Synthesis of Some Novel N-alkylated 2-chloro-Benzimidazole Derivatives and their Biological Evaluation

Ashish Yasin Hawaldar1 and Sanjay Dashrath Vaidya 2*

1 & 2 Department of Chemistry, Jagdishprasad Jhabarmal Tibrewala University, Jhunjhunu, Rajasthan, INDIA

* Correspondence: E-mail: sanjayjjtu@gmail.com

(Received 13 Jan, 2019; Accepted 03 Feb, 2019; Published 15 Feb, 2019 )

ABSTRACT: A series of novel N-substituted benzimidazole derivatives have been synthesized by the condensation of ortho-phenylenediamine and urea, subsequent reaction of the product obtained with Phosphoryl chloride to give 2-Chloro-1H-benzimidazole. Using this compound alkylation done at the benzimidazole NH position with different benzyl halide reagent leading to the functionalized derivatives. Newly synthesized derivatives (6a to 6h) have been elucidated on the basis of spectral and analytical data like melting point, IR, 1H-NMR, Mass spectroscopy and elemental analysis. All the synthesized compounds were screened for their antimicrobial activity. This displayed some promising results towards testing organism in-vitro.

Keywords: Anti-bacterial activity; Anti-fungal activity; Alkylation; Benzimidazole and Condensation.

INTRODUCTION: Benzimidazole and its derivatives are marked as prominent heterocyclic compounds that exhibit a large number of biological activities 1-4.Significantly, the benzimidazole moiety is a constituent part of Vitamin-B12 core structure5. Some of them are used as antihelmintic drugs6, 7, anti-ulcer8, anti-psychotic9,10, anti-fungal11, anti-dopaminergic12. The 2-substituted analogs of benzimidazoles are known to be potent biologically active compounds 13-14, some of the important benzimidazole derivatives have been reported as thyroid receptor agonists15. The activity and structural diversity exhibited by compounds containing benzimidazole moiety has led to the discovery and development of novel and useful bioactive benzimidazole libraries from time to time. We have been interested in the synthesis of novel benzimidazole ring systems in connection with our on-going project on benzimidazoles16-17. In continuation of our work on bioactive benzimidazole libraries, we herein describe our efforts towards the synthesis of a novel class of benzimidazole derivatives and their biological activity screening studies.

MATERIALS AND METHODS: o -phenylenediamine acid, Urea, Phosphoryl chloride, Conc HCl, Sodium hydroxide, Potassium carbonate and alkylating agent from commercial supplier. All the solvents used were of commercial grade only. Melting points recorded on a MRVIS Series, Lab. India Instrument. Thin layer Chromatography analysis was carried out using pre-coated silica gel plates and visualizatised using iodine/UV lamp. Infrared spectra were recorded on Jasco, FT/IR-4100 type-A using the KBr disc. Proton-NMR spectra of the compounds were recorded on JEOL 500 MHz NMR spectrometer. Elemental analysis was carried out on a Perkin Elmer Series II Elemental Analyzer 2400.

RESULTS AND DISCUSSION: 1, 3-dihydro-benzimidazol-2-one 2 was synthesized by known method using o-phenylene diamine, urea in Dimethyl formamide at temperature 135-140C with 94 % yield having m.p.99-101C. A mixture of 2, Phosphoryl chloride and catalytic amount of phenol was heated 103-107C for 12 hrs.and subsequent work-up resulted in the formation of 2-chloro-1H-benzimidazole18 3 white solid having m.p. 208-209oC and in 97% yield. Based on the spectral and analytical data the compound was assigned to be 2-chloro-1H-benzimidazole 3 (Scheme-1). The alkylation of 3 with various electrophilic reagents in presence of potassium carbonate as base yielded the N-alkylated derivatives obtained Compounds 6a-6h. (Scheme-2)

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Table 1: Antibacterial activity (minimal inhibition concentration; MIC g/ml) of 6a-6h.

Antibacterial activity (MIC, g/ml)

Compound

E. coli

(Gram negative)

P. Aeruginosa

(Gram negative)

S.Aureus

(Gram positive)

S.Pyogenus

(Gram positive)

6a

100

100

125

100

6b

100

100

100

150

6c

75

100

125

150

6d

100

100

100

150

6e

100

100

75

100

6f

75

62.5

62.5

100

6g

62.5

250

200

125

6h

62.5

100

100

150

Gentamycin

0.05

1

0.25

0.5

Ampicillin

100

-

250

100

Chloramphenicol

50

50

50

50

Ciprofloxacin

25

25

50

50

Norfloxacin

10

10

10

10

Table 2: Antifungal activity (minimal inhibition concentration; MIC g/ml) of 6a-6h.

Compound

C. albicans

A. niger

A. clavatus

6a

>1000

>1000

>1000

6b

1000

500

1000

6c

>1000

1000

1000

6d

>1000

>1000

500

6e

1000

500

1000

6f

500

250

>1000

6g

500

1000

>1000

6h

250

>1000

250

Nystatin

100

100

100

Greseofulvin

500

100

100


General Procedure of synthesis:

Synthesis of 1, 3-dihydro-benzimidazol-2-one (2): To a solution of o-phenylene diamine 1(5gm, 0.046 mole) in DMF was charged urea (5.52 gm., 0.092 mole) and mixture refluxed for 12 hrs. After ensuring the completion of reaction, DMF was removed off by distillation under vacuum; isolated solid was washed with water and then dissolved in 10% aqueous sodium hydroxide solution. The aqueous alkaline solution was filtered and neutralized with aqueous Hydrochloride solution (35%). The Precipitated product was filtered, washed and dried. After this workup obtain pure 2 (1, 3-dihydro-benzimidazol-2-one) 18 with 5.8gm,94% yield and good purity, having melting point 99-101C (Scheme-1).

Synthesis of 2-chloro-1H-benzimidazole (3): A mixture of 2 (10 gm, 0.07 mole), Phosphoryl chloride (22.88 gm, 0.14 mole) and catalytic amount of phenol was heated 103-107 0C for 12 hrs. After confirmation of completion of the reaction, the mixture was chilled in ice and neutralized with 40% aqueous sodium hydroxide solution to pH-10. The obtained crude material was then recrystallized to obtain pure product 3 (2-chloro-1H-benzimidazole)18 with yield 11 gm.97%, Melting point 208-209C (Scheme-1).

General procedure for the synthesis of N- alkylated derivatives of 2-chloro-1H-benzimidazole compounds (6a-6h): To a solution of 2-chloro-1H-benzimidazole 3 ( 13.10 mmoles) and potassium carbonate (19.65 mmoles) in acetone (80 ml) was added compound (5a-5h, 15.75 mmoles) at RT. The reaction mixture was then warmed to 50-55 0C for 4-6 hrs. Completion of reaction monitored on TLC. After completion of reaction, solvent was removed by evaporation and added 20 ml water and 25ml ethyl acetate, stirred for 15 min. layers were separated. Ethyl acetate layer containing expected compound was washed with 20 ml water, dried over sodium sulfate. After concentration of solvent under vacuum and recrystallization using aq. ethanol yielded corresponding N-substituted derivative (6a-6h) as a white solid. (Scheme-2)

Analytical Characterization:

1-(4Bromo-2fluoro-benzyl)-2chloro1Hbenzimidazole (6a): Yield 84%; mp.115-1180 C;IR(KBr):596(C-Br),741(CCl),1370(CF),1515(C=C),1604(C=N),2357(-CH 2)cm1;1 HNMR(CDCl3):d5.39(s,2H,CH2),6.80-6.83(t,1H,J=7.9Hz ,ArH),7.19-7.21(dd,1H ,J1=8.3Hz, J2=1.4Hz, ArH), 7.24-7.30(m,1H, ArH), 7.30-7.33(m, 3H, ArH), 7.71-7.73(dd, 1H, J1=6.5Hz, J2=1.7Hz ,ArH) MS (m/z): 340.20 (M++1). Elemental Anal. calc,- For C14H9BrClFN2 : C, 49.52; H, 2.67; N, 8.25 Found: C=49.5; H=2.72; N=8.27.

2-chloro-1-(4trifluoromethylbenzyl)1Hbenzimidazole (6b): Yield 75%; mp.92-990C; IR (KBr): 742(C-Cl),1328(C-F),1461(C=C),1617(C=N)2357(-CH2) cm -1; 1H-NMR (CDCl3): d 5.45 (s, 2H,CH2),7.18-7.20 (m,1H, ArH),7.28-7.32 (m, 4H, ArH),7.58-7.60 (d, 2H, J=8.3Hz, ArH). 7.73-7.74(m,1H,ArH).MS (m/z): 311.32 (M+ +1); Elemental Anal.-calcd. For C15H10ClF3 N2.C,57.99;H,3.24;N,9.02 Found:C=58.01; H=3.30; N=9.05.

2-chloro-1-naphthalene-1-ylmethyl1Hbenzimidazole (6c): Yield 75%; melting point- 142-1480C; IR (KBr): 745(C-Cl),1464(C=C),1614(C=N),2355(-CH2) cm-1; 1H-NMR(CDCl3): d 5.55(s, 2H, CH2,),7.20-7.23 (m, 1H,ArH),7.26-7.31(m,2H, ArH), 7.45-7.49 (td,2H, J1=6.5Hz, J2=3.9Hz , ArH), 7.57 (s, 1H, ArH), 7.73-7.75 (m, 2H , ArH) 7.80-7.81(t, 2H, J=4.1Hz, ArH).MS (m/z): 293.28 (M++1); Elemental Anal.-calcd. For C18H13ClN2: C, 73.85; H, 4.48; N, 9.57 Found: C=73.98; H=4.40; N=9.52.

2-choloro-1-(3-methylbenzyl-1H benzimidazole (6d ): Yield 80%; mp. 105-1100C; IR (KBr): 740(C-Cl),1461(C=C),1607(C=N),2357(-CH2) cm-1; 1H-NMR (CDCl3): d 2.29 (s, 3H, CH 3), 5.35(s,2H,CH2), 6.96-6.98(d, 2H,J=9.6Hz, ArH), 7.09-7.10 (d, 1H, J=7.6Hz, ArH), 7.19-7.22 (t, 1H, J=7.6Hz, ArH), 7.23-7.28 (m, 3H, ArH), 7.71-7.72 (d, J=6.9Hz 1H, ArH).MS (m/z): 256.96 (M++1); Elemental Anal. calcd. for C15H13ClN2: C, 70.18; H, 5.10; N, 10.91 Found: C=70.25; H=5.20; N=10.85.

2(2-chloro-benzimidazol-1yl-3trifluromethyl-phenyl) ethanone (6e): Yield 69%; mp. 190-1950 C;IR(KBr):747(CCl),1329(CF),1469(C=C),1617(C=N),1708(C=O),2358(CH 2)cm-1;11H-NMR (CDCl3): d 5.59 (s, 2H, -CH2), 7.11-7.12 (m, 1H, ArH), 7.27-7.32 (m , J1=7.6Hz, J2=1.4Hz, 2H, ArH), 7.74-7.75 (d, 1H, J=7.6Hz, ArH), 7.83-7.84 (d,2H, J=8.3Hz, ArH), 8.14-8.16 (d,2H, J=8.3Hz, ArH). MS (m/z): 339.32 (M++1); Elemental Anal.-calcd. For C16H10ClF3N2O: C, 56.74; H, 2.98; N, 8.27; Found: C=56.80; H=3.07; N=8.25.

2-chloro-1-(2-(4-nitrophenoxy)ethyl)-1H-benzo[d]imidazole (6f): Yield 85%; mp. 181-186C.; FT-IR(KBr,n,cm-1): 3064(CH), 1469(NO2) 1108(C-N) cm-1, 1H-NMR (DMSO-D6): d 4.50-4.53(m, 2H, -CH2), 4.72-4.74(t, 2H, J=5.2Hz, -CH2), 7.05-7.07(d, 2H, J=9Hz, ArH), 7.24-7.27(t, 1H, J=7.6Hz, ArH), 7.31-7.34(t, 1H, J=7.6Hz, ArH), 7.59-7.60(t, 1H, J =8.3Hz, ArH), 7.71-7.72(d, 1H, J=7.6Hz, ArH), 8.15-8.17(d, 1H, J=9.6Hz, ArH). MS (EI, m/z (%): 318.08 (M+1), Anal.-calcd. For C15H12ClN3O 3: C, 56.70; H, 3.81; N, 13.23 Found: C=56.75; H=3.76; N=13.85.

2-(3-(2-chloro-1H-benzo[d]imidazol-1-yl)propyl)-6,7-dimethoxyisoquinolin-1(2H)-one (6g): Yield 68%; mp. 148-1520C; FT-IR (KBr, n, cm-1): 2934(CH), 1664(C=0), 1110(C-N) cm-1 . 1H-NMR(CDCl3): d 1.93-1.99(m, 2H, -CH2), 3.49 (bs, 2H, -CH2 ), 3.74 (bs, 2H, -CH2 ), 3.89-3.91(d, 6H, J=10.3Hz , -CH3), 3.95-3.99(t, J=8.3Hz , 2H, -CH2), 6.14-6.16(d, J=9Hz , 1H, -CH=CH), 6.46-6.48(d, J=9Hz , 1H, -CH=CH), 6.78 (s, 1H, ArH), 6.79-6.81 (d, J=8.3Hz 1H, ArH), 6.83 (s, 1H, ArH), 7.11 -7.14 (t, J=7.9Hz 1H, ArH), 7.19-7.22 (t, J=7.6Hz 1H, ArH), 7.62-7.63 (d, J=7.6Hz, 1H, ArH). MS (EI, m/z (%): 412.08 (M+1); Anal.-calcd. For C22H22ClN3O3: C, 64.15; H, 5.38; N, 10.20. Found: C=64.20; H=5.34; N=10.26

7-(4-(2-chloro-1H-benzo[d]imidazol-1-yl)butoxy)quinolin-2(1H)-one (6h): Yield 85%; mp. 171-1740C; FT-IR (KBr, n, cm-1): 2948(CH), 1654(C=0), 1136(C-N) cm-1; 1H-NMR(DMSO-D6): d 1.74-1.80(m, 2H,-CH2), 1,89-1.95(m, 2H,CH 2), 4.02-4.04(t, 2H, J= 6.2Hz, CH2), 4.34-4.37(t, 2H, J= 7.2Hz, CH2), 6.27-6.29(d, 2H, J= 9.6Hz, ArH), 6.76-6.77(d, 2H, J= 6.2Hz, ArH), 7.24-7.27(t, 1H, J= 7.6Hz, ArH), 7.30-7.33(t, 1H, J= 7.6Hz, ArH), 7.52-7.54(d, 1H, J= 9.6Hz, ArH), 7.60-7.62(d, 1H, J=7.6Hz, ArH),7.65-7.67(d, 1H, J=8.3Hz, ArH),7.78-7.80(d,1H,J= 9.6Hz, ArH),11.56(s, 1H, N-H).MS (EI, m/z (%): 368.05 (M+1); Elemental Anal.-calcd. For C20H18ClN3O2: C, 65.31; H, 4.93; N, 11.42. Found: C=65.35; H=4.90; N,=11.46.

Antimicroial activity: The microbial activity was undertaken to evaluate the effect of the synthesized compounds on different bacterial and fungal strains. The compounds 6a-6h were screened for their antibacterial activity19 against human pathogenic Gram negative bacteria such as Escherichia coli MTCC442, Pseudomonas aeruginosa MTCC441 and Gram positive bacteria Staphylococcus aureus MTCC96, and Streptococcus pyogenes MTCC443. DMSO was used as diluents and Gentamycin, Ampicillin, Chloramphenicol, Ciprofloxacin and Norfloxacin as standard. The compounds 6a-6h was also screened for their antifungal activity20 against Candida albicans MTCC227, Aspergillus Niger MTCC282 and Aspergillus clavatus MTCC1323. Broth dilution method was used to evaluate the antibacterial activity. It is carried out in tubes. Mueller Hinton Broth21was used as nutrient medium. Serial dilutions were prepared in primary and secondary screening. Each synthesized drug was diluted obtaining 2000 g/mL concentration, as a stock solution. In primary screening 1000, 500 and 250 g/mL concentrations of the synthesized drugs were taken. The drugs found active in primary screening were similarly diluted to obtain 200, 100, 50, 25, 12.5, and 6.250 g/mL, and concentrations. The highest dilution showing at least 99% inhibition zone was taken as MIC.

CONCLUSION: We have synthesized N-substituted 2-chloro-1H -benzimidazole derivatives by using different condensation conditions and were thoroughly evaluated biological activity of compound 6a-6h such as antibacterial and antifungal (Table-1 and Table-2). Some of the compounds were found to have promising antibacterial activity against E. coli such as 6c,6f,6g and 6h.Whereas all the compounds were highly active against S. aureus when compared to the Ampicillin as a standard. These compounds were also screened against C. albicans, A. niger and A. clavatus for antifungal activity. Unfortunately, not a single compound shows the prominent antifungal activity when compared to the Greseofulvin and Nystatin as standard.

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