The stereocontrolled asymmetric synthesis of alpha -hydroxy-beta -amino acids has been investigated via the Lewis acid-promoted cyanation of (5R, 6S)-2-acetoxy-4-(benzyloxycarbonyl)-5,6-diphenyl-2,3,5,6-tetrahydro-4H-1,4-oxazines with trimethylsilyl cyanide. Base-catalyzed hydrolysis of the resulting cyano compounds proceeds with excellent stereoselectivity, providing access to diastereomerically pure oxazine-2-carboxylic acids which were readily converted to each enantiomer of the alpha -hydroxy-beta -amino acids isothreonine and nor-C-statine.