[CH3:1][N:2]1[C:7](=[O:8])[C:6]2[CH2:9][N:10]([CH2:13][CH2:14][CH2:15][CH2:16][O:17]C3CCCCO3)[CH2:11][CH2:12][C:5]=2[NH:4][C:3]1=[O:24].O.Cl>CO>[OH:17][CH2:16][CH2:15][CH2:14][CH2:13][N:10]1[CH2:11][CH2:12][C:5]2[NH:4][C:3](=[O:24])[N:2]([CH3:1])[C:7](=[O:8])[C:6]=2[CH2:9]112 g of 3-methyl-6-[4-(tetrahydropyran-2-yloxy)butyl]-5,6,7,8-tetrahydro-1H-pyrido[4,3-d]pyrimidine-2,4-dione were dissolved in 150 ml of methanol and 150 ml of water and the pH was adjusted to i with conc. hydrochloric acid at 20°-25° C., with stirring. After 7 h at room temperature, the mixture was concentrated to dryness under reduced pressure, the residue was taken up with water and the solution was adjusted to pH 7.8. It was concentrated under vacuum and the residue was extracted exhaustively with hot ethanol. The ethanolic extract was evaporated under reduced pressure and the crude 6-(4-hydroxybutyl)-3-methyl-5,6,7,8-tetrahydro-1H-pyrido-[4,3-d]pyrimidine-2,4-dione was purified by column chromatography on silica gel with dichloromethane/methanol (volume ratio 90:10) and by recrystallization from isopropanol.
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