To a solution of tert-butyl (1R,3S,4S)-3-[6-(5-methyl-1H-pyrazol-4-yl)-4-oxo-3,4-dihydrothieno[3,2-d]pyrimidin-2-yl]-2-azabicyclo[2.2.1]heptane-2-carboxylate (690 mg) in methanol (10 mL) was added 4M hydrochloric acid/ethyl acetate solution (2.0 mL) with stirring at room temperature. The reaction system was stirred with heating at 50° C. for 30 min, and the mixture was concentrated under reduced pressure. The residue was neutralized with saturated aqueous sodium hydrogen carbonate. Insoluble material was removed by filtration. The mixture was extracted with ethyl acetate, and dried over anhydrous sodium sulfate. Insoluble material was removed by filtration, and the filtrate was concentrated under reduced pressure. The residue was purified by basic silica gel column chromatography (ethyl acetate/hexane and methanol/ethyl acetate), and the obtained colorless solid was crystallized from methanol/ethyl acetate to give the title compound (220 mg) as a colorless solid. The optical purity was 86% ee. The analysis was performed by high performance liquid chromatography (column: CHIRALPAK OD-H (4.6 mm i.d.×250 mm L, manufactured by DAICEL CHEMICAL INDUSTRIES, LTD.), mobile phase: methanol, flow rate: 0.5 mL/min, column temperature: 30° C., detection 254 nm).
