C([Li])CCC.[Cl:6][C:7]1[CH:8]=[C:9]2[C:13](=[CH:14][C:15]=1[F:16])[NH:12][CH:11]=[CH:10]2.[C:17](=[O:19])=[O:18].O>CCCCCC.C1COCC1>[Cl:6][C:7]1[CH:8]=[C:9]2[C:13](=[C:14]([C:17]([OH:19])=[O:18])[C:15]=1[F:16])[NH:12][CH:11]=[CH:10]2
1.56 ml (2.49 mmol) of a 1.6 M butyl lithium solution in hexane were diluted with 3.2 ml THF at −78° C. under nitrogen. 211 mg (1.24 mmol) of 5-chloro-6-fluoro-1H-indole as a 3:1 mixture of regioisomers dissolved in 0.8 ml THF were added within 15 min keeping the temperature below −70° C. After 5 min 279 mg (2.49 mmol) of potassium-tert-butylate dissolved in 1 ml THF were added within 15 min. The reaction mixture was stirred for 2 h at −75° C. to give a clear orange solution. Solid carbon dioxide was added at that temperature and the reaction mixture was allowed to warm to rt within 30 min. Water was added and the reaction mixture was extracted twice with diethyl ether. The aqueous layer was acidified with 1N aqueous HCl solution and extracted twice with diethyl ether. The combined organic layers were washed with saturated aqueous NaCl solution, dried over sodium sulfate, filtered and the solvent was evaporated. The residue was stirred with n-hexane and a trace of diethyl ether for 2 h and filtered to yield 121 mg (46 of 5-chloro-6-fluoro-1H-indole-7-carboxylic acid as a light brown solid. 1H NMR (DMSO-d6, 300 MHz): δ 13.6 (br, 1H), 11.3 (br, 1H), 8.00 (d, 1H), 7.42 (s, 1H), 6.52 (s, 1H).
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