Stereoselective reactions. XXIV. Chlorotrimethylsilane promoted asymmetric Michael reaction of chiral lithioenamines derived from α-alkyl β-keto esters

Chlorotrimethylsilane promoted asymmetric Michael reaction of the chiral lithioenamines derived from α-alkyl β-keto esters and (S)-valine tert-butyl ester is described. Complementary asymmetric syntheses producing either enantiomers from the same starting material have been realized by changing the solvent system. That is, the lithioenamines react with methyl vinyl ketone or ethyl acrylate in THF in the presence of chlorotrimethylsilane to give, after hydrolysis, the Michael adducts in 57–90% ee. On the other hand, the reaction using toluene-HMPA as a solvent system, instead of THF, affords the corresponding antipodes with enantiomeric purities of 41–77% ee.