Synthesis and Antimicrobial Activity of 1,2-Benzothiazine Derivatives

A number of 1,2-benzothiazines have been synthesized in a three-step process. Nine chalcones 1-9 bearing methyl, fluoro, chloro and bromo substituents were chlorosulfonated with chlorosulfonic acid to generate the chalcone sulfonyl chlorides 10-18. These were converted to the dibromo compounds 19-27 through reaction with bromine in glacial acetic acid. Compounds 19-27 were reacted with ammonia, methylamine, ethylamine, aniline and benzylamine to generate a library of forty-five 1,2-benzothiazines 28-72. Compounds 28-72 were evaluated for their antimicrobial activity using broth micro dilution techniques against two Gram-positive bacteria (Bacillus subtilis and Staphylococcus aureus), and two Gram-negative bacteria (Proteus vulgaris and Salmonella typhimurium). The results demonstrated that none of the compounds showed any activity against Gram-negative bacteria, P. vulgaris and S. typhimurium, however compounds 31, 33, 38, 43, 45, 50, 53, 55, 58, 60, 63 and 68 showed activity against Gram-positive bacteria, Bacillus subtilis and Staphylococcous aureus. The range of MIC and MBC was 25-600µg/ml; though some of the MIC and MBC concentrations were high indicating weak activity. Structure activity relationship studies revealed that the compounds with a hydrogen atom or an ethyl group on the nitrogen of the thiazine ring exerted antibacterial activity against Gram-positive bacteria. The results also showed that the compounds where the benzene ring of the benzoyl moiety contained a methyl group or chlorine or bromine atom in the para position showed higher antimicrobial activity. Similar influences were identified where either a bromine or chlorine atom was in the meta position.