50.93 g (820 mmol) of triphenyl[[1-(triphenylmethyl)-1H-imidazol-4-yl]methyl]phosphonium chloride in solution in 333 ml of tetrahydrofuran are introduced, under argon, into a three-necked round-bottomed flask. 51.2 ml of a 1.6M solution of n-butyllithium in hexane (820 mmol) are added dropwise, at -70° C. After stirring for 30 minutes at -70° C., the reaction mixture is quickly poured into 270 ml of a 0.253M solution, cooled to -70° C., of 1,1-dimethylethyl (S)-4-oxo-2-[[(phenylmethoxy)carbonyl]amino]butanoate in tetrahydrofuran (683 mmol). The mixture is allowed to return to room temperature overnight. The mixture is hydrolysed with 280 ml of a saturated aqueous sodium chloride solution. The aqueous phase is separated from the organic phase and extracted with 2 times 140 ml of ethyl acetate. The organic phases are combined, dried over magnesium sulphate and evaporated to dryness. Purification is carried out by chromatography on a column of silica gel, the eluent being a hexane/ethyl acetate gradient.
