Methyl 3,5-difluoromandelate was prepared following General Procedure G below and using commerically available 3,5-difluoromandelic acid. The resultant α-hydroxy methyl ester was fluorinated according to the general procedure described in W. J. Middleton, et al., Org. Synth. CoL Vol. VI, 835. Specifically, a solution of diethylaminosulfur trifluoride (1.1 eq) in CH2Cl2 was cooled to 0° C. and treated with methyl 3,5-difluoromandelate (1.0 eq) as a solution in CH2Cl2. After 10 min. the cooling bath was removed and the reaction was stirred at ambient temperature for 30 min. The reaction was monitored by tlc (Rf=0.65, 1:1 ethyl acetate/hexanes). The mixture was then poured onto ice and the layers separated. The organic phase was washed with saturated aqueous NaHCO3 and brine. The organic layer was dried over Na2SO4, filtered, and concentrated in vacuo, The product was purified by LC2000 chromatograpy (180 mL/min) using 10% EtoAc/hexanes as the eluent. The resulting methyl 3,5-difluorophenyl-α-fluoroacetate was hydrolyzed by dissolving the ester in 70% aqueous dioxane and treating with lithium hydroxide (2.0 eq.). No starting material remained by tlc after 2 h. The dioxane was removed via rotary evaporation. The aqueous mixture was first washed with ethyl acetate and then acidified with 0.01 N HCl. The aqueous layer was extracted with ethyl acetate. The organic phase was washed with brine, dried over Na2SO4, filtered, and concentrated. The crude solid was recrystallized from ethyl acetate/hexanes affording 3,5-difluorophenyl-α-fluoroacetic acid as a white solid having a melting point of 90-110° C.
