Thesis (M.Sc.)--City University of Hong Kong, 2005. / At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Sept. 4, 2006) Includes bibliographical references.

Ph.D. / The first part of the research described in this thesis involves the development of a new methodology for the synthesis of N-hydroxy pyrrolidines, starting from carbohydrates as building blocks. The products were identified as possible synthons for the stereocontrolled synthesis of isosteric analogues of polyhydroxylated indolizidine alkaloids. The consecutive reduction and cyclisation of selectively protected 5-0-mesyl hexose O-(tert-butyldiphenylsilyl) oximes to afford chiral N-hydroxy pyrrolidines is discussed. The mechanism involves a cascade of neighbouring group participation steps by the O-benzoyl protecting groups. This protocol gave rise to novel chiral N-hydroxy pyrrolidines in good overall yield. The choice of leaving group as well as a labile oxime protecting group proved to be of great import in the outcome of the cyclisation reactions. The second part of the research concerns the ongoing development in our laboratories of the synthesis of analogues of biologically active compounds. In this regard, we were interested in synthesising the aza analogues of f3-C-nucleosides and f3-C-glycosides. Our strategy involves the synthesis of a D-ribose derived chiral cyclic nitrone as the key synthon. A facile route towards cyclic nitrones was developed starting from suitably protected hemiacetals of Dribofuranose. Readily available tri-O-benzyl-D-ribofuranose was allowed to react with hydroxylamine hydrochloride to afford an acyclic oxime. Selective silylation followed by iodonation at C-5 (with inversion of configuration) furnished the cyclisation precursor. Anhydrous TBAF-mediated desilylation and subsequent intramolecular nucleophilic attack afforded a cyclic nitrone in excellent yield. Following the same protocol, 2,3-isopropylidene-5- 0-trityl-D-ribofuranose was converted into the corresponding nitrone. The 1,3-dipolar cycloaddition reaction of a nitrone to an alkene is an extremely powerful synthetic method for the creation of complex heterocyclic structures. The reaction of the Dribose derived nitrones with a,(3-unsaturated carbonyl compounds furnished the corresponding cycloadducts in good diastereomeric excess. The exo-product was isolated as the major isomer in each case. The reaction of a variety of carbon nucleophiles, including a Grignard reagent, with the nitrones led to the formation of interesting p-C-glycoside analogues. One of the carbohydrate nitrones was also converted into its thymine C-nucleoside analogue. This work clearly shows that the construction of chiral cyclic nitrones from D-ribose derivatives is an extremely efficient and simple procedure. The final part of the work described in this thesis involve the construction of CD-ring analogues of the natural metabolite, streptonigrin. The use of metalation and palladium catalysed cross-coupling reactions were investigated for the synthesis of highly functionalised biaryls. The synthesis and crystal structure determination of [3-(tert-butoxycarbonylamino)-4- pyridyl]-trimethyltin(IV) is discussed. The Stille cross-coupling reaction between an electron rich arylstannane and an electron poor aryl halide proved to be the most successful. The use of co-catalytic copper(I) had a dramatic effect on the overall yield and rate of this Stille crosscoupling reaction. This methodology would , in principle, allow the construction of the natural product and appropriate structural analogues.

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The purpose of this investigation has been to make a comparison of the experimental and theoretically calculated electron spin resonance spectra of nitrogen heterocyclic radical ions. Because of the recent nature of the field, and because of recent advances in theoretical aspects, the theory of ESR and of the calculation of spin density distributions is reviewed in some detail in Chapter I. In Chapter III the ESR spectra of methyl substituted pyrazine and pyridine anions prepared by chemical reduction with alkali metals are described. Huckel and McLachlan calculations have been performed using various theoretical models and it has been found that presently used simple theories of spin density distributions give a reasonably good explanation of the ESR spectra of these aromatic radical ions. The modified ESR spectra of the ion-pairs of methyl substituted pyrazine, pyridine, and benzene anions with alkali metal cations are examined in Chapter IV. Factors influencing the stability of the ion-pairs are discussed and the theory of alkali metal hyperfine interaction is considered in some detail. It is found likely that in future work attention will have to be directed at the estimation of the energy of excitation from the anion to the cation. Finally, the ESR spectra of some aromatic heterocyclic N-oxide anions generated electrolytically are examined in Chapter V and the results of Huckel and McLachlan calculations of the spin density distributions in the ions are discussed. Because of the perturbed nature of the molecules agreement with the experimental distributions is found to be not quite as satisfactory as with the pyrazine anions. The mechanism of the nitrogen hyperfine interaction in N-oxide anions is investigated and it is suggested that this interaction is overwhelmingly due to the spin density on the nitrogen itself. This last aspect of the ESR spectra of N-oxide anions awaits more complete experimental confirmation. / Science, Faculty of / Chemistry, Department of / Graduate

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A search of the literature revealed that almost nothing is known concerning the class of compounds known as decahydroquinoxalines. Since this posed a problem involving a search for a systematic synthetic method of general application as well as a method for establishing the characteristic class and individual properties, a research project under the guidance of Dr. H. Smith Broadbent was initiated. Of particular interest aside from matters of purely chemical interest was the recognition that the hydroquinoxalines possess obvious structural similarities with atropine and other well-known drugs which have valuable application as hypotensive agents, analgesics, spasmolytic agents, parasympathetic depressants, antihistaminics, bronchodilators, etc. A catalyst consisting of 5% rhodium on alumina powder was reported by Baker and Co., Inc., to have certain special activity toward nitrogen heterocycles. This catalyst was applied in hydrogenation of alcohol solutions of quinoxaline and some of its substituted derivatives. At hydrogen pressures which ranged from 2000-5500 p.s.i. and temperatures of 100-200° C it was found that a variety of quinoxalines could be reduced completely to the decahydro compounds. The following decahydroquinoxalines were synthesized, described, and identified for the first time: decahydroquinoxaline, 6-methyldecahydroquinoxaline, 2,3,5,7-tetramethyldecahydroquinoxaline, 6-methoxydecahydroquinoxaline, 2,3-dimethyl-6-methoxydecahydroquinoxaline, and 6,7-dimethoxydecahydroquinoxaline. The existence of stereoisomeric forms of the decahydroquinoxalines is indicated, and it is believed that a separation of the cis and trans forms can be successfully accomplished by a combination of fractional crystallization and solvent recrystallization. Another method of separation depending on the differences of picrate solubilities in ethyl alcohol and in water is suggested. During the course of investigation five additional new compounds not previously reported in the literature were prepared and identified by elemental analysis. These were 6-phenylquinoxaline, 2,3,5,7-tetramethylquinoxaline, 6-phenyl-1,2,3,4-tetrahydroquinoxaline, 6-amino-1,2,3,4-tetrahydroquinoxaline, and 6,7-dimethoxy-1,2,3,4-tetrahydroquinoxaline. This fundamental research has opened up a convenient synthetic route for a whole new class of compounds and has in part indicated their characteristic properties.

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Ph.D., Faculty of Science, University of the Witwatersrand, 2011 / Atmospheric deposition of sulfur (S) and nitrogen (N) as a result of fossil fuel combustion is known to impact ecosystem structure and function. Potential impact includes acidification of soil and surface water and mobilisation of metal ions, with the resultant loss of plant productivity, changes in plant species diversity and changes in biotic communities in aquatic ecosystems. Rates of S (~8 kg S ha-1 year-1) and N (>6 kg S ha-1 year-1) deposition to the grasslands of the South African Highveld are comparable to other industrialised areas where ecosystem impacts have been observed. As part of a larger project, this work investigated four aspects of ecosystem impact: changes in soil and river water chemistry as well as S and N mineralisation rates. Reassessment of the soil chemistry at 18 sites on the South African Highveld after a 16-year period showed increases in both acidic and basic ion concentrations for individual sites and when the values for these sites were averaged to represent the study region. Grouping the soils by clay content showed that all sites with less than 25% clay (16 of 18 sites) showed significantly reduced pH(H2O) values. Sites with less than 4% clay showed increased exchangeable acidity and decreased acid neutralising capacity. Spatial scaling and mapping from site to soil form and land type, showed that across 92% of the study area the pH(H2O) values had been reduced. This method identified the sandier soils, near the southern and eastern boundaries of the study area where rainfall is higher, as sensitive to additional acidic inputs via atmospheric deposition. Clay-rich soils occur in the drier central part of the study area, close to emission sources. It is suggested that this proximity to emission sources results in the co-deposition of basic and acidic ions, adding to the buffering capacity of the soils, resulting in small but significant increases in soil acidity status over the 16 years. Sulfur and N mineralisation rates, using the in situ incubation method at 11 sites, were found to range between -0.66 and 1.09 μg SO42- g-1 soil day-1 and -0.97 and 1.21 μg N g-1 soil day-1. This translated into an annual flux of between -40 and 9.9 kg S ha-1 and between 27 and 81 kg N ha-1 from the soil organic pools. The use of the in situ incubation technique to determine S mineralisation is a new Theresa Bird 9505067D development and is proposed for in-field studies where S and N cycling are of interest as the method allows for concurrent mineralisation rate determination. It was found that from a biogeochemical perspective the Highveld grasslands are under researched with respect to S and N and complete assessments of the S and N cycles are proposed. The S budget proposes accretion of S in the soil organic pool due to continued inputs via deposition and low losses to the atmosphere or deeper soil horizons. Nitrogen, however, appears to limit productivity in these grasslands because atmospheric inputs and mineralisation rates are approximately equal to plant uptake. In the assessment of river water quality it was hypothesised that between 1991 and 2008 concentrations of dissolved salts, sulfate, nitrate and ammonium would increase in surface waters at five sites draining the Highveld grasslands. The Department of Water Affairs water quality monitoring database was accessed to assess for spatial and temporal differences in water quality. Significant spatial differences were found; however, over time few significant increases were found to support the hypothesis: sulfate, nitrate-plus-nitrite, and ammonium were observed to increase at one site each. In addition, the export of nitrogen, as mass load, from natural grasslands was found to be negligible at <2 kg N ha-1year-1. A conceptual framework proposes that soil texture, distance from emissions and land use are key drivers in the response of the grassland soils and surface waters to atmospheric S and N deposition. Although the study identified the soils most sensitive to deposition, it is proposed that processes in the Highveld grasslands are not yet negatively affected by the additional sulfur and nitrogen inputs. Continued monitoring for impacts on ecosystem structure and function is advocated.

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