Influence of Levulinoyl Protecting Groups on Glycosylation Stereoselectivity and Glycosyl Cation Structure

The stereoselective synthesis of 1,2-cis glycosidic bonds is historically challenging, and a common synthetic approach is based on the participation of remote protecting ester groups. Common intermediates of glycosylation reactions are glycosyl cations, whose structures are difficult to characterize. Here, the glycosylation reactions and structure of the glycosyl cations of galactose and mannose are investigated when protected with levulinic acid (Lev) at C4 and/or C6, respectively. The glycosyl cations can be assigned to rearranged structures as a consequence of ring opening, as well as dioxolenium ions that suggest remote participation. Some evidence for the long-range interaction of the Lev keto group is found as previously proposed, which could explain unusual solution-phase stereoselectivities observed.