[CH2:1]([O:3][P:4]([CH2:9][C:10]1[CH:15]=[CH:14][C:13]([C:16](=[O:18])[CH3:17])=[CH:12][CH:11]=1)(=[O:8])[O:5][CH2:6][CH3:7])[CH3:2].[Br-:19].[Br-].[Br-].C([N+](CCCC)(CCCC)CCCC)CCC.C([N+](CCCC)(CCCC)CCCC)CCC.C([N+](CCCC)(CCCC)CCCC)CCC.Cl>C(Cl)Cl.CO>[CH2:6]([O:5][P:4]([CH2:9][C:10]1[CH:11]=[CH:12][C:13]([C:16](=[O:18])[CH2:17][Br:19])=[CH:14][CH:15]=1)(=[O:8])[O:3][CH2:1][CH3:2])[CH3:7]
To a stirred solution of (4-acetylbenzyl)-phosphonic acid diethyl ester (2.56 g, 9.48 mmol) in DCM (100 ml) and MeOH (40 ml) at room temperature was added tetra-n-butylammonium tribromide (5.03 g, 10.43 mmol) followed by a drop of concentrated hydrochloric acid. The reaction mixture was stirred at room temperature for 18 h. The solvents were then removed under reduced pressure and the residue was partitioned between Et2O (100 ml) and sat. NaHCO3 (60 ml). The aqueous phase was separated and the organic phase was washed with brine (60 ml) and dried (Na2SO4). The solvent was removed under reduced pressure to give a pale yellow oil which was purified by flash chromatography (3:1 EtOAc/DCM) to give the title compound (2.01 g, 61%) as a colourless oil.
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