Compound 1b (232 mg) was dissolved in ethanol (7.89 ml). A 2N sodium hydroxide aqueous solution (5.26 ml) was added thereto and the mixture was stirred at room temperature overnight. The reaction mixture was ice-cooled, and a 2N hydrochloric acid aqueous solution (5.26 ml) was added to neutralize the mixture. Then the solvent was distilled off. The reaction mixture was ice-cooled, and a 2N hydrochloric acid aqueous solution (0.5 ml) was added to adjust the pH of the mixture to 1. The mixture was stirred for 1 hour. The precipitated solid was filtered off and vacuum-dried to give Compound 1 (88.0 mg, 41%) as a white solid.
