Cl[C:2](OC1C=CC([N+]([O-])=O)=CC=1)=[O:3].[C:14]([O:18][CH2:19][CH3:20])(=[O:17])[NH:15][NH2:16].C(N(CC)CC)C.[C:28]([NH:35][CH2:36][CH2:37][CH2:38][CH2:39][NH2:40])([O:30][C:31]([CH3:34])([CH3:33])[CH3:32])=[O:29]>C1COCC1.O>[CH3:34][C:31]([CH3:32])([CH3:33])[O:30][C:28](=[O:29])[NH:35][CH2:36][CH2:37][CH2:38][CH2:39][NH:40][C:2](=[O:3])[NH:16][NH:15][C:14]([O:18][CH2:19][CH3:20])=[O:17]To 4-nitrophenyl chloroformate (1.008 g, 5.00 mmol) in THF (25.00 mL) at 0° C. was added ethyl carbazate (0.521 g, 5 mmol) and triethylamine (1.525 mL, 11.00 mmol). The mixture was stirred at 0° C. for 1 hour. Then N-Boc-1,4-butanediamine (1.148 mL, 6.00 mmol) was added. The mixture was stirred at room temperature for 1 hour. Water was then added, extracted twice with ethyl acetate, dried over MgSO4, filtered, and concentrated in vacuo. The crude product was purified by silica gel chromatography (0-10% MeOH/CH2Cl2) giving ethyl 13,13-dimethyl-4,11-dioxo-12-oxa-2,3,5,10-tetraazatetradecan-1-oate (I-8a: 1.045 g, 66%) as a white solid. LC-MS (M+1) 319.1, t=0.92 minute.
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