New Results on the Stereoselective Alkylations of Malic Acid Derivatives Supported by Molecular Modeling

Abstract

AbstractThe stereoselectivity of the alkylation of dialkyl malates is dependent on steric hindrance of both ester alkyl groups. It was found that the two alkyl groups have opposite effects on diastereoselectivity. Increased steric hindrance at the C(1) carboxy group increases the anti‐selectivity, whereas increased steric hindrance at the C(4) carboxy group decreases it. The results are explained by comparing the structures of the enolates, which were obtained by molecular modeling. Alkylation at C(4′) of dioxolanones, derived from benzyl‐substituted malic acids, with an additional stereogenic center on the side chain is dependent on the stereogenic centers of the ring acetal and of the side chain. Alkylation at low temperatures occurs only with cis‐dioxolanones having an (R)‐configured side‐chain stereogenic center. The corresponding trans‐dioxolanone and the cis‐dioxolanone with a (S)‐configured side‐chain stereogenic center were recovered unchanged. A rationale is presented with models of monolithiated dioxolanones obtained by ab initio calculations.