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Novel methods for allylic amination by an intramolecular nitroso ene reaction

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posted on 31.01.2014, 11:58 authored by Duncan Atkinson
C-H functionalisation reactions aim for the selective cleavage of C-H bonds, and formation of a new carbon or heteroatom bond, often with the use of a transition metal catalyst. These reactions offer potential for functionalisation of hydrocarbons in fewer steps than conventional methods, and with high atom efficiency. They are therefore a subject of intense research in organic synthesis. Carbon-heteroatom bond forming reactions are particularly sought after, and useful in the efficient synthesis of many biologically significant groups such as oxazolidinone rings, 1,2 or 1,3 amino alcohols and amino acid analogues. An efficient, cheap and robust method for C-H amination would also be adaptable to varied syntheses of important large molecules. The necessity for complex and efficient transformations with a minimal number of steps means that heteroatom ring closures are also attractive and widely used reactions in such large molecule syntheses. The nitroso group is a highly reactive species which is normally generated in situ by oxidation of a hydroxylamine, for carbon-nitrogen bond forming reactions including the nitroso Heteroatomic Diels Alder reaction, nitroso ene reaction, and nitroso aldol reaction. Nitroso group reactions often show high stereo- and regioselectivity, and have formed key components of the syntheses of important biological molecules. Enantioselectivive protocols for the nitroso-ene reaction and, to a lesser extent, the nitroso HDA reaction, are poorly developed, however, and the range of available intramolecular nitroso reactions is limited. We aimed to establish efficient single-step intermolecular C-H amination reactions, to give 1,2 and 1,3 heteroatom functionalised molecules, and to develop the capacity for enantioselective induction in this reaction, if possible. Having synthesised a model set of unsaturated hydroxycarbamates, we identified a suitable system for nitroso generation, using a catalytic metal and stoichiometric oxidant. This resulted in in situ generation of cyclised product, with the olefin functionality intact. This cyclisation was then optimised and used to obtain a range of new heterocycles. The possibility of enantioselective induction via chiral catalysts was explored, as well as catalytic systems to increase the stereoselectivity of the reaction. In summary, a cheap, novel and reliable method was developed for formation of oxazolidinone rings from unsaturated hydroxycarbamates using an original intramolecular nitroso ene reaction, and a range of unsaturated heterocycles were synthesised in fairly good yields. Distereoselectivity of the nitroso ene cyclisations was optimised. However, to-date, we were unable to develop an enantioselective varient of the reaction. Several related aminations, as well as transformations of N-hydroxyoxazolidinone products, were also attempted during the project.



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© Duncan Atkinson

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A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.

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