Synthesis and antibacterial evaluation of some novel mannich bases of benzimidazole derivatives

Article abstract of DOI:10.4314/bcse.v30i3.10

Substituted benzimidazoles are known for their chemotherapeutic importance and many pharmacological properties. In this paper, we synthesized some novel Mannich bases of benzimidazole derivatives. The synthesized compounds were characterized by their physical and spectral data and in vitro antibacterial activity of these compounds tested against Bacillus subtilis, Bacillus pumilus, Escherichia coli and Pseudomonas aeruginosa organisms. The potency of the synthesized compounds was determined against standard drug Ciprofloxacin by measuring the zone of inhibition.

Full text of DOI:10.4314/bcse.v30i3.10

Bull. Chem. Soc. Ethiop. 2016, 30(3), 421-425.
Printed in Ethiopia
ISSN 1011-3924
 2016 Chemical Society of Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v30i3.10
SYNTHESIS AND ANTIBACTERIAL EVALUATION OF SOME NOVEL MANNICH
BASES OF BENZIMIDAZOLE DERIVATIVES
*
A. Ahmadi
Department of Chemistry, Faculty of Science, Karaj Branch, Islamic Azad University, Karaj,
Iran
(
Received January 29, 2016; revised May 19, 2016)
ABSTRACT. Substituted benzimidazoles are known for their chemotherapeutic importance and many
pharmacological properties. In this paper, we synthesized some novel Mannich bases of benzimidazole
derivatives. The synthesized compounds were characterized by their physical and spectral data and in vitro
antibacterial activity of these compounds tested against Bacillus subtilis, Bacillus pumilus, Escherichia coli and
Pseudomonas aeruginosa organisms. The potency of the synthesized compounds was determined against standard
drug Ciprofloxacin by measuring the zone of inhibition.
KEY WORDS: Benzimidazole, Mannich base, Antimicrobial activity
INTRODUCTION
Infectious microbial diseases remain pressing problems worldwide, because resistance to a
number of antimicrobial agents among variety of clinically significant species of
microorganisms has become an important global health problem. One way to battle with this
challenge is the conscious usage of the currently marketed antibiotics; the other is the
development of novel antimicrobial agents [1-8]. Hence, there will always be a vital need to
discover new chemotherapeutic agents to avert the emergence of resistance and ideally shorten
the duration of therapy. Benzimidazole nucleus is an important heterocyclic ring. Substituted
benzimidazole have received considerable attention during last decades as they are endowed
with variety of biological activities and have wide range of therapeutic properties. A literature
survey indicates that benzimidazole derivatives possess different pharmacological and
biological properties like antibacterial, anti-inflammatory, antifungal, anti-tubercular,
anticancer, anthelmintic activity, etc. Combination of two or more active moieties into one is a
common procedure of manipulation and this can possibly results in augmenting the activity,
removal of untoward side effects and particularly prevent the development of resistance by the
infectious micro-organisms [9-14].
N
N
N
S
S
S
N
COOH
N
COOH
N
COOH
NH
NH
N
NH
NH
NH
N
N
N
N
N
N
N
III
VI
V
_
*
_________
Corresponding author. E-mail: a-ahmadi@kiau.ac.ir; ahmadikiau@yahoo.com

422
A. Ahmadi
Figure 1. The target compounds for synthesizing.
By considering the above facts, we plan to synthesize a bi heterocyclic system comprising of
benzimidazole nucleus and biologically important heterocyclic systems like triazoles and
tetrazoles system (Figure 1). We have also planned to evaluate the synthesized compounds for
anti-bacterial activity.
EXPERIMENTAL
Material and equipments
All of chemicals and solvents were purchased from Merck (Darmstadt, Germany) and Sigma-
Aldrich chemical Co. (USA). Melting points (uncorrected) were determined with a digital
Electrothermal melting point apparatus (model 9100, Electrothermal Engineering Ltd., Essex,
1
13
UK). H and C-NMR spectra were recorded with a Bruker 300 MHz (model AMX, Karlsruhe,
Germany) spectrometer (internal reference: TMS). IR spectra were recorded with a Thermo
Nicolet FT-IR (model Nexus-870, Nicolet Instrument Corp, Madison, Wisconsin, USA)
spectrometer. Mass spectra were recorded with an Agilent Technologies 5973, Mass Selective
Detector (MSD) spectrometer (Wilmington, USA).
Preparations (Scheme 1)
2-Mercapto benzimidazole (I). A mixture of o-phenylenediamine (0.1 mol), potassium
hydroxide (0.1 mol), and carbon disulfide (0.1 mol), 100 mL of 95% ethanol and 15 mL of
water were refluxed for 3 h. Then warm water was added and acidified with dilute acetic acid.
The product was filtered and dried and is recrystalized by ethanol [15]. White needle crystals;
o
yield 73%; m.p. 300-302 C.
(1H-benzimidazol-2-ylthio) acetic acid (II). A mixture containing 2-mercaptobenzimidazole
(
0.013 mol), 20 mL of ethanol, potassium hydroxide (0.016 mol) was refluxed for 1 h. After
o
cooling the resulting solution to 30 C added chloroacetic acid (0.012 mol). After stirring at 25-
3
1
chloride, air dried at 50 C and recrystalized from water [16-18]. White needle crystals; yield
7
1
o
0 C for 18 h, the reaction mixture was added to 100 g of ice-water and stirred for 30 min at 0-
o
0 C. The obtained precipitate was collected by filtration, washed with water until free of
o
o
-1
5%; m.p. 214-216 C. IR (KBr) (cm ): 3153 (OH), 3114 (NH), 2980, 2917 (Ar-CH=CH), and
675 (C=O absorption band of COOH). H-NMR (δ, ppm): 12.7 (1H, bs, 1H of benzimidazole
NH), 12.5 (1H, s, 1H of OH of COOH), 7.0-7.43 (4H, m, 4H of Ar-H), 4.12 (2H, s, 2H of CH
of CH COOH). C S. MS: m/z (regulatory intensity): 208 (100), 209 (8), 210 (4).
2
2
9 8 2 2
H N O
General procedure for the synthesis of Mannich Bases (III-V). Benzimidazolylthioacetic acid
II) (0.002 mol) dissolved in ethanol and 3-4 drops of conc. HCl was added and reaction
(
mixture was kept for stirring. To the stirring reaction mixture, formaldehyde (0.002 mol) was
added drop wise and stirring was continued for 10 min. Meanwhile 0.002 mol appropriate amine
(
2
R-NH ) was dissolved in ethanol and was added into the above reaction mixture drop wise with
continuous stirring. After stirring the reaction, mixture was refluxed for 12 h. The mixture was
allowed to cool at room temperature. The solid thus separated was filtered and dried. The
obtained products (III-V) were re-crystallized from ethanol [19, 23].
{
1-[(4H-[1,2,4]Triazol-3-ylamino)-methyl]-1H-benzimidazol-2-ylsulfanyl}-acetic acid (III).
o -1
White needle crystals; yield 65 %; m.p. 254-256 C. IR (KBr) (cm ): 3180 (NH), 2950, 2870
Ar-CH=CH), and 1678 (C=O absorption band of COOH). H-NMR (δ, ppm): 9.6 (1H,s, 1H of
OH of COOH ), 7.6 (1H,s, 1H of NH of triazole), 6.4-7.3 (6H, m, 4H of Ar-H, 1H of CH of
triazole, 1H of NH), 4.8 (2H, d, 2H of N-CH -NH-), 3.9-4 (2H, d, 2H of CH COOH). Anal.
(
2
2
Bull. Chem. Soc. Ethiop. 2016, 30(3)

Synthesis and antibacterial evaluation of Mannich bases of benzimidazole derivatives 423
calcd. for C12
12 6 2
H N O S; C, 47.36; H, 3.97; N, 27.62%. Found: C, 47.19; H, 3.79; N, 27.54%.
MS: m/z (regulatory intensity): 304 (100), 305 (13.6), 306 (5).
[
1-([1,2,4]Triazol-4-ylaminomethyl)-1H-benzimidazol-2-ylsulfanyl]-acetic acid (IV). White
o
-
1
needle crystals; yield 55%; m.p. 224-226 C. IR (KBr) (cm ): 3200 (NH), 2943, 2861 (Ar-
CH=CH), and 1652 (C=O absorption band of COOH). H-NMR (δ, ppm): 10.16 (1H, s, 1H of
OH of COOH), 6.4-8.3 (6H, m, 4H of Ar-H, 2H of CH of triazole), 5.1 (2H, d, 2H of N-CH
NH-), 3.8-4.1 (2H, d, 2H of CH COOH). Anal. calcd. for C12 S; C, 47.36; H, 3.97; N,
7.62%. Found: C, 47.33; H, 3.71; N, 27.63%. MS: m/z (regulatory intensity): 304 (100), 305
13.5), 306 (5).
2
-
2
12 6 2
H N O
2
(
{
1-[(1H-Tetrazol-5-ylamino)-methyl]-1H-benzimidazol-2-ylsulfanyl}-acetic acid (V). White
o -1
needle crystals; yield 50%; m.p. 234-236 C. IR (KBr) (cm ): 3340 (NH), 2932, 2856 (Ar-
CH=CH), and 1702 (C=O absorption band of COOH). H-NMR (δ, ppm): 10.76 (1H, s, 1H of
OH of COOH), 7.4-7.9 (4H, m, 4H of Ar-H), 5.4 (2H, d, 2H of N-CH -NH-), 3.87-4.15 (2H, d,
2
2
4
H of CH
2 11 7 2
COOH). Anal. calcd. for C11H N O S; C, 43.27; H, 3.63; N, 32.11%. Found: C,
3.33; H, 3.71; N, 32.23%. MS: m/z (regulatory intensity): 305 (100), 306 (15), 307 (6).
NH
2
N
EtOH
KOH
+
CS
2
SH
N
H
NH2
o-phenylene diamine
I
ClCH COOH
2
KOH
N
S
N
H
COOH
II
R-NH2
HCHO
Ethanol
N
N
N
III:
N
H
N
N
IV:
R
S
N
N
COOH
N
N
V: N
(III-V)
NH
R
N
H
Scheme 1. Scheme of synthesis for intermediates (I and II) and final compounds (III-V).
Bull. Chem. Soc. Ethiop. 2016, 30(3)

424
A. Ahmadi
Antimicrobial activity
Antibacterial activity was carried out by disc diffusion method using Bacillus subtilis, Bacillus
pumilus, Escherichia coli and Pseudomonas aeruginosa organisms for antibacterial activity. The
potency of the synthesized compounds was determined against standard drug Ciprofloxacin by
measuring the zone of inhibition [24-26].
RESULTS AND DISCUSSION
The starting compound 2-mercapto benzimidazole (I) was prepared from o-phenylene diamine,
potassium hydroxide and carbon disulfide upon refluxing for 3 h in single step, respectively.
The I was refluxed for 60 min with potassium hydroxide, followed by chloroacetic acid and
stirred for 18 h to furnish (1H-benzimidazol-2-ylthio) acetic acid (II). The different Mannich
bases (III-V) were synthesized by refluxing the appropriate substituted amines, formaldehyde
with II in ethanolic medium for 12 h. The synthesized compounds were characterized by their
physical and spectral data.
The resulted synthesized compounds (III-V) were screened for antibacterial activity studies
at a concentration of 100 µg/mL using DMF as a control against Bacillus subtilis, Bacillus
pumilus, Escherichia coli and Pseudomonas aeruginosa by disc diffusion method on agar
nutrient media. Ciprofloxacin was used as standard drug for the comparison at the concentration
of 100 µg/mL against Gram (+ve) and Gram (-ve) bacteria used for the study.
The data in the Table 1 indicates that the compounds were found to possess moderate to
weak activity although several benzimidazoles were reported for good antibacterial activity. The
compound III was active against Pseudomonas aeruginosa, and the compounds III, IV and V
were active against Bacillus subtilis, whereas the compounds IV and V were active against
Bacillus pumilus, rest of the compounds showed only weak activity when compared to the
standard ciprofloxacin [26].
Table 1. Anti-bacterial activity of synthesized compounds (III-VI).
Entry
*Inhibition of zone diameter in mm
B. subtilis
B. pumilus
E. coli
P. aeruginosa
100 µg/mL
100 µg/mL
100 µg/mL
100 µg/mL
III
IV
V
17
19
18
28
-
12
17
19
30
-
8
12
11
32
-
16
10
14
30
-
Ciprofloxacin
DMF
CONCLUSIONS
A series of some novel benzimidazoles were synthesized and evaluated for their potential
antimicrobial activities. Based on results, it can be concluded that all the synthesized
compounds showed good to moderate antimicrobial activities. The results indicated that new
antimicrobial compounds could be prepared by changing of different substrates on various
benzimidazole derivatives. These new compounds could be evaluated for further
pharmacological activities in other studies.
Bull. Chem. Soc. Ethiop. 2016, 30(3)

Synthesis and antibacterial evaluation of Mannich bases of benzimidazole derivatives 425
ACKNOWLEDGEMENTS
The author gratefully acknowledges financial support of Islamic Azad University for this
research project.
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