[F:1][C:2]1[CH:3]=[CH:4][C:5]2[O:10][C:9]([C:11]([OH:13])=[O:12])=[CH:8][C:7](=O)[C:6]=2[CH:15]=1.[H][H]>[Pd].C(O)(=O)C>[F:1][C:2]1[CH:3]=[CH:4][C:5]2[O:10][CH:9]([C:11]([OH:13])=[O:12])[CH2:8][CH2:7][C:6]=2[CH:15]=1A mixture of 63.4 parts of 6-fluoro-4-oxo-4H-1-benzopyran-2-carboxylic acid and 400 parts of acetic acid was hydrogenated at normal pressure and at room temperature with 3 parts of palladium-on-charcoal catalyst 10%. After the calculated amount of hydrogen was taken up, the catalyst was filtered off and the filtrate was evaporated. The residue was stirred in petroleumether. The product was filtered off and dried in vacuo at 70° C., yielding 49 parts (83%) of 6-fluoro-3,4-dihydro-2H-1-benzopyran-2-carboxylic acid (int. 1).
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