[OH-].[Na+].[S:3]1[CH2:8][CH2:7][CH2:6][S:5][C:4]1=[C:9]([C:16]1[CH:21]=[CH:20][C:19]([OH:22])=[CH:18][CH:17]=1)[C:10]1[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=1.Cl[CH2:24][C:25]([OH:27])=[O:26].Cl>O>[C:25]([CH2:24][O:22][C:19]1[CH:18]=[CH:17][C:16]([C:9](=[C:4]2[S:5][CH2:6][CH2:7][CH2:8][S:3]2)[C:10]2[CH:11]=[CH:12][CH:13]=[CH:14][CH:15]=2)=[CH:21][CH:20]=1)([OH:27])=[O:26]To a solution of sodium hydroxide (1.3 g) in water (30 ml), (1,3-dithian-2-ylidene)-(4-hydroxyphenyl)-phenylmethane (3.0 g) and chloroacetic acid (1.4 g) are added. The mixture is refluxed for 3 hours. The water is slowly acidified with concentrated hydrochloric acid to pH 3. The aqueous phase is extracted with water (3×20 ml). The combined organic phases are washed with water (2×10 ml) and dried over magnesium sulphate. The solvent is evaporated in vacuo. The residue is purified by column chromatography on silicagel using dichloromethane/formic acid (99:1) as eluant. The fractions containing the pure product are washed with water to neutrality, dried over magnesium sulphate and evaporated. The residue is dried in vacuo to constant weight to give (4-carboxymethoxyphenyl)-(1,3-dithian-2-ylidene)-phenylmethane, m.p. 176°-177°.
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