New insights into the asymmetric Diels-Alder reaction: the endo- and S-selective retro-Diels-Alder reaction

The endo- and S-selective retro-Diels-Alder reactions in an imidazolethione-catalyzed asymmetric Diels-Alder reaction were verified and investigated, and account for the low ee values in a CH3CN-H2O catalytic system. This reverse process could be controlled by forming the dimethyl acetal of the aldehyde product with methanol. Both the exo- and endo-isomers were obtained in the CH3OH-H2O system in high yields with good to excellent enantioselectivities.