Epimerization and kinetic resolution in copper-catalyzed enantioselective 1,4-additions of organozinc reagents to 6-substituted cyclohex-2-enones

Enantioselective 1,4-addition reactions of diethyl-, dimethyl-, and di-n-butylzinc to 6-methylcyclohex-2-enone (1) and 6-t-butylcyclohex-2-enone (4), catalyzed by Cu(OTf)2 and phosphoramidites L1–L4 were examined. The additions to enone 1 proceeded with high enantioselectivity; by acid- or base-catalyzed epimerization, adduct (S,S)-2 can be obtained from racemic 1 in diastereo- and enantiomerically pure form. In contrast, Michael additions to substrate 4 were rather slow and could be used for the kinetic resolution of the enone.