Synthesis of new simplified hemiasterlin derivatives with α,β-unsaturated carbonyl moiety

Article abstract of DOI:10.1016/j.bmcl.2014.03.091

In this Letter, we report a convenient and efficient method for the synthesis of new simplified derivatives of hemiasterlin in which the α,α-dimethylbenzylic moiety A is replaced by α,β- unsaturated aryl groups as Michael acceptor. Most of these derivatives have a strong cytotoxic activity on three human tumor cell lines (KB, Hep-G₂ and MCF₇). Analogs 17b and 17f showed a high cytotoxicity against KB and Hep-G₂ cancer cell lines comparable to paclitaxel and ellipticine.

Full text of DOI:10.1016/j.bmcl.2014.03.091

Bioorganic & Medicinal Chemistry Letters 24 (2014) 2244–2246
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of new simplified hemiasterlin derivatives
with a,b-unsaturated carbonyl moiety
Chinh Pham The a, Tuyet Anh Dang Thi a, Thi Phuong Hoang a, Quoc Anh Ngo a, Duy Tien Doan a,
Thu Ha Nguyen Thi a, Tham Pham Thi a, Thu Ha Vu Thi a, M. Jean b, P. van de Weghe b,
,
⇑
Tuyen Nguyen Van a,
⇑
a Institute of Chemistry–Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam
b Equipe Produits Naturels, Synthèses et Chimie Médicinale (PNSCM), UMR CNRS 6226–Institut des Sciences Chimiques de Rennes, Université de Rennes 1,
UFR Sciences Pharmaceutiques et Biologique, 2, Avenue du Prof L. Bernard, F-35043 Rennes Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this Letter, we report a convenient and efficient method for the synthesis of new simplified derivatives
Received 26 February 2014
Revised 26 March 2014
Accepted 27 March 2014
Available online 5 April 2014
of hemiasterlin in which the a,a-dimethylbenzylic moiety A is replaced by a,b-unsaturated aryl groups as
Michael acceptor. Most of these derivatives have a strong cytotoxic activity on three human tumor cell
lines (KB, Hep-G2 and MCF7). Analogs 17b and 17f showed a high cytotoxicity against KB and Hep-G2
cancer cell lines comparable to paclitaxel and ellipticine.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Hemiasterlin
Cytotoxicity
Tripeptides
Hemiasterlins belong to a family of naturally occurring tripep-
tides from marine sponges.1 The important derivatives of hemias-
terlins are hemiasterlin A, hemiasterlin B, and hemiasterlin C,
which were isolated from marine sponge Auletta and Cymbastella
(Fig. 1) and exhibited potent cytotoxicity in vitro against murine
leukemia P388 and human breast, ovarian, colon, and lung cancer
cell lines.2,3 Hemiasterlins suppress microtubule depolymerization
presumably by binding to the vinca-alkaloid binding-domain of
tubulin and leading to mitotic arrest and cell death.4a The synthetic
analog HTI-286 (2) displayed especially potent cytotoxicity against
paclitaxel (Taxol™) resistant turmor cell lines in vitro and in vivo
and is currently in clinical trials.4b
There are several reports on the synthesis of new derivatives of
hemiasterlin in which the indole aromatic ring in the moiety A of
the original molecule was replaced by aryl functional groups.5,6a
However, asymmetric synthesis of the stereospecific amine group
and especially the gem-dimethyl moiety were proved to be highly
problematic. To overcome this difficulty, several studies explored
modifications of segment A in which the gem-dimethyl moiety
has been eliminated. Some of these derivatives showed promising
cytotoxic activity.6
the biological activity compared to the initial compound.7,8 Explana-
tions may include either a limitation of free energy of the peptide by
addition of a constrained system or by an electrophilic character of
the compound leading to the possibility to form covalent bond with
the protein target. Based on the idea that hemiasterlin derivatives
containing
remarkable cytotoxicity, we decided to synthesize new hemiasterlin
derivatives in which the -dimethylbenzylic group (fragment A) is
replaced by a ,b-unsaturated carbonyl group.
a,b-unsaturated carbonyl systems could induce a
a,a
a
New hemiasterlin derivatives were synthesized via classical
peptide coupling reactions between two fragments 7a–h and
14a,b. A general procedure for the synthesis of compound 7 is out-
lined in Scheme 1. Compounds 7 were prepared from glycine in
three steps.9 The synthesis started by acetylation of glycine with
acetic anhydride in water at room temperature following by con-
densation with aryl aldehydes using sodium acetate in the pres-
ence of acetic anhydride at 90 °C for 12 h which afforded
azalactones 6a–h in 72–83% yields.9 Finally, azalactones 6a–h were
hydrolyzed in aqueous sodium hydroxide, followed by treatment
with hydrochloric acid (12 N) at 100 °C for 4 h to give compounds
7a–h in 65–88% yields.
In recent years, previous works have shown that the presence of a
,b-unsaturatedcarbonylsystem in peptide derivativescan improve
Compounds 14a,b were obtained from Boc-L-valine in 6 steps as
a
depicted in Scheme 2.4 In the first step, Boc-L-valines 8a,b were
converted to Weinreb amide 9a,b in good yields (86% and 81%) by
treatment with a mixture of N,O-dimethylhydroxylamine hydro-
chloride, 1-ethyl-3-(3-dimethylaminoproryl)carbodiimide (EDC)
⇑
Corresponding authors. Tel.: +33 6 32 83 54 04 (P.v.d.W.).
E-mail address: pierre.van-de-weghe@univ-rennes1.fr (P. van de Weghe).
http://dx.doi.org/10.1016/j.bmcl.2014.03.091
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

C. P. The et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2244–2246
2245
2
R
O
O
O
O
N
OH
N
N
H
OH
N
H
NH
N
R
O
1
NH
1
O
R
B
A
C
2
2
2
2
2 (HTI-286
R = H)
R
= Me
= H
= H
R
R
R
R
= Me
= Me
= H
Hemiasterlin (1)
Hemiasterlin A
Hemiasterlin B
Hemiasterlin C
3 (HTI-286 analog R = OMe)
1
1
1
R
R
R
= Me
= H
Figure 1. Structures of hemiasterlin derivatives and HTI analogs.
O
6a Ar = naphthan-2-yl
6b Ar = furan-2-yl
(73%)
(72%)
(73%)
(70%)
(75%)
(74%)
(83%)
(80%)
(i)
(ii)
H N
CO H
2
AcHN
CO H
Ar
2
2
6c Ar = furan-3-yl
O
4
N
5 (65%)
6d Ar = thiophen-3-yl
6e Ar = 3-methoxyphenyl
6f Ar = 4-methoxyphenyl
6g Ar = 4-bromophenyl
6h Ar = 3-bromophenyl
CH
3
(iii)
7a, 82% - 7b, 80% - 7c, 75%
7d, 65% - 7e, 85% - 7f, 83%
7g, 88% - 7h, 87%
CO H
2
Ar
NHAc
Scheme 1. Reagents and conditions: (i) 2.0 equiv Ac2O, H2O, rt, 20 h. (ii) 0.75 equiv ArCHO, 1.0 equiv AcONa, Ac2O, 90 °C, 12 h. (iii) NaOH (1 N) then HCl (12 N), 100 °C, 4 h.
and hydroxybenzotriazole (HOBt) in the presence of N-ethylisopro-
pylamine (i-PrNHEt) at room temperature for 12 h.10 Reduction of
9a,b using LiAlH4 in THF was carried out at room temperature for
1 h to give aldehydes 10a,b in 78% and 65% yields, respectively.11
Afterward, Wittig reaction of aldehydes 10a,b with ethyl 2-(triphe-
nylphosphoranylidene)propanoate was carried out at reflux in CH2-
Cl2 for 6 h to afford the alkenoates 11a and 11b in 86% and 88%
yields, respectively.6,12 Then, removal of the Boc group using triflu-
oroacetic acid in CH2Cl2 at room temperature for 1 h led to 12a,b in
high yields (90% and 95%). The expectedcompounds 14a,b are finally
Eleven hemiasterlin derivatives showed strong activity against
the KB, Hep-G2 cell line with IC50 value below 100 nM. Analogs
16b, 17b and 18f exhibited potent cytotoxicity against the KB cell
line with IC50 = 8.2, 3.5 and 3.7 nM, respectively. Meanwhile ana-
logs 16b, 2217c and 17f displayed potent cytotoxicity against the
Hep-G2 cell line with IC50 value 17.8, 16.3 and 3.7 nM, respectively.
Derivatives 15a, 15e and 17a presented a cytotoxic activity against
MCF7 cell line with IC50 value 42.3, 60.0 and 67.9 nM. Concerning
the last cell line, the LU cell line, the hemiasterlin analogs showed
weak activities with IC50 values above 269 nM. It is noteworthy to
mention that two derivatives 17a and 17f present a cytotoxic
activity against two cancer cell lines (KB, Hep-G2) comparable with
those of ellipticine and paclitaxel.
obtained after coupling 12a,b and Boc-L-leucine in the presence of
EDC/HOBt in DMF at room temperature for 12 h following by Boc
deprotection in classical reaction conditions.
The final compounds 16a–h were then prepared in two steps
after peptide coupling reactions of 14a with amides 7a–h following
by the saponification of ester using a 1 N lithium hydroxide solu-
tion (Scheme 3a). Similarly, the hemiasterlin derivatives 18a–g
were obtained from 14b (Scheme 3b).
All compounds 15a–h, 16a–h, 17a–g and 18a–g were evaluated
in vitro for their cytotoxic activity against four human tumor cell
lines (KB, Hep-G2, LU and MCF7) and the results were summarized
in Table 1 (in bold are indicated the most significative results of
inhibition activities).
In conclusion, a concise synthetic approach for new modified
hemiasterlin derivatives was achieved in which the
ylbenzylic group and amino NHMe moiety were replaced respec-
tively by a ,b-unsaturated aryl and an amide NH-Ac group
a,a-dimeth-
a
avoiding the synthesis of the chiral fragment A. Most of these
derivatives possess strong cytotoxic activity on three human tumor
cell lines (KB, Hep-G2 and MCF7) and two analogs, 17b and 17f,
showed a comparable cytotoxicity activity to paclitaxel and ellipti-
cine against KB and Hep-G2 cancer cell lines. Based on previously
reported work,6b we can envisage that our hemiasterlin derivatives
O
O
Boc
N
Boc
N
Boc
N
Boc
N
(i)
(ii)
(iii)
CO H
Me
CO Et
2
2
R
R
N
R
H
R
OMe
8a R=H
9a R=H (86%)
9b R=Me (81%)
10a R=H (78%)
10b R=Me (65%)
11a R=H (86%)
8b R=Me
11b R=Me (88%)
(iv)
CF COO
3
R
N
R
N
H
N
2
(vi)
(v)
CO Et
2
CO Et
2
CO Et
2
R
H N
BocHN
3
O
O
CF COO
3
14a R=H (92%)
14b R=Me (91%)
13a R=H (73%)
13b R=Me (74%)
12a R=H (90%)
12b R=Me (95%)
Scheme 2. Reagents and conditions: (i) 1.1 equiv NH(Me)OMeÁHCl, 1.1 equiv EDC, 1.1 equiv HOBt, 2.0 equiv i-PrNHEt, DMF, rt, 12 h. (ii) 4.0 equiv LAH, THF, rt, 1 h. (iii)
Ph3P = C(CH3)CO2Et, CH2Cl2, reflux, 6 h. (iv) TFA, CH2Cl2, rt, 1 h. (v) 1.0 equiv Boc-
rt, 1 h.
L-Leucine, 1.1 equiv EDC, 1.1 equiv HOBt, 2.0 equiv i-PrNHEt, DMF, rt, 12 h. (vi) TFA, CH2Cl2,

2246
C. P. The et al. / Bioorg. Med. Chem. Lett. 24 (2014) 2244–2246
O
O
H
N
H
N
14a
(i)
(i)
(ii)
CO2Et
CO2H
(a)
(b)
Ar
Ar
N
H
Ar
Ar
N
H
O
O
NHAc
NHAc
15a, 60% - 15b, 50% - 15c, 55%
15d, 75% - 15e, 70% - 15f, 65%
15g, 71% - 15h, 60%
16a, 90% - 16b, 94% - 16c, 90%
16d, 98% - 16e, 90% - 16f, 95%
16g, 91% - 16h, 93%
O
O
(ii)
N
CO2Et
N
CO2H
14b
N
H
N
H
O
O
NHAc
NHAc
17a, 70% - 17b, 73% - 17c, 67%
17d, 86% - 17e, 65% - 17f, 68%
17g, 70%
18a, 94% - 18b, 98% - 18c, 92%
18d, 91% - 18e, 92% - 18f, 90%
18g, 93%
Scheme 3. Reagents and conditions: (i) 1.0 equiv 7a–h, 1.1 equiv EDC, 1.1 equiv HOBt, 2.0 equiv i-PrNHEt, DMF, rt, 12 h. (ii) 10 equiv LiOH, MeOH/H2O (2:1), rt, 10 h.
Table 1
Cytotoxicity evaluation
References and notes
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Entry
Compound
IC50 (nM)
Hep-G2
KB
LU
MCF7
1
2
3
4
5
6
7
8
15a
15b
15c
15d
15e
30.8
160.5
86.2
107.4
77.4
>269
>269
>269
63
39.7
214.2
67.3
57.6
47.7
>269
>269
>269
33.4
17.8
>269
>269
>269
>269
>269
>269
23.1
31.4
16.3
15.0
>269
3.7
>269
13.0
>269
149.4
63.2
>269
63.8
>269
1.26
0.19
>269
>269
>269
>269
228.4
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
231.0
158
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
1.82
42.3
>269
>269
203
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62
15f
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
>269
67.9
178.4
112.8
>269
>269
215
>269
>269
>269
>269
>269
>269
>269
>269
2.15
—
15g
15h
16a
16b
16c
16d
16e
16f
16g
16h
17a
17b
17c
17d
17e
17f
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
8.2
155.4
>269
>269
>269
>269
>269
23.0
3.5
24.1
202
>269
3.7
49.0
69.7
188.0
>269
234
>269
15.8
>269
1.26
3.9
17g
18a
18b
18c
18d
18e
18f
18g
Ellipticine
Paclitaxel
—
act as tubulin polymerization inhibitors. From our best com-
pounds, more detailed biological studies will be undertaken and
will be reported in due course.
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Acknowledgment
This work was financially supported in part by scientific re-
search and technological development project (code: ÐT.NCCB-
ÐHUD.2011-G/07).
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.03.091.