[S:1]1[CH2:6][CH2:5][CH2:4][S:3][C:2]1=[C:7]([C:19]1[CH:24]=[CH:23][CH:22]=[C:21]([C:25]([F:28])([F:27])[F:26])[CH:20]=1)[C:8]([O:10]CCCCCCCC)=[O:9].[OH-].[K+]>C(O)C.O>[S:1]1[CH2:6][CH2:5][CH2:4][S:3][C:2]1=[C:7]([C:19]1[CH:24]=[CH:23][CH:22]=[C:21]([C:25]([F:26])([F:27])[F:28])[CH:20]=1)[C:8]([OH:10])=[O:9]To a solution of n-octyl (1,3-dithian-2-ylidene)-(3-trifluoromethylphenyl)-acetate (15 g) in ethanol (375 ml), a 30% solution of potassium hydroxide in water (375 ml) is added. The mixture is refluxed for 2 hours. The solvents are removed in vacuo. Water (500 ml) and ether (500 ml) are added. The water solution is acidified to pH 4 with 5N sulphuric acid. The phases are separated and the water solution is extracted with ether (3×350 ml). The combined organic solutions are dried over magnesium sulphate and the solvent is evaporated in vacuo. The residue is triturated in petrol ether (b.p. 40°-65° ) and dried in vacuo to constant weight to give (1,3-dithian-2-ylidene)-(3-trifluoromethylphenyl)-acetic acid, m.p. 165°-166° C.
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