6.77 g (21 mmol) of benzyl 2-(2R)-t-butoxycarbonylamino-5-hydroxypentanoate was dissolved in 105 ml of tetrahydrofuran. 2.74 g (23 mmol) of 3-cyanophenol, 6.6 g (25 mmol) of triphenylphosphine and 10 g (23 mmol) of diethyl azodicarboxylate (40% solution in toluene) were added to the solution, and they were stirred at room temperature for one hour. The solvent was distilled off, and the residue was treated with ethyl acetate as the extractant in an ordinary manner. The solvent was distilled off, and the residue was purified by the silica gel column chromatography to obtain the title compound.
