Radiofluorination methodology for automated chemistry

Abstract

Microfluidic technology has recently emerged as an invaluable tool in the development of PET radiochemical methods allowing the quantities of precursors/reagents/solvents to be significantly reduced thus facilitating timely isolation of the final radiolabelled product. 1-5 The Advion NanoTek microfluidic system has been used to optimise model reactions designed to evaluate the status of the system and validate investigations. It was noted that the use of conventional phase-transfer agent (Kryptofix® 222/K2CO3) causes blockages to the micro-channels wherein the radiochemical reactions take place. We have demonstrated that tetraethylammonium bicarbonate is a viable alternative to the traditional phase-transfer agent, for the radiosynthesis of [18F]fluoroarenes, using a microreactor, via a key range of radiosynthetic strategies (Figure 1). Of particular benefit, in a microreactor, was the performance of tetraethylammonium bicarbonate where the occurrence of blocked reactors experienced with the conventional phase-transfer agent was eliminated thus dramatically increasing productivity.6 Figure 1: Key methods use to evaluate tetraethylammonium bicarbonate as a phase-transfer agent in the radiosynthesis of ethyl 4-[18F]fluorobenzoate. Various diaryliodonium salts have been synthesised via conventional methods and reacted with tetraethylammonium [18F]fluoride, to generate ethyl 4- and 3- [18F]fluorobenzoate with the synthesis of the 4-position regioisomer, generally, producing higher radiochemical yields. Excellent radiochemical yields and selectivity v were achieved for both isomers when using precursors bearing a 4-anisyl ring. The selectivity of the nucleophilic attack of [18F]fluoride on a diaryliodonium salt has been clearly demonstrated using electron-neutral and electron-rich derivatives. In addition, the Eckert & Ziegler Modular-Lab was used to generate a simple [18F]fluoroarene from a diaryliodonium salt using tetraethylammonium [18F]fluoride as the radiofluorinating agent. Radiochemical yields were low but comparable to those achieved when using the conventional phase-transfer agent, thus, proving that this new reagent may be used as a radiofluorinating agent in automated batch radiofluorinations.