Ring Transformations of 1,3 -Benzothiazines, 5 1 synthesis of benzisothiazoles by the oxidative ring contraction of 2-aryl-4 H-and 4-ARYL-2 H-1,3-benzothiazines

Abstract

6,7-Dimethoxy-2-phenyl-4 H-1,3-benzothiazine ( 1 ¯ ¯ a ¯ ¯ ) was oxidized with hydrogen peroxide to yield [2,2′-bis(benzoylaminomethyl)-4,4′,5,5'-tetramethoxyldiphenyl disulfide ( 4 ¯ ¯ ) . Potassium permanganate oxidation of 1 ¯ ¯ a ¯ ¯ gave 6,7-dimethoxy-2-phenyl-4 H-1,3-benzothiazin-4-one ( 5 ¯ ¯ a ¯ ¯ ) , which was oxidized with the calculated amounts of perbenzoic acid to obtain the corresponding 1-oxide ( 6 ¯ ¯ a ¯ ¯ ) and 1,1-dioxide ( 7 ¯ ¯ ) . Oxidation of 5 ¯ ¯ a ¯ ¯ − c ¯ ¯ with sodium periodate involved ring contraction, affording 1,2-benzisothiazol-3(2 H)-one 1-oxides ( 8 ¯ ¯ a ¯ ¯ − c ¯ ¯ ) . A similar ring transformation was observed in the oxidation of 4-aryl-2 H-1,3-benzothiazines ( 1 ¯ ¯ 3 ¯ ¯ a ¯ ¯ − f ¯ ¯ ) , resulting in the formation of 3-aryl-5,6-dialkoxy-1,2-benzisothiazoles ( 1 ¯ ¯ 4 ¯ ¯ a ¯ ¯ − f ¯ ¯ ) . The assumed mechanism of these ring transformations is discussed. The conversion of 1,3-benzothiazines to 1,2-benzisothiazoles actually represents a ring transformation reaction of 1,3-benzothiazines and a new synthesis of 1,2-benzisothiazole derivatives. The structures of the new compounds were confirmed by ir, 1H and 13C nmr spectroscopy.