Design, microwave-assisted synthesis, and spasmolytic activity of 2-(alkyloxyaryl)-1H-benzimidazole derivatives as constrained stilbene bioisosteres

Article abstract of DOI:10.1016/j.bmcl.2006.05.082

A simple, fast, and efficient method for the preparation of several 2-(alkyloxyaryl)-1H-benzimidazole derivatives is reported. Compounds were synthesized through a rapid one-pot three component reaction via microwave irradiation, starting from commerciall

Full text of DOI:10.1016/j.bmcl.2006.05.082

Bioorganic & Medicinal Chemistry Letters 16 (2006) 4169–4173
Design, microwave-assisted synthesis, and spasmolytic activity
of 2-(alkyloxyaryl)-1H-benzimidazole derivatives
q
as constrained stilbene bioisosteres
a,
a
*
Gabriel Navarrete-V a´ zquez, Hermenegilda Moreno-Diaz,
a
b
c
Francisco Aguirre-Crespo, Ismael Le o´ n-Rivera, Rafael Villalobos-Molina,
d
a
Omar Mu n˜ oz-Mu n˜ iz and Samuel Estrada-Soto
a
Facultad de Farmacia, Universidad Aut o´ noma del Estado de Morelos, Cuernavaca, Mor. 62210, M e´ xico
Centro de Investigaciones Qu ı´ micas, Universidad Aut o´ noma del Estado de Morelos, Cuernavaca, Mor. 62210, M e´ xico
b
c
Unidad de Biomedicina, FES Iztacala, Universidad Nacional Aut o´ noma de M e´ xico, Tlalnepantla 54090, M e´ xico
d
Unidad de Servicios de Apoyo en Resoluci o´ n Anal ı´ tica, Universidad Veracruzana, Xalapa, Ver. 91190, M e´ xico
Received 7 April 2006; revised 23 May 2006; accepted 30 May 2006
Available online 19 June 2006
Abstract—A simple, fast, and efficient method for the preparation of several 2-(alkyloxyaryl)-1H-benzimidazole derivatives is
reported. Compounds were synthesized through a rapid one-pot three component reaction via microwave irradiation, starting from
commercially available aldehydes and o-phenylenediamine, in the presence of Na S O and solvent-free conditions. The design of
2
2
5
these compounds explore the hypothesis that the stilbene framework could be mimicked with an appropriate 2-(Alkyloxyphe-
nyl)benzimidazole scaffold. This framework has a similar structural motif as the 6-phenylnaphthalene and behaves like stilbene
bioisosteres. The spasmolytic activity of these compounds was recorded using isolated rat ileum test. Compound 12 was the most
active of the series, showing an IC50 of 1.19 lM.
ꢀ
2006 Elsevier Ltd. All rights reserved.
In an attempt to identify novel compounds to treat dis-
orders of smooth muscle function, our group initially fo-
cused on the modestly selective structure of stilbene and
bibenzyl (Scheme 1) as starting points. Stilbenoids are
compounds that show a variety of biological activities
such as antineoplasic and vasorelaxing properties (e.g.,
given structure represents one of the useful methods in
the search for biologically active conformation. The
end result is a more constrained molecule, with an im-
posed conformation. It is well known that the benzimid-
azole pharmacophore is an important structural core in
medicinal chemistry that shows a broad spectrum of
6–8
1
–4
resveratrol).
Moreover, gigantol (Scheme 1) also be-
pharmacological activities.
taining the benzimidazole scaffold have been used as
Several compounds con-
longs to this class of natural products and it induces a
concentration-dependent inhibition of the spontaneous
contractions of the rat ileum with potency higher than
9
10
11
antiparasitic, antimicrobial, antitumoral, antihista-
1
minic, antifungal, and vasorelaxant agents.
2
13
14
5
or comparable to that of papaverine. Our initial efforts
focused on whether the stilbene framework could be
mimicked with an appropriately substituted 2-(alkyloxy-
aryl) benzimidazole scaffold (Scheme 1). Cyclizing open
structures or creating an additional ring system in a
Usually, 2-arylbenzimidazoles have been prepared by
classical cyclocondensation of o-phenylenediamines
with the corresponding carboxylic acids under harsh
1
5,16
dehydrating reaction conditions
or aldehydes under
1
7
oxidative conditions. The condensation of o-phenyl-
enediamines and the aldehydes requires an oxidative re-
agent to generate the benzimidazole core. Various
Keywords: Benzimidazole; Spasmolytic; Bioisosteric replacement;
Stilbenoids.
q
Taken in part from the MPharm Thesis of Hermenegilda Moreno-
Diaz.
1
8
19
reagents such as nitrobenzene, benzoquinone, sodi-
22
1
0,17,20
21
um metabisulfite,
2
In(OTf) , I /KI, and even
3 2
3
*
Corresponding author. Tel./fax: +52 777 3297089; e-mail:
gabriel_navarrete@uaem.mx
air have been employed for this purpose. Due to the
availability of commercial aldehydes, this method has
0
960-894X/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.05.082

4170
G. Navarrete-V a´ zquez et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4169–4173
OH
OH
R
OH
R
OH
HO
OMe
N
R
R
OMe
Gigantol
NH
OMe
R
R
OH
Phenylnaphtalene-type
stilbene analogue
Ring contraction
(Indene analogue)
Bioisosteric replacement
(Benzimidazole analogue)
HO
Resveratrol
OH
Stilbene framework
Scheme 1. Design and scaffold evolution of 2-(alkyloxyaryl)-1H-benzimidazole derivatives as constrained stilbene bioisosteres.
been chosen as a general procedure for their prepara-
tion. However, in most of the cases the reaction requires
at least 4–48 h, giving yields between 30 and 75%.
toward the correct binding pockets of the targets. Addi-
tionally, by systematically reducing the rotatable bonds
in the lead compounds, the resulting compounds may
have better chance to have oral bioavailability. The con-
cept of transforming a chain structure into a heterocy-
clic ring moiety is another well-explored method of
bioisosteric replacement. Applying a chain-to-ring
transformation usually induces some conformational
restriction in the lead compound, keeping key binding
functionalities in a fixed position. This may improve
the selectivity of the lead compound, as a flexible mole-
cule may interact with several closely related molecular
Microwave as heating source has been used for the rapid
synthesis of a variety of heterocyclic compounds both in
2
4
solution phase as well as under solvent-free conditions.
Using microwave irradiation, the rates of reactions
involving polar components are usually very fast. Reac-
tions that require hours or even days by conventional
heating may often be accomplished in seconds by micro-
2
5
wave heating, and that is the reason why this technol-
ogy is widely applied to drug discovery.
2
7
targets.
Taking this into consideration, we plan to apply this
methodology in our research projects aimed at the dis-
covery of new spasmolytic agents based on benzimid-
azole scaffold. As a part of our search for basic
information about the structural requirements for
smooth muscle relaxant activity, we have synthesized a
series of eighteen 2-(alkyloxyaryl)-1H-benzimidazole
derivatives related to the natural stilbenoid family,
reported in Table 1. The in vitro spasmolytic activity
of these compounds on the spontaneous contractions
of the rat ileum is also reported. A preliminary study
concerning the structure–activity relationships in these
compounds was also performed.
In this study, 17 benzimidazole derivatives (1–17, Table
1) have been synthesized by the reaction of 1,2-phenyl-
enediamine with adequate aromatic substituted alde-
hydes, utilizing sodium metabisulfite under microwave
2
8
irradiation (Scheme 2). All reactions were performed
without solvent in only 60 s as a maximum time, con-
firming that the focused microwave irradiation is a very
effective technique for accelerating thermal organic reac-
tions in solvent-free conditions. Solid compounds were
purified by recrystallization and the structure of the pure
compounds was established by spectroscopic and spec-
2
9
trometric data. All prepared compounds showed blue
emissions under UV irradiation in methanol solutions.
The reaction between o-phenylenediamine and the
corresponding aromatic aldehyde was carried out in
36–60 s under microwave irradiation and afforded the
corresponding products 1–17 in good yields (Table 1).
After the first irradiation for 10 s, the reaction mixture
was taken out, mixed again, and then heated at the same
power level for an additional 10 s. This step was repeat-
ed until the starting materials were consumed, moni-
tored by TLC analysis. Compound 18 was obtained
with low yield from 1, using an excess of alkylating
agent iodoethane (Scheme 3). When microwave irradia-
tion time was extended, it was possible to observe a
decrease of the yield due to formation of several by-
products. For comparison, a classical method for the
preparation of the benzimidazoles 1–17 was also done
by refluxing the adequate o-phenylenediamine, the alde-
hyde and sodium metabisulfite in DMF for 3–4.5 h. It
was demonstrated that the classical heating afforded
low yields of almost all compounds, in addition to other
The design of these benzimidazole derivatives explores
the hypothesis that the stilbene framework could be
mimicked with an appropriate 6-phenylnaphthalene
scaffold. The toxicity related to the naphthalene core
caused us to design a constrained indene ring. However,
the synthetic inaccessibility of certain functional groups
prompted us to investigate the 2-arylbenzimidazole scaf-
fold (Scheme 1), which has a similar structural motif as
the 6-phenylnaphthalene. The facile assembly of benz-
imidazole core allows us to further explore the effects
of substitution at positions 2, 3, 4, and 5 of the 2-phenyl
ring with oxygenated radicals.
Restriction of conformation is the most commonly used
2
6
technique in SAR development. The conformational
restrictions of a lead structure are often associated with
decreased entropy and increased binding affinity and
specificity with targets if the elements are oriented

G. Navarrete-V a´ zquez et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4169–4173
4171
Table 1. Physicochemical properties and pharmacological activity of synthesized 2-(alkyloxyaryl)-1H-benzimidazole derivatives 1–17
a
Compound
R
1
Ar
Mp (ꢁC)
C logP
Reaction Yield
time
Spasmolytic activity
IC50 (lM) max (%)
5.15 ± 0.58 42.4 ± 1.93 1.13
Potency
b
(%)
index
E
1
2
3
4
5
6
7
8
9
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
–H
CH
—
2-Hydroxyphenyl
2-Methoxyphenyl
242.1–243.5 3.97 ± 0.34 36 s
171.7–173.9 3.43 ± 0.30 49 s
149.4–150.3 3.97 ± 0.30 48 s
219.6–220.2 4.50 ± 0.30 50 s
141.2–143.3 5.09 ± 0.32 47 s
194.0–196.0 5.68 ± 0.34 48 s
229.2–231.1 5.55 ± 0.32 56 s
168.0–170.0 3.20 ± 0.30 48 s
254.1–256.6 3.34 ± 0.29 50 s
229.9–231.4 3.87 ± 0.30 60 s
81
89
88
87
75
78
68
88
92
90
87
83
85
94
86
88
75
46
—
—
7.7 ± 1.17 52.8 ± 1.8
0.75
2-Ethoxyphenyl
2-Propoxyphenyl
1.94 ± 0.20
1.9 ± 0.20
28.2 ± 9.4
50 ± 2.46 3.0
80 ± 7.11 3.06
51.4 ± 2.03 0.21
2-(Benzyloxy)phenyl
2-(4-Chlorobenzyloxy)phenyl
2-(4-Methylbenzyloxy)phenyl
2-Nitrophenyl
59 ± 9.14 38.2 ± 2.18 0.09
15 ± 0.29 47.7 ± 0.98 0.38
78.6 ± 0.16 11.1 ± 0.99 0.07
5.25 ± 0.39 61.2 ± 1.32 1.11
10.6 ± 0.22 54.9 ± 3.06 0.55
4-Hydroxyphenyl
4-Methoxyphenyl
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
4-(N,N-Dimethylamino)phenyl 294.2–296.3 4.12 ± 0.31 60 s
30 ± 5.6
1.19 ± 0.20
5.6 ± 0.32
1.8 ± 0.39
4.0 ± 0.86
3.4 ± 0.83
190 ± 19.8
1.72 ± 0.23
34.9 ± 5.28 0.19
70 ± 1.66 4.90
55 ± 3.33 1.04
65 ± 5.89 3.23
90 ± 1.45 1.45
59 ± 3.34 1.71
50 ± 1.94 0.03
60 ± 1.23 3.39
4-Hydroxy-3-methoxyphenyl
3,4-(Dimethoxy)phenyl
2,3,4-(Trimethoxy)phenyl
2,4,5-(Trimethoxy)phenyl
3,4-(Methylenedioxy)phenyl
4-Pyridyl
224.7–225.4 3.29 ± 0.33 60 s
235.5–236.7 3.94 ± 0.33 60 s
259.9–262.1 3.89 ± 0.39 50 s
258.7–259.7 3.64 ± 0.39 40 s
251.6–253.0 3.12 ± 0.37 40 s
153.0–155.0 2.44 ± 0.29 50 s
232.1–233.0 4.68 ± 0.59 8 h
2
CH
3
2-Ethoxyphenyl
Gigantol
Papaverine
—
—
—
—
2.96 ± 0.25
3.42 ± 1.03
—
—
5.83 ± 0.55 93.5 ± 3.02
1.55 ± 0.12 96.7 ± 5.02 3.76
1
—
a
1
13
Isolated yield, characterized by HR-MS, H NMR, C NMR.
Potency related to gigantol.
b
NH2
NH2
O
H
antispasmodic effect in a concentration-dependent man-
ner. These compounds were three to five times more ac-
tive than gigantol. In order to establish a preliminary
structure–activity relationship, the contribution of oxy-
genated radical substituting the C-2 (compounds 3, 4,
and 18) or C-4 (compounds 12, 14, and 16) of the phenyl
ring was analyzed for their selective and potent relaxant
smooth muscle activity. On the other hand, the presence
of the –N(CH3)2 and –NO2 groups in C-4 or C-2
N
Microwave
+
Ar
+
Na S O irradiation
Ar
2
2
5
(
36-60 sec)
N
H
1
9
1
-17
(
75-94%)
Scheme 2. Preparation of 2-(alkyloxyaryl)-1H-benzimidazole deriva-
tives 1–17.
(compounds 8 and 11, respectively) and those bearing
a hindered substituent (5–7) proved detrimental to spas-
molytic activity. Finally, the absence of oxygenated sub-
stituent groups, as in compound 17, with a pyridyl
instead of alkyloxyphenyl substituent, did not show
activity. This suggests that bioactivity of compounds de-
pends on the presence of oxygenated radicals attached in
positions 2 and/or 4 of the phenyl ring. Further experi-
ments are in progress to determine the mechanisms
underlying spasmolytic activity of these compounds.
N
K CO
N
N
2
3
+
CH CH I
3 2
(Excess)
Acetone
N
H
HO
O
1
18
Scheme 3. Synthesis of 2-(2-ethoxyphenyl)-1-ethyl-1H-benzimidazole
18).
(
In conclusion, we have developed a simple, rapid, and
efficient method for the preparation of 2-(alkyloxy-
aryl)-1H-benzimidazole compounds under solvent-free
conditions using readily available and inexpensive
reagents utilizing microwave irradiation. These
compounds behave like stilbene bioisosteres, retaining
spasmolytic activity, and could be considered leads for
the development of new therapeutic agents, including
drugs to treat disorders of smooth muscle function.
by-products, in which separation was difficult compared
with products obtained by microwave heating. The reac-
tion was found to proceed smoothly under microwave
irradiation within 36–60 s. whereas under reflux condi-
1
0,16
tions in 3–4.5 h. In previous reports
benzimidazole
derivatives were obtained by the reflux of 1,2-phenylen-
ediamines with the Na S O adduct of appropriated
2
2
5
aldehyde, which was previously prepared and kept in a
refrigerator for several hours. The most important result
of our approach is the optimization of yields and reac-
tion times using microwave irradiation.
Acknowledgments
3
0
The isolated rat ileum test allowed us to evaluate the
spasmolytic activity of all the synthesized compounds
This work was supported in part by grants from PROM-
EP-SEP, UAEMOR-PTC-131 (GNV), and CONACyT,
43440 (SES). We are grateful to Gumersindo Mir o´ n-
L o´ pez from CIQ, UAEM, for the determination of all
(Table 1). Pharmacological assay results indicate that
compounds 3, 4, 12, 14, and 18 produced a significant

4172
G. Navarrete-V a´ zquez et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4169–4173
spectra. Hermenegilda Moreno-Diaz acknowledges the
fellowship awarded by CONACyT to carry out graduate
studies.
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benzimidazoles 1–17. Microwave irradiation conditions: A
mixture of 1,2-phenylenediamine (0.0313 mol), 1.01 equiv-
alents of appropriate aldehyde, and 1.01 equivalents of
sodium metabisulfite was mixed and introduced in an open
Erlenmeyer flask. The mixture was irradiated in a house-
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24–60 s. After irradiation, the mixture was poured onto
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. Estrada-Soto, S.; L o´ pez-Guerrero, J. J.; Villalobos-Moli-
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A
mixture
of
1,2-phenylenediamine
(0.0313 mol), 1.01 equivalents of appropriate aldehyde,
and 1.01 equivalents of sodium metabisulfite in 10 mL
DMF was heated to reflux for 3–4.5 h. After cooling, water
(20 mL) was added and the mixture was extracted with
AcOEt (3 · 15 mL). The organic layer was dried over
magnesium sulfate and removed under vacuum. Purifica-
tion was done by chromatography on silica gel eluting with
chloroform and recrystallization from adequate solvent.
29. Selected data for compounds. 2-(1H-Benzimidazol-2-
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H NMR (300 MHz, DMSO-d
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) d 6.73 (dd, 1H, H-3 ,
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J = 7.91, J = 1.3 Hz), 7.11 (td, 1H, H-5 , J = 7.7, J = 7.7,
0
J = 1.3 Hz), 7.18 (td, 1H, H-4 , J = 7.7, J = 7.9,
J = 1.7 Hz), 7.21–7.26 (m, 2H, H-5, H-6, J = 8.5,
J = 1.3 Hz), 7.59–7.63 (m, 2H, H-4, H-7, J = 8.5, J = 6.8,
0
J = 1.4 Hz), 7.76 (dd, 1H, H-6 , J = 7.7, J = 1.7 Hz), 11.76
(br s, 2H, N–H, O–H) ppm; C NMR (75.5 MHz,
1
3
1
1
1
DMSO-d
0
6
) d 113.93 (C-1), 116.05 (C-4, C-7), 117.56 (C-
0
0
3 ), 120.65 (C-5 ), 123.48 (C-5, C-6), 128.77 (C-6 ), 132.72
0
0
2003, 58, 431.
(C-4 ), 141.11 (C-3a, C-7a), 156.39 (C-2), 157.60 (C-2 )
ppm; EIMS: m/z (% rel. int.) 210 (M , 100), 192 (2), 181
+
4. Estrada-Soto, S.; Villalobos-Molina, R.; Aguirre-Crespo
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10 2
(25); HR-MS: calcd for C13H N O: 210.0793. Found:
210.0795. 2-(4-Methoxyphenyl)-1H-benzimidazole (10):
1
White solid, mp 229.9–231.4 ꢁC (ethanol). H NMR
1
(300 MHz, DMSO-d ) d 3.82 (s, 3H, CH –O–), 7.05–7.09
6 3
0
0
(m, 2H, H-3 , H-5 , J = 8.5, J = 2.2 Hz), 7.21–7.25 (m, 2H,
H-5, H-6, J = 8.5, J = 7.0, J = 1.4 Hz), 7.57–7.61 (m, 2H,
0
H-4, H-7, J = 8.5, J = 1.4 Hz), 8.03–8.06 (m, 2H, H-2 , H-
0
13
1
1
1
1
6 , J = 8.5, J = 1.4 Hz), 10.88 (br s, 1H, N–H) ppm;
NMR (75.5 MHz, DMSO-d ) d 55.43 (CH –O–), 114.59
(C-3 , C-5 ), 116.08 (C-4, C-7), 123.48 (C-5, C-6), 125.28
C
6
3
0
0
0
0
0
(C-2 , C-6 ), 125.22 (C-1 ), 139.85 (C-3a, C-7a), 152.12 (C-
0
+
8. Ben-Alloum, A.; Bakkas, S.; Soufiaoui, M. Tetrahedron
Lett. 1998, 39, 4481.
2), 161.06 (C-4 ) ppm; EIMS: m/z (% rel. int.) 224 (M ,
100), 209 (35), 181 (25); HR-MS: calcd for C14 O:
224.0949. Found: 224.0952. 4-(1H-Benzimidazol-2-yl)-2-
12 2
H N
9. Verner, E.; Katz, B. A.; Spencer, J. R.; Allen, D.; Hataye, J.;
Hruzewicz, W.; Hui, H. C.; Kolesnikov, A.; Li, Y.; Luong,
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methoxyphenol (12): pale yellow solid, mp 224.7–225.4 ꢁC
1
(methanol). H NMR (300 MHz, DMSO-d
6
) d 3.97 (s, 3H,
O–), 6.91(d, 1H, H-5 , J = 8.4 Hz), 7.11–7.16 (m, 2H,
0
CH
3
0
H-5, H-6, J = 9.6, J = 3 Hz), 7.53 (br s, 1H, H-6 ,
J = 0.9 Hz), 7.61 (dd, 2H, H-4, H-7, J = 8.1, J = 1.8 Hz),
7.74 (d, 1H, H-2, J = 1.5 Hz), 9.57 (s, 2H, N–H, O–H)
2
2
1
3
1452.
ppm; C NMR (75.5 MHz, DMSO-d
0
6
) d 55.67 (CH
3
),
0
1. Trivedi, R.; De, S. K.; Gibbs, R. A. J. Mol. Catal. A:
Chem. 2006, 245, 8.
2. Gogoi, P.; Konwar, D. . Tetrahedron Lett. 2006, 47, 79.
3. Lin, S.; Yang, L. Tetrahedron Lett. 2005, 46, 4315.
4. Mavandadi, F.; Lidstr o¨ m, P. Curr. Top. Med. Chem. 2004,
, 773.
5. (a) Yu, H.; Kawanishi, H.; Koshima, H. Heterocycles
003, 60, 1457; (b) Su, Y. S.; Lin, M. J.; Sun, M. C.
110.24 (C-2 ), 116.07 (C-4, C-7), 117.28 (C-5 ),123.08 (C-
0
0
1 ), 123.48 (C-5, C-6), 124.72 (C-6 ), 139.73 (C-3a, C-7a),
0
0
2
2
2
146.81 (C-4 ), 150.22 (C-3 ), 151.67 (C-2) ppm; EIMS: m/z
(% rel. int.) 240 (M , 100), 225 (20), 210 (60), 197 (25);
+
HR-MS: calcd for
240.0900.
C H N O : 240.0898. Found:
14 12 2 2
4
2
30. Rat ileum test. Determination of spasmolytic activity:
male Wistar rats (200–250 g) were used. The animals were
2

G. Navarrete-V a´ zquez et al. / Bioorg. Med. Chem. Lett. 16 (2006) 4169–4173
4173
killed by ether exposure. The ileum was dissected and
placed in Krebs solution, pH 7.4, with the following
composition (in mM): NaCl, 118; KCl, 4.7; CaCl , 2.5;
of the compounds and positive control was determined
by comparing the muscular tone, inscribed by the
frequency and amplitude of the ileum contractions
before and after the application of the test materials.
Muscular tone was calculated from the tracings, using
2
MgSO
0
4 2 4 3
, 1.2 mM; KH PO , 1.2; NaHCO , 25.0; EDTA,
.026 and glucose, 11.1. Ileum strips (1–1.5 cm long)
were dissected and mounted in organ baths containing
Krebs solution gassed with a mixture of O /CO (19:1)
ꢂ
Acknowledge software (BIOPAC Systems Inc.). All the
results are expressed as means of six experiments ± SEM.
Concentration–response curves (CRC) for compounds
were plotted and the experimental data from the CRC
were adjusted by the nonlinear, curve fitting program
2
2
and under a constant tension of 1 g. After 30 min
stabilization time, a 10 min control period was recorded.
The test compounds dissolved in DMSO were added to
the bath in a volume of 100 lL at different concentra-
tions. Cumulative concentration–response curves were
obtained for each ileum, from 0.01 to 500 lM. The effect
ꢂ
(ORIGIN 6.0). The statistical significance (p < 0.05) of
differences between means was assessed by an analysis of
variance (ANOVA).