[NH2:1][C:2]1[NH:6][N:5]=[C:4]2[C:7]([CH3:18])([CH3:17])[N:8]([C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[CH2:9][C:3]=12.C(N(CC)C(C)C)(C)C.Cl[C:29]([O:31][CH2:32][CH3:33])=[O:30]>C1COCC1.CCOC(C)=O>[NH2:1][C:2]1[N:6]([C:29]([O:31][CH2:32][CH3:33])=[O:30])[N:5]=[C:4]2[C:7]([CH3:18])([CH3:17])[N:8]([C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[CH2:9][C:3]=1215 g of tert-Butyl 3-amino-6,6-dimethyl-2,6-dihydropyrrolo[3,4-c]pyrazole-5(4H)-carboxylate (59.4 mmol) were dissolved in anhydrous THF (150 ml) and treated, at 0° C. under Ar atmosphere, first with N,N-diisopropylethylamine (50 ml) and then with ClCO2Et (4.65 ml, 1 eq.) dropwise. 90 minutes later, the solvent was diluted with EtOAc (1 l), washed with water and then with brine, dried over sodium sulfate and evaporated. The crude product was purified by flash chromatography (hexane/EtOAc 2/8) to afford 7.3 g of 5-tert-butyl 2-ethyl 3-amino-6,6-dimethylpyrrolo[3,4-c]pyrazole-2,5(4H,6H)-dicarboxylate as the major compound in 38% yield, together with 5.7 g of 5-tert-butyl 1-ethyl 3-amino-6,6-dimethyl-4,6-dihydropyrrolo[3,4-c]pyrazole-1,5-dicarboxylate in 30% yield.
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