Amino acid based enantiomerically pure 3-substituted benzofused heterocycles: A new class of antithrombotic agents

Article abstract of DOI:10.1016/j.bmcl.2009.10.126

A diverse group of novel medium ring heterocycles derived from naturally abundant proteinogenic amino acids were evaluated for their potency towards antithrombotic activity. The more potent benzofused oxazepine and oxazocine scaffolds were diversified by incorporating different amino acids at the position number 3. Further the effect of ring size has also been taken into account and it was observed that the eight-membered oxazocines ane more potent compared to the corresponding oxazepines.

Full text of DOI:10.1016/j.bmcl.2009.10.126

Bioorganic & Medicinal Chemistry Letters 20 (2010) 244–247
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Amino acid based enantiomerically pure 3-substituted benzofused
heterocycles: A new class of antithrombotic agents
a,
Jitendra Kumar Mishra a, Krishnananda Samanta a, Manish Jain b, Madhu Dikshit b, , Gautam Panda
*
*
a Medicinal & Process Chemistry Division, Central Drug Research Institute, Lucknow 226 001, UP, India
b Pharmacology Division, Central Drug Research Institute, Lucknow 226 001, UP, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A diverse group of novel medium ring heterocycles derived from naturally abundant proteinogenic amino
acids were evaluated for their potency towards antithrombotic activity. The more potent benzofused oxa-
zepine and oxazocine scaffolds were diversified by incorporating different amino acids at the position
number 3. Further the effect of ring size has also been taken into account and it was observed that the
eight-membered oxazocines ane more potent compared to the corresponding oxazepines.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 16 March 2009
Revised 11 August 2009
Accepted 28 October 2009
Available online 30 October 2009
Keywords:
Amino acids
Benzodiazepines
Benzoxazepines
Benzothiazepines
Benzoxazocine
Antithrombotic activity
Intravascular thrombosis leads to many acute cardiovascular dis-
eases.1 The resulting clinical ramifications of occlusive thrombus
development, including myocardial infarction, deep vein thrombo-
sis, pulmonaryembolism, andstroke, annuallyaffectmillionsofpeo-
ple worldwide and are the leading causes of mortality and morbidity
in the industrialized world.2 The origin of thrombosis is the failure of
platelets to distinguish a damaged blood vessel wall in need of repair
from a thrombogenic surface. In this case, self-aggregation of plate-
lets occurs and that lead to vessel occlusion and the interruption of
blood flow.3 The current therapies include the use of anti-platelet
agents; aspirin, ticlopidine, clopidogrel and anticoagulants such as
heparin and warfarin. However, these drugs do not work in acute
conditions and require extended periods of closely monitored ther-
apy mainly because of the high risk of bleeding.4–6
The medium ring heterocycles constitute the core motif struc-
tures in a variety of natural products7 and also well established phar-
macophore in medicinal chemistry. Among the medium ring
heterocycles, the benzofused seven and eight-membered heterocy-
cles are having significant contribution. The benzodiazepines are a
class of privileged structures having a wide range of biological activ-
ities.8 Benzoxazepines show pharmacological activities such as anti-
psychotic, central nervous system along with antibreast cancer.9
Benzothiazepines are active constituents of an important class of
biologically active compounds such as bradykinin agonists.10
Eight-membered benzannulated heterocycle benzodiazocine, such
as Teleocidines activate protein kinase C (PKC) isozymes.11 Ben-
zoxazocine, such as Nefopamhydrochloride12 is a non-narcoticanal-
gesic drug with antidepressant properties.13 Although various
applications are reported, antithrombotic activities of this class of
compounds are not explored much. In continuation of our research
program towards synthesis and biological activity of amino acid
based heterocycles14 and natural products15, some of the abun-
dantly available benzofused heterocycles were evaluated for their
efficacy as antithrombotic agents. The discovery program oriented
towards the identification of antithrombotic agents was initiated
by conducting in vivo screen of our chemical library of medium ring
heterocycles.
Synthesis of seven and eight-member chiral heterocycles was
undertaken. S-amino acid and substituted benzene derivatives
were used as building blocks for the construction of benzannulated
chiral heterocycles. The treatment of S-amino alcohol derivatives
1a–b with N-tosyl derivative of methyl anthranilate and N-tosyl
derivative of 2-nitro phenylacetic acid methyl ester under DEAD/
PPh3 condition furnished 2a–b, 3a–b. The deprotection of Boc in
2a–b, 3a–b by 6 N HCl in methanol furnished amine hydrochlo-
rides 4a–b, 5a–b. The primary amine functionality was converted
to its tosyl derivatives 6a–b, 7a–b. The reduction of 6a–b, 7a–b
by LAH at 0 °C afforded the alcohols 8a–b, 9a–b which under
Mitsunobu16 cyclization conditions yielded the desired enantio-
merically pure 3-substituted 1,4-benzodiazepines, benzo[e][1,4]
diazocines 10a–b, 11a–b (Scheme 1). To begin with, methyl
* Corresponding authors.
E-mail address: gautam.panda@gmail.com (G. Panda).
0960-894X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2009.10.126

J. K. Mishra et al. / Bioorg. Med. Chem. Lett. 20 (2010) 244–247
245
NHTs
R1
R
R
Ts
N
Ts
N
Ts
N
R
Ts
N
R
Ts
N
CO2CH3
( )
n
a
NH 2.HCl
c
NHTs
d
NHTs
e
NHBoc
b
n = 0, 1
HO
R1
R1
R1
R1
R1
+
R
R
( )
( )
( )
nNTs
( )
n
n
n
n
( )
n = 1, 2
NHBoc
CO2CH3
6a-b
7a-b
CO2CH3
4a-b
5a-b
CH2OH
8a-b
9a-b
CO2CH3
2a-b , n = 0, X = N
3a-b , n = 1, X = N
10a-b
11a-b
1a-b
1a , R = CH2C6H4OBn
1b , R = CH2C6H5
Scheme 1. Reagents and conditions: (a) DEAD, PPh3, 0 °C (2 h) to rt (10 h); (b) 6 N HCl, MeOH, rt, 45 min; (c) TsCl, Et3N, DCM, 1 h; (d) LAH, dry THF, 1 h; (e) PPh3, DEAD, dry
THF.
thiosalicylate 12 and Boc protected tyrosinol 1a under Mitsunobu
condition furnished the ester derivative 13. The deprotection of
Boc group with 6 N HCl afforded the free amine hydrochloride
14, which was again converted to its tosyl derivative 15. The
reduction of ester group in 15 by LAH gave alcohol 16. The intra-
molecular Mitsunobu cyclization furnished enantiomerically pure
benzothiazepine derivative 17 in good yield (Scheme 2). To syn-
thesize 3-substituted benzoxazepines and benzo[f][1,4]oxazepine
derivatives, 18a, 18b, 18c and N-tosyl amino acid methyl esters
19a–f were used. The Mitsunobu reaction of S-amino acid deriva-
tives 19a–f with 18a, 18b, 18c provided the esters 20a–f, 21a–b,
22a, 23a–b. The Lithium aluminium hydride (LAH) reduction of
these esters afforded the corresponding alcohols 24a–f, 25a–b,
26a, 27a–b which on subsequent debenzylation by H2/Pd (10%
on carbon) gave 28a–f, 29a–b, 30a, 31a–b containing free alco-
holic and phenolic hydroxyl groups. Exposure of 28a–f, 29a–b,
30a, 31a–b to Mitsunobu reaction conditions, that is, diethylazo-
dicarboxylate (DEAD), triphenylphosphine (TPP) at 0 °C resulted
in the formation of desired enantiomerically pure 3-substituted
benzo[f][1,4]oxazepine, benzoxazepine derivatives 32a–f, 33a–
b, 34a, 35a–b (Scheme 3). Therefore, the selection of scaffold
was initially guided by the application of the Topliss tree ap-
proach (Table 1) with the goal of gaining as much information
from the least numbers of selected analogues.
activity compared to the benzodiazepines and benzothiazepine.
The benzothiazepines showed poor activity among all three types
(benzofused seven-membered) of ring system. The synthesized
benzodiazepines based on O-protected tyrosine 10a and phenylal-
anine 10b showed comparable antithrombotic activity of 20% pro-
tection with increase in bleeding time of 4% and 13%, respectively.
The benzothiazepine scaffold derived from O-protected tyrosine
gave only 10% protection. The benzoxazepine scaffold derived from
alanine showed the antithrombotic activity comparable to benzo-
diazepines. Surprisingly, the compound 32b was fully inactive
while the corresponding benzodiazepine 10a showed activity of
20%. On moving from methyl group at position-3 32a to 2-methyl
propyl group 32c further reduction in biological activity was
observed. Furthermore, changing groups from methyl 32a to iso-
propyl 32e and benzyl 32d gave the similar protection of 20%.
The 3-substituted benzoxazepine derived from tryptophan which
is having 3-methine indole as a side chain, showed the most prom-
ising antithrombotic activity of 40% protection which was compa-
rable with aspirin. After getting lead scaffold among benzofused
seven-membered heterocycles, we were interested to observe the
antithrombotic activity of benzofused eight-membered heterocy-
cles. In this context we tested few 3-substituted benzodiazocines
and benzoxazocines. The benzodiazocine derived from O-protected
tyrosine 11a showed somewhat improved activity of 30% compare
to corresponding diazepine 10a which showed the protection of
20%. While there was no change in biological activity of ben-
zodiazocine derived from phenylalanine 11b with the correspond-
ing benzodiazepine 10b. In comparison of benzoxazepines the
benzoxazocines were found to be more potent as indicated by
the results shown in table; the benzoxazocine derived from alanine
33a showed 40% protection compare to corresponding benzoxaze-
Structure–activity relationships: The biological result of the med-
ium ring heterocycles derived from naturally occurring Samino
acids are shown in the Table 1. Our SAR studies begin with the
comparison of the biological activity of various kinds of 3-substi-
tuted medium ring heterocycles towards antithrombotic activity.19
Among the benzo- fused seven-member heterocycles, the com-
pounds containing benzoxazepine ring showed the promising
SH
R1
R
R
CO2CH3
R
R
S
S
NHBoc
b
NH2.HCl
c
a
S
12
HO
S
S
R1
R1
NHTs
d
+
NHTs
e
R1
R
R1
R1
R
CO2CH3
CO2CH3
CO2CH3
CH2OH
NTs
NHBoc
13
14
1a
16
15
17
R = CH2C6H4OBn
Scheme 2. Reagents and conditions: (a) DEAD, PPh3, 0 °C (2 h) to rt (10 h); (b) 6 N HCl, MeOH, rt, 45 min; (c) TsCl, Et3N, DCM, 1 h; (d) LAH, dry THF, 1 h; (e) PPh3, DEAD, dry
THF.
R
CO2CH3
19a-f
TsHN
OBn
OH
CH2OH
R
OBn
CH2OH
R
O
OBn
CO2CH3
R
R1
R1
R1
b
c
R1
d
R
R1
NTs
NTs
( ) n
NTs
( ) n
OH
a
( ) n
NTs
( ) n
n = 1, 2
( ) n
n = 1, 2
18a, R1 = H
20a-f, n = 1, 21a-b, n = 2
22a, n = 1
23a-b, n = 1
24a-f, 25a-b
26a
28a-f, 29a-b
30a
32a-f, 33a-b
34a
35a-b
18b, R1 = 3-OMe
18c, R1 = 4-OMe
27a-b
31a-b
Scheme 3. Reagents and conditions: (a) DEAD, PPh3, 0 °C (2 h) to rt (10 h); (b) LAH, dry THF, 0 °C; (c) H2, 10% Pd/C, MeOH, rt, 2 h, 50 Psi; (d) DEAD, PPh3, dry THF.

246
J. K. Mishra et al. / Bioorg. Med. Chem. Lett. 20 (2010) 244–247
Table 1
In vivo assay of benzofused heterocycles derived from amino acids for antithrombotic activity
Ts
N
Ts
S
N
O
O
R
R1
R
R
R1
R1
R
R1
1
R
R
NTs
NTs
NTs
NTs
NTs
10a-b
11a-b
17
32a-f, 34a,
35a-b
33a-b
Compds
R1
Ra
Antithrombotic activity17 (% protection)
Bleeding time18 (% increase)
10a
10b
11a
11b
17
32a
32b
32c
32d
32e
32f
33a
33b
34a
35a
35b
36
H
H
H
H
H
H
H
H
H
H
H
H
CH2C6H4OBn
CH2C6H5
CH2C6H4OBn
CH2C6H5
CH2C6H4OBn
CH3
CH2C6H4OH
CH2CH(CH3)2
CH2C6H5
CH2(CH3)2
CH2C8H6N
CH3
CH2C6H4OH
CH2C6H4OH
CH2CH(CH3)2
CH2C8H6N
Aspirin
20.00
20.00
30.00
20.00
10.00
20.00
0.00
10.00
20.00
20.00
40.00
40.00
40.00
45.00
30.00
20.00
4
13
No change
22
28
25
No change
60
No change
22
33
No change
No change
24
6
12
100 20
H
3-OCH3
4-OCH3
4-OCH3
37
3
a
R means alkyl derivatives of different amino acids.
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to 32b and 32c, respectively.
In summary, a diverse group of novel medium ring heterocycles
derived from naturally abundant proteinogenic amino acids were
evaluated for their potency towards antithrombotic activity. The
more potent oxazepine and oxazocine scaffolds were diversified
through incorporation of different amino acids at the position-3.
Further, the effect of ring size on the biological activity was also ex-
plored and we found that the eight-membered benzoxazocines are
more potent compared to the corresponding seven-membered
benzoxazepines.
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Acknowledgements
Jitendra and Krishnananda thank CSIR for providing fellow-
ships. The DST, New Delhi, India, supported this project. Excellent
technical assistance of Mrs. K. Bhutani and Mr. S. Singh in the bio-
evaluation of test compounds is gratefully acknowledged. This has
CDRI communication number 7730.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2009.10.126.
12. The Merck Index; Budavari, S., Ed., 12th ed.; Merck, Rahway: NJ, 1996; p 1105.
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J. K. Mishra et al. / Bioorg. Med. Chem. Lett. 20 (2010) 244–247
247
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