Substituted Thiophenes via Intramolecular Thia-anti-Michael Addition

Significance

An efficient synthesis of 2,5-disubstituted 4-oxo-4,5-dihydrothiophene-3-carbox­amides via an intramolecular thia-anti-Michael addition is reported. For this mechanistically interesting reaction, it is proposed that eliminative thio­acetal opening is followed by amine addition (via thioketene?) and intramolecular 5-exo- trig anti-Michael addition of the sulfur anion to the α,β-enone to give the highly substituted thiophene derivatives. Although the scope of R¹ substituents (aryl bearing EWGs and EDGs, heteroaryl and alkyl groups) was adequately tested, only three amines were shown to participate in the reaction. X-ray crystal structures of two products were obtained which showed exclusive ketone tautomer forms.