[F:1][C:2]1[CH:7]=[CH:6][CH:5]=[C:4]([O:8][CH3:9])[CH:3]=1.[Cl-].[Al+3].[Cl-].[Cl-].[C:14]([N:17]1[CH2:22][CH2:21][CH:20]([C:23](Cl)=[O:24])[CH2:19][CH2:18]1)(=[O:16])[CH3:15].Cl>ClCCCl>[C:14]([N:17]1[CH2:18][CH2:19][CH:20]([C:23](=[O:24])[C:7]2[CH:6]=[CH:5][C:4]([O:8][CH3:9])=[CH:3][C:2]=2[F:1])[CH2:21][CH2:22]1)(=[O:16])[CH3:15]A mixture of 155 parts of 1-fluoro-3-methoxybenzene, 75 parts of aluminium chloride and 650 parts of 1,2-dichloroethane is stirred and 113 parts of 1-acetyl-4-piperidinecarbonyl chloride are added portionwise. Upon completion, stirring is continued for 1 hour at 40°-50° C. The reaction mixture is poured onto a mixture of crushed ice and hydrochloric acid. The product is extracted with methylbenzene. The extract is dried, filtered and evaporated. The residue is purified by high pressure liquid-chromatography over silica gel using a mixture of trichloromethane, hexane and methanol (47.5:47.5:5 by volume) as eluent. The first fraction (A-isomer) is collected and the eluent is evaporated, yielding 1-acetyl-4-(2-fluoro-4-methoxybenzoyl)-piperidine. The second fraction (B-isomer) is collected and the eluent is evaporated, yielding 40 parts (25%) of 1-acetyl-4-(4-fluoro-2-hydroxybenzoyl)piperidine.
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