0.492 g (1.71 mmol) of 4-[(4-dipropylaminobutyl)amino]methylbenzonitrile (2a), 9.8 ml of ethanol, and 0.325 g (3.42 mmol, 2.0 equivalents) of pyrrole-2-aldehyde were charged in a 50 ml recovery flask under a nitrogen stream. The mixture was then cooled to 0° C. After the addition of 0.870 g (4.11 mmol, 2.4 equivalents) of sodium triacetoxyborohydride, the mixture was stirred at room temperature for 17 hours. After confirming the disappearance of the raw material by TLC, the reaction solution was added to 50 ml of a saturated sodium bicarbonate aqueous solution, and extracted twice with 50 ml of chloroform. The chloroform layer was washed with 100 ml of a saturated sodium chloride aqueous solution, dried over anhydrous sodium sulfate, concentrated, and purified by silica gel column chromatography (Chromatorex NH, chloroform:n-hexane=1:1), thereby obtaining 0.342 g of 4-{[(4-dipropylaminobutyl)-(1H-pyrrol-2-ylmethyl)amino]methyl}benzonitrile (18) as a light yellow oily substance (yield: 54.5%). [HPLC: 87.6% (HPLC measurement conditions B)]
