Reactions of 1H-2-benzothiopyran 2-oxides with active methylene compounds: a novel ring contraction of 1-aryl derivatives to benzo[c]thiophenes. X-Ray molecular structure of 1-(2,2-diacetylvinyl)-3-phenylbenzo[c]thiophene

Abstract

1-Aryl-1H-2-benzothiopyran 2-oxides (5) reacted with active methylene compounds in acetic anhydride to undergo a novel ring contraction, affording benzo[c]thiophene derivatives (6), whose structures have been established by X-ray crystallography of compound (6a). In contrast, 1-unsubstituted 1H-2-benzothiopyran 2-oxide (1), under the similar reaction conditions, afforded no benzothiophene derivatives, but instead gave 1-substituted 1H-2-benzothiopyrans (2) in good yield. It is proposed that the novel ring contraction was caused by the steric effects of the bulky aryl group at the 1-position of 2-benzothiopyran 2-oxides which prevent active methylene compounds from attacking at C-1.