Copper-Catalyzed Enantioselective Synthesis of trans-1-Alkyl-2-substituted Cyclopropanes via Tandem Conjugate Addition-Intramolecular Enolate Trapping

Cu-TolBINAP-catalyzed conjugate addition of Grignard reagents to 4-chloro-alpha,beta-unsaturated esters, thioesters, and ketones leads to 4-chloro-3-alkyl-substituted thioesters and ketones in up to 84% yield and up to 96% ee upon protonation of the corresponding enolates at low temperature. Tandem conjugate addition-enolate trapping, however, yields trans-1-alkyl-2-substituted cyclopropanes in up to 92% yield and up to 98% ee. The versatility of this reaction is illustrated by the formation of key intermediates for the formal syntheses of cascarillic acid and grenadamide.