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This thesis focuses on the use of unsaturated aldols as useful substrates in natural product synthesis. Two methodologies have been investigated for the asymmetric synthesis of highly substituted lactones containing multiple contiguous stereocentres from unsaturated aldol precursors. These lactones have potential application as building blocks for natural product synthesis. Firstly, synthetic applications of the retro aldol reaction are reviewed. The second chapter describes a novel methodology for the asymmetric synthesis of highly substituted δ-lactones from syn-aldol cyclopropanes iii. Mercury mediated cyclopropane ringopening of the methyl ester cyclopropanes iv followed by concomitant cyclisation produced organomercurial δ-lactones v, which subsequently undergo reductive demercuration in basic sodium borohydride to afford the highly substituted δ-lactones vi in good yield and excellent diastereoselectivity. The scope of this method was investigated with variation of the R1 and R2 groups. The synthetic utility of this process was also demonstrated with the synthesis of a series of (+)-Prelactone natural products. The third chapter decribes a method of preparing hydroxy-γ-butyrolactones (viii-x) containing multiple contiguous stereocentres in high yield with good diastereoselectivity. Upjohn dihydroxylation conditions using catalytic osmium tetroxide were employed to β-alkenyl-β- hydroxy-N-acyloxazolidin-2-ones vii with different alkene substitution patterns. This resulted in the formation of triols that underwent spontaneous intramolecular 5-exo-trig cyclisation reactions to afford hydroxy-γ-butyrolactones viii, ix or x depending on the substitution pattern of the alkene precursor.

La réaction de Kulinkovich, découverte en 1989, permet la transformation d’esters encyclopropanols par réaction avec un complexe oléfinique de titane. Cette réaction a été appliquée en 2001 à la conversion des nitriles en cyclopropylamines primaires.Nous avons tout d’abord effectué une étude de la cyclopropanation asymétrique d’un cyanoester par un complexe chiral de titane. Une méthode d’évaluation rapide de l’induction asymétrique de ligands chiraux a été mise au point, et a permis de réaliser un screening dediols chiraux. Les meilleurs résultats ont été obtenus avec le TADDOL, qui a fourni des excès énantiomériques moyens, mais notre méthode d’évaluation reste un atout pour l’évaluation d’autres types de ligands chiraux.Une méthode originale de formation de 1,4-dicétones via un complexe oléfinique de titane a été découverte puis optimisée, et ses limites ont été explorées. La préparation des 1,4-dicétones s’effectue en deux étapes à partir d’acides carboxyliques avec des rendements modestes à satisfaisants. L’utilisation de réactifs peu coûteux et peu toxiques ainsi que la grande diversité structurale accessible rendent notre méthode de synthèse compétitive vis-àvis de celles décrites dans la littérature.Plusieurs études réalisées précédemment par notre équipe ont permis de synthétiser de façon exclusive les isomères (Z) de divers 2,3-méthanoaminoacides. Nous avons développé au cours de cette thèse une voie de synthèse des isomères (E) et (Z) de la 2,3-méthanolysineprotégés de façon orthogonale, via la cyclopropanation diastéréosélective d’une cyanhydrinebenzylée. La flexibilité de cette stratégie permet d’envisager la préparation des isomères (E)et (Z) de nombreux autres analogues contraints d’acides aminés. / The Kulinkovich reaction, discovered in 1989, allows the titanium-mediatedtransformation of esters into cyclopropanols using Grignard reagents. This reaction wasextended to other substrates, notably nitriles which afford primary cyclopropylamines. First, a study of the asymmetric cyclopropanation of a cyanoester using a chiral titaniumcomplex has been carried out. A method allowing the fast assessment of the asymmetricinduction of chiral ligands has been worked out, and used to conduct a screening of chiraldiols. The best results were obtained with the original TADDOL, which furnished averageenantiomeric excesses, but our method remains an asset for the evaluation of other types ofchiral ligand.An original method for the formation of 1,4-diketones using an olefinic titanium complexhas been discovered, optimized, and explored. We established that 1,4-diketones are availablein two steps from carboxylic acids with modest to correct yields. The use of inexpensive andminimally toxic reagents as well as an access to a vast structural diversity allow our method tobe competitive beside those reported in literature.Several studies carried out previously by our team allowed the exclusive synthesis of the(Z) isomers of various 2,3-methanoaminoacids. In this present work, we developped asynthetic pathway towards (E) and (Z) isomers of orthogonaly protected 2,3-methanolysine,using the diastereoselective cyclopropanation of a benzylated cyanhydrin as key reaction. Theflexibility of this strategy allows to consider the preparation of (E) and (Z) isomers of variousother constrained aminoacid analogues.

Multi-substituted optically pure cyclopropanes are important motifs present in many agrochemicals, pharmaceuticals and materials employed in manifold applications. Their synthesis is challenging due to both the strained conformation and the need to control both the relative and absolute stereochemistry. This thesis describes an investigation of the scope of the Wadsworth-Emmons cyclopropanation, highlighting it as a potential efficient methodology for diastereoselective and enantiospecific synthesis of these valuable ring systems. Chapter 1 is an introduction to the cyclopropanation protocols and is split in two subsections. The first is a description of the currently most exploited synthetic pathways to cyclopropanes, with analysis of their substrate scope and critical analysis of their limitations as compared to potential of the Wadsworth-Emmons cyclopropanation (Sections 1.1 to 1.3). The latter section consists of a review on the history and the evolution of this procedure and introduces the preconditions for the development of the project (Sections 1.4 to 1.17). Chapter 2 describes the results and discussion, introducing the use of novel alkyl-substituted triethyl phosphonoacetates to yield cyclopropyl-esters containing quaternary stereocentres. The high yields and the excellent trans-diastereoselectivity values obtained (proved by X-ray crystallography) allowed the proposal of a suggested reaction mechanism, which is supported by the subsequent experiments carried out in the presence of other functional groups, e.g. other carbonyls, nitriles, hetero-aromatics, substituted phenyl rings. This study was further extended to examine the effect of fluorine atom, due to its importance in biologically active environments. The study on the stereochemistry of the Wadsworth-Emmons cyclopropanation has been strongly supported by a range of X-ray crystal structures of the cyclopropane products. The chapter ends with an empirical set of guidelines, helpful for the design of successful cyclopropanations. Chapter 3, describes the experimental methods in full as well as full characterisation of the products obtained. Literature references, noted throughout the thesis, are listed in the final chapter.

Biflavonoids form a ubiquitous class of compounds in nature, which are well known for their numerous medicinal benefits. Biflavonoids such as chamaejasmine and isochamaejasmine have been shown to exhibit pharmacological activity such as anitidiabetic, anti-malaria, anti- HIV, anticancer etc. Biflavanones, chamaejasmine and isochamaejasmine are dimers of the flavanone narigenin at the C-3 position, which gives them a rare 3/3" C - C linkage. They are structurally similar in that they both have four stereocenters. The difference between these two compounds is the stereochemistry at the C-2" and C-3". In this study, installation of each of these stereocenters will be targeted at different stages of the synthesis. Installation of the stereocenters at C-2 and C-3 was achieved through the cyclopropanation of a 2-aryl-2H-chromene, followed by the tin (II) triflate catalyzed rearrangement of the cyclopropane into a gamma-lactone. This gamma-lactone provided the core structure for one of the flavanone units in the targeted biflavonoids, as well as providing the building block for the second flavanone unit. For the construction of the second flavanone unit the gamma-lactone was transformed into a chalcone precursor, which served as the platform for the installation of the other two stereocenters in the biflavonoids. The construction of the chalcone precursor was achieved first through the synthesis of an alpha-benzylidene lactone. Several attempts were made to open this alpha-benzylidene lactone via the addition of an aryllithium, which proved to be a challenge. This problem was resolved through the opening of the alpha-benzylidene lactone via reduction with Red-Al, subsequent formation of an aldehyde, and then addition of the aryllithium to the aldehyde. Successful synthesis of the chalcone precursor was achieved through the oxidation of the product of the aryllithium addition, albeit in a modest yield (43 %). Herein, studies related towards the synthesis of chamaejasmine and isochamaejasmine are presented.

Soluble polymer supports facilitate the recovery and recycling of expensive transition metal complexes. Recently, polyisobutylene (PIB) oligomers have been found to be suitable polymer supports for the recovery of a variety of transition metal catalysts using liquid/liquid biphasic separations after a homogeneous reaction. Our work has shown that PIB-supported Ni(II) and Co(II) β-diketonates prepared from commercially available vinyl terminated PIB oligomers possess catalytic activity like that of their low molecular weight analogs in Mukaiyama epoxidation of olefins. Carboxylic acid terminated PIB derivatives can act as carboxylate ligands for Rh(II) cyclopropanation catalysts. An achiral PIB-supported Rh(II) carboxylate catalyst showed good activity in cyclopropanation of styrene in hydrocarbon solvents, and could be easily recycled nine times by a post reaction extraction. Further application of PIB supports in asymmetric cyclopropanation reactions were investigated using PIBsupported arenesulfonyl Rh(II) prolinates derived from L-proline as examples. The PIBsupported chiral Rh carboxylates demonstrated moderate activity and were recovered and reused for four to five cycles. The prolinate catalyst prepared from PIB-anisole also showed encouraging enantioselectivity and about 8% ee and 13% ee were observed on trans- and cis-cyclopropanation product respectively. Finally, PIB oligomers can be modified in a multi step sequence to prepare PIBsupported chiral bisoxazolines that can in turn be used to prepare active, recyclable PIBsupported Cu(I) bisoxazoline complexes for olefin cyclopropanation. These chiral copper catalysts showed moderate catalytic activity and good stereoselectivity in cyclopropanation of styrene. A chiral ligand prepared from D-phenylglycinol provided the most effective stereo control and gave the trans- and cis-cyclopropanation product in 94% ee and 68% ee respectively. All three PIB-supported chiral bisoxazoline-Cu(I) catalysts could be reused five to six times.

Transition metal catalyzed reactions have an important place in synthetic chemistry, but the mechanistic details for many of these reactions remain undetermined. Through a combination of experimentally determined 13C kinetic isotope effects (KIEs) and density functional theory (DFT) calculations, some of these reactions have been investigated. The cyclopropanation of an olefin catalyzed by rhodium (II) tetrabridged complexes has been shown to proceed through an asynchronous, but concerted mechanism. DFT does not provide an accurate transition structure for the reaction of an unstabilized carbenoid with an olefin, but it does predict an early, enthalpically barrierless transition state which is consistent with the reactivity of unstabilized carbenoids. For the case of stabilized carbenoids, the theoretical structures predict the KIEs accurately and a new model is proposed to explain the selectivity observed in Rh2(S-DOSP)4-catalyzed cyclopropanations. The chain-elongation step of atom transfer radical polymerization (ATRP) has been shown to be indistinguishable from that of free radical polymerization (FRP) for the CuBr/2,2??-bipyridine system. While DFT calculations predict an earlier transition state than observed, the calculations suggest that with increasing levels of theory the predicted KIEs come closer to the observed KIEs. A recently proposed [2 + 2] mechanism for the cyclopropenation of alkynes catalyzed by Rh2(OAc)(DPTI)3 has been shown not to be a viable pathway. Rather, the experimental KIEs are predicted well by canonical variational transition state theory employing the conventional mechanism for cyclopropenation via a tetrabridged rhodium carbenoid. DFT calculations also suggest an alternative explanation for the observed enantioselectivity. The 13C KIEs for metal-catalyzed aziridination have been measured for three separate catalytic systems. While the KIEs do not completely define the mechanism, all of the reactions exhibit similar KIEs, implying similar mechanisms. A surprising feature of this system is the presumed nitrene intermediate??s triplet spin state. This complicates the DFT analysis of this system.

Environmentally friendly reactions yielding cis-cyclopropanes are desired in the insecticide industry as these are most effective at reducing the population of insects while remaining benign to the environment. We are exploring the best parameters for the most effective cis-cyclopropanation reactions. An alkene and a diazo compound react together in the presence of a dirhodium catalyst to produce cyclopropanes. First however, the starting materials must be pure. Alfa Aesar supplies cyclohexene which is reported to be 99% pure, however, Gas column chromatography/Mass spectroscopy reveals that the sample is only 3% cyclohexene. Using an adopted published procedure, we discovered that distillation under nitrogen produces cyclohexene in greater than 95% pure. This is suitable for use in cyclopropanation reactions.