Anomerisation is relevant for stereoselective glycoside synthesis. The 1,2-trans glycoside is generally isolated in reactions with 2-acyl containing donors using Lewis acids as promoters due to acyl group participation, yet in some reactions the 1,2-cis product is obtained. The formation of 1,2-cis products is explained when glycosidation and anomerisation occur in tandem (glycosidation-anomerisation) with certain factors leading to enhancement of the rate of 1,2-cis product formation. A greater understanding of the factors that influence anomerisation could help in developing applications of this reaction.
Factors that alter the rates of SnCl4and TiCl4 promoted anomerisation of acylated glycosides will be presented in this lecture. Anomeric ratios (stereoselectivity) will be shown to depend on the anomeric effect, saccharide residue, additives, catalyst, catalyst concentration, temperature, protecting group and electron withdrawing power of the aglycon. Very high ratios of the 1,2-cis anomer can be achieved for O- and even for S- or N-glycosides (glycosyl azides) and glycosyl thiols, where the anomeric effect is not as strong as for O-glycosides. A variety of applications have been developed which include glycolipid synthesis, disaccharide synthesis & glycocluster synthesis and regioselective anomerisation will be demonstrated.
References
1. (a)Poláková, M.; Pitt, N.; Tosin, M.; Murphy, P. V.Angew. Chem. Int. Ed. 2004,43, 2518-21. (b)Tosin M.; Murphy, P. V.Org.Lett.,2002,4, 3675-78.
2. O’Brien, C.; Polakova, M.; Pitt, N.; Tosin, M.; Murphy, P. V.Chem. Eur. J.2007,13, 902-909.
3. Pilgrim, W.; Murphy P.V.J. Org. Chem.2010,75, 6747-55.
