Practical preparation of α-hydroxy-β-amino ester units; Stereoselective synthesis of taxol side chain and norstatine

Abstract

An asymmetric reaction of chiral imines with α-silyloxy ketene acetals mediated by chiral boron reagents is described. The key to its success is the use of the chiral boron complex prepared in situ from ( R)- or ( S)-binaphthol and B(OPh) 3. Both diastereomers of α-hydroxy-β-amino ester units are successfully prepared with high selectivities by the chiral boron reagents depending on the geometry of the silyl ketene acetals. The optically pure anti α-hydroxy-β-amino ester is obtained from ( E)-silyl ketene acetal, while the corresponding syn α-hydroxy-β-amino ester is obtained from (Z)-silyl ketene acetal. The method can be efficiently applied to the stereoselective synthesis of taxol C-13 side chain and the norstatine family.