Compound 20 (108 mg, 0.262 mmol) obtained in Example 19 was suspended in a solvent mixture of 2 ml of tetrahydrofuran, 2 ml of ethanol, and 1 ml of water. To the suspension was added 55 mg (1.311 mmol) of lithium hydroxide monohydrate, and the resulting mixture was stirred at room temperature for 12 hours. The reaction mixture was acidified with 2N HCl, and the precipitated crystals were collected by filtration. The obtained crude crystals were purified by column chromatography (eluent: chloroform/methanol/acetic acid=40/1/1) to give 25 mg (yield 24%) of Compound 22, which was further recrystallized from isopropanol to give a yellow powder.
