[CH3:1][CH:2]1[CH:7]([C:8]2[CH:13]=[CH:12][C:11]([O:14]C)=[CH:10][CH:9]=2)[CH2:6][CH:5]([CH3:16])[CH2:4][NH:3]1.Br.N>C(O)(=O)C>[CH3:1][CH:2]1[CH:7]([C:8]2[CH:9]=[CH:10][C:11]([OH:14])=[CH:12][CH:13]=2)[CH2:6][CH:5]([CH3:16])[CH2:4][NH:3]1A solution of 2.2 g (10 mmol) of 2,5-dimethyl-3-(4-methoxyphenyl)piperidine in 15 ml of a 45% solution of hydrogen bromide in acetic acid was heated at 95° C. for 1 hour. After the addition of another 5 ml of the 45% solution of hydrogen bromide in acetic acid the mixture was heated at 95° C. for a further 90 minutes. After cooling, the reaction mixture was poured on ice, rendered basic with concentrated ammonia and extracted twice with dichloromethane. The organic layer was dried on sodium sulphate, filtered and evaporated under reduced pressure. In this manner 1.7 g (83%) of substantially pure 2,5-dimethyl-3-(4-hydroxyphenyl)piperidine were obtained.
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