9.38 g of 2-methoxyphenylacetic acid are added, at room temperature, to a suspension of 1.89 g of sodium hydride (80% dispersion in vaseline) in 200 cm3 of dry tetrahydrofuran. This suspension is cooled to -30° C., 7.77 cm3 of pivaloyl chloride are added and then a solution, cooled to -78° C., which is obtained by adding 35.27 cm3 of a 1.6M solution of butyllithium in hexane to a solution, cooled to -78° C., of 10 g of (4S,5S)-4-methyl-5-phenyl-2-oxazolidinone in 200 cm3 of dry tetrahydrofuran, is finally added. The reaction mixture is stirred for 45 minutes at -30° C. and then after reequilibrating to room temperature, 200 cm3 of a saturated aqueous solution of ammonium chloride is added followed by 500 cm3 of ethyl acetate; after decantation, the organic phase is washed twice with 100 cm3 of water and then twice with 100 cm3 of a saturated aqueous solution of sodium chloride, dried over magnesium sulphate and concentrated to dryness under reduced pressure (2.7 kPa). The residue is chromatographed on a silica gel column (particle size 0.04-0.06 mm, diameter 4.8 cm, height 36 cm), eluting under a nitrogen pressure of 0.6 bar with a cyclohexane and ethyl acetate mixture (85/15 and then 80/20 by volume) and collecting fractions of 50 cm3. Fractions 14 to 31 are pooled and concentrated to dryness under reduced pressure (2.7 kPa). 13.6 g of (4S,5S)-4-methyl5-Phenyl-3-(2-methoxyphenylacetyl)-2-oxazolidinone are obtained in the form of a yellow oil.
