New Insight into Dearomatization and Decarbonylation of Antitubercular 4H‐Benzo[e][1,3]thiazinones: Stable 5H‐ and 7H‐Benzo[e][1,3]thiazines

8‐Nitro‐4H‐benzo[e][1,3]thiazinones (BTZs) are potent in vitro antimycobacterial agents. New chemical transformations, viz. dearomatization and decarbonylation, of two BTZs and their influence on the compounds’ antimycobacterial properties are described. Reactions of 8‐nitro‐2‐(piperidin‐1‐yl)‐6‐(trifluoromethyl)‐4H‐benzo[e][1,3]thiazin‐4‐one and the clinical drug candidate BTZ043 with the Grignard reagent CH(3)MgBr afford the corresponding dearomatized stable 4,5‐dimethyl‐5H‐ and 4,7‐dimethyl‐7H‐benzo[e][1,3]thiazines. These methine compounds are structurally characterized by X‐ray crystallography for the first time. Reduction of the BTZ carbonyl group, leading to the corresponding markedly non‐planar 4H‐benzo[e][1,3]thiazine systems, is achieved using the reducing agent (CH(3))(2)S ⋅ BH(3). Double methylation with dearomatization and decarbonylation renders the two BTZs studied inactive against Mycobacterium tuberculosis and Mycobacterium smegmatis, as proven by in vitro growth inhibition assays.