(R)-1,2,2-TRIPHENYL-2-(TRIMETHYLSILOXY)ETHYL PROPIONATE - ANTI-SELECTIVE AND DIASTEREOFACIALLY SELECTIVE ALDOL ADDITION - DIASTEREOSELECTIVE SILYLATION AND ALKYLATION

The propionates (R)-5 and (R)-6 which are derived from the readily available chiral auxiliary reagent (R)-triphenylglycol (4) have been applied in stereoselective aldol reactions. Whereas the enolate 7 and the silyl ketene acetal 12, both generated from the ester (R)-6, display only moderate diastereoselectivity when treated with benzaldehyde, beta-hydroxyesters 8b and 16a, b are formed in diastereomeric ratios up to 95:5 (ratio of the main product to the sum of all other stereoisomers) when the propionate (R)-5 is subsequently deprotonated, transmetalated into the zirconium enolate and allowed to react with aldehydes. Alkaline hydrolysis or reduction with LiAlH4 enables the conversion of the adducts 8a and 16a, b into the carboxylic acid 15a and the diols 17a, b, respectively, which are obtained in >95% e.e. When the enolate 13 is treated with chlorotrimethylsilane, carbon silylation occurs so that the alpha-silyl propionate 18a is formed diastereoselectively. On the other hand, alkylations of the enolate 13 with primary alkyl halides display only moderate disastereoselectivities. The structures of the propionate 5, the alpha-silyl ester 18a, and the alkylation product 18b are proven unambiguously by crystal structure analyses.