[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].[CH3:18]S(O)(=O)=O>C1COCC1.[Pd]>[CH3:18][O:13][C:11]([CH:9]1[CH:8]=[CH:7][C:6]2[CH:15]=[C:2]([F:1])[CH:3]=[CH:4][C:5]=2[O:10]1)=[O:12]20 g of 6-fluoro-4-oxo-4H-1-benzopyran-2-carboxylic acid (96 mmol) were hydrogenated under 4 bars of hydrogen in 200 ml THF with 2 g of methanesulfonic acid (20 mmol) and 2 g of 5% Pd on charcoal (Type Escat 112, Engelhard) at 45-55° C. until consumption of hydrogen was no longer observed. The reaction progress was monitored by TLC. The catalyst was then removed by filtration and the reaction mixture concentrated to 40 ml under reduced pressure. 100 ml of methanol were then added and the mixture (solution) was stirred at 20-25° C. until less than 1% of 6-fluoro-3,4-dihydro-2H-chromen-2-carboxylic acid was detected (TLC). The solution was then concentrated under reduced pressure and the residue was dissolved in 60 ml of THF then concentrated under reduced pressure to remove water by azeotropic distillation to give 20 g of the title compound (yield: 100%).
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