Synthesis of (3S,4R)-bengamide e

Article abstract of DOI:10.1016/j.cclet.2010.11.023

(3S,4R)-Bengamide E (2) was synthesized starting from d-glucono-δ- lactone (3) and the key deoxygenation step from 13 to 15 was achieved by the application of NaBH₃CN and ZnI₂. Compared with natural bengamide E (1), the synthetic compound (3S,4R)-bengamide E (2) was inactive against the cell growth of HUVEC and cancer cells. These data represent the significance of the stereochemistry at C-3 and C-4 of bengamides for structural recognition and binding with the target(s).

Full text of DOI:10.1016/j.cclet.2010.11.023

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 505–507
www.elsevier.com/locate/cclet
Synthesis of (3S,4R)-bengamide E
Qi Jun Liu a,b, Hong Li a,b, Shao Peng Chen a, Guo Chun Zhou a,b,c,
*
a Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences, Guangzhou 510530, China
b Graduate School of Chinese Academy of Sciences, Beijing 100039, China
c College of Pharmaceutical Sciences, Nanjing University of Technology, Nanjing 210009, China
Received 16 September 2010
Available online 3 March 2011
Abstract
(3S,4R)-Bengamide E (2) was synthesized starting from D-glucono-d-lactone (3) and the key deoxygenation step from 13 to 15
was achieved by the application of NaBH3CN and ZnI2. Compared with natural bengamide E (1), the synthetic compound (3S,4R)-
bengamide E (2) was inactive against the cell growth of HUVEC and cancer cells. These data represent the significance of the
stereochemistry at C-3 and C-4 of bengamides for structural recognition and binding with the target(s).
# 2010 Guo Chun Zhou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: (3S,4R)-Bengamide E; Stereocenter; Deoxygenation
Bengamides were first isolated by Crews and co-workers from Jaspis sponges [1,2], which are classified as fused
ketide-amino acid derivatives. It was demonstrated that bengamides are a family of potent cancer cell growth
inhibitors [3,4] and anti-angiogenesis agents [5] as unique inhibitors of methionine aminopeptidase type I and type II
(MetAP 1 and MetAP 2). Bengamide E (1) (Fig. 1), a representative member of bengamides, exhibits 3.3 mmol/L
against MDA-MB-435 (human breast carcinoma cell) [4]. To investigate the importance of the stereochemistry at C-3
and C-4 of bengamide ketide side chain, we herein report the synthesis and biological evaluation of (3S,4R)-
bengamide E (2) (Fig. 1).
The (3S,4R)-bengamide E was synthesized starting from D-glucono-d-lactone (3) (Scheme 1). Bis-acetonide 4 was
prepared from 3 according to the literature [6]; O-Methylation of 4 was done by MeI and NaH to afford 5 in 72% yield
at ꢀ20 8C in THF without C-2 epimerization. After the reduction of carboxyl group of 5 into terminal alcohol 6 by
LiAlH4, compound 7 was prepared from 6 by using benzyl bromide and NaH with the catalysis of Bu4NI and diol 8
was generated by removal of terminal acetonide protection of 7. Then primary and secondary hydroxyl groups of 8
were protected by acetyl group and tetrahydropyranyl (THP) group, respectively. After a new chiral center was
introduced by the THP protection, every succeeding product bearing THP has a diastereomer counterpart.
Subsequently, treatment of the mixture of 9 with K2CO3 hydrolyzed acetyl group to give separable products of 10A
(more polar, 51% yield) and 10B (less polar, 41% yield) and oxidation of 10A and 10B by Swern oxidation afforded
aldehyde 11A and 11B, respectively. Julia olefination of aldehyde 11A or 11B gave low E-selectivity and yield (data
not shown), whereas, Wittig–Horner reaction [7] of aldehyde 11A or 11B produced a single product of E-olefin 12A or
* Corresponding author at: Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences, Guangzhou 510530, China.
E-mail address: allspringzhou@yahoo.com (G.C. Zhou).
1001-8417/$ – see front matter # 2010 Guo Chun Zhou. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.11.023

[()TD$FIG]
506
Q.J. Liu et al. / Chinese Chemical Letters 22 (2011) 505–507
9
OH OMe
9
OH OMe
O
H
O
H
7
7
3
N
5
1
N
3
5
1
8
6
2
4
8
6
2
4
1'
NH
1'
NH
2'
2'
OH OH
O
OH OH
O
(3S,4R)-Bengamide E (2)
Bengamide E (1)
Fig. 1. Structures of bengamide E (1) and (3S,4R)-bengamide E (2).
12B in a good yield as expected, which was the required geometrical isomer. Reaction of 12A with Grignard reagent
MeMgI in Et2O at 0 8C generated a tertiary alcohol 13A. Strategically, removal of allylic tertiary hydroxyl group in 13A
could achieve the same olefin moiety as bengamide E side chain. Many approaches including Pd/C in EtOH, Pd(OH)2 in
EtOH, Raney Ni in EtOH or in H2O and Et3SiH in CH2Cl2 failed to deoxygenate 13A. At the end, deoxygenation of 13A
was accomplished by the reduction with NaBH3CN and ZnI2 in CH2Cl2, which was modified from the literature [8].
Reductive deoxygenation of 13A by 3.0 equiv. NaBH3CN and 1.0 equiv. ZnI2 afforded desired key intermediate 15 as
well as THP-protected 14A that was easily transformed into 15 by PPTS in 72% yield. As a result, the two-step process
generated 15 in 62% yield in total without obvious dimerized and olefin isomerized byproducts. Through the same
reactions as 12A to 15, the key intermediate 15 was furnished with the similar yields starting from less polar 12B. The
diastereomer mixtures were used for the subsequent production of 15 on a larger scale.
With the key intermediate 15 in hand, the free secondary hydroxyl group was protected with acetyl group, and
subsequent debenzylation of terminal protection was carried out by DDQ at room temperature to afford C-1 primary
alcohol 17. Sequential oxidations of 17 with Dess-Martin periodinane (DMP) and NaClO2 buffered with KH2PO4
generated carboxylic acid 18 [9]. Activation of carboxylic acid 18 was carried out by N-hydroxysuccinimide (NHS)
with N,N0-dicyclohexylcarbodiimide (DCC) and then coupling reaction was conducted by treatment of NHS-activated
18 with freshly prepared L-(ꢀ)-a-amino-d-caprolactam [10] to furnish the coupled compound 19. Finally, (3S,4R)-
bengamide E (2) [11] was achieved by the treatment of 19 with dilute TFA at 0 8C (Scheme 2).
[()TD$FIG]
Scheme 1. Condition and regent: (a) Ref. [6]; (b) NaH, MeI, THF, ꢀ20 8C, 72%; (c) LiAlH4, THF, 0 8C, 70%; (d) BnBr, NaH, Bu4N+Iꢀ, THF, 86%;
(e) 50% HOAc, 67%; (f) pyridine, AcCl, CH2Cl2, ꢀ80 8C, 63%; (g) DHP, PPTS, CH2Cl2; (h) K2CO3, MeOH/H2O, 51% for 10A (the more polar
product) and 41% for 10B (the less polar product); (i) Swern oxidation; (j) NaH, THF, (EtO)2P(O)CH2CO2Et, 0 8C to rt, 82% for12A from 10A and
72% for 12B from 10B; (k) MeMgI, Et2O, 0 8C, 77% for 13A, 86% for 13B; (l) NaBH3CN, ZnI2, CH2Cl2, rt, 22% for 14A and 47% for 15 from 13A
or 21% for 14B and 38% for 15 from 13B; (m) PPTS, EtOH, 50 8C, 72%.

[()TD$FIG]
Q.J. Liu et al. / Chinese Chemical Letters 22 (2011) 505–507
507
O
OMe
COOH
O
OMe
O
OMe
c, d
b
a
15
OH
OBn
OAc
OAc
OAc
O
O
O
16
17
18
OH OMe
O
OMe
O
O
H
e, f
O
H
N
N
g
NH
NH
OH OH
O
OH
O
2
19
Scheme 2. Condition and regent: (a) Ac2O, TEA, CH2Cl2, 89%; (b) DDQ, CH2Cl2, 61%; (c) DMP, CH2Cl2; (d) NaClO2, KH2PO4, acetone/H2O,
86%; (e) NHS, DCC, DMAP, CH2Cl2, rt; (f) dioxane, saturated K2CO3, L-(ꢀ)-a-amino-d-caprolactam, 42%; (g) 3:1:3 THF–H2O–TFA, 0 8C, 61%.
Compound 2 was inactive up to 50.0 mmol/L against the cell growth of HUVEC (human umbilical vein endothelial
cell), A549 (human non-small cell lung cancer cell), Bel-7402 (human hepatocarcinoma cell) and MCF-7 (human
breast cancer cell) [12,13]. These structural modifications at C-3 and C-4 of bengamide side chain damaged biological
activity severely, thus, suggesting that strict stereochemistry at C-3 and C-4 of bengamides is important for structural
recognition and binding with the target(s).
Acknowledgments
We are grateful for the financial supports from National Basic Research Program of China (No. 2007CB914504)
and Natural Science Foundation of China (No. 30973621).
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d6): d 174.6, 169.9, 137.8, 129.0, 83.8, 73.8, 71.7, 70.9, 58.7, 51.8, 41.2, 31.2, 30.7, 29.1, 28.0, 22.8, 22.7; HRESIMS calcd. for C17H30N2NaO6
381.1996, found 381.1991 [M+Na]+.
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