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Diethyl 1-(3-tert-butoxycarbonylaminoethyl)-1H-pyrazole-3,5-dicarboxylate (2.4 g, 6.7 mmol) was dissolved in a dioxane solution (25 mL) of 4 N hydrochloric acid under nitrogen atmosphere. The reaction mixture was stirred at room temperature for 1 h before being alkalized with saturated sodium carbonate solution and extracted with dichloromethane. The organic phase was separated, washed with saturated saline, dried over anhydrous sodium sulfate, filtered and the solvent concentrated under reduced pressure. The crude product was purified by fast column chromatography (silica gel as stationary phase, eluent dichloromethane/methanol solution containing 7 M ammonia, gradient from 0/100 to 5/95). The target fraction was collected and the solvent was removed under reduced pressure to afford ethyl 4-oxo-4,5,6,7-tetrahydropyrazolo[1,5-a]pyrazine-2-carboxylate (1.1 g, 79% yield) as a white solid. The molecular formula of the compound is C9H11N3O3 and the LCMS analysis resulted in a retention time of 0.49 min and a mass-to-charge ratio of m/z 210 [M+H]+ (refer to LCMS method 1).

[1] Patent: WO2012/78817, 2012, A1. Location in patent: Page/Page column 140
[2] Patent: WO2012/83224, 2012, A1. Location in patent: Page/Page column 266
[3] Patent: WO2013/192350, 2013, A1. Location in patent: Paragraph 00591
[4] Bioorganic and Medicinal Chemistry Letters, 2016, vol. 26, # 2, p. 429 - 434
[5] Patent: US2015/361081, 2015, A1. Location in patent: Paragraph 0480