STEREOSELECTIVE REACTIONS .21. ASYMMETRIC ALKYLATION OF ALPHA ALKYL BETA-KETO-ESTERS TO ALPHA,ALPHA-DIALKYL BETA-KETO-ESTERS HAVING EITHER (R)-CHIRAL OR (S)-CHIRAL QUATERNARY CENTER DEPENDING ON THE SOLVENT SYSTEM

Asymmetric alkylation reaction of chiral enamines prepared from alpha-alkyl beta-keto esters and (S)-valine tert-butyl ester leading to either enantiomer is described. Lithiated chiral enamines can be alkylated with alkyl halides in a toluene solvent in the presence of HMPA to give, after hydrolysis, alpha,alpha-dialkyl beta-keto esters in 70-99%ee. The reactions in the presence of THF, dioxolane, or trimethylamine. instead of HMPA, afford the corresponding antipodes with enantiomeric purities of 44-92%ee. The present method provides a procedure for the synthesis of both enantiomers of alpha,alpha-dialkyl beta-keto esters in high enantiomeric purities starting from the same chiral enamines.