Glycosyl Dithiocarbamates: β‑Selective Couplings without Auxiliary Groups

In
this article, we evaluate glycosyl dithiocarbamates (DTCs) with
unprotected C2 hydroxyls as donors in β-linked oligosaccharide
synthesis. We report a mild, one-pot conversion of glycals into β-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-β-1,6-linked
tetrasaccharide. Glycosyl DTC couplings are highly β-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.