To a solution of the compound 3 (810 mg) and sodium carbonate (383 mg) in water (5 mL) and tetrahydrofuran (2 mL), benzyl chloroformate (413 μl) was added at 0° C. The reaction mixture was stirred at 0° C. for one hour. The reaction mixture was diluted with ethyl acetate. The organic layer was washed with water and saturated brine, dried over anhydrous sodium sulfate and then concentrated. The obtained residue was purified by silica gel column chromatography (hexane:ethyl acetate=4:1→3:1) to obtain the title compound (1.16 g) having the following physical properties.
