PRACTICAL PREPARATION OF ALPHA-HYDROXY-BETA-AMINO ESTER UNITS STEREOSELECTIVE SYNTHESIS OF TAXOL SIDE-CHAIN AND NORSTATINE

An asymmetric reaction of chiral imines with alpha-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 form (R)- or (S)-binaphthol and B(OPh)(3). Both diastereomers of alpha-hydroxy-beta-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 alpha-hydroxy-beta-amino ester is obtained from (E)-silyl ketene acetal, while the corresponding syn alpha-hydroxy-beta-amino ester is obtained from (2)-silyl ketene acetal. The method can be efficiently applied to the stereoselective synthesis of taxol C-13 side chain and the norstatine family.