Organocatalyzed Michael-Michael Cascade Reaction: Asymmetric Synthesis of Polysubstituted Chromans

An enantioselective cascade Michael Michael reaction between chalcones enolates and nitromethane catalyzed by a bifunctional thiourea is developed. This reaction provides a mild but efficient approach to chiral benzopyrans bearing three consecutive stereocenters in high yields with excellent stereoselectivities, and the benzopyrans can be easily transformed to the corresponding tricyclic product.