600 research outputs found
Novel methodology for the synthesis of ¹³C-Labelled phenols and its application to the total synthesis of polyphenols
No full textElectronic redacted version does not contain associated previously published materialThe base-catalysed reaction of 4H-pyran-4-one with a range of nucleophiles, namely diethyl malonate, ethyl acetoacetate, nitromethane, acetylacetone and ethyl cyanoacetate, was developed as a reliable, high yielding method for the preparation of para-substituted phenols.
The methodology was extended to include the use of the substituted pyranones, maltol, 2,6-dimethyl-4H-pyran-4-one and diethyl chelidonate. Reactions were studied using conventional heating methods and microwave irradiation. Microwave irradiation had definite beneficial effects, with improved yields, reduced reaction times and cleaner reaction profiles.
The potential of this methodology was examined for the regioselective placement of ¹³C-atoms into benzene rings using ¹³C-labelled nucleophiles or ¹³C-labelled 4H-pyran-4-ones. [3,5-13C₂]4H-Pyran-4-one and [2,6-13C₂]4H-pyran-4-one were prepared from various ¹³C-labelled versions of triethyl orthoformate and acetone. This methodology was applied to the synthesis of
[1,3,5-¹³C₃]gallic acid, via the base-catalysed reaction of [3,5-¹³C₂]4H-pyran-4-one with diethyl [2-¹³C]malonate, followed by subsequent transformations to yield [1,3,5-¹³C₃]gallic acid.
The preparation of [2-¹³C]phloroglucinol was carried out via [2-¹³C]resorcinol, with
regioselective placement of a single ¹³C-atom into the aromatic ring. This was accomplished from non-aromatic precursors, with the source of the ¹³C-atom being [¹³C]methyl iodide. The key step in this synthesis was the introduction of the third hydroxyl group, which was achieved using a modified iridium-catalysed C-H activation/borylation/oxidation procedure. The scope of an existing C-H activation/borylation reaction was modified and expanded to include a range of protected resorcinol derivatives. A catalyst system was developed which allowed high conversion to the intermediate arylboronic acids, followed by oxidation using aqueous Oxone®
to yield the corresponding phenols.
Finally, to demonstrate the potential of these new methods for application in the synthesis of isotopically labelled natural products and polyphenols, the syntheses of ¹³C-labelled anthocyanins were studied. A route was developed that could be applied to the synthesis of either cyanidin-3-glucoside or delphinidin-3-glucoside. Only the final coupling/cyclisation step to yield the desired anthocyanin targets remains to be carried out
Enzyme immobilisation and catalysis in ordered mesoporous silica
No full textA range of mesoporous materials based on SBA-15 have been prepared and characterised. The materials were templated by neutral block copolymer P123, and typically have a hexagonal (p6mm) pore structure, with high surface areas and narrow pore size distributions. The removal of the surfactant template by calcination and solvent extraction has been investigated. The aqueous stability of this material, and the hydrolysis of the surface was studied.
Organic functional groups were incorporated into the silica surface by co-condensation, or by post synthesis grafting. A range of functional groups were incorporated, including amine, carboxy, allyl and thiol groups. The pore size of the materials was controlled by the addition of trimethoxybenzene during synthesis, which significantly increased the pore size and uptake capacity of the materials.
The adsorption of CALB by SBA-15 was investigated, with support materials extracted by calcination or solvent extraction. Rapid uptake at high loading was observed, with a maximum loading of 450 mg g-1 measured. The leaching of the enzyme from the support was investigated, and found to be high with unfunctionalised supports. The leaching from functionalised supports incorporating sulfur groups was significantly reduced.
The activity of the immobilised CALB was measured by tributyrin hydrolysis in aqueous media, and by enantioselective transesterification of (R)-1-phenylethanol in organic media. The effect of surface functionalisation for reusability and thermal stability in aqueous systems was investigated.
Preliminary studies of supported CALB for dynamic kinetic resolution were carried out, with an investigation of acidic zeolites and a mesoporous supported catalyst for 1-phenylethanol racemisation. The encapsulation of immobilised CALB was investigated, and the activity and reusability of these systems studied
The synthesis of natural and novel glucosinolates
No full textThis thesis describes the synthesis of natural and novel glucosinolates 1.Chapter 1 gives an overview of glucosinolates, from isolation and biosynthesis
to previous chemical syntheses. Chapter 2 describes an improved method for
the formation of thiohydroximate bonds and its application over a variey of
substrates. Chapter 3 focuses on exploring new chemistry on intact glucoinolates and details oxidative chemistry on an alkene containing side chain glucosinolate. Futhermore, the total synthesis of novel glucosinolates 150, 156 and 161 is illustrated.Chapter 4 describes a model study for the solid phase synthesis of
glucosinolates. Finally experimental procedures for the compounds synthesised
in the thesis are described in Chapter 5
Also By The Same Author: AKTiveAuthor, a Citation Graph Approach to Name Disambiguation
No full textThe desire for definitive data and the semantic web drive for inference over heterogeneous data sources requires co-reference resolution to be performed on those data. In particular, name disambiguation is required to allow accurate publication lists, citation counts and impact measures to be determined. This paper describes a graph-based approach to author disambiguation on large-scale citation networks. Using self-citation, co-authorship and document source analyses, AKTiveAuthor clusters papers, achieving precision of 0.997 and recall of 0.818 over a test group of eight surname clusters
Synthesis of labelled desulfoglucosinolates and novel glucosinolate analogues
No full textA number of novel deuterated desulfoglucosinolates have been prepared. These have been shown to be suitable internal standards for the quantitative LC-MS analysis of glucosinolates. Use of the deuterated desulfoglucosinolates results in a 100-fold improvement in the sensitivity over existing methods. This should allow improved analysis of leaf surface glucosinolales, including determination of the amount present on a single leaf.
Desulfo-[²H₅]gluconasturtiin was the first compound prepared to establish the methodology. Then two indolyl desulfoglucosinolates were prepared, desulfo-4- [²H₃]methoxyglucobrassicin and desulfo-l-[²H₃]methoxyglucobrassicin, as the corresponding glucosinolates of these derivatives are more potent as oviposition cues when present on the leaf surface. These structures contained deuterium in the aglycone. Finally, β-D-[1-²H₁, 6-²H₂]glucopyranosyl phenethyl thiohydroximate was synthesised where the deuterium was incorporated into the glucose fragment. This synthetic methodology should thus be applicable to any glucosinolate of analytical interest.
Some novel glucosinolates and glucosinolate analogues have been prepared in order to probe the chemical mechanism of glucosinolate cleavage by myrosinase. This reaction gives (3-D-glucose and an aglycone which undergoes Lossen rearrangement to give sulfate and an isothiocyanate. Three novel substituted phenyl glucosinolates were prepared to probe the involvement of the rearrangement in the overall rate of reaction. Changes in the electronic nature of the side chain were expected to alter the rate of rearrangement (p-MeO > H > P-NO₂). However the opposite trend was observed, implying that changes in the pKa of the leaving group were more important. An analogue designed to mimic glucosinolate binding to myrosinase but unable to rearrange was designed and synthesised. Surprisingly this was neither a substrate nor an inhibitor for myrosinase
Mechanistic studies on quinolinate phosphoribosyltransferase
No full textQuinolinate phosphoribosyltransferase (QPRTase, EC 2.4.2.19) is an intriguing enzyme which appears to catalyse two distinct chemical reactions; transfer of a phosphoribosyl moiety from 5-phosphoribosyl-1-pyrophosphate to the nitrogen of quinolinic acid and decarboxylation at the 2-position to give nicotinic acid mononucleotide. The chemical mechanism of QPRTase is not fully understood. In particular, enzymatic involvement in the decarboxylation step is yet to be conclusively proven. QPRTase is neurologically important as it degrades the potent neurotoxin, quinolinic acid, implicated in diseases such as Huntington’s disease and AIDS related dementia. Due to its neurological importance and unusual chemistry the mechanism of QPRTase is important. Described here is a mechanistic study on human brain QPRTase.
Human brain QPRTase was successfully expressed in E. coli BL21 (DE3) from the pEHISTEV-QPRTase construct and the protein was efficiently purified by nickel affinity chromatography. The crystal structure was solved using multiwavelength methods to a resolution of 1.9 Å. Human brain QPRTase was found to adopt an energetically stable hexameric arrangement. The enzyme was also found to exist as a hexamer during gel filtration under physiological conditions.
Kinetic studies allowed the measurement of the kinetic parameters for quinolinic acid. The data gave a Km of 13.4 ± 1.0 μM and a Vmax of 0.92 ± 0.01 μM min-1. There was no evidence for cooperative binding of quinolinic acid to the six subunits of the QPRTase hexamer. The enzyme showed maximum activity at approximately pH 6.
The active site of human brain QPRTase is a deep pocket with a highly positive electrostatic surface composed of three arginine residues, two lysine residues and one histidine residue. Mutation of these residues resulted in either complete loss or significant reduction in enzymatic activity showing they are important for binding and/or catalysis. A possible mechanism involving QPRTase in the decarboxylation of quinolinic acid mononucleotide was proposed.
A series of quinolinic acid analogues were synthesised and tested as inhibitors of QPRTase. The inhibition studies highlighted some key interactions in the active site
Studies on the synthesis of dicaffeoylquinic acid conjugates
No full textDicaffeoylquinic acid (DCQA) is a natural polyphenolic compound widely distributed in plants such as coffee beans, which possesses a range of pharmacological activities. Herein, is reported studies undertaken towards the first total synthesis of 3,5-DCQA conjugates. Two synthetic routes were investigated. The first route involves a seven step sequence beginning from quinic acid. The overall yield via this synthetic approach was 30%. The key steps involved in the sequence were a regioselective benzylation of the C-3-hydroxyl group followed by silyl protection of the C-1 and C-4 hydroxyl groups. Deprotection of the benzyl group by hydrogenolysis and opening of the lactone afforded the 3,5-diol. Esterification of the 3,5-diol with 3,4-tert-butyldimethylsilyl caffeoyl chloride afforded the di-ester. Removal of the protecting groups afforded 3,5-DCQA. The second route involved selective protection of the C-3-hydroxyl group with silyl followed by benzylation of the C-1 and C-3 hydroxyl groups. Saponification of the lactone ring followed by benzylation of the carboxylic acid gave the benzyl ester. Silyl deprotection afforded the 3,5-diol. The 3,5-diol was subsequently esterified by refluxing in toluene with commercially available Meldrum’s acid. In the final step, the synthesis of 3,5-DCQA was achieved by a Knoevenagel condensation of 3,4-dihydroxybenzaldehyde and a malonate ester of quinic acid. An efficient method for the synthesis of possible metabolites of quinic acid conjugates was also described. This protocol employs N-(4-methoxyphenyl)-trifluoroacetimidate glucuronyl as the donor. The key reaction in this sequence was the coupling of N-(4-methoxyphenyl)-trifluoroacetimidate glucuronyl with 4-hydroxy-3-methoxy-benzaldehyde
Studies on alkylresorcinols and novel analogues
No full textAlkylresorcinols such as 1,3-dihydroxy-5-alkylbenzenes have been found to have a number of biological activites including antiparasitic, cytotoxic, fungicidal and bacteriocidal properties that act against various pathogens. These resorcinols have been found to be present and have been isolated from a range of plants in the Ancircardiaceae Ginkgoaceae and Graminae families.
Alkylresorcinols have previously been synthesised by various routes, with the attachment of the side chain being the most challenging aspect of the synthesis. For comprehensive studies on their structure-activity relationships, an efficient synthetic route therefore needs to be found. Alkylresorcinols were to be used as standards in GC-MS and LC-MS based analytical methods for the detection and quantification of alkylresorcinols in human fluids. Carboxyalkylresorcinols were required for the development of new automated immunoassays of alkylresorcinols from biological samples.
The work towards the synthesis of a number of novel analogues of resorcinol was achieved, namely 15-(3',5'-dihydroxyphenyl)pentadecane via a Suzuki coupling reaction; 16-(3',5'-dihydroxyphenyl)hexadecanoic acid via an iron(III) catalysed carbon-carbon coupling reaction employing a grignard reagent; 16-(3',5'-dihydroxyphenyl)hexadecane, 17-(3' ,5' -dihydroxyphenyl)heptadecanoic acid, 21 -(3',5'-dihydroxyphenyl)henicosanoic acid and 19-(3',5'-dihydroxyphenyl)nonadecanoic acid employing standard conditions of Wittig reactions
N-hydroxyguanidines and related compounds as nitric oxide donors
No full textThe design of new, improved NO-donor drugs is an important pharmacological objective
due to the biological importance of nitric oxide. N-Hydroxyguanidines represent a useful class of
NO donors where the mechanism of action is based on the biosynthetic pathway for NO.
Thirty new N-arylalkyl-N’-hydroxyguanidines were synthesized and their vasodilatation
activity examined by myography in rat aortic rings. The observed relaxations were reversed by
ODQ, which is an inhibitor of the guanylate cyclase, implying that this was an NO dependent
vasodilatation. The most active compounds were also tested in the isolated perfused kidney
(IPK) giving the vasodilatation properties. Preliminary results indicated that N-phenyl-N’-
hydroxyguanidine showed the best pharmacological profile with EC₅₀= 19.9 μM and ca. 100%
reversibility with ODQ. A series of N-phenylalkyl-N’-hydroxyguanidines were synthesised. NO
donor activity was found to be fairly constant up to three methylene groups, and then decreased.
Substitutions in the benzene ring of N-phenylethyl-N’-hydroxyguanidine demonstrated that
various electron-withdrawing and electron-donating groups in the para position did not
significantly affect the NO donor activity of this series of analogues. The nitro and
trifluoromethyl substituted compounds gave the best biological profiles. Additionally, a novel
heterocyclic, N–furfuryl-N’–hydroxyguanidine possessed very promising vasodilatation
properties. In general, almost all the N-arylalkyl-N’-hydroxyguanidines behaved as potent NO
donors in the rat aorta assay.
In order to establish the influence of the free NH₂ group in the hydroxyguanidine
functionality on the vasodilatation properties, N,N-dimethyl and N-methyl-N’-
hydroxyguanidines were successfully synthesised. Unfortunately, they have not been tested yet in
the biological assay. However, their NMR spectra showed some unusual features and their
detailed analysis and X-ray data are presented herein.
In addition a series of hydroxamic acids was synthesised and the NO donor activity
investigated using the same biological methodology. It was found that the
3-phenylpropionohydroxamic acid was the most potent compound with EC₅₀ = 6 μM and ODQ = 96%. However, behavior in the IPK indicated that hydroxamic acids did not undergo
the same biological pathway as in the rat aorta.
Two different types of enzyme-activated pro-drugs were designed using
N-hydroxyguanidines as the NO donating molecule. Synthetic studies towards these targets were
carried out using various synthetic approaches. The desired molecules have not yet been
synthesised but the chemistry explored so far has indicated potentially more successful
approaches that could be attempted
Mechanistic studies on myrosinase
No full textMyrosinase is the beta-thioglucosidase enzyme which catalyses the hydrolysis of glucosinolates, a group of naturally occurring plant metabolites. Glucosinolates are S-glucosides which occur predominantly in the family Cruciferae, and are abundant in the Brassica vegetables. The hydrolysis reaction, specifically activated by L-ascorbic acid, gives beta-D-glucose and an aglucone fragment, which then rearranges to give sulfate and an isothiocyanate. The aim of this study was the investigation of the chemical mechanism of myrosinase, in order for it to be compared with those of the much more widely studied (3-glycosidases. Myrosinase catalysed transglycosylation reactions were examined as a potential synthetic method. However, no transglycosylation was detected, even though a wide range of glycosyl acceptors was examined, including simple alcohols and thiols and examples with charged side chains to mimic the glucosinolate. Such reactions are commonly observed with beta-glycosidases. However, the stability and activity of myrosinase was not substantially affected by the presence of the acceptors. A small amount of transglycosylation was observed using azide, a charged glycosyl acceptor. This is consistent with the other transglycosylation reactions failing due to the lack of a suitable basic residue at the active site to deprotonate the acceptor. A range of potential substrates was synthesised, although only the nitrophenyl-p-D-glucosides showed any substrate activity. All S-glucosides were inactive. Myrosinase therefore appears to be very specific. Inhibition studies revealed that D-glucono-y-lactone, a potent competitive inhibitor and transition state mimic of p-glucosidases, was a poor noncompetitive inhibitor of myrosinase. It was proposed that this inhibitor binds at the L-ascorbic acid activator site. The inhibitory properties of a number of other compounds were examined, including reaction products and potential substrates (S-glucosides). [1-2H]-Sinigrin, p-nitrophenyl- and 2,4-dinitrophenyl-beta-D-[1-2H]-glucoside were prepared to measure the secondary deuterium isotope effects. The isotope effect obtained for 2,4-dinitrophenyl-p-D-[1-2H]-glucoside (DV= 1.26 +/- 0.11) indicated an sp2 hybridised intermediate via an SN1-Iike mechanism. The solvent kinetic isotope effect for sinigrin hydrolysis was determined (DV = 1.54 +/- 0.07, DV/K = 1.24 +/- 0.15) implying that a proton transfer was partially rate limiting
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