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

The simple gamma-pyranones 4, 5, and 6 undergo 4 pi + 2 pi cycloaddition re actions with Danishefsky's diene 8 and azomethine ylides 12, 16, and 25 to give a range of cycloadducts. The position of the electron-withdrawing grou p is the decisive factor in these reactions, with the 2-substituted derivat ives 5 and 6 being less reactive than the 3-substituted gamma-pyranone 4. T he esters, 4 and 5, react ria addition across the 2,3-C=C bond of the pyran one ring, whilst the aldehyde 6 reacts ria addition across the C=O of the c arbaldehyde group.