[Li+].CCC[CH2-].C(NC(C)C)(C)C.[Cl:13][C:14]1[CH:19]=[CH:18][N:17]=[CH:16][C:15]=1[F:20].[C:21](=[O:23])=[O:22]>C1COCC1>[Cl:13][C:14]1[C:19]([C:21]([OH:23])=[O:22])=[CH:18][N:17]=[CH:16][C:15]=1[F:20]
N-Butyllithium, 1.6 M in hexanes (9.50 mL, 15.21 mmol) was added to THF (20 mL) and cooled to −78° C. To this solution was added diisopropylamine (2.167 mL, 15.21 mmol) followed by 4-chloro-3-fluoropyridine (2 g, 15.21 mmol) and the solution stirred for 6 h at −78° C. The reaction mixture was then poured on to crushed dry ice and stirred until the reaction mixture warmed to room temperature. The reaction was quenched by addition of aqueous ammonium chloride solution and acidified to pH=2 using conc. HCl and extracted with ethyl acetate (3×10 mL). The combined organic layers were washed with brine (10 mL), dried over sodium sulphate and concentrated under reduced pressure to afford the 4-chloro-5-fluoronicotinic acid (1.7 g, 9.68 mmol, 64% yield) as a pale yellow solid. LCMS (ESI) m/e 175.9 [(M+H)+, calcd for C6H4ClFNO2 176.0]; LC/MS retention time (Method F): tR=0.57 min.
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