Regiospecific Anomerisation of Acylated Glycosyl Azides and Benzoylated Disaccharides by Using TiCl4

Chelation induced anomeri- sation is promoted when Lewis acids, such as TiCl4 or SnCl4, coordinate to the pyranose ring oxygen atom and an- other site, giving rise to endocyclic cleavage and isomerisation to the more stable anomer. In this research regio- specific site-directed anomerisation is demonstrated. TiCl4 (2.5 equiv) was employed to induce anomerisation of 15 glycosyl azide and disaccharide sub- strates of low reactivity, and high yields (> 75 %) and stereoselectivies (a/b > 9:1) were achieved. The examples in- cluded glucopyranuronate, galactopyra- nuronate and mannopyranuronate as well as N-acetylated glucopyranuro- nate and galactopyranuronate deriva- tives. A disaccharide with the a1!4 linkage found in polygalacturonan was included. The use of benzoylated sac- charides was found to be important in disaccharide anomerisation as attempts to isomerise related acetyl protected and 2,3-carbonate protected derivatives were not successful.