[OH:1][C:2]1[C:7](=[O:8])[N:6]([CH:9]([CH3:11])[CH3:10])[C:5](=[O:12])[N:4]2[CH2:13][CH2:14][NH:15][C:16](=[O:17])[C:3]=12.[CH3:18][Si](C=[N+]=[N-])(C)C>CO.CCOCC>[CH:9]([N:6]1[C:7](=[O:8])[C:2]([O:1][CH3:18])=[C:3]2[C:16](=[O:17])[NH:15][CH2:14][CH2:13][N:4]2[C:5]1=[O:12])([CH3:10])[CH3:11]To a solution of 9-hydroxy-7-isopropyl-3,4-dihydro-2H-pyrazino[1,2-c]pyrimidine-1,6,8(7H)-trione (1.0 g, 4.2 mmol) in MeOH (30 mL) was added trimethylsilyldiazomethane (7 mL of a 2.0 M solution in ether, 14 mmol). The mixture was stirred at ambient temperature for 24 h. The solvent was removed in vacuo and the residue was triturated in MeOH and the solid was collected by filtration to give the title compound as a solid. HPLC RT=2.07 min (Method A); ES MS (M+H)=254; 1H NMR (400 MHz, DMSO-d6) δ 8.64 (2, 1H), 5.03 (septet, J=7 Hz, 1H), 3.86 (m, 2H), 3.66 (s, 3H), 3.37 (m, 2H), 1.38 (d, J=7 Hz, 6H).
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