Anomeric and Exo-Anomeric Effects in Carbohydrate Chemistry

Abstract

This chapter discusses the several possibilities regarding the specification of the energy associated with the anomeric effect, and the experimental data concerning the configurational and conformational equilibria of the anomeric center and the related variations of valence geometry. The free-energy aspect of the anomeric effect as displayed by the equilibria of isomers is the area wherein the very concept was incepted and developed, and which still dominates its investigation. A considerable amount of data has been accumulated during the past two decades on the anomeric effect in terms of structure, energy, reactivity, and other properties. The description and understanding of the nature of stereoelectronic effects is an appropriate field for the application of organic quantum chemistry. Although several molecular orbital (MO) methods successfully describe the stereochemical behavior of pyranose models, their application to the more complex carbohydrates and oligosaccharides is limited at present due to economical reasons. The various rationalizations of the anomeric effect have been offered over the past years; they may be roughly divided into two main groups—namely, the electrostatic and the delocalization rationalization of the anomeric effect. It may be expected that the intramolecular interactions involved in the anomeric effect also influence the other points on the generalized reaction hypersurface.