Doctoral Degrees (Chemistry)
Permanent URI for this collection
Browse
Browsing Doctoral Degrees (Chemistry) by Author "Bezuidenhout, B. C. B."
Now showing 1 - 1 of 1
Results Per Page
Sort Options
Item Open Access Structure and synthesis of bioactive polyphenols from Cyclopia subternata (Honeybush) and Aspalathus linearis (Rooibos) tea: conformational analysis of selected chiral derivatives(University of the Free State, 2009-10-23) Brand, Dirk Jacobus; Brandt, E. V.; Bezuidenhout, B. C. B.; Steenkamp, J. A.English: Endorsed by results from our initial investigations of the unique South African tea, brewed from Cyclopia intermedia (Honeybush), demonstrating the presence of polyphenolic flavonoids and glycosides, the tea is gaining popularity as a health beverage. The presence of these compounds which are claimed to have beneficial pharmacological properties, supported by the fact that the tea contains no caffeine and has low tannin content, as well as its usage as a medicinal plant by the indigenous people of the Western and Eastern Cape, prompted continued investigations also into the metabolic content of Cyclopia subternata. Selected extracts of the unfermented shoots and stems of C. subternata were subjected to chromatographic separations that afforded a novel flavan, flavonols, flavanones, flavones, isoflavone and C6.C2- and C6.C1-type compounds. The content of the acetone extract has previously been reported as well as the exploration of various synthetic routes in an attempt to confirm the structure of the novel flavan O- glucoside isolated from the tea. A continuation of the investigation of fractions from the methanol extract of the plant yielded a few compounds that were not found in the acetone extract. This work has been summarized in Chapter one and published as the phenolic profile of Cyclopia subternata. The unambiguous confirmation of the structure of the flavan O-glucoside and essentially the novel aglycone isolated from the tea, necessitated the use of synthetic, Nuclear Magnetic Resonance spectrometric (1H NMR), mass spectroscopic and circular dichroism methods. Mass spectrometry (‘soft’ ionization) was utilized to produce signature fragment ions of the synthesized flavan and the aglycone of the natural flavan for direct comparison. Hydroxylation patterns of the synthetic flavan and the natural flavan aglycone, as well as the relative configuration and point of attachment of the sugar moiety was deduced from NMR experiments. The absolute configuration of the aglycone was determined by circular dichroism. The total synthesis of Aspalathin, a potent antioxidant found in the South African plant, Aspalathus linearis (Rooibos Tea), as the free phenol, was demonstrated. The aglycone of aspalathin, phloretin, also a good antioxidant, was synthesized in aqueous medium without the need for protective groups. This one-step synthesis of phloretin, utilizing a recyclable and environmentally friendly catalyst, has potential commercial applications. For the past 30 years, Mosher esters have extensively been utilized as chiral derivatizing agents (CDA’s) for secondary alcohols and amides with great success. The empirical model employed to deduce the absolute stereochemistry of the resultant ester or amide requires a certain degree of conformational rigidity for the CDA, the origin of which has not yet been determined. The stabilizing atomic orbital hyperconjugative interactions and the location of the prevalent molecular orbitals in such esters were identified with the aid of ab initio calculations, 1H, 19F NMR and infrared spectrometry. Although flavonoid crystal structures are very rare, a novel (-)-epicatechin-Mosher ester crystal was obtained and analyzed by x-ray crystallography with interesting conformational observations. The theoretical model used to investigate the hyperconjugative effects in the Mosher ester, was published and utilized on a recently developed novel CDA that shows a greater degree of rigidity. This CDA, α-cyano-α-fluoro-p-tolyl-acetic acid (CFTA) has greater reactivity and was utilized in cases where the Mosher acid proved inadequate. The investigation regarding the employment of CFTA was carried out with Japanese collaboration and commercial production of the reagent is envisaged. The hyperconjugative molecular model used in the modeling of the Mosher reagent was refined and extended to CFTA. The origin of the very large fluorine chemical shift difference between the epicatechin-CFTA diastereomers was investigated and the results published.