Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

A new method for α-specific glucosylation and its application to the one-pot synthesis of a branched α-glucan

We have developed a new and highly efficient α-specific glucosylation method based on the synergistic α-directing effects of a TolSCl/AgOTf promoter system and the steric β-shielding or the remote participation of protecting groups at the donor 6-O-position.

Dehydrative glycosylation with the Hendrickson reagent

The Hendrickson reagent is able to perform efficiently dehydrative glycosylation of 1-hydroxyglycosyl donors. The reaction occurs under mild conditions through an anomeric oxophosphonium intermediate detected by nuclear magnetic resonance. Further insight into the mechanism was gained by (18)O labeling of anomeric OH.

Selective Cleavage of Methoxy Protecting Groups in Carbohydrates

The selective cleavage of methoxy protecting groups next to hydroxy groups is achieved using a radical hydrogen abstraction reaction as the key step. Under the reaction conditions, the hydroxy group generates an alkoxyl radical that reacts with the sterically accessible adjacent methoxy group, which is transformed into an acetal. In the second step, the acetals are hydrolyzed to give alcohols or diols. A one-pot hydrogen abstraction-hydrolysis procedure was also developed. Good yields were usually achieved, and the mild conditions of this methodology were compatible with different functional groups and sensitive substrates such as carbohydrates.

An expedient synthesis of benzyl 2,3,4-tri-O-benzyl-β-d-glucopyranoside and benzyl 2,3,4-tri-O-benzyl-β-d-mannopyranoside

An efficient three-step synthesis of benzyl 2,3,4-tri-O-benzyl-beta-D-glucopyranoside, a widely used building block in carbohydrate chemistry, is described. The key step is the selective debenzylation-acetylation of perbenzylated beta-glucose using ZnCl2-Ac2O-HOAc. This approach was also used to affect an efficient three-step synthesis of benzyl 2,3,4-tri-O-benzyl-beta-D-mannopyranoside.

[Dehydrative glycosidation and partially benzylated sugar derivatives]

Dehydrative glycosidation reactions reported by the authors' group are reviewed. The authors' efforts were concentrated on developing reagent systems usable for one-stage-one-pot glycosidation. Such systems could simplify the glycosidation step using 1-OH sugar derivatives, since any preactivation stage for the hemiacetal OH group could be omitted. The systems, utilizing the dehydration potential of sulfonyl chloride, such as the p-nitrobenzenesulfonyl chloride-silver trifluoromethanesulfonate-triethylamine system as well as the p-nitrobenzenesulfonyl chloride-silver trifluoromethanesulfonate-N,N-dimethylacetamide-triethylamine system, were useful for the syntheses of many kinds of oligosaccharides. As a system free from any metals, the authors developed the trimethylsilyl trifluoromethanesulfonate-pyridine (TP) system. During the study of the system containing cobalt (II) bromide, the authors found that the bromide converts 1-OH sugar into the corresponding 1-Br derivative, which is then activated with the cobalt salt to undergo glycosidation with alcohol. To prepare partially benzylated sugar derivatives used as acceptors in the authors' studies, controlled benzylation and forced tritylation were carried out. Short syntheses of a variety of useful sugar derivatives using such convenient procedures are described. As a novel protecting group for the hemiacetal OH group, the authors used the 2-methoxyethyl group. Many kinds of trehalose-type disaccarides we prepared.

Generation of Alkoxycarbenium Ion Pools from Thioacetals and Applications to Glycosylation Chemistry

[reaction: see text] Alkoxycarbenium ions have been generated and accumulated as "cation pools" by the low-temperature electrochemical oxidation of alpha-phenylthioethers. Although an unsuccessful attempt to accumulate glycosyl cations was made, a one-pot method for electrochemical glycosylation, which involves anodic oxidation of thioglycosides to generate glycosyl cation equivalents followed by their reactions with glycosyl acceptors, has been developed.

Sulfide-mediated dehydrative glycosylation

The development of a new method for glycosylation with 1-hydroxy glycosyl donors employing dialkyl sulfonium reagents is described. The process employs the reagent combination of a dialkyl sulfide and triflic anhydride to effect anomeric bond constructions. This controlled dehydrative coupling of various C(1)-hemiacetal glycosyl donors and nucleophilic acceptors proceeds by way of a sulfide-to-sulfoxide oxidation process in which triflic anhydride serves as the oxidant.