[F:1][C:2]1([F:20])[C:5]([O:6][C:7]2[CH:8]=[C:9]([CH:14]=[CH:15][C:16]=2[O:17][CH3:18])[C:10]([O:12]C)=[O:11])=[CH:4][C:3]1=[CH2:19].C(=O)([O-])[O-].[K+].[K+]>CO.O>[F:1][C:2]1([F:20])[C:5]([O:6][C:7]2[CH:8]=[C:9]([CH:14]=[CH:15][C:16]=2[O:17][CH3:18])[C:10]([OH:12])=[O:11])=[CH:4][C:3]1=[CH2:19]A solution of methyl 3-(4,4-difluoro-3-methylenecyclobut-1-enyloxy)-4-methoxybenzoate (1.39 g) in methanol (22 mL) is treated with a solution of potassium carbonate (0.83 g) in water (8 mL) and the solution is heated at 60-70° C. for 8 hours, cooled and evaporated. The residue is dissolved in water (30 mL), extracted with diethyl ether (50 mL) and acidified to pH 4 by treatment with glacial acetic acid. The precipitated solid is extracted with ethyl acetate (2×75 mL). The combined organic extracts are dried over magnesium sulfate and concentrated, to give 3-(4,4-difluoro-3-methylene-cyclobut-1-enyloxy)-4-methoxybenzoic acid, in the form of a white solid, m.p. 163-165° C.
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