(2-Methyl-4-oxo-4H-furo[3,2-c]pyridin 5-yl)acetaldehyde(0.18 g) and acetic acid(0.1 ml) were added to a 1,2-dichloroethane solution (5 ml) of 1-ethyl-3,3,5-trimethyl-7-{4-[(pyridin-4-ylmethyl)amino]butyl}-1,5-dihydrobenzo[b][1,4]diazepine 2,4-dione(0.38 g). The mixture was stirred for 30 minutes at room temperature. sodium triacetoxyborohydride(0.32 g) was added to the mixture, and the mixture was stirred at room temperature overnight. The reaction mixture was condensed under reduced pressure. The residue was purified by silica gel column chromatography (ethyl acetate: methanol=1:0→9:1). The purified product was condensed under reduced pressure. A 6N-hydrogen chloride ethyl acetate solution(1.0 ml) was added to an ethyl acetate solution (20 ml) of the residue, and the liquid was stirred at room temperature. The precipitated insoluble matter was separated, washed with ethyl acetate, and dried to give the title compound(0.43 g) as a white solid.
