Switching of product's chirality in diastereodifferentiating [2+2] photocycloaddition of (E)- versus (Z)-stilbene to chiral fumarate upon direct and charge-transfer-band excitation

Diastereodifferentiating [2+2] photocycloadditions of (E)- and (Z)-stilbenes to bis((R)-1-methylpropyl) fumarate were performed through the direct excitation of stilbenes and the selective excitation of the charge-transfer (CT) complex at various temperatures. The geometrical isomers of stilbene afforded the opposite diastereomers of mu-truxinate in both excitation modes, with a dramatic decrease in the product's diastereoselectivity upon prolonged irradiations.