alpha-silyl controlled asymmetric Michael additions of acyclic and cyclic ketones to nitroalkenes

Virtually enantiopure alpha-t-butyldimethylsilyl ketones (R)-1 and (S)-5 ar e converted regioselectively to the corresponding trimethylsilyl enol ether s (R)-2 and (S,Z)-6. Subsequent asymmetric Michael addition to nitroalkenes in the presence of SnCl4 affords the 1,4-adducts 3 and 7 in good yields an d high diastereo- and enantiomeric excesses. Removal of the t-butyldimethyl silyl group with n-Bu4NF, THF, NH4F/HF leads to the alpha,beta-disubstitute d gamma-nitro ketones (S,R)-4 (de = 91-92 %, ee > 98 %) and (R,S)-8 (de > 9 6 %, ee > 98 %) in very good overall yields.