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.

Natural polysaccharides with health benefits are characterized by a large structural diversity and differ in building blocks, linkages, and lengths. They contribute to human health by functioning as anti-adhesives preventing pathogen adhesion, stimulate immune maturation and gut barrier function, and serve as fermentable substrates for gut bacteria. Examples of such beneficial carbohydrates include the human milk oligosaccharides (HMOs). Also, specific non-digestible carbohydrates (NDCs), such as galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS) are being produced with this purpose in mind, and are currently added to infant formula to stimulate the healthy development of the newborn. They mimic some functions of HMO, but not all. Therefore, many research efforts focus on identification and production of novel types of NDCs. In this review, we give an overview of the few NDCs currently available [GOS, FOS, polydextrose (PDX)], and outline the potential of alternative oligosaccharides, such as pectins, (arabino)xylo-oligosaccharides, and microbial exopolysaccharides (EPS). Moreover, state-of-the-art techniques to generate novel types of dietary glycans, including sialylated GOS (Sia-GOS) and galactosylated chitin, are presented as a way to obtain novel prebiotic NDCs that help shaping the infant microbiome.

An efficient strategy for synthesis of a spacer‐armed tetrasaccharide related to galactan I of Klebsiella pneumoniae is described, which uses newly developed acid‐free conditions for the pyranoside‐into‐furanoside (PIF) rearrangement of a digalactoside bearing a 4‐pentenyl group at the anomeric position. The 4‐pentenyl aglycon was successfully used both as a leaving group in the glycosylation of 3‐(trifluoroacetamido)propanol, and as a temporary anomeric protecting group, allowing conversion into an imidate donor. Regioselective coupling of the disaccharide blocks gave the desired tetrasaccharide sequence required for investigation of the interaction of galactan I with immune‐system proteins.

A straightforward glycosylation method is described to regio‐ and stereoselectively introduce two α‐ l ‐fucose moieties directly to the secondary rim of β‐cyclodextrin. Using NMR and MS fragmentation studies, the nonasaccharide structure was determined, which was also visualized using molecular dynamics simulations. The reported glycosylation method proved to be robust on gram‐scale, and may be generally applied to directly glycosylate β‐cyclodextrins to make well‐defined multivalent glycoclusters.

Invited for the cover of this issue is the group of Marthe Walvoort at the University of Groningen. The image depicts a cyclodextrin ring as a dartboard, which is hit with fucose darts at specific positions to give a di‐fucosylated product. Read the full text of the article at 10.1002/chem.201806090.