Asymmetric aldol additions catalyzed by chiral phosphoramides: Electronic effects of the aldehyde component

The trichlorosilyl enolates of cyclopentanone and cycloheptanone were prepared by a mercury (II)-catalyzed metathesis from their TMS enol ethers in good yield. These enolates undergo spontaneous addition to aldehydes to provide the aldol adducts syn-4, 5a-e in high yield and selectivity (dr 19/1 to >49/1). More electron rich aldehydes tend to be more diastereoselective. The reaction of these enolates with aldehydes is also catalyzed by the chiral phosphoramide (S,S)-1 to provide anti-4, 5a-e in good to excellent diastereo- (dr 15/1 to 35/1) and enantioselectivity (er 84.4/15.6 to 95.9/4.1). In these cases, a trend is apparent only with sterically similar benzaldehyde derivatives. In addition, optimization studies suggest that two mechanistically distinct pathways may be operating in the catalyzed reaction. (C) 1998 Elsevier Science Ltd. All rights reserved.