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A mixture of 6.5 g (41.1 mmol) of 4,5-difluoro-2-hydroxybenzaldehyde, 10.4 ml (61.7 mmol) of diethyl bromo-malonate, 11.3 g (82.2 mmol) of potassium carbonate and 50 ml of ethyl methyl ketone was boiled at reflux for 3 hours while stirring, filtered and concentrated in a vacuum. The brown oil obtained was dissolved in 65 ml of ethanol, treated with 6.5 g of potassium hydroxide pellets and heated at reflux for one hour while stirring. The mixture was concentrated in a vacuum and the residue was treated with 65 ml of water and 65 ml of 3N sulfuric acid and heated at reflux over 30 minutes. Subsequently, the mixture was filtered and the residue was washed with water and triturated in 50 ml of hexane over 30 minutes. The solid was filtered off and dried. There were obtained 4.95 g (60%) of 5,6-difluorobenzofuran-2-carboxylic acid as a light yellow solid with m.p. 270°.

1. temperatureat reflux for 3 hours
2. filtrationfiltered
3. concentrationconcentrated in a vacuum
4. customThe brown oil obtained
5. temperatureheated
6. temperatureat reflux for one hour
7. stirringwhile stirring
8. concentrationThe mixture was concentrated in a vacuum
9. additionthe residue was treated with 65 ml of water and 65 ml of 3N sulfuric acid
10. temperatureheated
11. temperatureat reflux over 30 minutes
12. filtrationSubsequently, the mixture was filtered
13. washthe residue was washed with water
14. customtriturated in 50 ml of hexane over 30 minutes
15. filtrationThe solid was filtered off
16. customdried