[CH2:1]([N:5]1[C:9]2=[N:10][C:11]3[N:12]([CH3:19])[C:13](=[O:18])[NH:14][C:15](=[O:17])[C:16]=3[N:8]2[CH:7]=[C:6]1[C:20]1[CH:25]=[CH:24][CH:23]=[CH:22][CH:21]=1)[CH2:2][CH2:3][CH3:4].[N+:26]([O-])([OH:28])=[O:27].O>C(O)(=O)C.CO>[CH2:1]([N:5]1[C:9]2=[N:10][C:11]3[N:12]([CH3:19])[C:13](=[O:18])[NH:14][C:15](=[O:17])[C:16]=3[N:8]2[C:7]([N+:26]([O-:28])=[O:27])=[C:6]1[C:20]1[CH:21]=[CH:22][CH:23]=[CH:24][CH:25]=1)[CH2:2][CH2:3][CH3:4]To a solution of commercially available 8-butyl-1-methyl-7-phenyl-1H-imidazo[2,1-f]purine-2,4(3H,8H)-dione (3, 100 mg, 0.296 mmol) in acetic acid (500 μL) was added HNO3 (fuming, 24 μL, 0.592 mmol), and the resulting mixture was stirred at room temperature for 12 h. Water was added, and the resulting precipitate was filtered off. Cristallization in methanol afforded the desired product as a yellow solid (26 mg, 23% yield). 1H NMR (400 MHz, DMSO-d6): δ=11.05 (s, 1H), 7.60-7.69 (m, 5H), 3.93 (t, J=7.3 Hz, 2H), 3.43 (s, 3H), 1.62 (quint, J=7.3 Hz, 2H), 1.14 (sext, J=7.3 Hz, 2H), 0.70 (t, J=7.3 Hz, 3H); 13C NMR (100 MHz, DMSO-d6): δ=153.4, 152.1, 150.5, 144.6, 134.4, 130.9, 130.7, 128.5, 126.9, 124.7, 101.6, 44.3, 29.5, 29.0, 18.6, 12.9; IR (film) ν=3172, 3052, 2964, 1693, 1502, 1356, 1294, 732, 577 cm−1; HRMS (ESI): m/z: calcd for C18H18N6O4Na+: 405.1287, found: 405.1288.
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