9.38 g of 2-methoryphenylacetic acid are added at room temperature to a suspension of 1.89 [lacuna] of sodium hydride (80% dispersion in petroleum jelly) in 200 cm3 of dry tetrahydrofuran. This suspension is cooled to -30° C., 7.77 cm3 of pivaloyl chloride are added, and then finally a solution, cooled to -78° C., is added which is obtained by adding a solution of 35.27 cm3 of 1.6M butyllithium in hexane to a solution, cooled to -78° C., of 10 g of (4S,5S)-4-methyl-5-phenyl-oxazolidin-2-one in 200 cm3 of dry tetrahydrofuran. The reaction mixture is stirred at -30° C. for 45 minutes and then, after returning to room temperature, 200 cm3 of saturated aqueous ammonium chloride solution are added followed by 500 cm3 of ethyl acetate; after separation of the phases by decanting, the organic phase is washed twice with 100 cm3 of water and then twice with 100 cm3 of saturated aqueous sodium chloride solution, 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 pressure of 0.6 bar of nitrogen with a mixture of cyclohexane and ethyl acetate (85:15 then 80:20 by volume) and collecting fractions of 50 cm3. Fractions 14 to 31 are combined and concentrated to dryness under reduced pressure (2.7 kPa). 13.6 g of (4S,5S)-4-methyl-5-phenyl-3-(2-methoxyphenylacetyl)oxazolidin-2-one are obtained in the form of a yellow oil.
