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Title: Ring opening/cyclisation in oxiranyl carbinyl free radicals
Authors: Zaidi, Naveed A.
Issue Date: 1991
Publisher: © Naveed Ahmed Zaidi
Abstract: A study aimed to examine the electronic and possible stereoelectronic factors controlling the opening of the epoxide C-O versus C-C bond in 4<. ,5-epoxy-5<' -cholestan-3-yl-radicals. formed by photolysis of 3B -acetoxy-4<' ,5-epoxy-5<' -cholestanes in HMPAlH20 and by BU3SnH reduction of 4<. ,5-epoxy- 3B -[jmidazol-l-yl(thiocarbonyJ)oxyl-5<' -cholestanes. These derived radicals gave cholest-3-en-5B -01 via the cholest-3-en-5<' -oxy-radicals and reversible cleavage of the C5- CIO bond. The 4<. ,5-epoxy- 5<' -cholestan-6-yl-radicals, formed by photolysis of 6a -acetoxy-4<' ,5-epoxy-5E. -cholestanes in HMPA/H20, and by BU3SnH reduction of 4E. ,5-epoxy-6a -[jmidazol-l-yl(thiocarbonyJ)oxyl-5E. - cholestanes gave the cholest-5-en-4E. -oxy-radicals, which were reduced to give the cholest- 5-en-4E. -ols. In the study of bicycloI2.2.llheptane systems, the reduction with BU3SnH of eOO0-2-[jmidazol-l-yl( thiocarbonyJ)oxyl-anti-3 '-phenylbicycloI2.2.1 Iheptan-3-spiro-2 '-oxiranes and the ant;-3 '-p-chlorophenyl and 3'-methylene analogues failed to generate a radical at C-2 irrespective of the stereochemistry (exo- or eOOo-) of the epoxides. Unexpected products were derived from the reduction of the thioester group to methoxy and acetal groups; the epnxide groups remained unaltered. This novel reduction has not been observed before. The photolysis in HMPA/H20 of end0-2-acetoxy-anti-3 '-phenylbicycloI2.2.llheptan-3-spiro- 2' -oxiranes and the anti-3' -p-chlorophenyl and 3' -methylene analogues failed to generate a radical at C-2 irrespective of the stereochemistry (exo- or endo-) of the epoxides; but instead gave cleavage of the C-O bonds of the aryl epnxides which stereoselectively gave the eOO0-2-acetoxy-4f. -phenylbicyclo- 13.2.J1octan-3-ones. The methylene epoxides were obtained unchanged after photolysis, in low yield. The reaction between BU3SnH and the ex0-2-bromo-anti-3 '-phenylbicycloI2.2.J1heptan-3-spiro- 2' -oxiranes and the anti-3' -p-chlorophenyl analogue (exo- and endo-epoxides) generated a radical at C-2 and the products, norcamphor, benzyl alcohol and p-chlorobenzyl alcohol, were derived exclusively from C-C bond cleavage.
Description: A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.
URI: https://dspace.lboro.ac.uk/2134/22083
Appears in Collections:PhD Theses (Chemistry)

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