The compound (263 mg) obtained in Example 1-2 was dissolved in dichloromethane (4.0 ml) and a 15% trimethyl aluminum/hexane solution (1.08 ml) was dropped thereto. The whole was stirred at room temperature for 15 minutes. The solution was added with the compound (300 mg) obtained in Example 19-1 and stirred for additional 20 hours. The resultant was heated to 40° C. and stirred for additional 7 hours, and then 1 mol/l hydrochloric acid was added to stop the reaction, followed by extraction with chloroform. The organic layer was washed with a saturated aqueous sodium hydrogen carbonate solution, water, and a saturated saline solution and dried with anhydrous sodium sulfate. The residue obtained by distilling the solvent off was purified through silica gel column chromatography (chloroform/methanol), thereby obtaining the subject compound (237 mg) as a yellow solid.
