[O:1]=[C:2]1[NH:16][C:15](=[O:17])[C:4]2([CH2:7][N:6](C(OC(C)(C)C)=O)[CH2:5]2)[NH:3]1.[ClH:18]>O1CCOCC1>[CH2:5]1[C:4]2([C:15](=[O:17])[NH:16][C:2](=[O:1])[NH:3]2)[CH2:7][NH:6]1.[ClH:18]tert-Butyl 6,8-dioxo-2,5,7-triazaspiro[3.4]octane-2-carboxylate (Preparation 18A, 620 mg, 2.57 mmol) was dissolved in 15 mL of hot dioxane until a homogeneous yellow solution formed. To this solution was added 30 mL of 4 M hydrochloric acid in dioxane. The reaction was stirred for 3 h and then concentrated in vacuo to give 2,5,7-triazaspiro[3.4]octane-6,8-dione, hydrochloric acid (420 mg, 92%). MS (M+1)=142.0. 1H NMR (400 MHz, DMSO-d6) δ ppm 10.9 (1H, s), 9.30 (2H, br d), 8.70 (1H, s), 4.15 (1H, br s), 4.05 (1H, br s), 3.39 (2H, br s).
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