In an atmosphere of argon, 0.95 ml of n-butyl lithium (1.59M/hexane solution, 1.51 mmol) was added dropwise at −78° C. to a 10 ml anhydrous tetrahydrofuran solution of 300 mg (0.72 mmol) of the compound obtained in Example 15. The mixture was stirred at −78° C. for 30 minutes, and then 133 μl (1.08 mmol) of 1-methyl-4-piperidone was added, followed by heating the mixture gradually to 0° C. over the course of 1 hour. Then, water was added to the reaction mixture, and then the mixture was extracted with ethyl acetate. The extract was dried over anhydrous sodium sulfate, and then the solvent was distilled off under reduced pressure. The residue was purified by silica gel column chromatography (chloroform/methanol=100/1) to obtain 179 mg (55%) of the captioned compound.
