Intermolecular (5+3) Cycloaddition of Bicyclo[1.1.0]butanes with Oxidopyrylium

A formal (5+3) cycloaddition of bicyclo[1.1.0]butanes (BCBs) with maltol derivatives is achieved by employing oxidopyrylium species as 1,5-dipoles. This transformation provides a new pathway for synthesizing all-carbon bridged bicyclo[5.1.1]nonanones, a structurally constrained medium-ring scaffold. The reaction proceeds under mild conditions and exhibits high functional group tolerance alongside a broad substrate scope.