Dissertations / Theses on the topic 'Biomimetic synthesis'

The first chapter of this thesis discusses the biomimetic total synthesis of clivonine. Clivonine is a member of the Amaryllidaceae family of alkaloids and was isolated from the plant Clivia miniata Regel by Wildman in 1956. Barton first proposed the biosynthesis of the Amaryllidaceae alkaloids in 1958, hypothesising that the key diversifying step in the biosynthesis was an intramolecular phenolic oxidative coupling of a common precursor, norbelladine. However, it was not until 2 years later, in 1960, that Barton was able to account for the biosynthesis of the lycorenine class of the Amaryllidaceae alkaloids, the class in which clivonine is a member. He proposed that a ‘ring-switch’ must occur after the initial intramolecular phenolic oxidative coupling from a lycorine type progenitor to that of a lycorenine type skeleton. Our synthesis of clivonine demonstrates for the first time the synthetic interconversion between these two classes of compounds in a manner that can be considered biomimetic. The second chapter of this thesis details our synthesis of the acetonide protected derivative of cis-3,5-cyclohexadien-1,2-diol. The first section of this chapter highlights the importance of these types of molecules and reviews the previous syntheses of this compound. The second section then details the development of a five-step three-pot synthesis of this compound in an overall 50% yield starting from commercially available 1,3-cyclohexadiene. The final chapter of this thesis discusses the studies towards the development of a catalytic asymmetric retro-Cope elimination reaction. This chapter will first highlight recent advances that have been made towards the asymmetric hydroamination (AHA) reaction before reviewing the recent progress made towards expanding the scope of the retro-Cope elimination reaction. Finally, our proposed method of developing an asymmetric retro-Cope reaction will be outlined, along with details of preliminary studies focussed on the development of a suitable system with which the asymmetric retro-Cope elimination can be studied.

The asymmetric synthesis of the C₁₃-C₂₇ moiety 44 of the polyether antibiotic etheromycin is described. The final step in the synthesis was the formation of the tricyclic fragment 44 by a biomimetic triepoxide cyclisation cascade. The cyclisation cascade, 144 to 44, is stereospecific and entirely dependent upon the epoxides stereochemistry. The absolute stereochemistry of each of the three epoxides was independently controlled by the Sharpless asymmetric epoxidation methodology. The carbon skeleton of 44 was constructed from geraniol, (R)-methyl 3-hydroxy-2-methylpropionate 117, and two units of t-butyl acetate. Apart from C₂₆, the chiral centres were all controlled by the Sharpless asymmetric epoxidation. The synthetic strategy was designed to effect the stepwise enantioselective introduction of the three epoxides while building the C₁₃-C₂₇ carbon skeleton, and directing a subsequent cascade reaction by an internal nucleophile. Two trisubstituted epoxides were introduced stepwise with >20:1 stereoselectivity by asymmetric epoxidation of a geraniol derived segment. The fragment was manipulated between epoxidations to allow stepwise introduction of the epoxides, and to ensure terminal differentiation of the groups. Both hydroxyl groups used to control epoxidation were subsequently and separately utilised, after conversion to the iodide, in alkyation reactions with the lithium enolate of t-butyl acetate to extend the carbon chain. No other conditions investigated to selectively react α to epoxides were satisfactory. Of the two t-butyl ester groups introduced, one (C_24) was reduced to the aldehyde and coupled in a Julia reaction with a sulphone derived from 117. The resulting trans olefin was converted into a trans homoallylic alcohol, which was epoxidised by Sharpless methodology with 3 : 1 stereoselectivity. The second of the t-butyl esters (C_13) was used as an internal nucleophile to induce the cascade reaction. The natural ring stereochemistry of 44 was assumed from the high predictability and stereocontrol of the epoxidation reactions and confirmed by ^1H NMR nOe difference experiments. The synthesis of the sulphone 161, in which the three contiguous chiral centres and methyl ketone represents a common polyether terminus, was also demonstrated using a stereocontrolled aldol reaction.

Agelastatin A is a tetracyclic alkaloid isolated from the marine sponge Agelas dendromorpha. It exhibits cytotoxicity towards KB and lymphocytic leukaemia cells, arthropod toxicity and insecticidal activity and selectively inhibition of glycogen synthase kinase-3. Agelastatin A is one of a structurally related group of pyrrole- imidazole alkaloids derived from the "linear" skeleton of oroidin and it is postulated that it is biologically derived from oroidin via the generation of an N-acyliminium ion, which undergoes two cyclisations, via a second N-acyliminium ion, followed by hydration to afford the natural product. This thesis describes the work undertaken towards the biomimetic synthesis of Agelastatin A and efforts towards the synthesis of an oroidin-like precursor from which the N-acyliminium ion could be generated. Disconnection of the precursor gave a 3-pyrroline A-ring, which was prepared using a Birch reduction, but efforts to synthesise the D-ring imidazolone fragment were hampered by low solubility, necessitating the use of protecting groups. Selective introduction of the Z-alkene was made difficult by conjugation to the imidazolone and efforts to maintain the Z-alkene geometry failed. Parikh-Doering oxidation of a propargylic alcohol intermediate afforded a novel thioaldehyde that underwent an acid-catalysed cyclisation via an N-acyliminium ion analogous to the second N-acyliminium ion in the proposed biomimetic synthesis. The thioaldehyde forced the required alkene conformation and was therefore applied in the synthetic route to the imidazolone fragment. The introduction of a leaving group was ultimately unsuccessful, but a precursor for generation of the ion by oxidation was prepared by reductive amination of the corresponding aldehyde. The precursor was treated with trityl tetrafluoroborate but the expected oxidation did not occur, instead an acid-catalysed cyclisation was observed. This cyclisation again proceeded via an N-acyliminium ion analogous to that predicted to be generated in the biomimetic synthesis, to afford a novel bicycle. Therefore although the double cyclisation required for the biomimetic synthesis was not mimicked, the two novel cyclisations provide good evidence that both cyclisations are plausible.

The resorcylate unit (2, 4-dihydroxybenzoic acid) is found in numerous biologically active natural products. This thesis outlines the application of a novel biomimetic synthesis strategy to the syntheses of resorcylate natural products and analogues. A synthetic pathway to the meroterpenoid antibiotic Hongoquercin B has been successfully developed in nine steps from trans, trans-farnesyl acetate using a double biomimetic strategy. A regioselective decarboxylative farnesyl migration and cycloaromatisation gave the resorcylate, which undergoes a lewis acid mediated diastereoselective cationic epoxy-diene cascade cyclisation to give the tetracyclic core. The single epoxy-farnesyl stereocentre was used to control the remaining 4 stereocentres of the tetracyclic core. This cascade tetracyclisation sequence simplifies the synthesis of terpenoid resorcylate natural products. Efficient syntheses of a range C-5 substituted resorcylates and resorcinamides from functionalised keto-dioxinones are also described. Functionalized keto-dioxinones, generated via enolate acylation or alkylation reactions, were subsequently C-formylated and cyclised to the corresponding arenes. Further manipulations gave a wide range of structures of potential pharmaceutical interest including C-5-substituted, C-4,5-cyclo-fused and C-5,6-cyclo-fused resorcylates, as well as resorcinamides. The syntheses are noted for brevity with a maximum of 5 synthetic steps and without the need for protection of phenol groups (b).

Thesis (Ph. D.)--Chemistry and Biochemistry, Georgia Institute of Technology, 2010.
Committee Chair: Bridgette Barry; Committee Member: David Collard; Committee Member: Ingeborg Schmidt-Krey; Committee Member: Jake Soper; Committee Member: Mira Josowicz. Part of the SMARTech Electronic Thesis and Dissertation Collection.

The C4 benzoyloxy substituted β-lactams (65)-(68) were formed by the copper-promoted reaction of β-lactarns (42)-(45) with t-butyl perbenzoate. The benzoyloxylation of the azetidin-2-one ring occurs at C4 with no competing reaction at the C3 position. The relative ease of abstraction by t-butoxy radicals of hydrogens bonded to the endocyclic and exocyclic carbons adjacent to the amide nitrogen was determined from the reaction of the β-lactam (47). This reaction gave the endocyclic substituted β-lactam (69), the exocyclic substituted β-lactam (70) and the disubstituted β-lactam (71). Substitution solely at the exocyclic carbon occurred in the reaction of the β-lactam (48), which gave the β-lactam (73). The mode of substitution of the β-lactams (42)-(45), (47) and (48) is discussed. Reaction of the γ-lactam (80) with t-butyl perbenzoate gave the C5 substituted γ-lactam (90) and the exocyclic substituted γ-lactam (87). The mode of formation and ratio of these two products is discussed. Bromination of N-benzoylvaline methyl ester (19) with N-bromosuccinimide, followed by reaction with tri-n-butyltin hydride gave N-benzoyl-3-bromovaline methyl ester (93). The synthesis of this compound is discussed. Reaction of 1-(1-methoxycarbonyl-2-methypropyl)azetidin-2-one (31) with sulphuryl chloride gave the azetidin-2-one (102). An independent synthesis of 1-(3-chloro-1-methoxycabonyl-2-methylpropyl)-3,3-dimethylazetidin-2-one (102) from N-(3-bromo-2,2-dimethylpropanoyl)valine methyl ester (100) is described and the mechanism of these reactions is discussed.

Resorcylic acid lactones (RALs) are a biologically important and structurally interesting class of natural products. Recently the Barrett group has developed a novel, biomimetic strategy to access RALs that bypasses many common inefficiencies inherent to more classical synthetic approaches. The focus of this thesis consists of a synthetic study of the RALs radicicol and paecilomycin B that utilised this methodology.

Le passage des médicaments a travers la membrane cellulaire représente souvent une limitation majeur dans un grand nombre de thérapies (anti-cancéreuse, anti-virale par exemple). Des peptides vecteurs connus comme les CPPs (cell penetrating peptides) ont été utilises avec succès pour introduire a l’intérieur des cellules diverses molécules (protéines, peptides, siRNA, quantum dots) et présentent un fort potentiel dans l'adressage de médicaments. Parmi les différents CPPs décrits dans la littérature la plupart sont des peptides basiques ou amphiphiles.Nous nous sommes intéressés a l'utilisation d’oligomères non charges construits a partir de motifs contraints mimes de dipeptides comme vecteurs de pénétration cellulaire. L'internalisation cellulaire et leur localisation ont été établies a l'aide de dérivés fluorescents par microscopie confocale. L' étude de pénétration cellulaire par mesure de fluorescence a montre que des oligomères de (3S)-amino-5-carbonylmethyl-2,3-dihydro-1,5-benzothiazepine-4(5H)-one] (DBT) sont aussi puissants que les oligomères d'arginine (oligoArg), vecteurs de référence. Par microscopie confocale nous avons montré que ces composés sont internalisés dans les lysosomes. L’efficacité d'internalisation de nos composés a été confirmé par une méthode de quantification par spectrométrie de masse MALDI-TOF développée dans notre groupe. Cette méthode repose sur l'utilisation conjointe d'un marqueur UV-absorbant dérivé de l'acide alfa-cyano-4-hydoxycinnamique (HCCA) et d'une matrice MALDI adaptée. Un effet important de discrimination spectrale est obtenu, permettant une amplification du signal de la molécule d' intérêt dans un mélange complexe. Ainsi les faibles concentrations internalisées peuvent être détectées. Grâce a cette technique et l'utilisation d'un étalon deutéré, nous avons calculé la concentration intracellulaire de deux CPP de référence l'octa-arginine et la pénétratine. Nous avons aussi étudier l’internalisation de petits oligomères construits a partir d'acide 2-aminomethyl-phenyl-acetique (AMPA). Par microscopie confocal nous avons constaté que ces petits oligomères sont internalisés par voie endo-lysosomale.L’efficacité de la pénétration cellulaire de ces petits oligomères aromatiques (oligoAMPA et oligoDBT) offre une nouvelle classe de vecteurs qui ont la particularité d’être non-cationiques et hydrophobes. De tels composés pourraient être utilisés pour la délivrance de médicaments dans le traitement des maladies comme le cancer, les maladies lysosomales ou la maladie d'Alzheimer. Afin de montrer que cette nouvelle classe de vecteurs est capable d'internaliser des composés biologiquement actifs, nous les avons associés a un inhibiteur puissant de la Cathepsine D (CD) la pepstatine. CD est une endopeptidase lysosomale qui dans des conditions normales est localisée dans les endosomes et les lysosomes. Pour certains cancers, la CD est surexprimée et secrétée a l’extérieur de la cellule. La CD est probablement impliquée dans la prolifération des cellules cancéreuses par l'activation de certains facteurs de croissances dans les endosomes. La pepstatine est une inhibiteur puissant de la CD. Cependant son efficacité thérapeutique potentielle est limitée par une faible capacité de pénétration des membranes cellulaires et une faible solubilité nécessitant de fortes doses pour l'inactivation de la CD in vitro et in vivo. Afin d’améliorer son efficacité et sa biodisponibilité, des conjugues de la pepstatine avec nos vecteurs de pénétration cellulaire, oligo (AMPA)4 et (DBT)4, et une partie solubilisante ont été développés. Certains de ces bioconjugués ont montre une toxicité élevée (IC50 = 2.10-6) in vitro sur différentes lignées cellulaires tumorales. Des tests in vivo sur des souris sont prévus pour le futur
As a part of a program for foldamer design two ¦Â-turn mimetics (3S)-amino-5-(carboxylmethyl)-2,3-dihydro-1,5-benzothiazepin-4(5H)-one or DBT and 2-aminomethyl-phenyl-acetic acid or AMPA were selected as frameworks from a molecular modeling study for their suitability to adopt helical structure. At first we developed a highly efficient scale up synthesis of the DBT moiety protected by 9-fluorenylmetoxycarbonyl (Fmoc) group. By standard solid phase peptide synthesis (SPPS) we synthesized DBT oligomers of different lenghts and modifications were introduced at their N-terminus. Our first task was to perform structural analysis of the oligomers by NMR and X-Ray. Numerous NOE interactions in the DBT pentamer and hexamer molecules were detected by NMR 2D NOESY experiments. These data strongly suggest the organization of these DBT oligomers. Small crystals were obtained from the same molecules in DMSO but at the time being their size is not importan t enough for X-Ray crystallography studies. In a parallel study we hypothesized that short oligomers constructed by DBT or AMPA frameworks could translocate the cellular membrane and could be used as new cell penetrating non-peptides - CPNP. Even though these compounds are not charged as most cell penetrating peptides (CPP)5 or CPNP, we considered that by virtue of their aromaticity, hydrophobicity and their well-organized structure they could have a non-specific interaction with the lipid bilayer and thus be internalized into the cell. Short oligomers were synthesized on Rink amide (RA) resin following SPPS methodology and labelled at their N-terminus with fluorescein isothiocyanate (FITC). At first the cellular uptake of the (DBT)2-4 oligomers in MDA-MB-231 breast cancer cells was analyzed by fluorescence emission measurement and compared to the potent and well-studied CPP octa-arginine (Arg)8 as a positive control and carboxyfluorescein as a negative control. The highest intracellular fluorescence intensity was found for (DBT)4 with a drastic decrease (>4-times) for (DBT)3 and (DBT)2 oligomers. Thus, the cellular uptake appeared length-dependent with an increase of the internalization with the oligomer size. Moreover, the amount of (DBT)4 that was internalized was more significant than that of (Arg)8 despite the fact that it is uncharged. By confocal microscopy we determined that (DBT)4 is mainly localized in the endosomes after 3 hours of incubation and in the lysosomes after 16 hours of incubation. Altogether, these data indicate the ability of these oligomers to target the endolysosomal pathway. Although most of the initial drug delivery studies aimed to avoid lysosomal addressing to prevent subsequent drug degradation, more recent studies demonstrated the relevant clinical utility to target this compartment for drug delivery in the treatment of lysosomal storage diseases, Alzheimer¡¯s disease, and cancer.While analyzing the internalization efficiency of our CPNP we decided to straightforward evaluate their concentration inside the cells. We studied our compounds internalization by total fluorescence emission measurement and by confocal microscopy but none of these techniques gave us the possibility to determine the exact amount of compound internalized per cell. A study reported by Burlina et al. brought a great improvement in proposing a highly reproducible quantification method based on MALDI-TOF MS to measure the concentration of the internalized peptides. However, after cell lysis, this method requires the capture of the biotin-labelled CPP by streptavidin coated magnetic beads. This step is particularly critical for the accuracy of the quantification. This is the reason why we decided to develop a new general methodology based on MALDI-TOF mass spectrometry (MS) which does not require any purification or separation steps. We studied the internalization of CPP/CPNP compou nds by using an UV light-absorbing tag alpha-cyano-4-hydroxycinnamic acid (HCCA) and preparing the samples in a neutral matrix such as alpha-cyano-4-hydroxycinnamic methyl ester (HCCE). This combination (HCCA tag and HCCE matrix) enabled us to discriminate MS signals induced by peptides of interest that were present in low concentration from those of unlabelled more abundant peptides. By addition of a precise amount of deuterated-HCCA-tagged CPP/CPNP prior the MALDI TOF MS experiment, the internalized CPP/CPNP could be quantified on the basis of the ratio between the [M+H]+ peaks of the deuterated and nondeuterated HCCA-tagged CPP.Another direction for research was to synthesize bioconjugates between our newly discovered CPNP and some biologically active compounds that are unable to cross the cell membrane. We selected pepstatine which is a powerful transition state inhibitor of the Cathepsin D (CD). Pepstatine while a very potent inhibitor of the CD is unable to cross the cellular membrane. Moreover pepstatine activity in vitro or in vivo is hampered by its poor solubility in water. CD is a soluble lysosomal aspartic endopeptidase synthesized in rough endoplasmic reticulum as preprocathepsin D (pCD).12 Upon entering the acidic endosomal and lysosomal compartments proteolytic cleavages of the pCD result in the formation of the active enzymatic form of CD. Under normal physiological conditions pCD is sorted to the lysosomes and found intracellularly but in some pathological and physiological conditions like cancer pCD/CD escape the normal targeting mechanism and is secreted from the cell. Once secreted to the outside, pCD can be endocytosed via M6PR or yet unknown receptor by both cancer cells and fibroblasts. The endocytosed pCD undergoes maturation into the enzymatically active CD. An enzymatic activity of CD outside of the cell or inside the endosomes could be responsible for the activation of several growth factors and growth factor receptors. Several groups have proven that the tumour growth is not inhibited by the powerful CD inhibitor pepstatine. These results exclude the importance of the CD enzymatic activity outside of the cell but as already mentioned pepstatine is unable to penetrate into the cell thus CD activation of growth factors inside the endosomes or the lysosomes is still a possibility. Different CPNP-Pepstatine conjugates were synthesized and tested in vitro for their ability to inhibit MDA-MB-231 breast cancer cells growth. Some of these conjugates showed high cytotoxicity, probably via a Cathepsin D inhibition in the endosomes or the lysosomes. One o f the most potent tested compounds was JMV4463. This compound was obtained by the conjugation of pepstatine with a CPNP as delivery system (AMPA4) and with solubilizing moiety composed of polyethylene glycol and D-Arginine residue. The good in vitro results obtained with the vectorized pepstatine encouraged us to perform in vivo tests. We performed scale up synthesis of JMV4463 in order to obtain enough product for anti-cancer activity on mice in the near future

L’impiego in campo farmacologico di peptidi bioattivi presenti in natura é ostacolato da numerosi svantaggi. Allo scopo di ottenere nuovi analoghi di amminoacidi per lo sviluppo di peptidomimetici piú stabili ed efficaci, sono stati sintetizzati due composti con restrizioni conformazionali, diastereomericamente puri. Uno di essi ha una struttura basata su di un anello γ-lattamico e contiene un centro quaternario. Esso puó essere utilizzato come precursore di analoghi della (R)-metilomoserina e dell’acido (R)-metilaspartico. Il secondo composto é un derivato di un α-hydrazino acido con restrizione conformazionale data da un anello imidazolidinonico. Tali composti possono essere anche impiegati nella produzione di foldameri. A tale scopo alcuni dei corrispondenti oligomeri sono giá stati sintetizzati e sono in atto i relativi studi strutturali e computazionali. Nello scenario globale del rapido sviluppo del fenomeno della resistenza batterica agli antibiotici, nuovi mimetici sintetici dei peptidi antimicrobici (SMAMPs) sono stati sintetizzati e testati come possibili farmaci antimicrobici. Il nuovo design ha facilmente permesso di variare gradualmente sia la rigiditá conformazionale che la idrofobicitá di tali molecole, allo scopo di costruire un modello struttura-attivitá (SAR). E’ stato cosí individuato un nuovo composto che presenta una MIC di 0.78μg/mL contro S. aureus e di 6.25μg/mL contro E. coli. E’ stato inoltre notato che per questa classe di composti, a differenza di altre precedentemente studiate, una elevata rigiditá conformazionale non é essenziale per una buona attivitá farmacologica
The use of natural bioactive peptides in medical therapies is challenged by their low oral availability, potential immunogenicity and poor metabolic stability in vivo. With the aim to provide new analogues of amino acids to be used in the creation of more stable and effective peptidomimetics, we synthesized two diastereomerically pure compounds with conformational restriction. One is based on a γ-lactam ring with a quaternary carbon, and is the precursor of (R)-methylhomoserine and (R)-methylaspartic acid mimetics. The other compound is based on an imidazolidinone ring and is analogue of α-hydrazino acids. These compounds can also be used as monomers for foldamers. Some corresponding oligomers have already been synthesized whereas structural and conformational studies are taking place. Considering the rapid and widespread development of antibiotic resistances, Antimicrobial Peptides (AMPs) are receiving ever more interest and importance as new drug candidates. Because many difficulties are associated with peptide drugs, synthetic mimics of AMPs (SMAMPs) have been designed and synthesized. The new design presented allows for easy tuning of both the conformational restriction and the overall hydrophobicity with the aim to Fbuild a structure-activity relationship model (SAR). A novel compound was discovered which has MICs of 0.78μg/mL against S. aureus and 6.25μg/mL against E. coli. It was also found that, despite what was seen in previous studies on different classes of SMAMPs, in this case a conformational rigidity is not essential for a good antimicrobial activity and selectivity.

Our work is devoted to the synthesis of new biomimetic molecular switches, capable to convert light-energy into a E/Z isomerization process. Computational studies performed on Rhodopsin have defined the penta-2,4-dieniminium cation as the minimal chromophore whose excited-state mimics the one of the natural pigment responsible of the vision mechanism. By embedding this π system into conformational rigid molecules we developed a new class of molecular switches featuring a selective and ultra-fast light-driven E/Z photoisomerization. An intriguing process, we named “cyclopropyl ring-opening/nitrilium ion ring-closing tandem reaction”, has been conveniently used to approach “one-pot” the pivotal chromophore. The successful functionalization of these molecules potentially allows their grafting to a peptide domain.

Biomimetic synthesis is the branch of synthetic organic chemistry which attempts to learn from nature into order to solve the challenges of chemical synthesis. This thesis describes application of biomimetic principles to the total synthesis of three phenylethanoid alkaloid natural products: incargranine B; millingtonine and incargranine A. Chapter 1 provides a general introduction to the area. Specific introductions can be found at the start of each chapter. Chapter 1 introduces the concept of biomimicry and provides a brief overview of the development of the underlying concepts and terminology. The major biosynthetic pathways involved in the production of incargranine B, millingtonine and incargranine A (shikimic acid, ornithine alkaloids) are also introduced. Chapter 2 discusses the synthesis of incargranine B. Biosynthetic analysis of this dimeric alkaloid led us to question its structural assignment and suggest a structural revision. This speculative reassignment was validated through a biomimetic total synthesis of our proposed structure. Incargranine B was successfully prepared in a longest linear sequence of six steps, forming three new rings, four bonds and three contiguous stereocentres in a single biomimetic domino condensation/Mannich/SEAr sequence. Chapter 3 describes the synthesis of millingtonine. We proposed that millingtonine is biosynthetically related to incargranine B through a divergent/re-convergent network of pathways. Synthetic exploration of this hypothesis culminated in the total synthesis of millingtonine and discovery of an unanticipated biosynthetic intermediate, dia-millingtonine, which we propose as a previously unidentified natural product. . Chapter 4 details the synthesis of incargranine A. Incorporating dia-millingtonine into our biosynthetic hypothesis allowed the development of a four step bioimimetic total syntheses of incargranine A which was scaled-up to provide over one gram of natural product. Chapter 5 summarises the work presented and provides a perspective on its contribution to the field.

Biomineralisation is the synthesis of inorganic materials in biological systems. Many biominerals - such as bone! teeth, and shells - are high-performance composites synthesised with extreme precision under physiological conditions. Understanding biomineralisation is expected to inspire 'green' methods for the manufacture of novel materials. Diatoms are eukaryotic algae that mineralise an external cell wall, or frustule, composed of hydrated silica. Silicification depends upon the uptake of soluble silicon (silicic acid) from the local environment by specific silicic acid transport proteins (SITs). This unusual family of integral membrane proteins are relatively uncharacterised. This project aimed to express and purify 5113 from the diatom Thalassiosira pseudonana (TpSIT3) for further characterisation in vitro, and to explore whether synthetic SIT3 proteoliposomes could be used as a model mineralisation system with potential applications in nanotechnology. TpSIT3 was successfully overexpressed in yeast and purified in the solubilising detergents Fos-choline 12 and octyl glucoside. The purified protein was successfully reconstituted into synthetic liposomes and silicic acid uptake was assessed using two fluorescent assays including a novel method which utilised zinc silicate fluorescence. This method was used to determine that silicic acid transport by TpSIT3 displayed Michaelis-Menten kinetics with a Km of 6.1 ± 2.7 μM, similar to silicic acid uptake studies in diatom cultures. The structure and function of a silicifying cationic peptide were also characterised for the first time. Peptide-mediated silicification only proceeded at ≥2 mM silicic acid when pH was >6.4 and peptide concentration was ≥2.5 mM. These results underpinned efforts to synthesise silica within the int erior lumen of peptide-loaded SIT proteoliposomes. Preliminary electron microscopy and elemental analysis suggested that such an approach was feasible. This thesis thus establishes a series of novel methods that can be used to study silicic acid transport and silica mineralisation in vitro

This thesis describes work towards the biomimetic synthesis and understanding the biosynthesis of two families of natural products: prodiginines and quartromicins. Prodiginines are a large family of red pigmented tripyrrole antibiotics. Although they have not been used clinically, the promising anti-cancer, immunosuppressive and antimalarial activity they display at non-toxic doses has generated renewed interest in their utilisation. The synthesis of an analogue of the proposed pyrrole-2-carboxyl-RedO intermediate in prodiginine biosynthesis has been achieved. The resulting NAC thioester and analogues of it have been used to investigate the prodiginine biosynthetic pathway in Streptomyces coelicolor, and to examine the production of prodiginine analogues by mutasynthesis. Quartromicins, novel anti-viral antibiotics, are a structurally unique group of spirotetronate natural products produced by Amycolatopsis species. They are unusual symmetric macro cyclic compounds which possess a 32-membered carbocyclic structure with four spirotetronic acid units connected by enone or dienone linkers in a head-to-tail fashion. These macrocyclic compounds are intriguing because they have alternating endo- and exo- spirotetronic acid units, with the opposite "comers" being identical. Although the quartromicins have therapeutic potential, very little is known about their biosynthesis. In this research a biosynthetic pathway to the quartromicins has been proposed based on hypothetical pathways to related natural products. The synthesis of the two putative key intermediates in quartromicin biosynthesis has been achieved. An improved method for the synthesis of exomethylene tetronates has been developed, and novel rearrangements have been discovered. The two putative key intermediates have been used to investigate the biomimetic synthesis of the carbon skeleton of the quartromicin algycone, and mass spectrometric evidence for formation of homo- and heterodimers, and a heterotetramer of the key intermediates has been obtained.

This thesis describes studies towards the biomimetic synthesis of the lomaiviticin aglycone. Also described in this thesis are studies towards the reductive dimerisation of ahaloketones with the aim of applying this strategy to the synthesis of the lomaiviticin aglycone. The introduction comprises of an overview of the natural products lomaiviticins A and Band their parent family of natural products, the kinamycins. A brief overview of biomimetic synthesis is given, along with a proposed biomimetic hypothesis for the formation of lomaiviticins A and B. Also presented in the introduction is the previous work conducted by others with regards to the total synthesis of the lomaiviticin aglycone. The results and discussion begins with a brief over view of the photochemistry of epoxyketones and quinones. Next, the synthesis of several models of the proposed monomer of the lomaiviticin aglycone is presented. The attempted dimerisation of these models by photochemical and metal-based reductive means is discussed. A discussion of the reductive dimerisation of haloketones with regards to natural product synthesis is followed by the synthesis and attempted dimerisation of a model a-bromoketone. The final part of the results and discussion describes the synthetic efforts towards the proposed monomer. The monomer was synthesised up to the penultimate step, namely deprotection of an acetonide and epoxide formation. An attempted dimerisation was made on an advanced bromoketone, however, no dimer could be isolated. Several efforts were made in light of the failed final step, however, all efforts to install an epoxide motif earlier on in the synthesis also failed. l The next section of this thesis begins with a discussion of the dimeric natural product kingianin A, and then moves on to cover a proposed biosynthesis of the molecule. Next, a possible biomimetic synthesis is discussed, followed by a section regarding the synthesis of a proposed monomer. The final part of this section includes a brief discussion concerning the attempts to dimerise a proposed monomer. The penultimate chapter of prose in this thesis regards novel studies conducted on radical azide chemistry. The first part of the chapter discusses a novel method for the reduction of nitroaromatic azides. The serendipitous discovery of this reaction is discussed followed by an optimisation of conditions to promote effective reduction of a model system. The preferred conditions are then applied to a series of substrates. The mechanism is probed though radical quenching experiments and the synthesis and evaluation of a likely intermediate is discussed. The section then ends with a proposed mechanism, followed by a conclusion . The second part of this chapter is an extension of the methodology whereby a proposed intermediate of the reduction is trapped by an aldehyde rather than a proton, leading to a novel synthesis of electron poor amides. The section begins with a brief overview of amide synthesis and the discussion of two relevant recent a~vances in the field. The reaction is optimised for a model substrate and the reaction is shown to be amenable to a one-pot diazotisation-azidation-amidation sequence leading to high yields of amides. The methodology is then extended to several substrates. The mechanism of the reaction is probed by radical quenching experiments, nitrogen 15 labelling studies and the evaluation of a likely intermediate. The chapter ends with a plausible mechanism followed by a conclusions section. The final chapter of prose in this thesis relates to the development of a novel aryne sigma bond insertion reaction. The chapter begins with a brief overview of aryne chemistry followed by a summary of the common methods for the stannylation of aromatic molecules. The development of the reaction using a model is then discussed, followed by the application of the methodology to several substrates. The final part of the thesis contains an experimental section, which gives detailed procedures for the synthesis and characterisation of the compounds discussed throughout this thesis.

The work described in this thesis is divided into two parts, both of which are related to the synthesis of lindenane sesquiterpenoid natural products. Shizukaol A 1, cycloshizukaol A 2 and trishizukaol A 3 represent three of many novellindenane bi-and trisesquiterpene natural products isolated from plants of the Chloranthus genus. Biologically, a number of monomeric and dimeric structures containing the lindenane skeleton have been shown to exhibit antitumor, hepatoprotective and tyrosinase inhibitory activities. It was proposed that the majority of dimeric and trimeric structures could be accessed biomimeticaUy from the as yet unisolated reactive intermediate lindenatriene 4, itself a possible biosynthetic oxidative product of lindenene 5, a natural product isolated from the taxonomically unrelated species Undera Strychnijolie. The first part of this project is concerned with the expedient total synthesis of (±)-lindenene 5 from Hagemann's ester 7 via a key intramolecular cyclopropanation reaction of diazoketone 6. Initial investigations into the cyclopropanation resulted in an unexpected sterochemical outcome, allowing the total synthesis of two diasteromers of (±)-lindenene 5: (±)-epi-lindenene and {±)-iso-lindenene. The total synthesis of (±)-lindenene 5 was finally achieved by the thermally induced olefin insertion of diazoketone 6, marking the first example of such an intramolecular cyclopropanation carried out in the absence of a transition metal catalyst, along with the first example of the synthesis of a lindenane natural product. The second part of the project describes the approaches towards the biomimetic conversion of {±)-lindenene 5 into {±)-l indenatriene 4. Model studies on the conversion of furans into decorated butenolide structures were then applied to lindenene systems with varying degrees of success. (±)-Undenene 5 was converted into several chlorantholactone natu ral products, postulated as biosynthetic oxidative intermediates en route to lindenatriene 4. Although lindenat riene 4 was not synthesised, valuable insight was gained into the reactivity of many of the butenolide intermediates, and future work in the area seems promising.

The first steps towards the long term objective of building entirely synthetic organic/inorganic composite materials in a biomimetic manner have been achieved. Following an introduction and discussion of the background to the work (Chapter 1), the syntheses and characterisation of various molecular weights of both poly(exo, exo-1,4-cyclopentenylene-5,6-ethylidene-2,3-sodium dicarboxylate) and poly(exo,endo-l,4-cyclopentenylene-5,6-ethylidene-2,3- sodium dicarboxylate) and model compounds of their repeat units have been described (Chapters 2 and 4). These compounds were used as additives in the crystallisation of CaCO(_3) from supersaturated aqueous solutions of Ca(HC0(_3))(_2) (Chapters 3 and 5). The work described in Chapter 3 showed that the diacid model compounds used as additives controlled the morphology of calcite crystals grown from supersaturated solutions of Ca(HCO(_3))(_2) over a large range of concentrations of model compound; [Ca(^2+)]: [model compound] 10 to 1000:1. The polymers of these monomers appeared to give the same type of crystal morphology as the isolated model repeat units, however, modification was observed on only one face of the CaCO(_3) crystals. This observation, and the relatively small crystal size distributions measured, indicated that the calcite crystallisation was nucleated beneath the polymer films at the truncated modified face and growth continued down into the solution.

En chimie biomimétique des hélices, plusieurs squelettes amides non naturels (peptoides, -peptides, -peptides) ont récemment été décrits pour s'auto-organiser au niveau moléculaire et adopter des structures hélicoidales, permettant de mimer des structures secondaires de protéines. Nous avons montré que la fonction urée qui favorise la formation de liaison hydrogene, peut être introduite avec succès dans des oligomères adoptant une structure en hélice. Nous avons étudié en détail plusieurs paramètres gouvernant la formation et la stabilisation de cette hélice : la longueur de chaine, la nature des résidus incorporés, le type de solvant et le blocage de l'amine terminale. Nous avons ainsi démontré que des oligomères d'urées N,N'-liées de la taille d'un heptamère bloqué peuvent adopter une structure hélicoidale dans un solvent protique comme le methanol. L'ensemble de ces données structurales nous a permis d'envisager pour la première fois l'utilisation des oligomères d'urée dans le domaine thérapeutique. La découverte que des oligomères d'urée cationiques et amphiphiles comportant seulement 7-8 résidus présentent une activité antimicrobienne significative ainsi qu'une bonne sélectivité vis-à-vis des différentes membranes cellulaires (eukaryotes/procaryotes) est prometteuse
In the field of biomimetic chemistry, several unnatural oligoamide backbones (peptoids, -peptides, -peptides) have been described that self-organize at the molecular level to form stable helices useful to mimic protein secondary structure elements. We have shown that the urea moiety, by its capacity to form auto-complementary and bi-directional hydrogen bonds can be substituted for the amide linkage to generate oligomeric strands with strong propensity for helix formation. We have studied in depth the requirements for helix formation and stabilization, in term of chain-length, side-chains nature, solvent effect and terminal charge influence. We have thus demonstrated that N,N'-oligoureas as short as N-capped heptamer can adopt a stable helical fold in a protic solvent (e. G. Methanol). Altogether, these valuable structural data have provided a rationale to tackle biological issues using oligoureas. The present work is the first attempt to design bioactive oligoureas based on their propensity to adopt 2. 5-helical structures. The finding that cationic amphiphilic oligoureas as short as 7-8-residues display significant antibacterial activity in vitro as well as membrane selectivity is promising

This thesis describes the total synthesis of the polyketide derived marine natural product (-)-maurenone (14) and synthetic studies of a model system for the marine polyketides, the spiculoic acids (20, 22-24). A biomimetic approach involving cyclisation of linear polyketide precursors to install the complex chemical frameworks was employed. Maurenone is a polypropionate derived metabolite isolated from pulmonate molluscs collected off the coast of Costa Rica. While structural assignment following isolation revealed a relatively uncommon tetra-substituted dihydropyrone moiety the only stereochemical information deduced was the trans-relative relationship between the C8 and C9 protons. The total synthesis of a series of eight stereoisomeric putative structures was achieved in order to assign the stereochemistry of (-)-maurenone (14), as that depicted above. A time and cost efficient strategy was developed utilising common intermediates providing access to the eight stereoisomeric structures in a convergent manner. Six key fragments, four aldehydes (109) and two ketones (110), were synthesised using highly diastereoselective syn- and anti-boron aldol reactions and were coupled using a lithium-mediated aldol reaction. Trifluoroacetic acid-promoted cyclisation/dehydration enabled installation the ƒ×-dihydropyrone ring. All eight isomers of one enantiomeric series were synthesised by coupling two ketones with each of four aldehydes. By comparison of the NMR data for the eight isomers with that reported for the natural product, the relative stereochemistry was established as shown. The (-)-enantiomer of maurenone was synthesised in nine linear steps (13 % overall yield) from (R)-2-benzylpentan-3-one ((R)-40) and (R)-2-benzoyloxypentan-3-one ((R)-39). The spiculoic acid family of polyketide derived natural products, isolated from plakortis sponges, possess a unique [4.3.0]-bicyclic core which is proposed to be formed via an enzyme catalysed Intramolecular Diels-Alder (IMDA) cycloaddition reaction of linear polyene precursors 25. Model linear precursors (114), possessing various olefin geometries at C2 and both stereochemical orientations of the C5 stereocentre, were synthesised in order to examine stereoselectivity of the thermally induced IMDA cycloaddition reaction. The two alternative C4-C6 stereotriads of the linear precursors 114 were achieved by employing highly diastereoselective substrate-controlled aldol reactions; an anti-boron aldol reaction, controlled by the facial preference of (R)-2-benzoyloxypentan-3-one ((R)-39), and a syn-titanium aldol reaction, under the control of chiral N-acylthiazolidinethione ((R)-43a). The diene and dienophile moieties were installed using either standard Wittig, H.W.E. or ¡§modified¡¨ Julia olefination reactions. A thorough stereochemical assignment of the cycloadducts of the thermally induced IMDA reaction of each linear precursor was accomplished employing 2D NMR techniques. Comparison of the stereochemistry of each of the cycloadducts with the spiculoic acids revealed that the linear precursor (2E,5S)-114 produced a cycloadduct 232 with stereochemistry analogous to the natural products in 94 % diastereoselectivity. Thus, a synthetic approach to the spiculoic acids via synthesis of a linear precursor 285 possessing a TBS ether at C5 in the S configuration was proposed. Unfortunately, problems encountered in the synthesis of the proposed linear precursors to the spiculoic acids ultimately prevented the total synthesis from being achieved.