Metal free [3+2] cycloadditions of alkylidene cyclopropanes via a non-classical “bow-tie” cation

Abstract

Abstract: Treatment of cyclopropylidene-containing 1,5-hexadienes with either diazepane carboxylate organocatalysts or Lewis acids initiates a tandem Michael addition – cyclopropane ring-opening (MACRO) sequence resulting in the formation of [3.2.1]-bridged bicycles. DFT calculations suggest the reaction proceeds by a non-classical corner-alkylated cyclopropane which facilitates an uncommon 1,3-ring expansion of the cyclopropane. The rearrangement generates a reactive bridgehead enone/eniminium which can be trapped intermolecularly by HCl or intramolecularly by electron-rich arenes to generate bridged tetracycles. The method has the potential to furnish functionalized polycycles applicable to natural product synthesis.