Efficient regioselective labelling of the CFC alternative 1,1,1,2-tetrafluoroethane (HFC-134a) with fluorine-18

Efficient chemistry is described for the regioselective labelling of the CFC alternative 1,1,1,2-tetrafluoroethane with cyclotron-produced positron-emitting fluorine-18 (t_1/2 = 109.7 min). 1-¹⁸F]1,1,1,2-Tetrafluoroethane was prepared by nucleophilic addition of no-carrier-added ¹⁸F]fluoride to trifluoroethylene and 2-¹⁸F]1,1,1,2-tetrafluoroethane by nucleophilic displacement of tosylate with ¹⁸F]fluoride in 2,2,2-trifluoroethyl p-toluenesulphonate. Each reaction was mediated by a potassium cation-Kryptofix^® 2.2.2 complex, with or without acetonitrile as solvent, in a sealed glassy carbon vessel. The selectivities were 97.2±0.4% for labelling in the 1-position by nucleophilic addition and 91.2±1.2% for labelling in the 2-position by nucleophilic substitution. GC separation afforded each labelled tetrafluoroethane in high radiochemical purity (>99.995%) and high chemical purity (>99.6%). Specific radioactivities of about 37 MBq (1 mCi) per μmol were obtained. Each synthesis was fully automated to cope safely with the high initial radioactivity and delivered purified product within one physical half-life of the fluorine-18 The products are suitable for pharmacokinetic studies in man.