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.