Masters Degrees (Chemistry)
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Browsing Masters Degrees (Chemistry) by Author "Bonnet, S. L."
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Item Open Access Photochemistry of (+)-catechin and (-)-epicatechin(University of the Free State, 2008-01) Wilhelm, Anke; Van der Westhuizen, J. H.; Bonnet, S. L.Despite the well known fact that photolysis of free phenolic catechins give rise to isomerisation at the C-2 position (e.g. (-)-cis epicatechin converts to the sterically less hindered (-)-trans isomer), researchers have failed to isolate any ring opened compounds via trapping of intermediates with nucleophiles such as methanol or ethanol and radical trap solvents such as 2-propanol. Re-closing of the ring was slow enough to allow bond rotation to yield the observed isomerisation at C-2 but too fast to allow trapping of the intermediate by methanol or 2-propanol. This is unexpected given that thermal ring opening under mild conditions with acid, base or BF3 catalysis had resulted in the isolation of many ring opened species. Our aim was to reinvestigate the photochemistry of free phenolic (+)-catechin, (-)- epicatechin and (+)-fisetinidol at 250 nm and to trap the putative ring opened intermediates with a soft carbon centred nucleophile such as phloroglucinol. Photolysis of (+)-catechin in the presence of phloroglucinol with methanol as solvent resulted in the isolation of the optically active product 1,3-di(2,4,6-trihydroxyphenyl)-1- (3,4-dihydroxyphenyl)propan-2-ol with (1S,2S) absolute configuration and unreacted optically active starting material. Photolysis of (-)-epicatechin under the same conditions resulted in the isolation of the optically active product 1,3-di(2,4,6-trihydroxyphenyl)-1-(3,4-dihydroxyphenyl)propan- 2-ol with (1R,2R) absolute configuration, unreacted optically active starting material (-)- epicatechin, as well as (-)-ent-catechin. The two above mentioned products are enantiomers and have identical NMR spectra, but mirror image CD spectra. The two starting materials, (+)-catechin and (-)-epicatechin, are diastereoisomers and do not have identical NMR spectra. Acetylated (-)-ent-catechin from photolysis of (-)-epicatechin has the same NMR spectra as acetylated (+)-catechin but mirror image CD spectra. Identification of the methoxy-trapped products, 2-((2S,3R)-2-acetoxy-3-(3',4'- diacetoxyphenyl)-3-methoxypropyl)benzene-2',4',6'-triyl triacetate and 2-((2S,3S)-2- acetoxy-3-(3',4'-diacetoxyphenyl)-3-methoxypropyl)benzene-2',4',6'-triyl triacetate, indicates an ionic mechanism, as a radical mechanism would result in a —CH2OH substituted product. The absence of any coupling products in photolysis of (+)-3',4',5,7-tetra-Omethylcatechin, indicates that a free phenolic OH on the 1-position of the B-ring is essential to stabilize the carbocation intermediate long enough for condensation to take place via a quinone methide. Remarkable is the complete stereoselectivity. This indicates that the 3-hydroxy group allows the bulky phloroglucinol group to attack the quinone methide from the antiposition only. Photolysis of (+)-fisetinidol under the same conditions as irradiation of (+)-catechin, yielded the expected propan-2-ol, (1S,2S)-3-(2,4-dihydroxyphenyl)-1-(3,4- dihydroxyphenyl)-1-(2,4,6-trihydroxyphenyl)-propan-2-ol. Our photolytic synthesized products, (1S,2S)-1,3-di(2,4,6-trihydroxyphenyl)-1-(3,4- dihydroxyphenyl)propan-2-ol and (1R,2R)-1,3-di(2,4,6-trihydroxyphenyl)-1-(3,4- dihydroxyphenyl)propan-2-ol, also have a diaryl chromophore in the 1-position. We established an aromatic quadrant based rule to correlate the stereochemistry of the biaryl moiety on C-1 with the sign of the Cotton effect of the CD spectra. This rule is in agreement with previous rules established for 4-arylflavan-3-ols. Photolysis of (+)-3',4',5,7-tetra-O-methylcatechin in methanol and in the presence of 5 eq. phloroglucinol gave no coupling. Isolation of 2-(2-hydroxy-2-methylpropyl)-3,5- dimethoxyphenol in the presence of acetone represents trapping of the o-quinone methide. Irradiation of (+)-3-O-tosyl-3',4',5,7-tetra-O-methylcatechin (better leaving group on C-3) at 300 nm gave (+)-3',4',5,7-tetra-O-methylcatechin. Retention of the absolute configuration at C-3 indicates that fission of the O-S bond took place and not the C-O bond. We postulated that the sulfone group acted as chromophore of the photochemically active compound and not the aromatic rings.Item Open Access Photochemistry of pentoxifylline: a xanthine derivative(Unversity of the Free State, 2006-11) Han, Ze; Van der Westhuizen, J. H.; Bonnet, S. L.English: Pentoxifylline [1-(5'-oxohexyl)-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione], sold under the trade name Trental®, is a methylxanthine derivative used in treatment of peripheral and cerebrovascular diseases and poor regional microcirculation (intermittent claudication). It has recently been investigated as an antitumor agent. It improves tumor perfusion and influences cytokine –mediated inflammation. Our objectives were to synthesise 1-(3-oxobutyl)-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione and some of its derivatives for use as internal standards in the determination of biological fluids by liquid chromatography and for pharmaceutical/ biological screening as enzyme inhibitors. These efforts were hampered by the low reactivity of the N-1 position on the theobromine towards alkylation with electrophiles. As an alternative method to achieve the aforementioned goals, we investigated the photochemistry of pentoxifylline. Of particular interest was the fact that pentoxyphylline has two chromophores, i.e. carbonyl and xanthine, separated by a linear butyl alkyl chain. We now report a series of photochemical reactions of pentoxifylline and reaction conditions that were used to synthesise novel analogues. The carbonyl moiety reacted predictably to yield three products in toluene. Norrish II fission yielded 1-allyl-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2,6-dione (A) in yields of up to 40%, and Yang cyclisation yielded (R*, R*,)-(±)-1-{[2-Hydroxy-2-methylcyclobutyl]methyl}-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione/(B) (10% yield). The ratio of these two products was always 4:1. The expected racemic 1-(5-hydroxyhexyl)-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione / lisophylline (C) (6.5% yield) was isolated via photo-reduction of the carbonyl group to an alcohol. From TLC chromatograpy it appeared that tributyltin hydride increased the yield of these three products. A subsequent HPLC analysis proved this to be wrong, but affirmed the 4:1 ratio of A: B. In benzene as solvent, no lisophylline was obtained. This, together with the fact that the highest yield of (A) was obtained in benzene, indicated that the methyl group of toluene acted as a hydrogen donor during reduction of the carbonyl group. The photo-sensitisation and photo-initiation of pentoxifylline in methanol, ethanol and 2-propanol in the absence of oxygen led to the formation of the C-8 α-hydroxylalkyl analogues of pentoxifylline. Yet, in the presence of oxygen all these C-8 substituted products 8-(1-hydroxy-1-methylethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2, 6-dione (D), 8-(1-hydroxymethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihy- dro-1H-purine-2,6-dione (E) and 8-(1-hydroxyethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2,6-dione (F) were not produced, while the carbonyl photo- chemical products A, B and C were formed in the same yields as those in the toluene reaction. These facts can be explained that triplet ground state oxygen quenches a triplet-excited state of xanthine but not the singlet-excited state of the carbonyl functionality. The yield of the reduction product (lisophylline) was not improved by the addition of tri-butyltin hydride (TBTH). This observation indicated that the pentoxifylline carbonyl group reacted via singlet-excited states and yielded products A, B and C. The improvement of the yield from 32 to 48% with naphtalene and the decrease in the yield with benzophenone supports a singlet intermediate in the Norrish II type reaction of the carbonyl moiety in pentoxifylline. The tri N-substituted xanthine moiety coupled photochemically with isopropanol to yield 8-(1'-hydroxy-1-methyl)ethyl pentoxifylline (D). This reaction involves substitution of the aromatic 8-hydrogen with an isopropyl group, probably via radical initiated aromatic substitution. The highest yield of this product (55%) was obtained in the presence of 50% acetone. This supports a triplet mechanism for the excited xanthine chromophore. Several unknown products were isolated in low yields from the 2-propanol, EtOH/acetone photochemical reaction mixtures where further purification and structure elucidation will be performed. These are likely products derived from some new rearrangements of 8-substituted products. We have developed methods to expand the range of derivatives of pentoxifylline that can be synthesised in reasonable yields. These products will be used as internal standards for bio-analytical purposes and in our biological assays. Conditions have been established that selectively encourage reactions at the carbonyl moiety (toluene, triplet quencher) or the xanthine moiety (protic solvents, photosensitiser or radical initiator).Item Open Access Reactions of polyphenols with α-keto acids(University of the Free State, 2007-01) Montsho, Rosinah Maiyane; Van der Westhuizen, J. H.; Bonnet, S. L.English: Novel methods of carbon-carbon bond formation are of considerable theoretical and practical interest to synthetic organic chemists. This work investigates the formation and synthetic potential of a methine bond (one carbon link) between two aromatic moieties to form diphenylmethane derivatives. This methine link is of industrial importance when the aromatic moiety is hydroxylated. The colour stability of red wine is attributed to a methine bond that is the result of condensation between glyoxylic or pyruvic acid and an anthocyanidin. This bond may be formed spontaneously during the ageing of wine. Wattle extract based adhesives rely on the reaction between formaldehyde and polyphenols to form methine linked polymers. Patented antioxidants rely on the availability of a benzylic proton on a methine link, ortho to a hydroxy group (Irganox®HP-136). The proximity of the two carbonyl double bonds in α-dicarbonyl compounds enhances the reactivity of each other towards nucleophiles. In the case of α-keto acids the α- keto group is more electrophilic than the carboxylic group and susceptible to attack by nucleophiles. The hydroxy groups of phloroglucinol and other polyhydroxybenzenes donate electrons to the aromatic ring to increase the nucleophilicity of the aromatic carbons. Polyphenols thus become ambident nucleophiles that can react either via oxygen or carbon and have the ability to form new carbon-carbon bonds with suitable electrophiles. As part of our ongoing investigation into the importance of p-quinone methides in flavonoid chemistry the reaction of a variety of polyhydroxyphenols with α-keto acids were investigated. Addition of an aromatic ring to a carbonyl group creates a benzylic hydroxy group. With strongly nucleophilic aromatic rings this benzylic substituent is replaced with a second aromatic ring to yield the anticipated methine linked biaryl compound. Phloroglucinol reacts with pyruvic acid to give 4,6-dihydroxy-3-methyl-3-(2,4,6- trihydroxyphenyl)-1-benzofuran-2(3H)-one and with glyoxylic acid to yield 4,6- dihydroxy-3-(2,4,6-trihydroxyphenyl)-1-benzofuran-2(3H)-one. These products are lactones between the phenolic- and carboxylic acid moiety of an intermediate biaryl organic acid. With oxaloacetic acid a 4,5′,6,7′-tetrahydroxy-2H-spiro[benzofuran-3,4′- chroman]-2,2′-dione is isolated. With unreactive aromatic nucleophiles the benzylic hydroxy group is eliminated before substitution can take place if hydrogen is available in the α-position. Tri-omethylphloroglucinol reacts with pyruvic acid to give methyl-2-(2,4,6- trimethoxyphenyl)-acrylate via the elimination of water. This acrylic acid reacts with ozone to form methyloxo-(2,4,6-trimethoxyphenyl)-acetate and with diazomethane to form 2-methoxy(2,4,6-trimethoxyphenyl)-4,5-dihydrofurane. To demonstrate the potential of this reaction we reacted resorcinol with phydroxyphenylpyruvic acid and obtained both the Z and E isomers of 6-hydroxy-3-(4- hydroxybenzylidene)-3H-benzofuran-2-one. This isoaurone synthesis represents an improvement on the recently published synthesis of this natural product. We have developed a novel reaction to form carbon-carbon bonds and synthesize methine linked diaryl compounds. We have developed this reaction into a new procedure to synthesize free phenolic 3-substituted benzofuran-2-ones. We adapted this reaction to improve a recently published method to synthesize a free phenolic isoaurone. We can use our reaction to synthesize acrylic acids with a phenolic substitutuent in the α-position and have started to explore the potential of this α,β- unsaturated carboxylic acid as intermediates for various synthetic procedures.Item Open Access A solid state NMR and MS characterisation of the chemical composition of mimosa bark extract(University of the Free State, 2011-01) Senekal, Nadine D.; Van der Westhuizen, J. H.; Bonnet, S. L.; Reid, D.English: Mimosa (Acacia mearnsii) also known as black wattle, and quebracho (Schinopsis balansae, Schinopsis lorentzii) are the major commercial sources of natural condensed tannins (proanthocyanidin oligomers) used today. Mimosa bark is harvested from commercial plantations in South Africa which, according to a survey done by the Department of Water Affairs and Forestry for 2001, cover an area of about 107 000 hectares in South Africa. Quebracho is extracted from the wood of natural forests in Brazil and Argentina. Mimosa bark is extracted with water (about 50% by weight). Tara (Cæsalpinia spinosa) and Italian chestnut (Castanea sativa) are the major commercial sources of hydrolysable tannins. The ability of water soluble hydrolysable and condensed tannins (polyphenols) to react with proteins, presumably via hydrogen bonds, lies at the heart of their ability to transform raw hide into leather and their commercial application as tannin agents. It explains their existence in nature as anti-feeding agents as it renders plants indigestible to insects and herbivores. It also explains the use of milk in tea where the complexation of milk proteins with tea tannins reduces astringency. The chemistry of this process however remains uncertain. The polyphenolic nature also renders tannin extracts very susceptible to oxidation and further polymerisation and rearrangements that render the extracts even more complex. This is evident in the transformation of green tea (high flavan-3-ol content and low condensed tannin content) into Indian or black tea (low flavan-3-ol content and high condensed tannin content). The quality of red wine is to a large extent determined by the amount and composition (which changes during ageing in a poorly understood way) of its condensed tannin. The tannins react with protein receptors on the tongue to impart “mouth feel” characteristics. Wood-aged wine not only contains condensed tannins from grape skin, but also hydrolysable tannins from the wooden barrels it is aged in. The polyphenolic nature of the aromatic rings allows reaction with electrophiles. This forms the basis of adhesive manufacturing, where formaldehyde is used to polymerise tannin extracts to form adhesives. Other commercial applications of tannin extracts include the use as anti-foaming agents in oil drilling and the manufacturing of amine containing resins (via the Mannich reaction) for water purification applications (removal of heavy metals). The production of mimosa condensed tannin is a sustainable process as trees are harvested every eight years. Tannins will become a more important source of feedstock nutrients, as crude oil, which is currently used, becomes depleted. It also creates employment in rural areas. Higher oligomers of condensed tannins are built up by successive addition of flavan-3-ol monomer extension units via C-4 to C-8 or C-4 to C-6 interflavanyl bonds. Higher oligomers are impossible to purify by chromatography and other methods of analysis are required. Acid catalysed fission of the interflavanyl bonds and trapping of the monomer intermediates with toluene-α-thiol or floroglucinol followed by analysis of the trapped products with HPLC is normally used to analyse condensed tannin composition. The analysis of mimosa and quebracho tannins is however compounded by the resorcinol type A-ring in these compounds. The absence of a 5-OH group imparts stability to the interflavanyl bond against acid hydrolysis. The high temperatures thus required to hydrolyse the interflavanyl bond in mimosa and quebracho tannins leads to decomposition. Mass spectrometry and 13C NMR (nuclear magnetic resonance) spectrometry in solution have also been used with varying degrees of success. The analysis of hydrolysable tannins is even more complex than that of condensed tannins. As a result, the composition of condensed and hydrolysable tannin extracts remains uncertain, after more than 50 years of research. Of particular interest are the average chain length of tannin extracts from different sources and the composition of the constituent monomers. In this thesis the potential of solid state NMR and electrospray mass spectroscopy to solve vexing problems in tannin chemistry was investigated. Solid state NMR is particularly useful to investigate insoluble samples, overcoming problems associated with selective extraction, chemical modifications during extraction and sample preparation and uncertainty regarding compounds that are not extracted. Electrospray mass spectrometry complements MALDITOF mass spectrometry in that molecules with masses below 500 Dalton are detected. We were able to assign all the resonances in solid state NMR of hydrolysable and condensed tannins by comparing liquid and solid state spectra of pure flavonoids and tannin extract. This allowed us to distinguish unequivocally between condensed tannins and hydrolysable tannins with a simple routine experiment, avoiding laborious chemical tests. A method was developed to identify and distinguish with confidence between quebracho and mimosa condensed tannins. This method is the only available method to identify quebracho, which is of interest to oenology (quebracho tannins are added to wine) and could hitherto only be identified chemically because it tests negatively for all the available tests for tannins. We established that no insoluble higher oligomeric condensed tannins or tannins covalently bonded to other insoluble bark components remain in spent mimosa bark (after extraction of tannins). It promises an easy way for the wattle industry to investigate lower extraction temperatures and extraction time and the associated energy savings. A fingerprinting method for mimosa was developed and is already used by the industry (Annex A). As the gum resonances do not overlap with the tannin resonances, the bark can be analysed directly without the requirement of manufacturing an extract. The only sample preparation required is to grind the bark (about 100mg) finely and pack the solid state NMR rotor. As carbon is magnetised via hydrogen, less than 30 minutes NMR time is required per sample. This provides an easy way to identify the bark of quebracho, mimosa and hydrolysable tannins. A solid state NMR spectrum of the spent bark not only indicated that no condensed tannins remain, but also supports the conclusion that spent bark consists of water insoluble gums (polymers of glucose and other sugars). We believe that this method will find application in identifying novel sources of tannins from indigenous plants. We expanded our investigation into tanned leather and developed an easy method to determine whether leather was tanned with mimosa, quebracho, Italian chestnut, tara, synthetic tanning material, chromium or aluminium. We believe this method can be used by the leather industry to determine tannin loading of tanned leathers. By combining our electrospray mass spectrometry data with published MALDI-TOF mass spectrometry data we could calculate the relative composition of monomers, dimers, trimers, tetramers etc. in condensed tannin sample. These calculations were used by the mimosa and quebracho tannin industry to comply with new European Union (EU) REACH (Registration, Evaluation, Authorisation and Restriction of Chemical substances) legislation. Without compliance mimosa extract cannot be exported to the EU. Sulfitation (treating mimosa and particularly quebracho extract with bisulfite) is routinely used in industry to enhance the extract’s properties (e.g. increase water solubility) and products with different levels of sulfitation are commercially available. The chemical changes associated with sulfitation remain speculation. The solid state NMR indicated that the C-ring is opened during the process. The electrospray MS conclusively demonstrated the existence of condensed tannin-sulfonate molecules for the first time. The m/e values correspond with ring opening and introduction of a sulfonate group on the C-2 position.