Thesis (MSc)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

Vitamin B6 was discovered almost 90 years ago, and since then it has received a lot of interest from the scientific community due to its role in human health and its impact on several biochemical processes. One of the most interesting aspect of vitamin B6 studied in the past decade is its role in cancer. From the research on this subject so far, the following can be suggested: early-stage cancer cells have a higher vitamin B6 content than normal cells due to its role in metabolic processes. As the cancer makes progress, there is a change in vitamin B6 activation and trapping in the cell, decreasing the amount of active vitamin B6 in the cell in order to resist cell death. From these conclusions, we can see that vitamin B6 could potentially be an interesting radiotracer to use for diagnosis and staging of cancers. One of the most predominant form of imaging which is done nowadays to detect and diagnose cancers is Positron Emission Tomography (PET) imaging. Research on PET imaging is driven by the potential of new radiotracers which can be added to the current arsenal of tools for the fight against cancer. Therefore, this project focuses on the attempted synthesis of two potential radiotracers derived from vitamin B6 based on the insertion of fluorine-18. None of the two proposed radiotracers were successfully synthesized but we successfully synthesized one cold standard and difluorinated pyridoxine with cold conditions similar to radiochemistry. The main issues which were faced were the degradation of the potential precursors when attempting fluorination, the lack of reactivity of intermediates for the formation of precursors and an acetyl migration leading to the wrong precursors. By using a milder fluorination strategy to avoid degradation (room temperature, no free fluoride source: AgF2 as fluorinating agent), the 6-Fluoropyridoxine cold standard was synthesized. By changing the protection strategy (not using any acetyl groups), acetyl migration was avoided which led to the synthesis of a difluorinated pyridoxine using mild conditions (room temperature). The difluorination was also successful using harsher conditions (heat). There is still a lot of work to do to synthesize a radiotracer derived from vitamin B6 but there are some signs that this may be possible with additional work.

Thesis advisor: Jianmin Gao / My graduate research has used engineered peptides to perturb the non-covalent interactions in protein folding, protein-protein association and protein-membrane association. We have focused on understanding the fundamental principles of molecular recognition behind protein-protein and protein-membrane interactions, and further using these principles in protein engineering. This thesis includes three projects. I) Towards Small Molecule Receptors for Membrane Lipids: A Case Study on Phosphatidylserine The lipid composition and distribution of cell membranes play important roles in regulating the physiology of the cell. The lipid composition of plasma membranes is one characteristic feature that can be used to identify cell types and functions. Molecules that specifically recognize a particular lipid are useful as imaging probes for targeting cells or tissues of interest. Protein based lipid binding probes have intrinsic limitations due to their large size and poor pharmacokinetic properties such as slow clearance rate and poor in vivo stability. A plausible strategy to achieve a probe with small size and high binding affinity and selectivity is to use a peptide to mimic the protein lipid-binding domains. As a case study, a cyclic peptide that specifically targets phosphatidylserine containing membranes has been developed. This cyclic peptide is potentially capable of imaging apoptosis in vivo, and the strategy of developing this cyclic peptide can be generalized to the design of peptide-based probes for other lipid species. My research has pointed out a challenging but feasible way to design a peptide that achieves specificity and affinity similar to lipid-binding proteins. (II) Study of Apoptotic Cell Membrane (ACM) Permeant Molecules Noninvasive imaging of apoptosis is highly desirable for the diagnosis of a variety of diseases, as well as for the early prognosis of anticancer treatments. One characteristic feature of apoptotic cells that has been targeted for developing specific biomarkers is enhanced membrane permeability compared to that of healthy cells. Several unrelated molecules that are capable of selectively penetrating the apoptotic cell membrane (ACM) have recently been reported. However, the origin of the altered ACM permeability is poorly understood, as is the scope of molecular structures that can permeate through the ACM. Herein, we report a systematic investigation on the altered ACM permeability. Our results show that simple modifications of commonly used dyes (e.g. fluorescein) afford specific entry into cells at the early stages of apoptosis. The ACM appears to be permeable to molecules of various functional groups and charge, but does discriminate against molecules of large size. The new findings reported here greatly expand the pool of small molecules for imaging cell death, thus facilitating the development of noninvasive imaging agents for apoptosis. (III) Study of Aromatic-Fluorinated Aromatic Interactions in Peptide Systems Therapeutic proteins have been through a remarkable expansion in the last two decades. A general problem that they are facing is poor stability. Protein engineering focuses on solving this problem by incorporating unnatural amino acids into protein sequences to purposefully modify protein structures. Fluorinated aliphatic amino acids have been demonstrated to be effective in stabilizing protein structures and functioning as recognition motifs. In contrast, fluorinated aromatic amino acids are less studied. We investigated the effect of perturbation of fluorination on aromatic residues on the stability of protein model systems, as well as the influence on protein-protein association behavior. The results of this study provided a fundamental understanding of aromatic interactions in protein systems, and guidelines for protein engineering with fluorinated aromatics for stabilizing protein structures or directing specific protein-protein interactions. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Positron emission tomography (PET) is a powerful, non-invasive in vivo imaging technique used for diagnostics and drug development. The synthesis of \(^{18}F\)-PET tracers is challenging due to the short half-life of the unnatural isotope that necessitates late-stage fluorination, and the limited reactivity of nucleophilic fluoride, the preferred and widely accessible form of \(^{18}F\). This thesis describes the development of an electrophilic fluorination reagent derived from fluoride. The reagent can be employed in a late-stage fluorination reaction of palladium aryl complexes to give access to small molecule aryl fluorides. The reagent can be made from \([^{18}F]\)fluoride and used to synthesize radiolabeled small molecules for PET imaging experiments. Two small molecules known to interact with the serotonergic system were synthesized, radiolabeled, and imaged in rats and non-human primates and evaluated for use as PET tracers. / Chemistry and Chemical Biology

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009. / Full text also available online. Scroll down for electronic link.

This thesis has focussed on ¹⁸F-fluorination reactions activated by an organomediator, with the aim of broadening the scope of metal-free reactions in ¹⁸F-radiosynthesis. <b>Chapter 1</b> provides an introduction to positron emission tomography (PET) and ¹⁸Fradiochemistry, including radioisotope production and modes of activation in ¹⁸Fradiosynthesis. In <b>Chapter 2</b>, the concept of chirality and its relevance in the context of radiotracer design is introduced. The previously disconnected fields of organomediated asymmetric fluorination and ¹⁸F-radiosynthesis are merged for the first time via investigation of three distinct activation modes: chiral non-racemic secondary amine-mediated asymmetric &alpha;-¹⁸F-fluorination of aldehydes employing [¹⁸F]N-fluorobenzenesulfonimide; use of a phase transfer reagent for asymmetric ¹⁸F-fluorocyclisation and application of a chiral nonracemic ¹⁸F-fluorinating agent, chiral [¹⁸F]Selectfluor bis-triflate. Application of the first of these approaches to the radiosynthesis of the PET tracer (2S,4S)-4-[¹⁸F]fluoroglutamic acid with high d.r. is described. <b>Chapter 3</b> explores the use of hypervalent iodine reagents to mediate the oxidative fluorination of N-arylsulfonamides with nucleophilic fluoride via an umpolung approach. Preliminary studies on translation to radiosynthesis with [¹⁸F]fluoride are also disclosed. In <b>Chapter 4</b>, experimental data is provided for all compounds, as well as analytical and chiral HPLC traces for ¹⁸F-reactions.

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12290号 / 論工博第4006号 / 新制||工||1450(附属図書館) / 26628 / UT51-2008-T61 / (主査)教授 檜山 爲次郎, 教授 松原 誠二郎, 教授 中條 善樹 / 学位規則第4条第2項該当

Highly polar organic fluorinated motifs are of interest in materials chemistry, for example, in liquid crystal applications. Cyclohexane is an important and widely used structural motif within organic chemistry. Work has been carried out to prepare single stereoisomers of multivicinal fluorinated cyclohexanes, a class of compounds that has not been previously produced. A synthesis of the all-syn-1,2,3,4-tetrafluorocyclohexane, in 9 steps from cyclohexa-1,3-diene will be presented. The ¹⁹F NMR spectra of the all-syn-1,2,3,4-tetrafluorocyclohexane shows interesting dynamic conformational effects. This is a small polar organic molecule, which was crystalline at room temperature. The structure of the compound was confirmed by single crystal X-ray diffraction studies. The synthesis of the all-syn-1,2,4,5-tetrafluorocyclohexane from cyclohexa-1,4-diene is also presented. The synthesis of a single diastereoisomer of 1,2,3,4,5,6-hexafluorocyclohexane, derived from benzene in 5 steps, is presented. As with the tetrafluoro compounds, the ¹⁹F NMR spectra of this compound shows dynamic conformational effects. The structure of the compound was confirmed by single crystal X-ray diffraction studies. The 1,2,4,5-tetrafluorocyclohexane motif was elaborated to contain a phenyl group, producing “rod-like” molecules. This motif was synthesised in view of potential applications for liquid crystalline materials.

Nitroxides are intermediates in the accepted reaction mechanisms of the antioxidant activity of diarylamines and hindered alkyl amines. The parent amines are used as additives to preserve synthetic and natural hydrocarbon-based materials from oxidative degradation. New methodology which enables monitoring of hydrocarbon autoxidations at low rates of radical generation has revealed that diarylnitroxides and hindered nitroxides are far better inhibitors of unsaturated hydrocarbon autoxidation than their precursor amines, implying intervention of a previously overlooked mechanism. Experimental and computational investigations suggest that the nitroxides catalyze the cross-dismutation of alkylperoxyl and hydroperoxyl radicals to yield a hydroperoxide and O2, thereby halting the autoxidation chain reaction. The hydroperoxyl radicals – key players in hydrocarbon combustion, but essentially unknown in autoxidation – are proposed to derive from a tunneling-enhanced intramolecular (1,4)- hydrogen-atom transfer/elimination sequence from oxygenated radical addition intermediates. These insights suggest that nitroxides are preferred additives for the protection of unsaturated hydrocarbonbased materials from autoxidation since they exhibit catalytic activity under conditions where their precursor amines are less effective and/or inefficiently converted to nitroxides in situ. Polyunsaturated fatty acids (PUFAs) are highly autoxidizable lipids that are integral structural components of biological membranes as well as substrates for enzymes the produce inflammatory mediators implicated in a host of degenerative diseases. In particular, the interactions between these substrates and their respective native enzymes are hotly pursued since elucidation of the underlying mechanisms could lead to the discovery of better small molecule inhibitors for the ailments to which they contribute. In the past decade, an additional mode of cellular degeneration has been unveiled in the process of ferroptosis whose hallmark includes a sharp increase in the cellular pool of PUFA derived hydroperoxides. As a result, there is further incentive to uncover all mechanisms by which these inflammatory precursors are developed. Herein, progress toward the synthesis of fluorinated PUFAs is presented. These are proposed to be useful to probe the interactions of PUFAs with lipoxygenase enzymes, which metabolize polyunsaturated fatty acids to their hydroperoxide derivatives.

<P> Fluorine-18 labeled 6-fluoro-3, 4-dihydroxy-phenyl-L-alanine (6-FDOPA) has been used in conjunction with Positron Emission Tomography (PET) to study the dopamine metabolism in the living human brain and also to monitor gastrointestinal carcinoid tumors. Elemental fluorination of L-DOPA in anhydrous HF (aHF) or aHF/BF3 has been shown to be an efficient method for the synthesis of 6-fluoro-L-DOPA. Utilization of aHF, however, is not desirable in a hospital environment owing to its hazardous nature. This work has consequently focused on the development of new methodologies for the direct electrophilic fluorination of aromatic amino acids, which circumvent the use of aHF. </p> <p> The present work has shown that the reactivity and selectivity of F2 towards L-DOPA in CF3S03H is comparable to that in aHF. The discovery and versatility of this new synthetic procedure has led to the production of 6-[18F]fluoro-L-DOPA, 6[18F]fluoro-D-DOPA, 4-[18F]fluoro-L-m-tyrosine (4-FMT) and 6-(8F]fluoro-L-m-tyrosine (6-FMT) in high radiochemical yields that are not only suitable for small animal imaging, but are also suitable for clinical use in human subjects. Because of the low volatility of CF3S03H, its removal from the reaction mixture was accomplished by use of an anion exchange resin in acetate form. The syntheses of2-, 4-and 6-FMT were also achieved by the direct fluorination of m-tyrosine (MT) in H20. The effect of temperature on the fluorination of MT was investigated and it was shown that, unlike CF3S03H, optimal conditions in H20 were attained at elevated reaction temperatures. </p> <p> There have been several reports relating to the formation of [18F]OF2 as a major byproduct (up to 20%) in the gas phase nuclear reaction, 180(p,n)18F. This reaction is used for the routine production of [18F]F2 which, in tum, is utilized for the syntheses of PET tracers such as radiofluorinated aromatic amino acids. Because the reactivity of OF2 has been reported to be similar to that of F2, its selectivity as a fluorinating agent towards aromatic amino acids was investigated. The effect of solvent acidity on the fluorination of MT using OF2 was studied and it was shown that, in contrast with the reactivity of F2 in superacids, OF2 is a more efficient fluorinating agent in less acidic solvent media. The use of H20 as the solvent medium for fluorination ofMT resulted in the formation of 19FFMT isomers in 4.35 ±0.04% yield. Consequently, the potential use of OF2 as a fluorinating agent for aromatic amino acids was also investigated for L-phenylalanine, 3nitro-L-tyrosine, 4-nitro-DL-phenylalanine, L-DOPA, 3-0-methyl-L-DOPA, 3,4dimethoxy-L-phenylalanine, m-, p-and a-tyrosine. In these studies, the only aromatic system fluorinated by OF2 was MT, indicating that the presence of [18F]OF2 as a byproduct resulting from the nuclear reaction, 180(p,n)18F, does not have a significant impact on the syntheses of radio fluorinated aromatic amino acids that have applications in PET imaging. </p> / Thesis / Master of Science (MSc)