Gajee, Roohi; (1993) Probing stereoselectivity with carbohydrates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the use of carbohydrates to probe the effects of oxygenated substituents on the stereoselectivity of free radical and osmium tetroxide dihydroxylation reactions. The first, introduction, chapter briefly surveys literature methods employed in the synthesis of 2-deoxy-β-glycosidic linkages in carbohydrate chemistry. The use of O-acyl thiohydroxamate chemistry on ulosonic acid derivatives in the arabino-series is then presented as a method for the preparation of β-C- and β-O-glycosides. The evidence for an accelerating or stabilizing β-oxygen effect in radical reactions is then outlined. Finally, the stereodirecting effect of an allylic alcohol or ether on the osmylation of alkenes is discussed with particular reference to glycal derivatives. In the second chapter, on the basis of chemical correlation with products of known absolute configuration and/or of proton coupled 13C-nmr spectroscopy, the anomeric configuration of a number of 3-deoxyheptulosonic acid derivatives previously prepared in this laboratory is reassigned. The effect of temperature and of axial substituents in the 3- position on the stereoselectivity of alkoxyglycosyl radical reactions is then briefly examined. The third chapter describes a series of probes designed to identify any stereochemical component to the β-oxygen effect in free radical reactions. By comparison with non-oxygenated analogues it is determined that β-oxygen bonds do indeed accelerate radical reactions. However, competition between stereoisomers reveals that the stereoelectronic component, while real is very small. The fourth chapter outlines a brief investigation into the stereochemistry of osmylation reactions of glycals. In arabino-glycals reaction occurs on the β-face and it is postulated that this is a function of the lone pairs on the ring oxygen. This chapter describes the reversal of this stereoselectivity in going to the ribo-series. Some attempts at the reversal of stereoselectivity by use of chiral ligands for osmium are described. The final chapter is the complete experimental part.

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