[O:1]=[C:2]1[N:6]([CH2:7][CH2:8][CH2:9][CH2:10][NH:11][C:12](=[O:18])[O:13][C:14]([CH3:17])([CH3:16])[CH3:15])[C:5](=[O:19])[NH:4][NH:3]1>C(Cl)Cl>[O:1]=[C:2]1[N:6]([CH2:7][CH2:8][CH2:9][CH2:10][NH:11][C:12](=[O:18])[O:13][C:14]([CH3:15])([CH3:16])[CH3:17])[C:5](=[O:19])[N:4]=[N:3]1To tert-butyl 4-(3,5-dioxo-1,2,4-triazolidin-4-yl)butylcarbamate (15 mg, 0.055 mmol) in CH2Cl2 (0.551 mL) was added SiO2—HNO3 (35 mg). The mixture was stirred at room temperature for 15 minutes, then filtered, concentrated in vacuo and dried, giving tert-butyl 4-(3,5-dioxo-3H-1,2,4-triazol-4(5H)-yl)butylcarbamate (I-8c: 15 mg, quantitative yield) as a red oil.
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