A Solution to Chemical Pseudaminylation via a Bimodal Glycosyl Donor for Highly Stereocontrolled α- and β-Glycosylation.

A robust methodology for the stereocontrolled chemical glycosylation of pseudaminic acid has been developed to afford both α- (axial) and β- (equatorial) glycosides reliably with complete stereoselectivity, using a common glycosyl donor (7 N-Cbz/5 N-azido Pse thioglycoside) simply by changing the reaction conditions. In the CH2Cl2/MeCN cosolvent, highly β-selective pseudaminylation was observed, while addition of 5.0 equiv DMF in CH2Cl2 gave the α-pseudaminosides.