An Array of BengamideE Analogues Modified at the Terminal Olefinic Position: Synthesis and Antitumor Properties

Article abstract of DOI:10.1002/cmdc.201300033

Based on our previously described synthetic strategy for bengamideE, a natural product of marine origin with antitumor activity, a small library of analogues modified at the terminal olefinic position was generated with the objective of investigating the effect of structural modifications on antitumor properties. Biological evaluation of these analogues, consisting of IC₅₀ determinations against various tumor cell lines, revealed important aspects with respect to the structural requirements of this olefinic position for activity. Interestingly, the analogue possessing a cyclopentyl group displayed greater potency than the parent bengamideE, representing a key finding upon which to base further investigations into the design of new analogues with promising biological activities. Better than the real thing: A small library of bengamideE analogues was synthesized and evaluated for cytotoxicity against various tumor cell lines. This led to the identification of an analogue more potent than natural bengamideE, in which the isopropyl group, located at the terminal olefinic position, is replaced by a cyclopentyl group. Copyright

Full text of DOI:10.1002/cmdc.201300033

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DOI: 10.1002/cmdc.201300033
An Array of Bengamide E Analogues Modified at the
Terminal Olefinic Position: Synthesis and Antitumor
Properties
Francisca Martꢀn-Gꢁlvez,[a] Cristina Garcꢀa-Ruiz,[a] Antonio Sꢁnchez-Ruiz,[a, c]
Frederick A. Valeriote,[b] and Francisco Sarabia*[a]
Dedicated to Professor A. Vasella on the occasion of his 70th birthday.
Based on our previously described synthetic strategy for ben-
gamide E, a natural product of marine origin with antitumor
activity, a small library of analogues modified at the terminal
olefinic position was generated with the objective of investi-
gating the effect of structural modifications on antitumor
properties. Biological evaluation of these analogues, consisting
of IC50 determinations against various tumor cell lines, revealed
important aspects with respect to the structural requirements
of this olefinic position for activity. Interestingly, the analogue
possessing a cyclopentyl group displayed greater potency than
the parent bengamide E, representing a key finding upon
which to base further investigations into the design of new an-
alogues with promising biological activities.
Introduction
Marine sponges of the Jaspidae genus have proven to be an
extraordinary source of bioactive secondary metabolites. These
notable metabolites include the cyclodepsipeptide jaspa-
mide,[1] the polyketides bengamides,[2] and bengazoles.[3] Partic-
ularly striking are the bengamides, which represent a wide
family of natural products with promising antitumor properties
that were isolated by Crews and co-workers in the late 1980s.[2]
This discovery was followed by the identification of new ben-
gamide family members,[4] including the recent isolation of
bengamides E, E’, and F’, from Myxococcus virescens bacteria
(Figure 1).[5] Their prominent antitumor, antibiotic, and antihel-
minthic activities[6] have prompted intense research activity in
the chemical and biological fields. Their antitumor properties,
demonstrated by preliminary potent activity against larynx epi-
thelial carcinoma (1.0 mgmLꢀ1),[2] were further evaluated
against human breast MDA-MB-435 carcinoma cells. These bio-
logical studies revealed that bengamides A and B were the
most potent members of the family, with IC50 values of 0.001
and 0.0024 mm, respectively, and an IC50 value of 3.3 mm for
bengamide E.[2d] In order to unravel the mechanism of action
of these natural products, Towbin et al. conducted proteomic
[a] Dr. F. Martꢀn-Gꢁlvez, C. Garcꢀa-Ruiz, Dr. A. Sꢁnchez-Ruiz, Prof. Dr. F. Sarabia
Department of Organic Chemistry, Faculty of Sciences
University of Malaga, Campus de Teatinos s/n. 29071 Mꢁlaga (Spain)
E-mail: frsarabia@uma.es
[b] Dr. F. A. Valeriote
Josephine Ford Cancer Center
Henry Ford Health System, Detroit, MI 48202 (USA)
[c] Dr. A. Sꢁnchez-Ruiz
Current address: School of Chemistry
Manchester Interdisciplinary Biocenter
University of Manchester, Manchester M1 7DN (UK)
Figure 1. Molecular structures of natural bengamides.
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studies[7] demonstrating that both methionine aminopeptidas-
es types 1 and 2 (MetAp1 and MetAp2), enzymes responsible
for the cleavage of the N-terminal initiator methionine residue
during protein synthesis,[8] were the direct cellular targets of
the bengamides. This mechanism of action was supported by
measuring the inhibitory activity exerted by representative nat-
ural bengamides against MetAp1 and 2 (Table 1),[9] together
design of new potential leads for the treatment of tuberculo-
sis.[22]
Our interest in these molecules prompted us to engage in
a project directed toward the establishment of an efficient syn-
thesis of the bengamides and related analogues. At the outset
of our work, we used a Sharpless asymmetric epoxidation fol-
lowed by an oxirane ring-opening reaction to generate the C2/
C3 system.[23,24] In addition, an olefin cross-metathesis allowed
the stereoselective introduction of a terminal olefinic substitu-
ent. Later, the design and synthesis of a new class of chiral sul-
fonium salts in our labs,[25] such as 24, proved to be efficient
and high-yielding tools for asymmetric synthesis of epoxya-
mides and encouraged us to use them for synthesis of the
bengamides.[26] Therefore, starting from alcohols 22 or 23, we
prepared the corresponding epoxyamides 25 and 26 in good
yields and excellent stereoselectivities. From these epoxya-
mides, we synthesized compounds 27–29, which were finally
directed toward bengamide E and a wide array of analogues,
such as 30–38, via olefin cross-metathesis or palladium-mediat-
ed couplings (Scheme 1). The invention of this new asymmetric
Table 1. Inhibition of MetAp enzymatic activity.
Compound
IC50 [mm][a]
MetAp1
MetAp2
Bengamide A (1)
Bengamide B (2)
Bengamide M (8)
Bengamide O (10)
1.9ꢁ0.2
29.3ꢁ10.4
5.4ꢁ2.3
2.7ꢁ0.4
10.5ꢁ3.8
17.9ꢁ7.9
>50
>50
[a] Data from Liu et al.[9]
with the crystallization and subsequent X-ray analysis of the
complex enzyme–bengamide.[7] The biological effect of inhibit-
ing MetAps is the blockade of cell cycle division in endothelial
cells at the G1 and G2 phases[10] as well as an anti-angiogenic
effect in epithelial cells.[11] Interestingly, a similar mode of
action is displayed by the anti-angiogenic agents fumagillin
and ovalicin,[12] despite their structural differences. However,
while these compounds selectively inhibit MetAp2,[13,14] the
bengamides bind to both MetAp types. This lack of selectivity
may be the reason for the poor pharmacokinetic properties
and undesired side effects exhibited by bengamide analogue
LAF389,[15] which was considered as a clinical candidate but
has not been approved as a treatment for cancer.[16] Despite
these studies toward the identification of the molecular targets
of fumagillin and the bengamides, the proposed mechanism
of action remains controversial.[17]
At the molecular level, Liu discovered that inhibition of me-
thionine aminopeptidase by bengamide A hampered N-myris-
toylation of proto oncogene c-Src, which produced a significant
decrease in its tyrosine kinase activity, resulting in a remarkable
delay in cell cycle progression.[9] Therefore, inhibition of these
enzymes could indirectly impair the functions of c-Src and
likely other oncogenes that are essential for tumor growth. In
addition to these biological findings, Crews and colleagues re-
cently discovered that the bengamides behave as immune-
modulating agents, owing to their inhibition of nuclear factor
kB (NF-kB).[5] These studies suggest that the bengamides may
serve as therapeutic leads for the treatment of inflammatory
diseases. These biological activities exhibited by the benga-
mides, combined with their interesting molecular structures,
have generated intense synthetic pursuits in the last few
years.[18] Similarly, the antitumor activity possessed by the ben-
gamides stimulated the design and synthesis of bengamide
analogues, which led to the discovery of potent compounds
with cytotoxicities in the low nanomolar range and improved
solubilities in water with respect to those displayed by the nat-
ural counterparts.[19–21] The ability of the bengamides to inhibit
methionine aminopeptidase has also been exploited in the
Scheme 1. Synthetic strategy for bengamides based on chiral sulfur ylides.
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epoxidation methodology, combined with palladium-assisted
coupling from vinyl iodides 28 or 29, made possible an effi-
cient and straightforward synthesis of the bengamides com-
pared to our previous synthesis.[24]
lines. To accomplish this goal, we relied on our previous syn-
thetic strategy, using chiral sulfonium salts, to construct the
targeted bengamide analogues.
Having established efficient and convergent routes toward
the bengamides, our next goal was the design of a library of
bengamide analogues for biological evaluation. The structures
of the bengamides are amenable to modification by changing
the configuration of certain stereocenters of the polyketide
chain, the geometry of the double bond, the substituents of
the terminal olefinic position, and the nature of the caprolac-
tam residue. Our synthetic strategy toward these molecules
was therefore designed on the premise of modifying these ele-
ments so as to reach optimum molecular diversity and obtain
the maximum number of library members. Biological screening
of these compounds was expected to lead to the establish-
ment of sufficient structure–activity relationships to facilitate
the next step of this research, consisting of the design, synthe-
sis, and identification of potential drug candidates.
Results and Discussion
Chemistry
Our previous synthetic studies on the bengamides provided
a basis for the generation of a wide range of analogues modi-
fied at the terminal olefinic position. Together with the benga-
mide analogues previously prepared,[26] as indicated in
Scheme 1, we decided to complete this initial list with the in-
clusion of new derivatives. In particular, compounds 46–50
were synthesized from vinyl iodide precursors 28 or 29 via
Suzuki[27] (46–48), Sonogashira[28] (49), and Negishi[29] (50) cou-
plings in modest to good yields. Removal of the protecting
groups by direct acidic hydrolysis for 46–49, or sequential
TBAF and acidic treatments for 50, afforded the analogue
series of bengamide E, compounds 51–54 and 56 (Scheme 2).
Among the various modifications at the terminal olefinic po-
sition, it was determined that additional alkyl groups could
provide a positive interaction with the hydrophobic pocket of
the methionine aminopeptidase active site. This assumption
was supported by the recent discovery of LAF-389-related
Apart from the bengamide analogue LAF-389 (39)[19] and
other related compounds, such as 40[20] and 41[21] (Figure 2), in
which the isopropyl moiety was replaced with a tert-butyl
group, no other structural modifications have been undertaken
Figure 2. Representative tert-butyl bengamide analogues and their cytotox-
icities toward MDA-MB-435 human breast cancer cells.
Scheme 2. Synthesis of bengamide E analogues 51–54 and 56. Reagents and
conditions: a) 1.2 equiv 42, 0.2 equiv Pd(PPh3)4, 2.0 equiv Tl2CO3, THF/H2O (3/
1), 608C, 18 h, or 1.2 equiv 43, 0.2 equiv Pd(dpephos)Cl2, 2.0 equiv Tl2CO3,
THF/H2O (3:1), 608C, 5 h, or 1.2 equiv 44, 0.2 equiv Pd(dpephos)Cl2, 2.0 equiv
Tl2CO3, THF/H2O (3:1), 508C, 1 day, 36% for 46, 67% for 47, 54% for 48;
b) 1.3 equiv TMSCꢂCH, 0.2 equiv CuI, 0.2 equiv PPh3, 0.1 equiv Pd(OAc)2,
2.0 equiv NEt3, C6H6, 258C, 2 h, 79% for 49; c) 5.0 equiv 45, 0.2 equiv
Pd(PPh3)4, THF, 258C, 10 h, 61%; d) 70% AcOH in H2O, MeOH, 708C, 1–2 h,
or 5 days at 258C, 53% for 51, 58% for 52, 45% for 53, 28% for 54;
e) 2.0 equiv TBAF (1.0m in THF), THF, 258C, 1 h, 97%; f) 70% AcOH in H2O,
MeOH, 708C, 1–2 h, 49% for 56.
in this region of the molecule. Our synthetic strategy allows us
to introduce a wide array of substituents at this terminal ole-
finic position via a ring closing metathesis or a palladium-
mediated coupling reaction. In the present article, we wish to
report the synthesis of an array of terminal olefinic position-
modified analogues of bengamide E and the corresponding
biological evaluation of these compounds, including natural
bengamides E and E’, against a panel of different tumor cell
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compound 57, named LBM648,[16] which was identified as
a potent and selective inhibitor of MetAp2 with an extra
methyl group located at the olefinic C6 position. To more fully
probe the biological significance of the presence of additional
methyl groups at the olefinic position, we initially targeted the
7-methyl analogue of bengamide E, compound 58. For the
synthesis, we selected gem-dibromoalkenyl alcohol 59[30] as
a suitable precursor for a subsequent controlled double alkyla-
tion of the double bond. Alcohol 59 was then subjected to
a Swern oxidation,[31] followed by reaction with sulfonium salt
24 under basic conditions according to our two-phase
method[32] to afford epoxyamide 60 as only one diastereomer
and in 62% overall yield from 59. Compound 60 was then
transformed following the synthetic strategy delineated for the
bengamides, which entailed a) super-H reduction[33] to obtain
epoxy alcohol 61, b) an oxirane ring-opening of the resulting
epoxy alcohol with methanol by modification of the Miyashita
methodology,[34] c) selective oxidation of the resulting diol to
the carboxylic acid with TEMPO/BAIB in the presence of
water,[35] and d) coupling with the commercially available
amino caprolactam 62 to obtain the advanced precursor 63
without difficulties. With this compound in hand, we proceed-
ed with the sequential Negishi reactions in which an initial
coupling should involve preferentially a bromine at the trans
olefinic position where an isopropyl group should be initially
installed.[36] However, to our dismay, the attempted controlled
coupling, by treatment of 63 with one equivalent of diisopro-
pylzinc in the presence of Pd(dpephos)Cl2, did not produce the
desired monocoupled alkene. Instead, isopropyl alkyne 64 was
the only product formed in 78% yield. Other reaction condi-
tions were tested in order to avoid this undesired elimination;
however, all attempts were completely unsuccessful. Despite
this, the resulting alkyne was considered of interest for biologi-
cal evaluation. Thus, compound 64 was treated with aqueous
acetic acid to obtain alkyne analogue 65 albeit in poor yield
(Scheme 3).
Scheme 3. Toward the synthesis of bengamide E analogue 58: Synthesis of
alkynyl analogue 65. Reagents and conditions: a) 2.0 equiv (COCl)2, 4.0 equiv
DMSO, 6.0 equiv NEt3, CH2Cl2, ꢀ788C, 40 min; b) 1.1 equiv 24, 1.1 equiv
3.0 m NaOH in H2O, CH2Cl2, 258C, overnight, 62% over 2 steps; c) 2.5 equiv
Super-H (1.0m in THF), THF, 08C, 0.5 h, 68%; d) 1.0 equiv DBU, MeOH/
B(OMe)3 (1:1), 708C, 1 day; e) 6.0 equiv BAIB, 0.8 equiv TEMPO, CH3CN/H2O
(1:1), 258C, 3 h; f) 1.5 equiv 62, 1.2 equiv BOP, 2.0 equiv DIPEA, DMF, 258C,
2 h, 40% over three steps; g) 1.0 equiv ZniPr2 (1.0m in toluene), 0.2 equiv
Pd(dpephos)Cl2, THF/DMF (1:1), 258C, 6 h, 78%; h) 70% AcOH in H2O, MeOH,
708C, 2 h, 35%.
In light of these discouraging results, we opted to introduce
a methyl group at the terminal olefinic position earlier in the
synthesis via a pivotal iodine intermediate for subsequent in-
stallation of an isopropyl group via palladium chemistry. To
this aim, the alkyne derivative of d-tartaric acid, compound
66,[37] was chosen as the starting material to achieve the geo-
metrically controlled construction of the targeted trisubstituted
alkene. Methylation of alkyne 66, mediated by nBuLi/HMPA,[38]
provided methyl alkynyl derivative 67, which was subjected to
stereocontrolled stannylation[39] by reaction with Bu3SnH in the
presence of PdCl2(PPh3)2 for an excellent (97%) yield. Exchange
of the tributyltin moiety with iodine[39] was similarly successful
in delivering compound 68, which was treated with TBAF to
obtain alcohol 69. Starting from this alcohol, transformation
into amide 72 followed the same synthetic sequence as de-
scribed for the bengamides, involving a reaction of the result-
ing aldehyde from 69 with sulfonium salt 24 to provide epox-
yamide 70 as a single diastereomer and in good yield (55%
over two steps from 69). Further synthetic transformations of
70, including reduction, an oxirane ring-opening reaction with
methanol, selective oxidation and coupling with lactam 62,
gave compound 72. This vinyl iodide was then reacted with
diisopropylzinc under Negishi coupling conditions to afford
the desired trisubstituted alkene 73 in good yield (65%). Final
acetal cleavage by aqueous acetic acid yielded bengamide E
analogue 58 (Scheme 4).
Biology
Following the preparation of the bengamide E analogues (30–
38, 51–54, 56, 58, and 65), and natural bengamides E (15) and
E’ (17), the next step in this research was to evaluate the anti-
tumor properties of these compounds to determine the influ-
ence of the described structural modifications on antiprolifera-
tive activity. Determination of the cytotoxic properties of all
these compounds was performed by measuring their IC50
values against an HT29 human colon adenocarcinoma cell line
using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
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More promising results were obtained from analogues bearing
a tert-butyl or a cyclic group in place of the isopropyl. tert-
Butyl analogue 31 retained significant cytotoxic activity, slight-
ly superior to that of bengamide E. This was not a surprising
result, as Novartis previously noted this improvement in activi-
ty with compounds such as LAF-389. In our case, however, we
did not detect a remarkable increase in cytotoxic activity as ini-
tially expected. For the cyclic analogues, we observed impor-
tant antitumor activity for compound 38, which exhibited
a very similar cytotoxic profile to bengamide E. To our delight,
cyclopentyl analogue 56 exhibited the best cytotoxic result
among all of our evaluated analogues, with a fourfold im-
provement in antitumor activity over bengamide E. In contrast,
introduction of a bulkier cyclohexane moiety, as in analogue
35, led to complete loss of cytotoxic activity. Similarly, phenyl
analogue 32 did not display significant biological activity. Final-
ly, the introduction of an extra methyl group at the C7 olefinic
position (compound 58) led to a sevenfold loss in antitumor
activity, indicating limited tolerance for structural modifications
in this region of the molecule. Assuming that the antitumor ac-
tivity of the bengamides is a consequence of MetAp inhibition,
according to the interaction mode established by Towbin
et al.,[7] we can conclude that hydrophobic pocket P1 of the
active site is highly sensitive to steric volume, in accordance
with the biological results obtained for the terminal olefinic-
modified bengamides (Table 2).
As we have previously mentioned, bengamide E (15) has
been shown to inhibit the in vitro growth of a tumor cell line
at micromolar concentrations (3.3 mm against MDA-MB-435).[2d]
A more detailed characterization of its activity and selectivity
profile is missing, however, with the exception of the in vitro
antitumor studies carried out by Banwell et al., who reported
activity against two tumor cell lines and against human umbili-
cal vein endothelial cells.[42]
Scheme 4. Synthesis of bengamide E analogue 58. Reagents and conditions:
a) 1.4 equiv nBuLi (1.6 m in hexanes), 1.03 equiv HMPA, 2.0 equiv MeI, THF,
ꢀ788C!258C, 8 h, 80%; b) 5.0 equiv Bu3SnH, 0.1 equiv PdCl2(PPh3)2, THF,
258C, 97%; c) 1.1 equiv I2, CH2Cl2, 08C, 1 h, 97%; d) 1.2 equiv TBAF (1.0 m in
THF), THF, 258C, 1 h, 89%; e) 2.0 equiv (COCl)2, 4.0 equiv DMSO, 6.0 equiv
NEt3, CH2Cl2, ꢀ788C, 40 min; f) 1.1 equiv 24, 1.1 equiv 3.0m NaOH in H2O,
CH2Cl2, 258C, overnight, 55% over two steps; g) 2.5 equiv Super-H (1.0 m in
THF), THF, 08C, 20 min; h) 1.0 equiv DBU, MeOH/B(OMe)3 (1:1), 708C, 1 day,
42% over two steps; i) 6.0 equiv BAIB, 0.8 equiv TEMPO, CH3CN/H2O (1:1),
258C, 10 h; j) 1.5 equiv 62, 1.2 equiv BOP, 2.0 equiv DIPEA, DMF, 258C, 1 h,
58% over two steps; k) 1.2 equiv ZniPr2 (1.0 m in toluene), 0.2 equiv Pd-
(dpephos)Cl2, THF/DMF (1:1), 258C, 6 h, 65%; l) 70% AcOH in H2O, MeOH,
708C, 2 h, 57%.
To establish a more complete antiproliferative profile either
for bengamide E or for the most potent bengamide analogues
identified in our preliminary biological evaluation (Table 2), we
decided to examine the cytotoxicities of these compounds
against other different cancer cell lines, namely MDA-MB-231
(human breast carcinoma), HT1080 (human fibrosarcoma), and
HL60 (human promyelocytic leukemia), as well as against a pri-
mary culture of non-transformed bovine aorta endothelial
(BAE) cells. Bengamide E and fumagillin were used as controls
to compare the activity of the newly synthesized analogues.
According to these biological assays, the antiproliferative activ-
ity obtained for bengamide E was in agreement with the previ-
ously reported cytotoxic activity, with IC50 values for the five
cell types in the low micromolar range. On the other hand, fu-
magillin, a fungal metabolite that potently inhibits angiogene-
sis by blocking endothelial cell proliferation and that has ad-
vanced to clinical trials for multiple cancers, showed a biphasic
effect on the growth of proliferating endothelial cells. At lower
concentrations, there is an initial decrease in cell number, likely
due to a cytostatic effect; followed by a plateau across several
orders of magnitude in concentration and then a second cyto-
toxic effect. This biphasic dose–response curve, typical of fu-
magillin and its derivatives, was obtained for the tumor cell
bromide (MTT) dye reduction assay.[40] These results are com-
piled in Table 2.[41]
According to these biological results, lengthening the poly-
ketide chain at this olefinic position essentially led to a com-
plete loss of cytotoxic activity, in the case of analogues 34, 37,
and 52, or a significant decrease in activity (a 30-fold decrease
compared with bengamide E), as in the case of natural benga-
mide E’ (17) or analogues 33, 53, and 54. However, the pres-
ence of the terminal olefinic substituent is essential for antitu-
mor activity, as demonstrated by methylene analogue 30,
which was completely devoid of activity. On the other hand,
slight modifications at this olefinic position led to important
changes in biological activities. For example, replacement of
the isopropyl group by an iodine (compound 36) or by an iso-
propenyl group (compound 51) led to a tenfold loss of activity.
Interestingly, alkyne analogue 65 exhibited only a twofold loss
in antitumor activity with respect to natural bengamide E.
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servations indicating that inhibition of MetAp2 by the benga-
mides does not result in selective inhibition of endothelial cell
proliferation.[16] Dose–response curves obtained with bengami-
de E and its analogues showed a sharp decrease in cell survival
at concentrations near the IC50 value (Figure 3). From these re-
sults, we confirmed that cyclopentyl analogue 56 was the
Table 2. In vitro antitumor activities of bengamide E analogues against
HT29 human colon adenocarcinoma cells.
Compound
R
IC50 [mm][a]
0.95ꢁ0.16
Bengamide E (15)
Bengamide E’ (17)
Analogue 30
27.5ꢁ1.5
>100
Analogue 31
0.87ꢁ0.2
68.8
Analogue 32
Analogue 33
Analogue 34
38.5
100
Analogue 35
Analogue 36
Analogue 37
62.5
14.8ꢁ2.0
61
Analogue 38
Analogue 51
2.1ꢁ0.4
15.2ꢁ2.5
Analogue 52
Analogue 53
100
33.7ꢁ3.0
Analogue 54
Analogue 56
33.3ꢁ10.0
0.22ꢁ0.05
Figure 3. Dose-dependent effect on the in vitro growth of tumor and endo-
thelial cells by a) bengamide E (15) and b) cyclopentyl analogue 56 [cell sur-
vival (CS) is represented as percentage of control-cell growth in cultures
containing no drugs; each point represents the mean of quadruplicates; SD
values were all <10% and are omitted for clarity].
Analogue 65
Analogue 58
2.2ꢁ0.4
6.6ꢁ0.7
most potent compound, exhibiting an antitumor activity three-
to fourfold more potent than natural bengamide E, with a very
similar cytotoxic profile and remarkably improved cytotoxicity
with respect to tert-butyl derivative 31, which displayed a po-
tency very similar to that of bengamide E.
[a] In vitro cytotoxicities were determined by MTT assay as detailed in the
Experimental Section; IC50 values were obtained from semilogarithmic
dose–response plots as the concentrations of compounds yielding 50%
cell survival; values represent the average ꢁSD.
lines studied, indicating that the antiproliferative activity of fu-
magillin is not endothelial-specific, which is in agreement with
previously reported data (Table 3).[43] In contrast to fumagillin,
the selected bengamide analogues (31, 36, 38, 51, 56, 58, and
65) displayed well-defined cytotoxic activity in tumor as well
as in endothelial cells. This is in agreement with previous ob-
Conclusions
The present work describes the synthesis of a series of benga-
mide E analogues modified at the terminal olefinic position.
This synthetic undertaking was accomplished by use of our de-
lineated synthetic strategy for the bengamides, which enjoys
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from CaH2. Yields refer to chroma-
tographically and spectroscopical-
ly (1H NMR) homogeneous materi-
als, unless otherwise stated. All
solutions used in workup proce-
dures were saturated unless oth-
erwise noted. All reagents were
purchased at highest commercial
quality and used without further
Table 3. In vitro antitumor activities of bengamide E, fumagillin, and bengamide analogues against various
tumor cell lines and BAEC.
Compound
Cell Lines and IC50 [mm][a]
HCT116[g]: 0.6
Bengamide E (15)[b]
Bengamide E (15)[c]
MDA-MB-435[e]: 3.3
A549[f]: 1.9
HUVEC[h]: 0.3
MDA-MB-231[i]
1.64ꢁ0.54
2.2ꢁ0.1
HT29[j]
0.95ꢁ0.16
0.87ꢁ0.2
14.8ꢁ2.0
2.1ꢁ0.4
HT1080[k]
0.29ꢁ0.03
0.25ꢁ0.05
3.6ꢁ1.0
HL60[l]
0.68ꢁ0.10
1.1ꢁ0.2
7.1ꢁ0.5
2.1ꢁ0.4
6.3ꢁ0.2
0.19ꢁ0.01
5.5ꢁ1.0
1.7ꢁ0.1
36ꢁ7.5
BAEC[m]
0.28ꢁ0.03
0.17ꢁ0.01
3.9ꢁ0.4
Bengamide E (15)[d]
Analogue 31
Analogue 36
Analogue 38
Analogue 51
Analogue 56
Analogue 58
Analogue 65
Fumagillin
purification
unless
otherwise
14.2ꢁ1.4
3.7ꢁ0.5
stated. All reactions were moni-
tored by thin-layer chromatogra-
phy carried out on 0.25 mm silica
gel plates (60 F254) using UV light
as a visualizing agent and 7%
ethanolic phosphomolybdic acid
or para-anisaldehyde solution and
heat as developing agents. Silica
gel 60 (particle size 0.040–
0.063 mm) was used for flash
column chromatography. Prepara-
tive thin-layer chromatography
(PTLC) separations were carried
out on 0.25, 0.50, or 1 mm silica
gel plates (60F-254). NMR spectra
1.1ꢁ0.5
1.6ꢁ0.5
16.9ꢁ3.7
0.44ꢁ0.06
11.1ꢁ3.0
5.7ꢁ2.0
15.2ꢁ2.5
0.22ꢁ0.05
6.6ꢁ0.7
1.7ꢁ0.7
2.3ꢁ0.2
0.12ꢁ0.05
1.21ꢁ0.13
0.5ꢁ0.1
0.10ꢁ0.02
2.4ꢁ1.3
2.2ꢁ0.4
1.5ꢁ0.2
54.3ꢁ10.2
38.3ꢁ12.5
biphasic curve
biphasic curve
[a] In vitro cytotoxicities were determined by MTT assay as detailed in the Experimental Section; IC50 values
were obtained from semilogarithmic dose–response plots as the concentrations of compounds yielding 50%
cell survival; values represent the average ꢁSD. [b] Data from Crews et al.[2d] [c] Data from Banwell et al.[42]
[d] Data determined by our research group. Tumor cell lines: [e] MDA-MB-435: human breast carcinoma;
[f] A549: non-small-cell lung cancer; [g] HCT116: colon cancer cells; [h] HUVEC: primary human umbilical vein
endothelial cells; [i] MDA-MB-231: human breast carcinoma; [j] HT29: human colon adenocarcinoma;
[k] HT1080: human fibrosarcoma; [l] HL60: human promyelocytic leukemia; [m] BAEC: non-transformed bovine
aorta endothelial cells.
were recorded on
a
Bruker
Avance 400 MHz instrument and
were calibrated using residual undeuterated solvent as an internal
reference. The following abbreviations were used for multiplicities:
s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; band, sev-
eral overlapping signals; b, broad. Optical rotations were recorded
on a PerkinElmer 241 polarimeter. High resolution mass spectra
(HRMS) were recorded on an ESI-TOF mass spectrometer in posi-
tive mode. Analytical and preparative HPLC were carried out with
a Jasco instrument in reverse-phase using a reflection index detec-
tor. For preparative HPLC, a C8 5m-Luna column (250ꢃ10.00 nm)
convergency and flexibility for structural diversity. Using this
strategy, we were able to generate a wide array of diverse ana-
logues for biological evaluation against different tumor and
endothelial cell lines.
The biological evaluation of these analogues, including the
natural bengamides E and E’, led us to establish: 1) a complete
cytotoxic profile for bengamide E and 2) the identification of
a more potent analogue, compound 56, in which a cyclopentyl
group replaced the isopropyl group at the terminal olefinic po-
sition. It is important to note the poor tolerance of the benga-
mides to structural modification as demonstrated by the lack
of cytotoxicity for many of the synthesized analogues. These
biological findings suggest a nearly perfect and very precise fit
for the entire framework of the bengamides at the enzyme
active site. However, this extensive structure–activity relation-
ship study enabled the identification of a three- to fourfold
more potent analogue, compound 56, which should facilitate
the identification and development of a new class of benga-
mide analogues featuring a cyclopentyl moiety. The design of
new analogues that combine this cyclopentyl group with cap-
rolactam-type units corresponding to the most potent mem-
bers, such as bengamides A and B or LAF-389, should provide
promising new inhibitors of endothelial or tumor cell growth
as potential anticancer compounds. Syntheses of this newly
proposed class of bengamide analogues, as well as evaluation
of their cytotoxic activities, are currently in progress.
was employed with a flow rate of 4.7 mLminꢀ1
.
Compound 46:
A solution of vinyl iodide 28 (47.0 mg,
0.098 mmol) and pinacol boronic ester 42 (23 mL, 0.117 mmol,
1.2 equiv) in a 3:1 mixture of THF/H2O (4.0 mL) was treated with
Pd(PPh3)4 (23.0 mg, 0.019 mmol, 0.2 equiv) and Tl2CO3 (91 mg,
0.195 mmol, 2.0 equiv). The reaction mixture was heated at 608C
for 18 h, then was cooled to room temperature, diluted with Et2O,
and washed with a saturated aqueous KHSO4 solution. The aque-
ous phase was extracted with Et2O and the combined organic ex-
tracts were washed with brine, dried over anhydrous MgSO4, and
the solvent was evaporated under reduced pressure. The crude
product was purified by flash column chromatography (silica gel,
10% MeOH, 45% EtOAc in hexanes) to afford compound 46
(14.0 mg, 36%) as a yellow foam: Rf =0.25 (silica gel, EtOAc);
[a]25D = +93.4 (c=0.7 in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=
1.48 (s, 6H), 1.51–1.63 (m, 2H), 1.81–1.91 (m, 2H), 1.87 (s, 3H),
2.02–2.13 (m, 2H), 3.27–3.32 (m, 2H), 3.52 (s, 3H), 3.65 (dd, J=8.3,
1.7 Hz, 1H), 3.73 (d, J=8.3 Hz, 1H), 3.92 (dd, J=8.6, 1.7 Hz, 1H),
4.57 (ddd, J=11.5, 6.6, 1.8 Hz, 1H), 4.61 (dd, J=8.7, 8.0 Hz, 1H),
5.01–5.02 (m, 2H), 5.60 (dd, J=15.7, 8.0 Hz, 1H), 6.07 (t, J=6.1 Hz,
1H), 6.44 (d, J=15.7 Hz, 1H), 7.88 ppm (d, J=6.1 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=18.9, 27.2, 27.7, 28.3, 29.3, 31.7, 42.5, 52.3,
60.1, 69.4, 78.1, 79.7, 81.7, 109.5, 118.4, 125.8, 138.0, 141.7, 171.7,
175.3 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C20H32N2O6:
397.2339, found: 397.2344.
Experimental Section
General: 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 CH2Cl2 and benzene were distilled
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2013, 8, 819 – 831 825

CHEMMEDCHEM
FULL PAPERS
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1
Compound 47: A solution of vinyl iodide 28 (28.0 mg, 0.058 mmol)
and boronic MIDA ester 43 (18 mg, 0.064 mmol, 1.1 equiv) in a 3:1
mixture of THF/H2O (4.0 mL) was treated with Pd(dpephos)Cl2
(8.0 mg, 0.012 mmol, 0.2 equiv) and Tl2CO3 (55 mg, 0.116 mmol,
2.0 equiv). The reaction mixture was heated at 608C for 5 h, then
cooled to room temperature, diluted with Et2O, and washed with
a saturated aqueous KHSO4 solution. The aqueous phase was ex-
tracted with Et2O, and the combined organic phases were washed
with brine, dried over anhydrous MgSO4, and the solvent was
evaporated under vacuum. The crude product was purified by
flash column chromatography (silica gel, 45% EtOAc in hexanes) to
afford compound 47 (18.0 mg, 67%) as a yellow foam: Rf =0.25
(silica gel, EtOAc); [a]25D = +3.4 (c=0.9 in CH2Cl2); 1H NMR
(400 MHz, CDCl3): d=1.43 (s, 3H), 1.44 (s, 3H), 1.46–1.56 (m, 5H),
1.63–2.13 (m, 12H), 3.24–3.35 (m, 2H), 3.40 (s, 3H), 3.61–3.63 (m,
1H), 3.68–3.72 (m, 1H), 3.87 (ddd, J=8.6, 1.3, 0.8 Hz, 1H), 4.52–4.57
(m, 1H), 4.53 (dd, J=8.7, 7.9 Hz, 1H), 5.51 (dd, J=15.2, 8.1 Hz, 1H),
5.66 (dd, J=15.4, 6.9 Hz, 1H), 5.94 (t, J=6.2 Hz, 1H), 5.98 (dddd,
J=15.4, 10.4, 1.2, 0.7 Hz, 1H), 6.28 (dd, J=15.2, 10.4 Hz, 1H),
7.84 ppm (d, J=6.5 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=26.0,
26.1, 26.7, 27.3, 27.9, 28.9, 31.3, 32.6, 32.7, 40.7, 42.1, 51.9, 59.6,
69.1, 77.7, 79.4, 81.6, 109.0, 126.4, 126.7, 135.7, 142.4, 171.2,
174.9 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C25H40N2O6:
465.2965, found: 465.2958.
(c=1.3 in CH2Cl2); H NMR (400 MHz, CDCl3): d=0.17 (s, 9H), 1.41
(s, 3H), 1.44 (s, 3H), 1.46–1.55 (m, 2H), 1.78–1.91 (m, 2H), 2.00–2.10
(m, 2H), 3.24–3.30 (m, 2H), 3.48 (s, 3H), 3.60 (ddd, J=8.3, 5.5,
1.7 Hz, 1H), 3.69 (d, J=8.3 Hz, 1H), 3.79 (d, J=5.4 Hz, 1H), 3.89
(dd, J=8.4, 1.5 Hz, 1H), 4.52–4.57 (m, 2H), 5.80 (dd, J=15.9, 1.1 Hz,
1H), 6.12 (dd, J=15.9, 7.0 Hz, 1H), 6.34 (t, J=6.1 Hz, 1H), 7.86 ppm
(d, J=6.3 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=ꢀ0.2, 26.7, 27.1,
27.9, 28.9, 31.2, 42.1, 51.9, 59.7, 69.2, 79.3, 81.3, 96.1, 102.6, 109.7,
113.3, 140.4, 171.2, 174.9 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd
for C22H36N2O6Si: 453.2421, found: 453.2415.
Compound 50: A dry and argon-flushed 10 mL flask was charged
with anhydrous ZnCl2 (46 mg, 0.34 mmol, 1.0 equiv) and THF
(0.7 mL). The resulting solution was cooled at 08C, then cyclopen-
tylmagnesium chloride (177 mL, 2.0m in Et2O, 0.35 mmol,
1.05 equiv) was added dropwise at 08C. After addition, the reaction
mixture was stirred for an additional 15 min and then used directly
in the next reaction. Pd(PPh3)4 (18 mg, 0.015 mmol, 0.2 equiv) was
added to
a solution of vinyl iodide 29 (40 mg, 0.06 mmol,
1.0 equiv) in THF (2.5 mL), followed by a fresh cyclopentylzinc chlo-
ride (45) solution (0.7 mL, 0.5m in THF, 5.0 equiv) at 258C. After
stirring for 10 h at this temperature, the crude mixture was diluted
with EtOAc and quenched with a saturated aqueous NH4Cl solu-
tion. The organic phase was washed with brine, dried (MgSO4), and
filtered, and the solvent was evaporated under reduced pressure.
The crude product was purified by flash column chromatography
(silica gel, 50% EtOAc in hexanes) to afford compound 50 (22 mg,
61%) as a yellow oil: Rf =0.40 (silica gel, 60% EtOAc in hexanes);
[a]25D = +34.8 (c=0.1 in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=
0.08 (s, 3H), 0.09 (s, 3H), 0.84 (s, 9H), 1.23–1.34 (m, 3H), 1.37 (s,
3H), 1.38 (s, 3H), 1.46–1.64 (m, 5H), 1.75–1.88 (m, 4H), 1.97–2.02
(m, 2H), 2.11–2.15 (m, 2H), 2.44–2.55 (m, 1H), 3.21–3.29 (m, 2H),
3.42 (s, 3H), 3.75 (d, J=1.8 Hz, 1H), 4.02 (dd, J=7.9, 7.0 Hz, 1H),
4.07 (dd, J=7.0, 1.8 Hz, 1H), 4.27 (dd, J=8.0, 7.9 Hz, 1H), 4.46
(ddd, J=11.1, 5.5, 1.6 Hz, 1H), 5.55 (ddd, J=15.3, 8.0, 0.8 Hz, 1H),
5.75 (dd, J=15.2, 7.8 Hz, 1H), 5.94 (bs, 1H), 7.86 ppm (d, J=5.2 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=ꢀ4.7, ꢀ4.6, 18.2, 25.1, 25.8,
27.0, 27.1, 27.9, 29.0, 31.4, 32.8, 32.9, 42.1, 43.0, 51.9, 58.6, 74.6,
79.5, 80.8, 83.2, 108.1, 126.2, 141.1, 168.7, 175.0 ppm; HRMS (ESI-
TOF) m/z [M+H]+ calcd for C28H50N2O6Si: 539.3516, found:
539.3522.
Compound 48:
A solution of vinyl iodide 28 (29.0 mg,
0.060 mmol) and catechol boronic ester 44 (14 mL, 0.072 mmol,
1.2 equiv) in a 3:1 mixture of THF/H2O (4.0 mL) was treated with
Pd(dpephos)Cl2 (9.0 mg, 0.012 mmol, 0.2 equiv) and Tl2CO3 (56 mg,
0.12 mmol, 2.0 equiv). The reaction mixture was heated at 508C for
1 day, then was cooled to room temperature, diluted with Et2O,
and washed with a saturated aqueous KHSO4 solution. The aque-
ous phase was extracted with Et2O, and the organic phase was
washed with brine, dried over anhydrous MgSO4 and filtered, and
the solvent was evaporated under reduced pressure. The crude
product was purified by HPLC (preparative column, C8, 5 mm, 60%
CH3CN in H2O, 4.7 mLminꢀ1, tR =6.2 min) to afford compound 48
(14.0 mg, 54%) as a yellow foam: Rf =0.28 (silica gel, EtOAc);
[a]25D = +56.4 (c=0.7 in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=
0.96–1.01 (m, 6H), 1.45 (s, 6H), 1.48–1.61 (m, 2H), 1.79–1.88 (m,
2H), 1.98–2.10 (m, 1H), 2.07–2.14 (m, 3H), 2.17–2.26 (m, 2H), 3.22–
3.32 (m, 2H), 3.49 (s, 3H), 3.60–3.65 (m, 1H), 3.68 (d, J=6.6 Hz,
1H), 3.72 (d, J=8.0 Hz, 1H), 3.88 (dd, J=8.6, 1.5 Hz, 1H), 4.52–4.57
(m, 1H), 4.56 (dd, J=8.3, 8.1 Hz, 1H), 5.42 (t, J=7.3 Hz, 1H), 5.50
(dd, J=15.7, 8.0 Hz, 1H), 6.07 (bs, 1H), 6.21 (d, J=15.7 Hz, 1H),
7.85 ppm (d, J=5.9 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=13.7,
14.1, 19.8, 21.3, 26.8, 27.4, 27.9, 28.9, 31.3, 42.2, 51.9, 59.6, 69.1,
78.4, 79.5, 81.6, 108.9, 121.6, 135.7, 138.5, 171.3, 174.9 ppm; HRMS
(ESI-TOF) m/z [M+H]+ calcd for C23H38N2O6: 439.2808, found:
439.2811.
General procedure for bengamide E analogues 36, and 51–54: A
solution of the corresponding acetal derivative (0.01 mmol) in
MeOH (0.5–1.0 mL) was treated with a 70% aqueous AcOH solu-
tion (0.5–1.0 mL) at 708C for 1–2 h. After this time, the solvents
were removed by evaporation under reduced pressure. Purification
by HPLC (preparative column, C8, 5 mm, 40–50% CH3CN in H2O,
4.7 mLminꢀ1) afforded the corresponding bengamide E analogues.
Bengamide E analogue 36: 44%; white solid: Rf =0.14 (silica gel,
6% MeOH in CH2Cl2); tR =2.76 min (C8, 5 mm, 4.7 mLminꢀ1, 50%
CH3CN in H2O); [a]25D = +90.6 (c=0.3 in CH2Cl2); H NMR (400 MHz,
1
Compound 49: NEt3 (25 mL, 0.178 mmol, 2.0 equiv) and (trimethyl-
silyl)acetylene (16 mL, 0.116 mmol, 1.3 equiv) were added to a solu-
tion of compound 28 (43 mg, 0.089 mg), CuI (4.0 mg, 0.018 mmol,
0.2 equiv), PPh3 (5.0 mg, 0.018 mmol, 0.2 equiv) and Pd(OAc)2
(2.0 mg, 0.009 mmol, 0.1 equiv) in benzene (2.0 mL) at 258C. After
stirring for 2 h at 258C, the reaction mixture was diluted with
EtOAc and quenched with a saturated aqueous NH4Cl solution. The
aqueous phase was extracted with EtOAc, and the combined or-
ganic extracts were washed with brine, dried over MgSO4, filtered,
and concentrated under reduced pressure. The resulting crude
product was purified by flash column chromatography (silica gel,
45% EtOAc, 5% MeOH in hexanes) to afford compound 49 (32 mg,
79%) as a yellow solid: Rf =0.27 (silica gel, EtOAc); [a]25D = +85.4
CDCl3): d=1.41–1.47 (m, 1H), 1.55–1.64 (m, 1H), 1.76–1.90 (m, 2H),
2.04–2.07 (m, 2H), 3.27–3.31 (m, 2H), 3.54 (s, 3H), 3.65 (d, J=
3.3 Hz, 1H), 3.79–3.83 (m, 2H), 4.28 (t, J=5.0 Hz, 1H), 4.54 (dd, J=
11.1, 6.5 Hz, 1H), 6.33 (bs, 1H), 6.50 (d, J=14.6 Hz, 1H), 6.62 (dd,
J=14.6, 5.9 Hz, 1H), 7.95 ppm (d, J=6.3 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=28.0, 28.8, 30.9, 42.1, 52.1, 60.0, 71.3, 73.2,
75.7, 79.6, 80.5, 144.3, 172.0, 174.8 ppm; HRMS (ESI-TOF) m/z [M+
H]+ calcd for C14H23IN2O6: 443.0679, found: 443.0685.
Bengamide E analogue 51: 53%; white solid; Rf =0.10 (silica gel,
5% MeOH in CH2Cl2); tR =5.54 min (C8, 5 mm, 2.98 mLminꢀ1, 40%
CH3CN in H2O); [a]25D = +45.7 (c=0.2 in CH2Cl2); H NMR (400 MHz,
1
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2013, 8, 819 – 831 826

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CDCl3): d=1.76–1.90 (m, 3H), 1.85 (s, 3H), 2.02–2.10 (m, 2H), 3.26–
3.32 (m, 2H), 3.55 (s, 3H), 3.66 (dd, J=5.0, 1.2 Hz, 1H), 3.79 (d, J=
7.4 Hz, 1H), 3.83 (dd, J=7.4, 1.4 Hz, 1H), 4.37 (dd, J=6.2, 5.8 Hz,
1H), 4.54 (ddd, J=11.3, 6.4, 1.5 Hz, 1H), 5.00 (s, 2H), 5.68 (dd, J=
15.7, 7.0 Hz, 1H), 6.04 (bs, 1H), 6.44 (d, J=15.7 Hz, 1H), 8.00 ppm
(d, J=6.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=18.6, 28.0, 28.9,
31.0, 42.1, 52.1, 60.1, 72.3, 73.1, 74.1, 80.5, 117.3, 127.8, 135.6,
141.4, 172.2, 174.6 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for
C17H28N2O6: 357.2026, found: 357.2032.
+66.7 (c=0.6 in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=1.26–1.32
(m, 3H), 1.43 (s, 6H), 1.52–1.55 (m, 3H), 1.57–1.67 (m, 2H), 1.72–
1.79 (m, 2H), 1.81–1.89 (m, 2H), 2.10–2.12 (m, 2H), 2.39–2.49 (m,
1H), 3.24–3.30 (m, 2H), 3.49 (s, 3H), 3.58–3.61 (m, 1H), 3.66–3.67
(m, 1H), 3.71 (d, J=8.1 Hz, 1H), 3.84 (dd, J=8.6, 1.6 Hz, 1H), 4.46
(dd, J=8.4 Hz, 1H), 4.55 (ddd, J=11.2, 6.4, 1.6 Hz, 1H), 5.38 (ddd,
J=15.3, 8.3, 1.0 Hz, 1H), 5.81 (dd, J=15.3, 7.6 Hz, 1H), 6.14 (bs,
1H), 7.85 ppm (d, J=6.3 Hz, 1H); 13C NMR (100 MHz, CDCl3) d=
25.1, 26.8, 27.3, 27.9, 28.9, 31.3, 32.7, 32.8, 42.1, 43.0, 51.9, 59.6,
60.0, 78.0, 79.2, 81.5, 108.8, 124.3, 142.1, 171.3, 174.9 ppm; HRMS
(ESI-TOF) m/z [M+H]+ calcd for C22H36N2O6: 425.2652, found:
425.2648.
Bengamide E analogue 52: 58%; white solid; Rf =0.24 (silica gel,
6% MeOH in CH2Cl2); tR =5.54 min (C8, 5 mm, 4.7 mLminꢀ1, 50%
1
CH3CN in H2O); [a]25D = +35.0 (c=0.1 in CH2Cl2); H NMR (400 MHz,
CDCl3): d=1.06–1.50 (m, 7H), 1.65–2.12 (m, 10H), 3.06 (bs, 1H),
3.11 (d, J=6.7 Hz, 1H), 3.26–3.33 (m, 2H), 3.55 (s, 3H), 3.63 (dd, J=
6.2, 5.2 Hz, 1H), 3.78 (d, J=7.4 Hz, 1H), 3.82 (d, J=7.5 Hz, 1H), 4.30
(dd, J=6.4, 6.1 Hz, 1H), 4.42 (bs, 1H), 4.53 (dd, J=10.5, 5.6 Hz, 1H),
5.59 (dd, J=15.3, 7.1 Hz, 1H), 5.67 (dd, J=15.3, 6.8 Hz, 1H), 5.90–
6.00 (m, 1H), 6.00 (dd, J=15.4, 10.4 Hz, 1H), 6.30 (dd, J=15.3,
10.4 Hz, 1H), 8.00 ppm (d, J=6.1 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d=25.9, 26.1, 27.9, 28.9, 31.1, 32.7, 40.7, 42.1, 52.0, 60.1,
72.3, 73.0, 74.1, 77.7, 80.6, 126.9, 128.7, 133.8, 141.8, 172.3,
174.6 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C22H36N2O6:
425.2652, found: 425.2656.
Bengamide E analogue 56: Hydroxyamide 55 (12.5 mg,
0.029 mmol) was subjected to acidic hydrolysis according to the
general procedure described above to obtain, after purification by
HPLC (preparative column, C8, 5 mm, 50% CH3CN in H2O,
4.7 mLminꢀ1), bengamide E analogue 56 (5.5 mg, 49%) as a yellow
solid: Rf =0.30 (silica gel, 8% MeOH in CH2Cl2); tR =3.12 min (C8,
5 mm, 4.7 mLminꢀ1, 50% CH3CN-50% H2O); [a]25D = +17.4 (c=0.2
1
in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.24–1.35 (m, 2H), 1.38–
1.47 (m, 1H), 1.52–1.66 (m, 5H), 1.75–1.80 (m 3H), 1.86–1.90 (m,
1H), 2.03–2.08 (m, 2H), 2.44 (sext, J=8.2 Hz, 1H), 3.22–3.36 (m,
2H), 3.53 (s, 3H), 3.60 (dd, J=5.3, 0.8 Hz, 1H), 3.78 (d, J=6.9 Hz,
1H), 3.83 (dd, J=6.9, 0.9 Hz, 1H), 4.22 (dd, J=6.3, 6.2 Hz, 1H), 4.54
(ddd, J=11.5, 6.8, 1.2 Hz, 1H), 5.48 (ddd, J=15.4, 7.2, 0.8 Hz, 1H),
5.79 (dd, J=15.2, 7.6 Hz, 1H), 6.17 (bs, 1H), 8.00 ppm (d, J=6.2 Hz,
1H); 13C NMR (100 MHz, CDCl3): d=25.1, 28.0, 28.8, 31.0, 32.8, 32.9,
42.1, 42.9, 52.0, 59.9, 72.4, 72.7, 74.1, 81.1, 126.4, 139.5, 172.1,
174.7 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C19H32N2O6:
385.2339, found: 385.2329.
Bengamide E analogue 53: 45%; white solid; Rf =0.53 (silica gel,
10% MeOH in CH2Cl2); tR =3.82 min (C8, 5 mm, 4.7 mLminꢀ1, 50%
1
CH3CN in H2O); [a]25D = +55.2 (c=0.2 in CH2Cl2); H NMR (400 MHz,
CDCl3): d=0.96–1.01 (m, 6H), 1.38–1.59 (m, 2H), 1.77–1.90 (m, 2H),
2.02–2.08 (m, 2H), 2.12 (q, J=7.5 Hz, 2H), 2.22 (q, J=7.6 Hz, 2H),
3.12 (bs, 1H), 3.10–3.26 (m, 3H), 3.53 (s, 3H), 3.64–3.67 (m, 1H),
3.79 (d, J=7.0 Hz, 1H), 3.84 (d, J=6.8 Hz, 1H), 4.32 (dd, J=6.3,
6.1 Hz, 1H), 4.40 (bs, 1H), 4.53 (ddd, J=11.2, 6.6, 1.4 Hz, 1H), 5.43
(t, J=7.4 Hz, 1H), 5.60 (dd, J=15.8, 7.2 Hz, 1H), 6.10 (bs, 1H), 6.22
(d, J=15.8 Hz, 1H), 7.97 ppm (d, J=6.2 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d=13.7, 14.1, 19.8, 21.3, 27.9, 28.8, 31.0, 42.1, 52.1, 59.9,
72.6, 72.7, 74.6, 81.0, 115.5, 121.0, 124.0, 135.1, 136.6, 138.6, 143.8,
172.1, 174.7 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for
C20H34N2O6: 399.2495, found: 399.2484.
Epoxyamide 60: A solution of oxalyl chloride (0.59 mL, 6.65 mmol,
2.0 equiv) in CH2Cl2 (12 mL) was cooled to ꢀ788C, and DMSO
(0.94 mL, 13.3 mmol, 4.0 equiv) was added dropwise. After 10 min,
a solution of alcohol 59 (1.05 g, 3.32 mmol, 1.0 equiv) in CH2Cl2
(6 mL) was added. The reaction mixture was stirred at ꢀ788C for
40 min, then NEt3 (2.8 mL, 19.94 mmol, 6.0 equiv) was added at
this temperature. After 10 min at ꢀ788C, the reaction was allowed
to reach room temperature and then diluted with Et2O and
washed with a saturated aqueous NH4Cl solution. The organic
phase was washed with H2O and brine, dried over anhydrous
MgSO4, filtered, and the solvent was evaporated under reduced
pressure. The resulting crude aldehyde was used in the next step
without purification. An aqueous NaOH solution (3.0 m 1.22 mL,
3.65 mmol, 1.1 equiv) was added to a solution of sulfonium salt 24
(1.15 g, 3.65 mmol, 1.1 equiv) in H2O (15 mL). A solution of crude
aldehyde (~3.32 mmol) in CH2Cl2 (15 mL) was added, and the reac-
tion mixture was stirred vigorously overnight at 258C. After this
time, both phases were separated, and the aqueous layer was ex-
tracted twice with CH2Cl2. Combined organic extracts were then
washed with H2O and brine, dried over anhydrous MgSO4, filtered,
and concentrated. Purification of the crude product by flash
column chromatography (silica gel, 30% EtOAc in hexanes) provid-
ed epoxyamide 60 (1.09 g, 62% over two steps) as a yellow oil:
Rf =0.35 (silica gel, 30% EtOAc in hexanes); [a]25D = +68.6 (c=1.2
Bengamide E analogue 54: 28%; white solid; Rf =0.13 (silica gel,
5% MeOH in CH2Cl2); tR =4.7 min (C8, 5 mm, 4.7 mLminꢀ1, 50%
1
CH3CN in H2O); [a]25D = +19.7 (c=0.2 in CH2Cl2); H NMR (400 MHz,
CDCl3): d=0.17 (s, 9H), 1.41–1.48 (m, 1H), 1.57–1.63 (m, 1H), 1.77–
1.91 (m, 2H), 2.06 (d, J=12.1 Hz, 2H), 3.26–3.31 (m, 2H), 3.54 (s,
3H), 3.64 (dd, J=4.7, 1.4 Hz, 1H), 3.79 (d, J=7.3 Hz, 1H), 3.82 (dd,
J=7.3, 1.4 Hz, 1H), 4.35 (ddd, J=6.0, 4.4, 1.4 Hz, 1H), 4.54 (dd, J=
10.5, 6.6 Hz, 1H), 5.86 (dd, J=15.9, 1.5 Hz, 1H), 6.20 (dd, J=15.9,
5.8 Hz, 1H), 6.20–6.23 (m, 1H), 7.97 ppm (d, J=6.4 Hz, 1H);
13C NMR (100 MHz, CDCl3): d=ꢀ0.1, 27.9, 28.8, 31.0, 42.1, 52.1,
60.1, 71.8, 73.3, 77.2, 80.4, 95.6, 103.1, 112.1, 142.4, 172.2,
174.7 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C19H32N2O6Si:
413.2108, found: 413.2096.
Hydroxyamide 55:
A
solution of compound 50 (20 mg,
0.037 mmol, 1.0 equiv) in THF (3.0 mL) was treated with TBAF
(74 mL, 1.0 m solution in THF, 2.0 equiv) at 258C. 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, extracted twice with Et2O, and the combined
organic phases were washed with H2O and brine, dried over anhy-
drous MgSO4, and concentrated under reduced pressure. Purifica-
tion of the resulting crude product by flash column chromatogra-
phy (silica gel, 90% EtOAc in hexanes) afforded hydroxyamide 55
1
in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.40 (s, 6H), 1.55 (s, 3H),
1.64 (s, 3H), 1.76–1.86 (m, 1H), 2.07–2.11 (m, 1H), 2.13 (s, 3H), 2.47
(ddd, J=13.3, 9.0, 6.7 Hz, 1H), 2.70 (ddd, J=13.3, 7.1, 5.0 Hz, 1H),
3.50 (dd, J=3.3, 1.9 Hz, 1 Hz, 1H), 3.70 (d, J=1.9 Hz, 1H), 3.90–3.92
(m, 1H), 3.93 (d, J=3.4 Hz, 1H), 4.05 (ddd, J=9.2, 5.2, 1.4 Hz, 1H),
4.35 (ddd, J=10.5, 4.8, 3.2 Hz, 1H), 4.74 (t, J=8.1 Hz, 1H),
6.56 ppm (d, J=8.1 Hz, 1H); HRMS (ESI-TOF) m/z [M+H]+ calcd for
C18H27Br2NO5S: 528.0055, found: 528.0062.
(15.3 mg, 97%) as a yellow oil: Rf =0.27 (silica gel, EtOAc); [a]25
=
D
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Epoxy alcohol 61: Epoxyamide 60 (221 mg, 0.42 mmol, 1.0 equiv)
in THF (5.0 mL) was treated with Super-H (1.05 mL, 1.0m in THF,
1.05 mmol, 2.5 equiv) at 08C. After 20 min at this temperature, the
reaction mixture was diluted with Et2O and washed with a saturat-
ed aqueous NH4Cl solution. The aqueous phase was separated, ex-
tracted twice with Et2O, and the combined organic phase washed
with H2O and brine, dried over anhydrous MgSO4, and concentrat-
ed under reduced pressure. Purification of the resulting crude
product by flash column chromatography (silica gel, 30% EtOAc in
hexanes) provided epoxy alcohol 61 (102 mg, 68%) as a yellow oil:
Rf =0.38 (silica gel, 40% EtOAc in hexanes); [a]25D = +23.7 (c=0.8
action mixture was treated with a saturated aqueous NH4Cl solu-
tion. The aqueous solution was extracted with Et2O, and the com-
bined organic phases were washed with brine, dried over MgSO4,
filtered, and the solvent was evaporated under reduced pressure.
The crude product was purified by flash column chromatography
(silica gel, 5% MeOH in CH2Cl2) to afford alkyne 64 (25.3 mg, 78%)
as a white solid: Rf =0.32 (silica gel, 5% MeOH in CH2Cl2); tR =
4.0 min (C8, 5 mm, 4.7 mLminꢀ1, 60% CH3CN-40% H2O); [a]25D = +
1
86.7 (c=0.6 in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.152 (d, J=
6.9 Hz, 3H), 1.154 (d, J=6.9 Hz, 3H), 1.42 (s, 3H), 1.48 (s, 3H), 1.53–
1.56 (m, 1H), 1.63–1.69 (m, 1H), 1.84–1.89 (m, 2H), 2.00–2.05 (m,
1H), 2.08–2.12 (m, 1H), 2.54–2.63 (m, 1H), 3.25–3.31 (m, 2H), 3.51
(s, 3H), 3.72–3.73 (m, 2H), 3.70–3.79 (m, 1H), 4.12 (dd, J=7.8,
1.5 Hz, 1H), 4.56 (ddd, J=11.2, 6.3, 1.7 Hz, 1H), 4.75 (dd, J=7.8,
1.7 Hz, 1H), 6.15 (bs, 1H), 7.91 ppm (d, J=6.1 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=20.5, 22.7, 26.6, 26.7, 27.9, 28.9, 31.3, 42.1,
51.9, 59.7, 66.7, 69.5, 75.5, 80.7, 81.1, 93.0, 109.9, 171.4, 174.8 ppm;
HRMS (ESI-TOF) m/z [M+H]+ calcd for C20H32N2O6: 397.2339,
found: 397.2348.
1
in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.41 (s, 3H), 1.43 (s, 3H),
1.70 (bs, 1H), 3.17 (ddd, J=5.8, 3.7, 2.5 Hz, 1H), 3.20 (dd, J=5.0,
2.3 Hz, 1H), 3.68 (dd, J=8.3, 5.0 Hz, 1H), 3.72–3.75 (m, 1H), 4.01
(ddd, J=12.8, 4.0, 1.8 Hz, 1H), 4.65 (t, J=8.2 Hz, 1H), 6.52 ppm (d,
J=8.2 Hz, 1H); HRMS (ESI-TOF) m/z [M+H]+ calcd for C10H14Br2O4:
356.9337, found: 356.9345.
Hydroxyamide 63: Epoxy alcohol 61 (1.2 g, 3.35 mmol, 1.0 equiv)
was dissolved in a 1:1 mixture of MeOH/B(OMe)3 (34 mL). The re-
sulting solution was treated with DBU (0.5 mL, 3.35 mmol,
1.0 equiv) and heated at 708C for 1 day. After this time, the reac-
tion mixture was allowed to reach room temperature, cooled to
08C, and then treated with a saturated aqueous NaHCO3 solution.
After stirring for 30 min at 08C, EtOAc was added, and both phases
were separated. The aqueous phase was extracted with EtOAc, and
the combined organic extracts were washed with H2O and brine,
dried over anhydrous MgSO4, and the solvent was evaporated
under reduced pressure. The resulting crude product (1.28 g) was
used in the next step without purification. The crude diol (63 mg,
0.16 mmol, 1.0 equiv) was dissolved in a mixture of CH3CN/H2O
(4.0 mL, 1:1), and the resulting solution was treated with BAIB
(312 mg, 0.97 mmol, 6.0 equiv) followed by TEMPO (21 mg,
0.13 mmol, 0.8 equiv) at 258C. After 3 h, the crude mixture was di-
luted with EtOAc, quenched by the addition of a saturated aque-
ous Na2S2O3 solution and, after separation of both layers, the aque-
ous phase was extracted with EtOAc. The combined organic solu-
tion was washed again with a saturated aqueous Na2S2O3 solution,
then dried over anhydrous MgSO4, and the solvent was evaporated
under reduced pressure. The crude acid was dissolved in DMF
(4 mL) and treated with DIPEA (55 mL, 0.32 mmol, 2.0 equiv), l-Lys-
lactam 62 (40 mg, 0.24 mmol, 1.5 equiv), and BOP (87 mg,
0.19 mmol, 1.2 equiv) at 258C. After 2 h, the crude mixture was di-
luted with Et2O and washed with a saturated aqueous NH4Cl solu-
tion. The aqueous phase was washed with Et2O, and the combined
organic extracts were washed with brine, dried over anhydrous
MgSO4, and the solvent was evaporated under vacuum. Purification
of the resulting crude product by flash column chromatography
(silica gel, EtOAc) provided amide 63 (33 mg, 40% over three
steps) as a white solid: Rf =0.27 (silica gel, EtOAc); [a]25D = +65.3
Alkynyl bengamide E 65:
A solution of acetal derivate 64
(11.7 mg, 0.03 mmol, 1.0 equiv) in MeOH (0.5 mL) was treated with
a 70% aqueous AcOH solution (1.5 mL) at 708C for 2 h. The solvent
was then removed by evaporation under reduced pressure. Purifi-
cation of the crude product by HPLC (preparative column, C8,
5 mm, 30% CH3CN in H2O, 4.7 mLminꢀ1) afforded the correspond-
ing alkynyl bengamide E analogue 65 (10 mg, 35%) as a white
solid: Rf =0.30 (silica gel, 5% MeOH in CH2Cl2); tR =4.25 min (C8,
5 mm, 4.7 mLminꢀ1, 30% CH3CN in H2O); [a]25D = +73.5 (c=0.2 in
CH2Cl2); 1H NMR (400 MHz, CDCl3): d=1.152 (d, J=6.9 Hz, 3H),
1.154 (d, J=6.9 Hz, 3H), 1.41–1.53 (m, 2H), 1.80–1.90 (m, 2H),
2.03–2.10 (m, 2H), 2.55–2.63 (m, 1H), 3.26–3.32 (m, 2H), 3.55 (s,
3H), 3.73 (dd, J=6.9, 1.2 Hz, 1H), 3.80 (d, J=6.4 Hz, 1H), 4.04 (dd,
J=6.4, 0.8 Hz, 1H), 4.10–4.22 (m, 1H), 4.51 (ddd, J=11.4, 6.3,
1.8 Hz, 1H), 4.76 (dd, J=7.9, 1.7 Hz, 1H), 6.1 (t, J=5.7 Hz, 1H),
7.91 ppm (d, J=6.2 Hz, 1H); HRMS (ESI-TOF) m/z [M+H]+ calcd for
C17H28N2O6: 357.2026, found: 357.2034.
Methyl alkyne 67: nBuLi (1.6 m in hexane, 708 mL, 1.13 mmol,
1.4 equiv) was added dropwise to a solution of alkyne 66 (219 mg,
0.81 mmol, 1.0 equiv) in THF (6 mL) at ꢀ788C. The reaction mixture
was stirred at ꢀ788C for 1 h, followed by addition of HMPA
(145 mL, 0.83 mmol, 1.03 equiv) and methyl iodide (103 mL,
1.62 mmol, 2.0 equiv). The resulting solution was stirred for 8 h
while gradually warming to 258C. A saturated aqueous NH4Cl solu-
tion was then added, and the mixture was extracted with EtOAc.
The combined organic phases were washed with brine, dried over
anhydrous MgSO4, and concentrated under reduced pressure. The
resulting residue was purified by flash column chromatography
(silica gel, 8% EtOAc in hexanes) to afford methyl alkyne 67
(184 mg, 80%) as a yellow oil: Rf =0.48 (silica gel, 10% EtOAc in
hexanes); 1H NMR (400 MHz, CDCl3): d=0.07 (s, 6H), 0.90 (s, 9H),
1.39 (d, J=0.6 Hz, 3H), 1.48 (d, J=0.6 Hz, 3H), 1.85 (d, J=2.1 Hz,
3H), 3.77 (dd, J=4.0, 3.0 Hz, 2H), 4.00 (ddd, J=7.7, 4.0, 3.6 Hz, 1H),
4.54 ppm (dq, J=7.6, 2.1 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=
ꢀ5.5, ꢀ5.3, 3.7, 18.3, 25.8, 26.5, 26.8, 61.9, 67.5, 75.9, 82.2, 82.8,
109.8 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C15H28O3Si:
285.1886, found: 285.1864.
1
(c=0.3 in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.41 (s, 3H), 1.45
(s, 3H), 1.49–1.59 (m, 1H), 1.79–1.89 (m, 3H), 2.00–2.10 (m, 2H),
3.25–3.30 (m, 2H), 3.50 (s, 3H), 3.68–3.70 (m, 2H), 3.88 (dd, J=2.8,
1.8 Hz, 1H), 4.00 (dd, J=8.1, 1.2 Hz, 1H), 4.55 (ddd, J=11.2, 6.3,
1.6 Hz, 1H), 4.80 (dd, J=8.3 Hz, 1H), 6.43 (t, J=5.9 Hz, 1H), 6.49 (d,
J=8.4 Hz, 1H), 7.87 ppm (d, J=6.3 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d=26.7, 27.0, 28.0, 28.9, 31.2, 42.1, 52.0, 59.7, 69.6, 76.6,
78.7, 81.2, 94.4, 110.1, 135.8, 171.1, 174.9 ppm; HRMS (ESI-TOF) m/z
[M+H]+ calcd for C17H26Br2N2O6: 513.0236, found: 513.0229.
Vinyl iodide 68: Bu3SnH (2.1 mL, 7.57 mmol, 5.0 equiv) was added
slowly (0.1 mLhꢀ1) to a solution of methyl alkyne 67 (431 mg,
1.51 mmol, 1.0 equiv) and PdCl2(PPh3)2 (106 mg, 0.151 mmol,
0.1 equiv) in THF (10 mL). After addition, the reaction mixture was
concentrated under reduced pressure, and the crude product was
purified by flash column chromatography (silica gel, 2% EtOAc in
Alkyne 64: Diisopropylzinc (53 mL, 0.053 mmol, 1.0 m in toluene,
0.65 equiv) was added to a solution of 63 (42 mg, 0.082 mmol,
1.0 equiv) and Pd(dpephos)Cl2 (12 mg, 0.016 mmol, 0.2 equiv) in
THF/DMF (4 mL, 1:1) at 258C. After 6 h at this temperature, the re-
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hexanes) to afford the corresponding stannyl derivative (844 mg,
97%) as a yellow oil: Rf =0.25 (silica gel, 2% EtOAc in hexanes);
1H NMR (400 MHz, CDCl3): d=0.05 (s, 3H), 0.06 (s, 3H), 0.88 (t, J=
7.3 Hz, 9H), 0.89 (s, 9H), 1.25–1.35 (m, 6H), 1.41 (s, 3H), 1.44 (s,
3H), 1.45–1.52 (m, 12H), 1.94 (d, J=1.8 Hz, 3H), 3.61–3.64 (m, 1H),
3.66 (dd, J=5.5, 2.7 Hz, 1H), 3.77–3.81 (m, 1H), 4.88 (dd, J=8.2 Hz,
1H), 5.54 ppm (dq, J=8.3, 1.8 Hz, 1H). This stannyl derivative
(844 mg, 1.47 mmol, 1.0 equiv) was dissolved in CH2Cl2 (20 mL) and
cooled to 08C. Then, I2 (409 mg, 1.61 mmol, 1.1 equiv) was added,
and the resulting mixture was stirred at this temperature for 1 h.
After this time, saturated aqueous Na2S2O3 solution was added and
the reaction mixture was extracted with Et2O. The combined or-
ganic layers were washed with brine, dried over anhydrous MgSO4,
filtered, and concentrated under reduced pressure. The crude
product was purified by flash column chromatography (silica gel,
2% EtOAc in hexanes) to afford vinyl iodide 68 (590 mg, 97%) as
a yellow oil: Rf =0.30 (silica gel, 2% EtOAc in hexanes); 1H NMR
(400 MHz, CDCl3): d=0.06 (s, 3H), 0.07 (s, 3H), 0.89 (s, 9H), 1.39 (s,
3H), 1.40 (s, 3H), 2.49 (d, J=1.5 Hz, 3H), 3.70–3.71 (m, 1H), 3.71–
3.74 (m, 1H), 3.75–3.80 (m, 1H), 4.65 (dd, J=8.7, 7.9 Hz, 1H),
6.21 ppm (dq, J=8.7, 1.5 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=
ꢀ5.5, ꢀ5.3, 18.3, 25.8, 26.9, 27.1, 28.4, 61.6, 74.6, 80.6, 100.4, 109.1,
138.1 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C15H29IO3Si:
413.1009, found: 413.1007.
gel, 50% EtOAc in hexanes); [a]25D = +15.8 (c=0.5 in CH2Cl2);
1H NMR (400 MHz, CDCl3): d=1.43 (s, 6H), 2.13 (bs, 2H), 2.51 (d, J=
1.5 Hz, 3H), 3.19 (ddd, J=7.9, 4.0, 3.4 Hz, 1H), 3.44 (s, 3H), 3.56
(dd, J=8.1, 1.6 Hz, 1H), 3.81 (dd, J=12.0, 3.4 Hz, 1H), 3.88-3.92 (m,
2H), 4.73 (dd, J=8.7, 8.6 Hz, 1H), 6.26 ppm (dq, J=8.9, 1.5 Hz, 1H);
13C NMR (100 MHz, CDCl3): d=26.8, 27.2, 28.6, 58.0, 60.7, 67.8, 74.0,
79.0, 81.5, 101.9, 109.4, 137.2 ppm; HRMS (ESI-TOF) m/z [M+H]+
calcd for C12H21IO5: 373.0512, found: 373.0528.
Amide 72: The selective oxidation of diol 71 (22 mg, 0.059 mmol)
to the carboxylic acid and subsequent coupling with l-Lys-lactam
62 (15 mg, 0.09 mmol, 1.5 equiv) was carried out exactly as de-
scribed above for 63 to yield amide 72 (17 mg, 58% over two
steps) as a yellow foam: Rf =0.30 (silica gel, EtOAc); [a]25D = +38.6
1
(c=0.7 in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.42 (s, 3H), 1.44
(s, 3H), 1.50–1.60 (m, 2H), 1.68–1.89 (m, 2H), 2.10–2.11 (m, 2H),
2.49 (d, J=1.5 Hz, 1H), 3.26–3.33 (m, 2H), 3.51 (s, 3H), 3.57 (dd, J=
8.6, 1.6 Hz, 1H), 3.71 (d, J=8.6 Hz, 1H), 3.86 (dd, J=8.5, 1.7 Hz,
1H), 4.55 (ddd, J=11.3, 6.3, 1.6 Hz, 1H), 4.82 (dd, J=8.8, 8.5 Hz,
1H), 6.15–6.20 (m, 1H), 6.19 (dd, J=8.8, 1.5 Hz, 1H), 7.93 ppm (d,
J=6.2 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=26.7, 27.2, 27.9, 28.5,
28.9, 31.3, 42.1, 51.9, 59.9, 68.9, 73.4, 78.6, 80.9, 101.3, 109.3, 137.5,
171.6, 174.8 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for
C18H29IN2O6: 497.1148, found: 497.1156.
Alcohol 69: TBAF (1.9 mL, 1.0m in THF, 1.90 mmol, 1.2 equiv) was
added to a solution of 68 (651 mg, 1.58 mmol, 1.0 equiv) in THF
(15 mL) at 258C. After 1 h at this temperature, the reaction mixture
was diluted with Et2O and treated with a saturated aqueous NH4Cl
solution. The aqueous phase was extracted with Et2O, and the
combined organic phases were washed with H2O and brine, dried
over MgSO4, filtered, and the solvent was evaporated under re-
duced pressure. The obtained crude product was purified by flash
column chromatography (silica gel, 20% EtOAc in hexanes) to
afford alcohol 69 (420 mg, 89%) as a yellow oil: Rf =0.23 (silica gel,
Hydroxyamide 73: Pd(dpephos)Cl2 (5.0 mg, 0.007 mmol, 0.2 equiv)
was added to a solution of vinyl iodide 72 (17.0 mg, 0.03 mmol,
1.0 equiv) in a 1:1 mixture of DMF/THF (2.0 mL), followed by diiso-
propylzinc (22 mL, 1.0m solution in THF, 0.022 mmol, 0.65 equiv) at
258C. After stirring for 10 h at 258C, the reaction mixture was treat-
ed with H2O, diluted with Et2O and, after separation of both layers,
the aqueous phase was extracted with Et2O twice. The resulting or-
ganic solution was then washed with brine, dried over MgSO4, fil-
tered, and concentrated under reduced pressure. Purification of
the crude product by flash column chromatography (silica gel,
EtOAc) provided compound 73 (8.0 mg, 65%) as a yellow foam:
Rf =0.30 (silica gel, EtOAc); [a]25D = +25.0 (c=0.2 in CH2Cl2);
1H NMR (400 MHz, CDCl3): d=1.00 (d, J=6.8 Hz, 6H), 1.44 (s, 3H),
1.45 (s, 3H), 1.48–1.59 (m, 2H), 1.69 (dd, J=4.6, 1.3 Hz, 3H), 1.81–
1.90 (m, 2H), 1.97–2.03 (m, 1H), 2.05–2.14 (m, 1H), 2.23–2.30 (m,
1H), 3.25–3.34 (m, 2H), 3.51 (s, 3H), 3.54 (dd, J=8.4, 1.7 Hz, 1H),
3.70 (dd, J=8.3, 1.5 Hz, 1H), 3.79 (dd, J=8.6, 1.6 Hz, 1H), 4.55
(ddd, J=11.1, 6.4, 1.7 Hz, 1H), 4.83–4.89 (m, 1H), 5.16–5.18 (m, 1H),
6.02 (t, J=6.4 Hz, 1H), 7.90 ppm (d, J=6.0 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=13.8, 14.2, 21.1, 21.2, 26.8, 27.4, 27.9, 28.9,
31.3, 36.8, 42.1, 51.8, 59.7, 69.1, 73.3, 79.5, 81.4, 108.6, 118.6, 171.6,
174.9 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd for C21H36N2O6:
413.2652, found: 413.2671.
1
20% EtOAc in hexanes); H NMR (400 MHz, CDCl3): d=1.41 (s, 6H),
2.06–2.07 (m, 1H), 2.49 (d, J=1.5 Hz, 3H), 3.54 (ddd, J=12.1, 8.6,
3.4 Hz, 1H), 3.77 (ddd, J=8.4, 3.2, 2.3 Hz, 1H), 3.84 (ddd, J=12.3,
2.8, 2.1 Hz, 1H), 4.62 (dd, J=8.6 Hz, 1H), 6.20 ppm (dq, J=8.8,
1.5 Hz, 1H).
Epoxyamide 70: Epoxyamide 70 (193 mg, 55% over two steps)
was prepared from alcohol 69 (203 mg, 0.681 mmol) by a Swern
oxidation, followed by reaction with sulfonium salt 24 (236 mg,
0.749 mmol, 1.10 equiv) according to the same procedure de-
scribed above for the preparation of 60, to afford 70 as a yellow
oil: Rf =0.55 (silica gel, 30% EtOAc in hexanes); [a]25D = +63.6 (c=
0.8 in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=1.35 (s, 3H), 1.38 (s,
3H), 1.53 (s, 6H), 2.12 (d, J=1.6 Hz, 3H), 2.43–2.50 (m, 2H), 2.54 (s,
3H), 2.56–2.65 (m, 2H), 3.31 (d, J=1.9 Hz, 1H), 3.69 (dd, J=1.9,
1.8 Hz, 1H), 3.86–3.89 (m, 2H), 4.00–4.03 (m, 1H), 4.30–4.35 (m,
1H), 4.65–4.70 (m, 1H), 6.24 ppm (dq, J=8.8, 1.6 Hz, 1H); 13C NMR
(100 MHz, CDCl3): d=26.8, 27.2, 28.6, 58.0, 60.7, 67.8, 74.0, 79.0,
81.5, 101.9, 109.4, 137.2 ppm; HRMS (ESI-TOF) m/z [M+H]+ calcd
for C19H30INO5S: 512.0968, found: 512.0972.
Bengamide E analogue 58: A solution of acetal derivative 73
(3.0 mg, 0.007 mmol, 1.0 equiv) in MeOH (0.5 mL) was treated with
a 70% aqueous AcOH solution (1.0 mL) at 708C for 2 h. After this
time, the solvent was removed by evaporation under reduced
pressure. The resulting crude product was purified by HPLC (prepa-
rative column, C8, 5 mm, 30% CH3CN in H2O, 4.7 mLminꢀ1) to afford
bengamide E analogue 58 (1.5 mg, 57%) as a white solid: Rf =0.30
(silica gel, 10% MeOH in CH2Cl2); tR =2.94 min (C8, 5 mm,
Diol 71: Epoxyamide 70 (193 mg, 0.378 mmol, 1.0 equiv) in THF
(5 mL) was treated with Super-H (950 mL, 1.0m in THF, 0.95 mmol,
2.5 equiv) according to the same procedure described above for
epoxyamide 60. The resulting crude epoxy alcohol was used in the
next step without further purification. The crude alcohol (~
0.378 mmol) was dissolved in a 1:1 mixture of MeOH/B(OMe)3
(2.0 mL) and treated with DBU (56 mL, 0.378 mmol, 1.0 equiv) ac-
cording to the same procedure described above for 61, to afford
diol 71 (59 mg, 42% over two steps) as a yellow oil: Rf =0.22 (silica
4.7 mLminꢀ1
,
50% CH3CN in H2O); [a]25D = +42.3 (c=0.1 in
1
CH2Cl2); H NMR (400 MHz, CDCl3): d=1.01 (d, J=6.8 Hz, 6H), 1.41–
1.49 (m, 2H), 1.69 (dd, J=5.8, 1.2 Hz, 3H), 1.84–1.90 (m, 2H), 2.00–
2.10 (m, 2H), 2.26–2.29 (m, 1H), 3.28–3.32 (m, 2H), 3.53 (s, 3H),
3.56–3.58 (m, 1H), 3.77 (bs, 2H), 4.36 (bs, 1H), 4.52–4.56 (m, 2H),
5.23 (d, J=8.7 Hz, 1H), 6.17 (bs, 1H), 7.94 ppm (bs, 1H); HRMS (ESI-
TOF) m/z [M+H]+ calcd for C18H32N2O6: 373.2339, found: 373.2325.
ꢂ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Acknowledgements
This work was financially supported by the Ministerio de Ciencia
e Innovaciꢂn (ref. CTQ2010-16933), the Junta de Andalucꢀa
(FQM-03329), and fellowships from Junta de Andalucꢀa (F.M.-G.)
and Ministerio de Ciencia e Innovaciꢂn (C.G.-R.). We thank Dr. J. I.
Trujillo (St. Louis, MO, USA) for assistance in the preparation of
this manuscript. We thank Unidad de Espectroscopꢀa de Masas
de la Universidad de Granada for exact mass spectrometric assis-
tance.
Keywords: analogues
·
antitumor agents
·
asymmetric
synthesis · bengamides · epoxyamides
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Received: January 23, 2013
Published online on March 19, 2013
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ChemMedChem 2013, 8, 819 – 831 831