The high diastereoselectivity of the base-catalysed epimerization of oxazolidin-2-ones is shown to depend on the nature of the N-substituent (R group):  when R = Bn, the 4,5-trans-product (4S,5R)-9 is formed, whereas when R = H the 4,5-cis-product (4S,5S)-10 is formed, both with >99 : 1 d.r.  The successful hydrolysis of the oxazolidin-2-one group in both cis- and trans-derivatives show this to be a stereodivergent route to enantiopure a-hydroxy-b-amino isopentanoic acids (2R,3S)-1 and (2S,3S)-2. Enantiomeric purities of target compounds were determined by means of Mosher¢¥s ester, which were established the enantiomeric ratio in 1 and 2 to be >99:1