Synthesis of bengamide E analogues and their cytotoxic activity

Article abstract of DOI:10.1016/j.tetlet.2017.03.077

A series of bengamide E analogues were prepared from the corresponding polyketide chain and amino acids via amide coupling reactions. Opening of the polyketide chain lactone ring with α-aminolactams was successfully achieved under microwave irradiation in the presence of sodium 2-ethyl hexanoate. A cytotoxic activity evaluation against a panel of cancer cell lines (KB, HepG-2, Lu-1, MCF-7, HL-60 and Hela) indicated that the 2′R analogues were generally more cytotoxic than the 2′S analogues. Additionally, several analogues exhibited selective inhibition against various cancer cell lines: compounds 32a and 32b selectively inhibited MCF-7 cells, while 33b and 35b were more sensitive toward Lu-1 and HepG-2, respectively. Notably, some of the synthetic analogues possess cytotoxic activities with IC₅₀ values less than 1?μM.

Full text of DOI:10.1016/j.tetlet.2017.03.077

Accepted Manuscript
Synthesis of bengamide E analogues and their cytotoxic activity
Thi Dao Phi, Huong Doan Thi Mai, Van Hieu Tran, Van Loi Vu, Bich Ngan
Truong, Tuan Anh Tran, Van Minh Chau, Van Cuong Pham
PII:
S0040-4039(17)30400-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.03.077
TETL 48778
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 January 2017
6 March 2017
22 March 2017
Please cite this article as: Phi, T.D., Mai, H.D.T., Tran, V.H., Vu, V.L., Truong, B.N., Tran, T.A., Chau, V.M.,
Pham, V.C., Synthesis of bengamide E analogues and their cytotoxic activity, Tetrahedron Letters (2017), doi:
http://dx.doi.org/10.1016/j.tetlet.2017.03.077
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Tetrahedron Letters
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Synthesis of bengamide E analogues and their cytotoxic activity
Thi Dao Phia,b, Huong Doan Thi Maia, Van Hieu Trana, Van Loi Vua, Bich Ngan Truonga, Tuan Anh
Trana,c, Van Minh Chaua and Van Cuong Phama, ∗
aAdvanced Center for Bioorganic Chemistry of the Institute of Marine Biochemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet,
Caugiay, Hanoi, Vietnam.
bGraduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Caugiay, Hanoi, Vietnam.
cUniversity of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Caugiay, Hanoi, Vietnam.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A series of bengamide E analogues were prepared from the corresponding polyketide chain and
amino acids via amide coupling reactions. Opening of the polyketide chain lactone ring with α-
aminolactams was successfully achieved under microwave irradiation in the presence of sodium
2-ethyl hexanoate. A cytotoxic activity evaluation against a panel of cancer cell lines (KB,
HepG-2, Lu-1, MCF-7, HL-60 and Hela) indicated that the 2’R analogues were generally more
cytotoxic than the 2’S analogues. Additionally, several analogues exhibited selective inhibition
against various cancer cell lines: compounds 32a and 32b selectively inhibited MCF-7 cells,
while 33b and 35b were more sensitive toward Lu-1 and HepG-2, respectively. Notably, some
of the synthetic analogues possess cytotoxic activities with IC50 values less than 1 µM.
Available online
Keywords:
Bengamide
Synthesis
Cytotoxic
Caprolactam
Valerolactam
2009 Elsevier Ltd. All rights reserved.
1. Introduction
caprolactam ring including changes of ring size and
stereochemistry at C-2’ of the lactam units. Additionally,
Marine sponges of the family Jaspidae have proven to be an
important source of bioactive secondary metabolites. The
sponge-derived bengamides, first reported in 1986,1 have a
unique molecular structure and were found to possess a broad
spectrum of biological activities such as antitumor, antibiotic,
and anthelmintic properties.2a-d The structural modification of
bengamides has focused mainly on altering the different
stereocenters of the polyketide side-chain;3a-e the substituent
located at the terminal olefinic position;4a-c or modification of the
caprolactam unit.5a-c These modifications have led to the
obtainment of more potent bengamide derivatives. Modification
by replacement of isopropyl by tert-butyl at the terminal of the
polyketide chain has proven successful and simplified the
synthesis of analogues. Also, the presence of tert-butyl instead of
isopropyl in the structures of bengamide analogues makes these
structures more stable by avoiding olefin isomerization. A
bengamide analogue, LAF389 (Fig. 1) with tert-butyl at the
terminal polyketide side-chain,6 has been used in a clinical trial.
However, poor pharmacokinetic properties and unclear side
effects, which appeared early in the trial, have prevented its
further development.7 This prompted the search for new
bengamide derivatives.
cytotoxic activity evaluation of the synthetic bengamide
analogues was also reported.
Figure 1. Reported structural modification of bengamides
2. Result and Discussion
In the first steps, lactams 3a-b, 4a-b were synthesized from
the corresponding enantiomeric amino acids. Several methods for
the synthesis of α-amino-lactams from amino acids have been
reported,8a-c however, it was evident that these involved the use
of expensive reagents such as Me3SiCl and (Me3Si)2NH, or
Herein, we report the synthesis of a series of bengamide
analogues with replacement of isopropyl at terminal polyketide
by the tert-butyl group, and modification of the
c—ha—in —
∗ Corresponding author. Tel.: +84-4-37917049; fax: +84-4-37917054; e-mail: phamvc@imbc.vast.vn

2
Tetrahedron
needed to be performed under high pressure, at high
O
O
O
temperature and with long reaction times. In this work, the α-
amino-lactams were synthesized under microwave irradiation
(Scheme 1, 2). Compounds 4a and 4b were obtained in 79% and
82% yield under microwave irradiation (284 W) for 60 min in
ethylene glycol from L-lysine and D-lysine, respectively. Since,
D- and L-ornithines were commercially available in the
monohydrochloride salt forms, addition of a base to the reaction
mixture would be required. In the first attempt, pyridine was used
for the cyclization reaction of D-ornithine monohydrochloride
under microwave irradiation (284 W) for 60 min in ethylene
glycol to give compound 3b in moderate yield (55%). A higher
yield (78%) was obtained when aqueous NaHCO3 (10%) was
used instead of pyridine. Applying the same procedure,
1
H2N
2
b
N
a
NH
NH
O
n
n
3a (2S): n = 1
3b (2R): n = 1
4a (2S): n = 2
4b (2R): n = 2
5a (2S): n = 1 (38%)
5b (2R): n = 1 (52%)
6a (2S): n = 2 (57%)
6b (2R): n = 2 (52%)
O
O
O
H2N
R
N
R
2
c
N
N
O
n
n
7a (2S): n = 1, R = A (45%)
7b (2R): n = 1, R = A (42%)
8a (2S): n = 1, R = B (42%)
8b (2R): n = 1, R = B (40%)
9a (2S): n = 1, R = C (50%)
13a (2S): n = 1, R = A (90%)
13b (2R): n = 1, R = A (72%)
14a (2S): n = 1, R = B (70%)
14b (2R): n = 1, R = B (68%)
15a (2S): n = 1, R = C (72%)
compound
monohydrochloride in 66% yield.
3a
was
synthesized
from
L-ornithine
A =
B =
The primary amino groups of aminolactams 3a-b, 4a-b were
protected using phthalic anhydride which was achieved under
microwave irradiation (Scheme 3). It was found that these
reactions conducted under conventional heating at 110 oC
required much longer times. For example (data not shown),
treatment of 4a with phthalic anhydride in acetic acid in the
presence of 4Å molecular sieves at 110 oC for 15 h, afforded 6a
in 41% yield, while compound 6a was obtained in 57% yield
when the reaction was irradiated (284 W) for 60 min. With
microwave irradiation, compounds 5a-b and 6b were prepared in
38–52% yield from the corresponding aminolactams 3a-b and 4b
(Scheme 3). The protected compounds 5a-b, 6a-b were then
reacted with alkylbromides in the presence of K2CO3, KOH and
KI, to obtain the corresponding amino-protected N-
alkylaminolactams 7a-b–12a-b. Hydrazine mediated phthalimide
deprotection of 7a-b–12a-b gave compounds 13a-b–18a-b,
respectively.
(2R): n = 1, R = C (45%)
(2R): n = 1, R = C (67%)
9b
15b
10a (2S): n = 2, R = A (46%)
10b (2R): n = 2, R = A (45%)
11a (2S): n = 2, R = B (55%)
11b (2R): n = 2, R = B (50%)
12a (2S): n = 2, R = C (50%)
12b (2R): n = 2, R = C (46%)
16a (2S): n = 2, R = A (92%)
16b (2R): n = 2, R = A (79%)
17a (2S): n = 2, R = B (81%)
17b (2R): n = 2, R = B (75%)
18a (2S): n = 2, R = C (82%)
18b (2R): n = 2, R = C (73%)
C =
Scheme 3. Reagents and conditions: a) phthalic anhydride, acetic acid, MS
4Å, MW (284 W), 1 h; b) bromomethylcyclohexane, benzyl bromide or
cinnamyl bromide, K2CO3, KOH, KI, DMSO, 0 to 60 oC, 23 h; c) aq. hydrazine
(35%), acetonitrile, rt, 2 h.
Scheme 1. Reagents and conditions: (a) aq. NaHCO3 (10%), MW (284 W),
60 min; 3a (78%), 3b (72%).
Scheme 2 Reagents and conditions: (a) Ethylene glycol, MW (284 W), 60
min; 4a (79%), 4b (82%).
Scheme 4. Reagents and conditions: (a) N-alkylaminolactams, 13a-b – 18a-b,
sodium 2-ethyl hexanoate, THF, MW (100 W) 60 min; (b) TFA, H2O, THF, 0
oC, 1 h.
With polyketide 19 which was prepared according to the
literature method,6,9 and the N-alkylaminolactams in hand, the
amide coupling reaction was achieved by treatment of 19 with N-
alkylaminolactams 3a-b, 4a-b, 13a-b–18a-b under microwave
irradiation (100 W) to afford the desired coupling compounds
20a-b–27a-b (Scheme 4). It was found that the coupling reaction

of polyketide 19 with aminolactams without microwave
deprotection with TFA in THF at
corresponding bengamide analogues 28a-b–35a-b.
0
oC afforded the
assistance took much longer (15-20 h).6 Finally, acetonide
Table 1. Cytotoxicity of compounds 28a-b – 35a-b (IC50: µM)
Compound
28a
R
KB
HepG-2
>50
Lu-1
13.8
2.6
MCF-7
>50
HL-60
>50
Hela
>50
>50
22.1
11.1
28b
22.0
>50
>50
29a
29b
30a
30b
31a
31b
32a
32b
33a
33b
34a
34b
35a
8.2
2.4
2.9
4.4
2.3
1.7
22.0
1.1
11.8
0.4
5.7
1.0
1.9
11.8
6.6
12.7
2.6
7.5
2.7
5.8
2.3
4.3
0.6
17.1
0.3
7.7
1.1
23.6
20.1
8.8
>50
>50
55.8
29.0
36.8
21.5
>50
5.4
>50
>50
39.7
19.7
36.8
16.1
>50
2.3
11.3
12.0
6.6
17.8
20.4
4.3
2.4
15.1
1.5
22.8
1.2
0.2
>50
1.0
>50
10.4
32.5
8.6
>50
19.5
45.6
10.6
13.4
>50
39.1
41.5
16.2
26.9
4.8
1.9
1.5
4.9
35b
1.1
0.3
0.4
0.4
25.1
0.3
2.7
0.6
8.9
0.5
9.5
0.4
Ellipticine
All synthetic derivatives were evaluated for their cytotoxicity
against a panel of cancer cell lines, KB (mouth epidermal
carcinoma cells), HepG2 (human liver hepatocellular carcinoma
cells), LU (human lung adenocarcinoma cells), MCF7 (human
breast cancer cells), HL-60 (human promyelocytic leukemia
cells), and Hela (human cervical carcinoma cells). Cytotoxicity
comparison of the tested compounds revealed that, the 2’R series
were generally more cytotoxic than the corresponding 2’S-epimer
which possessed similar C-2’ configuration as the natural
bengamides. These synthetic analogues were found to be less
active against HL-60 and Hela cell lines in comparison with the
other tested cell lines (KB, HepG-2, Lu-1 and MCF-7).)
Additionally, the cytotoxicity data (Table 1) indicated that
several compounds exhibited selective inhibition against some
cancer cell lines: compounds 32a and 32b selectively inhibited
MCF-7 cells, while 33b and 35b were more sensitive toward Lu-
1 and HepG-2, respectively. Additionally, compounds 32b, 33b
and 35b possessed cytotoxic activities with IC50 values less than
1 µM against several cancer cell lines.
Acknowledgments
The authors thank the Vietnam National Foundation for
Science and Technology Development (NAFOSTED) and the
Ministry of Science and Technology of Vietnam (MOST) for
financial support (Project code: ĐT. NCCB-ĐHƯD.2012-G/05).
Supplementary data

4
Tetrahedron
Supplementary data (detailed experimental procedures and
NMR spectra for the synthetic bengamide E derivatives)
associated with this article can be found, in the online version, at
http://
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Highlights
•
•
•
Bengamide analogues were prepared via amide
coupling reactions under MW irradiation
The 2’R analogues were generally more cytotoxic
than the 2’S analogues
Some of the synthetic analogues had cytotoxicities
with IC50 values less than 1 µM

6
Tetrahedron
Graphical Abstract
Synthesis of bengamide E analogues and
their cytotoxic activity
Leave this area blank for abstract info.
Thi Dao Phi, Huong Doan Thi Mai, Van Hieu Tran, Van Loi Vu, Bich Ngan Truong, Tuan Anh Tran, Van Minh
Chau and Van Cuong Pham