No description available.

Stereochemistry and chirality are arguably two of the most important subjects pertaining to the development of new pharmaceutical drugs. Since enantiomers have the potential to encompass different pharmacological effects in biological systems, both enantiomers have to be tested for pharmacological activity. Not only has obtaining these single enantiomers become crucial, but formulation of the pure enantiomer of a drug also has the potential to contain advantages for both pharmaceutical formulation and therapeutic effect. Epoxide hydrolase is an enzyme commonly found in nature that catalyses the hydrolysis of epoxides, resulting in the formation of the corresponding vicinal diol. Over the last few years a large amount of research has been completed on these enzymes from sources such as mammals, insects, bacteria and fungi. Micro-organisms especially have enjoyed ample attention because of their abundant supply. Recently it was found that certain yeasts contain this enzyme and have the ability to enantioselectively catalyse certain hydrolysis reactions. Styrene oxides are terminal epoxides that are, due to the reactivity of the epoxide ring, useful synthons in the organic synthesis of pharmaceutical products. The first objective of this project was to synthesize three nitro derivatives of styrene oxide namely para-, meta-, and ortho-nitrostyrene oxide. Al three products were obtained from the corresponding nitrophenacyl bromide in yields of 52%, 90% and 57% respectively. The second objective was lo find a suitable yeast slrain containing the epoxide hydrolase enzyme to enantioselectively hydrolyse the synthesised products and unsubstituted styrene oxide. A screening was completed during which 410 yeast strains from more than 44 genera were tested. Epoxide hydrolase activity was found to be widespread throughout the screened yeast domain, while the genera Candida, Debaryomyces, Pichia, Rhodosporidium, Rhodotorula and Trichosporon specifically were very successful in catalysing the hydrolysis of the substrates. Rhodosporidium toruloides UOFS Y-0471 and Rhodotorula glutinis UOFS Y-0653 were chosen for further studies because of their superior enantioselectivity. The final objective was to optimise these reactions in terms of pH, temperature and substrate concentration. It was found that a pH value of 7.2 and a temperature of 45’C yielded optimal enzyme activity. Increased temperatures (45’C), however, lead to a decrease in enantioselectivity and, in the case of R. toruloides together with the substrate puranitrostyrene oxide, reversed enantioselectivity. Lower temperatures (15’C) increased enantioselectivity, resulting in a remarkable improvement from a 10% yield of the single enantiomer (45’C) to a 35% yield. Surprisingly this temperature decrease had a very small affect upon the reaction time. / Thesis (M.Sc. (Pharmaceutical Chemistry)--Potchefstroom University for Christian Higher Education, 2002.

Heidelberg, Univ., Diss., 2006.

No description available.

Protonated epoxides feature prominently in organic chemistry as reactive intermediates. Gas-phase calculations studying the structure and ring-opening energetics of protonated ethylene oxide, propylene oxide and 2-methyl-1,2-epoxypropane were performed at the B3LYP and MP2 levels (both with the 6-311++G** basis set). Structural analyses were performed for 10 protonated epoxides using B3LYP, MP2, and CCSD/6-311++G** calculations. Protonated 2-methyl-1,2-epoxypropane was the most problematic species studied, where relative to CCSD, B3LYP consistently overestimates the C2-O bond length. The difficulty for DFT methods in modeling the protonated isobutylene oxide is due to the weakness of this C2-O bond. Protonated epoxides featuring more symmetrical charge distribution and cyclic homologues featuring less ring strain are treated with greater accuracy by B3LYP. Ion-pair separation (IPS) of THF-solvated fluorenyl, diphenylmethyl, and trityl lithium was studied computationally. Minimum-energy equilibrium geometries of explicit mono, bis and tris-solvated contact ion pairs (CIPs) and tetrakis-sovlated solvent separated ion pair (SSIPs) were modeled at B3LYP/6-31G*. Associative transition structures linking the tris-solvated CIPs and tetrakis-solvated SIPs were also located. In vacuum, B3LYP/6-31G* ΔHIPS values are 6-8 kcal/mol less exothermic than the experimentally-determined values in THF solution. Incorporation of secondary solvation in the form of Onsager and PCM single-point calculations showed an increase in exothermicity of IPS. Application of a continuum solvation model (Onsager) during optimization at the B3LYP/6-31G* level of theory produced significant changes in the Cα-Li contact distances in the SSIPs. An increase in of ion pair separation exothermicity was observed upon using both PCM and Onsager solvation models, highlighting the importance of both explicit and implicit solvation in modeling of ion pair separation. / Ph. D.

Epoxide hydrolase I from Solanum tuberosum (StEH1) and isolated variants thereof has been studied for mapping structure-function relationships with the ultimate goal of being able to in silico predict modifications needed for a certain activity or selectivity. To solve this, directed evoultion using CASTing and an ISM approach was applied to improve selectivity towards either of the enantiomeric product diols from (2,3-epoxypropyl)benzene (1). A set of variants showing a range of activites and selectivities was isolated and characterized to show that both enantio- and regioselectivity was changed thus the enrichment in product purity was not solely due to kinetic resolution but also enantioconvergence. Chosen library residues do also influence selectivity and activity for other structurally similar epoxides styrene oxide (2), trans-2-methyl styrene oxide (3) and trans-stilbene oxide (5), despite these not being selected for.    The isolated hits were used to study varying selectivity and activity with different epoxides. The complex kinetic behaviour observed was combined with X-ray crystallization and QM/MM studies, powerful tools in trying to explain structure-function relationships. Crystal structures were solved for all isolated variants adding accuracy to the EVB calculations and the theoretical models did successfully reproduce experimental data for activities and selectivities in most cases for 2 and 5.  Major findings from calculations were that regioselectivity is not always determined in the alkylation step and for smaller and more flexible epoxides additional binding modes are possible, complicating predictions and the reaction scheme further. Involved residues for the catalytic mechanism were confirmed and a highly conserved histidine was found to have major influence on activity thus suggesting an expansion of the catalytic triad to also include H104. Docking of 1 into the active site of the solved crystal structures was performed in an attempt to rationalize regioselectivity from binding. This was indeed successful and an additional binding mode was identified, involving F33 and F189, both residues targeted for engineering. For biocatalytic purpose the enzyme were was successfully immobilized on alumina oxide membranes to function in a two-step biocatalytic reaction with immobilized alcoholdehydrogenase A from Rhodococcus ruber, producing 2-hydroxyacetophenone from racemic 2.

In this thesis, different approaches in synthesizing molecular resist are examined, and structure-property relations for the molecular resist properties are studied. This allows for design of resists that could be studied further as either negative or positive tone resists in photolithography. A series of compounds having different number of acrylate moiety, and different backbones were investigated for photoresist application. Thermal curing of acrylate compounds in organic solvent was also examined. Film shrinkage, as well as auto-polymerization was observed for these compounds that make them unsuitable as photoresist material. Furthermore, calix[4]resorcinarenes (C4MR) was chosen as backbone, and the functional groups was selected as oxetane and epoxy. Full functionalized C4MR compounds with oxetane, epoxy and allyl were synthesized. Variable-temperature NMR of C4MR-8Allyl was studied in order to get a better understanding of the structure’s conformers. Energy barrier of exchange (ΔG#) was determined from coalescence temperatures, and was 57.4 KJ/mol for aromatic and vinyl hydrogens and 62.1 KJ/mol for allylic hydrogens.

Biocatalysis and enzyme technology represent significant research topics of contemporary biotechnology. The immobilisation of these catalysts on or in static supports serves the purpose of transforming the catalyst into a particle that can be handled through effortless mechanical operations, while the entrapment within a membrane or capsule leads to the restraint of the enzyme to a distinct space. This confinement leads to a catalyst with a superior stability, and cell durability under reaction conditions. Epoxide hydrolase is a widely available co-factor independent enzyme, which is known to have remarkable chemio-, regio- and stereoselectivity for a wide range of substrates. Recently it was found that certain yeasts, including Rhodosporidium toruloides, contain this enzyme and are able to enantioselectively catalyse certain hydrolysis reactions. The objective of this project was four-sided: a) to immobilise Rhodospridium toruloides in an optimised immobilisation matrix (calcium alginate beads), for the kinetic resolution of 1.2- epoxyoctane in order to obtain an optically pure epoxide and its corresponding vicinal diol, b) to determine the effect of immobilisation on activity as well as stability of the enzyme and gain better understanding of the parameters that influence enzyme activity in a support, c) to determine the effect of formulation parameters on some of the bead characteristics and, d) to gain some insight in the distribution of epoxide and diol in the water and bead phases and the formulation parameters that have an effect thereon. Rhodospridium toruloides was immobilised in calcium alginate beads consisting of different combinations of alginate and CaCl2 concentrations. Best results were obtained with a combination of 0,5 % (m/v) alginate and 0,2 M CaC12. The immobilised cells exhibited lower initial activity. but more than 40 times the residual activity of that of the free cells after a 12-hour storage period. Both the immobilised and free cells exhibited an increase in reaction rate (V) with an increase in substrate concentration. An increase in the alginate concentration lead to the formation of smaller beads, but a decrease in enzume activity, while an increase in the CaCl2 solution concentration had no effect on bead diameter or enzyme activity. Epoxide diffused preferentially into the beads (± 96 %), and the diol into the water phase, which leads to the natural separation of the epoxide and the diol. The CaCl2 concentration affected epoxide diffusion with no effect on diol diffusion, which opens up the possibility to regulate the diffusion of epoxide into the beads. Although only a very small fraction of the epoxide inside the beads could be extracted, the alginate proved to be chirally selective for the (R)-epoxide, improving the reaction efficiency by increasing the % ee, of the epoxide extracted from the beads between 26 % and 43 %. The possibility to develop a system where the product is formed, purified and concentrated in a one-step reaction by extracting the product from the bead phase was clearly demonstrated. / Thesis (M.Sc. (Pharm.) (Pharmaceutical Chemistry))--Potchefstroom University for Christian Higher Education, 2003.

Fluorine atom plays a very important role in pharmaceuticals, agricultural chemicals, and medical imaging and it has become one of the most popular area in organic chemistry. For example, in modern medicinal chemistry introducing fluorine atom could potentially improve absorption, metabolism and potency of drug candidates. As a result, methods that allow the selective and efficient formation of the carbon-fluorine bond are highly desirable. An evolving approach is the utilization of transition-metals to catalyze the nucleophilic substitution of fluoride ion. This thesis described several novel and efficient methods to generate allylic and benzylic C-F bonds using rhodium/iridium catalyst.

Photopolymerization, which uses light rather than heat to initiate polymerization, is a facile technique used to fabricate adhesives, protective coatings, thin films, photo-resists, dental restoratives, and other materials. Epoxide monomers, which are polymerized via cationic photoinitiation, have received less attention in fundamental research in comparison to free radical polymerized acrylate monomers. The characterization of propagation mechanisms, network structures, and physical properties is yet lacking. This project focused on the reactivity and physical properties of 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate (EEC), and the kinetic and physical effects of chain transfer agents (CTAs) in EEC based formulations were characterized. This characterization was carried out using real-time Raman spectroscopy, real-time infrared spectroscopy, dynamic mechanical analysis, simple gel fraction measurements, and atomic force microscopy. The effects of water, organic alcohols, processing conditions (e.g., UV light intensity, humidity, post-illumination curing temperature), and photoinitiation systems were investigated. In general, increasing the concentration of CTAs in a crosslinking epoxide resin increases the rate of polymerization and the overall epoxide conversion level. High CTA levels also correspond to lower glass transition temperatures (Tg) and lower crosslink densities. A post-illumination annealing was critical in obtaining stable physical properties for high Tg epoxide materials. In addition, humidity (water being the most universal contaminant type of CTA) was found to impact the surface properties of an epoxide polymer negatively by reducing the surface hardness. Hybrid acrylate-epoxide systems are much more complex and unpredictable in curing behavior. The use of hydroxy acrylates in hybrid systems allows for grafting between the epoxide and the acrylate domains, via the AM mechanism. Another intricacy of hybrid systems is the initiation system. In order to maximize the conversion of both the epoxide and the acrylate moieties, the free-radical photoinitiator must not hinder the polymerization of the epoxide monomer. Some very efficient free-radical photoinitiators limit the epoxide polymerization by absorbing the majority of the deep-UV incident photons. Finally, a renewable acrylate oligomer was synthesized to provide a green alternative to petroleum-based oligomers currently used. The oligomer was freely miscible and readily photopolymerized with a wide range of commercial monomers. The Tg relationship between the commercial monomers and the parent resin followed the Fox equation. The results of this research provide strategies for controlling epoxide kinetics and physical properties in neat and hybrid systems. This information is useful for tailoring resin formulations to specific end-use applications, especially in films, coatings, and adhesives. Hybrid epoxide-acrylate photopolymerization affords the unique opportunity to structure polymer networks in time and to engineer advanced material properties. These hybrid systems are based on formulations that contain both an epoxide moiety, which undergoes cationic ring-opening photopolymerization, and an acrylate moiety, which undergoes free-radical photopolymerization. Through the combination of these two independent reactive systems, hybrid polymers exhibit lower sensitivity to oxygen and moisture and offer advantages such as increased cure speed and improved film-forming properties. The ability to design the polymer network architecture and to tune mechanical properties can be realized through control of the cationic active center propagation reaction relative to the cationic chain transfer reaction. Specifically, grafted polymer networks can be developed through the covalent bonding of the epoxide chains to the acrylate chains via hydroxyl substituents. This work demonstrates the formation of these grafted polymer networks and overviews the physical properties obtained through control of hydroxyl content and hybrid formulation composition.