Copper(II)-Catalyzed C-C Bond-Forming Reactions of alpha-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 alpha-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.