[Cl:1][C:2]1[C:11]2[C:6](=[CH:7][CH:8]=[CH:9][CH:10]=2)[O:5][CH:4]([C:12]2[CH:17]=[CH:16][CH:15]=[CH:14][CH:13]=2)[C:3]=1[CH:18]=[O:19].Cl([O-])=[O:21].[Na+].[Na]>C(O)(C)(C)C.CC(=CC)C.O>[Cl:1][C:2]1[C:11]2[C:6](=[CH:7][CH:8]=[CH:9][CH:10]=2)[O:5][CH:4]([C:12]2[CH:13]=[CH:14][CH:15]=[CH:16][CH:17]=2)[C:3]=1[C:18]([OH:21])=[O:19]To a stirred solution of 5.8 g 4-chloro-3-formyl-flav-3-ene in 500 ml tert-butanol and 100 ml 2-methylbut-2-ene at room temperature is added dropwise a solution of 17.9 g sodium chlorite and 17.9 g sodium dihydrogenophosphate in 200 ml water. Stirring is continued for 5 hours. The solution is concentrated in vacuo and water is added to the residue to give a suspension. The mixture is extracted with hexane and the clear organic extracts are discarded. Subsequently, the aqueous suspension is acidified to pH 3 and the mixture is extracted with chloroform. The combined chloroform extracts are washed with water, dried over magnesium sulfate, filtered and evaporated. The residual solid is recrystallized in a mixture of chloroform and hexane. Pure 4-chloro-3-carboxy-flav-3-ene is obtained as yellow crystals; m.p. 210° C.
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