Synthesis and biological activity of 2-alkylbenzimidazoles bearing a N-phenylpyrrole moiety as novel angiotensin II AT1 receptor antagonists

Article abstract of DOI:10.1016/j.bmcl.2007.02.042

A series of 2-alkylbenzimidazoles bearing a N-phenylpyrrole moiety were synthesized and evaluated as a novel class of AT₁ receptor antagonists. Among them, compounds 10a and 10g inhibited [¹²⁵I] AngII-binding affinity to AT₁

Full text of DOI:10.1016/j.bmcl.2007.02.042

Bioorganic & Medicinal Chemistry Letters 17 (2007) 2921–2926
Synthesis and biological activity of 2-alkylbenzimidazoles bearing a
N-phenylpyrrole moiety as novel angiotensin II AT1
receptor antagonists
Jin Yi Xu,a Yi Zeng,a Qian Ran,a Zhen Wei,a Yi Bi,a Qian Hui He,a Qiu Juan Wang,b
Song Hu,b Jing Zhang,b Ming Yue Tang,b Wei Yi Huaa and Xiao Ming Wua,*
aDepartment of Medicinal Chemistry, College of Pharmacy, China Pharmaceutical University, 24 Tongjia Xiang,
Nanjing 210009, China
bDepartment of Physiology, College of Pharmacy, China Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009, China
Received 21 October 2006; revised 9 February 2007; accepted 16 February 2007
Available online 20 February 2007
Abstract—A series of 2-alkylbenzimidazoles bearing a N-phenylpyrrole moiety were synthesized and evaluated as a novel class of
AT1 receptor antagonists. Among them, compounds 10a and 10g inhibited [125I] AngII-binding affinity to AT1 receptor at nanomolar
level and potently inhibited the Ang II-induced pressor response by oral administration. Moreover, evaluation in spontaneously
hypertensive rats showed that 10a is an orally active AT1 receptor antagonist.
Ó 2007 Elsevier Ltd. All rights reserved.
The renin–angiotensin system (RAS) is known to play a
pivotal role in the regulation of fluid, electrolyte bal-
ance, and blood pressure, and is a modulator of cellular
growth and proliferation.1 Inhibitors of the RAS would
be effective for the treatment of hypertension and con-
gestive heart failure.2 Among them, angiotensin-con-
verting enzyme (ACE) inhibitors have been very
successful in the treatment of hypertension and conges-
tive heart failure during the last few decades. However,
these inhibitors suffer from some side effects such as dry
cough and angioedema caused by their nonspecific
actions.3 On the other hand, angiotensin II (AngII)
receptor antagonists block the RAS at the AngII receptor
level. This provides a more specific attempt to inhibit the
activity of the RAS and has become the main pharmaco-
logical approach.4
of potent, selective, and orally active AT1 receptor
antagonists exemplified by losartan (DuP 753, 1).6
The available data from the literature7 indicated four
key structural requirements for good binding affinity
in the imidazole derivatives: a biphenyl tetrazole moiety
linked to the imidazole nucleus; a short lipophilic alkyl
chain at the 2-position of the heterocycle; a basic
nitrogen acting as a hydrogen-bond acceptor in the
3-position of the heterocycle; polar substituents in the
5-position of imidazoles. These pharmacophore features
can be best exemplified by the extremely potent benz-
imidazole antagonist CV-11194 (2)8 and imidazo[4,5-
b]pyridine antagonist L-158,809 (3).9
A large body of existing literature has described the
extensive SAR work focusing on either the replacement
of the imidazole with other heterocyclic and non-hetero-
cyclic groups (e.g., valsartan10) or the modification of
the acid functional group. We became interested in
exploring new surrogates for the biphenyl tetrazole
group because (1) there is a paucity of work studying
the modification of biphenyl ring bearing an acid group,
(2) it is extremely intriguing to us that compounds lost
the AT1 receptor affinity when a nitrogen atom was
introduced in the distal phenyl ring, while little effects
were observed if a nitrogen was placed in the proximal
The first non-peptide AngII receptor antagonists,
N-benzylimidazole-5-acetic acid derivatives, were
originally reported from Takeda laboratories.5 Since
then, this imidazole lead has been developed into a series
Keywords: 2-Alkylbenzimidazole; AT1 receptor antagonists; Synthesis;
Activity; Hypertension.
*
Corresponding author. Tel.: +86 25 83242366; fax: +86 25
83302827; e-mail addresses: jinyixu@china.com; xmwu@cpu.edu.cn
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.02.042

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J. Y. Xu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2921–2926
phenyl group.11 Recently we have noticed a report of
novel AT1 receptor antagonists based on the 4-phenyl-
quinoline such as compound 4 with moderate to good
activity.12
of 8 with tributyltin chloride and sodium azide through
a 1.3-dipolar cycloaddition.6 The N-(tributylstannyl) tet-
razoles were converted to the free tetrazole by treatment
with anhydrous hydrogen chloride in MeOH to give
Me
Me
Cl
N
N
N
N
N
N
Et
Et
Bu
Bu
N
N
N
N
HOH2C
N
N
N
Me
N
N
N
N
Me
N
N
NH
N
NH
N
NH
N
NH
COOH
N
2
3
1
4
In order to search for a novel class of AT1 receptor
antagonists, our efforts were focused on the replace-
ments of the biphenyl tetrazole moiety. Computer-
assisted modeling techniques were used to evaluate
structural parameters in comparison to the related
biphenyl system of some potent compounds. Our previ-
ous QSAR analysis13 of 42 AT1 receptor antagonists
suggested that the distance between the center of two
phenyl rings of biphenyl fragment is optimal at about
0.40–0.65 nm, while the distance between the center of
proximal ring and acidic group is at about 0.35–
0.60 nm. According to our computational calculations,
we found N-phenylpyrrole bearing an acidic group sat-
isfies the above conditions, and could compare to the
biphenyl bearing an acidic group spacers in terms of dis-
tances between comparable atoms. These results led us
to propose novel 2-alkylbenzimidazole-based AT1
receptor antagonists bearing a N-phenylpyrrole moiety.
We wish to report herein our synthesis, and in vitro and
in vivo biological evaluation of the designed
compounds.
10a–f in 38–55% yields. Then saponification of 10a–b
with aqueous LiOH in THF produced 10g–h in 40–
42% yields. Saponification of 13 with NaOH in MeOH
gave the products 14a–d in 59–71% yields.
Angiotensin II receptor (AT1)-binding assay. The pre-
pared compounds were evaluated for their activity to
competitively inhibit [125I] AngII binding to the AT1
receptor by a conventional ligand-binding assay using
a bovine adrenal cortex as described previously.15 The
binding affinity is expressed as IC50 value (shown in
Table 1), which is the concentration of compound which
inhibits [125I] AngII (0.1 nM) binding to AT1 receptor by
50%. Initial analysis on the SAR demonstrated that
benzimidazole, in agreement with the results of
CV-11194 series described in the literature,8,16 was an
excellent replacement for the imidazole ring in our
series. More interestingly, replacement of the biphenyl
tetrazole by N-phenyl-1H-pyrrole-2-tetrazole appeared
to improve in vitro potency by more than one order of
magnitude, comparing 10g to the parent structure
N
R
N
N
N
N
2
N
NH
R
COOH
N
3
1
N
N
R
COOR
3
R = COOH
or
N
NH
N
As shown in Schemes 1 and 2, 2-acylamino-3-nitro-
benzoate 5 was reacted with 1-[4-(bromomethyl)-phe-
nyl]-1H-pyrrole-2-carbonitrile 6 and methyl 1-[4-(bro-
momethyl)-phenyl] 1H-pyrrole-2-carboxylate 11 (6 and
11 were prepared based on a described procedure in
the literature14) using NaH in DMF to give the corre-
sponding alkylated products 7 and 12, respectively.
Reductive cyclization of 7 and 12 was accomplished in
good yields with iron powder and concentrated hydro-
chloric acid in boiling MeOH to provide 8 and 13.
The tetrazole derivatives 9 were obtained by reaction
CV-11149. The compound 10g showed single-digit
nanomole potency, with an increase of 17- and 79-fold
over losartan and CV-11194, respectively. While the
replacement of the tetrazole group with a carboxylic
acid moiety produced a decrease in the binding affinity
of these compounds (comparing 10g with 14a; 10h with
14b), this is similar to the results described for the
biphenyl system (e.g., losartan).6
Angiotensin II receptor functional antagonism in rabbit
aorta. The synthesized compounds were also evaluated

J. Y. Xu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2921–2926
2923
CN
COOR1
COOR1
N
N
BrCH2
R2
ii
6
CN
iii
CN
NCOR2
N
NHCOR2
i
COOR1
NO2
NO2
N
N
7
8
5
N
N
N
N
N
N
N
N
N
N
N
N
N
R2
R2
iv
R2
NSnBu3-n
N
NH
v
N
NH
COOR1
COOR1
COOH
N
N
N
9
10a-f
10g-h
Scheme 1. Reagents and conditions: (i) NaH, DMF, rt (69–78%); (ii) Fe/HCl, MeOH, reflux (75–84%); (iii) n-Bu3SnCl, NaN3, toluene, reflux; (iv)
HCl, MeOH, rt (38–55%); (v) LiOH, THF/H2O, 80 °C, then HCl, rt (40–42%).
COOMe
COOR1
N
COOR1
BrCH2
N
R2
ii
11
COOMe
iii
COOMe
NCOR2
N
NHCOR2
i
COOR1
NO2
NO2
N
N
5
12
13
N
R2
COOH
N
COOH
N
14a-d
Scheme 2. Reagents and conditions: (i) NaH, DMF, rt (65–76%); (ii) Fe/HCl, MeOH, reflux (80–88%); (iii) NaOH, MeOH/H2O, reflux; then HCl, rt
(59–71%).
Table 1. Angiotensin II antagonistic activity of the target compounds10a–h and 14a–d
N
2
R
3
R
N
1
COOR
N
10a-h, 14a-d
a
Compound
R1
R2
R3
Binding IC50(nM) SEM
pA2
10a
10b
Me
Me
Me
Me
Et
Et
H
Bu
Pr
CN4H
CN4H
CN4H
CN4H
CN4H
CN4H
CN4H
CN4H
CO2H
CO2H
CO2H
CO2H
9.8 0.2
32 1.5
140 8.6
200 14
19 3.1
41 5.2
6.9 0.1
16 1.8
33 2.4
92 4.6
160 38
250 47
120 16 (150b)
550b
8.4
7.9
7.5
7.0
7.8
7.1
8.5
8.2
8.0
7.0
6.5
5.9
7.9
10c
10d
Et
Me
Bu
Pr
10e
10f
10g
10h
Bu
Pr
H
14a
14b
H
Bu
Pr
H
14c
14d
H
Et
Me
H
1 (losartan)
2 (CV-11194)
a Antagonism of AngII-induced contraction is expressed as pA2, which was obtained as described in the Experimental Section (Supporting
Information).
b Data taken from the literature (Ref. 19).

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J. Y. Xu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2921–2926
in a functional assay for their antagonism of AngII-
induced contractions in the rabbit thoracic aortic
rings.17,18 Losartan was taken as a positive control drug
in the assays. These compounds and losartan inhibited
AngII (10 nM)-induced contraction in a concentration-
related manner, as exemplified by compounds 10a and
10g in Figure 1. The antagonistic activity of the com-
pounds was expressed as pA2 and are listed in Table 1.
The assay results showed that some of our compounds
exhibited potent antagonistic activity of AT1receptor
in a functional assay, which correlated highly with our
binding affinity results. Among them, compounds 10a
and 10g had a pA2 value of 8.4 and 8.5, respectively,
and showed more potent activity than the positive drug
losartan (pA2 = 7.9) in the assay. Compounds 10a and
10g selectively inhibited AngII-induced contractions of
rabbit aortic strips in a competitive manner and had
no effect on the contraction induced by norepinephrine
(10 nM), KCl (10 nM), and histamine (10 nM).
Figure 2. Inhibitory effects of 10a,10g, and losartan (1 mg/kg po) on
AngII (100 ng/kg iv)-induced pressor response in conscious normo-
tensive rats.
Antagonism of AngII-induced pressor response by oral
administration and oral activity in the spontaneously
hypertensive rats (SHR). Based on the results of
in vitro AngII-binding assay and functional antagonism,
10a and 10g were selected for further evaluation in
in vivo models. In assessing the inhibition of pressor
response induced by AngII (100 ng/kg iv) in conscious
normotensive rats, 10a and 10g were given by oral
administration.8,15 As shown in Figure 2, the inhibitory
activity of 10a was more potent and longer-acting than
that of 10g and losartan at a dose of 1 mg/kg po.
When evaluated orally in conscious SHR, compounds
10a and 10g at doses of 1–3 mg/kg significantly
decreased blood pressure in a dose-dependent manner
and were more efficacious than losartan. Compound
10a at 1 mg/kg po reduced the mean arterial blood pres-
sure by more than 40 mmHg with a duration of action
exceeding 24 h (Figs. 3 and 4). Furthermore, it produced
no observable alteration in the basal heart rate at these
doses.
Figure 3. Effects of 10a and 10g at 1 mg/kg po on mean arterial
pressure in conscious SHR. Values represent means SEM.
It should be pointed out here that compound 10g
showed less efficacy in vivo with respect to 10a, although
it had the higher binding affinity in vitro than 10a. We
Figure 4. Effects of 10a and losartan (3 mg/kg po) on mean arterial
pressure in conscious SHR after oral administration. Values represent
means SEM.
believe that this might be attributed to its lower oral bio-
availability due to the highly polar character of molecule
with one tetrazole and one carboxylic acid group.
For the purpose of understanding the SAR of synthe-
sized compounds from standpoint of computational
chemistry, the MM2 program calculation of these com-
Figure 1. Concentration–inhibition curves of 10a, 10g, and losartan on
the AngII (10 nM)-induced contraction in isolated rabbit aorta
(n = 5–6).

J. Y. Xu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 2921–2926
2925
Table 2. The minimal energy conformational parameters for 10a and losartan
Bond angle (deg) Distance (nm)
Dihedral angle (deg)
Losartan
C27–C22–N19
127.080
130.202
122.488
122.100
121.547
126.504
N19–N16
N19–C2
N17–C8
0.7599
0.5276
0.7416
174.895a
58.017b
45.340c
C22–N19–C18
N19–C18–C5
C8–C7–C2
C7–C8–C13
C8–C13–N14
10a
C27–C21–N18
C21–N18–C17
N18–C17–C5
C8–N7–C2
127.125
129.528
118.787
124.197
124.963
125.851
N18–N15
N18–C2
0.7589
0.5173
0.7320
178.110a
57.786b
38.746c
N7–C8–C12
C8–C12–N13
a Expressed as the dihedral angle between imidazole (or benzimidazole) ring and the proximal phenyl ring.
b Expressed as the dihedral angle between the proximal phenyl ring and the distal phenyl (or pyrrole) ring.
c Expressed as the dihedral angle between the distal phenyl (or pyrrole) ring and tetrazole ring.
Figure 5. The dominant conformations of 10a and losartan.
pounds was investigated on SGI Indigo R 40000 work-
station as described previously.13 In the computational
studies on structural parameters, we found that the min-
imal energy conformations of the most active com-
pound10a and losartan are similar and their structure
parameters are relatively close, indicating that the two
molecules have similar stereo electronic characteristics
(Table 2). The dominant conformations of 10a and
losartan are shown as Figure 5. A more detailed study
of the mode of action and their QSAR of this series of
2-alkylbenzimidazole derivatives is underway.
10a has interesting pharmacological properties and this
compound is currently under further investigation.
Acknowledgments
We are thankful to National Natural Science Foundation
of China (No. 30371688) and Key fund of Ministry of
Education of China (No. 03089) for financial assistance.
Supplementary data
In conclusion, aseries of 2-alkylbenzimidazole derivatives
bearing a N-phenylpyrrole moiety were designed and syn-
thesized as a novel class of non-peptide AT1 receptor
antagonists, and their biological activities were evaluated.
The study results revealed that the bioisosteric replacement
of biphenyl tetrazole with N-phenylpyrrole-2-tetrazole,
and N-phenylpyrrole-2-carboxylic acid in this series, pro-
duced extremely potent novel analogues. As demon-
strated with in vitro and in vivo results, compound 10a
is an orally active AT1 receptor antagonist that is more
potent and efficacious than losartan. We believe that
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.bmcl.
2007.02.042.
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