Reactivity and selectivity of captodative olefins as dienes in hetero-Diels-Alder reactions

The reactivity and selectivity of the the captodative olefins 1-acylvinyl benzoates 1a-1f and 3a as heterodienes in hetero-Diels-Alder reactions in the presence of electron-rich dienophiles is described. Heterodienes 1 undergo regioselective cycloaddition with the alkyl vinyl etherdienophiles 6a,b and 9 to give the corresponding dihydro-2H-pyrans 7, 8, and 10 under thermal conditions. The reactivity of these cycloadditions depends, to a large extent, on the electronic demand of the substituent in the aroyloxy group of the heterodiene. Frontier-molecular-orbital (FMO; ab initio) and density-functional-theory (DFT) calculations of the ground and transition states account for the reactivity and regioselectivity observed in these processes.