Single‐Step Radiosynthesis of “18F‐Labeled Click Synthons” from Azide‐Functionalized Diaryliodonium Salts

Abstract

Positron emission tomography (PET) is an increasingly important biomedical imaging technique that relies on the development of radiotracers labeled with positron-emitters to achieve biochemical specificity. Fluorine-18 (t1/2 = 109.7 min) is an attractive positron-emitting radiolabel for organic radiotracers, primarily because of its longer half-life and greater availability relative to those for the main alternative, carbon-11 (t1/2 = 20.4 min). Rapid simple methods are sought for labeling prospective PET radiotracers with fluorine-18 from cyclotron-produced aqueous [18F]fluoride ion, which must often be converted first into a suitably reactive labeling synthon for use in a subsequent labelling reaction. Use of 18F-labeled synthons in 'click chemistry' attracts increasing attention for labeling PE Tradiotracers. Here we describe rapid single-step radiosyntheses of azido- or azidomethyl-bearing [18F]fluoroarenes from the reactions of diaryliodonium salts with no-carrier-added [18F]fluoride ion within a microfluidic apparatus to provide previously poorly accessible 18F-labeled click synthons in radiochemical yields of 15% for [18F]4-fluorophenyl azide and about 40% for each of the [18F](azidomethyl)-fluorobenzenes. The radiosyntheses of the latter synthons was possible under 'wet conditions', so obviating the need to dry the cyclotron-produced [18F]fluoride ion and greatly enhancing the practicality of the method.