Fluorine-18 as a tracer in inorganic and organic syntheses and its application to positron emission tomography

Abstract

<p>The application of fluorine-18 to the study of inorganic and organic reaction mechanisms has been investigated. From the distribution of [¹⁸F] among the decomposition products of NF₄⁺H*F₂⁻ it has been shown that the attack of H*F₂⁻ on NF₄⁺ occurred exclusively on the fluorine and not on the nitrogen contrary to predictions based on bond polarities. An [¹⁸F] changes experiment between ClF₅ and [¹⁸F]FNO showed complete randomization of the [¹⁸F] between the two molecules. A ¹⁹F NMR study using neat ClF₅ and ClF₅ in anhydrous HF solution in the presence and absence of excess CsF showed slow chemical exchange between ClF₅ and CsF in anhydrous HF at room temperature. Evidence from both [¹⁸F] radiotracer and ¹⁹F NMR studies has provided conclusive evidence for the existence of the ClF₆⁻ anion. Fluorine-18 has also been used as a probe to study the selectivity of F₂ towards aromatic amino acids in different solvents. Differences in reactivity and selectivity of dilute fluorine towards tyrosine and protonated tyrosine in HF and HF/BF₃ respectively, have been exploited for the synthesis of 2- and 3-fluorotyrosine. Results from the radiofluorination of m-tyrosine in different solvents showed that anhydrous HF is an ideal solvent for the synthesis of [¹⁸F] fluoro-m-tyrosine. Fluorine-18 labelled fluoro-m-tyrosine, has been synthesised and used, in conjunction with Positron Emission Tomography, to study striatal aromatic acid decarboxylase activity in living human brain. It has been demonstrated that [¹⁸F] fluoro-m-tyrosine will probably supersede [¹⁸F] fluorodopa as the compound of choice to study the dopaminergic pathway in man.</p>