[F:1][C:2]1[C:7]([F:8])=[CH:6][C:5]([F:9])=[C:4]([F:10])[C:3]=1[CH3:11].C([Li])CCC.[C:17](=[O:19])=[O:18].Cl>CCCCCC.C(OCC)C>[F:1][C:2]1[C:7]([F:8])=[C:6]([C:17]([OH:19])=[O:18])[C:5]([F:9])=[C:4]([F:10])[C:3]=1[CH3:11]The product of step (a) above (4.3 g) was mixed with diethyl ether (30 ml), the mixture cooled to -70° C., and maintained at this temperature whilst a solution of n-butyllithium in n-hexane (1.6 M, 16.4 ml) was slowly added. The mixture was stirred for a period of 1 hour during which time a fine white precipitate was formed. Dry carbon dioxide gas was then passed into the mixture for 30 minutes whilst the temperature was maintained within the range -70° to -40° C., and continued to be passed in thereafter whilst the mixture was allowed to warm to the ambient temperature. After acidifying with dilute hydrochloric acid (6 N, 40 ml) the organic phase was separated, washed with water and dried over anhydrous magnesium sulphate. After evaporation of the solvents under reduced pressure the residual solid was recrystallised from toluene to yield 2,3,5,6-tetrafluoro-4-toluic acid, m.p. 170° C. identified by infra red and nuclear magnetic resonance spectroscopy.
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