Thesis-1991-Zaidi.pdf (3.77 MB)
Ring opening/cyclisation in oxiranyl carbinyl free radicals
thesis
posted on 2016-07-25, 10:08 authored by Naveed A. ZaidiA 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.
History
School
- Science
Department
- Chemistry
Publisher
© Naveed Ahmed ZaidiPublisher statement
This work is made available according to the conditions of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0) licence. Full details of this licence are available at: https://creativecommons.org/licenses/by-nc-nd/4.0/Publication date
1991Notes
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.Language
- en