Lewis acid promoted anomerisation: recent developments and applications

Anomerisation, defined as the epimerisation of anomers, is useful for the stereoselective synthesis of glycosides. Nearly all recent studies have been conducted with pyranosides and increased interest in this topic has been due to identification of strategies that have led to enhanced anomerisation rate. The use of 2,3-trans carbamate or 2,3-trans carbonates, as cyclic protecting groups on pyranosides, generates a fused ring system, leading to inner strain and lowering the energy barrier to anomerisation. Separately, chelation induced anomerisation of glucuronic acid or galacturonic acid derivatives, involving TiCl₄ or SnCl₄, occurs with increased rate compared to gluco- or galactopyranosides. Careful choice of protecting groups or substituents on the saccharide can lead to further increased reactivity. The incorporation of benzoyl protecting groups instead of acetyl groups, for example, leads to faster TiCl₄ or SnCl₄ promoted anomerisation in glucopyranosides and glucuronic acids. Anomer distributions at equilibrium, and by inference, the anomeric effect, are influenced by saccharide structure, Lewis acid, concentration of the Lewis acid, protecting groups and temperature. Anomerisation has been applied to stereoselective synthesis of (neo)glycolipids for immunological study and used to generate various types of glycosides including O-linked disaccharides. All four anomeric linkages of a chitin type tetrasaccharide were epimerised, using BF₃ OEt_2, with high stereoselectivity using 2,3-trans carbamate protected precursors. Regioselective or site directed anomerisation has been achieved. There have been reports of TMSI, Au(i), Au(iii) and Bi(iii) based promoters of anomerisation. Cross-over experiments, DFT based searching of transition states, trapping of intermediates and structure-reactivity studies have been employed to study the reaction mechanism and evidence for pathways involving both endocyclic cleavage and exocyclic cleavage have been obtained.