Synthesis and antibacterial activity of novel fused 1,3-thiazoles and 1,3-thiazines incorporating a 2,4-dihydroxyphenyl residue.

In an attempt to find a new class of antimicrobial agents, a series of benzothiazoles, 1,3-thiazolo[5,4-b]pyridines, 4H-3,1-benzothiazines, naphtho[2,3-d][1,3]thiazole-4,9-diones and other related compounds containing a 2,4-dihydroxyphenyl moiety were prepared. They were obtained via the reaction of aryl-modified sulfinyl[bis(2,4-dihydroxyphenylmethanethione)]s with appropriate commercial chemical reagents in the endocyclization processes. The MIC values of the compounds towards eight reference bacterial strains were assessed by the two-fold serial micro-dilution broth method. They exhibited inhibitory effects against the Gram-positive strains tested opposite to Gram-negative ones. Some compounds were more effective than the reference drug. 4-(6-Chloro-4H-3,1-benzothiazin-2-yl)-6-methylbenzene-1,3-diol (5b) due to its very good activity (MIC from 1.56 to 3.13 µg/mL) and low cytotoxicity (IC(50) > 50 µg/mL) may be regarded as a promising precursor for the development of novel antibacterial agents.