Glycosyl Dithiocarbamates: β-Selective Couplings without Auxiliary Groups

In this article, we evaluate glycosyl dithiocarbamates (DTCs) with unprotected C2 hydroxyls as donors in beta-linked oligosaccharide synthesis. We report a mild, one-pot conversion of glycals into beta-glycosyl DTCs via DMDO oxidation with subsequent ring opening by DTC salts, which can be generated in situ from secondary amines and CS2. Glycosyl DTCs are readily activated with Cu(I) or Cu(II) triflate at low temperatures and are amenable to reiterative synthesis strategies, as demonstrated by the efficient construction of a tri-beta-1,6-linked tetrasaccharide. Glycosyl DTC couplings are highly beta-selective despite the absence of a preexisting C2 auxiliary group. We provide evidence that the directing effect is mediated by the C2 hydroxyl itself via the putative formation of a cis-fused bicyclic intermediate.