[CH3:1][O:2][C:3](=[O:16])[CH2:4][C:5]1[CH:10]=[CH:9][C:8]([Cl:11])=[CH:7][C:6]=1[C:12]([O:14][CH3:15])=[O:13].C1C(=O)N([Br:24])C(=O)C1.C(OOC(=O)C1C=CC=CC=1)(=O)C1C=CC=CC=1>C(Cl)(Cl)(Cl)Cl>[CH3:1][O:2][C:3](=[O:16])[CH:4]([C:5]1[CH:10]=[CH:9][C:8]([Cl:11])=[CH:7][C:6]=1[C:12]([O:14][CH3:15])=[O:13])[Br:24]18E (16.48 mmol) was dissolved in CCl4 (50 mL), and was added NBS (N-bromosuccinimide, 3.23 g, 18.13 mmol), followed by catalytic amount of BPO (benzoyl peroxide). The reaction mixture was refluxed for about 20 hours under stirring. When no more product was produced any more, the reaction mixture was cooled and filtered. To the filtrate was added NBS (1.6 g, 8.24 mmol) and catalytic amount of BPO. The reaction mixture was refluxed for additional 10 hours. After the reaction completed, the mixture was cooled and filtered. The filtrate was concentrated to give 6.4 g of a crude product (19A), which was used directly for the next step.
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