[C:1]([C:5]1[CH:15]=[C:14]([CH3:16])[C:8]([CH:9](O)[C:10]([OH:12])=[O:11])=[C:7]([CH3:17])[CH:6]=1)([CH3:4])([CH3:3])[CH3:2].C(O)(=O)C.C(C1C=C(C)C(C(O)C(O)=O)=C(C)C=1)(C)(C)C.Cl>C(O)(=O)C>[C:1]([C:5]1[CH:6]=[C:7]([CH3:17])[C:8]([CH2:9][C:10]([OH:12])=[O:11])=[C:14]([CH3:16])[CH:15]=1)([CH3:4])([CH3:3])[CH3:2]A mixture of 2.89 g of 4-tert-butyl-2,6-dimethylmandelic acid and 7.75 g of 4-tert-butyl-2,6-dimethylmandelic acid acetate, 4.5 g of 37% strength hydrochloric acid, 1.86 g of red phosphorus and 0.66 g of KI in 30 ml of glacial acetic acid is heated at 100° C. for 16 hours. Excess phosphorus is filtered off with suction and washed three times with in each case 10 ml of glacial acetic acid. The filtrate is substantially concentrated on a rotary evaporator at a bath temperature of 50° C./60 mbar. The resulting residue is diluted with 25 ml of water and, by addition of 10% strength aqueous sodium hydroxide solution, dissolved. This solution is extracted twice with in each case 20 ml of MTBE and then adjusted to pH 1 using 48% strength sulfuric acid. The resulting greasy solid is taken up in methylene chloride. This solution is extracted with 25 ml of water, dried over sodium sulfate and concentrated using a rotary evaporator. This gives 7.66 g of 4-tert-butyl-2,6-dimethylphenylacetic acid in a purity of 99.0 GC area % (yield about 86% of theory).
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