Design and synthesis of new 2-substituted-5-(2-benzylthiophenyl)-1,3,4- oxadiazoles as benzodiazepine receptor agonists

Article abstract of DOI:10.1016/j.bmcl.2005.04.018

A series of new 2-substituted-5-(2-benzylthiophenyl)-1,3,4-oxadiazoles was designed and synthesized as anticonvulsant agents. Conformational analysis and superimposition of energy minima conformers of the designed molecules on estazolam, a known benzodiazepine receptor agonist, revealed that the main proposed benzodiazepine pharmacophores were well matched. Electroshock and pentylenetetrazole-induced lethal convulsion tests showed that the introduction of an amino group in position 2 of 1,3,4-oxadiazole ring and a fluoro substituent at para position of benzylthio moiety had the best anticonvulsant activity. It seems this effect is mediated through benzodiazepine receptors mechanism.

Full text of DOI:10.1016/j.bmcl.2005.04.018

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3126–3129
Design and synthesis of new 2-substituted-5-(2-benzylthiophenyl)-
1,3,4-oxadiazoles as benzodiazepine receptor agonists
Afshin Zarghi,a,* Mehrdad Faizi,b Bijan Shafaghi,b Avideh Ahadian,a
Hamid R. Khojastehpoor,a Vahideh Zanganeh,b Sayyed A. Tabatabaia and Abbas Shafieec
aDepartment of Medicinal Chemistry, Faculty of Pharmacy, Shaheed Beheshti University of Medical Sciences,
Tehran 14155-6153, Iran
bDepartment of Pharmacology, Faculty of Pharmacy, Shaheed Beheshti University of Medical Sciences, Tehran 14155-6153, Iran
cDepartment of Medicinal Chemistry, Faculty of Pharmacy and Pharmaceutical Research Center,
Tehran University of Medical Sciences, Tehran 14155-6451, Iran
Received 6 March 2005; revised 5 April 2005; accepted 8 April 2005
Available online 3 May 2005
Abstract—A series of new 2-substituted-5-(2-benzylthiophenyl)-1,3,4-oxadiazoles was designed and synthesized as anticonvulsant
agents. Conformational analysis and superimposition of energy minima conformers of the designed molecules on estazolam, a
known benzodiazepine receptor agonist, revealed that the main proposed benzodiazepine pharmacophores were well matched. Elec-
troshock and pentylenetetrazole-induced lethal convulsion tests showed that the introduction of an amino group in position 2 of
1,3,4-oxadiazole ring and a fluoro substituent at para position of benzylthio moiety had the best anticonvulsant activity. It seems
this effect is mediated through benzodiazepine receptors mechanism.
Ó 2005 Elsevier Ltd. All rights reserved.
Benzodiazepines (BZDs) are widely used in the treat-
ment of central nervous system (CNS) disorders.1 The
pharmacological effects of BZDs result from their affin-
ity for a specific binding domain on the GABAA recep-
tors, known as the BZD receptor.2,3 BZD agonists
increase the frequency of the opening of the chlorine
channel in response to GABA action, causing anxiolytic,
sedative and muscle relaxant effects.4 Several pharmaco-
phore models have been proposed for BZDs, and
amongst all models suggested for binding to the BZD
receptor at least two features are common: an aromatic
ring and a coplanar proton accepting group in suitable
distance. Also, the presence of a second out-of-plane,
aromatic ring could potentiate binding to the recep-
tor.5–9 On this basis, we reported 2-substituted-5-(2-benz-
yloxyphenyl) and (2-phenoxyphenyl)-1,3,4-oxadiazoles
which showed considerable anticonvulsant activity.10,11
In the present study, some new 2-substituted-5-(2-benz-
ylthiophenyl)-1,3,4-oxadiazoles with a flexible second
out-of-plane aromatic ring, benzylthio group, were de-
signed (Fig. 1) which had all the suggested requirements
for binding to the BZD receptors. In order to confirm
that the designed compounds can mimic the proper con-
formation for binding to the BZD receptor, conforma-
tional analysis of designed molecules as well as a
known benzodiazepine agonist, estazolam (Fig. 1) was
performed followed by superimposition of energy
minima conformers. As an in vivo model for evaluating
BZD effects, pentylenetetrazole (PTZ)-induced lethal
convulsion12 and maximal electroshock13 (MES) tests
Keywords: 1,3,4-Oxadiazole; Benzodiazepine receptor agonists; PTZ;
MES.
*
Corresponding author. Tel.: +98 21 87735215; fax: +98 21
8795008; e-mail: azarghi@yahoo.com
Figure 1. The structure of designed compounds and estazolam.
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.04.018

A. Zarghi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3126–3129
3127
were performed on synthesized compounds. To clarify
the mode of action of the synthesized compounds, the
effect of flumazenil, a BZD receptor antagonist, on the
anticonvulsant activity of the compounds was
determined.
force field method implemented in PCMODEL 6.0 soft-
ware.20 The conformers were optimized further by AM1
calculation using MOPAC 6.0 program.21 Global en-
ergy minima conformers of the designed compounds
were superimposed on corresponding conformer of
estazolam molecule, which was considered as a reference
BZD agonist.
The designed compounds were synthesized according to
Scheme 1. Reaction of thiosalicylic acid 1 with the
appropriate benzyl chloride in alkaline hydromethanolic
solution afforded corresponding 2-benzylthio benzoic
acid 2.14 Esterification of 2 by methanol using acidic
condition gave 2-benzylthio benzoic acid methyl ester
3.15 2-Benzylthio benzoic acid hydrazides 4 were readily
prepared by treatment of 3 with hydrazine hydrate
in methanol.16 The hydrazides were converted to
2-amino-5-(2-benzylthiophenyl)-1,3,4-oxadiazoles 5 using
cyanogen bromide in methanol (56–68%).17 5-(2-Benzyl-
thiophenyl)-2-mercapto-1,3,4-oxadiazole 6 was prepared
by the reaction of hydrazide 4a with carbon disulfide
under basic condition.18 Sonication of compound 6 in
the presence of suitable alkyl halide in alkaline
media afforded 2-alkylthio-5-(2-benzylthiophenyl)-1,3,4-
oxadiazoles 7a–c (71–88%).19 The compounds were
The BZD activity of the synthesized compounds was
determined through the evaluation of the ability of the
compounds to protect mice against convulsion induced
by a lethal dose of PTZ and electroshock as two routine
models. Diazepam was considered as a reference BZD
agonist with anticonvulsant effect in both models. Each
one of the synthesized compounds, diazepam or vehicle
was administered 30 min before injection of PTZ
100 mg/kg or application of electroshock (60 Hz,
37.2 mA and 0.25 s). After 30 min, the dead mice were
counted in PTZ test and occurrences of HLTE (hind
limb tonic extension) were observed in MES Model.
As shown in Table 1, compound 5b with amino group
on position 2 of oxadiazole ring and fluoro substituent
at para position of benzylthio group has the best
anticonvulsant activity in both PTZ and MES models.
The activity was antagonized with flumazenil, a benzo-
diazepine antagonist, which establishes the involvement
of benzodiazepine receptors in this effect. Figure 2 shows
that energy minima conformers of the compound 5b, the
1
characterized by H nuclear magnetic resonance, infra-
red, mass spectrometry and CHN analysis.
Conformational analysis of the synthesized compounds
and estazolam were preliminarily performed by MMX
Scheme 1. Reagents and conditions: (i) BzCl, KOH 10%; MeOH, rt, 3 h; (ii) MeOH, H2SO4, reflux, 8 h; (iii) NH2NH2ÆH2O; EtOH, rt, 6 h; (iv) BrCN,
NaHCO3; MeOH, rt, 3 h; (v) CS2, KOH, EtOH, reflux, 6 h; (vi) RI, NaOH 10%, EtOH, sonication, 20 min.

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A. Zarghi et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3126–3129
Table 1. Pharmacological evaluation of the synthesized compounds
tron-withdrawing group such as Cl (5c) abolished the
activity. This effect appears to be a steric hindrance ef-
fect since the unsubstituted analogue 5a had a mild anti-
convulsant activity. These results are in good agreement
with the classical SAR data of BZDs22 and our previous
studies on 1,3,4-oxadiazole derivatives.10,11 However,
the benzylthiophenyl-1,3,4-oxadiazoles had less anticon-
vulsant activity compared with benzyloxy phenyl deriva-
tives11 in the both models. In the series of 2-alkylthio
oxadiazoles, only compound 7a had a weak anticonvul-
sant activity. Compounds 7b and c did not have any
significant anticonvulsant activity in both models. In
conclusion, the results of this investigation indicate that
some synthesized 2-amino-5-aryl-1,3,4-oxadiazoles with
a simple non-rigid structure in which the flexible second
out-of-plane aromatic ring, benzylthio group, has a suit-
able substituent can show benzodiazepine activity con-
firming the suggested SARs for benzodiazepine agonists.
a
ED50 (mg/kg)
Compd
R1 R2
PTZ
MES
5a
5b
H
F
NH2
NH2
89.2 (60.1–135.9)b 78.9 (54.9–102.9)b
29.1 (16.8–42.1)b
>100
>100
43.3 (30.8–54.6)b
>100
>100
5c
Cl NH2
6
7a
H
H
H
H
SH
SCH3 93.2 (65.1–188.4)b >100
7b
7c
SC2H5 >100
SBz
>100
>100
1.8 (1.1–2.6)b
>100
1.4 (1.1–2.0)b
Acknowledgements
Diazepam
a n = 10, 95% confidence limits in parentheses, LD50 of all compounds
>300 mg/kg.
b ED50 significantly increased in the presence of flumazenil 10 mg/kg
(P < 0.05).
This work was partially supported by a grant from the
research council of Shaheed Beheshti University of
Medical Sciences.
References and notes
most potent synthesized analogue, and estazolam
are superimposed. Obviously, the main BZD
pharmacophores, aromatic rings and proton accepting
groups, number 2 nitrogen of the 1,3,4-oxadiazole
and triazolobenzodiazepine rings, are well matched.
Replacement of fluoro substituent with a larger elec-
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Figure 2. Stereoview of the superimposition of the energy minima conformers of estazolam (top left) and compound 5b (top right).

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