Acid-Catalyzed O-Glycosylation with Stable Thioglycoside Donors

Efficient O- and S-glycosylation with ortho-2,2-dimethoxycarbonylcyclopropylbenzyl thioglycoside donors by catalytic strain-release

We herein present a strain-release glycosylation method employing a rationally designed ortho-2,2-dimethoxycarbonylcyclopropylbenzyl (CCPB) thioglycoside donor. The donor is activated through the nucleophilic ring-opening of a remotely activable donor-acceptor cyclopropane (DAC) catalyzed by mild Sc(OTf)3. Our new glycosylation method efficiently synthesizes O-, N-, and S-glycosides, providing facile chemical access to the challenging S-glycosides. Because the activation conditions of conventional glycosyl donors and our CCPB thioglycoside are orthogonal, our novel donor is amenable to controlled one-pot glycosylation reactions with conventional donors for expeditious access to complex glycans. The strain-release glycosylation is applied to the assembly of a tetrasaccharide of O-polysaccharide of Escherichia coli O-33 in one pot and the synthesis of a 1,1'-S-linked glycoside oral galectin-3 (Gal-3) inhibitor, TD139, to demonstrate the versatility and effectiveness of the novel method for constructing both O- and S-glycosides.

Visible-light-mediated synthesis of non-anomeric S-aryl glycosides via a photoactive electron-donor-acceptor complex

A visible-light-mediated glycosylation reaction between glycosyl redox-active esters and disulfides has been reported, through which a series of S-aryl glycosides were obtained in good yields with satisfactory stereoselectivity. The preliminary mechanistic studies revealed that this transformation proceeded via an EDA complex. Moreover, the potential application value was demonstrated in the late-stage functionalisation of drug molecules and a gram-scale experiment.

S-o-(p-Methoxyphenylethynyl)benzyl (SMPEB) Glycosides for Catalytic Glycosylation and Their Application in the Synthesis of Polyporus Umbellatus Polysaccharides

We herein reported a new type of S-o-(p-methoxyphenylethynyl)benzyl donor for a highly efficient glycosylation method. The donor was activated by 10% Tf2O and underwent glycosylation with various acceptors to provide the corresponding glycosides in excellent yield. Furthermore, two repetitive fragments of Polyporus umbellatus polysaccharides (PUPs), isolated from traditional Chinese medicine "Polyporus umbellatus", were prepared by combining the "single-catalyst one-pot" and "latent-active" strategies for the first time for future clear studies on the structure-activity relationship of PUPs.

Efficient and versatile formation of glycosidic bonds via catalytic strain-release glycosylation with glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoate donors

Catalytic glycosylation is a vital transformation in synthetic carbohydrate chemistry due to its ability to expediate the large-scale oligosaccharide synthesis for glycobiology studies with the consumption of minimal amounts of promoters. Herein we introduce a facile and efficient catalytic glycosylation employing glycosyl ortho-2,2-dimethoxycarbonylcyclopropylbenzoates (CCBz) promoted by a readily accessible and non-toxic Sc(III) catalyst system. The glycosylation reaction involves a novel activation mode of glycosyl esters driven by the ring-strain release of an intramolecularly incorporated donor-acceptor cyclopropane (DAC). The versatile glycosyl CCBz donor enables highly efficient construction of O-, S-, and N-glycosidic bonds under mild conditions, as exemplified by the convenient preparation of the synthetically challenging chitooligosaccharide derivatives. Of note, a gram-scale synthesis of tetrasaccharide corresponding to Lipid IV with modifiable handles is achieved using the catalytic strain-release glycosylation. These attractive features promise this donor to be the prototype for developing next generation of catalytic glycosylation.

Efficient Synthesis of 2-OH Thioglycosides from Glycals Based on the Reduction of Aryl Disulfides by NaBH4

An improved method to efficiently synthesize 2-OH thioaryl glycosides starting from corresponding per-protected glycals was developed, where 1,2-anhydro sugars were prepared by the oxidation of glycals with oxone, followed by reaction of crude crystalline 1,2-anhydro sugars with NaBH4 and aryl disulfides. This method has been further used in a one-pot reaction to synthesize glycosyl donors having both “armed” and “NGP (neighboring group participation)” effects.

Leveraging Trifluoromethylated Benzyl Groups toward the Highly 1,2-Cis-Selective Glucosylation of Reactive Alcohols

Here, we demonstrate that substitution of the benzyl groups of glucosyl imidate donors with trifluoromethyl results in a substantial increase in 1,2-cis-selectivity when activated with TMS-I in the presence of triphenylphosphine oxide. Stereoselectivity is dependent on the number of trifluoromethyl groups (4-trifluoromethylbenzyl vs 3,5-bis-trifluoromethylbenzyl). Particularly encouraging is that we observe high 1,2-cis-selectivity with reactive alcohol acceptors.

Recent advances in glycan synthesis

Carbohydrates play important roles in life science, but their synthesis is always hampered by their complicated chemical structures. Scientists have never stopped trying to solve the problem of glycan synthesis from various aspects. Here a brief overview of recent progress in glycan synthesis, including chemical approaches, chemoenzymatic approaches, and automated synthesis, will be discussed, focusing on the efficiency of new glycosylation methods, the stereoselectivity of coupled products, and their applications in the assembly of complex glycan chains.

1,2-cis-Selective glucosylation enabled by halogenated benzyl protecting groups

We report on our initial results from a systematic effort to implement electron-withdrawing protecting groups and Lewis basic solvents/additives as an approach to 1,2-cis(α)-selective O-glucosylation. 1,2-cis-Selective O-glucosylations are reported with thioglucosides and glucosyl trichloroacetimidates and a range of acceptors. A correlation between electron-withdrawing effects and 1,2-cis selectivity has been established. This phenomenon may prove to be broadly applicable in the area of chemical O-glycosylation.

Triflic acid-mediated synthesis of thioglycosides

An efficient synthesis of thioglycosides from per-acetates in the presence of triflic acid is described. The developed protocol features high reaction rates and product yields. Some reactive sugar series give high efficiency in the presence of sub-stoichiometric trifluoromethanesulfonic acid (TfOH) in contrast to other known protocols that require multiple equivalents of Lewis acids to reach high conversion rates.