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Surface Chemistry Has Reached Radiofluorination

Everyone who has done radiofluorinations knows the painful, lengthy process of drying down aqueous fluoride, which is part of almost every fluorination sequence used these days. The goal is to render fluoride anions sufficiently active to perform nucleophilic substitution chemistry, which is not possible as long as fluoride is heavily solvated. In its desovated state fluoride becomes highly basic.  Therefore, along with the pain of the physical dry down comes an optimization process for additives and bases to prevent fluoride from evaporating as HF while keeping it sufficiently soluble. 

Few exceptional reactions that operate under “aqueous conditions” have provided radiotracers used in imaging [1]. Usually, those reactions don’t go through nucleophilic substitution pathways and come with their own limitations and challenges. Now the van Dam group at UCLA has published their work on a new approach to the problem:

Titania nanoparticles are capable of coordinating fluoride to their surface, stripping off the hydration sphere and rendering it nucleophilic. The basicity is tamed through the coordination to the lewis acidic surface. The only problem lies in delivering the electrophile to coordinated fluoride, which the group achieved through another coordination event. Tosylate as a leaving group – just like fluoride – is also easily held coordinated on the titania surface. Bringing those two components together on nanoparticles now enables highly efficient, fast radiofluorination without having to dry down fluoride prior to the reaction. The chemists at UCLA have taken the conceptual approach one step further and performed a complete synthesis of [18F]-Fallypride on a scale relevant for imaging. Their isolated dose conforms with all QC criteria for PET radiotracers.

Our ability to perform radiofluorinations is still limited, but once again remarkable creativity and technical finesse has added a method to our toolbox. Hopefully the field will remain in the focus of so much work and excitement to enable many more great experiments in the imaging domain.

- MGS -

[1] a) J. Am. Chem. Soc. 2012, 134, 17456–17458, b) ACS Chem. Neurosci. 2014, 5, 611

[2] J. Am. Chem. Soc. 2015, 137, 5686-5694

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