Synthesis of 1,3-thiazine derivatives and their evaluation as potential antimycobacterial agents

A series of eight 5,6-dihydro-4H-1,3-thiazine derivatives was synthesized by the BF3 x Et2O-catalyzed reaction of selected alpha,beta-unsaturated ketones with thiobenzamide at room temperature. The antimycobacterial activities of these compounds were determined against Mycobacterium tuberculosis H37Rv (ATCC 27294) using the Alamar blue susceptibility assay. Three compounds, 5-hydroxy-3-phenyl-4-aza-2-thiabicyclo[3.3.1]none-3-ene 3a, 4-hydroxy-4-methyl-6-pentyl-2-phenyl-5,6-dihydro-4H-1,3-thiazine 3b, and 4-ethyl-4-hydroxy-2-phenyl-5,6-dihydro-4H-1,3-thiazine 3c exhibited inhibitory activities of 97, 77 and 76%, respectively, at a concentration of 6.25 microg/ml. The actual MIC99 for the most active of these compounds, 3a, was also determined to be >6.25 microg/ml. These results, and especially those for 3a, suggest that 1,3-thiazines are potential lead compounds in the search for new antitubercular agents.