The halogenation and epoxidation of some 3-substituted cholest-5-enes

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dc.contributor.advisor De la Mare, P.B. en Wilson, Raymond Donald en 2007-08-11T06:27:17Z en 2007-08-11T06:27:17Z en 1976 en
dc.identifier THESIS 77-049 en
dc.identifier.citation Thesis (PhD--Chemistry)--University of Auckland, 1976 en
dc.identifier.uri en
dc.description Full text is available to authenticated members of The University of Auckland only. en
dc.description.abstract The mechanisms of halogenation and epoxidation of simple olefins and the known results of halogenation, epoxidation, and other electrophilic addition reactions involving 5, 6-unsaturated steroids are reviewed. The isomerization and autoxidation of cholest-5-en-3-one also are reviewed. The results of a detailed investigation of the mechanisms of halogenation and epoxidation of unsubstituted, 3α-substituted, 3β-substituted, and 3-keto-cholest-5-ene are described, and are discussed in terms of transition-state structures, and electronic, steric, and conformational effects. Epoxidation of these substrates using m-chloroperbenzoic acid is stereoselective. It is proposed that the oxidation involves a highly asymmetrical transition state and is sterically controlled. In the presence of pyridine, 5, 6-epoxycholestan-3-ones rearrange to 6-hydroxycholest-4-en-3-ones with retention of configuration at the 6-carbon. The mechanism is discussed. Halide-catalysed halogenation of cholest-5-enes proceeds with predominantly α-attack by the electrophile. It is proposed that the stereochemistry of attack is controlled mainly by steric effects upon the pre-equilibrium complexation of the halogen and olefin. Non-catalysed and pyridine-catalysed halogenation of 3-tetrahedral cholest-5-enes via halonium ions is highly stereoselective; but similar halogenation of the 3-ketone is almost non-stereoselective. These features are explained in terms of substantially bridged transition states and, where appropriate, pre-equilibria, It is proposed that addition to the 3-tetrahedral cholest-5-enes is sterically controlled. Similar steric effects must influence the halogenation of the ketone; but it is proposed that β-attack is accelerated through the accessibility of a boat conformation of the A-ring. The fates of the halonium intermediates are deduced and discussed. Those derived from 3-tetrahedral cholestenes give adducts; those derived from the 3-ketone are in part trapped by nucleophiles to give adducts and in part deprotonated to give 6-halocholest-4-en-3-ones. Halogenation of the ketone also gives some 6β-halocholest-4-en-3-one by a fully concerted SE2' mechanism. 5-Bromocholestan-3-ones readily undergo 1, 2-dehydrobromination by a free-radical chain mechanism. en
dc.language.iso en en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.relation.isreferencedby UoA9921803714002091 en
dc.rights Restricted Item. Available to authenticated members of The University of Auckland. en
dc.rights.uri en
dc.title The halogenation and epoxidation of some 3-substituted cholest-5-enes en
dc.type Thesis en Chemistry en The University of Auckland en Doctoral en PhD en
dc.rights.holder Copyright: The author en

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