Photochemistry of tetramethyl‐4‐pyrone in alcohol solvents

Irradiation of tetramethyl‐4‐pyrone in alcohol solvents leads to the formation of tetramethyl‐2‐pyrone and to a variety of cyclopent‐2‐enone and cyclopent‐3‐enone solvent adducts. The structure and mode of product formation is consistent with photochemical generation of a short‐lived 2,6‐bridged oxabicyclohexenyl zwitterion which undergoes sigmatropic rearrangement to a longer‐lived epoxycyclopentenone. Although the zwitterion is efficiently trapped by methanol solvent, in less nucleophilic alcohol solvents it rearranges to the epoxycyclopentenone which isomerizes to the 2‐pyrone or reacts with the alcohol solvent to form the observed solvent adducts.