[OH:1][C:2]1[CH:12]=[CH:11][C:5]([CH:6]([OH:10])[C:7]([OH:9])=[O:8])=[CH:4][CH:3]=1.[C:13]1(C)C=CC(S(O)(=O)=O)=C[CH:14]=1.C([O-])(O)=O.[Na+]>C(O)C.O>[OH:1][C:2]1[CH:12]=[CH:11][C:5]([CH:6]([OH:10])[C:7]([O:9][CH2:13][CH3:14])=[O:8])=[CH:4][CH:3]=1A mixture of 5 g of 4-hydroxymandelic acid and 1.28 g of p-toluenesulfonic acid in 200 ml of ethanol is heated at 50° C. for 4 hours. The mixture is poured into saturated aqueous NaHCO3 solution, and a white precipitate is formed. The mixture is diluted with water until the precipitate disintegrates, and then is extracted with diethyl ether. The organic phases are dried over sodium sulfate. Filtration and evaporation of the solvent give ethyl 4-hydroxymandelate (4.2 g, 82%) as a pale yellowish solid.
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