Aliphatic Nucleophilic Radiofluorination

Abstract

In this review we are looking at some aspects of nucleophilic aliphatic radiofluorination, notably the labelled fluoride source, design aspects, the leaving group and the solvent. It should be clear that there is more to this branch of radiolabelling than one would suspect from the frequently used standard tosylate replacement with kryptofix/[18F]fluoride in acetonitrile or DMSO. Competitive elimination can be a serious problem that can affect both yield and purification. Deprotection of sensitive groups after radiolabelling and its possible side reactions can complicate purification. The right choice of leaving group and protecting groups may be crucial. Newer developments such as the use of tertiary alcohols or ionic liquids as solvents, long-chain polyfluorinated sulphonate leaving groups facilitating fluorous solid phase extraction, or immobilisation of the precursor on a solid phase support may help to solve these problems, for example the long-standing problems with [18F]FLT, whereas older concepts such as certain cyclic reactive entities for ring opening or even an abandoned reagent as [18F]DAST should not be forgotten.