A solution of 2.5 g of ethyl (2R,3S)-3-tert-butoxycarbonylamino-2-hydroxy-3-phenylpropionate, 1.12 g of 3,3-dimethoxypentane and 25 mg of pyridinium p-toluene-sulphonate in 25 cm3 of toluene is stirred for 3 hours at a temperature in the region of 20° C. The reaction mixture is heated to boiling and the distillate is collected in a graduated vessel. After 20 cm3 of distillate have been collected, a solution of 1.12 g of 3,3-dimethoxypentane and 25 mg of pyridinium p-toluenesulphonate in 10 cm3 of toluene is added to the reaction medium and the mixture is heated to reflux of the toluene for 5 hours. The reaction medium is cooled to a temperature in the region of 20° C. and 10 cm3 of saturated aqueous sodium hydrogen carbonate solution are then added. The aqueous phase is separated after settling has taken place and then extracted with 2 times 10 cm3 of dichloromethane. The organic phases are combined, dried over magnesium sulphate, filtered and concentrated to dryness under reduced pressure (2.7 kPa) at a temperature in the region of 40° C. 2.3 g of ethyl (4S,5R)-3-tert-butoxycarbonyl-2,2-diethyl-4-phenyl-5-oxazolidinecarboxylate are thereby obtained in the form of a yellow oil.
