INTRODUCTION:

A large number of studies have been carried out on benzo(b)thiophene (1); there are still numerous problems to be solved. This is not the case with the hydroxy benzo(b)thiophenes and quinines. Since these materials are the essential intermediates in the synthesis of commercial thieno indigo dyes, they have been the subject of an intensive research program in several industrial laboratories and many academic institutions throughout the world. Almost every conceivable preparation and reaction of these materials has been reported. It occurs naturally as a constituent of petroleum-related deposits such as lignite tar. Benzothiophene has no household use. It is used primarily in industry and research¹. Benzothiazole is a biheterocyclic system containing benzene ring fused with thiazole ring. It is a colorless liquid, B.P. 227 °C, sparingly soluble in water. It is an aroma constituent of cocoa beans, coconuts and walnuts. Benzothiazole derivatives have attracted continued interest over the years because of their varied biological activities.

In bioorganic and medicinal chemistry 2-aminobenzothiazole derivatives are broadly found with applications in drug discovery and development for the treatment of diabetes, epilepsy, inflammation, ulcer, anthelmintic, analgesia, tuberculosis, viral, bacterial infections etc.

Substituted benzothiazoles² 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 benzothiophene 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 infectious micro-organisms. By considering the above facts we plan to synthesize a biheterocyclic system comprising of benzothiophene nucleus and biologically important heterocyclic system like benzothiazole moiety. We have also planned to evaluate the synthesized compounds for anti-bacterial, activity.

MATERIAL AND METHODS:^6-9

ANTIBACTERIAL ACTIVITY:

The compounds were tested in-vitro for their antibacterial activity against four microorganisms viz. S. Aureus, Escherichia coli, B. subtilis, P. aeruginosa which are pathogenic in human beings.

Method: Disc diffusion methods using Staphylococcus aureus, E. coli.

ANTIFUNGAL ACTIVITY:

The compound were tested in-vitro for their antifungal activity against Candida albicans and Aspergillus niger.

Method: Disc Agar diffusion method using Fluconazole as a standard drug using C. albicans and A. niger.

EXPERIMENTAL: All the reactions should be carried out was anhydrous conditions were conducted in flame dry apparatus. Solvents and reagents were purified by standard methods.

Melting points were determined by using Toshniwal apparatus in open capillaries and are uncorrected. The purity of the compounds was checked by TLC on silica gel G plates using Methanol: Chloroform (7:3) solvent system and U.V lamp used as a visualizing agent. IR spectra were recorded using KBr pellets on a JASCO FT/IR-5300 spectrophotometer. ¹H-NMR spectra on a Varian EM-200 spectrophotometer using DMSO and CDCl₃ solvent and TMS as internal standard (chemical shift values expressed in δ ppm). Mass spectra were recorded by LCMS technique.

PROCEDURE

I) Synthesis of 3-chlorobenzo [b] thiophene-2-carbonylchloride (1):

A stirred mixture of cinnamic acid (0.5 mol), pyridine (0.05 mol), thionyl chloride (0.77 mol) and chlorobenzene (300 ml) was heated at reflux for 48 hr. excess of thionyl chloride was removed under reduced pressure and the remaining material was suspended in hot hexane (800 ml) the filtered. The hot filtrate was treated with charcoal, filtered and allows cooling; the obtained precipitate was collected as yellow needled, m.p. 114-116 ⁰C.

II) Synthesis of 2-amino-6-substituted benzothiazole (2):

To a solution of 4-substituted aniline (0.2 mol) and potassium thiocyanate (0.8 mol) in acetic acid (360 ml) added drop wise with stirring a solution of bromine (0.2 mol) dissolved in acetic acid (150 ml) by maintaining the temperature 30^oC by keeping in ice bath. After all the bromine solution added the mixture was stirred for 10-15 h, the mixture was filtered neutralized with aqueous ammonia solution. The obtained precipitate was filtered washed with water and dried. The crude product was recrystallized with suitable Solvents.

III) N-(6-Substituted-1,3-benzothiazol-2-yl)-3-chloro-1-benzothiophene-2-carboxamide (3 a-j):

The compound 3-chlorobenzo[b]thiophene-2-carbonylchloride (1) (0.002 mol) and 2-amino-6-substituted benzothiazole (2) / 2-aryl substituted amine (0.002 mol) dissolved in 25-30 ml of pyridine and refluxed for 10-15 h. The reaction mixture was cooled and poured into the ice cooled water. The solid separated was filtered washed with water, dried. The crude product was recrystallized from ethanol. The physical data was presented in the table no2.

METHODOLOGY:^3,4,5

The proposed project was carried out by following Schemes^3,4,5 1, 2 and 3. In the Scheme-I the starting material cinnamic acid was reacted with thionyl chloride, in presence of pyridine in chlorobenzene medium to yield 3-chlorobenzo [b] thiophene-2-carbonylchloride (1).

Where as in the Scheme-II the 2-amino-6-substituted benzothiazole is prepared by the reaction of 4-substituted aniline and potassium thiocyanate in presence of bromine in acetic acid.

In the scheme-III the compound N-(6-substituted-1,3-benzothiazol-2-yl)-3-chloro-1-benzothiophene-2-carboxamide (3a-j) was prepared by reacting 3-chlorobenzo[b] thiophene-2-carbonylchloride (1) and 2-amino-6-substituted benzothiazole (2) / 2-amino substituted pyridine in pyridine medium refluxed for 10-15 h furnished title compound N-(6-Substituted-1,3-benzothiazol-2-yl)-3-chloro-1-benzothiophene-2-carboxamide (3 a-j).

The structures of the all synthesized compounds were confirmed by their spectral data like ¹H NMR, Mass and IR Spectra. The synthesized compounds were evaluated for the antibacterial activity.

Where as in the Scheme-II the 2-amino-6-substituted benzothiazole is prepared by the reaction of 4-substituted aniline and potassium thiocyanate in presence of bromine in acetic acid.

Table-1. Anti-bacterial activity-Sensitivity testing of compounds 3(a-j)⁶.

Compound No.	Zone of Inhibition in mm
	Antibacterial activity				Antifungal activity
	*S. aureus*	*B. subtilis*	*E. coli*	*P. aeruginosa*	*C. albicans*	*A. niger*
3a	14	21	10	17	9	10
3b	18	19	12	15	10	11
3c	30	27	14	18	9	11
3d	19	22	11	18	9	11
3e	28	28	14	14	10	9
3f	14	19	10	15	10	10
3g	21	23	13	19	11	9
3h	14	20	10	16	9	10
3i	11	12	10	9	11	11
3j	10	12	9	11	12	12
Ciprofloxicin	26	26	28	25	-	-
Fluconazole	-	-	-	-	26	25

Standard: Ciprofloxacin

*Each value is an average of three independent determination ± Standard deviation.

Note: ‘--’ denotes no activity, 8-12 mm poor activity, 13-17 mm moderate activity, 18-20 mm and above good activity.

Table-2.Analytical data of synthesized compounds.

Compound Code	Compound	Molecular Formula	Yield (%)	Molecular weight	Melting Point (⁰C)
3a		C₁₇H₁₁N₂OS₂Cl	62.00	258	215-220
3b		C₁₉H₁₃N₂O₃S₂Cl	75.00	416	235-240
3c		C₁₇H₁₁N₂O₂S₂Cl	72.00	374	205-208
3d		C₁₆H₈N₂OS₂Cl₂	80.00	379	215-220
3e		C₁₆H₈N₃O₃S₂Cl	85.00	389	225-230
3f		C₁₆H₈N₂OS₂ClF	79.00	362	225-226
3g		C₁₄H₉N₂OSCl	72.00	288	180-186
3h		C₁₅H₁₁N₂OSCl	80.00	302	205-206
3i		C₁₅H₁₁N₂OSCl	82.00	302	203-204
3j		C₁₅H₁₁N₂OSCl	78.00	302	203-210

The structures of the all synthesized compounds were confirmed by their spectral data like ¹H NMR, Mass and IR Spectra. The synthesized compounds were evaluated for the antibacterial activity.

Compound 3a

¹H NMR (DMSO, dppm): 2.3 to 2.4 (3H, s, 3H of CH₃), 7.0 to 8.1 (8H, m, 7H of Ar-H and 1H of CONH).

LCMS (m/z): Molecular ion peak appear at 358 as (M+).IR (KBr cm^-1): Absorption band appears at 754 and 862 substituted benzene, 1638 C=O, 2856 and 2918 Ar-CH stretching, 3288, 3360 and 3395 NH stretching.

Compound 3f

¹H NMR (DMSO, dppm): 6.9 to 7.3 (3H, m, 3H of Ar-H), 7.4 to 7.5 (1H, s, 1H of CONH which was disappear in D₂O exchange), 7.5 to 7.9 (4H, m, 4H of Ar-H).

LCMS (m/z): Molecular ion peak appear at 363 as (M+1).

IR (KBr cm^-1): Absorption band appears at 752 and 858 substituted benzene, 1657 C=O, 2820 and 2918 Ar-CH stretching, 3366 NH stretching.

Compound 3h

¹H NMR (DMSO, dppm): 2.1 to 2.2 (3H, s, 3H of CH₃), 5.2 to 5.5 (1H, bs, 1H of CONH which was disappear in D₂O exchange), 6.6 to 8.0 (7H, m, 7H of Ar-H).).

LCMS (m/z): Molecular ion peak appear at 303 as (M+1). IR (KBr cm^-1): Absorption band appears at 752 and 858 substituted benzene, 1668 C=O, 2850 and 2957 Ar-CH stretching, 3325 NH stretching.

RESULTS AND DISCUSSIONS:

A series of Benzohtiophene derivatives were synthesized their structures were characterized by elemental analysis 1H NMR, IR and Mass spectral datas.

The synthesized compounds were subjected to antimicrobial evolution by disc diffusion method. Physical data all derivatives given in the Table No 2.

All the compounds have shown very potent antifungal activity, which can be comparable with the standard drug.

In fact all the bacterial strains used for the study were susceptible to the synthesized compounds but the E. coli is much more sensitive and P. aerugimose is less sensitive to the compound shown in Table No.1.

Among the above synthesized compounds 3a, 3f, 3g, 3h, 3i and 3j were shows more potent activities. While others are 3b, 3c, 3d and 3e were shows poor activities.

All the synthesized compounds were screened for antibacterial activity. The data in the Table No 2 indicate that compound 3d, 3e and 3f were exhibited a broad spectrum antibacterial activity. While other synthesized compounds of this series shown poor antibacterial activity.

All the synthesized compounds were screened for antibacterial activity against Bacillus subtilis, Bacillus pumilus, Escherichia coli and Pseudomonas aureginosa by disc diffusion method using ciprofloxacin as a standard against Gram positive and Gram negative bacteria.

The synthesized compounds 3d, 3e and 3f were exhibited a broad-spectrum antibacterial activity. While other synthesized compounds of this series shown poor antibacterial activity.

CONCLUSION:

From the data of the antibacterial activity and antifungal activity it is clearly concluded that some of the synthesized compounds are promisingly significant antibacterial agents.

From the above results one can establish that the synthesized substituted benzothiophene can be rich source for the exploitation. Therefore in search of new generation of the active compounds modification of the benzothiophene nucleus have resulted in a large number of compounds having diverse pharmacological activities. The synthesis, structure and biological activities of benzothiophene derivatives have been long focused for research interest in the field of medicine, due to potential activities exhibited by them.

Further the detail structure activity relationship studies are required along with the molecular manipulation i.e. molecular modeling may give better drugs and further toxicological study is needed. Molecules prepared for the biological testing do not always turn out as potential new molecules, but may be intended to serve as models for evaluation of the hypothesis.

ACKNOWLEDGMENT:

Authors are thankful to professor and principal, Department of Pharmaceutical Chemistry, for providing laboratory facilities to carry out the present research work.

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Received on 30.04.2011 Modified on 22.05.2011

Asian J. Research Chem. 4(10): Oct., 2011; Page 1544-1548