Design, synthesis, and evaluation of novelly substituted benzimidazole compounds as angiotensin II receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2005.05.054

5-Nitrobenzimidazole derivatives with varying substituents at 2-position have been designed, synthesized, and evaluated for angiotensin II antagonistic activity. A drug-receptor interaction model has been proposed.

Full text of DOI:10.1016/j.bmcl.2005.05.054

Bioorganic & Medicinal Chemistry Letters 15 (2005) 3962–3965
Design, synthesis, and evaluation of novelly substituted
benzimidazole compounds as angiotensin II
receptor antagonists
Alka Bali,a Yogita Bansal,b M. Sugumaran,b Jatinder Singh Saggu,b P. Balakumar,b
Gurpreet Kaur,b Gulshan Bansal,b,* Ajay Sharmac and Manjeet Singhb
aUniversity Institute of Pharmaceutical Sciences, Punjab University, Chandigarh, India
bDepartment of Pharmaceutical Sciences and Drug Research, Punjabi University, Patiala 147 002, India
cDivision of Pharmacology, Zydus Cadila Limited, Ahmadabad, India
Received 15 April 2005; revised 6 May 2005; accepted 16 May 2005
Available online 20 July 2005
Abstract—5-Nitrobenzimidazole derivatives with varying substituents at 2-position have been designed, synthesized, and evaluated
for angiotensin II antagonistic activity. A drug–receptor interaction model has been proposed.
Ó 2005 Published by Elsevier Ltd.
1. Introduction
site L3 accommodates the benzyl group whereas site
B/H interacts with carboxylate group.6 A critical analy-
sis of structure of 5-substituted benzimidazole com-
pounds suggests that a similar group at this position
may take an orientation analogous to that taken up by
carboxylate of 1 (Fig. 1b). Hence, compounds 8 are
designed with nitro group at position 5, and a correla-
tion of relative orientations of nitro and carboxylate of
8 and 1, respectively, is established by determining the
distances between O1 and N4 of 1 and O1 and C1 of 8
in their energy minimized conformations using com-
puter software ChemOffice 6.0. These distances have
been found to be very similar (4.638 A in 1 and
˚
4.236 A in 8) (Fig. 1). Hence these compounds are ex-
pected to have greater activity than losartan. Further,
different alkyl groups are substituted at position 2 of
benzimidazole nucleus to optimize an appropriate alkyl
group in 5-substituted benzimidazole derivatives.
Angiotensin II (Ang II) receptor antagonists are widely
accepted as novel antihypertensives clinically because of
lesser side effects and better therapeutic profiles than
ACE inhibitors. Losartan,1,2 the protypical agent of this
category, served as lead for the development of newer
Ang II receptor antagonists.3 Varied substitutions in
benzimidazole nucleus have been extensively studied
for this purpose. Among the different substituents, a
lipophilic group with H-bond accepting capability
(acylureas) at 6-position,4 and a carboxylic function at
7-position5 of benzimidazole nucleus have been found
to be favorable for Ang II antagonism. Further, a linear
butyl chain and an ethoxy group is required at position 2
in 6-substituted benzimidazole and in benzimidazole-7-
carboxylic acid derivatives, respectively. However, posi-
tion 5 in this nucleus has not been exploited much. A
series of triazole derivatives with different substituents
at C-5 of the triazole nucleus has been reported where
a benzylthio group increases the activity substantially.
A further presence of carboxy group at ortho position
of benzylthio moiety makes the compound 1 even more
active than losartan. The binding profile of 1 with AT1
receptors proposed in literature (Fig. 1a) depicts that
˚
Hence, the present study has been conducted to design,
synthesize, and evaluate 5-nitrobenzimidazole deriva-
tives 8a–d bearing n-butyl, ethoxymethyl, n-propyl,
and ethyl chain at 2-position. For the synthesis of pen-
dant carboxylbiphenyl methyl moiety of the target com-
pounds, a novel three-step method has been devised
with improved yields. This novel synthetic route did
not lead to formation of some carcinogenic by-products
as reported with the other methods.7–12 The biological
activity of these compounds has been determined taking
losartan and candesartan as reference compounds.
Keywords: 5-Substituted benzimidazole; Angiotensin II; Antagonists;
Candesartan; Losartan.
*
Corresponding author. Tel.: +91 9855166102; fax: +91
1752283073; e-mail: gulshanbansal@rediffmail.com
0960-894X/$ - see front matter Ó 2005 Published by Elsevier Ltd.
doi:10.1016/j.bmcl.2005.05.054

A. Bali et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3962–3965
3963
Figure 1. (a) Binding profile of 1 where L1–L3 are lipophilic pockets, B is basic site, H is H-bond donor site, and B/H is either a basic or H-bond
donor site.(b) Proposed binding profile of benzimidazole derived compounds where L3 can accommodate a bulky substituent at 6-position and B/H
acts either as a basic or H-bond donor site for substituents at 5-position.
2. Chemistry
yl-20-carboxylic acid 5. The reaction mixture showed
three components in TLC, which were separated by col-
umn chromatography using silica gel (100–120 mesh) as
stationary phase and chloroform as mobile phase. The
required component was identified as third fraction
which was subjected to substitution reaction with phos-
phorus oxychloride in xylene and dimethyl formamide15
to produce the pendant moiety 4-(chloromethyl)biphen-
yl-20-carboxylic acid 6. Coupling of intermediates 2a
and 3 with 6 using anhydrous potassium carbonate in
dimethyl formamide resulted in target compounds 7a
and 8, respectively, and finally reduction of 8a with zinc
in sodium hydroxide produced the final target compound
9a. The compounds synthesized were characterized on
the basis of analytical data and spectral evidences.16
Synthetic scheme (Fig. 2) for target compounds was
divided into two steps. Step I involved synthesis of 2-
alkylbenzimidazoles 2 by condensation reaction of
o-phenylenediamine with the respective carboxylic acids
1. The corresponding 5-nitro derivatives 3 were prepared
by nitration under controlled temperature conditions.13
Step II includes the novel sequential combination of
three routine reactions to synthesize 20-carboxybiphenyl
methylene chloride. Biphenyl-2-carboxylic acid 4 was
prepared by potash fusion of 9H-flourenone,14 which
was then subjected to aromatic substitution reaction
using paraformaldehyde and acetamide in concd sulfuric
acid15 to affect intermediate, 4-acetamidomethyl biphen-
Figure 2. Synthetic scheme. (i) KOH, 180–200 °C, dil HCl; (ii) acetamide, paraformaldehyde, H2SO4, heat, stirring; (iii) POCl3, xylene, DMF, reflux,
H2O.

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A. Bali et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3962–3965
Table 2. Comparative molecular surface areas of target and reference
compounds
3. Biological activity
2
Molecular surface area (A )
˚
Ang II receptor antagonism was determined on endothe-
lium removed isolated rat aortic ring using force transduc-
ers and BIOPAC four-channel recorder (BIOPAC
Systems Inc., Santa Barbara, CA, USA). The activity
was expressed as pA2 values (Table 1). pA10 values were
also determined to establish the mode of antagonism.17
Compound
8a
8b
388.322
370.754
367.638
375.166
470.272
Losartan
Candesartan
1
3.1. Statistical analysis
to the receptor site. The noncompetitive mode of anta-
gonism of 8 further suggests that such receptor pocket
may not be accessible to the natural ligand, Ang II.
Moreover, comparison of molecular surface areas
(MSAs) of losartan, candesartan and 1 indicate that in-
crease in activity can be directly correlated with MSA
(Table 2). Hence, the higher activity of 8a over candesar-
tan can be attributed to this MM parameter. Lesser
activity of 9a with respect to candesartan and 8a further
supports this proposition. Hence, in the light of these
inferences, a drug–receptor interaction model has been
proposed (Fig. 1b) which includes an additional receptor
pocket (B/H site) capable of accommodating a bulky
group and act as a H-bond donor leading to greater
drug–receptor interaction area and hence activity.
Linear regression was employed to plot the CRC
between developed tension and negative log molar con-
centration of angiotensin II. Paired Student ÔtÕ test was
employed for statistical analysis, p < 0.05 was consid-
ered to be statistically significant.
4. Results and discussions
Ang II antagonism by compounds with same functional
group at 2-position has been found to be a function of
substituent at 5-position. Presence of amino group has
increased the activity substantially over the unsubsti-
tuted one (9a vs 7a). The maximum activity has been
observed with nitro group (compound 8a). This suggests
that there are some sites in the receptor pocket, which
can interact with the functional groups at position 5.
The higher activity of 8a over 9a suggests that this group
at 5-position should be either ionic or H-bond acceptor.
Decrease in length of alkyl chain from n-butyl to n-pro-
pyl 8c and to ethyl 8d has decreased the activity indicat-
ing that n-butyl group is required to be present at
2-position when 5-position of benzimidazole nucleus is
substituted. The less activity of 2-ethoxymethyl analog
8b than the corresponding 2-n-butyl analog 8a suggests
that presence of any electronic element in the linear alkyl
chain is not conducive to activity. Compound 8a has
been found to be even more active than the reference
compounds losartan and candesartan. The higher activi-
ty of 5-nitro derivatives may be ascribed to the ability of
nitro group to act as H-bond acceptor with respect to
the receptor site. Moreover, the bulk of nitro group
may optimally ‘‘fill up’’ the receptor pocket and hence
results in closer proximity to the interacting surface of
the receptor. It may consequently increase their affinity
5. Summary
Substituted benzimidazole nucleus coupled to carb-
oxylbiphenyl methyl group has been designed, synthe-
sized, and evaluated for Ang II antagonism.
Compound with nitro group at 5-position and n-butyl
chain at 2-position have been found to be more potent
than candesartan. Hence, a new binding profile has been
proposed where an additional receptor pocket in the
binding site can accommodate bulky but H-bond accep-
tor group and this pocket may not be accessible to nat-
ural ligands and even losartan and candesartan.
Additionally, a novel and simple method for synthesis
of pendant biphenyl moiety has been devised to improve
safety and yield. Further designing and synthesis of
compounds with other such functional groups at 5-posi-
tion are underway.
References and notes
1. Dunica, J. V.; Chiu, A. T.; Carini, D. J.; Gregory, G. B.;
Johnson, A. L.; Timmermans, P. B. M. W. M. J. Med.
Chem. 1990, 33, 1312.
2. Carini, D. J.; Duncia, J. V.; Aldrich, P. E.; Chiu, A. T.;
Johnson, A. L.; Pierc, M. E.; Price, W. A.; Santella, J. B.,
III; Wells, J.; Wexler, R.; Wong, C.; Yoo, S. E.;
Timmermans, P. B. M. W. M. J. Med. Chem. 1991, 34,
2525.
3. Wexler, R. R.; Greenleen, W. J.; Irvin, J. D.; Goldberg,
M. R.; Prendergast, K.; Smith, R. D.; Timmermans, P. B.
M. E. M. J. Med. Chem. 1996, 39, 625.
4. Ries, U. J.; Mihm, G.; Narr, B.; Hasselbach, K. M.;
Wittneben, H.; Entzeroth, M.; Van Meel, J. C. A.;
Wienen, W.; Kavel, N. H. J. Med. Chem. 1993, 36, 4040.
5. Kubo, K.; Kohara, Y.; Imamiya, E.; Sugiura, Y.; Inada,
Y.; Furukawa, Y.; Nishikawa, K.; Naka, T. J. Med.
Chem. 1993, 36, 2182.
Table 1. The comparative Ang II antagonism of the target and
reference compounds
Compound
R
pA2
pA10 (pA2 ꢀ pA10
5.81 0.65
6.9a,b 0.9
)
Mode#
*
7a
9a
–n-C4H9
–n-C4H9
–n-C4H9
6.45
7.8a
8.50a,b 1.8
C
C
8a
8b
6.7
5.6
4.2
3.6
0.9
0.82
NC
NC
NC
NC
C
–CH2OEt 7.3
1.7
2.9
3.4
6.5
7.1
8c
8d
–n-C3H7
–Et
7.1
7.0
7.4
8.02
Losartan
Candesartan
C
a p < 0.05 vs losartan.
b p < 0.05 vs candesartan.
* n = 5.
# Mode of antagonism (NC, noncompetitive; C, competitive).

A. Bali et al. / Bioorg. Med. Chem. Lett. 15 (2005) 3962–3965
3965
6. Ashton, W. T.; Cantone, C. L.; Chang, L. L.; Hutchins, S.
M.; Strelitz, R. A.; Mcross, M.; Chang, R. S. L.; Lotti, V.
J.; Faust, K. A.; Chen, T.; Bunting, P.; Schorn, T. N.;
Kivlighn, S. D.; Siegl, P. K. S. J. Med. Chem. 1993, 36,
591.
7. Kageyama, H.; Miyazaki, T.; Kimura, Y. Synlett 1994,
371.
8. Fanta, P. E. Chem. Rev. 1964, 64, 613.
9. Gomberg, M.; Bachmann, W. E. J. Am. Chem. Soc. 1924,
46, 2339.
10. Meyers, A. I.; Mihelich, E. D. J. Am. Chem. Soc. 1975, 97,
7383.
11. Lo, Y. S.; Rossano, L. T. U.S. Patents, 130,439, 1992.
12. Brown, H. C.; Nelson, K. L. J. Am. Chem. Soc. 1953, 75,
6292.
13. Grimmett, M. R. In Comprehensive Heterocyclic Chemis-
try. The Structure, Reactions, Synthesis and Uses of
Heterocyclic Compounds; Potts, K. T., Ed.; Pergamon:
Oxford, 1984; Vol. 5, p 429, Part 4a.
2H, ArH); 7.01 (d, J = 8, 2H, ArH); 7.53 (dd, J = 8;2,
2H, ArH); 7.58 (dd, J = 8;2, 2H, ArH); 7.87 (dd, J = 8;2,
2H, ArH); 7.92 (dd, J = 8;2, 1H, ArH); 8.01 (dd, J = 8;2,
1H, ArH); 8.90 (b, 1H, COOH). MS (EI): 384 (MÅ+). 2-n-
Butyl-5-nitro-1-[(20-carboxybipheny-4-yl) methyl] benz-
imidazole (8a): yield 66.6%, mp 160–162 °C, IR (KBr):
3600–3200 (O–H); 1670 (C@O); 1440 and 1370 (N@O).
Anal. Calcd for C25H23N3O4: C, 69.63; H, 5.36; N, 9.79.
Found: C, 69.24; H, 5.81; N, 9.58. dH (300 MHz,
CDCl3): 0.91 (t, J = 7.5, 3H, CH2CH2CH2CH3); 1.43
(sx, J = 7.5, 2H, CH2CH2CH2CH3); 1.97 (qv, J = 7.5,
2H, CH2CH2CH2CH3); 2.95 (t, J = 7.5, 2H,
CH2CH2CH2CH3); 3.25 (s, 2H, CH2); 6.82 (d, J = 8,
1H, ArH); 7.46 (d, J = 2, 2H, ArH); 7.48 (d, J = 2, 2H,
ArH); 7.84 (dd, J = 8;2, 2H, ArH); 7.94 (d, J = 2,
1H, ArH); 8.08 (s, 1H, ArH); 8.24 (d, J = 8, 1H, ArH);
8.65 (s, 1H, ArH); 9.58 (b, 1H, COOH). MS (EI): 429
(MÅ+).
5-Amino-2-n-butyl-1-[(20-carboxybipheny-4-yl)
methyl] benzimidazole (9a): yield 64%, mp 168–170 °C,
IR (KBr): 3600–2700 (O–H and N–H); 1690 (C@O). Anal.
Calcd for C25H25N3O2: C, 75.19; H, 6.26; N,
10.53. Found: C, 74.65; H, 6.89; N, 9.96. dH (300 MHz,
CDCl3): 0.90 (t, J = 7.5, 3H, CH2CH2CH2CH3);
1.40 (sx, J = 7.5, 2H, CH2CH2CH2CH3); 1.75 (qv,
J = 7.5, 2H, CH2CH2CH2CH3); 2.82 (t, J = 7.5, 2H,
CH2CH2CH2CH3); 3.10 (s, 4H, CH2 and NH2); 6.61 (m,
1H, ArH); 6.77 (d, J = 2, 1H, ArH); 7.32 (m, 1H, ArH);
7.43 (d, J = 6, 2H, ArH); 7.51 (d, J = 6, 2H, ArH); 7.64
(m, 3H, ArH); 7.99 (d, J = 8, 1H, ArH); 9.72 (b, 1H,
COOH). MS (EI): 399 (MÅ+).
14. Kenner, G. W.; Robinson, M. J. T.; Tylor, C. M. B.;
Webster, B. R. J. Chem. Soc., Part II 1962, 1756.
15. Manlding, D. R.; Lotts, K. D.; Robinson, S. A. J. Org.
Chem. 1983, 48, 2938.
16. 2-n-Butyl-1-[(20-carboxybipheny-4yl)methyl]
benzimid-
azole (7a): yield 42.0%, mp 122–124 °C, IR (KBr):
3600–2700 (O–H); 1660 (C@O); 1420 (C–O–H). Anal.
Calcd for C25H24N2O2: C, 78.13; H, 6.25; N, 7.29.
Found: C, 77.29; H, 6.84; N, 7.79. dH (300 MHz,
CDCl3): 0.90 (t, J = 7.5, 3H, CH2CH2CH2CH3); 1.35
(sx, J = 7.5, 2H, CH2CH2CH2CH3); 1.67 (qv, J = 7.5,
2H, CH2CH2CH2CH3); 2.95 (t, J = 7.5, 2H,
CH2CH2CH2CH3); 3.04 (s, 2H, CH2); 6.95 (d, J = 8,
17. Schild, H. O. Br. J. Pharmacol. Chemother. 1947, 2,
189.