Synthesis
Example 2 Synthesis of methyl 6-fluoro-1-benzopyran-2-carboxylate: 20 g of 6-fluoro-4-oxo-4H-1-benzopyran-2-carboxylic acid (96 mmol) was mixed with 2 g of methanesulfonic acid (20 mmol) and 2 g of 5% Pd/C catalyst (Escat 112 type, Engelhard) in THF at 45-55°C under 4 bar of hydrogen pressure The hydrogenation reaction was carried out until the consumption of hydrogen stopped. The reaction progress was monitored by TLC. Upon completion of the reaction, the catalyst was removed by filtration and the reaction mixture was concentrated under reduced pressure to 40 ml. Subsequently, 100 ml of methanol was added and the mixture was stirred at 20-25 °C until the TLC assay showed less than 1% of 6-fluoro-3,4-dihydro-2H-chromene-2-carboxylic acid. Next, the solution was concentrated under reduced pressure, the residue was dissolved in 60 ml of THF and concentrated again under reduced pressure and the water was removed by azeotropic distillation. Finally 20 g of methyl 6-fluorochroman-2-carboxylate was obtained (yield: 100%). δH (400 MHz; CDCl3) 6.89-6.79 (2H, m, Ar), 6.77-6.76-6.72 (1H, m, Ar), 4.73-4.69 (1H, m), 3.79 (3H, s), 2.87-2.69 (2H, m) , 2.31-2.12 (2H, m).
