The structure and synthesis of oligoflavanoids and oligostilbenes from cassia abbreviata
| dc.contributor.advisor | Malan, E. | |
| dc.contributor.advisor | Coetzee, J. C. | |
| dc.contributor.author | Mthembu, Makhosazana Claribel | |
| dc.date.accessioned | 2018-06-04T10:14:03Z | |
| dc.date.available | 2018-06-04T10:14:03Z | |
| dc.date.issued | 2000-01 | |
| dc.description.abstract | English: During the present study the acetone extract of the heartwood of Cassia abbreviata afforded the known flavan-f-ols, catechin, epicatechin, epiafzelechin, afzelechin and the new (2R,3S) guibourtinidol. This was the first natural occuring 4',7-dihydroxy substituted flavan-f-ol to be isolated. The protocol recently developed in our laboratories using asymmetric dihydroxylation of 1,3-diarylpropenes and subsequent acid-catalyzed cyclization was used to yield the four diastereomers and amongst them the one isolated from C. abbreviata. Proguibourtinidin dimers were identified as the permethyl ether acetates namely the four known dimers only isolated now for the second time, guibourtinidol-( 4α →8)-catechin, - epicatechin, -afzelechin and epiafzelechin. (2S,3R,4R)-Guibourtinidol-( 4 β →8)- afzelechin and (2R,3S,4S)-guibourtinidol-(4α →6)-afzelechin are two new dimers from the heartwood. With extensive NOESY and COSY NMR-experiments it was possible not only to elucidate the structures and the stereochemistry of all the proguibourtinidin dimers but also the preferred conformations of the two rotamers of each compound. The assignment of the structures and configuration of the two dimeric stilbenes (cyclobutane derivatives) required a special effort due to the similarity and very small differences in their 1H NMR spectra, l3C NMR spectra, mass spectral fragmentation pattern and the inability to grow crystals suitable for X-ray studies. From the synthesis using photodimerization of the monomenc 3,3',4',5- tetramethoxystilbene and limited structural information from the literature, 1H NMR and MS it was possible to present structures for the two dimeric 3,3',4',5- tetramethoxystilbenes as α-trixillic and β-truxinic configurations. The monomeric 3,3',4',5-tetrahydroxystilbene, the assumed precursor for the cyclobutane compound was also detected in the acetone extract. Partnering the stilbene were the two bibenzyls namely 3,3',4',5-tetrahydroxy- and 3,4',5-trihydroxydihydrostilbene. Extensive investigative attempts were conducted to synthesize the trimeric procassinidin, cassiaflavan-( 4→8)-epigallacatechin-( 4→6)-cassiaflavan isolated from Cassia petersiana. From the model reactions executed excellent control regarding regio- and stereoselectivity during C4→C8 formation and good yields were obtained in the synthesis of different cassiaflavan-flavan-3-o1 dimers. However, when the same procedure was appplied to accomplish the C4→C6 coupling between the top dimeric unit and the bottom cassiaflavan, it was not successful. | en_ZA |
| dc.description.abstract | Afrikaans: Gedurende die huidige ondersoek van die asetoonekstrak van die kernhout van Cassia abbreviata is die bekende flavan-3-ole, katesjien, epikatesjien, afzelesjien, epiafzelesjien en die nuwe (2R,3S)-guibourtinidol geïsoleer. Hierdie was die eerste 4',7 -dihidroksi flavan-3-ol wat uit die natuurlike materiaal kom. Deur gebruik te maak van 'n metodologie wat in hierdie laboratoriums ontwikkel is, deur asimmetriese dihidroksilering van 1,3-diarielpropene, gevolg deur suurgekataliseerde siklisering is die vier diastereomere berei en die een is soortgelyk aan die verbinding wat vanuit C. abbreviata geïsoleer is. Proguibourtinidien dimere is geïdentifiseer as metieleterasetaat derivate waarvan vier bekendes slegs vir die tweede keer geïsoleer is nl. guibourtinidol-(-( 4α →8)-katesjien, - epikatesjien, -afzelesjien en -epiafzelesjien. (2S,3R,4R)-Guibourtinidol-( 4β →8)-afzelesjien en (2R,3S,4S)-guibourtinidol-(4α →6)- afzelesjien is twee nuwe dimere wat vanuit die kernhout gehaal is. Met behulp van omvattende NOESY en COSY KMR-eksperimente was dit moontlik om die strukture sowel as die stereochemie op te klaar maar hiermee tesame is die konformasies van die twee rotamere van elke dimeer ook vasgestel. Die toekenning van die strukture en die konfigurasie van die twee dimeriese stilbene (siklobutaan derivate) het 'n grootse poging geverg omrede die klein verskille wat daar is in die 1H KMR, 13C KMR, massaspektrometriese fragmentasie patroon en die feit dat geen bruikbare kristalle vir X-straal analise verkry kon word nie. Fotodimerisasie van die monomeriese 3,3',4',5' -tetrametoksi-stilbeen en beperkte informasie vanuit die literatuur, 1H KMR en MS was dit moontlik om die strukture van die twee dimeriese 3,3',4',5-tetrametoksistilbene as o-trixillies en ~-truxinies voor te stel. Die monomeriese 3,3',4' ,5-tetrahidroksistilbeen wat aanvaar word as die voorloper van die twee siklobutaan verbindings is ook in die asetoonekstrak opgespoor. Tesame met die stilbeen is die twee bibensiliese verbindings nl. 3,3',4' ,5-tetrahidroksidihidrostilbeen en 3,4' ,5-trihidroksidihidrostilbeen gevind. Omvattende ondersoekende pogings is uitgevoer om die trimeriese procassinidien, cassiaflavaan-( 4→8)-epigallokatesjien-( 4→6)-casssiaflavaan wat vanuit Cassia petersiana geïsoleer is te sintetiseer. Verskeie model reaksies is uitgevoer wat baie goeie regio- en stereoselektiwiteit vir die C4→C8 binding vorming gelewer het. Verskeie cassiaflavaan-flavan-S-ol dimere is as modelle gebruik. Die finale koppeling van die C4→C6 binding tussen die dimeriese top eenheid en die onderste cassiaflavaan was na verskeie pogings nie suksesvol nie. | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/8370 | |
| 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 | Cassia abbreviata | en_ZA |
| dc.subject | Leguminosae | en_ZA |
| dc.subject | Flavan-3-ols | en_ZA |
| dc.subject | Bibenzyls | en_ZA |
| dc.subject | Stilbenes | en_ZA |
| dc.subject | Dimeric stilbenes | en_ZA |
| dc.subject | Proguibourtinidins | en_ZA |
| dc.subject | Biomimetic synthesis | en_ZA |
| dc.subject | Flavonoids | en_ZA |
| dc.subject | Biomimetics | en_ZA |
| dc.subject | Plant pigments | en_ZA |
| dc.subject | Dissertation (M.Sc. (Chemistry))--University of the Free State, 2000 | en_ZA |
| dc.title | The structure and synthesis of oligoflavanoids and oligostilbenes from cassia abbreviata | en_ZA |
| dc.type | Dissertation | en_ZA |