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.

M.Sc. (Chemistry) / Please refer to full text to view abstract

In 1896, Becquerel discovered the natural radioactivity in potassium uranyl sulphate. Since then, Pierre and Marie Curie, E. Rutherford and F. Soddy all made tremendous contributions to the discovery of many other radioactive elements. The work of all these scientists has shown that all elements found in nature with an atomic number greater than 83 (bismuth) are radioactive. Artificial radioactivity was first reported by I. Curie and F. Joliot in 1934. These scientists irradiated boron and aluminium targets with a particles from polonium and observed positrons emitted from the target even after removal of the a particle source. This discovery of induced or artificial radioactivity opened up a brand new field of tremendous importance. Around the same time, the discovery of the cyclotron, deuteron and neutron by various scientists facilitated the discovery of many more artificial radioactivities. At present time more than 2700 radionuclides have been produced artificially in the cyclotron, the nuclear reactor, the neutron generator and linear accelerator. Radiopharmaceuticals are drugs that contain a radionuclide and are used for imaging if the radionuclide is a photon emitter (gamma-g or positron-b+) or for therapy if the radionuclide is a particle emitter (alpha-a or beta-b- or Auger/conversion e-). / Prof. A. Roodt

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The chemistry of homogeneous transition metal systems offer parallels to the reactions on the surfaces of industrial hydrodesulphurization catalysts. The reactions of several ruthenium complexes with sulphur-containing reagents are described, with an emphasis on the kinetics and mechanisms thereof. The complex Ru(CO)₂(PPh₃)₃ (2), for example, reacts quickly with thiols and disulphides, producing cct-RuH(SR)(CO)₂(PPh₃)₂ (9) and cct-Ru(SR)₂(CO)₂(PPh₃)₂ (14), respectively, although 2 fails to react with unstrained thioethers. Reactions of the related complex Ru(CO)₂(PPh₃)(dpm) (dpm=Ph₂PCH₂PPh₂) are complicated by the lability of all of the three different ligands. The two dihydrides cct-RuH₂(CO)₂(PPh₃)₂ (3) and RuH₂(dpm)₂, as a cis/trans mixture (7), react with thiols to produce the hydrido-thiolato complexes 9 and RuH(SR)(dpm)₂ (13). respectively. The mechanisms appear to depend on the basicity of the hydride ligands; the more basic dihydride, 7, is probably protonated by the thiol, giving an unobserved molecular hydrogen intermediate, while 3 reacts by slow reductive elimination of H₂. The same rate constant, rate law, and activation parameters are found for the reaction of 3 with thiols, CO or PPh₃. The reaction of 3 with RSSR produces mostly 9, with small amounts of 14. The complete characterization of several members of the series 9 and 14 is described, including the crystal structure of the p-thiocresolate example of each. The reactions of 9 with other thiols, P(C₆H₄pCH₃)₃, CO, RSSR, HCl, PPh₃, and H₂, are also reported. The first three of these reactions share the same rate law and rate constant, the common rate determining step probably being initial loss of PPh₃. Some equilibrium constants for the exchange reactions of 9d (R=CH₂CH₃) with other thiols were tetermined, the Keq values increasing with the acidity of the incoming thiol. The mercapto hydrogens of 9a and 14a (R=H) exchange with the acidic deuterons of added CD₃OD. The hydridic and ortho-phenyl hydrogens exchange more slowly, presumably by intramolecular processes. Complex 14b (R=C₆H₄pCH₃) is unstable in the presence of light, exchanges phosphines rapidly with added P(C₆H₄pCH₃)₃, exchanges thiolate groups with added thiols, and is converted by high pressures of H₂ to a mixture of 9b and 3. Intermediates proposed for the mechanism of the thiol exchange reactions of 9 and 14 contain two or three thiolate groups sharing a proton. A related complex, [Ru(CO)₂(PPh₃)(μSEt)₂(μ₃SEt)Na(THF)]₂, which contains three thiolate groups on a ruthenium centre sharing a sodium cation, was isolated from the reaction of cct-RuCl₂(CO)₂(PPh₃)₂ with sodium ethanethiolate. In acetone, 9b and 14b can be formed cleanly from cct-RuHCl(CO)₂(PPh₃)₂ and cct-RuCl₂(CO)₂(PPh₃)₂, respectively, by reaction with p-thiocresolate. Complex 3 or cheaper analogues could be used as catalysts for the reduction of disulphides by H₂, or as recyclable reagents for the non-oxidative extraction of thiols from thiol-containing mixtures such as oil fractions. The chemistry described above will help to guide future researchers to systems that more closely parallel the processes occurring on the surfaces of industrial hydrodesulphurization catalysts. / Science, Faculty of / Chemistry, Department of / Graduate

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published_or_final_version / Chemistry / Master / Master of Science