[CH3:1][NH:2][C:3](=[O:11])[C:4]1[CH:9]=[CH:8][CH:7]=[N:6][C:5]=1[NH2:10].[C:12](=O)(OC1C=CC=CC=1)[O:13]C1C=CC=CC=1>CN(C)C1C=CN=CC=1.CO>[CH3:1][N:2]1[C:3](=[O:11])[C:4]2[CH:9]=[CH:8][CH:7]=[N:6][C:5]=2[NH:10][C:12]1=[O:13]10.0 g (66 mmol) of 2-aminonicotinic acid methyl amide, 21.4 g (100 mmol) diphenyl carbonate and 8.5 g (66 mmol) 4-dimethylaminopyridine were melted at 140 to 150° C. The reaction was allowed to continue for 20 minutes at that temperature. The mixture is allowed to cool down to 50 to 60° C. and 300 ml methanol was added while stirring. The mixture is allowed to cool down to room temperature. 3-methyl-1H-pyrido[2,3-d]pyrimidine-2,4-dione crystallized from the medium, was isolated by filtration and washed with water, methanol and tert.-butyl methyl ether. 7.80 g (67%) of 3-methyl-1H-pyrido[2,3-d]pyrimidine-2,4-dione was isolated (m.p. 272-4° C.).
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