Synthesis of 2H-1,2-Benzothiazine 1,1-Dioxides via Heteroannulation Reactions of 2-Iodobenzenesulfonamide with Ketone Enolates under SRN1 Conditions§

2-Iodobenzenesulfonamide (1a) underwent photostimulated SRN1 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 β-hydrogen atoms present in the ketone enolate. Isotopic labeling studies with 2,4-dimethyl-3-pentanone-d14 (24) confirmed the major role of the β-hydrogens in the reduction process. Reactions of 1a with cyclopentanone, cyclohexanone, and cyclooctanone enolates afforded new tricyclic benzothiazine derivatives 2629. 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 SRN1-based synthesis of benzothiazine 3 (R = Ph).