Substituent effects on the 4π + 2π cycloadditions of 4H-pyran-4-one derivatives

The simple γ-pyranones 4, 5, and 6 undergo 4π + 2π cycloaddition reactions with Danishefsky’s diene 8 and azomethine ylides 12, 16, and 25 to give a range of cycloadducts. The position of the electron-withdrawing group is the decisive factor in these reactions, with the 2-substituted derivatives 5 and 6 being less reactive than the 3-substituted γ-pyranone 4. The esters, 4 and 5, react via addition across the 2,3-CC bond of the pyranone ring, whilst the aldehyde 6 reacts via addition across the CO of the carbaldehyde group.