane (Ph3PCH2) [sodium hexamethyldisilylamide
(NaHMDS, 0.12 mL, 0.12 mmol, 1.0 M solution in THF, 4.0
equiv) was slowly added to a suspension of methyltriphenylphospho-
nium bromide (44 mg, 0.12 mmol, 4.0 equiv) in THF (4.0 mL) at 0
°C and stirring at this temperature for 15 min] at 0 °C. After being
stirred for 0.5 h, the reaction mixture was diluted with Et2O and
washed with a saturated aqueous NH4Cl solution. The aqueous phase
was washed with Et2O, the combined organic phases were washed with
brine and then dried over anhydrous MgSO4, and the solvent was
evaporated under reduced pressure. Purification of the obtained crude
yellow oil: Rf = 0.28 (silica gel, 40% EtOAc in hexanes); [α]25
=
D
1
+1.97 (c 0.4, CH2Cl2); H NMR (400 MHz, CDCl3) δ 0.06 (s, 6 H),
0.88 (s, 9 H), 1.30 (t, J = 7.7 Hz, 3 H), 1.39 (s, 3 H), 1.42 (s, 3 H),
3.02−3.05 (m, 2 H), 3.24−3.27 (m, 1 H), 3.42−3.44 (m, 1 H), 3.47
(d, J = 0.9 Hz, 3 H), 3.73 (dd, J = 10.7, 5.4 Hz, 1 H), 3.82−3.86 (m, 1
H), 3.91 (d, J = 9.6 Hz, 1 H), 4.06−4.15 (m, 3 H), 4.25−4.31 (m, 2
H), 4.39 (d, J = 6.1 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ −5.5,
−5.4, 14.1, 18.3, 25.9, 26.9, 27.2, 59.3, 62.0, 63.5, 68.1, 70.6, 72.3, 78.8,
80.7, 109.4, 173.7; HRMS (ESI-TOF) m/e 453.2526, M + H+ calcd for
C20H40O9Si 453.2520.
Acid 62. NaIO4 (24 mg, 0.11 mmol, 1.1 equiv) was added to a
solution of trihydroxy ester 61 (46 mg, 0.10 mmol, 1.0 equiv) in a 1:1
THF/H2O mixture (4.0 mL). The mixture was stirred for 8 h, then
diluted with Et2O, and washed with water. The aqueous layer was
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product by flash column chromatography (silica gel, 30% EtOAc in
hexanes) provided alkene 65 (5.0 mg, 46% over two steps) as a pale
yellow oil: Rf = 0.25 (silica gel, 20% EtOAc in hexanes); [α]25D = −5.4
(c 0.1, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 1.43 (s, 3 H), 1.44 (s,
3 H), 1.42−1.57 (m, 2 H), 1.79−1.90 (m, 2 H), 1.95−2.11 (m, 2 H),
3.23−3.32 (m, 2 H), 3.48 (s, 3 H), 3.63 (ddd, J = 7.7, 5.8, 1.7 Hz, 1
H), 3.70 (dd, J = 11.1, 6.8 Hz, 1 H), 3.88 (dd, J = 8.5, 1.7 Hz, 1 H),
4.47−4.53 (m, 1 H), 4.53−4.58 (m, 1 H), 5.24 (dd, J = 10.3, 0.7 Hz, 1
H), 5.37 (d, J = 17.1 Hz, 1 H), 5.82 (ddd, J = 17.5, 10.3, 7.4 Hz, 1 H),
6.36 (t, J = 6.2 Hz, 1 H), 7.84 (d, J = 6.1 Hz, 1 H); 13C NMR (100
MHz, CDCl3) δ 26.6, 27.0, 27.7, 28.1, 31.8, 41.9, 51.2, 61.4, 71.3, 77.6,
78.2, 81.3, 108.9, 119.9, 135.0, 172.4, 174.9; HRMS (ESI-TOF) m/e
357.2018, M + H+ calcd for C17H28N2O6 357.2026.
E-Alkene 66. To a solution of alkene 65 (4.3 mg, 0.012 mmol, 1.0
equiv) in a 1:2 CH2Cl2/3-methyl-1-butene mixture (3.0 mL) was
added second generation Hoveyda−Grubbs catalyst 48 (2.3 mg,
0.0036 mmol, 0.3 equiv). The flask was then capped and heated at 40
°C overnight. After this time, the reaction mixture was allowed to
reach room temperature, and the solvents were removed by
concentration under reduced pressure. The crude product was purified
by flash column chromatography (silica gel, 6% MeOH in CH2Cl2) to
obtain alcohol E-alkene 66 (3.6 mg, 75%) as a yellow oil: Rf = 0.30
(silica gel, 20% EtOAc in hexanes); [α]25D = −9.8 (c 0.4, CH2Cl2); 1H
NMR (400 MHz, CDCl3) δ 0.98 (d, J = 6.7 Hz, 3 H), 1.01 (d, J = 6.7
Hz, 3 H), 1.39 (s, 6 H), 1.41−1.52 (m, 2 H), 1.77−1.89 (m, 2 H),
1.99−2.04 (m, 1 H), 2.16 (dd, J = 1.4, 1.2 Hz, 1 H), 2.32 (od, J = 6.7,
1.5 Hz, 1 H), 3.24−3.26 (m, 2 H), 3.48 (s, 3 H), 3.60 (d, J = 9.5 Hz, 1
H), 3.75−3.79 (m, 1 H), 3.81 (ddd, J = 9.5, 4.4, 2.0 Hz, 1 H), 4.42
(dd, J = 8.5, 8.4 Hz, 1 H), 4.56 (ddd, J = 11.1, 7.0 Hz, 1 H), 5.38 (ddd,
J = 15.4, 8.3, 1.3 Hz, 1 H), 5.85 (dd, J = 15.4, 6.5 Hz, 1 H), 6.26 (t, J =
6.2 Hz, 1 H), 7.86 (d, J = 7.0 Hz, 1 H); 13C NMR (100 MHz, CDCl3)
δ 21.9, 22.1, 26.7, 27.4, 27.9, 28.9, 30.8, 30.9, 42.1, 51.7, 59.1, 68.3,
78.8, 79.6, 81.3, 109.1, 123.5, 144.6, 169.8, 175.2; HRMS (ESI-TOF)
m/e 399.2487, M + H+ calcd for C20H34N2O6 399.2495.
Hz, 3 H), 1.06 (d, J = 6.8 Hz, 3 H), 1.44 (s, 3 H), 1.47 (s, 3 H), 2.37
(od, J = 6.6, 1.0 Hz, 1 H), 2.89 (dd, J = 13.5, 9.2 Hz, 1 H), 3.31 (dd, J
= 13.6, 3.2 Hz, 1 H), 3.40 (s, 3 H), 3.96−4.04 (m, 2 H), 4.22 (dd, J =
9.1, 2.2 Hz, 1 H), 4.36−4.42 (m, 2 H), 4.79−4.85 (m, 1 H), 4.96 (d, J
= 1.2 Hz, 1 H), 5.43 (ddd, J = 15.4, 8.6, 1.4 Hz, 1 H), 5.99 (dd, J =
15.4, 6.3 Hz, 1 H), 7.20 (d, J = 7.5 Hz, 2 H), 7.30−7.35 (m, 3 H); 13C
NMR (100 MHz, CDCl3) δ 21.8, 21.9, 26.7, 27.0, 30.9, 37.7, 55.5,
58.3, 67.6, 72.2, 79.3, 79.9, 80.6, 109.3, 123.9, 127.6, 129.1, 129.5,
134.9, 145.2, 153.6, 170.6; HRMS (ESI-TOF) m/e 448.2329, M + H+
calcd for C24H33NO7 448.2335.
Silyl Ether 69. To a solution of compound 68 (197 mg, 0.44
mmol, 1.0 equiv) in CH2Cl2 (5.0 mL) were added 2,6-lutidine (108
μL, 0.924 mmol, 2.0 equiv) and TBSOTf (162 μL, 0.704 mmol, 1.6
equiv) at 0 °C. After 1 h at this temperature, the mixture was
quenched by addition of MeOH, diluted with Et2O and washed with a
saturated aqueous NH4Cl solution. The aqueous phase was extracted
with Et2O, the organic layers were washed with brine and dried over
MgSO4, and the solvent was evaporated under reduced pressure. The
crude product was then subjected to purification by flash column
chromatography (silica gel, 20% EtOAc in hexanes) to yield silyl ether
69 (227 mg, 92%) as a yellow oil: Rf = 0.31 (silica gel, 20% EtOAc in
hexanes); [α]25 = +41.0 (c 0.3, CH2Cl2); 1H NMR (400 MHz,
D
CDCl3) δ 0.11 (s, 3 H), 0.22 (s, 3 H), 0.92 (s, 9 H), 1.02 (d, J = 6.8
Hz, 6 H), 1.40 (s, 3 H), 1.41 (s, 3 H), 2.34 (od, J = 6.7, 1.3 Hz, 1 H),
2.88 (dd, J = 13.4, 9.4 Hz, 1 H), 3.34 (s, 3 H), 3.31−3.37 (m, 1 H),
3.92 (dd, J = 8.3, 6.7 Hz, 1 H), 4.08 (dd, J = 6.7, 2.9 Hz, 1 H), 4.19
(dd, J = 9.0, 7.2 Hz, 1 H), 4.24 (dd, J = 9.1, 2.0 Hz, 1 H), 4.37 (dd, J =
8.3 Hz, 1 H), 4.62 (ddt, J = 11.1, 7.7, 3.5 Hz, 1 H), 4.92 (d, J = 2.9 Hz,
1 H), 5.42 (ddd, J = 15.4, 8.5, 1.5 Hz, 1 H), 5.91 (dd, J = 15.4, 6.0 Hz,
1 H), 7.23−7.26 (m, 2 H), 7.28−7.37 (m, 3 H); 13C NMR (100 MHz,
CDCl3) δ −4.9, −3.7, 18.5, 22.0, 22.1, 26.1, 27.1, 27.2, 30.9, 37.8, 55.7,
58.0, 66.8, 71.9, 78.3, 81.5, 81.6, 108.4, 123.8, 127.2, 128.9, 129.0,
129.2, 136.0, 144.1, 153.6, 169.0; HRMS (ESI-TOF) m/e 562.3186, M
+ H+ calcd for C30H47NO7Si 562.3200.
2-epi-Bengamide E (29). A solution of hydroxyl amide 66 (3.5
mg, 0.009 mmol, 1.0 equiv) in MeOH (2.0 mL) was treated with a
70% aqueous AcOH solution (1.5 mL) at 70 °C for 1 h. After this
time, the solvent was removed by evaporation under reduced pressure.
Purification of the crude product by flash column chromatography
(silica gel, 6% MeOH in CH2Cl2) afforded the 2-epimer of bengamide
Alcohol 70. To a solution of oxazolidinone 69 (62 mg, 0.11 mmol,
1.0 equiv) in THF (3.0 mL) was added LiBH4 (276 μL, 2.0 M in THF,
0.55 mmol, 5.0 equiv) at 0 °C. The reaction was allowed to reach
room temperature. After 6 h at this temperature, the reaction mixture
was diluted with EtOAc and quenched with a saturated aqueous
NaHCO3 solution. The aqueous phase was extracted with EtOAc, the
combined organic phases were washed with brine, dried over MgSO4,
and filtered, and the solvent was evaporated under reduced pressure.
The obtained crude product was purified by flash column
chromatography (silica gel, 25% EtOAc in hexanes) to afford alcohol
70 (31 mg, 72%) as a yellow oil: Rf = 0.27 (silica gel, 20% EtOAc in
E, compound 29 (2.5 mg, 76%) as a white solid: Rf = 0.36 (silica gel,
1
10% MeOH in CH2Cl2); [α]25 = −7.5 (c 0.1, CH2Cl2); H NMR
D
(400 MHz, CDCl3) δ 0.98 (d, J = 6.7 Hz, 3 H), 1.00 (d, J = 6.7 Hz, 3
H), 1.36−1.43 (m, 1 H), 1.51−1.57 (m, 1 H), 1.74−1.88 (m, 2 H),
1.99−2.08 (m, 2 H), 2.30 (od, J = 6.7, 1.2 Hz, 1 H), 3.06 (bs, 2 H),
3.25−3.31 (m, 2 H), 3.51 (s, 3 H), 3.58−3.59 (m, 1 H), 3.81 (d, J =
5.5 Hz, 1 H), 3.94−3.95 (m, 1 H), 4.00 (bs, 1 H), 4.23 (dd, J = 6.3, 6.2
Hz, 1 H), 4.56 (ddd, J = 11.2, 7.1, 1.4 Hz, 1 H), 5.42 (ddd, J = 15.5,
7.2, 1.4 Hz, 1 H), 5.79 (ddd, J = 15.5, 6.5, 0.9 Hz, 1 H), 6.28 (t, J = 5.3
Hz, 1 H), 7.85 (d, J = 6.7 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ
22.1, 22.2, 28.0, 28.9, 30.8, 31.1, 42.1, 51.7, 59.2, 72.0, 72.9, 74.0, 81.9,
125.4, 141.9, 170.2, 175.1; HRMS (ESI-TOF) m/e 359.2175, M + H+
calcd for C17H30N2O6 359.2182.
hexanes); [α]25 = +31.8 (c 0.9, CH2Cl2); 1H NMR (400 MHz,
D
CDCl3) δ 0.14 (s, 6 H), 0.96 (s, 9 H), 1.01 (d, J = 6.7 Hz, 3 H), 1.02
(d, J = 6.7 Hz, 3 H), 1.41 (s, 3 H), 1.42 (s, 3 H), 2.34 (od, J = 6.7, 1.3
Hz, 1 H), 3.30 (dt, J = 5.0, 4.1 Hz, 1 H), 3.41 (s, 3 H), 3.75 (dd, J =
5.3, 1.6 Hz, 1 H), 3.81 (t, J = 4.4 Hz, 2 H), 3.87 (dd, J = 8.7, 1.5 Hz, 1
H), 4.36 (dd, J = 8.5, 8.4 Hz, 1 H), 5.40 (ddd, J = 15.5, 8.2, 1.3 Hz, 1
H), 5.80 (ddd, J = 15.5, 6.6, 0.6 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ −4.5, −4.0, 18.2, 22.0, 22.1, 25.9, 26.8, 27.3, 31.0, 57.3, 58.9,
68.4, 78.2, 78.9, 82.1, 108.8, 123.6, 144.6; HRMS (ESI-TOF) m/e
389.2718, M + H+ calcd for C20H40O5Si 389.2723.
Aldol Product 68. To a stirred solution of oxazolidinone 67 (257
mg, 1.03 mmol, 1.0 equiv) in CH2Cl2 (5.0 mL) at 0 °C was added a
freshly prepared 1.0 M solution of n-Bu2BOTf in CH2Cl2 (1.23 mL,
1.23 mmol, 1.2 equiv) dropwise followed by freshly distilled Hunig’s
base (269 μL, 1.54 mmol, 1.5 equiv), and the mixture was stirred for 1
h at 0 °C. This mixture was cooled to −78 °C, and a solution of crude
aldehyde, obtained by oxidation of alcohol 50 (226 mg, 1.13 mmol, 1.1
equiv), in CH2Cl2 (3.0 mL) was added. The resulting solution was
then stirred for 8 h while gradually being warmed to 25 °C. An
aqueous phosphate buffer solution (pH = 7.0, 3.0 mL) was added, and
the mixture was stirred for 30 min. The aqueous phase was separated
and extracted with CH2Cl2 twice. The combined organic phases were
dried (MgSO4) and concentrated under reduced pressure. The crude
product was purified by flash column chromatography (silica gel, 50%
→ 70% EtOAc in hexanes) to afford oxazolidinone 68 (386 mg, 84%)
Amide 71. The oxidation of alcohol 70 (62 mg, 0.159 mmol, 1.0
equiv) and subsequent coupling with L-Lys-lactam 46 (39 mg, 0.239
mmol, 1.5 equiv) was carried out exactly as described for 45 above to
yield amide 71 (51 mg, 63% over two steps) as a yellow oil: Rf = 0.32
1
(silica gel, EtOAc); [α]25 = +5.6 (c 0.3, CH2Cl2); H NMR (400
D
MHz, CDCl3) δ 0.07 (s, 3 H), 0.08 (s, 3 H), 0.90 (s, 9 H), 0.97 (d, J =
6.8 Hz, 3 H), 0.98 (d, J = 6.8 Hz, 3 H), 1.39 (s, 3 H), 1.41 (s, 3 H),
1.45−1.60 (m, 2 H), 1.76−1.87 (m, 3 H), 2.00−2.10 (m, 1 H), 2.30
(od, J = 6.6, 1.3 Hz, 1 H), 3.20−3.33 (m, 2 H), 3.37 (s, 3 H), 3.61 (d, J
= 3.9 Hz, 1 H), 3.83 (dd, J = 8.3, 5.4 Hz, 1 H), 3.97 (dd, J = 5.4, 3.9
Hz, 1 H), 4.30 (t, J = 8.3 Hz, 1 H), 4.55 (ddd, J = 11.3, 6.7, 1.4 Hz, 1
H), 5.40 (ddd, J = 15.5, 8.3, 1.4 Hz, 1 H), 5.80 (ddd, J = 15.5, 6.2, 0.4
Hz, 1 H), 6.14−6.17 (m, 1 H), 7.60 (d, J = 6.7 Hz, 1 H); 13C NMR
(100 MHz, CDCl3) δ −4.3, −4.0, 18.5, 21.8, 21.9, 26.2, 27.0, 27.1,
27.9, 28.9, 30.7, 31.4, 42.0, 51.9, 58.5, 73.1, 78.8, 81.0, 83.5, 108.4,
as a yellow oil: Rf = 0.23 (silica gel, 60% EtOAc in hexanes); [α]25
=
D
+12.7 (c 0.2, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 1.05 (d, J = 6.8
L
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The Journal of Organic Chemistry
Article
124.5, 144.1, 169.8, 175.2; HRMS (ESI-TOF) m/e 513.3365, M + H+
calcd for C26H48N2O6Si 513.3360.
(dd, J = 8.3, 1.7 Hz, 1 H), 3.58 (d, J = 2.0 Hz, 1 H), 3.90 (dd, J = 9.2,
0.6 Hz, 1 H), 4.00 (ddd, J = 9.2, 5.2, 1.4 Hz, 1 H), 4.31 (ddd, J = 8.5,
4.8, 3.3 Hz, 1 H), 5.09 (ddd, J = 15.6, 8.3, 1.4 Hz, 1 H), 5.92−6.00 (m,
1 H); 13C NMR (100 MHz, CDCl3) δ 15.9, 21.8, 21.9, 23.0, 26.2, 30.7,
30.9, 34.5, 51.2, 55.6, 56.0, 56.4, 56.5, 67.1, 95.9, 122.8, 145.4, 163.0;
HRMS (ESI-TOF) m/e 356.1872, M + H+ calcd for C18H29NO4S
356.1896.
Diepoxy Alcohol 77. Diepoxy amide 76 (100 mg, 0.28 mmol, 1.0
equiv) was reduced by treatment with Super-H (0.70 mL, 1.0 M in
THF, 2.5 equiv) according to the procedure described above for 42 to
yield diepoxy alcohol 78 (36 mg, 70%) as a yellow oil: Rf = 0.26 (silica
gel, 40% EtOAc in hexanes); 1H NMR (400 MHz, CDCl3) δ 1.00 (d, J
= 6.8 Hz, 6 H), 2.27−2.40 (m, 1 H), 2.91 (dd, J = 4.6, 2.1 Hz, 1 H),
2.97−3.09 (m, 1 H), 3.16−3.20 (m, 1 H), 3.33 (dd, J = 8.2, 2.0 Hz, 1
H), 3.65−3.75 (m, 1 H), 3.93−4.01 (m, 1 H), 5.10 (ddd, J = 15.6, 8.3,
1.3 Hz, 1 H), 5.95 (ddd, J = 15.6, 6.5, 3.3 Hz, 1 H); 13C NMR (100
MHz, CDCl3) δ 21.8, 21.9, 30.9, 53.5, 55.7, 56.3, 57.9, 60.6, 123.2,
145.1; HRMS (ESI-TOF) m/e 185.1206, M + H+ calcd for C10H16O3
185.1178.
Hydroxy Amide 66. The treatment of silyl ether 71 (28 mg, 0.055
mmol, 1.0 equiv) with TBAF (81 μL, 1.0 M in THF, 0.082 mmol, 1.5
equiv) was carried out exactly as described before for 49 to obtain
alcohol 66 (22 mg, 100%), whose physical and spectroscopic
properties were identical to those obtained from alkene 65.
2-epi-Bengamide E (29). Treatment of hydroxyl amide 66 (22
mg, 0.055 mmol, 1.0 equiv) with a 70% aqueous AcOH solution was
carried out exactly as described above to obtain 2-epi-bengamide E
(29) (15 mg, 76%).
2,3-Epoxy Bengamide 36. The treatment of acetal 73 (22 mg,
0.060 mmol, 1.0 equiv) with AcOH was carried out exactly as
described before for 29 to obtain 2,3-epoxy bengamide E 36 (17 mg,
85%) as a white foam: Rf = 0.34 (silica gel, 10% MeOH in EtOAc);
[α]25D = −15.3 (c 0.2, CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.98
(d, J = 6.9 Hz, 3 H), 0.99 (d, J = 6.9 Hz, 3 H), 1.31−1.51 (m, 2 H),
1.72−2.08 (m, 4 H), 2.27−2.37 (m, 1 H), 3.17 (t, J = 2.2 Hz, 1 H),
3.21−3.28 (m, 2 H), 3.49 (s, 1 H), 3.58−3.64 (m, 1 H), 4.15 (t, J = 6.5
Hz, 1 H), 4.49 (dd, J = 10.2, 5.9 Hz, 1 H), 5.47 (dd, J = 15.6, 7.5 Hz, 1
H), 5.82 (dd, J = 15.6, 6.5 Hz, 1 H), 6.03−6.19 (m, 1 H), 7.43−7.49
(m, 1 H); 13C NMR (100 MHz, CDCl3) δ 21.9, 22.1, 27.8, 28.8, 30.8,
31.4, 42.1, 51.7, 52.2, 58.7, 71.7, 74.3, 124.6, 143.2, 167.2, 174.8;
HRMS (ESI-TOF) m/e 327.1908, M + H+ calcd for C16H26N2O5
327.1920.
Epoxy Amide 75. To a solution of allylic alcohol 74 (102 mg, 1.0
mmol, 1.0 equiv) in CH2Cl2 (10 mL) was added MnO2 (14.0 g, 16.3
mmol, 16.0 equiv). After stirring for 12 h at 25 °C, the crude mixture
was filtered through Celite, and the resulting clear solution was
concentrated under reduced pressure at 20 °C to obtain the
corresponding α,β-unsaturated aldehyde, which was employed for
the next step without further purification. This aldehyde was reacted
with sulfonium salt 22 (350 mg, 1.12 mmol, 1.1 equiv) and NaOH
(3.0 M aqueous solution, 0.34 mL, 1.02 mmol, 1.0 equiv) according to
the procedure described above for 42 to yield epoxy amide 75 (184
Epoxy Bengamide E Analogue 37. Diepoxy alcohol 77 (15 mg,
0.08 mmol, 1.0 equiv) was treated with MeOH/B(OMe)3 and DBU
according to the same procedure described above for the preparation
of 44. Crude opening product 78 (∼0.08 mmol) was oxidized with
TEMPO/BAIB and coupled with L-Lys-lactam 46 (20 mg, 0.12 mmol,
1.5 equiv) exactly as described above for 47 to yield epoxy amide 37
(13.0 mg, 47% over 3 steps) as a colorless oil: Rf = 0.21 (silica gel,
1
EtOAc); H NMR (400 MHz, CDCl3) δ 1.02 (d, J = 6.8 Hz, 6 H),
1.56−1.67 (m, 2 H), 1.79−1.91 (m, 2 H), 1.95−2.09 (m, 2 H), 2.30−
2.39 (m, 1 H), 3.19−3.40 (m, 4 H), 3.86 (s, 3 H), 4.02−4.05 (m, 1 H),
4.50−4.59 (m, 2 H), 5.36 (ddd, J = 15.5, 7.1, 1.4 Hz, 1 H), 5.84 (ddd, J
= 15.5, 6.5, 1.0 Hz, 1 H), 5.87−5.94 (m, 1 H), 7.78 (d, J = 12.3 Hz, 1
H); 13C NMR (100 MHz, CDCl3) δ 22.0, 22.1, 27.9, 28.9, 30.2, 31.4,
42.1, 51.6, 52.1, 53.4, 57.0, 57.8, 82.9, 122.4, 143.4, 175.2, 175.6;
HRMS (ESI-TOF) m/e 341.2058, M + H+ calcd for C17H28N2O5
341.2077.
Diepoxy Amide 38. The oxidation of diepoxy alcohol 77 (50 mg,
0.27 mmol, 1.0 equiv) and subsequent coupling with L-Lys-lactam 46
(67 mg, 0.41 mmol, 1.5 equiv) was carried out exactly as described
above for 45 to yield diepoxy amide 38 (46 mg, 55% over two steps)
mg, 58% over 2 steps) as a yellow oil: Rf = 0.18 (silica gel, 30% EtOAc
1
in hexanes); [α]25 = +15.9 (c 0.4, CH2Cl2); H NMR (400 MHz,
D
CDCl3) δ 0.98 (d, J = 6.7 Hz, 3 H), 0.99 (d, J = 6.7 Hz, 3 H), 1.53 (s,
3 H), 1.63 (s, 3 H), 1.72−1.80 (m, 1 H), 1.97−2.05 (m, 1 H), 2.07 (s,
3 H), 2.28−2.37 (m, 1 H), 2.37−2.46 (m, 1 H), 2.51−2.59 (m, 1 H),
3.49−3.54 (m, 2 H), 3.88 (d, J = 9.3 Hz, 1 H), 4.01 (ddd, J = 9.1, 5.3,
1.4 Hz, 1 H), 4.27 (ddd, J = 8.5, 4.8, 3.2 Hz, 1 H), 5.13 (ddd, J = 15.7,
8.0, 1.4 Hz, 1 H), 6.02 (dd, J = 15.6, 6.5 Hz, 1 H); 13C NMR (100
MHz, CDCl3) δ 15.8, 21.7, 21.8, 23.0, 26.3, 30.8, 30.9, 34.3, 55.5, 55.8,
58.5, 67.0, 95.9, 122.5, 146.1, 163.6; HRMS (ESI-TOF) m/e 314.1784,
M + H+ calcd for C16H27NO3S 314.1790.
as a white solid: Rf = 0.43 (silica gel, EtOAc); [α]25 = +49.4 (c 0.2,
D
1
DMSO); H NMR (400 MHz, CDCl3) δ 1.00 (d, J = 6.8 Hz, 6 H),
1.36−1.50 (m, 2 H), 1.79−1.90 (m, 2 H), 1.96−2.04 (m, 2 H), 2.28−
2.38 (m, 1 H), 2.90 (dd, J = 4.5, 2.1 Hz, 1 H), 3.05 (dd, J = 4.5, 2.1 Hz,
1 H), 3.24−3.30 (m, 2 H), 3.35 (dd, J = 8.2, 1.8 Hz, 1 H), 3.45 (d, J =
2.1 Hz, 1 H), 4.50 (ddd, J = 11.4, 6.0, 1.4 Hz, 1 H), 5.09 (ddd, J =
15.6, 8.2, 1.4 Hz, 1 H), 5.95 (dd, J = 15.6, 6.6 Hz, 1 H), 6.10 (t, J = 6.0
Hz, 1 H), 7.46 (d, J = 5.5 Hz, 1 H); 13C NMR (100 MHz, CDCl3) δ
21.8, 27.9, 28.9, 30.9, 31.4, 42.2, 51.7, 52.7, 56.4, 57.0, 57.2, 122.7,
145.4, 166.8, 174.6; HRMS (ESI-TOF) m/e 309.1802, M + H+ calcd
for C16H24N2O4 309.1814.
Diepoxy Amide 76. To a solution of epoxy amide 75 (320 mg,
1.02 mmol, 1.0 equiv) in THF (20 mL) was added dropwise Red-Al
(0.7 mL, 70% w/v in toluene, 2.24 mmol, 2.2 equiv) at 0 °C. After 1 h
at 0 °C, the reaction mixture was quenched by addition of a saturated
aqueous NH4Cl solution. After separation of both layers, the aqueous
phase was extracted with EtOAc, the organic extracts were washed
with brine and dried over MgSO4 and the solvent was evaporated
under reduced pressure. The crude epoxy aldehyde was used for the
next step without further purification. To a solution of sulfonium salt
22 (350 mg, 1.12 mmol, 1.1 equiv) in H2O (15 mL) was added a 5.0
M aqueous NaOH solution (0.20 mL, 1.02 mmol, 1.0 equiv). Then, a
solution of crude epoxy aldehyde (∼ 1.02 mmol) in CH2Cl2 (10 mL)
was added, and the reaction mixture was vigorously stirred overnight at
25 °C. After this time, both phases were separated, and the aqueous
layer was extracted with CH2Cl2 twice. Combined organic extracts
were then washed with water and brine, dried over anhydrous MgSO4,
filtered, and concentrated. Purification of the crude product by flash
column chromatography (silica gel, 20% EtOAc in hexanes) provided
diepoxy amide 76 (310 mg, 85% over 2 steps) as a yellow oil: Rf = 0.37
Epoxy Amide 80. Epoxy amide 80 (1.82 g, 79% over two steps)
was prepared from allylic alcohol 79 (880 mg, 6.70 mmol, 1.0 equiv)
by oxidation with MnO2, followed by reaction with sulfonium salt 22
(1.90 g, 6.70 mmol, 1.0 equiv) according to the same procedure
described above for the preparation of 75. 80: yellow oil; Rf = 0.44
(silica gel, 40% EtOAc in hexanes); [α]25 = +25.1 (c 0.3, CH2Cl2);
D
1H NMR (400 MHz, CDCl3) δ 0.03 (s, 9 H), 1.49 (s, 3 H), 1.59 (s, 3
H), 1.69−1.75 (m, 1 H), 1.94−2.01 (m, 1 H), 2.02 (s, 3 H), 2.38
(ddd, J = 13.4, 8.9, 6.7 Hz, 1 H), 2.52 (ddd, J = 13.2, 7.0, 5.1 Hz, 1 H),
3.48−3.53 (m, 2 H), 3.85 (d, J = 9.2 Hz, 1 H), 3.98 (ddd, J = 9.1, 5.2,
1.4 Hz, 1 H), 4.25 (ddd, J = 10.2, 4.9, 3.2 Hz, 1 H), 5.65−5.72 (m, 1
H), 6.24−6.30 (m, 1 H); 13C NMR (100 MHz, CDCl3) δ −1.6, 15.7,
22.9, 26.2, 30.7, 55.7, 55.8, 59.5, 67.0, 68.7, 95.8, 138.6, 140.4, 166.1;
HRMS (ESI-TOF) m/e 344.1709, M + H+ calcd for C16H30NO3SSi
344.1716.
1
(silica gel, 40% EtOAc in hexanes); H NMR (400 MHz, CDCl3) δ
Diepoxy Amide 81. Diepoxy amide 81 (104 mg, 84% over two
steps) was prepared from epoxy amide 80 (110 mg, 0.32 mmol, 1.0
equiv) by reduction with Red-Al, followed by reaction with sulfonium
salt 22 (111 mg, 0.35 mmol, 1.1 equiv) according to the same
procedure described above for the preparation of 76. 81: yellow oil; Rf
0.99 (d, J = 6.8 Hz, 6 H), 1.52 (s, 3 H), 1.63 (s, 3 H), 1.76−1.86 (m, 1
H), 2.01−2.09 (m, 1 H), 2.11 (s, 3 H), 2.27−2.37 (m, 1 H), 2.47
(ddd, J = 13.2, 8.4, 7.2 Hz, 1 H), 2.57 (ddd, J = 13.0, 7.7, 5.1 Hz, 1 H),
3.01 (dd, J = 3.5, 2.1 Hz, 1 H), 3.34 (dd, J = 3.6, 1.9 Hz, 1 H), 3.36
M
dx.doi.org/10.1021/jo4003272 | J. Org. Chem. XXXX, XXX, XXX−XXX

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Article
= 0.19 (silica gel, 40% EtOAc in hexanes); [α]25 = +31.4 (c 0.1,
AUTHOR INFORMATION
Corresponding Author
*E-mail: frsarabia@uma.es.
D
■
CH2Cl2); 1H NMR (400 MHz, CDCl3) δ 0.06 (s, 9 H), 1.52 (s, 3 H),
1.63 (s, 3 H), 1.76−1.85 (m, 1 H), 2.02−2.08 (m, 1 H), 2.10 (s, 3 H),
2.46 (ddd, J = 13.3, 8.4, 7.2 Hz, 1 H), 2.58 (ddd, J = 13.0, 7.5, 5.1 Hz,
1 H), 3.03 (dd, J = 3.6, 2.0 Hz, 1 H), 3.35 (dd, J = 3.6, 2.0 Hz, 1 H),
3.36−3.40 (m, 1 H), 3.58 (d, J = 1.9 Hz, 1 H), 3.89 (d, J = 9.2 Hz, 1
H), 4.00 (ddd, J = 9.1, 5.2, 1.2 Hz, 1 H), 4.28−4.34 (m, 1 H), 5.67
(dd, J = 18.7, 7.6 Hz, 1 H), 6.23 (dd, J = 18.7, 0.5 Hz, 1 H); 13C NMR
(100 MHz, CDCl3) δ −1.6, 15.9, 22.9, 26.2, 30.7, 34.4, 51.2, 55.5, 56.0,
56.7, 57.6, 67.0, 95.9, 137.9, 140.9, 162.9; HRMS (ESI-TOF) m/e
386.1820, M + H+ calcd for C18H31NO4SSi 386.1821.
Present Address
†School of Chemistry, Manchester Interdisciplinary Biocenter,
University of Manchester, Manchester M1 7DN, U.K.
Notes
The authors declare no competing financial interest.
Diepoxy Alcohol 82. Diepoxy amide 81 (50 mg, 0.13 mmol, 1.0
equiv) was reduced by treatment with Super-H (0.33 mL, 1.0 M in
THF, 2.5 equiv) according to the procedure described above for 42 to
yield diepoxy alcohol 82 (15 mg, 55%) as a yellow oil: Rf = 0.22 (silica
gel, 60% EtOAc in hexanes); [α]25D = +16.4 (c 0.2, CH2Cl2); 1H NMR
(400 MHz, CDCl3) δ 0.08 (s, 9 H), 2.93 (dd, J = 4.7, 2.1 Hz, 1 H),
3.08 (dd, J = 4.7, 2.2 Hz, 1 H), 3.19 (dd, J = 5.7, 2.3 Hz, 1 H), 3.33−
3.38 (m, 1 H), 3.67−3.75 (m, 1 H), 3.94−4.00 (m, 1 H), 5.69 (dd, J =
18.7, 7.6 Hz, 1 H), 6.23 (d, J = 18.7 Hz, 1 H); 13C NMR (100 MHz,
CDCl3) δ −1.6, 53.4, 55.7, 57.5, 58.0, 60.4, 137.5, 141.2; HRMS (ESI-
TOF) m/e 215.1112, M + H+ calcd for C10H18O3Si 215.1104.
Epoxy Bengamide E Analogue 84. Diepoxy alcohol 82 (45 mg,
0.21 mmol, 1.0 equiv) was treated with MeOH/B(OMe)3 and DBU
according to the same procedure described above for the preparation
of 44. Crude opening product 83 (∼0.21 mmol) was oxidized with
TEMPO/BAIB and coupled with L-Lys-lactam 46 (52 mg, 0.32 mmol,
1.5 equiv) exactly as described above for 47 to yield epoxy amide 84
(20 mg, 26% over 3 steps) as a colorless oil: Rf = 0.21 (silica gel,
ACKNOWLEDGMENTS
This work was financially supported by Ministerio de Economia
y Competitividad (ref CTQ2010-16933), Junta de Andalucia
(FQM-03329), and fellowships from Junta de Andalucia (F.M.-
G.) and Ministerio de Ciencia e Innovacion (C.G.-R.). We
■
́
́
́
́
thank Dr. J. I. Trujillo (St. Louis, MO) for assistance in the
preparation of this manuscript. We thank Unidad de
́
Espectroscopia de Masas de la Universidad de Granada for
exact mass spectroscopic assistance.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
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dx.doi.org/10.1021/jo4003272 | J. Org. Chem. XXXX, XXX, XXX−XXX

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