Compound 1 (1.43 g, 2.28 mmol) obtained in Example 1 was dissolved in a mixed solvent of tetrahydrofuran (20 ml) and ethanol (20 ml), and 5 ml of 2N hydrochloric acid was added thereto, followed by heating under reflux for 0.5 hours. The solvent was distilled off under reduced pressure, and a saturated aqueous solution of sodium bicarbonate was added thereto followed by extraction with ethyl acetate. The resulting organic layer was washed with a saturated aqueous solution of sodium chloride and dried over magnesium sulfate, and the solvent was distilled off under pressure. The resulting residue was washed with diethyl ether/ethyl acetate and crystallized to give 1.1 g (yield: 88%) of the title compound.
