[C:1]1(=[O:19])[N:5]([CH2:6][CH2:7][CH2:8][CH2:9][CH2:10][C:11]([OH:13])=[O:12])[C:4](=[O:14])[C:3]2=[CH:15][CH:16]=[CH:17][CH:18]=[C:2]12.[Br:20]Br.OS([O-])=O.[Na+].C([O-])(O)=O.[Na+]>O>[Br:20][CH:10]([CH2:9][CH2:8][CH2:7][CH2:6][N:5]1[C:4](=[O:14])[C:3]2=[CH:15][CH:16]=[CH:17][CH:18]=[C:2]2[C:1]1=[O:19])[C:11]([OH:13])=[O:12]
Treat dropwise a mixture of 6-phthalimidocaproic acid (12.7 g, 48 mmol) and dry red phosphorous (1.95 g, 63 mmol) with bromine (12.7 mL, 246 mmol) at 0° C. Warm the resulting lumpy mixture to room temperature and heat to 80° C. for 3 hours. Cool the reaction mixture to room temperature, pour into water (300 mL) containing NaHSO3, and neutralize using solid NaHCO3. Wash the aqueous layer with diethyl ether (150 mL) and acidify with concentrated hydrochloric acid. Collect the pale yellow precipitate by filtration and dry to give 2-bromo-6-phthalimidocaproic acid (15 g, 91.5% yield).
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