Oxime derivatives : versatile reagents for radical-mediated syntheses of heterocycles

Abstract

A summary of tin hydride mediated reactions in generating radicals in organic synthesis is presented, together with some of the many alternative methods now available for conducting radical reactions. Particular attention has been given to the iminyl radical and the development of tin-free organic radical precursors. This introduction is followed by three chapters describing research on the development of two new sources of iminyl radicals and their application in syntheses of aza- heterocyles. O-Phenyl oxime ethers are the first iminyl radical precursors described in the thesis. Microwave thermolyses of oxime ethers released iminyl and phenoxyl radicals under comparatively mild conditions and with short reaction times. Few microwave-assisted synthetic methods, based around radical intermediates, are known. The mild and neutral conditions associated with radical chemistry, and the ability of radicals to perform intramolecular cyclisations, together with the virtues of MAOS, make their combination a very useful tool in syntheses of aza-heterocycles. A comprehensive study of intramolecular additions of iminyl radicals onto several radical acceptors, alkenes, alkynes, phenyl rings and indoles, is described. Furthermore, a wide range of nitrogen heterocyles with potential biological activity was prepared making use of this methodology. Intramolecular iminyl radical cyclisation onto imines via microwave irradiation was another process extensively studied. Microwave assisted syntheses of dihydroquinazolines and quinazolines are described. The precursor O-phenyl oxime ethers enable imine formation to be assimilated with iminyl radical generation before subsequent cyclisation. Clean, fast and high yielding methodology was therefore developed for the syntheses of these highly interesting heterocycles which form the basis of many pharmaceutical products. Dioxime oxalates were the second type of precursor investigated as sources of iminyl radicals. Homolytic cleavage of their N-O oxime bonds occurred on photolysis releasing two molecules of CO₂ and two iminyl radicals in a clean and atom-efficient process. A facile route to dioxime oxalates with a range of radical acceptors in suitable positions is described. ESR spectroscopy was used to demonstrate that dioxime oxalates dissociate on photolysis to give iminyl radicals in the presence of photosensitizer. This technique also confirmed the proposed mechanisms of radical cyclisation onto double bonds and several 2-azacyclopentylmethyl radicals were characterized by ESR spectroscopy. In several instances both the uncyclized iminyl radical, and the cyclised C-radical, could be simultaneously detected, and their concentrations determined. ESR spectroscopy was then profitably used to determine 5-exo-cyclization rate constants of iminyl radicals onto double bonds. Finally, the syntheses of several heterocycles from dioxime oxalates are described. Photolytic dissociation of dioxime oxalates containing alkenyl groups yielded iminyl radicals that ring closed to 3,4-dihydro-2H-pyrroles in toluene solution. The syntheses of phenanthridines, and the natural product trisphaeridine, were also accomplished by UV irradiation of dioxime oxalates containing aromatic rings as the radical acceptor.