Great interest in natural furanoside-containing compounds has challenged the development of preparative methods for their synthesis. Herein a novel reaction in carbohydrate chemistry, namely a pyranoside-into-furanoside (PIF) rearrangement permitting the transformation of selectively O-substituted pyranosides into the corresponding furanosides is reported. The discovered process includes acid-promoted sulfation accompanied by rearrangement of the pyranoside ring into a furanoside ring followed by solvolytic O-desulfation. This process, which has no analogy in organic chemistry, was shown to be a very useful tool for the synthesis of furanoside-containing complex oligosaccharides, which was demonstrated by synthesizing disaccharide derivatives α-D-Galp-(1→3)-β-D-Galf-OPr, 3-O-s-lactyl-β-D-Galf-(1→3)-β-D-Glcp-OPr, and α-L-Fucf-(1→4)-β-D-GlcpA-OPr related to polysaccharides from the bacteria Klebsiella pneumoniae and Enterococcus faecalis and the brown seaweed Chordaria flagelliformis.

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Sulfated polysaccharides, fucoidans, from brown algae are built up mainly of α-L-fucopyranosyl units and form a group of natural biopolymers with a wide spectrum of biological activities. Systematic synthesis of oligosaccharides representing fucoidans' fragments gives molecular probes for detecting pharmacophores within fucoidan polysaccharide chains. Recently, it was discovered that the fucoidan from brown seaweed Chordaria flagelliformis contains not only α-L-fucopyranosyl units but also α-L-fucofuranosyl ones. To establish the influence of the unusual α-L-fucofuranose residue on the biological activity and conformational properties of fucoidans, the synthesis of selectively O-sulfated pentasaccharides, which represent the main repeating unit of the fucoidan from C. flagelliformis, was performed. The features of the synthesis were the use of the pyranoside-into-furanoside rearrangement to prepare the fucofuranoside precursor and remote stereocontrolling participation of O-acyl groups to manage stereoselective α-bond formation in glycosylation reactions.

The pyranoside-into-furanoside (PIF) rearrangement was recently reported as a useful tool for the synthesis of furanoside-containing complex oligosaccharides. Until now, this transformation has only been described for some protected allyl-β-D-galactosides and one L-fucoside. In this communication its applicability is expanded. The formation of furanosides was observed under acid-promoted sulfation for a series of galactosides with varying alkyl substituents at the anomeric position as well as for several fucosides and glucosides. Meanwhile, furanoside formation was not detected in the case of mannopyranosides. The different reactivity of substrates was explained by values of activation barriers of endocyclic C1-O5 bond cleavage calculated by ab initio RHF methods. The reported results clarify the scope and limitations of the PIF rearrangement, which is important for its application in the synthesis of carbohydrate structures of practical meaning.