Thesis (MSc)--Stellenbosch University, 2002. / ENGLISH ABSTRACT: Mycotoxins have assumed worldwide importance due to the ubiquitous occurrence of
toxigenic fungi, their infestation of plant-based foods and feeds and the subsequent
economical and health impact it because of their contamination of commercial
products. Ochratoxin A (OA) is a nephrotoxic mycotoxin produced by isolates of
Aspergillus ochraceus and Penicillium verrucosum and occurs frequently in nature.
The major target for toxicity of OA in mammalian species is the kidneys and it has
been the major cause of Danish Porcine Nephropathy. OA has also been extensively
implicated in the aetiology of Balkan Endemic Nephropathy and Chronic Interstitial
Nephropathy in Northern-Africa. Furthermore, OA has been identified as a
carcinogen, an immunosuppressant and a teratogen with respect to the foetal central
nervous system.
Although a large amount of research has been conducted into the chemical nature of
the toxicity of OA, the exact molecular mechanism of action of OA is not yet
conclusive. Numerous structure activity relationship studies have suggested that the
toxicity of OA may be assigned to three major processes: (i) inhibition of ATP
production; (ii) inhibition of protein synthesis; and (iii) the disruption of hepatic
microsomal calcium homeostasis through the promotion of membrane lipid
peroxidation. It is the aim of this thesis to gain a better understanding, through the
synthesis ofOA analogues, of the chemical structure responsible for the toxic function
of the ochratoxins.
The halogen-group has extensively been implicated in the toxicity of the ochratoxins.
This is evident in ochratoxin B (OB), the dechloro analogue of OA, which is
approximately ten times less toxic than OA. Preliminary tests have indicated that
bromo-ochratoxin B(BrOB), the bromo analogue of OA, is more toxic than ochratoxin
A to renal cells. Fluoro-ochratoxin B and other analogues of OA, where other amino
acids are incorporated, should provide invaluable information on the structure-activity
relationships and the mode of action of the ochratoxins. Our research effort addresses
both these aspects (i) fluorination of the dihydroisocoumarin moiety and (ii) the coupling of different amino acids and dipeptides to the non-toxic hydrolysed product
of OA, ochratoxin a.
Chapter one includes a review of the important biological aspects of OA that has
served as a guideline to the synthesis of effective OA analogues. An overview of the
relevant chemistry involved in the modification of OA will conclude the chapter.
Chapter two entails a discussion of fluorine in bio-organic chemistry. This includes an
overview of the impact that fluorine substitution has on the biological reactivity of
molecules. A review on the synthesis of organofluorine compounds, which forms the
emphasis of this study, concludes the chapter.
Chapter three elaborates on the different methodologies used in our attempts to
synthesise fluoro-ochratoxin B and other analogues. These included the direct
electrophilic fluorination of OB and different analogous aromatic model compounds
by xenon difluoride, N-fluorobenzenesulfonimides and Selectfluor™ as fluorinating
agents. Also involved is an investigation into an alternative route for the synthesis of
fluoro aromatic compounds from bromo and chloro analogues by means of palladium
catalysed trimethyl- and tributylstannyl and trimethylsilylation which in tum may be
substituted with fluorine by means of xenon difluoride. Efforts towards the direct
catalytic fluorosubstitution of aryl halides are also investigated. The synthesis of a key
intermediate, fluoroacetoacetaldehyde, in a de nova synthetic route to fluoroochratoxin
B is also discussed.
Furthermore, the synthesis of novel OA analogues with respect to the replacement of
the L-phenylalanine moiety is addressed. This includes the conversion of OA to Oa,
by acid hydrolysis, followed by the coupling of ortho-, meta- and para- substituted
DL-fluorophenylalanine to the lactone acid. This is followed by the synthesis of
histidylhistidine methyl ester and attempted coupling to Oa. The coupling of
halosalicylic acids and salicylic acid to L-phenylalanine, for use as model aromatic
substrates for fluorination, IS discussed. Peptide coupling by
dicyclohexylcarbodiimide carboxyl activation, with reference to the protection of the
phenolic hydroxyl group in 5-chlorosalicylic acid for application to Oa, concludes
this work. / AFRIKAANSE OPSOMMING: Mikotoksiene is van wêreld-wye belang as gevolg van die alomteenwoordige
voorkoms van toksigeniese fungi, hul besmetting van plantaardige kossoorte en
voerstowwe en die gevolglike ekonomiese en gesondheidsimpak deur die
besoedeling van kommersiële produkte. Ochratoksien A (OA) is 'n nefrotoksiese
mikotoksien wat geproduseer word deur isolate van Aspergillus ochraceus en
Penicillium verrucosum en kom algemeen in die natuur voor. Die niere is die hoof
teiken vir vergifiting deur OA in soogdierspesies en is as die vername oorsaak van
"Danish Porcine Nephropathy" aangewys. OA word verder aangedui as die oorsaak
vir "Balkan Endemic Nephropathy" en "Chronic Interstitial Nephropathy" in Noord-
Afrika. OA is verder geïdentifiseer as 'n karsinogeen, immuno-onderdrukker en is
teratogenies ten opsigte van die sentrale senuweestelsel van fetusse.
Alhoewel aansienlike navorsing alreeds gewei is aan die chemiese natuur van die
toksisiteit van OA, is die presiese molekulêre meganisme van OA reaktiwiteit
onbeslis. Verskeie struktuur-aktiwitweit verwantskaps studies dui daarop dat die
toksisiteit van OA hoofsaaklik toegeskryf kan word aan drie hoof prosesse: (i)
inhibisie van ATP produksie; (ii) inhibisie van proteïen sintese; en (iii) die
ontwrigting van hepatiese mikrosomale kalsiumhomeostase deur die bevordering van
membraanlipiedperoksidasie. Hierdie tesis het ten doel, deur die sintese van OA
analoë, om 'n beter insig oor die chemiese struktuur wat verantwoordelik is vir die
toksiese funksionaliteit van ochratoksiene te verkry.
Die halogeen substituent is grootliks geïmpliseer in die toksisiteit van OA. 'n Bewys
hiervan is ochratoksien B (OB), die dechlooranaloog van OA, wat ongeveer tien maal
minder toksies is as OA. Voorlopige ondersoeke het aangetoon dat bromoochratoksien
B (BrOB), die broomanaloog van OA, meer toksies is vir nierselle as
OA. Fluoorochratoksien B en ander analoë van OA, waar ander aminosure
geïnkorporeer word, behoort waardevolle inligting te voorsien met betrekking tot die
struktuur-aktiwiteitsverwantskappe en die wyse waarop ochratoksiene funksioneer.
Hierdie navorsingspoging spreek beide aspekte aan; (i) die fluorering van die dihidroïsokumarien gedeelte en, (ii) die koppeling van verskillende armnosure en
dipeptiede aan die nie-toksiese hidrolieseproduk van OA, nl. ochratoksien a.
Hoofstuk een vervat 'n oorsig van die belangrike biologiese aspekte van OA wat dien
as riglyn vir die sintese van doeltreffende OA analoë. Die hoofstuk word afgesluit met
'n oorsig van die relevante chemie betrokke by die modifisering van die struktuur van
OA.
Hoofstuk twee bevat 'n bespreking van die aanwending van fluoor in bio-organiese
chemie. Dit bevat 'n oorsig van die impak wat fluoorsubstitusie het op die biologiese
reaktiwiteit van molekules. 'n Opsomming oor die sintese van
organofluoorverbindings, wat die essensie van hierdie studie is, beëindig die hoofstuk.
Hoofstuk drie handeloor die veskillende metodes wat toegepas is in pogings om
fluoorochratoksien B en ander analoë te sintetiseer. Dit sluit in die direkte
elektrofiliese fluorering van OB en ander verwante aromatiese modelverbindings deur
gebruik te maak van xenondifluoried, N-fluoorbenseensulfonimied en Selectfluor™ as
fluoreringsreagense. Dit behels verder ook 'n ondersoek na 'n alternatiewe roete tot
die sintese van fluooraromatiese verbindings vanaf broom- en chlooranaloë. Vir die
doel word palladiumgekataliseerde trimetiel- en tributielstannilering, en
trimetielsililering wat vervolgens deur middel van xenondifluoried met fluoor
gesubstitueer kan word, aangewend. Pogings tot die direkte katalitiese
fluoorsubstitusie van arielhaliede word ook bespreek. Die sintese van 'n sleutelintermediêr,
fluoroasetoasetaldehied, in 'n de nova sintese roete tot fluoorochratoksien
B word bespreek.
Die sintese van nuwe OA analoë, met betrekking to die vervangmg van die Lfenielalanien
(L-Phe) groep word ondersoek. Dit bevat die omsetting van OA na Oa,
deur suurhidrolise, gevolg deur die koppeling van orto-, meta- en paragesubstitueerde
DL-fluoorfenielalanien aan die laktoonsuur, Oa. Daarna word die
sintese van histidielhistidienmetielester en die verdere pogings aangaande koppeling
met Oa bespreek. Die koppeling van halosalisielsure en salisielsuur aan L-Phe wat
dien as model aromatiese verbindings vir fluorering, word behandel. Peptiedkoppeling
met behulp van disikloheksielkarbodiimied-karboksielaktivering, met inbegrip van die beskerming van die fenoliese hidroksiel groep m 5-chloorsalisielsuur Vir die
toepassing op Oa, beëindig hierdie werk.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53070 |
| Date | 03 1900 |
| Creators | Gabrielli, William Fullard |
| Contributors | Bredenkamp, M. W., Steyn, P. S., Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science . |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 193 p. : ill. |
| Rights | Stellenbosch University |
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