γ-Methylidene-δ-valerolactones as a Coupling Partner for Cycloaddition: Palladium-Catalyzed [4 + 3] Cycloaddition with Nitrones

A new type of reagent, γ-methylidene-δ-valerolactones, has been devised, which acts as a four-carbon unit in a palladium-catalyzed cycloaddition reaction through the formation of a 1,4-zwitterionic species. The utility has been demonstrated in the context of stereoselective [4 + 3] cycloaddition with nitrones to provide highly functionalized 1,2-oxazepines, including the asymmetric variant with high enantioselectivity.