[C:1](Cl)(=[O:5])[O:2][CH2:3][CH3:4].[NH2:7][C:8]1[C:9]2[CH2:15][N:14]([C:16]([O:18][C:19]([CH3:22])([CH3:21])[CH3:20])=[O:17])[CH2:13][C:10]=2[NH:11][N:12]=1.C(N(C(C)C)CC)(C)C>O1CCCC1>[NH2:7][C:8]1[C:9]2[CH2:15][N:14]([C:16]([O:18][C:19]([CH3:22])([CH3:21])[CH3:20])=[O:17])[CH2:13][C:10]=2[N:11]([C:1]([O:2][CH2:3][CH3:4])=[O:5])[N:12]=1A solution of ethyl chlorocarbonate (8.9 ml, 93 mmol) in tetrahydrofuran (THF, 250 ml) was slowly added to a mixture of tert-butyl 3-amino-4,6-dihydropyrrolo[3,4-c]pyrazole-5(1H)-carboxylate (20 g, 89 mmol) and diisopropylethylamine (DIEA, 92 ml, 528 mmol) in THF (500 ml) at 0-5° C. The reaction was kept at the same temperature for two hours then allowed to reach room temperature and stirred overnight. The obtained mixture was evaporated to dryness under vacuum, and the resulting residue extracted with ethyl acetate (AcOEt) and water. The organic layer was separated, dried over sodium sulfate and evaporated to dryness. The mixture was purified by flash-chromatography (eluent: ethyl acetate/cyclohexane 4/6 to 7/3) to give 19 g of the title compound as a white solid. [M+H]+ 297
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