[C:1]([C:5]1[CH:6]=[C:7]([C:15]2[CH:23]=[CH:22][CH:21]=[C:20]3[C:16]=2[CH:17]=[CH:18][CH2:19]3)[CH:8]=[C:9]([C:11]([CH3:14])([CH3:13])[CH3:12])[CH:10]=1)([CH3:4])([CH3:3])[CH3:2].CS(C)=O.[Br:28]N1C(=O)CCC1=O.C1(C)C=CC(S(O)(=O)=O)=CC=1>O>[Br:28][C:18]1[CH2:19][C:20]2[C:16]([CH:17]=1)=[C:15]([C:7]1[CH:8]=[C:9]([C:11]([CH3:14])([CH3:13])[CH3:12])[CH:10]=[C:5]([C:1]([CH3:2])([CH3:3])[CH3:4])[CH:6]=1)[CH:23]=[CH:22][CH:21]=2A cold solution (5° C.) of compound 5 (49 g, 161 mmol, 1 equiv), dimethyl sulfoxide (500 mL) and water (5 mL) was treated in one portion with N-bromosuccinimide (43 g, 241 mmol, 1.5 equiv). The bath was removed and the reaction allowed to stir at room temperature overnight. The reaction was poured into water (1 L) and the mixture extracted with ethyl acetate (2×500 mL). The combined organic layers were washed with saturated brine (1 L), dried over sodium sulfate and concentrated under reduced pressure. The resulting residue was dissolved in toluene (500 mL) and p-toluenesulfonic acid (6.2 g, 32.6 mmol, 0.2 equiv) was added. The mixture was refluxed for 20 hours while removing water with a Dean-Stark trap. The reaction was cooled to room temperature and concentrated under reduced pressure. The resulting residue was purified over silica gel (1 Kg) eluting with heptanes to give compound 6 (41 g, 66% yield) as a white solid.
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