[OH:1][C:2]1[CH:10]=[C:9]([OH:11])[CH:8]=[CH:7][C:3]=1[C:4]([OH:6])=[O:5].[C:12]1(O)[CH:17]=[CH:16][CH:15]=[CH:14][CH:13]=1.C1(C)C=CC=CC=1.S(Cl)(Cl)=O>CN(C)C=O>[OH:1][C:2]1[CH:10]=[C:9]([OH:11])[CH:8]=[CH:7][C:3]=1[C:4]([O:6][C:12]1[CH:17]=[CH:16][CH:15]=[CH:14][CH:13]=1)=[O:5]A 1.5 l sulfonating flask equipped with stirrer, condenser, dropping funnel and gas outlet is charged with 154.1 g (1.0 mol) of 2,4-dihydroxybenzoic acid, 141.2 g (1.5 mol) of phenol, 500 ml of toluene and 1 ml of dimethylformamide. 178.5 g (1.5 mol) of thionyl chloride are then added dropwise at an internal temperature of 100°-105° C. over a period of 2.5 hours. The reddish clear solution is then refluxed overnight (110°-15° C.). The toluene and the phenol are distilled off in vacuo, the highly viscous residue (271.5 g) is stirred in 300 ml of 7:3 toluene/cyclohexane, and the product is allowed to crystallize overnight. The precipitate is filtered off with suction while cold (10° C.) and washed with 3×50 ml of 73 toluene/cyclohexane. It is dried at 80° C. in vacuo to give 125.5 g (54.4% of theory) of phenyl 2,4-dihydroxybenzoate (m.p. 135°-137° C.).
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