3-Pentanone (68.9 μL, 0.65 mmol) and sodium triacetoxyborohydride (165 mg, 0.78 mmol) were added four times every two hours to a mixed solution of ethyl 2-[(3S,4R)-3-amino-4-{[(4-chloro-5-ethyl-1H-imidazol-2-yl)carbonyl]amino}piperidin-1-yl]-1,3-benzothiazole-7-carboxylate obtained in Example (202e) (62.0 mg, 0.13 mmol) in methanol (1 mL) and tetrahydrofuran (1 mL) under ice-cooling, and the mixture was stirred at room temperature for 15 hours. Saturated aqueous sodium bicarbonate solution was added to the reaction solution, followed by extraction with ethyl acetate. The organic layer was washed with brine and dried over anhydrous sodium sulfate. The insoluble matter was separated by filtration, and the filtrate was concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (eluate: hexane:ethyl acetate=1:1 v/v) to obtain 48 mg of the title compound (68%) as a colorless solid.
