Diastereoselectivity in the preparation of β-silyl esters from αβ-unsaturated esters and amides attached to chiral auxiliaries

Abstract

The conjugate addition of the phenyldimethylsilyl-cuprate reagent to cinnamate and crotonate esters and amides 1 of various known chiral auxiliaries, a–e, is diastereoselective. The sense of the diastereoselectivity of silyl-cuprate addition to the esters 1b–d, 8 and 9 is different from established precedent based on carbon-cuprates, but is normal for silyl-cuprate addition to the amide 1a, the imides 1e and 21, and the oxazolidine 6. The chiral auxiliary e gives the best results of those tested, and the silicon-containing group can be removed from the chiral auxiliary using alkoxide ion in aprotic media, making available β-silyl esters 27–29 of high enantiomeric excess, with recovery of the chiral auxiliary 30.