[Br:1][C:2]1[C:6]([N+:7]([O-:9])=[O:8])=[C:5]([Br:10])[NH:4][N:3]=1.[H-].[Na+].[C:13]([O:17][C:18](=[O:23])[NH:19][CH2:20][CH2:21]Br)([CH3:16])([CH3:15])[CH3:14]>CN(C=O)C>[C:13]([O:17][C:18](=[O:23])[NH:19][CH2:20][CH2:21][N:3]1[C:2]([Br:1])=[C:6]([N+:7]([O-:9])=[O:8])[C:5]([Br:10])=[N:4]1)([CH3:16])([CH3:15])[CH3:14]A solution of 3,5-dibromo-4-nitropyrazole (2) (16.0 g, 59 mmol) in DMF (130 ml) was added dropwise over 20 min to a stirred solution of NaH (2.6 g, 65 mmol; 60% dispersion in oil washed beforehand with hexane under an inert atmosphere) in DMF (80 ml). After stirring for 10 min, a solution of (2-bromoethyl)carbamic acid tert-butyl ester (19.85 g, 88.5 mmol; obtained by reaction of 2-bromoethylamine with di(tert-butyl)dicarabonate) in DMF (35 ml) was added dropwise over 10 min. The reaction mixture was heated at 80° C. for 3 h and then the DMF was evaporated under reduced pressure. A mixture of DCM/water (200 ml, 1/1) was added to the residue and the organic phase was washed with water (100 ml). The organic phase was dried over Na2SO4 and the solvent was evaporated under reduced pressure. The [2-(3,5-dibromo-4-nitropyrazol-1-yl)ethyl]carbamic acid tert-butyl ester (3.6 g, 15%) was obtained in the form of a pale yellow solid.
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