Direct Synthesis of Glycans Containing Challenging ManNAcA Residues.

A method for direct, highly stereoselective synthesis of glycans containing β-linked d-mannosaminuronic acid (ManNAcA) residues is reported herein, among which is the capsular polysaccharide of type 8. Previous chemical syntheses of this glycan relied on indirect methods comprising glucosylation followed by a multistep epimerization and oxidation sequence. The high β-stereocontrol with direct glycosidation of 3--picoloylated ManNAcA donors was achieved using the H-bond-mediated aglycone delivery (HAD) reaction. A method to achieve complete α-ManNAcA stereoselectivity with 3--benzoylated donors is also reported.