ZnI-Mediated β-Galactosylation of C2-Ether-Type Donor.

Conventional glycosylation with galactosyl donors having C-2 benzyl (Bn) ether-type functionality often leads to anomeric mixtures, due to the anomeric and steric effects that stabilize the 1,2--α- and 1,2--β-glycosides, respectively. Herein we report a versatile ZnI-directed β-galactosylation approach employing a 4,6--tethered and 2--Bn galactosyl donor for the stereoselective and efficient synthesis of β--galactosides. With a broad substrate scope, the reaction tolerates a wide range of functional groups and complex molecular architectures, providing stereocontrolled β-galactosides in moderate to excellent yields. The practicality of this transformation is demonstrated through the synthesis of a tetrasaccharide arabinogalactan fragment with high stereoselectivity.