Reactions of thiophenes with acetylenes in polar solvents; a novel annulation reaction

Abstract

Although for a long time thiophenes have been considered to be inert in cycloaddition reactions, recently some have been reported to react with acetylenes via either a (2+2)or a (4+2)-cycloaddition reaction l-4 . Particularly some of the (2+2)-cycloadditions of thiopheneslr2, and of benzo[b]thiophenes5-7, proved to be versatile reactions in the synthesis of l-heterocycloheptatrienes. For example, reaction of 3-pyrrolidinothiophenes (1) with dimethyl acetylenedicarboxylate in apolar solvents at low temperature yielded the corresponding thiepins (A), a class of anti-aromatic compounds that could not be obtained by other routes. We now report that in polar solvents the reactions of A with dimethyl acetylenedicarboxylate take an entirely different course.