Intramolecular (4+3) Cycloadditions of Thiophenes

Abstract

In contrast to the rich (4+3) cycloaddition chemistry of furans [1], only one instance of an intermolecular (4+3) cycloaddition of a thiophene has appeared with an unreported yield [2]. This paucity of examples is reflective of the high aromaticity of thiophenes that make them generally incompetent as dienes for cycloadditions. We have been working on (4+3) cycloadditions of epoxy enolsilanes with various dienes [3]. Herein, we report the first series of effective (4+3) cycloadditions of thiophenes which occurred in the intramolecular context. Thiophenes tethered to epoxy or aziridinyl enolsilanes underwent (4+3) cycloadditions under silyl triflate catalysis, to afford endo and exo cycloadducts in overall yields up to 83%, and with diastereomeric ratio of up to 8:1 (Eq. 1). Subsequent derivatizations of the cycloadducts afforded functionalized desulfurized dienes in moderate to high yields (Scheme 1). This strategy allows us to further utilize this (4+3) cycloaddition for organic synthesis applications.