Dearomative Intramolecular (4+3) Cycloadditions of Thiophenes

Unexpectedly facile dearomative intramolecular (4+3) cycloadditions of thiophenes with epoxy enolsilanes, providing sulfur-bridged cycloadducts, are reported. A total of fifteen thiophene substrates have been found to undergo (4+3) cycloaddition smoothly to produce endo and exo (4+3) adducts in yields of up to 83 % with moderate to good diastereoselectivity. Complete conservation of enantiomeric purity was observed when the optically enriched epoxide was used. The desulfurizing transformations of the sulfur-bridged skeleton of the cycloadducts provide functionalized 6,7-fused bicyclic frameworks consisting of 1,3-cycloheptadiene subunits. Density functional theory calculations reveal the origins of the facile dearomatization of thiophenes in these (4+3) cycloadditions. Dearomative intramolecular (4+3) cycloadditions of thiophenes with epoxy and aziridinyl enolsilanes occur under silyl catalysis to generate sulfur-bridged cycloadducts. Conservation of enantiomeric purity was observed when optically-enriched epoxide was used. A desulfurizing transformation provides functionalized 6,7-fused bicyclic frameworks. DFT calculations reveal the origins of the facile dearomatization of thiophenes in these cycloadditions. image