Iterative One‐Pot α‐Glycosylation Strategy: Application to Oligosaccharide Synthesis

Based on the combined use of dimethylformamide (DMF) modulation and neighboring group participation, three iterative one‐pot α‐glycosylation methods, i.e., one‐pot (α,α)‐, one‐pot (β,α)‐, and one‐pot (α,β)‐glycosylations, were developed. These methods are applicable to a range of thioglycosyl donors, confer stereocontrol in α‐/β‐glycosidic bond formation, and thus provide for rapid access to oligosaccharides with various permutations of anomeric configurations. The utility of these one‐pot glycosylation methods is demonstrated in the synthesis of eight non‐natural and natural oligosaccharide targets, including the core 1 serine conjugate, core 8 serine conjugate, the D ‐Gal‐α(1→3)‐D ‐Glc‐α(1→3)‐L ‐Rha trisaccharide unit of the cell wall component in Streptococcus pneumoniae, and the D ‐Glc‐α(1→2)‐D ‐Glc‐α(1→3)‐D ‐Glc trisaccharide terminus of the N‐linked glycan precursor. Confirmation of the anomeric configurations of these oligosaccharides is evidenced by ¹H, ¹³C, ¹³C‐non‐proton decoupling, and heteronuclear correlation 2D NMR experiments. Global deprotection of selected oligosaccharide targets is illustrated.