Single-carbon insertion enables conversion of cyclopropenes into cyclobutenes: access to oxaspiro[2.3]hexenes

We report 1-oxaspiro[2.3]hex-4-enes as a novel strained spiro heterocycle. This motif was accessed via a formal C1 insertion into cyclopropenes. The method employs dihalocarbenes to trigger a skeletal editing process involving in situ bicyclo[1.1.0]butane formation and strain-release-driven ring expansion, delivering halogenated cyclobutenes under simple conditions. A broad substrate scope is demonstrated, enabling access to chloro-, bromo-, and fluoro-substituted spirocycles, including derivatives of bioactive molecules. Mechanistic studies supported by in silico calculations demonstrate an asynchronous, concerted carbene addition followed by intramolecular epoxidation and ring expansion as the rate-determining step.