alpha-silyl controlled asymmetric Mannich reactions of acyclic ketones with imines

Enantiomerically pure alpha-dimethylthexylsilyl ketones (S)-1a-e were conve rted regio- and stereoselectively to the corresponding alpha-silylated trim ethylsilyl enol ethers (Z,S)-2a-e. Stereoselective alpha-silyl controlled M annich reactions with BF3. Et2O activated benzylideneaniline afforded the a lpha'-silylated alpha,beta-disubstituted beta-amino ketones 3a-e in anti/sy n ratios up to 80/20 and with moderate to good yields (61 - 82%). The major diastereomers (S,R,S)-3a-e (de greater than or equal to 96%) were converte d to the a,beta-disubstituted beta-amino ketones (R,S)-4a-e in excellent yi elds (92 - 96%), diastereomeric (de greater than or equal to 96%) and enant iomeric excesses (ee = 95 - greater than or equal to 99%) by removal of the silyl group with NH4F/n-Bu4NF.