To a solution of tert-butyl[(4′R)-6′-{[(5-methoxypyrazin-2-yl)carbonyl]amino}-2′,2′-dimethyldispiro[cyclopropane-1,3′-chromene-4′,4″-[1,3]oxazol]-2″-yl]carbamate (13.75 g, 26.98 mmol) in chloroform (140 ml) was added trifluoroacetic acid (68 ml) in an ice-water bath, and the mixture was stirred at room temperature for 3 hours. The mixture was concentrated in vacuo, and the residue was purified by silica gel column chromatography (pre-column: NH-silica gel; main column: neutral silica gel, chloroform/methanol=100:0-10:1). To the purified product was added saturated aqueous sodium hydrogen carbonate and extracted with chloroform. The organic layer were washed with brine, dried over anhydrous sodium sulfate, filtered, and the filtrate was evaporated in vacuo. The residue was purified by silica gel column chromatography (pre-column: basic silica gel; main column: neutral silica gel, chloroform/methanol=100:0-10:1). The purified product was triturated with a mixture of hexane/ethyl acetate (4:1) (300 mL), and the mixture was stirred at 60° C. for 1 hour and room temperature for 4 days. The precipitate was collected, washed with mixture of hexane/ethyl acetate (4:1) (200 mL), and dried in vacuo at 50° C. to afford N-[(4′R)-2″-amino-2′,2′-dimethyldispiro[cyclopropane-1,3′-chromene-4′,4″-[1,3]oxazol]-6′-yl]-5-me thoxypyrazine-2-carboxamide (8.38 g) as a crystal.
