Synthesis of 2H-1,2-benzothiazine 1,1-dioxides via heteroannulation reactions of 2-iodobenzenesulfonamide with ketone enolates under S(RN)1 conditions.
[Reaction: see text]. 2-iodobenzenesulfonamide (1a) underwent photostimulated S(RN)1 reactions in liquid NH3 with the potassium enolates derived from acetone, pinacolone, butanone, and 3-methyl-2-butanone to give fair to good yields of 2H-1,2-benzothiazine 1,1-dioxides 2. Reductive dehalogenation of 1a was found to predominate in photoinduced reactions of 1a with 3-pentanone, 2-methyl-3-pentanone, and 2,4-dimethyl-3-pentanone, the extent of reduction being proportional to the number of beta-hydrogen atoms present in the ketone enolate. Isotopic labeling studies with 2,4-dimethyl-3-pentanone-d14 (24) confirmed the major role of the beta-hydrogens in the reduction process. Reactions of 1a with cyclopentanone, cyclohexanone, and cyclooctanone enolates afforded new tricyclic benzothiazine derivatives 26-29. Attempts to extend the heteroannulation reaction to the preparation of 2H-1,2-benzothiazin-3(4H)-one 1,1-dioxides 3 (R = H, Ph) through reactions of 1a with tert-butyl acetate and ethyl phenylacetate enolates resulted only in hydrodehalogenation of 1a. However, oxazoline anion 30, a synthetic equivalent of ethyl phenylacetate, was successfully employed in an alternative S(RN)1-based synthesis of benzothiazine 3 (R = Ph).