1H-1,4-benzothiazines. New cyclic sulphonium ylides

Abstract

Five derivatives of the 1H-1,4-benzothiazine ring system (6) have been prepared by cyclodehydration of the sulphoxides (7) with trifluoroacetic anhydride. The derivative (6a) was also prepared by methylation of 2,3-dihydro-2,2-dimethyl-10H-phenothiazin-4(1H)-one (11a). ¹H and ¹³C N.m.r. spectra of the compounds indicate that they can be regarded as stabilised sulphonium ylides, the substituents about the sulphur being in a non-planar arrangement. When heated in hydrochloric acid the ylides are demethylated and give the corresponding 4H-1,4-benzothiazines.