To a mixture of 3-(3-(6-benzyloxy-pyridin-3-ylmethyl)-isoxazol-5-yl)-pyridine-2,6-diamine (50 mg, 0.13 mmol) described in Example 25 and N,N-dimethylformamide (0.5 mL) were added a formaldehyde aqueous solution (14 mg, 0.17 mmol, content: 37%), α-picoline-borane (17 mg, 0.16 mmol), and acetic acid (50 mL) at room temperature, which was stirred overnight at the same temperature. A saturated sodium hydrogencarbonate aqueous solution was added to the reaction mixture, which was extracted with ethyl acetate. The organic layer was washed with saturated aqueous sodium chloride and dried over anhydrous magnesium sulfate, after which the organic layer was concentrated under a reduced pressure. The residue thus obtained was purified by reverse-phase high performance liquid chromatography (using an acetonitrile-water mobile phase containing 0.1% trifluoroacetic acid) to obtain a crude product, and then purified by silica gel thin-layer chromatography (diethyl ether: hexane=2:1) to obtain the title compound (2.3 mg, 4.4%).
