A new, efficient, cost-effective method for the preparation of the Evans' chiral auxiliary (4S)-4-isopropyl-2-oxazolidinone (4) is described. A Schotten-Baumann acylation of valine with phenyl carbonochloridate quantitatively affords the protected valine, which is then reduced with borane in tetrahydrofuran to give an alcohol. Cyclization with a catalytic amount, of potassium tert-butoxide gives 4 in 81% overall yield and in 41-43% after crystallization from ethyl acetate/n-hexane.
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Wuts et al. (1989) studied this question.