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
Tin(IV) chloride catalyzed inverse electron demand hetero-Diels-Alderreaction of substituted methyl 2-oxo-3-alkenoates with simple alkenes led to methyl 3,4-dihydro-2H-pyran-6-carboxylates in satisfactory yields. The addition proceeded stereospecifically through an exo transition state.