Organoboranes as 18F Fluoride Anion Captors
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Fluorine-18 (18F) is the most frequently used radionuclide in positron emission tomography (PET). This is, in part, due to the fact that fluorine is the element that is prevalent in organic molecules as well as easy to introduce directly into a molecule of interest. In addition, the labelling of biomolecules; such as, oligonucleotide, peptides, and proteins with radioisotopes for use in PET imaging is an increasingly important research aspect due to the use of these biomolecules in medical diagnostic or therapeutic applications. Unfortunately, the direct fluorination of peptides and proteins is difficult since harsh reaction conditions (high temperatures, acidic or basic conditions) that are required to get high radiochemical yields may destroy those biomolecules. To solve this problem, peptides and proteins are generally radiolabelled through a suitable prosthetic group under mild conditions. This dissertation focuses on the synthesis and properties of novel organoboranes designed for 18F fluoride anion capture for PET application. With the goal of developing new boron-based radiotracer, a series of ortho-phenylene phosphino-boranes and carbene-borane adducts were synthesized and studied. This general strategy of attaching cationic moiety close to trifluoroborates leads to a B-F bond stabilization in aqueous environment. Gratifyingly, these compounds can be radiolabeled via direct 18F-19F isotopic exchange reactions. With these results in hands, the radiolabeled organoborane/biomolecule conjugates have been synthesized and evaluated in vivo to use as potential PET probes. In addition to trifluoroborate species, we have also prepared a new class of fluorescent dyes called triazaborolopyridiniums and investigated their capability as PET/fluorescence dual modality probes. In a more fundamental aspect of the some of the ideas presented above, we have also decided to explore the synthesis and properties of ambiphilic molecules featuring adjacent phosphine and methylium groups. The structures and isomerization processes of these compounds are also described in this dissertation.
Chansaenpak, Kantapat (2015). Organoboranes as 18F Fluoride Anion Captors. Doctoral dissertation, Texas A & M University. Available electronically from