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2,3-Difluoro-4-methyl-benzoic acid methyl ester (7a): 2,3-Difluoro-4-methyl-benzoic acid (10.0 g, 58.1 mmol) was dissolved in MeOH (200 mL, 6.0 mol) and sulfuric acid (1.00 mL, 19.0 mmol), and was heated at reflux overnight. The mixture was then cooled and concentrated under reduced pressure. The resulting residue was taken up in EtOAc and washed with saturated aqueous NaCl, dried over MgSO4, filtered and concentrated to yield 9.0 g of intermediate (7a) as a white solid. 1H-NMR (CDCl3): 7.61 (1H, t), 7.00 (1H, t), 3.93 (3H, s), 2.35 (3H, s).
With dmap; In tert-butyl alcohol; at 45℃; for 5.0h;
0253] 7-Fluoro-benzo [dlisothiazole-6-carboxylic acid[0254] Step 1: 2.3-Difluoro-4-methyl-benzoic acid tert-butyl ester[0255] A mixture of <strong>[261763-37-5]2,3-difluoro-4-methyl-benzoic acid</strong> (20.0 g, 116 mmol), -tert- butyl dicarbonate (25.0 g, 116 mmol) and DMAP (2.0 g, 16.4 mmol) in tert-butanol (500 mL) was stirred at 45C for 5 hours before being concentrated in vacuo. The resultant residue was triturated in Et2O and filtered. The filtrate was concentrated in vacuo to give a residue which was partitioned between ethyl acetate and a IM aqueous solution of hydrochloric acid. The organic layer was separated and washed with a saturated aqueous solution of sodium hydrogen carbonate followed by brine, dried (Na2SO4), filtered and evaporated in vacuo to give the title compound as a colourless oil (17.3 g, 65%). 1H NMR (CDCl3, 400 MHz) 7.51 (1 H, ddd, J = 8.3, 6.6, 1.9 Hz), 6.95 (1 H, m), 2.33 (3 H, d, J = 2.3 Hz), 1.59 (9 H, s).
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