Enantiomerically Pure Acetals in Organic Synthesis. 2. Diastereoselective Alkylation of Enantiomeric Lithio Alkyl Lactyl Tetrahydropyranosides and Related Enolates

A concise approach for the rapid synthesis of enantiomerically pure α-alkylated derivatives of lactate esters and of other enantiomerically pure α-hydroxy esters is presented. This methodology, which makes use of enantiomeric lithium enolates prepared from diastereomeric tetrahydropyranyl ethers derived from alkyl lactates and other α-hydroxy esters, is used to prepare both enantiomers of frontaiin from (S)-(-)-methyl lactate.