Lewis Acid-Catalyzed Hetero-Diels-Alder Reactions of Methyl 2-Oxo-3- alkenoates with Alkenes.
1994
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.
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