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Employing parallel and combinatorial strategies, a new water-tolerant ruthenium based method for the in situ oxidation of a hydroxylamine to the corresponding nitroso compound has been developed. It has been discovered that 0.1 mol % of a ruthenium(II)-salen derived complex is able to catalyse the oxidation reaction of an N-Boc-hydroxylamine to the corresponding 7V-Boc-nitroso compound, which can then be trapped out in a [4+2] cycloaddition reaction with a 1,3-conjugated diene. The reaction is complete within 1 hour at room temperature and produces the corresponding cycloadduct, after work up and purification by column chromatography, in an 81 % yield. These same catalytic conditions have also been applied to cycloaddition reactions of the N-Boc-nitroso dienophile with other cyclic and acyclic 1,3-conjugated dienes. One of the fundamental aims of the project was to discover a catalyst that would not only catalyse the oxidation of the hydroxylamine to the nitroso species but also catalyse the cycloaddition reaction of the nitroso species with the conjugated 1,3-diene. Hence, the asymmetric ruthenium(ll)-salen derived catalyst was prepared and tested in the reaction. Unfortunately, although this catalyst works well for the oxidation step, it did not produce asymmetric induction for the cycloaddition reaction. A catalytic cycle and hypotheses for the lack of enantioselectivity have been proposed. It was also discovered that 15 % e.e. could be obtained when a ruthenium(ll)- DIOP system was used in the oxidation-cycloaddition reaction between N-Boc- hydroxylamine and cyclohexadiene, in the presence of 3 equivalents of TBHP. Unfortunately, only a 14 % yield of the corresponding cycloadduct was obtained. The use of nitrosobenzene and ortho-methoxynitrosobenzene as dienophiles in the nitroso Diels-Alder reaction has also been demonstrated. Attempts were made to also discover a water-tolerant asymmetric catalyst for these cycloaddition reactions. Work has also been earned out to show the applications of the acyl-nitroso Diels-Alder cycloaddition reactions towards the synthesis of spider venoms. A synthetic route towards substituted piperidine spider venoms was proposed but due to time constraints, work could not be completed in this area.

The principle objective of this study is to conduct a definitive investigation into the cycloaddition of allenes and ketenes, with particular emphasis on halogenated ketenes.

The synthesis of 5-membered heterocyclic rings that bear both amine and carboxylic acid functional groups has been investigated using a 1,3-dipolar cycloaddition reaction strategy. These molecules, on incorporation into a chain of amino acids, have the potential to restrict the conformational freedom of the peptide. Cycloaddition of a nitrile oxide, derived from a Boc-protected naturally-occurring a-amino acid, with a pyrrolidine en amine led to a Boc-protected 3-aminoalkylisoxazole amino acid ester. The nitrogen and carbon termini of this isoxazole were coupled to other a-amino acids. Analysis of the dipeptide from coupling to (S)-alanine indicated that the integrity of the chiral centre of the isoxazole had been retained during the synthesis. Molecular modelling of a tripeptide unit incorporating the isoxazole showed that the presence of the ring had, as intended, restricted the conformational freedom of the molecule. Analogous cycloadditions using azomethine imines as the dipole yielded the corresponding tetrahydropyrazoles (pyrazolidines). These dipoles were generated by reaction of an aldehyde with a 1,2-disubstituted hydrazine, followed by elimination of the elements of water from the resulting aminol. Reaction with a dipolarophile bearing an electron withdrawing substituent gave predominantly the 4-substituted pyrazolidine. A study of the scope of the reaction found that, although the required carboxylic acid group could easily be incorporated by use of methyl acrylate as the dipolarophile, it proved impossible to attach an amine group to the ring using this methodology. However, by using one of the nitrogen atoms in the ring as the N-terminus, two pyrazolidines with protected amine and carboxylic acid groups were prepared, and these can be thought of as conformationally restricted B-amino acids.

Steric effects are important in synthesis. Whilst steric hindrance is well known in hindering reactions, steric effects can also be employed to accelerate reactions, in particular cycloaddition reactions, and even to promote reactions that otherwise do not occur. A survey is included in previous work on steric effects in chemical reactions, principally cycloadditions. This includes a brief discussion on the importance of orientation and solvent effects on Diels-Alder cyclisations and ene reactions. The effect of substituents on the cyclisation of N-allyl furfurylamines has been studied. It was shown that bulky N-protecting groups enhance cyclisation, an effective buttress being the trityl (triphenylmethyl) group. The latter has the added advantage of being particularly easy to remove. A study of some ene reactions has also been carried out and steric acceleration of these processes has also been observed. A novel reaction involving an intermolecular cycloaddition followed by a sterically accelerated ene reaction has also been uncovered. Some attempts have also been made at carrying out these sterically accelerated reactions on a solid support as required in combinatorial chemistry. This involved the preparation of a new type of substituted support, with a view to utilising this in a combinatorial approach to synthesis. The structures have been supported by using a molecular modelling package, Alchemy 2000 from Tripos Associates Inc.

<p> This thesis describes an attempt to develop a potentially useful route to seven-membered carbocycles. α-Diketones and their enol ether derivatives were proposed to be suitable three-carbon cycloaddition dienophiles when complexed with a Lewis acid. Reaction with 1,3-dienes should then lead to the desired cycloheptyl system.</p> <p> 1H-NMR and 13C-NMR studies were used to determine the stoichiometry and charge delocalization of complexes formed between 2,3-butanedione, 1,2-cyclohexanedione, 2-methoxy-cyclohex-2-en-1-one and the Lewis acids TiCl4 and SnCl4. Observations from these studies suggested that 1,2-cyclohexanedione and 2-methoxy-cyclohex-2-en-1one could behave as substituted allyl cations when complexed with TiCl4 or SnCl4.</p> <p> Reaction of the 1,2-cyclohexanedione/TiCl4 and 2-methoxy-cyclohex-2-en-1one/ TiCl4 complexes with 1 ,3-butadiene gave rise to six-membered carbocycles from [4C+2C] cycloadditions. The reaction of 2,3-butanedione-mono-trimethylsilyl enol ether/TiCl4 complex with 1,3-butadiene gave a mixture of the [4C+2C] and [4C+3C] cycloaddition products.</p> / Thesis / Master of Science (MSc)

Les chimistes comme tous travailleurs ont besoin d'outils pour leur travail, en l'occurrence les réactions chimiques. Ces outils permettent aux chimistes de transformer la matière et ainsi de fabriquer des produits naturels ou non-naturels à partir de précurseurs aisément disponibles. Par exemple, le menthol est un additif naturel d'une grande importance industrielle et il est synthétisé à partir du pinne qui est extrait des aiguilles de pins à coût modéré. C'est pourquoi, il est fondamental de concevoir et développer de tels outils. Afin de connaître les étendues possibles d'application, le chimiste effectue un travail dit de méthodologie. De nombreux essais sont réalisés de façon précise et rationnelle pour déterminer les limites et les avantages de la réaction d'intérêt. Dans ce mémoire, la réaction d'intérêt est une cycloaddition (4+1) intramoléculaire formelle. Lors de cette réaction deux liens carbone-carbone sont formés. Cela permet de passer d'une structure simple linéaire à une structure complexe bicyclique, présentant un levier synthétique, la double liaison. Dans la première partie de l'introduction de ce mémoire sont rapportées toutes les utilisations des aminocarbènes, c'est-à-dire les carbènes avec un atome d'azote en position a du carbène. On s'est restreint aux carbènes métalliques, de type Fisher. On a détaillé plus précisément les aminocarbènes de chrome, qui sont au coeur de ce projet. La seconde partie de l'introduction présente brièvement les réactions de cycloaddition (4+1) rapportées dans la littérature, nous détaillons plus particulièrement les réactions de cycloaddition (4+1) utilisant des carbènes de chrome. Nous arrivons alors à la première réaction de cycloaddition (4+1) intermoléculaire, mettant en jeu un diène neutre et un aminocarbène de chrome et nous résumons ensuite les travaux de Louis-Philippe Lefebvre. Le deuxième chapitre du mémoire décrit les efforts effectués afin d'élargir l'étendue de la réaction et la compréhension du mécanisme réactionnel des réactions de cycloaddition (4+1) formelle où les substrats présentent des différences électroniques et de grandeur de cycles créés. Nous discutons des apports des résultats obtenus et du chemin qu'il reste à parcourir pour compléter l'étude de cette réaction et de l'appliquer en synthèse totale de produits naturels.

From a review of the published work in the field of cycloadditions, it is evident that further research is needed to establish the mechanism of ketene cycloadditions. This work was initiated with the intent of obtaining kinetic data which will contribute to the elucidation of the mechanism of ketene cycloadditions.