STEREOSELECTIVE SYNTHESIS OF CYCLOPROPANES WITH QUATERNARY CARBON CENTRES
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Multi-substituted optically pure cyclopropanes are important motifs present in many agrochemicals, pharmaceuticals and materials employed in manifold applications. Their synthesis is challenging due to both the strained conformation and the need to control both the relative and absolute stereochemistry. This thesis describes an investigation of the scope of the Wadsworth-Emmons cyclopropanation, highlighting it as a potential efficient methodology for diastereoselective and enantiospecific synthesis of these valuable ring systems. Chapter 1 is an introduction to the cyclopropanation protocols and is split in two subsections. The first is a description of the currently most exploited synthetic pathways to cyclopropanes, with analysis of their substrate scope and critical analysis of their limitations as compared to potential of the Wadsworth-Emmons cyclopropanation (Sections 1.1 to 1.3). The latter section consists of a review on the history and the evolution of this procedure and introduces the preconditions for the development of the project (Sections 1.4 to 1.17). Chapter 2 describes the results and discussion, introducing the use of novel alkyl-substituted triethyl phosphonoacetates to yield cyclopropyl-esters containing quaternary stereocentres. The high yields and the excellent trans-diastereoselectivity values obtained (proved by X-ray crystallography) allowed the proposal of a suggested reaction mechanism, which is supported by the subsequent experiments carried out in the presence of other functional groups, e.g. other carbonyls, nitriles, hetero-aromatics, substituted phenyl rings. This study was further extended to examine the effect of fluorine atom, due to its importance in biologically active environments. The study on the stereochemistry of the Wadsworth-Emmons cyclopropanation has been strongly supported by a range of X-ray crystal structures of the cyclopropane products. The chapter ends with an empirical set of guidelines, helpful for the design of successful cyclopropanations. Chapter 3, describes the experimental methods in full as well as full characterisation of the products obtained. Literature references, noted throughout the thesis, are listed in the final chapter.
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