Fluorine-18 labelled fluoropyridines have found increasing application in the medical imaging technique of Positron Emission Tomography. The fluorine-18 has largely been restricted to the 2-and 6-position where its introduction may be readily achieved, from a range of appropriate precursors, by nucleophilic aromatic substitution using [ 18 F]fluoride. However, fluorine at this position may be labile in vivo limiting the potential of the radiotracer. To date the more stable 3-fluoro and 5-fluoropyridines have not been exploited due to unsuitable methods of preparation. Pyridyliodonium salts provide a convenient way to overcome this restriction and allow the introduction of fluorine-18, as fluoride, into the 3-or 5-position.

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