[C:1]([N:5]1[CH:9]=[C:8]([C:10](=[O:12])[CH3:11])[CH:7]=[N:6]1)([CH3:4])([CH3:3])[CH3:2].[BrH:13].BrBr.S([O-])(O)=O.[Na+]>ClCCl.C(O)(=O)C>[Br:13][CH2:11][C:10]([C:8]1[CH:7]=[N:6][N:5]([C:1]([CH3:4])([CH3:3])[CH3:2])[CH:9]=1)=[O:12]To a solution of 1.66 g (10.0 mmol) 1-(1-tert-butyl-1H-pyrazol-4-yl)-ethanone in 18 ml dichloromethane is added 5.86 μl (0.10 mmol) of a 32% solution of hydrobromic acid in acetic acid. Then a solution of 3.20 g (20 mmol) bromine in 5 ml dichloromethane is added dropwise. The reaction mixture is stirred for 3 days at room temperature. Then aqueous sodium hydrogen sulfite is added to reduce the excess bromine. The reaction mixture is reduced in volume under vacuum and it is then partitioned between water and ethyl acetate. The organic phase is dried over sodium sulfate and evaporated. The residue is chromatographed on a silica gel column with cyclohexane/ethyl acetate as eluent to afford 2-bromo-1-(1-tert-butyl-1H-pyrazol-4-yl)-ethanone as yellow oil, which crystallizes on standing; HPLC/MS 2.28 min (B), [M+H] 245/247.
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