Conformationally Restricted GalN3 Donors Enable Zinc(II)-Mediated Stereocontrolled Galactosylation

We report a Zn(II)-mediated stereocontrolled galactosylation with two conformationally restricted GalN3 donors that enables divergent α- and β-glycosylation. The 4,6-O-TIPDS/3-O-Bz donor selectively affords α-GalN3 glycosides, while the 4,6-O-benzylidene/3-O-TIPS donor provides β-glycosides with high stereoselectivity. The methods show a broad substrate scope, good yields, and scalability and enable synthesis of GalNAc-α-Ser and the Acinetobacter baumannii CPS K45 tetrasaccharide repeating unit.