Enantioselective Synthesis of Chromans with a Quaternary Stereogenic Centre through Catalytic Asymmetric Cascade Reactions

A highly enantioselective cascade reaction of benzotriazoles with
nitroolefin-containing enonates catalyzed by a base/acid bifunctional
organocatalyst has been developed. This cascade sequence affords efficient
access to densely functionalized chiral chromans with a quaternary
stereogenic center in high yield (up to 91%) with excellent enantioselectivity
(up to 96% ee) and diastereoselectivity (up to 96:4 dr). The reaction
itself features simple experimental procedures under benign conditions
and is completely atom-economic in character.