Synthesis, antimicrobial and antimycobacterial evaluation of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones

Article abstract of DOI:10.3109/14756366.2010.548331

A series of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones (111) were synthesized and screened for their antimicrobial and antimycobacterial activities. Further, a series of [2-(substituted phenyl)-benzimidazol-1-yl]-pyridin-3-yl-methanones (1220) reported in our earlier study was also screened for their antimycobacterial activity. The antimycobacterial activity results indicated that [2-(4-Nitro-phenyl)-imidazol- 1-yl]-pyridin-3-yl-methanone (8, minimum inhibitory concentration [MIC]=3.13 g) was equipotent as standard drug ciprofloxacin and [2-(4-Nitro-phenyl)- benzimidazol-1-yl]-pyridin-3-yl-methanone (16, MIC=1.56 g) was equipotent as standard drug ethambutol. The results of antimicrobial screening demonstrated that 2-[1-(Pyridine-3-carbonyl)-1H-imidazol-2-yl]-benzoic acid (compound 11, MIC=0.002 g) was two times more effective than standard drug ciprofloxacin (MIC=0.004 g) against tested bacterial strains and [2-(2,5-Dimethyl-phenyl)- imidazol-1-yl]-pyridin-3-yl-methanone (compound 3, MIC=0.005 g) was equipotent to the reference compound, fluconazole against tested fungal strains.

Full text of DOI:10.3109/14756366.2010.548331

Journal of Enzyme Inhibition and Medicinal Chemistry, 2011; 26(5): 720–727
© 2011 Informa UK, Ltd.
ISSN 1475-6366 print/ISSN 1475-6374 online
DOI: 10.3109/14756366.2010.548331
RESEARCH ARTICLE
Synthesis, antimicrobial and antimycobacterial evaluation
of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-
methanones
Balasubramanian Narasimhan1, Deepika Sharma2, Pradeep Kumar1, Perumal Yogeeswari3, and
Dharmarajan Sriram3
1Faculty of Pharmaceutical Sciences, Maharshi Dayanand University, Rohtak 124001, India, 2University Institute of
Pharmaceutical Sciences, Panjab University, Chandigarh 160014, India, and 3Medicinal Chemistry Research Laboratory,
Pharmacy Group, Birla Institute of Technology and Science, Hyderabad 500078, India
Abstract
A series of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones (1–11) were synthesized and screened for
their antimicrobial and antimycobacterial activities. Further, a series of [2-(substituted phenyl)-benzimidazol-1-yl]-
pyridin-3-yl-methanones (12–20) reported in our earlier study was also screened for their antimycobacterial activity.
The antimycobacterial activity results indicated that [2-(4-Nitro-phenyl)-imidazol-1-yl]-pyridin-3-yl-methanone (8,
minimum inhibitory concentration [MIC]=3.13 µg) was equipotent as standard drug ciprofloxacin and [2-(4-Nitro-
phenyl)-benzimidazol-1-yl]-pyridin-3-yl-methanone (16, MIC=1.56 µg) was equipotent as standard drug ethambutol.
The results of antimicrobial screening demonstrated that 2-[1-(Pyridine-3-carbonyl)-1H-imidazol-2-yl]-benzoic acid
(compound 11, MIC=0.002 µg) was two times more effective than standard drug ciprofloxacin (MIC =0.004 µg)
against tested bacterial strains and [2-(2,5-Dimethyl-phenyl)-imidazol-1-yl]-pyridin-3-yl-methanone (compound 3,
MIC=0.005 µg) was equipotent to the reference compound, fluconazole against tested fungal strains.
Keywords: Imidazole/benzimidazole derivatives; antimycobacterial; antimicrobial
Introduction
new case of TB is reported every 4 sec and one patient
dies every 10 sec4. e factors responsible for this are
(i) patient non-compliance to existing drug regimens
which has resulted in the emergence of single drug-re-
sistant strains to all major anti-TB drugs; (ii) emergence
of multidrug resistant TB (MDR-TB), which is defined
as the disease caused by the strains of M. tuberculosis
resistant to two mainstay first line anti-TB drugs, iso-
niazid and rifampicin, and (iii) association of human
immunodeficiency virus (HIV) with TB, in which TB
is the leading cause of death among patients who are
HIV-positive5. e development of potent new anti-TB
agents with low toxicity profiles effective against both
drug-susceptible and drug-resistant strains of M. tuber-
culosis and capable of shortening the current duration
of therapy are the need of the hour6. ere are two basic
Disease-causing microbes that have become resistant
to drug therapy are an increasing public health problem
nowadays1. ere is a real need for discovery of new
chemical entities endowed with antimicrobial activity,
possibly acting through mechanisms which are distinct
from the well-known classes of antimicrobial agents, to
which many clinically relevant pathogens have become
resistant2.
Tuberculosis (TB), which is caused by single infec-
tious agent Mycobacterium tuberculosis, is one of the
most important infectious disease3. Recently, world
health organization (WHO) reported that the global fig-
ure of total deaths by infectious diseases is 17 million,
of which TB accounts for ~20 % of mortality. e grav-
ity of problem increases even more by the fact that one
Address for Correspondence: Balasubramanian Narasimhan, Faculty of Pharmaceutical Sciences, Maharshi Dayanand University,
Rohtak 124001, Haryana, India, Tel.: +91-1262-272535; Fax: +91-1262-274133. E-mail: naru2000us@yahoo.com
(Received 09 October 2010; revised 01 December 2010; accepted 12 December 2010)
720

Antitubercular activity of substitued imidazoles 721
approaches to develop a new drug for TB: (i) synthesis
of analogues, modification, or derivatives of existing
compounds for shortening and improving TB treat-
ment and (ii) searching for novel structures, that the TB
organism has never been presented with before, for the
treatment of MDR-TB7.
e compounds 1–3 and 5–11 were synthesized by
the similar procedure followed for 4 using corresponding
substituted anilines.
Compound 1. mp (°C) 79–81; Yield− 50.76%; 1H-NMR:
δ 7.0 (d, 2H, CH of imidazole), δ 7.20–7.31 (m, 3H, CH of
C3-C5 of benzene), δ 7.46–7.48 (d, 1H, CH of C2 of ben-
zene), δ 8.2 (d, 1H, CH of C6 of benzene), δ 7.51–7.57 (m,
CH of C4-C5 of pyridine), δ 8.81 (d, 1H, CH of C6 of pyri-
dine), δ 9.05 (s, 1H, CH of C2 of pyridine); 13C-NMR: 136,
122, 136.5, 126.8, 134.8, 128.5, 128.8, 127.2, 193, 131.9,
152.2, 155.2, 124.6, 134.9; IR (KBr pellets): cm−1 1724.2
(C = O str.), 3080.0 (CH str., aromatic), 1027.8 (Ring bend-
ing mode, imidazole), 1597.9 and 1680.0 (C = C and C = N
str. of pyridine ring); Anal. Calculated for C15H11N3O: C,
72.28; H, 4.45; N, 16.86; O, 6.42. Found: C, 72.19; H, 4.41;
N, 16.83; O, 6.41.
e prevalence of imidazoles in natural products and
pharmacologically active compounds has instituted a
diverse array of synthetic approaches to these hetero-
cycles. e imidazole nucleus reported to have a wide
range of pharmacological activities viz. anticancer, anti-
mycobacterial, antiviral, antibacterial, antifungal, antidi-
abetic, anti-inflammatory, analgesic, and antiobesity
activities8–15. In pursuit of this goal, our research efforts
are directed toward discovery of new chemical entities
that are effective as anti-TB agents viz. synthesis and
antitubercular evaluation of [2-(substituted phenyl)-im-
idazol-1-yl]-pyridin-3-yl-methanones. Further, we have
also screened the nicotinic acid-based benzimidazole
derivatives reported in our previous study16 for their anti-
mycobacterial activity.
Compound 4. mp (°C) 94–96; Yield− 68.75%; 1H-NMR:
δ 7.29–7.34 (d, 2H, CH of C4 and C5 of imidazole), δ 7.83–
7.86 (m, 2H, CH of C5 and C6 of ArCl), δ 7.99–8.02 (d, 1H,
CH of C6 of ArCl), δ 8.65–8.67 (d, 1H, CH of C3 of ArCl), δ
8.81–8.83 (t, 1H, CH of C5 of nicotinic acid), δ 8.98–8.99
(d, 1H, CH of C6 of nicotinic acid), δ 9.21 (s, 1H, CH of C2
of nicotinic acid), δ 8.94–8.95 (d, 1H, CH of C4 of nicotinic
acid); 13C-NMR: 136.4, 121.9, 135.7, 128.3, 127.2, 128.7,
129.9, 131.8, 191, 131.7, 152.5, 155.4, 124.6, 135.7; IR (KBr
pellets): cm−1 1731.96 (C = O str.), 3084.93 (CH str., aro-
matic), 1025.1 (Ring bending mode, imidazole), 735.79
(C-Cl str., aromatic), 1601.77 and 1699.0 (C = C and C = N
str. of pyridine ring); Anal. Calculated for C15H10ClN3O: C,
63.50; H, 3.55; N, 14.81; O, 5.64; Cl, 12.5. Found: C, 63.48;
H, 3.54; N, 14.78; O, 5.60; Cl, 12.3.
Compound 6. mp (°C) 99–101; Yield− 36.80%;
1H-NMR:7.13 (d, 1H, CH of C5 of imidazole), δ 7.16 (d,
1H, CH of C4 of imidazole), δ 7.26–7.56 (m, 4H, ArH of
ArCl), δ 7.87 (d, 1H, CH of C4 of pyridine), δ 7.88 (d, 1H,
CH of C5 of pyridine), δ 7.97 (d, 1H, CH of C6 of pyridine),
δ 9.04 (s, 1H, CH of C2 of pyridine); 13C-NMR: 135.6, 122.6,
137.2, 125.9, 131, 128.4, 134.2, 128, 190, 131.9, 152.6, 155.4,
124.3, 135.9; IR (KBr pellets): cm−1 1719.1 (C = O str.),
3070.0 (CH str., aromatic), 1019.8 (Ring bending mode,
imidazole), 710.2 (C-Cl str., aromatic), 1594.7 and 1690.0
(C = C and C = N str. of pyridine ring); Anal. Calculated for
C15H10ClN3O: C, 63.50; H, 3.55; N, 14.81; O, 5.64; Cl, 12.5.
Found: C, 63.48; H, 3.54; N, 14.78; O, 5.60; Cl, 12.3.
Compound 11. mp (°C) 145–147; Yield− 56.30%;
1H-NMR:δ 6.95–7.0 (d, 2H, CH of imidazole), δ 10.50
(s, 1H, COOH), δ 7.31 (t, 1H, CH of C4 of benzoic acid),
δ 7.94 (d, 1H, CH of C6 of benzoic acid), δ 7.32–7.35 (t,
1H, CH of C6 of benzoic acid), δ 7.54 (d, 1H, CH of C4 of
pyridine), δ 7.48–7.49 (t, 1H, CH of C5 of pyridine), δ 8.54
(d, 1H, CH of of C6 of pyridine), δ 9.2 (s, 1H, CH of C2 of
pyridine); 13C-NMR: 136.8, 122, 138.2, 127.4, 134.8, 128.9,
131, 129.8, 190, 131.5, 155.7, 152.6, 124.9, 136.1, 172.8; IR
(KBr pellets): cm−1 1720.0 (C = O str.), 3010.0 (CH str., aro-
matic), 1030.0 (Ring bending mode, imidazole), 1598.0
and 1653.4 (C = C and C = N str. of pyridine ring); Anal.
Calculated for C16H11N3O3: C, 65.53; H, 3.78; N, 14.33; O,
16.37. Found: C, 65.50; H, 3.74; N, 14.30; O, 16.35.
Experimental
Melting points were determined in open capillary tubes
on a Sonar melting point apparatus and are uncorrected.
Reaction progress was monitored by thin layer chroma-
tography (TLC) on silica gel sheets (Merck silica gel-G)
and the purity of the compounds is ascertained by single
1
spot on TLC sheet. H nuclear magnetic resonance (1H
NMR) and 13C-NMR spectra were recorded in Bruker
Avance II 400 NMR spectrometer using appropriate deu-
terated solvents and are expressed in parts per million (δ,
ppm) downfield from tetramethylsilane (internal stan-
dard). Infrared (IR) spectra were recorded on a Shimadzu
FTIR spectrometer.
General procedure for the synthesis of [2-(o-
Chlorophenyl)-imidazol-1-yl]-pyridin-3-yl-methanone4
o-Chloroaniline (0.13 mol) in hydrochloric acid/water
mixture (1:1) was diazotized using solution of sodium
nitrite at 0–10°C. To the diazotized mixture, imidazole
(0.004 mol) was added with vigorous shaking. A solution
of sodium acetate (40 g in 100 ml) was added drop wise to
the above mixture by maintaining temperature at 5–10°C.
e above solution was stirred initially for 3 h at cold
condition followed by continuation of stirring at room
temperature for 48 h. e product, 2-(o-Chlorophenyl)-
1-imidazole, obtained was filtered, dried, and recrystal-
lized from alcohol.
A
solution of 2-(o-Chlorophenyl)-1-H-imidazole
(0.002 mol) in diethyl ether (50 ml) was added with a
solution of nicotinyl chloride (0.002 mol) in diethyl ether
(50 ml). e above mixture was stirred for 24 h at room
temperature. eresultantproduct, [2-(o-Chlorophenyl)-
imidazol-1-yl]-pyridin-3-yl-methanone, was isolated by
evaporation of ether and purified by recrystallization
from methanol.
© 2011 Informa UK, Ltd.

722 Balasubramanian Narasimhan et al
clear in the MIC determination into fresh medium. MBC
and MFC values represent the lowest concentration of
compound that produces a 99.9% end point reduction19.
Evaluation of antimycobacterial activity
All compounds were screened for their in vitro antimyco-
bacterial activity against MTB, in Middlebrook 7H11agar
medium supplemented with OADC by agar dilution
method similar to that recommended by the National
Committee for Clinical Laboratory Standards for the
determination of minimum inhibitory concentration
(MIC) in triplicate. e MIC is defined as the minimum
concentration of compound required to give complete
inhibition of bacterial growth.
Results and discussion
Chemistry
e general procedure for the preparation of target com-
pounds 1–11 is described in Scheme 1. e key inter-
mediates, 2-(substituted phenyl)-1H-imidazoles, were
prepared by the condensation of imidazoles with cor-
responding substituted aryl diazonium chlorides which
in turn was prepared by the diazotization of substituted
anilines. However, based on our experience, the applica-
tion of the cupric chloride for the condensation of aryl-
diazonium chloride with benzimidazole as suggested by
Dahiyaetal.20 resultedinresinousproducts.erefore,the
coupling was carried out by using sodium acetate along
with stirring at cold conditions for the initial 3h followed
by 48 h stirring at room temperature, which resulted
in a solid product. For the synthesis of [2-(substituted
phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones
(1–11), the key intermediates, 2-(substituted phenyl)-1-
H-imidazoles, have been reacted with nicotinyl chloride
which was formed by the reaction of nicotinic acid with
thionyl chloride. e physicochemical characteristics of
the synthesized compounds are presented in Table 1. It
is important to note here that the % yield of most of the
synthesized compounds are below 50%. e low yield of
synthetic compounds may be attributed to any one or
more of the following reasons21: (i) the reaction may be
reversible and position of equilibrium is unfavorable to
the product; (ii) the incursion of side reactions leading to
Evaluation of antimicrobial activity
Determination of MIC
e antimicrobial activity was performed against Gram-
positive bacteria: S. aureus, B. sublitis, Gram-negative
bacterium: E. coli, and fungal strains: C. albicans and
A. niger by tube dilution method17. Dilutions of test
and standard compounds [ciprofloxacin (antibacterial)
and fluconazole (antifungal)] were prepared in double
strength nutrient broth—I.P. (bacteria) and Sabouraud
dextrose broth I.P.18 (fungi). e samples were incubated
at 37°C for 24 h (bacteria), at 25°C for 7 days (A. niger),
and at 37°C for 48 h (C. albicans), respectively, and the
results were recorded in terms of MIC (the lowest con-
centration of test substance which inhibited the growth
of microorganisms).
Determination of minimum bactericidal/fungicidal
concentration (MBC/MFC)
e minimum bactericidal concentration (MBC) and
fungicidal concentration (MFC) were determined by sub-
culturing 100 µL of culture from each tube that remained
H
N
R1
R1
R1
R2
R4
R2
R4
R2
R4
N
N
NaNO2/HCl
0-100 C
Cl−N2
+
R3
R3
H2N
R3
48 h
N
H
COOH
COCl
SOCl2
24 h
N
N
R1
R2
N
N
R3
R4
O
N
1-11
Scheme 1. Synthetic route followed to obtain target compounds
Journal of Enzyme Inhibition and Medicinal Chemistry

Antitubercular activity of substitued imidazoles 723
Table 1. Physicochemical characteristics and antimycobacterial activities of synthesized [2-(substituted phenyl)-imidazol-1-yl/
benzimidazole-1-yl]-pyridin-3-yl-methanones.
R1
R2
R1
R2
R4
N
N
N
N
R3
R3
R4
O
O
N
N
1-11
12-20a
MIC
Comp.
R1
H
R2
H
R3
R4
Mol. formula
C15H11N3O
M. wt.
249.27
277.33
277.33
283.72
283.72
283.72
328.17
294.27
294.27
294.27
293.28
333.83
333.83
333.83
315.37
344.30
344.30
344.30
343.41
329.39
m.p. (°C)
Rf value*
% yield
50.76
42.50
56.80
68.75
54.76
36.80
35.78
45.98
65.70
43.76
56.30
39.13
47.82
73.91
11.20
48.59
70.09
14.95
45.08
14.20
(µg/ml)
1
H
H
H
H
CH3
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
79–81
219–221
209–211
94–96
0.83
0.32
0.15
0.15
0.80
0.74
0.32
0.58
0.53
0.18
0.15
0.95
0.48
0.59
0.15
0.22
0.31
0.90
0.13
0.90
>25.0
25.0
25.0
12.5
6.25
12.5
6.25
3.13
6.25
6.25
>25.0
3.13
3.13
6.25
25
2
CH3
CH3
Cl
CH3
H
C17H15N3O
3
H
C17H15N3O
4
H
H
C15H10ClN3O
C15H10ClN3O
C15H10ClN3O
C15H10BrN3O
C15H10N4O3
C15H10N4O3
C15H10N4O3
C16H11N3O3
C19H12N3OCl
C19H12N3OCl
C19H12N3OCl
C19H13N3O2
C19H12N4O3
C19H12N4O3
C19H12N4O3
C20H13N3O3
C20H15N3O2
5
H
H
Cl
H
119–121
99–101
>260
6
H
Cl
H
7
H
Br
NO2
H
8
H
H
49–51
9
NO2
H
H
189–191
219–221
145–147
84–86
10
NO2
H
H
11
COOH
Cl
H
12
H
H
13
H
H
Cl
H
224–226
149–151
64–66
14
H
Cl
H
15
H
OH
NO2
H
16
H
H
244–246
239–241
49–51
1.56
3.13
3.13
25
17
H
NO2
H
18
NO2
COOH
OCH3
H
19
H
H
139–141
99–101
20
H
H
25
Isoniazid
Ethambutol
0.10
1.56
3.13
Ciprofloxacin
MIC, minimum inhibitory concentration.
*Toluene:Chloroform (7:3).
aReported in Reference 16.
the formation of by-products; (iii) the premature work-up
of the reaction before its completion; (iv) the volatiliza-
tion of products during reaction or work-up; (v) the loss of
product due to incomplete extraction, inefficient crystal-
lization, or other work-up procedures; (vi) the presence
of contaminants in the reactants or reagents leading to a
less efficient reaction.
e structures of compounds 1–11 were assigned by
IR and 1H-NMR spectroscopic data, which are consistent
with the proposed molecular structures. e appearance
of medium out of plane deformation bands (C-C bend-
ing) at 746.4 cm−1 and 756.6 cm−1 indicated the presence
of 1,2-disubstituted benzene ring in compound 4 and 11.
Similarly, the appearance of the C-C out of plane band
at 679.1 cm−1 indicated the presence of 1,3-disubtituted
benzene ring in compound 6. In contrast, the monosub-
stituted benzene ring (compound 1) showed C-C out
of plane band deformation at 684.6 cm−1. e presence
of 3-substituted pyridine in structures of compounds
(1–11) was confirmed by strong out of plane deformation
bands (C-H bending) at 820–770 cm−1 which were visible
from their IR spectra. e appearance of medium bands
at 3500 cm−1 in the IR spectra of compound 11 indicated
the presence of a free OH in the carboxylic acid group.
Further, the appearance of strong C=O stretching bands
at 1735–1710 cm−1 in the IR spectra of [2-(substituted
phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones (1–11)
demonstrated the presence of tertiary amide linkage
between the 3-substituted pyridine and the benzimi-
dazole nucleus. e presence of an additional IR band
© 2011 Informa UK, Ltd.

724 Balasubramanian Narasimhan et al
C=O stretching at 1702 cm−1 indicated the presence of
carboxylic acid group in compound 11. e appearance
of IR bands at 1602–1594 cm−1 and 1699− 1653 cm−1 in the
IR spectra synthesized compounds (1–11) indicated the
presence of C=C and C=N stretching of pyridine nucleus
of the nicotinic acid. e ring breathing mode of imi-
dazoles nucleus was indicated by the appearance of IR
bands at 1019–1030 cm−1 of the synthesized compounds
(1–11). Further, the presence of halogens in compound
4 and 6 was indicated by the presence of Ar-Cl stretching
vibrations at 735.7 cm−1 and 710.2 cm−1, respectively.
e appearance of δ at 6.9–7.8 ppm corresponds to
the aromatic protons of benzene and imidazole nucleus,
whereas the nicotinic acid nucleus showed δ at 8–9 ppm.
Further, it is important to note that the appearance of a
singlet at δ 10.50 ppm indicated the presence of COOH
group in compound 11. e absence of a singlet around
δ 11 ppm in the NMR spectra of compounds 1–10 indi-
cated the absence of the free COOH group. is confirms
that the compounds 1–11 are tertiary amides and not the
physical mixture of nicotinic acid. erefore, this assures
the reaction of nicotinyl chloride with the secondary
nitrogen of imidazole nucleus.
activity as evidenced by the fact that the 50% improve-
ment in MIC values of p, o, m− nitro substituted phenyl
imidazoles (8, 9, and 10) in comparison to o, p, m− chloro
substituted phenyl imidazoles (4, 5, and 6). Among the
different nitro substitutions, the p-nitro phenyl group
(8) conferred the antimycobacterial activity equivalent
to ciprofloxacin. Further, to study the effect of electron
withdrawing carboxylic acid group, we have designed
and synthesized compound 11 and tested for its antitu-
bercular activity. is change resulted in substantial loss
in activity (Compound 11, MIC = > 25.0 µM/ml). is is
similar to the results observed by Gill et al.23.
e antimycobacterial activity results of benzimida-
zole derivatives (12–20) are presented in Table 1. All the
synthesized compounds were found to be less active than
the standard drug isoniazid. Among the halo substituted
benzimidazole derivatives, ortho (12) and para chloro
(13) derivatives are more effective than the meta chloro
(14) derivative. is is in contrast to the results obtained
for imidazole derivatives in case of ortho chloro deriva-
tive, which is not equipotent to p-chloro derivative. e
improved antimycobacterial activity of o-chloro benz-
imidazole derivative (12) may be attributed to the pres-
ence of a fused benzene ring to the imidazole nucleus
which may favor the binding of o-chloro derivative to
its target site. As in case of imidazole derivatives, the
presence of electron donating group does not favor the
antimycobacterial activity of the benzimidazole deriva-
tives also (15 and 20). Among the p, o, m− nitro substi-
tuted phenyl benzimidazoles (16–18), the p-nitro phenyl
group (16) (Compound 16, MIC = 1.56 µM/ml) conferred
the antimycobacterial activity equivalent to the reference
compound ethambutol. As in case of imidazoles, the pres-
ence of electron withdrawing carboxylic acid group in
benzimidazole (19) resulted in substantial loss in activity
(Compound 19, MIC = 25.0 µM/ml). In general, the ben-
zimidazole derivatives are more potent than the imida-
zole derivatives against M. tuberculosis strain as they are
active at half the concentration of imidazole derivatives
i.e., fusion of benzene ring to imidazole nucleus greatly
improved the antimycobacterial activity.
Antimycobacterial activity
e in vitro antimycobacterial activity of synthesized
compounds against MTB was carried out in Middlebrook
7H11agar medium supplemented with OADC by agar
dilution method and the results are presented in Table
122. At the commencement of this study in the prelimi-
nary screening, compound (1) imidazole nucleus with
an unsubstituted phenyl ring displayed poor antimyco-
bacterial activity with an MIC of > 25 µM/mL. So we have
taken compound 1 as a lead compound and planned
to improve the antimycobacterial activity by making
substitutions on phenyl ring. e first step toward lead
optimization was incorporation of electron donating
group (CH3) on phenyl ring. e addition of electron
donating group does not improve the antitubercular
activity evidenced by the slight increase in the activity
of compound 2, 3 (MIC = 25 µM/ml) in comparison to
1. Based on these results, we have dropped the idea of
introducing electron donating groups to the phenyl ring
of the imidazole nucleus. e next structural modifica-
tion was introduction of electron withdrawing halogen
group viz. Cl and Br. e presence of p-chloro (5) phenyl
ring improved the antimycobacterial potential of imida-
zole derivative in comparison to its presence in ortho (4)
and meta (6) position as evidenced by their MIC values
presented in Table 1. is fact is true with the p-bromo
derivative (7) also. Enhancement in activity proves that
this modification is step up toward synthesis of a phar-
macophore. On the basis of biological data, improve-
ment in biological activity was observed with increase
in electronegativity of molecule. Hence, we planned
to introduce more electron withdrawing nitro group to
the phenyl ring of imidazole nucleus. is modification
produced excellent improvement in the antitubercular
From the aforementioned results, we can draw follow-
ing conclusions:
1. Electron withdrawing groups improve the antimy-
cobacterial activity compared to the electron donat-
ing groups. e presence of halo groups at para
position improved the antitubercular activity of the
synthesized compounds. is is similar to the results
observed by Mamolo et al.24, who stated that the
presence of bromo, chloro, and phenyl substituents
at the para position of the benzene ring attached to
imidazole improved the antitubercular activity.
2. Among the different electron withdrawing groups,
nitro group is most effective in conferring the anti-
mycobacterial activity to the imidazole derivative.
is is supported by the finding of Satyajit Dutta, who
stated that nitrophenyl group at the 2nd position in
Journal of Enzyme Inhibition and Medicinal Chemistry

Antitubercular activity of substitued imidazoles 725
Table 2. Antimicrobial activity of synthesized [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones.
MIC (µM/ml)
E. coli
0.003
ND
Comp.
S. aureus
0.012
ND
B. subtilis
0.003
ND
C. albicans
0.025
0.022
0.005
0.022
0.022
0.342
0.004
0.020
0.020
0.025
0.020
—
A. niger
0.050
0.005
0.005
0.044
0.044
0.044
0.019
0.040
0.040
0.025
0.040
—
1
2
3
ND
ND
ND
4
0.044
0.022
0.044
ND
0.044
0.044
0.044
ND
0.044
0.006
0.011
ND
5
6
7
8
0.010
0.040
0.013
0.002
0.004
—
0.010
0.002
0.005
0.002
0.004
—
0.040
0.040
0.002
0.002
0.004
—
9
10
11
Ciprofloxacin
Fluconazole
0.005
0.005
MIC, minimum inhibitory concentration; ND, not detected.
imidazole derivatives is responsible for high biologi-
cal activity19.
3. Fusion of benzene ring to imidazole nucleus greatly
improves the antimycobacterial activity.
p-Br group - Improves activity
against C. albicans
Fusion of benzene ring
improves antimycobacterial
activity by two fold
p-NO2 group - High
antimycobacterial activity
R
N
Antimicrobial activity
o, m -NO2 group - Improves
N
antibacterial activity
e synthesized imidazole derivatives were evaluated for
their in vitro antibacterial activity against Gram-positive
S. aureus, B. subtilis and Gram-negative E. coli and in
vitro antifungal activity against C. albicans and A. niger
by tube dilution method17. Double strength Nutrient
broth I.P. and Sabouraud dextrose broth I.P.18 have been
employed as media for growth of bacterial and fungal
species, respectively. e results of antimicrobial activity
are presented in Table 2.
COOH group - High
antibacterial activity
O
N
Essential for
antimycobacterial activity
In case of S. aureus, compound 11 was found to be
more active than the standard ciprofloxacin with an
MIC value of 0.002 µM/ml (Table 2). Against B. sub-
tilis, compounds 1, 9, and 11 were emerged as most
active ones with MIC value of 0.003, 0.002, and 0.002
µM/ml, respectively, in comparison to the standard
compound ciprofloxacin (MIC = 0.004 µM/ml, Table 2).
Compounds 1, 10, and 11 demonstrated high antibac-
terial activity against the Gram negative bacteria, E. coli
(Table 2).
In case of antifungal activity against C. albicans, com-
pound 7 (MIC = 0.004 µM/ml, Table 2) was found to be
more active than the reference compound fluconazole
(MIC = 0.005 µM/ml, Table 2) and compound 3 was equi-
potent to the reference drug fluconazole. For antifungal
activity against A. niger, compounds 2 and 3 exhibited
their antifungal potential at MIC values of 0.005 µM/ml,
which is same as the reference, fluconazole.
Figure 1. Structural requirements for the antimicrobial and
antimycobacterial activity of [2-(substituted phenyl)-imidazol-1-
yl]-pyridin-3-yl-methanones.
group in improving antimicrobial activity is similar to
the results of Sharma et al.25.
2. e compound 11 demonstrated high antibacterial
activity compared to the reference compound cipro-
floxacin against all the tested bacterial species. e
higher activity of compound 11 can be attributed
to the presence of imidazole as well as the pyridine
nucleus which was absent in the ciprofloxacin, even
though both compound 11 and ciprofloxacin have
the common free carboxylic acid group.
3. It is interesting to note an important observation that
the compound 11 demonstrated excellent antibac-
terial activity but exhibited poor antifungal activity.
e lower antifungal activity of 11 is supported by
the work of Goker et al.26 who reported that the pres-
ence of trifluoromethyl, carboxylic, ester, and amide
groups does not improve antifungal activity.
From the aforementioned antimicrobial activity
results, following structure activity relationship (SAR)
can be deduced:
4. e presence of electron donating CH3 group is
responsible for the high antifungal activity of the
imidazole derivative compound 3 against the tested
fungal strains. e role of electron donating group in
1. e antibacterial activity of compounds 9, 10, and 11
may be due to the presence of electron withdrawing
COOH and NO2 group. Role of electron withdrawing
© 2011 Informa UK, Ltd.

726 Balasubramanian Narasimhan et al
Table 3. MBC/MFC of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-3-yl-methanones.
MBC/MFC(µM/ml)
Comp.
S. aureus
0.025
ND
B. subtilis
0.025
ND
E. coli
0.025
ND
C. albicans
0.100
0.090
0.045
0.352
0.088
ND
A. niger
0.400
0.360
ND
1
2
3
ND
ND
ND
4
0.352
0.176
0.352
ND
0.352
0.352
0.352
ND
0.352
0.044
0.088
ND
ND
5
0.176
ND
6
7
0.076
0.169
0.169
0.169
0.09
0.019
0.169
ND
8
ND
ND
0.169
0.084
0.003
0.021
9
0.169
0.338
0.021
0.169
0.006
0.021
10
11
ND
ND
MBC/MFC, minimum bactericidal/fungicidal concentration; ND, not detected.
improving antifungal activity of imidazole derivatives
is supported by the studies of Emami et al.27.
5. In contradiction, the compound 7 which contains
an electron withdrawing bromo group showed an
appreciable antifungal activity against C. albicans.
is result indicated that the presence of the elec-
tron withdrawing Br substituent on the phenyl ring
attached to the 2nd position of imidazole nucleus
may be favorable for its binding with the fungal target
of C. albicans. is is similar to the results of one of
our previous study of antimicrobial activity of imida-
zole derivatives28.
6. e presence of electron withdrawing groups and
electron donating groups conferred antibacterial and
antifungal activity respectively to the synthesized
imidazole derivatives. is fact is also supported
by the antitubercular results, where the most active
compounds against the mycobacterium are the ones
which have the electron withdrawing groups in their
structure.
derivatives were 3-fold higher than the MIC values, which
indicated that the synthesized compounds were bacte-
riostatic and fungistatic in action. (A drug is considered
to be bacteriostatic/fungistatic when its MFC and MBC
values are 3-fold higher than its MIC value)30.
Conclusion
A series of [2-(substituted phenyl)-imidazol-1-yl]-pyridin-
3-yl-methanones (1–11) and [2-(substituted phenyl)-
benzimidazol-1-yl]-pyridin-3-yl-methanones
(12–20)
were screened for their antimycobacterial activity. e
antimycobacterial activity results indicated that [2-(4-
Nitro-phenyl)-imidazol-1-yl]-pyridin-3-yl-methanone (8)
and [2-(4-Nitro-phenyl)-benzimidazol-1-yl]-pyridin-3-yl-
methanone (16) were the most effective ones. e results of
antimicrobial screening of [2-(substituted phenyl)-imida-
zol-1-yl]-pyridin-3-yl-methanones (1–11) demonstrated
that
2-[1-(Pyridine-3-carbonyl)-1H-imidazol-2-yl]-
benzoic acid (11) was two times more effective than stan-
dard drug ciprofloxacin against the tested bacterial strains
and [2-(2,5-Dimethyl-phenyl)-imidazol-1-yl]-pyridin-3-
yl-methanone (3) was equipotent to the reference com-
pound, fluconazole, against tested fungal strains. Further,
the comparison of antimycobacterial results of imidazole
and benzimidazole derivatives indicated that the benzimi-
dazole derivatives are more effective in fighting against M.
tuberculosis and can be selected as lead compounds for
the development of NCEs.
7. e aforementioned results indicated the fact that
different structural requirements are essential for
a compound to be selected as antibacterial or anti-
fungal agent. is is similar to the results obtained by
Sortino et al.29.
e SAR of antimicrobial and antitubercular activity of
synthesized substituted imidazole derivatives are sum-
marized in Figure 1.
Based on the MIC values observed, the MBC and MFC
were determined by subculturing 100 µL of culture from
each tube that remained clear in the MIC determination
into fresh medium. It may be possible that the presence of
one or few organisms cannot be detected by determining
the turbidity of the solution. In this case, the determina-
tion of MBC and MFC may give useful information which
involves subculturing of 100 µL of culture from each tube
that remained clear in the MIC determination into fresh
medium. is is the reason for the fact that MBC/MFC
values (Table 3) of test compounds are observed at high
concentration than their MIC values (Table 2). In gen-
eral, the MFC and MBC values of synthesized imidazole
Declaration of interest
e authors report no conflicts of interest. e authors
alone are responsible for the content and writing of the
paper.
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