Copper(II)-Catalyzed CC Bond-Forming Reactions of α-Electron-Withdrawing Group-Substituted Ketene S,S-Acetals with Carbonyl Compounds and a Facile Synthesis of Coumarins

A new catalytic CC bond‐forming reaction has been developed. Catalyzed by cheap and commercially available copper(II) bromide (CuBr2; 10 mol%), the reactions of α‐electron‐withdrawing group (EWG)‐substituted ketene S,S‐acetals with aldehydes or ketones in acetonitrile at room temperature gave a variety of densely functionalized coupling products in excellent yields (80–98%). Based on this catalytic process, coumarin derivatives were efficiently synthesized when salicylaldehydes were selected as the carbonyl components.