Synthesis and Evaluation of New Benzothiazole Derivatives as Potential Antimicrobial Agents

 

Vrushali N.Patil, Ameya G.Yadav, A.S.Bobade*, S.V.Athlekar, L.S.Patil and Abhay Chowdhary

* Department of Chemotherapy, Haffkine Institute for Training, Research and Testing, Acharya Donde Marg, Parel, Mumbai-400012.

*Corresponding Author E-mail: patil_vrush1@yahoo.com

 

ABSTRACT:

A series of pyrazolyl benzothiazole derivatives containing different substituent were synthesized. These pyrazolyl benzothiazole derivatives were evaluated for their antibacterial activity against S. aureus (Gram positive) and S.typhi (Gram negative) bacterial strains and antifungal activity against C. albicans and A. niger species.

 

KEYWORDS: Antibacterial, antifungal, formylated pyrazolyl benzothiazole, Microwave method.

 


INTRODUCTION:

Recently much interest of the use of microwave oven for the synthesis of heterocyclic compounds has been shown and that is due to substantial reduction in reaction time and better yield of the product1. Coupling of two techniques, that is, organic reaction using microwave irradiation has been field, which had shown excellent results leading to development of many reaction procedures, which are environmental friendly falling in the domain of ‘Green Chemistry’.

 

Benzothiazole derivatives are reported to possess antibacterial2, anti-inflammatory and anticonvulsant activities. Therefore it was considered worthwhile to explore the synthesis of compounds built upon benzothiazole skeleton incorporating pyrazolyl moiety with the hope of potentiating the activity of two such units in the same compound.

 

For the present investigation we have prepared 2-{[(4-substituted phenyl azomethylene)-3-substituted phenyl] pyrazol-1-yl} benzothiazoles (5a-o) starting from 2-amino benzothiazole (1) which was converted to corresponding benzotiazol-2-yl hydrazine (2) using hydrazine hydrate. The available hydrazino group was exploited to give benzotiazol-2-yl hydrazones analogues (3) on reacting with different methyl phenyl ketones. Vilsmeier Haack reaction3 was applied to benzothiazole hydrazones to obtain corresponding formylated pyrazolyl benzothiazoles (4). Addition of substituted anilines to (4) resulted the corresponding products (5a-o) (Scheme I).

 

Anti-bacterial activity:

A total of 15 compounds were subjected to antibacterial screening against Staphylococcus aureus ATCC 3750, Salmonella typhi NCTC 786 using tube dilution technique4. Ampicillin and Trimethoprim and were used as the standard drugs against S. aureus and S. typhi, respectively.

 

Anti-fungal activity:

The SAR of antifungal agents has revealed that the basic nitrogen-carbon linkage is an essential feature for the activity5. All the 15 synthesized compounds were screened for antifungal activity against standard strain of Candida albicans ATCC 10231 by Sabouraud-Dextrose broth by tube dilution method using Miconazole as standard drug.

 

EXPERIMENTAL:

All raw materials used in the synthesis have been obtained from Fluka AG (Buchs, Switzerland) and Aldrich Chemical and Co. Inc. (Milwaukee, WI, USA). All the commercial solvents used in the experimental work were redistilled and dried before use. Melting points of the synthesized compounds was recorded on Thermonik Melting Point Apparatus (Campbell Electronics, Mumbai, India) and are uncorrected. TLC was performed on silica gel coated plates using iodine vapour as visualizing agent. IR spectra were recorded on Hartmann-Braun, MB series (Bomem, Quebec, Canada), 1H NMR spectra were recorded on 300 MHz instrument (Jeol Ltd., Tokyo, Japan), using deuterated solvent DMSO, at Institute of Science, Mumbai. Elemental analyses were carried out on at SAIF, IIT Powai, Mumbai.

 

General method for preparation of Benzothiazol-2-yl hydrazine (2)

Concentrated HCl (6ml) was added dropwise with stirring to hydrazine hydrate (6ml) at 5-10°c, to it ethylene glycol


Table-1: Physical properties and antimicrobial activity of compounds 5a-o.

Comp

Substituent

Yield

(%)

M.P.

(C)

Antimicrobial activity (µg/ml) (MIC)

S. aureus

ATCC 3750

S. typhi

NCTC 786

C. albicans

ATCC 10231

A..  niger

ATCC 16404

(R)

(R’)

5a

3,4-dichloro

2,5-dimethoxy

68

196

50

100

200

100

5b

3,4-dichloro

4-fluoro

71

148

100

100

100

100

5c

3,4-dichloro

3,4-dichloro

72

205

50

100

N.A

200

5d

3,4-dichloro

4-chloro

68

214

100

50

100

100

5e

3,4-dichloro

4-methoxy-2-nitro

73

143

100

100

200

200

5f

3,4-dichloro

4,5-dichloro-2-nitro

72

282

200

100

100

200

5g

3,4-dichloro

3,4-dimethyl

69

261

100

50

100

100

5h

2,4-dihydroxy

2,5-dimethoxy

79

263

200

N.A

200

100

5i

2,4-dihydroxy

2,4-difluoro

76

269

200

100

200

100

5j

2,4-dihydroxy

4-fluoro

70

150

N.A

200

100

50

5k

4-fluoro

3-chloro-4-fluoro

75

155

100

100

100

100

5l

4-fluoro

4-fluoro

80

99

50

50

50

50

5m

4-fluoro

2,4-difluoro

69

120

40

100

50

40

5n

4-fluoro

3,4-dichloro

71

274

100

50

100

100

5o

4-fluoro

2,5-dimethoxy

78

189

40

100

40

50

                 Elemental analysis (C, H, N) of the compounds was in the range of µ 0.4%.

                 Ampicillin (MIC-0.04 µ g/ml) used as standard against S. aureus.

                 Trimethoprim (MIC 0.01 µ g/ml) used as standard against S. typhi.

                 Miconazole (MIC 6.25 µ g/ml) as standard against C. albicans and A. niger.

                 N.A.- Inactive at 200 µ g/ml.

 


(24ml) and 2-amino benzothiazole (1) (0.03 mol), were added and refluxed for 3 hrs. On cooling solid separated out, which was filtered, washed with water and recrystallized form ethanol. M.P.=195°C IR (KBr); 3321, 3202, 1651, 1460 (C=N), 1195, 1159, 816. 1H NMR (DMSO); d5.04, (2H, s, NH2), d7.43 (1H, t, Ar-H), d7.60 (1H, q, Ar-H), d7.72 (1H, d, Ar-H), d9.34 (1H, s, NH).

 

Synthesis of    Benzothiazol-2-yl hydrazones (3a-c):

Compound 2 (1.5mmol), appropriate acetophenone (3,4-dichloro, 2,4-dihydroxy and 4-fluoro) (2.2mmol) and glacial acetic acid (2-3drops) were taken in absolute alcohol (20ml) and refluxed on a water bath for 5-13 hrs. On cooling solid separated out, which was filtered, washed with little water and crystallized from ethanol.

 

3a: M.P.= 216°C. IR (KBr); 3148, 3049, 2908, 1568, 1459 (NHNH=C), 1341, 1126, 918, 937. 1H NMR (DMSO); d2.45 (3H, s, CH3), d7.22 (1H, m, Ar-H), d7.47 (1H, d, Ar-H), d7.76 (1H, m, Ar-H), d11.63 (1H, s, NH).

 

Synthesis of Formylated pyrazole benzothiazole (4a-c)

·                  Method A (conventional)

Compound 3 (5mmol) was dissolved in Vilsmeier Haack reagent (DMF) (6ml) in POCl3 (1.5ml) and stirred on room temperature for 4 hrs, then the contents were poured over crushed ice (previously neutralized with NaHCO3), solid separated out, which was filtered washed with water, dried and crystallized from ethanol.

 

·                  Method B (MWI)

The hydrazone (3a-c) (0.004M) was added in mixture of Vilsmeier-Haack reagent in an open Erlenmeyer flask and irradiated in microwave oven for 5 sec. After completion of the reaction, product was worked up as described in method A.

 

4a: M.P.=200-202°C IR (KBr); 3131, 3062, 2923 (CHO), 2360, 2342, 1695, 1610, 1548, 1463(C=N),1356, 1207, 898. 1H NMR (DMSO); d7.49, (1H, m, Ar-H), d7.63 (3H, s, Ar-H), d8.03 (2H, s, Ar-H), d9.75 (1H, s, >C=CH), 10.14 (1H, s, CHO).

 

Synthesis of 2-{[(4-substituted phenyl azomethylene)-3-substituted phenyl] pyrazol-1-yl} benzothiazoles (5a-o)

Compound 4 (0.001M) was dissolved in 10-15ml benzene. Add (0.001M) of substituted aniline followed by addition of 1-2 drops of glacial acetic acid and reflux on water bath for 5-10hrs. The H2O formed is removed using Dean-Stark apparatus. On cooling solid separates out, which was filtered, washed with little water and recrystallised from ethanol/DMF. The physical characteristics are recorded in Table 1. Spectral data of the selected compounds are given in Table 2. The yields of the compounds were moderate to good (55-80%) ( Table 1). All other reactions were facile and afforded almost quantitative yield of the products. All the synthesized compounds were stable at room temperature.

 

Table 2: Spectral data of selected synthesized compounds.

Compd.

IR spectral data cm-1

NMR spectral data

d= ppm

5b

3156 (NH-stretching)

1619 (C=N)

1566 (C=C)

7.9 (s, 1H, 5-CH)

6.9-7.4 (m, 11H, ArH)

2.17 (s, 1H, N=CH)

5j

1601 (C=N)

3566 (NH-stretching)

3300 (-OH)

8.06 (s, 1H, 5-CH)

7.1-7.8 (m, 11H, ArH)

2.21 (s, 1H, N=CH)

5l

3288 (NH-stretching)

1621 (C=N)

1523 (C=C)

8.36 (s, 1H, 5-CH)

7.5-8.12 (m, 12H, ArH)

2.25 (s, N=CH)

 

Biological screening:

The results of the biological screening are given in Table 3. Ampicillin (MIC-0.04 mg/ml) and Trimethoprim (MIC-0.01 mg/ml) were used as the standard drugs for S. aureus and S. typhi resp. Also, Miconazole (MIC 0.01 mg/ml) was used as a standard drug against fungal species. Two compounds 5m and 5o-exhibited activity at 40mg/ml against S.aureus. Rest of the compounds exhibited activity in the range of 50-200mg/ml.


Scheme I:

 


 

Compounds 5l, 5m and 5o exhibited activity upto 40mg/ml while rest of the compounds exhibited moderate activity against the fungal species.

 

ACKNOWLEDGEMENT:

The authors are thankful to SAIF (Sophisticated Analytical Instrument Facility), IIT (Indian Institute of Technology), Powai, Mumbai for carrying out the elemental analysis (CHN) and also thankful to Institute of Science, Fort, Mumbai for recording the NMR spectra.

 

REFERENCES:

1.       Caddick S., Tetrahedron, 51, (1995), 10403.

2.       P. Gopkumar, B. Shivakumar, E. Jaychandran, A.N. Nagappa, Indian J. Heterocyclic Chem., 11 (2001), 39.

3.       S. Salvi, P.T. Perumal, Indian J. Chem., 39B (2000), 163.

4.       E. Joan Stokes, Clinical Bacteriology, 4th Edition (1995), 226.

5.       Wilson Giswold’s “Textbook of Organic Medicinal Chemistry and Pharmaceutical Chemistry”, 11th Edition.

 

 

 

Received on 03.11.2009       Modified on 25.03.2010

Accepted on 13.06.2010      © RJPT All right reserved

Research J. Pharm. and Tech.3 (4): Oct.-Dec.2010; Page 1044-1046