A suspension of 3-amino-1-(4-bromo-5-isoquinolinesulfonyl)pyrrolidine hydrochloride (172 mg) obtained in Example 1 in 1,2-dichloroethane (10 ml) is added with triethylamine (0.28 ml, Wako Pure Chemical Industries), and then with furfural (42 mg, Aldrich), and the mixture is stirred at room temperature for 30 minutes, then added with sodium triacetoxyborohydride (170 mg, Aldrich), and further stirred at room temperature for 30 hours. The reaction mixture is added with saturated aqueous sodium hydrogencarbonate (15 ml), and the organic layer is separated. The aqueous layer is extracted 3 times with chloroform (10 ml for each time), and the combined organic layer is washed with saturated brine (30 ml). The organic layer is dried over anhydrous magnesium sulfate, and then the solvent is evaporated under reduced pressure. The residue is purified by silica gel chromatography (chloroform:methanol=30:1) to obtain the title compound (52 mg (predictive yield)).
