[C:1]1([CH:7]([S:12][C:13]2[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=2)[CH2:8][C:9](O)=O)[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=1.P(Cl)(Cl)([Cl:21])=O.C([O-])(=O)C.[Na+].CN(C)[CH:31]=[O:32]>>[Cl:21][C:9]1[C:18]2[C:13](=[CH:14][CH:15]=[CH:16][CH:17]=2)[S:12][CH:7]([C:1]2[CH:6]=[CH:5][CH:4]=[CH:3][CH:2]=2)[C:8]=1[CH:31]=[O:32]A mixture of 1.3 g 3-phenyl-3-phenylmercapto-propionic acid and 4ml phosphorous oxychloride is stirred under nitrogen at 70° C. for four hours. The reaction mixture is then cooled to 4° C. with an ice bath and 4 ml dimethylformamide is added. Stirring is maintained at room temperature for five hours. After cooling with an ice bath, 30 ml of a cooled saturated sodium acetate aqueous solution is added. After a few minutes stirring, the reaction mixture is extracted with methylene chloride, the organic layer washed with a saturated sodium bicarbonate aqueous solution then with water, dried over magnesium sulfate, filtered and evaporated to dryness. The residual oil is purified by column chromatography and pure yellow 4-chloro-3-formyl-thioflav-3-ene is obtained which crystallised after a few days; m.p. 63°-65° C.
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