Synthesis of Benzofuran Derivatives and their Evaluation of Antimicrobial Activity

2-Acetyl benzofuran (1) was synthesized by refluxing salicylaldehyde and dry chloroacetone in presence of potassium carbonate. The product formed was reacted with 2-aminobenzothiazole with catalyst p-toluene sulphonic acid to form N-[(1Z)-1-(benzofuran-2-yl)ethylidene]1,3-benzothiazol-2-amine (2). In cold condition, Staudinger reaction was carried out for compound (2) with various acid chlorides in presence of triethylamine to synthesize 4-[1benzofuran-2-yl]-1-[1, 3-benzothiazol-2-yl]-4-methylazetidin-2-one (3a-3j). All the synthesized compounds were characterized on the basis of analytical data. The compounds were screened for their antimicrobial activities. Compound 3b, 3c and 3d exhibited moderate activity.


Introduction
Benzofuran derivatives posses a wide range of biological activities.They have been reported to possess antimicrobial [1][2][3][4] , antitumour 5,6 , anti-inflammatory 7 activity etc. Benzothiazoles play a significant role as antibacterial [8][9][10] and antifungal activity.Azetidinones with heterocyclic molecule has created an excellent drug for antimicrobial [11][12][13] activity.Based on these findings and in continuation of our research work, we report herein synthesis of a tri-heterocyclic molecule containing benzofuran, benzothiazole and azetidinone for enhanced activity.

Experimental
Melting points were determined in open capillary tube and are uncorrected.The TLC was taken in silica gel G precoated TLC plates.The solvents and reagents used for synthesis were of laboratory grade.The IR was recorded with Perkin Elmer FTIR spectrophotometer using KBr press pellet technique.The NMR was recorded in Bruker 400 MHz FTNMR spectrometer using DMSO as solvent and TMS as internal standard.

Synthesis of 2-aminobenzothiazole
To a solution of aniline (0.2 mole) and potassium thiocyanate (0.8 mole) in 360 mL of 96% acetic acid was added drop wise, with stirring, 0.2 mole of bromine dissolved in 250 mL of glacial acetic acid while the temperature was kept below 35 o C.After all the bromine solution had been added the mixture was stirred for 10 h and was then filtered and the residue washed with water.The combined filtrate and washings were neutralized with ammonium hydroxide.The precipitate was collected on a filter and dried.Yield 70% IR cm -1 : 3056 (CH aromatic), 3397 (NH) 1 H NMR δ ppm (400MHz): 7.14 to 7.6 aromatic protons, 5.55(1H, s, NH).

Antimicrobial activity
The minimum inhibitory concentration was determined by the method of serial dilutions and found to be 100 µg/mL for bacteria and 200 µg/mL for fungi.The newly synthesized compounds were screened for antibacterial activity using cup plate diffusion method; B. subtilis and E. coli were taken as test organisms.Among the tested compounds 3b, 3c and 3d were moderately active against standard Ampicillin while the remaining compounds were weakly active.The fungicidal activity of the compounds was evaluated against C. albicans using griseofulvin as standard, compounds 3b, 3c and 3d exhibited moderate activity.

Results and Discussion
The scheme of synthesis of titled compounds is depicted in the (Scheme 1).All the intermediate and targeted molecules were confirmed by TLC, IR, NMR and mass data.

Conclusion
Various tri-heterocyclic compounds 3a to 3j of benzofuran derivatives were synthesized containing benzofuran, benzothiazol and azetidinone rings.The compounds were characterized by IR, NMR and ESI MS spectral data.They were evaluated for antimicrobial activity, compounds 3b, 3c and 3d exhibited moderate activity when compared to standard Ampicillin and Griseofulvin 2acetyl benzofuran was synthesized by reacting salicylaldehyde and chloroacetone in presence of anhydrous potassium carbonate as reported.2-Aminobenzothiazole was prepared by stirring aniline and potassium thiocyanate in presence of bromine and acetic acid in cold condition.N-[(1Z)-1-(benzofuran-2-yl)ethylidene]1,3-benzothiazol-2-amine(2)  was obtained by refluxing 2-acetyl benzofuran and 2-aminobenzothiazole in presence of ptoluene sulphonic acid.The IR spectra of compound (2) showed presence of C=N stretching vibrations at 1643 cm -1 and disappearance of NH absorption bands.The targeted molecule 4-[1-benzofuran-2-yl]-1-[1,3-benzothiazol-2-yl]-3-bromo-4-methylazetidin-2-one (3d) was synthesized by mixing compound (2) with triethylamine in dioxane and bromoacetyl chloride in cold condition.The IR spectra revealed the peak at 1637 cm-1  showing the presence of C=O stretching vibration of cyclic amide.The C-H stretching frequency was at 2850 cm -1 and C-Br stretching frequency at 754 cm -1 indicated their presence in azetidinone ring.From 1 H NMR spectra δ value 1.41was assigned to methyl protons, a singlet at δ value 4.48 was integrated to methine of azetidinone and the multiplet δ value at 7.15 to 7.69 was due to aromatic protons.The structure of compound (3d) was further supported by its ESI MS Spectra indicating molecular ion peak was at 413 m/z and fragmented ion peak at 285 m/z.Cl -CH 3 -CH 3 CH 2etc.Scheme 1

Table 1 .
Physical data of synthesized compounds (

3a-3j).
All compounds gave satisfactory CH and N analysis.