To a solution of 0.260 g (0.799 mmol) of 6-(4-heptylbenzoyl)-4,5,6,7-tetrahydrofuro[2,3-c]pyridine in 20 ml of acetic acid, 0.108 ml (1.20 mmol) of 50% aqueous dimethylamine and 0.097 ml (1.20 mmol) of 37% aqueous formaldehyde were added, followed by stirring at 100° C. for 60 minutes. Additionally, 0.072 ml (0.799 mmol) of 50% aqueous dimethylamine and 0.064 ml (0.799 mmol) of 37% aqueous formaldehyde were added, followed by stirring for 30 minutes. After the solvent was distilled off under reduced pressure, the residual solution was alkalified with 5% aqueous solution of sodium hydrogen carbonate, and extracted with dichloromethane 2 times. The combined organic layer was washed with water and dried over anhydrous sodium sulfate; the solvent was distilled off under reduced pressure. The resulting crude product was purified by silica gel column chromatography (chloroform to chloroform/methanol=50/1 to 25/1) to yield the desired product.
