To a solution of N-[(benzyloxy)carbonyl]-O-(tert-butyl)-L-serine (25.0 g) in dry THF (100 mL) was added 1,1′-carbonylbis-1H-imidazole (16.5 g) at 0° C. by small portions, and the mixture was stirred at room temperature overnight (solution 1). To a solution of diisopropylamine (25.7 g) in dry THF (330 mL) was added dropwise a 1.6 mol/L n-butyllithium-hexane solution (158.7 mL) under an argon atmosphere at −78° C. The mixture was stirred at −78° C. for 30 min, and a solution of ethyl acetate (24.8 mL) in dry THF (100 mL) was added dropwise. After the completion of the dropwise addition, the mixture was stirred for 30 min, and solution 1 was added dropwise. The mixture was stirred at −78° C. for 1 hr, and acetic acid (25 mL) was added to the reaction mixture. After warming to room temperature, water was added and the mixture was extracted with ethyl acetate. The extract was washed with saturated brine, dried over magnesium sulfate, and concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography (eluent: hexane-ethyl acetate=50:1→1:1) to give the title compound as a colorless oil (yield: 19.2 g, 62%).
