Microwave‐Induced Synthesis and Regio‐ and Stereoselective Epoxidation of 3‐Styrylchromones

The microwave‐assisted solvent‐free synthesis of (E)‐3‐styrylchromones from 3‐formylchromones and phenylmalonic acid on sodium acetate support has been developed; the method affords the styryl derivatives in moderate to good yields and with complete diastereoselectivity. Protocols for the highly regioselective epoxidation of (E)‐ and (Z)‐3‐styrylchromones have been elaborated. Treatment of the alkenes with dimethyldioxirane led to the exclusive formation of 3‐(3‐aryloxiran‐2‐yl)chromones with complete diastereoselectivity, whereas treatment with hydrogen peroxide under alkaline conditions afforded the corresponding 2,3‐epoxy‐3‐styrylchromanones as the only products. Epoxidation performed in the presence of chiral, nonracemic cinchona‐alkaloid‐based quaternary ammonium salts allowed the synthesis of enantiomerically enriched 2,3‐epoxy‐3‐styrylchromanones, but with only moderate ee values. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)