[CH3:1][C:2]([CH3:22])([CH3:21])[O:3][C:4](=[O:20])[NH:5][CH2:6][CH2:7][CH2:8][CH2:9][NH:10][C:11](=[O:19])[NH:12][NH:13][C:14](OCC)=[O:15].C([O-])([O-])=O.[K+].[K+]>CCO>[O:19]=[C:11]1[N:10]([CH2:9][CH2:8][CH2:7][CH2:6][NH:5][C:4](=[O:20])[O:3][C:2]([CH3:1])([CH3:22])[CH3:21])[C:14](=[O:15])[NH:13][NH:12]1To ethyl 13,13-dimethyl-4,11-dioxo-12-oxa-2,3,5,10-tetraazatetradecan-1-oate (I-8a: 1.045 g, 3.28 mmol) in EtOH (13.13 mL) was added K2CO3 (1.815 g, 13.13 mmol). The mixture was stirred at 65° C. for 16 hours. Cooled to room temperature, filtered, and then 4N HCl in dioxane was added up to pH or approximately 4. The white solid was filtered, rinsed with some methanol, and then the filtrate was concentrated in vacuo. The residue was purified by silica flash chromatography (0-10% MeOH/CH2Cl2) giving tert-butyl 4-(3,5-dioxo-1,2,4-triazolidin-4-yl)butylcarbamate (388 mg, 43%) as a oily white solid. LC-MS (M−1) 271.1, t=0.73 minute. 1H NMR (400 MHz, CD3CN) δ ppm 1.39 (s, 9 H) 1.42-1.49 (m, 2 H) 1.52-1.65 (m, 2 H) 3.03 (q, J=6.40 Hz, 2 H) 3.42 (t, J=6.95 Hz, 2 H).
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