An acetonitrile (29.7 ml) solution of the benzyl 6-benzyloxycarbonylamino-2-((hydroxyphosphinoyl)oxy)hexanoate (2.97 g) obtained in the above step (a) was added with benzyl alcohol (3.53 ml) and a pyridine (11.9 ml)-acetonitrile (11.9 ml) mixed solution of pivaloyl chloride (4.20 ml), and the mixture was stirred at room temperature for 1 hour. Saturated aqueous ammonium chloride was added to the reaction system, and the organic substances were extracted with ethyl acetate. The organic layer was concentrated under reduced pressure, and pyridine was removed by azeotropic distillation using toluene. The residue was roughly purified by silica gel (Kanto Kagaku Co., 60 N, spherical, neutral) chromatography (300 g, hexane:ethyl acetate=3:2) to obtain crude benzyl 6-benzyloxycarbonylamino-2-((benzyloxyphosphinoyl)oxy)hexanoate (2.45 g, 68%). This compound was subjected to the measurement of molecular weight and then used in the following reaction without further purification.
