Pyranosides with 2,3-trans carbamate groups: Exocyclic or endocyclic cleavage reaction?

Pyranosides with 2,3-trans carbamate groups exhibit high 1,2-cis selectivity in glycosylation reactions. Using glycosyl donors with N-benzyl 2,3-trans carbamate groups, an anti-Helicobacter pylori oligosaccharide was synthesized in an efficient manner. Moreover, pyranosides with 2,3-trans carbamate groups readily undergo anomerization from the β to the α configuration under mild acidic conditions via endocyclic cleavage. Acyclic cations generated during the endocyclic cleavage reaction were captured using reduction and intramolecular Friedel-Crafts reaction. By exploiting this anomerization, multiply aligned 1,2-trans glycosyl bonds can be transformed to 1,2-cis glycosyl bonds in a single operation. The 1,2-cis selective preparation of aminoglycosides is crucial for biologically active oligosaccharides. Glycosyl donors containing the N-benzyl 2,3-trans carbamate group for 1,2-cis selective glycosylation were developed and used to prepare an anti-Helicobacter pylori oligosaccharide in an efficient manner. In addition, pyranosides containing the N-acetyl 2,3-trans carbamate group were found to be easily anomerized under mild acidic conditions. The anomerization occurred through endocyclic cleavage, and the generated acyclic cation was captured. Multiple existing 1,2-trans glycosyl bonds were changed to the 1,2-cis form in a single step involving endocyclic cleavage.