Lewis Acid-Catalyzed Hetero-Diels–Alder Reactions of Methyl 2-Oxo-3-alkenoates with Alkenes

Lewis acid-catalyzed cycloadditions of 4-substituted methyl 2-oxo-3-butenoates and -3-pentenoates with alkenes took place with high regio- and stereoselectivities to afford substituted 6-methoxycarbonyl-3,4-dihydro-2H-pyrans in good to moderate yields. None of the ene products were produced. Similar reactions of methyl 3-phenyl-2-oxo-3-butenoate with alkenes afforded both the corresponding 3,4-dihydro-2H-pyrans and a dimeric cycloadduct of the diene component. The regio- and stereoselectivities of the present inverse-electron-demand cycloadditions indicated that the reactions proceeded through a concerted exo transition state.