To crude 5-mercapto-8-methyl-1,3-diphenyl-1H,8H-pyrimido[4,5-d]pyrimidine-2,4,7-trione 24 (100 mg) obtained in Step 5 was added N,N-dimethylformamide (0.5 ml). Under a nitrogen atmosphere, potassium carbonate (44 mg) and methyl iodide 25 (20 μl) were successively added, and the mixture was stirred at room temperature for 3 hrs. Water was added, and the mixture was extracted with chloroform. The organic layer was washed with water and brine, and dried over anhydrous sodium sulfate. Anhydrous sodium sulfate was filtered off, and the filtrate was concentrated under reduced pressure. The residue was purified by column chromatography (hexane:ethyl acetate=4:1→3:1) to give 8-methyl-5-methylsulfanyl-1,3-diphenyl-1H,8H-pyrimido[4,5-d]pyrimidine-2,4,7-trione 26 (91 mg, yield 89%) as brown crystals.
