Zuberi, Sheena; (1995) Radical ring opening reactions of cyclopropylcarbinyl systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is divided into six chapters. Chapter one commences with a brief discussion of the physical organic chemistry of the monocyclic cyclopropylcarbinyl system, this is followed by an investigation of the regioselectivity observed in ring opening as a function of substituent on the cyclopropane moiety. This is succeeded by the application of the cyclopropylcarbinyl radical as a probe in establishing the reaction mechanism in enzyme catalysed reactions. This section concludes with a brief survey of the synthetic utility of such systems. In a similar fashion, chapter two discusses the ring-opening of bicyclo[n.1.0]alk-2-yl radicals. Chapter three is prefaced by a brief consideration of the previous work on the generation and synthetic utility of ketyl radicals, concentrating in particular, on the reductive ring opening reactions of the cyclopropylcarbinyl system with lithium in liquid ammmonia. This is followed by a brief review of the reduction of the carbonyl group by samarium(II) iodide before presenting a critical analysis of our own results using this reagent towards the studies of the reductive ring opening of both cis- and trans- cyclopropyl ketone substrates. Samarium(II) iodide ring opening of both the former and the latter gave, in each case the product of thermodynamic control arising from cleavage of the more substituted bond. This chapter finally concludes with the attempted formation of enolate derivatives under these reaction conditions. Chapter four introduces the development of a novel alkylidenecyclopropylcarbinyl rearrangement by taking advantage of a variety of radical triggers. The major part of this section concentrates on the generation of radical anions from the corresponding ketones specifically by lithium in liquid ammonia. By varying the substitution pattern on the ring, ring opened alcohol products, as well as the expected ketones were observed. Chapter five elaborates this methodology further by investigating the mode of ring- opening of a relatively simple bicyclic analogue under the same reaction conditions. Lithium in liquid ammonia ring opening of 7-methylenebicyclo[4.1.0]heptan-2-one gave products arising from cleavage of the exocyclic bond as a result of stereoelectronic control. However, cleavage of the endocyclic bond was also seen to occur, leading to cycloheptyl derivatives via the lower energy allylic radical. Finally, the adaptation of this methodology to a bicyclic system which incorporates a tandem cyclopropylcarbinyl rearrangement/cyclisation on route to bicyclic structures is presented. Chapter six provides a formal description of the experimental results and procedures.

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