Compound 1 (400 mg, 0.999 mmol) obtained in Example 1 was dissolved in a mixed solvent of methanol (6.0 mL) and dichloromethane (4.0 mL), and sodium borohydride (37.8 mg, 0.999 mmol) was added thereto at 0° C., followed by stirring at 20 minutes. A saturated aqueous ammonium chloride solution was added to the reaction mixture, followed by extraction with ethyl acetate twice. The organic layer was washed with saturated brine, dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to yield a residue. The residue was purified by silica gel column chromatography to obtain the title compound (363 mg, 90%).
