[Br:1][CH2:2][CH2:3][CH2:4][CH2:5][C:6]1[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=1.[C:12](Cl)(=[O:14])[CH3:13].[Cl-].[Al+3].[Cl-].[Cl-].[C:20](C1C=CC=CC=1)(=[O:22])C.Br[O-].[Na+].C(O)(=O)C1C=CC=CC=1>C(O)C>[Br:1][CH2:2][CH2:3][CH2:4][CH2:5][C:6]1[CH:11]=[CH:10][C:9]([C:20]([O:14][CH2:12][CH3:13])=[O:22])=[CH:8][CH:7]=1In this case, 4-bromobutylbenzene is acylated with acetyl chloride and aluminum chloride; the resulting acetophenone is oxidized with sodium hypobromite to the benzoic acid, and this compound is esterified with ethanol and mineral acid to afford ethyl 4-(4-bromobutyl)benzoate. This ester is treated with the anion prepared from methyl methylthiomethyl sulfoxide and a strong base such as sodium hydride. The resulting dimethylmercaptal S-oxide is hydrolyzed with acid catalysis to the aldehyde intermediate XXI.
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