[CH3:1][O:2][C:3]1[CH:4]=[C:5]([C:11]#[C:12][C:13]2[NH:21][C:20]3[C:19](=[O:22])[N:18]([CH2:23][C:24]#[CH:25])[C:17](=[O:26])[N:16]([CH2:27][CH3:28])[C:15]=3[N:14]=2)[CH:6]=[CH:7][C:8]=1[O:9][CH3:10].[C:29](=O)([O-])[O-].[K+].[K+].CI.ClCCl.CO>CN(C)C=O>[CH3:1][O:2][C:3]1[CH:4]=[C:5]([C:11]#[C:12][C:13]2[N:21]([CH3:29])[C:20]3[C:19](=[O:22])[N:18]([CH2:23][C:24]#[CH:25])[C:17](=[O:26])[N:16]([CH2:27][CH3:28])[C:15]=3[N:14]=2)[CH:6]=[CH:7][C:8]=1[O:9][CH3:10]A mixture of 2.0 g (5.3 mmol) of 8-(3,4-dimethoxyphenylethynyl)-3-ethyl-1-prop-2-ynyl-3,7-dihydropurine-2,6-dione, potassium carbonate (0.83 g, 6 mmol) and methyl iodide (0.6 ml, 10 mmol) in dimethylformamide (20 ml) was stirred at room temperature for 0.5 h (TLC-control: dichloromethane/methanol, 9.5:0.5). The product was precipitated by addition of water (100 ml), filtered under reduced pressure, was washed with water and dried at 70° C. Further purification was achieved by column chromatography on silica gel (eluent: dichloromethane/methanol, 9.5:0.5) and subsequent crystallization from dichloromethane/petroleum ether. Yield: 1.76 g, 80% (crude product); m.p.: 221.5° C.
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