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