One step synthesis of pyrido[1,2-a]benzimidazole derivatives of aryloxypyrazole and their antimicrobial evaluation

Article abstract of DOI:10.1016/j.cclet.2013.01.021

A new series of pyrido[1,2-a]benzimidazole derivatives bearing the aryloxypyrazole nucleus have been synthesized by base-catalyzed cyclocondensation reaction through multi-component reaction (MCR) approach. All the synthesized compounds were investigated

Full text of DOI:10.1016/j.cclet.2013.01.021

Chinese Chemical Letters 24 (2013) 123–126
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
One step synthesis of pyrido[1,2-a]benzimidazole derivatives of
aryloxypyrazole and their antimicrobial evaluation
*
Hardik H. Jardosh, Chetan B. Sangani, Manish P. Patel , Ranjan G. Patel
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388120, Gujarat, India
A R T I C L E I N F O
A B S T R A C T
Article history:
A new series of pyrido[1,2-a]benzimidazole derivatives bearing the aryloxypyrazole nucleus have been
synthesized by base-catalyzed cyclocondensation reaction through multi-component reaction (MCR)
approach. All the synthesized compounds were investigated against a representative panel of pathogenic
strains using broth microdilution minimum inhibitory concentration (MIC) method for their in vitro
antimicrobial activity. Reviewing the data, majority of the compounds were found to be active against
employed pathogens. SAR study explores that antimicrobial activity is strongly depends on the nature of
the substituents at the ether linked aryl ring attached to the pyrazole unit, together with the substituent
present on the C5 of the benzimidazole unit.
Received 3 September 2012
Received in revised form 16 October 2012
Accepted 31 December 2012
Available online 6 February 2013
Keywords:
Aryloxypyrazole
Pyrido[1,2-a]benzimidazole
Multicomponent reaction
Antimicrobial activity
ß 2013 Manish P. Patel. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
1. Introduction
compounds. On the basis of this hypothesis, we report herein the
synthesis and antimicrobial activity of novel pyrido[1,2-a]benz-
Over the past few decades, the problems posed by multi-drug
resistant microorganisms have reached an alarming level in many
countries around the world [1]. This has led to a serious challenge
to the medical community; hence, the development of new
effective antimicrobial agents is an important goal. In pursuit of
this goal, our research efforts are focused on the development of
novel structural moieties with promising antimicrobial properties
[2–7].
Recently, we have been particularly interested in the synthesis
of pyrazole incorporating structures for antimicrobial evaluations
[8–10], on the premise that functionalized pyrazole is a chemically
useful synthon bearing diverse biological activities like antimicro-
bial [11,12], anti-inflammatory [13], antitubercular [14], antitu-
mor [15] properties.
imidazole derivatives bearing ether linked-pyrazole moiety.
All the synthesized compounds were characterized using FT-IR,
1H NMR, 13C NMR, elemental analysis and mass spectrometry and
were subjected to antimicrobial screening in vitro against a
representative panel of eight human pathogens, of which three
Gram-positive bacteria (Bacillus subtilis, Clostridium tetani, Strepto-
coccus pneumoniae), three Gram-negative bacteria (Escherichia coli,
Salmonella typhi, Vibrio cholerae) and two fungi (Aspergillus
fumigatus, Candida albicans) using broth microdilution MIC
(minimum inhibitory concentration) method [19].
2. Experimental
All the reagents were obtained commercially and used after
further purification. All melting points were taken in open
capillaries in a paraffin bath and are uncorrected. The monitoring
of the progress of all reactions and homogeneity of the synthesized
compounds was carried out by thin layer chromatography (TLC).
TLC was run using TLC aluminum sheets silica gel 60F254 (Merck).
Elemental analysis (% C, H, N) was carried out by Perkin-Elmer
2400 series-II elemental analyzer (Perkin-Elmer, USA) and all
compounds are within ꢀ0.4% of theory specified. The IR spectra were
recorded in KBr on a Perkin-Elmer Spectrum GX FT-IR spectropho-
tometer (Perkin-Elmer, USA). 1H NMR and 13C NMR spectra were
recorded in DMSO-d6 on a Bruker Avance 400F (MHz) spectrometer
(Bruker Scientific Corporation Ltd., Switzerland) using TMS as an
internal standard at 400 MHz and 100 MHz respectively. Chemical
In the past few years, pyrido[1,2-a]benzimidazole derivatives
attracted organic as well as medicinal chemists due to their diverse
syntheses and various biological activities including perforin
inhibition [16], antimalarial [17], antimicrobial, anti-HIV-1 and
anticancer properties [18].
From the aforementioned reports, it seemed that a combination
of both therapeutically active moieties aryloxypyrazole and
pyrido[1,2-a]benzimidazole together in a single molecular frame-
work may enhance the pharmacological activity of the title
* Corresponding author.
E-mail address: patelmanish1069@yahoo.com (M.P. Patel).
1001-8417/$ – see front matter ß 2013 Manish P. Patel. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.01.021

124
H.H. Jardosh et al. / Chinese Chemical Letters 24 (2013) 123–126
R1
R1
R1
Compd.
4a
R1
R2
R3
R2
R2
H
H
H
DMF
K2CO3
DMF
POCl3
N
N
N
N
H
CH3
OCH3
Cl
H
4b
4c
+
N
N
H
H
O
Cl
H3C
H3C
HO
O
H
H
4d
4e
H3C
CH3
CH3
CH3
CH3
H
H
H
O
O
H
1a-h
H
CH3
OCH3
Cl
H
4f
R1
H
4g
H
4h
4i
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
R2
N
N
H
CH3
OCH3
Cl
4j
H
H3C
NC
Ethanol
O
R3
CN
H
4k
4l
N
N
CN
+
1a-h
4a-p
+
CN
H
CN
Piperidine
CH3
CH3
CH3
CH3
H
4m
4n
4o
N
NH
2
CH3
OCH3
Cl
N
4p
R3
Scheme 1. Synthetic pathway for the intermediates 1a–h and compounds 4a–p.
shifts are reported in parts per million (ppm). Mass spectra were
scanned on a Shimadzu LCMS 2010 spectrometer (Shimadzu, Tokyo,
Japan).
General procedure for synthesis of compounds 4a–p: A 100 mL
round bottomed flask, fitted with a reflux condenser, was charged
with a mixture of aryloxypyrazole-4-carbaldehyde 1a–h (5 mmol),
unit, singlet around
linked aryl ring and singlet around
methyl group of benzimidazole unit. Singlet around
stands for methoxy group of aryl ring attached to pyrazole unit.
Multiplets in the range of 6.60–8.58 appeared for aromatic
protons. Moreover, a distinctive broad singlet around 8.51–8.75
stands for primary amine of the 4a–p compounds. In the 13C NMR
spectral data of the title compounds 4a–p, the signal at around
78.40–88.35 is assigned to carbon attached with carbonitrile while
signals around 102.00–157.00 are attributed to all the aromatic
carbon of compounds 4a–p. Also, distinctive signals around
13.50–21.85 and 55.70–55.75 stands for methyl and methoxy of
d
2.31–2.38 stands for methyl group of ether
2.52–2.58 appeared for
3.38–3.42
d
d
d
d
malononitrile
2 (5 mmol), 2-cyanomethylbenzimidazole 3a,b
(5 mmol), and a catalytic amount of piperidine (1 mmol) in
ethanol (15 mL) (Scheme 1). The mixture was heated under reflux
for 3.5 h. and the progress of the reaction was monitored by TLC.
After the completion of reaction (as evidenced by TLC), the reaction
mixture was cooled to room temperature and stirred magnetically
for another 20 min, the solid mass was separated and collected by
filtration, washed well with ethanol (15 mL) and purified by
leaching in an equal volume ratio of chloroform and methanol
(20 mL) to obtain a pure solid sample.
d
d
d
d
4a–p derivatives, respectively. The obtained elemental analysis
values are in good agreement with theoretical data. Further, mass
spectra of all the title compounds showed molecular ion peak M+
corresponding to their exact mass.
Physical, analytical and spectroscopic characterization data of
compounds 4a–p are given in Supplementary Material.
3.2. Antimicrobial screening
3. Results and discussion
Examination of the antimicrobial data (Table 1) revealed that
againstGram-positive bacteriaB. subtilis,compounds 4e, 4f,4i and 4m
3.1. Chemistry
(MIC = 100
pounds 4b, 4j and 4k (MIC = 200
potent compared to ampicillin while compounds 4d, 4g, 4o and 4p
(MIC = 250 g/mL) were found to have comparable activity to
ampicillin. Against the species C. tetani, compounds 4l and 4o
(MIC = 100 g/mL) were found to possess excellent activity com-
paredto ampicillinandequivalentto ciprofloxacin.Compounds 4d,4i,
4m and 4n (MIC = 200 g/mL) were found to be more active than
ampicillin while compounds 4a, 4c, 4e, 4g, 4j and 4p (MIC = 250 g/
m
g/mL) found to be equipotent to ciprofloxacin. Com-
mg/mL) were found to be more
The synthetic route depicted in Scheme
chemistry part of the present work. The required intermediates,
1-aryl-5-chloro-3-methyl-1H-pyrazole-4-carbaldehydes were
prepared by Vilsmeier-Haack reaction (chloroformylation) of 1-
aryl-3-methyl-1H-pyrazol-5(4H)-one [20]. 1-Aryl-5-chloro-3-
methyl-1H-pyrazole-4-carbaldehyde undergoes nucleophilic sub-
stitution reaction with respective phenol at refluxing temperature
for 4 h in presence of basic catalyst (K2CO3) in DMF which resulted
in required 3-methyl-5-aryloxy-1-aryl-1H-pyrazole-4-carbalde-
hydes 1a–h according to literature procedure [21,22]. Pyr-
1 outlines the
m
m
m
m
mL) were found equally potent to ampicillin. Against the Gram
positive bacteria S. pneumoniae only compounds 4b, 4e and 4m
(MIC = 100
ampicillin. Toward Gram-negative strain E. coli, compounds 4g
(MIC = 50 g/mL) and 4p (MIC = 62.5 g/mL) were found to have
significant activity whereas, compounds 4c, 4f, 4i and 4l
mg/mL) were found to have equal potency compared to
ido[1,2,-a]benzimidazole
synthesized via base catalyzed three component cyclocondensa-
tion reaction of 1a–h, malononitrile 2 and 2-cyanomethylbenzi-
derivatives
4a–p
have
been
m
m
midazole 3a,b in ethanol containing
piperidine in good yield (58%–85%).
a
catalytic amount of
(MIC = 100
ampicillin.
mg/mL) were found to have comparable activity, to
The title compounds 4a–p were characterized by 1H NMR, 13C-
NMR, FT-IR, mass spectra and elemental analysis. The IR spectrum
of title compounds 4a–p revealed the presence of amino, cyano,
and ether groups due to the appearance of absorption bands at
Against S. typhi compounds 4i (MIC = 50
(MIC = 62.5 g/mL) were found to have fabulous activity whereas,
compounds 4c, 4h, 4j and 4k (MIC = 100 g/mL) were found to have
comparable activity to ampicillin. Against V. cholerae compound 4n
(MIC = 50 g/mL)wasfoundtobeequipotentwithchloramphenicol
and more potent than ampicillin whereas, compounds 4c, 4f and 4p
(MIC = 100 g/mL) were found to have comparable activity, to
ampicillin. Against fungal pathogen C. albicans, compounds 4c, 4g,
mg/mL) and 4f
m
m
around 3440–3360 and 3340–3170, 2250–2190, 1230–1180 cmꢁ1
respectively. In 1H NMR spectrum, a singlet around
1.90–2.27
appeared for methyl group of pyrazole unit, singlet around 2.30–
2.40 observed for methyl group of aryl ring attached to pyrazole
,
m
d
d
m

H.H. Jardosh et al. / Chinese Chemical Letters 24 (2013) 123–126
125
Table 1
In vitro antimicrobial activity MIC (
mg/mL) of pyrido[1,2-a]benzimidazole derivatives 4a–p.
Compound
Gram-positive bacteria
Gram-negative bacteria
Fungi
Bs. MTCC 441
Ct. MTCC 449
Sp. MTCC 1936
Ec. MTCC 443
St. MTCC 98
Vc. MTCC 3906
Af. MTCC 3008
Ca. MTCC 227
4a
500
200
500
250
100
100
250
500
100
200
200
500
100
500
250
250
250
50
250
500
250
200
250
500
250
500
200
250
125
100
200
200
100
250
250
100
50
200
100
500
250
100
250
500
200
250
500
500
500
100
200
250
250
100
50
250
250
100
250
500
100
50
200
500
100
200
500
250
500
100
250
200
100
250
125
250
250
500
250
250
50
1000
>1000
>1000
>1000
>1000
>1000
>1000
1000
>1000
>1000
>1000
>1000
>1000
200
500
500
250
1000
>1000
500
250
1000
>1000
1000
1000
500
250
>1000
250
250
–
4b
4c
4d
4e
4f
62.5
4g
250
100
50
4h
200
100
250
200
100
200
250
250
62.5
100
25
4i
4j
100
100
200
250
250
500
200
100
25
4k
4l
4m
4n
4o
200
100
100
25
1000
>1000
–
4p
Ampicillin
Ciprofloxacin
Norfloxacin
Chloramphenicol
Griseofulvin
–
–
100
50
10
10
10
10
–
–
50
50
50
50
50
–
–
–
–
–
–
–
–
100
500
Bs.: Bacillus subtilis; Ct.: Clostridium tetani; Sp.: Streptococcus pneumoniae; Ec.: Escherichia coli; St.: Salmonella typhi; Vc.: Vibrio cholerae; Af.: Aspergillus fumigatus; Ca.: Candida
albicans.
MTCC: Microbial type culture collection.
‘–’ represents ‘not tested’.
4m, 4o and 4p (MIC = 250
mg/mL) were found to be more potent,
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