A Solution to Chemical Pseudaminylation via Development of a Bimodal Glycosyl Donor to Enable Highly Stereocontrolled α- and β-Glycosylation

Bacterial pseudaminic acids (Pse) are present on the surface of many pathogenic bacteria. Herein, we report a robust methodology for the stereocontrolled chemical glycosylation of pseudaminic acid to afford both α- (axial) and β- (equatorial) glycosides reliably with complete stereoselectivity, using a common glycosyl donor (7N-Cbz/5N-azido Pse thioglycoside) simply by changing the reaction conditions (DCM-DMF, -40 oC and DCM/MeCN, -78 oC, respectively). Examples of such bimodal selectivity are both sparse and highly sought-after in carbohydrate chemistry. This method enables efficient access to pseudaminylated molecules, which will open up various opportunities in chemical glycobiology research of bacterial pseudaminic acids and carbohydrate-based antibacterial vaccine development.