Diastereoselectivity and enantioselectivity in the addition of chiral imines of 2-methylcyclohexanone to crotonic and methacrylic acid esters

Abstract

Additions of the chiral imine (reacting as its secondary enamine tautomer) obtained from (S)-1-phenylethylamine and 2-methylcyclohexanone were performed with the phenyl ester of crotonic and methacrylic acid as well as with their methyl ester. In each example, the stereochemical relationship of the substituents in the major adduct was shown to be the one predicted in a previous theoretical calculation which established that the reactants complex has a chairlike geometry. In all the examples, the diastereoselectivity is superior to 98%. The enantioselectivity of the reactions is excellent as is usually the case with unsubstituted electrophilic olefins, the example with phenyl methacrylate being particularly remarkable (de and ee >99%). In each case the favored diastereofacial selectivity is again in accordance with the rule elaborated previously. Relevant facts about the influence of the substituents upon the reactivity, the proportion of regioisomers, the stereoselectivity, and the enantioselectivity of the reaction are given.