The compound (380 mg) obtained in Example 18-3 was dissolved in DMF (7.6 ml) and 60% sodium hydride (60.0 mg) was added thereto, and the whole was stirred for 1 hour. After that, methyl iodide (213 mg) was gradually added thereto and the whole was stirred at room temperature for 2 hours. After completion of the reaction, the solvent was distilled off under reduced pressure. The residue was dissolved in chloroform and washed with a saturated aqueous sodium hydrogen carbonate solution, followed by extraction with chloroform. The organic layer was washed with a saturated saline solution and dried with anhydrous sodium sulfate. After filtration, the solvent was distilled off under reduced pressure. The residue was purified through silica gel column chromatography (chloroform/methanol/water), thereby obtaining the subject compound (83.0 mg) as a brown solid.
