Approaches to the synthesis of fluoromuscimol and applications of the enantiomers of [7-²H₁]-benzyl fluoride as chiral probes for enzyme reactions

Abstract

This thesis explores the synthesis and use of biologically active fluorinated organic compounds. Muscimol is the neuroactive species of the psychoactive mushroom Amanita muscaria. It binds at the same site as GABA on GABA[sub]A receptors, and fluorination of muscimol is therefore of particular interest, as it could allow the imaging of GABA binding sites by positron emission tomography using the fluorine-18 isotope. Chapter 1 describes research towards the synthesis of fluoromuscimol 33. Two approaches were taken. The first involved cyclisation of a fluorinated precursor, while the second aimed at the introduction of fluorine using a pre-existing synthesis of muscimol. ¹H and ¹⁹F NMR evidence of the formation of a fully protected fluoromuscimol was observed after a key lithium-halogen exchange step, followed by in situ fluorination. Previous work in the group allowed the synthesis of chiral [7-²H₁]-benzyl fluoride with high enantiopurity. In Chapter 2, (S)- and (R)-[7-²H₁]-benzyl fluorides (S)-78 and (R)-78 were resynthesized from benzaldehyde in two steps. They were used in competition experiments to explore the cryptic stereochemistry of the unspecific peroxygenase of Agrocybe aegerita, showing a weak preference for a pro-(S) proton abstraction in the first stage of the mechanism of benzyl fluoride oxidation. Moreover, an oxidation reaction of these benzyl fluorides to obtain enantiomers (S)-82 and (R)-82 of chiral [2-²H₁]-fluoroacetate was improved to remove acetate pollution in the product. Finally, chiral analysis of the hexyl ester of these enantiomeric fluoroacetates by a specific ²H NMR technique established their enantiopurity.