Protecting Groups as a Factor of Stereocontrol in Glycosylation Reactions

Abstract

Synthetic oligosaccharides are objects of interest as model compounds in studies on the biological activity of natural compounds, but also as components for new drugs, glycoconjugate vaccines, carbohydrate diagnostic agents and various other products. The key stage in oligosaccharide synthesis is the glycosylation reaction, which leads to the formation of a linkage between carbohydrate fragments. This review highlights a current problem in modern glycochemistry – the methods of stereocontrol in the glycosylation reaction. Protecting groups within the structure of glycosyl donors are considered as stereocontrolling factors, affecting the reaction mechanism by means of (1) participation or anchimeric assistance, (2) deactivation, (3) intramolecular aglycone delivery. The well-established mechanism of neighboring group participation at O-2 for the synthesis of 1,2- trans glycosides is shown, as well as its modern modifications, including activating ethers, chiral protecting groups and achiral bicyclic glycosyl donors. The mechanisms of remote participation from O-3, O-4 and O-6 of the glycosyl donor are discussed in detail. In addition, approaches to stereocontrol using deactivating protection (conformation constraining and electron withdrawing groups) are described. Finally, synthetic approaches based on intramolecular aglycone delivery are considered. Both classical and novel protecting groups used to control the steric outcome of glycosylation are presented, the mechanisms underlying the presented approaches are discussed in detail, while also showing how the described strategies apply to the synthesis of complex structures.