A closer look at the synthesis of 2-[(18)F]fluoroethyl tosylate to minimize the formation of volatile side-products

BACKGROUND: 2-[(18)F]Fluoroethyltosylate ([(18)F]FEtOTs) is a well-known (18)F-fluoroalkylating agent widely used to synthesize radiotracers for positron emission tomography. The widespread use of [(18)F]FEtOTs is due in part to its low volatility when compared to other halide and sulfonate building blocks. In this work, the radioactive volatile side-products formed during the synthesis of [(18)F]FEtOTs were identified and characterized for the first time, and an optimization of the reaction conditions to minimize their formation was proposed. RESULTS: In order to characterize the volatiles produced during [(18)F]FEtOTs synthesis, the reaction mixtures of both cold FEtOTs and [(18)F]FEtOTs were co-injected onto the HPLC system. The radioactive peaks corresponding to the volatile compounds were collected, analyzed through headspace gas chromatography mass spectrometry sampler (HS-GC–MS) and identified as vinyl fluoride ([(19)F]VF) and 2-fluoroethanol ([(19)F]FEOH). By using a rotatable central composite design with a two-level full factorial core of two factors (2(2)), it was determined that temperature and time are independent variables which affect the generation of [(18)F]VF and [(18)F]FEOH during the radiosynthesis of [(18)F]FEtOTs. In addition, in order to reduce the formation of the volatiles ([(18)F]VF and [(18)F]FEOH) and increase the yield of [(18)F]FEtOTs, it was demonstrated that the molar ratio of base to precursor must also be considered. CONCLUSION: [(18)F]VF and [(18)F]FEOH are volatile side-products formed during the radiosynthesis of [(18)F]FEtOTs, whose yields depend on the reaction time, temperature, and the molar ratio of base to precursor. Therefore, special care should be taken during the radiosynthesis and subsequent reactions using [(18)F]FEOTs in order to avoid environmental contamination and to improve the yield of the desired products. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s41181-022-00179-8.