[CH3:1][N:2]1[C:10](=[O:11])[C:9]2[NH:8][CH:7]=[N:6][C:5]=2[N:4]([CH2:12][O:13][C:14](=[O:19])[C:15]([CH3:18])([CH3:17])[CH3:16])[C:3]1=[O:20].C(=O)([O-])[O-].[K+].[K+].[CH2:27](Br)[C:28]#[C:29][CH3:30]>CN(C)C=O.C(OCC)(=O)C>[CH2:27]([N:8]1[C:9]2[C:10](=[O:11])[N:2]([CH3:1])[C:3](=[O:20])[N:4]([CH2:12][O:13][C:14](=[O:19])[C:15]([CH3:16])([CH3:17])[CH3:18])[C:5]=2[N:6]=[CH:7]1)[C:28]#[C:29][CH3:30]2,2-Dimethylpropionic acid [1-methyl-2,6-dioxo-1,2,6,7-tetrahydropurin-3-yl]methyl ester (1.871 g) was dissolved in N,N-dimethylformamide (30 ml), potassium carbonate (1.5 g) and 2-butynyl bromide (0.7 ml) were added to the solution, and the mixture was stirred at room temperature overnight. The reaction mixture was diluted with ethyl acetate, and washed with water and 1N hydrochloric acid. The organic layer was dried over anhydrous magnesium sulfate and filtered. Then, the solvent was removed by distillation. The residue was purified by silica gel column chromatography to give 2.12 g of the title compound from the elution fraction of hexane-ethyl acetate (3:2).
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