[NH2:1][C:2]1[O:3][N:4]=[C:5]2[C:10]=1[C:9](=[O:11])[N:8]([CH3:12])[C:7](=[O:13])[N:6]2[CH3:14].[C:15](Cl)(=[O:18])[CH2:16][CH3:17]>N1C=CC=CC=1>[CH3:12][N:8]1[C:9](=[O:11])[C:10]2=[C:2]([NH:1][C:15](=[O:18])[CH2:16][CH3:17])[O:3][N:4]=[C:5]2[N:6]([CH3:14])[C:7]1=[O:13]0.588 g of 3-amino-5,7-dimethyl-isoxazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione was dissolved in 15 ml of pyridine under heating. Then, 1.5 g of propionyl chloride was added to the solution, and the mixture was stirred at 80° C. for 4 hours. The solvent was removed from the reaction solution under reduced pressure. To the residue was added water, which was subjected to extraction with chloroform. The chloroform layer was taken and dried over anhydrous sodium sulfate and evaporated to dryness. The residue was chromatographed on a column of silica gel, and the column was eluted with chloroform:acetone:formic acid (20:1:0.1). The intended compound was recovered from the resultant eluate and recrystallized from isopropyl ether-acetone to give 0.345 g of colorless prisms, melting point: 150°-152° C.
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