1,1′-Carbonyldiimidazole (173 mg, 1.07 mmol) was added to a solution of the N′-hydroxy-6-[4-methoxy-3-(3,3,6,6-tetramethyl-1,8-dioxo-2,3,4,5,6,7,8,9-octahydro-1H-xanthen-9-yl)phenyl]pyridine-2-carboximidamide produced in Example 8-1 (424 mg, 0.82 mmol) in THF (8.5 ml) at room temperature, and the mixture thus obtained was then stirred at the same temperature as above for 2 hours. Thereafter, 1,8-diazabicyclo[5.4.0]undec-7-ene (0.160 ml, 1.07 mmol) was added to the resulting solution, and the mixture thus obtained was then stirred for 4.5 hours. Subsequently, an aqueous solution of ammonium chloride was added to the reaction solution, and the mixed solution was then extracted with dichloromethane twice. The organic layer thus obtained was dried over anhydrous sodium sulfate. The solvent was distilled away under reduced pressure, and the residue thus obtained was then purified by silica gel thin-layer chromatography (methanol:dichloromethane=1:10, v/v). The solid thus obtained was washed with a mixed solution of diethyl ether and hexane, followed by collection by filtration and drying, to obtain the title compound (309 mg, yield: 69%).
