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Subsequently, 6.1 g of the 1-benzyl-3-methyl-4-piperidone-N,N-dimethylhydrazone obtained above was dissolved in 30 ml of tetrahydrofuran. The solution were added dropwise to a solution of lithium diisopropylamide in tetrahydrofuran obtained from 23 ml of methyl lithium (1.2M ether solution), 3.8 ml of diisopropylamine and 30 ml of tetrahydrofuran at -8° C. over 15 minutes and the mixture was stirred at the same temperature for 2 hours. Thereafter, the reaction mixture was cooled to -60° C. and, to the reaction mixture was added 2.4 ml of methyl iodide. The temperature was gradually raised to 0° C. in 4 hours. The reaction mixture was poured into a mixed solvent of 120 ml of dichloromethane and 40 ml of water, and the dichloromethane layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The concentrate was purified by silica gel column chromatography (eluent: chloroform:methanol=20:1 V/V) to obtain 3.8 g of 1-benzyl-3,5-dimethyl-4-piperidone.