Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

Straightforward synthesis of protected 2-hydroxyglycals by chlorination-dehydrochlorination of carbohydrate hemiacetals

A straightforward and scalable method for the synthesis of protected 2-hydroxyglycals is described. The approach is based on the chlorination of carbohydrate-derived hemiacetals, followed by an elimination reaction to establish the glycal moiety. 1,2-dehydrochlorination reactions were studied on a range of glycosyl chlorides to provide suitable reaction conditions for this transformation. Benzyl ether, isopropylidene and benzylidene protecting groups, as well as interglycosidic linkage, were found to be compatible with this protocol. The described method is operationally simple and allows for the quick preparation of 2-hydroxyglycals with other than ester protecting groups, providing a feasible alternative to existing methods.

Direct Dehydrative Glycosylation of C1-Alcohols

Due to the central role played by carbohydrates in a multitude of biological processes, there has been a sustained interest in developing effective glycosylation methods to enable more thorough investigation of their essential functions. Among the myriad technologies available for stereoselective glycoside bond formation, dehydrative glycosylation possesses a distinct advantage given the unique properties of C1-alcohols such as straightforward preparation, stability, and a general reactivity compatible with a diverse set of reaction conditions. In this Focus Review, a survey of direct dehydrative glycosylations of C1-alcohols is provided with an emphasis on recent achievements, pervading limitations, mechanistic insights, and applications in total synthesis.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Mild one-pot preparation of glycosyl bromides

Mild one-pot protocols for the preparation of glycosyl bromides and alkyl bromides via in situ generation of HBr is reported here.

[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.

Simple preparations of alkyl and cycloalkyl α-glycosides of maltose, cellobiose, and lactose

Alkyl, cycloalkyl, allyl, 4-pentenyl, and benzyl alpha-glycosides of maltose, cellobiose, and lactose were prepared (17-77% yield; alpha/beta=70/30-96/4) via a direct reaction of the free disaccharides with a binary AcBr-AcOH mixture, followed by glycosidation with alcohol using FeCl3 in MeNO2 or CH2Cl2, Zemplén deacetylation, and resolution of the anomeric mixture of glycosides by chromatography. Using MeCN as solvent for the glycosidation step, the corresponding beta-biosides were also prepared (16-61% yield; alpha/beta=25/75-5/95).

Synthesis of alpha-lactosyl-(1-->3)-L-glycero-alpha-D-manno-heptopyranoside, a partial oligosaccharide structure expressed within the lipooligosaccharide produced by Neisseria gonorrhoeae strain 15253

The glycosyl donor, hepta-O-benzyl-beta-lactosyl trichloroacetimidate (4) was prepared by treating hepta-O-benzyl-lactose with trichloroacetonitrile in the presence of potassium carbonate. The acceptor, methyl 2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-alpha-D-manno-oct-7-enopyranoside (8) was synthesized by hydrolysis of a 3,4-butane diacetal of methyl L-glycero-alpha-D-manno-oct-enopyranoside and subsequent benzylidenation. Glycosidation of the donor 4 with the acceptor 8 in 1,4-dioxane using Me(3)SiOTf as a promoter for 1 h at room temperature gave methyl (2,3,4,6-tetra-O-benzyl-beta-D-galactopyranosyl)-(1-->4)-(2,3,6-tri-O-benzyl-alpha-D-glucopyranosyl)-(1-->3)-2-O-benzyl-4,6-O-benzylidene-7,8-dideoxy-alpha-D-manno-oct-7-enopyranoside (9) as a major product (59%). The oct-enopyranoside moiety of the trisaccharide 9 was converted to a heptopyranoside (80%) by oxidative cleavage with OsO(4)-NaIO(4) and subsequent reduction. Hydrogenolysis of the resulting trisaccharide and subsequent acetylation gave the peracetate of alpha-lactosyl-(1-->3)-Hep. Deacetylation of the peracetate afforded the title trisaccharide.