Speaker
Mr
Christian Drerup
(Forschungszentrum Jülich GmbH, Germany)
Description
Because of its extraordinary suitable decay properties fluorine-18 is the most widely used radionuclide in positron emission tomography (PET) [1]. Presently the radioorganic syntheses of no-carrier-added (n.c.a.) 18F-labelled products are practically limited to nucleophilic procedures. This complicates or excludes n.c.a. syntheses of many putative tracer compounds and intensifies a special demand for electrophilic 18F-labelling. There is the unanswered question whether an electrochemical oxidation of nucleophilic [18F]fluoride can lead to an electrophilic [18F]fluorine analogue. Since in organic fluorochemistry N-F compounds are known as highly effective and selective electrophilic fluorinating agents [2], they were chosen to be synthesized with n.c.a. [18F]fluoride.
Based on cyclic voltammetric measurements an electrochemical synthesis of
[18F]N-fluorobis(phenylsulfonyl)amine was attempted. Bis(trifluoromethylsulfonyl)imide with 1-butyl-1-methylpyrrolidinium as counter anion was chosen as a putative suitable precursor. Due to its excellent physical and electrochemical properties this ionic liquid serves as conducting salt, solvent and starting material at once and offers a simplified performance by avoiding major side reactions. Cyclic voltammetry of bis(trifluoromethylsulfonyl)imide showed two succeeding oxidation steps. The first step may lead to a resonance-stabilized radical before further oxidation causes the generation of an unstable cation and finally the decomposition of the compound.
Further electrosynthetic experiments were carried out in an established electrochemical cell [3]. After intercalating [18F]fluoride from aqueous solution into glassy carbon (Sigradur®G) the ionic liquid was added. At a tension of 6-8 V intercalated fluoride and bis(trifluoromethylsulfonyl)imide were expected to co-oxidize and recombine at the surface of the electrode. Products were separated from their ionic contents and analyzed by radio-HPLC. An unambiguous identification could not be achieved due to the lack of a reference compound. A resulting n.c.a. organic product, however, showed a similar HPLC-retention with co-injected bis(phenylsulfonyl)imide as “pseudostandard”. Thus, in principle a production of an n.c.a. electrophilic 18F-fluorinating reagent starting from [18F]fluoride appears probable.
References:
[1] H. H. Coenen, Fluorine-18 Labeling Methods in PET Chemistry, The Driving Force in Molecular Imaging, P. A. Schubiger, L. Lehmann, M. Friebe (eds.), Springer Verlag 2007, 16-50.
[2] G. G. Furin, A. A. Fainzilberg, Russ. Chem. Rev., 68, (1999), 653-684.
[3] K. Hamacher, T. Hirschfelder, H. H. Coenen, Appl. Radiat. Isot., 56, (2002), 519-523.
Primary author
Mr
Christian Drerup
(Forschungszentrum Jülich GmbH, Germany)
Co-authors
Dr
Daniel Roehrens
(Forschungszentrum Jülich GmbH)
Dr
Fabian Kuegler
(Forschungszentrum Jülich GmbH)
Prof.
Heinrich Hubert Coenen
(Forschungszentrum Jülich GmbH)
Dr
Johannes Ermert
(Forschungszentrum Jülich GmbH)
Dr
Kurt Hamacher
(Forschungszentrum Jülich GmbH)
Mr
Milosch Stumpf
(Forschungszentrum Jülich GmbH)