A stereoselective synthesis of (E)-alpha,beta-unsaturated ketones involving the reactions of organocerium reagents with secondary beta-enamino ketones

Stereoselective construction of trisubstituted alkenes has wide applicability to the synthesis of many natural products. Specifically, beta-disubstituted enones are important functionalized trisubstituted alkene targets. The reaction of organocerium reagents with secondary beta-enamino ketones affords beta-disubstituted alpha,beta-unsaturated ketones in fairly good yields. This process shows considerable stereoselectivity, and alpha,beta-unsaturated ketones of (E) configuration are predominantly observed. Organolithium-derived cerium reagents display better stereoselectivity than organomagnesium-based ones. The mechanism of the reaction varies with nitrogen substitution: N-phenyl groups give 1,2-addition products, whereas substitution products are observed with N-alkyl groups. When organocerium reagents were used with beta-enamino ketones bearing secondary alkyl groups at the nitrogen atom, a lack of reactivity was observed.