Structure and synthesis of bioactive polyphenols from Cyclopia subternata (Honeybush) and Aspalathus linearis (Rooibos) tea: conformational analysis of selected chiral derivatives
| dc.contributor.advisor | Brandt, E. V. | |
| dc.contributor.advisor | Bezuidenhout, B. C. B. | |
| dc.contributor.advisor | Steenkamp, J. A. | |
| dc.contributor.author | Brand, Dirk Jacobus | |
| dc.date.accessioned | 2015-10-20T08:46:01Z | |
| dc.date.available | 2015-10-20T08:46:01Z | |
| dc.date.copyright | 2007-12-31 | |
| dc.date.issued | 2009-10-23 | |
| dc.date.submitted | 2007-12-31 | |
| dc.description.abstract | 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. | en_ZA |
| dc.description.abstract | Afrikaans: Navorsingsresultate uit die ondersoek op Cyclopia intermedia (Heuningbostee), wat die teenwoordigheid van antioksidatiewe polifenoliese flavonïede en hul glikosiede as metaboliete toon, was bydraend tot die steeds toenemende gewildheid van hierdie tee as gesondheidsdrankie. Die teenwoordigheid van hierdie verbindings in die tee, wat belowende farmakologiese eienskappe besit, die afwesigheid van kaffeïen bevat en die medisinale gebruik van die plant deur die inwoners van die Oos en Wes Kaap het verdere navorsing ook van C. subternata geregverdig. Die ekstrakte van die ongefermenteerde blare en takke van C. subternata is onderwerp aan verskeie chromatografiese skeidings wat ‘n nuwe flavaan, flavonole, flavanole, isoflavone en verskeie ander C6.C2- en C6.C1-tipe verbindings uit die plant gelewer het. Tydens die aanvanklike studie is die asetoon ekstrak ondersoek en verskeie sintetiese roetes verken om die nuwe flavaan aglikoon se struktuur te bevestig. In hierdie voortgesette studie is die methanol ekstrak van die plant volledig ondersoek en hoewel verskeie verbindings oorvleul met die uit die asetoon ekstrak, is addisionele verbindings geïdentifiseer wat bydra tot die polifenoliese profiel van C. subternata. Die metaboliete uit die plant is gepubliseer en word in Hoofstuk een kortliks bespreek. Sintese en spektroskopiese metodes, insluitend Kern Magnetiese Resonans Spektroskopie (1H KMR), Sirkulêre Dichroïsme en Massaspektrometrie is gebruik om die flavaan aglikoon se struktuur te bevestig. ‘Sagte’ ionisasie metodes is tydens massaspektrometrie gebruik om unieke diagnostiese fragmentione te produseer. Sintese van die flavaan het direkte vergelyking met hierdie fragmentione van die geïsoleerde flavaan moontlik gemaak en die struktuur bevestig. Die absolute konfugurasie word deur middel van Sirkulêre Dichroïsme bepaal. Die totaal sintese van vryfenoliese Aspalatien, ‘n potente antioksidant teenwoordig in die Suid Afrikaanse plant Aspalathus linearis (Rooibos tee), is ontwikkel. Die sintese van die aspalatien aglikoon, floretin, in waterige medium sonder die gebruik van beskermende groepe is gedemonstreer. Hierdie een-stap sintese, wat ‘n omgewingsvriendelike herwinbare katalis benut, het kommersieële potensiaal. Gedurende die afgelope 30 jaar is die Mosher reagens suksesvol gebruik as chirale derivatiserings reagens vir alkohole, amiede en gevolglik ook geselekteerde flavonoïede. Die impiriese model wat gebruik word om die absolute stereochemie van alkohole en amiede af te lei maak staat op ‘n mate van rigiditeit in hierdie ester. Die oorsprong van hierdie rigiditeit kon tot dusver egter nie oortuigend verduidelik word nie. Die stabiliserende hiperkonjugatiewe atoomorbitaal interaksies verantwoordelik vir hierdie rigiditeit en die molekulêre orbitale waar dit gesetel is, is geïdentifiseer deur middel van kwantummeganiese berekenings, 1H, 19F KMR, en infrarooi spektrometrie. Gegewe dat kristalle van flavonoïede baie raar is, is die kristalle van die epicatechin- Mosher ester wat hier verkry is uniek. X-straal kristallografie toon interressante konformasionele gedrag in hierdie kristal. Die hiperkonjugatiewe molekulêre modellerings model wat ontwikkel is om die Mosher ester te ondersoek is reeds gepubliseer en ook toegepas op ‘n moderne chirale derivatiserings reagens wat onlangs ontwikkel is. Hierdie chirale derivatiserings reagens, α-cyano-α-fluoro-p-tolyl-acetic acid (CFTA), toon ‘n grooter mate van rigiditeit en reaktiwiteit en is suksesvol gebruik in gevalle waar die Mosher ester onsuksesvol was. Ondersoek na die CFTA ester is uitgevoer in samewerking met n Japanese groep wat die CFTA reagens ontwikkel het. Kommersieële vervaardiging van CFTA word in die vooruitsig gestel. Die hiperkonjugatiewe model wat vir die molekulêre modelering van die Mosher ester gebruik is, is verfyn en toegepas op CFTA. Die oorsprong van die opvallende groot verskil in chemiese verskuiwing van 19F in die KMR spektra van die epicatechin-CFTA diastereoisomere is ook ondersoek en die resultate gepubliseer. | af |
| dc.description.sponsorship | National Research Foundation (NRF) | en_ZA |
| dc.description.sponsorship | ARC Infruitec - Nietvoorbij | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/1406 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Thesis (Ph.D. (Chemistry))--University of the Free State, 2007 | en_ZA |
| dc.subject | Polyphenols -- Synthesis | en_ZA |
| dc.subject | Herbal teas | en_ZA |
| dc.subject | Rooibos tea | en_ZA |
| dc.title | Structure and synthesis of bioactive polyphenols from Cyclopia subternata (Honeybush) and Aspalathus linearis (Rooibos) tea: conformational analysis of selected chiral derivatives | en_ZA |
| dc.type | Thesis | en_ZA |