Diastereoselective addition of metal-coordinated and ''naked'' nucleophilic reagents to norephedrine derived 2-acyl-N-tosyl-oxazolidines

The addition of tri-s-butyl borohydrides to the 2-acetyl-1,3-oxazolidine 1 could be directed with high selectivity to either the Si or the Re pi-carbonyl face under chelating or non-coordinating conditions respectively. Addition of hydrides to the corresponding phenyl ketone 2 was highly Si selective only in the former conditions. Grignard reagents and organolithiums add to the methyl ketone 1 with remarkable Si and Re pi-face selectivity respectively. With phenyl ketone 2 only organomagnesium reagents follow the above trend. This data are in accord with a Felkin-type or a stereochemically opposite chelaled mode of addition of these nucleophiles in the absence or in the presence of chelating metal counterions respectively.