Synthesis and cytotoxicity of novel imidazo[4,5-d]azepine compounds derived from marine natural product ceratamine A

Article abstract of DOI:10.1016/j.bmcl.2018.02.004

A series of novel imidazo[4,5-d]azepine compounds derived from marine natural product ceratamine A were designed and synthesized in 7 steps. Most compounds exhibited comparable cytotoxicity against five human cancer cell lines (HCT-116, HepG2, BGC-823, A549 and A2780) to natural product ceratamine A. Compound 1k, bearing methoxy group at C-14, C-15 and C-16, showed the best in vitro cytotoxicity, which was better than ceratamine A. The structure and activity relationships study showed that the benzyloxymethyl group on N-3 played an important role on the cytotoxicity.

Full text of DOI:10.1016/j.bmcl.2018.02.004

Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and cytotoxicity of novel imidazo[4,5-d]azepine compounds
derived from marine natural product ceratamine A
a
a
⇑
Xuan Pan , Lulu Tao , Ming Ji, Xiaoguang Chen, Zhanzhu Liu
State Key Laboratory of Bioactive Substances and Functions of Natural Medicines, Institute of Materia Medica, Peking Union Medical College and Chinese Academy of Medical
Sciences, Beijing 100050, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel imidazo[4,5-d]azepine compounds derived from marine natural product ceratamine A
were designed and synthesized in 7 steps. Most compounds exhibited comparable cytotoxicity against
five human cancer cell lines (HCT-116, HepG2, BGC-823, A549 and A2780) to natural product ceratamine
A. Compound 1k, bearing methoxy group at C-14, C-15 and C-16, showed the best in vitro cytotoxicity,
which was better than ceratamine A. The structure and activity relationships study showed that the ben-
zyloxymethyl group on N-3 played an important role on the cytotoxicity.
Received 21 December 2017
Revised 1 February 2018
Accepted 2 February 2018
Available online xxxx
Keywords:
Ó 2018 Elsevier Ltd. All rights reserved.
Ceratamine A
Microtubule
Heck reaction
Anticancer
Over the past 30 years, marine natural products have gained a
1
lot of attention as an important source of drug candidates. Several
anticancer drugs derived from marine natural products have
2
achieved great successes in clinic. This trend indicates that marine
natural products would continue to play a key role in anticancer
3
drug discovery.
Ceratamine A (Fig. 1) is a heterocyclic alkaloid isolated from
marine sponge Pseudoceratina sp., which displayed potent antimi-
totic activity through promoting tubulin polymerization. The fact
that ceratamine A has a totally different binding site on micro-
tubule from paclitaxel makes it a promising lead compound in
anticancer drug discovery.4 However, the lack of availability from
natural source brings huge difficulty in its further biological
evaluation.
Fig. 1. Marine natural product ceratamine A.
(
HCT-116, HepG2, BGC-823 and A2780) to ceratamine A and better
cytotoxicity against A549 than ceratamine A (Fig. 2). Because the
synthesis of intermediate 1a was apparently easier than cer-
atamine A and its analogues, we attempted to synthesize some
derivatives with the imidazo[4,5-d]azepine skeleton in order to
discover potential anticancer candidate with simpler structure.
In this paper, a series of analogues of compound 1a as cer-
atamine A simplified derivatives were synthesized in 7 steps as
depicted in Scheme 1. The synthetic route generally follows that
In our previous work, we have reported an efficient approach to
synthesize ceratamine A5 and its analogues.6 Some analogues
exhibited slight increases in cytotoxicity compared with cer-
atamine A, and a preliminary structure and activity relationship
was obtained. It was found that the introduction of bulky groups
at C-14 and C-16 could increase the cytotoxity, and that the sub-
stituents on N-7 played a significant role on high potency. During
the evaluation of cytotoxicity, we were pleased to find that the
6
reported by us previously. The key step was to employ Heck reac-
6
intermediate 1a with the imidazo[4,5-d]azepine skeleton exhib-
7
tion to construct the imidazo[4,5-d]azepine core. Firstly, amine 5
was easily prepared from commercially available histamine dihy-
ited comparable cytotoxicity against four human cancer cell lines
drochloride over 4 steps. Treatment of amine 5 with substituted
cinnamic acids
6
generated by Knoevenagel condensation8
⇑
Corresponding author.
E-mail address: liuzhanzhu@imm.ac.cn (Z. Liu).
These authors contributed equally.
afforded compounds 7a-d in satisfying yields. Benzylation of 7a-
d with NaH afforded amides 8a-n. The target compounds 1a-n
a
https://doi.org/10.1016/j.bmcl.2018.02.004
960-894X/Ó 2018 Elsevier Ltd. All rights reserved.
0
Please cite this article in press as: Pan X., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.02.004

2
X. Pan et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
3 2 2
Scheme 2. Reagents and conditions: (a) AlCl , CH Cl , r.t., 1 h.
Table 1
Cytotoxicity of compounds 1a-n and 9a.
Compound
IC50 (l
mol/L)a
HCT-116
HepG2
BGC-823
A549
A2780
Ceratamine A
12.4
20.0
7.94
11.19
6.23
7.8
13.9
20.0
20.4
26.8
9.9
11.2
20.4
Fig. 2. Synthesis and cytotoxicity of ceratamine A analogues and the intermediate
b
1
1
1
1
1
a
b
c
d
e
1
a.
44.65
39.40
40.87
34.30
72.54
47.94
44.40
53.32
18.03
10.04
>100
51.41
23.71
>100
39.08
72.94
52.60
14.57
80.99
75.70
71.59
31.28
27.84
12.92
>100
42.62
93.26
>100
11.38
19.56
22.47
16.05
18.34
11.79
46.80
25.55
20.58
8.56
21.19
24.77
31.29
26.28
48.29
22.20
43.11
25.25
23.85
8.97
were then successfully prepared through Heck reaction. We also
synthesized compound 9a to check the effect of the benzy-
loxymethyl (BOM) group on the cytotoxicity (Scheme 2). The struc-
3.80
1f
13.95
10.50
31.05
24.69
8.46
1g
1
13
1h
tures of the target compounds were confirmed by HNMR, CNMR
and HRMS data. It should be noted that the methylene protons sig-
1
1
i
j
1
nals of 7-membered ring broadened in the H NMR spectra due to
1k
1l
8.90
the flipping of 7-membered lactam ring9 (see the Supplementary
67.06
36.00
19.36
>100
36.02
52.49
26.43
51.14
>100
28.24
72.83
>100
1m
data). This phenomenon was well studied by variable-temperature
1
6
1n
H
N
M
R
i
n
o
u
r
p
r
e
v
i
o
u
s
w
o
r
k
.
9a
The cytotoxicity of these compounds against five human cancer
cell lines (HCT-116, HepG2, BGC-823, A549 and A2780) was evalu-
ated by the standard MTT assay. The results are shown in Table 1.
a
The IC50 values represent the inhibitory concentration of 50% of cell growth.
b
Compound 1a was synthesized previously.6
Scheme 1. Reagents and conditions: (a) Boc
2 2
O, 4N NaOH, dioxane/H O (2:1), r.t., 2 h, 95%; (b) NBS, THF, r.t. 5 h, 92%; (c) BOMCl, Et
3 3
N, THF, r.t., overnight, 82%; (d) CF COOH,
CH Cl , r.t., 4 h, 93%; (e) EDCI, DMAP, CH Cl , r.t., 30 h; (f) NaH, DMF, r.t., overnight, 76–92% (over 2-step); (g) Pd(PPh )
2
2
2
2
3 4
, Methyl dicyclohexylamine, DMF, 120 °C, 20 h, 50–65%.
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X. Pan et al. / Bioorganic & Medicinal Chemistry Letters xxx (2018) xxx–xxx
3
As illustrated in Table 1, most compounds exhibited cytotoxic-
ity against these five cell lines with IC50 values of micromolar level.
It is worth noting that compound 1a exhibited better cytotoxicity
dates. Further structural optimization and biological studies of
compound 1k are in progress and will be reported in due course.
2
than the analogues 1b-j and 1n, indicating that substituents at R
Acknowledgment
(
whether bulky groups, electron-donating groups or electron-with-
1
drawing groups) may be not preferable. It is evident that R had an
This work was financially supported by CAMS Innovation Fund
for Medical Sciences (CIFMS, 2016-I2M-3-009).
appreciable influence on the cytotoxicity. In particular, compound
1
1
k (R = 3, 4, 5-OCH
3
) had the most potent inhibitory activities,
even better than the natural product ceratamine A. However, intro-
duction of CF group at C-15 (compound 1m) or pyridine ring
compound 1l) led to a decrease in cytotoxicity. Compound 9a,
with the benzyloxymethyl (BOM) group being cleaved, exhibited
obviously weaker cytotoxicity than compound 1a, which indicated
that the benzyloxymethyl (BOM) group on N-3 was critical for high
potency.
In conclusion, a series of ceratamine A simplified derivatives
possessing imidazo[4,5-d]azepine core was designed and synthe-
sized from histamine dihydrochloride in 7 steps. Compared with
natural product ceratamine A, these newly synthesized com-
pounds exhibited comparable cytotoxicity. Especially, compound
A. Supplementary data
3
(
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.bmcl.2018.02.004.
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Please cite this article in press as: Pan X., et al. Bioorg. Med. Chem. Lett. (2018), https://doi.org/10.1016/j.bmcl.2018.02.004