Pyranoside-into-Furanoside Rearrangement of Galactose Units within a Preassembled Oligosaccharide Chain

The pyranoside-into-furanoside (PiF) rearrangement is a valuable method for the synthesis of furanoside-containing oligosaccharides structurally related to antigenic polysaccharides from fungal and bacterial pathogens. To date, the PiF rearrangement has been mainly reported for monosaccharides. In this study, we successfully applied this process to transform galactopyranoside (Gal<i>p</i>) units at the nonreducing end of oligosaccharides, as well as inner Gal<i>p</i> units. Moreover, we achieved simultaneous transformation of both Gal<i>p</i> residues in unprotected β-(1→6)-linked disaccharides into the corresponding difuranoside derivatives. Notably, a pronounced dependence of the oligosaccharide substitution pattern and linkage to neighboring glycosyl residues on the efficiency of the PiF rearrangement was revealed. These findings provide a novel synthetic approach toward biologically relevant oligosaccharides containing Gal<i>f</i> units.