600 ml of a solution of NaHMDS (sodium salt of 1,1,1,3,3,3-hexamethyldisilazane) (1M) in tetrahydrofuran are added dropwise, at -78° C. under an inert atmosphere, to a solution of 100 g of Nα-benzyloxycarbonyl-D-tryptophan in a liter of anhydrous tetrahydrofuran. After one hour, the temperature is brought to 0° C. and a solution of 64 g of tert-butyl bromoacetate in 50 ml of anhydrous tetrahydrofuran is added dropwise. The reaction mixture is then brought to ambient temperature and the solvent is subsequently evaporated off in vacuo. The residue is diluted with dichloromethane and the solution is acidified by adding an aqueous 4N hydrochloric acid solution. After extraction with dichloromethane, the organic phases are dried over sodium sulphate and then concentrated in vacuo. 136 g of the expected product are obtained.
