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 Staphylococcus aureus 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-O-picoloylated ManNAcA donors was achieved using the H-bond-mediated aglycone delivery (HAD) reaction. A method to achieve complete α-ManNAcA stereoselectivity with 3-O-benzoylated donors is also reported.