A stirring mixture of 10 g of sodium bicarbonate in 50 ml dimethylformamide was treated with 4.0 g of 4-(2,3,4,5,6-pentafluorophenoxy)piperidine followed by a solution of 4.06 ml of 4,4-bis-(4-fluorophenyl)butyl chloride in 10 ml of dimethylformamide, and then stirred at 78° C. for 20 hours. After filtering, the filtrate was poured into water and extracted with ethyl acetate. The organic layer was washed with water followed by a saturated sodium chloride solution and dried over anhydrous magnesium sulfate. Filtration followed by evaporation of the volatiles afforded a residue which was purified by means of high pressure liquid chromatography (silica gel; elution with 20% ethyl acetate/dichloromethane) to afford 5.44 g of 1-[4,4-bis-(4-fluorophenyl)butyl]-4-(2,3,4,5,6-pentafluorophenoxy)piperidine as an oil. The oil was dissolved in hot isopropanol and acidified with an etheral solution of oxalic acid to precipitate the corresponding oxalate salt. Recrystallization from isopropanol afforded 2.75 g (30.0%) of 1-[4,4-bis-(4-fluorophenyl)butyl]-4-(2,3,4,5,6-pentafluorophenoxy)piperidine oxalate, m.p. 183°-185° C.
