Pyrones as Substrates for Palladium–Catalyzed [4 + 3] Cycloadditions

A novel reaction of pyrones was observed upon treatment with the palladium complex 1, a 4π-electron reagent. Whereas pyrones normally behave as 2π-electron substrates toward 4π-electron systems, they undergo [4 + 3] cycloaddition with 1 to give seven-membered ring compounds such as 2 and 3. A two-step mechanism for the cycloaddition is offered as an explanation for this behavior.