An Yb(OTf)₃-Mediated Indirect Activation Strategy for the Stereoselective Synthesis of α-Sialosides from 2-Fluorosialyl Donors

Most of chemical sialylation reactions are conducted at extremely low temperatures to achieve the formation of challenging sialic acid linkages with high stereoselectivities. Performing chemical sialylation at room temperature independent of enzymatic methods represents an effective approach, particularly significant in biological and biochemical research. Our study aims to develop a convenient method of providing α-sialyl glycosides. Herein, we carry out sialyation using Yb(OTf)3 as an activating promoter at room temperature in tetrahydrofurane and obtain excellent stereoselectivities when reacting the N-acetyl-5-N,4-O-carbonyl-2-fluorosialyl donor with galacto- or glucopyranosides. The advantages of this method include an over eight-month shelf life of the sialyl donors and minimal formation of the hydrolyzed or eliminated side-products. Sialylation of the C3 hydroxyl group in galactosides affords exclusive α-selectivities, and a one-pot synthesis of trisaccharide is accomplished by application of this method. Finally, we anticipate that this sialylation strategy can compensate for the limitations of the current enzymatic synthesis of complex glycans.

Direct Experimental Characterization of a Sialyl Cation

Sialic acids are monosaccharide residues involved in several biological processes. Controlling the stereoselectivity of sialylation reactions is challenging and mechanistic studies on the structure of its intermediate, the sialyl cation, are scarce. Here it is shown that a sialyl cation can be generated and isolated from an ionized sialic acid precursor. This short-lived species is structurally characterized for the first time using cryogenic infrared spectroscopy. In combination with quantum chemical calculations, the results reveal that the positive charge at the anomeric carbon of the sialyl cation is stabilized by remote participation of the C5-NHAc group leading to the formation of a bridged structure. In this structure, the β-side is shielded from nucleophilic attack, potentially explaining the α-selectivity of this building block in SN1-type sialylation reactions. Other modes of participation are energetically unfavored and cannot be observed experimentally.

A Ligand-Controlled Approach Enabling Gold(I)-Catalyzed Stereoinvertive Glycosylation with Primal Glycosyl ortho-Alkynylbenzoate Donors

A diarylurea-containing phosphine ligand-modulated stereoinvertive O-glycosylation with primal furanosyl and pyranosyl ortho-alkynylbenzoate (ABz) donors under gold(I) catalysis is disclosed. Both α- and β-configured glycosides could be obtained from the corresponding stereochemically pure β- and α-glycosyl donors with high yields and good to excellent stereoselectivities, respectively. This method accommodates a variety of glycosyl donors and alcoholic acceptors, leading to both 1,2-cis and 1,2-trans glycosidic linkages, and has been applied to the convenient preparation of a series of linear arabinan glycans. Mechanistic investigations reveal that the counteranion could bridge the diarylurea residue on the phosphine ligand with the alcoholic acceptor via hydrogen bond interactions, thereby permitting stereoinvertive displacement at the anomeric position.

Total Synthesis of a PSGL-1 Glycopeptide Analogue for Targeted Inhibition of P-Selectin

The high affinity interaction between P-selectin glycoprotein ligand-1 (PSGL-1) and P-selectin is mediated by a multimotif glycosulfopeptide (GSP) recognition domain consisting of clustered tyrosine sulfates and a Core 2 O-glycan terminated with sialyl LewisX (C2-O-sLeX). These distinct GSP motifs are much more common than previously appreciated within a wide variety of functionally important domains involved in protein-protein interactions. However, despite the potential of GSPs to serve as tools for fundamental studies and prospects for drug discovery, their utility has been limited by the absence of chemical schemes for synthesis on scale. Herein, we report the total synthesis of GSnP-6, an analogue of the N-terminal domain of PSGL-1, and potent inhibitor of P-selectin. An efficient, scalable, hydrogenolysis-free synthesis of C2-O-sLeX-Thr-COOH was identified by both convergent and orthogonal one-pot assembly, which afforded this crucial building block, ready for direct use in solid phase peptide synthesis (SPPS). C2-O-sLeX-Thr-COOH was synthesized in 10 steps with an overall yield of 23% from the 4-O,5-N oxazolidinone thiosialoside donor. This synthesis represents an 80-fold improvement in reaction yield as compared to prior reports, achieving the first gram scale synthesis of SPPS ready C2-O-sLeX-Thr-COOH and enabling the scalable synthesis of GSnP-6 for preclinical evaluation. Significantly, we established that GSnP-6 displays dose-dependent inhibition of venous thrombosis in vivo and inhibits vaso-occlusive events in a human sickle cell disease equivalent microvasculature-on-a-chip system. The insights gained in formulating this design strategy can be broadly applied to the synthesis of a wide variety of biologically important oligosaccharides and O-glycan bearing glycopeptides.

Integrated chemoenzymatic synthesis of a comprehensive sulfated ganglioside glycan library to decipher functional sulfoglycomics and sialoglycomics

Ganglioside glycans are ubiquitous and complex biomolecules that are involved in a wide range of biological functions and disease processes. Variations in sialylation and sulfation render the structural complexity and diversity of ganglioside glycans, and influence protein-carbohydrate interactions. Structural and functional insights into the biological roles of these glycans are impeded due to the limited accessibility of well-defined structures. Here we report an integrated chemoenzymatic strategy for expeditious and systematic synthesis of a comprehensive 65-membered ganglioside glycan library covering all possible patterns of sulfation and sialylation. This strategy relies on the streamlined modular assembly of three common sialylated precursors by highly stereoselective iterative sialylation, modular site-specific sulfation through flexible orthogonal protecting-group manipulations and enzymatic-catalysed diversification using three sialyltransferase modules and a galactosidase module. These diverse ganglioside glycans enable exploration into their structure-function relationships using high-throughput glycan microarray technology, which reveals that different patterns of sulfation and sialylation on these glycans mediate their unique binding specificities.

Recent progress in the synthesis of glycosphingolipids

To accelerate the biological study and application of the diverse functions of glycosphingolipids (GSLs), the production of structurally defined GSLs has been greatly demanded. In this review, we focus on the recent developments in the chemical and chemoenzymatic synthesis of GSLs. In the chemical synthesis section, the syntheses based on glucosyl ceramide cassette, late-stage sialylation, and diversity-oriented strategies for GSLs or ganglioside synthesis are highlighted, which delivered terpioside B, fluorescent sialyl lactotetraosyl ceramide, and analogs of lacto-ganglio-series GSLs, respectively. In the chemoenzymatic synthesis section, the synthesis of ganglioside GM1 by multistep one-pot multienzyme method and the total synthesis of highly complex ganglioside LLG-5 using a water-soluble lactosyl ceramide as a key substrate for enzymatic sialylation are described.

Diacetyl strategy for synthesis of NHAc containing glycans: enhancing glycosylation reactivity via diacetyl imide protection

The presence of NHAc groups in the substrates (both glycosyl donors and acceptors) significantly reduced the reactivity of glycosylation. This decrease was attributed to the NHAc groups forming intermolecular hydrogen bonds by the NHAc groups, thereby reducing molecular mobility. Hence, a diacetyl strategy involving the temporary conversion of NHAc to diacetyl imide (NAc2) was developed for the synthesis of NHAc-containing glycans. This strategy has two significant advantages for oligosaccharide synthesis. The NAc2 protection of NHAc substantially enhances the rate of glycosylation reactions, resulting in improved yields. Moreover, NAc2 can be readily reverted to NHAc by the simple removal of one acetyl group under mild basic conditions, obviating the necessity for treating the polar amino group. We have achieved the efficient synthesis of oligosaccharides containing GlcNHAc and N-glycans containing sialic acid using the diacetyl strategy.

Stereoselective Synthesis of Sialyl Lewisa Antigen and the Effective Anticancer Activity of Its Bacteriophage Qβ Conjugate as an Anticancer Vaccine

Sialyl Lewisa (sLea ), also known as cancer antigen 19-9 (CA19-9), is a tumor-associated carbohydrate antigen. The overexpression of sLea on the surface of a variety of cancer cells makes it an attractive target for anticancer immunotherapy. However, sLea -based anticancer vaccines have been under-explored. To develop a new vaccine, efficient stereoselective synthesis of sLea with an amine-bearing linker was achieved, which was subsequently conjugated with a powerful carrier bacteriophage, Qβ. Mouse immunization with the Qβ-sLea conjugate generated strong and long-lasting anti-sLea IgG antibody responses, which were superior to those induced by the corresponding conjugate of sLea with the benchmark carrier keyhole limpet hemocyanin. Antibodies elicited by Qβ-sLea were highly selective toward the sLea structure, could bind strongly with sLea -expressing cancer cells and human pancreatic cancer tissues, and kill tumor cells through complement-mediated cytotoxicity. Furthermore, vaccination with Qβ-sLea significantly reduced tumor development in a metastatic cancer model in mice, demonstrating tumor protection for the first time by a sLea -based vaccine, thus highlighting the significant potential of sLea as a promising cancer antigen.

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.

Biocatalysis versus Molecular Recognition in Sialoside-Selective Neuraminidase Biosensing

Sialic acid recognition and hydrolysis are essential parts of cellular function and pathogen infectivity. Neuraminidases are enzymes that detach sialic acid from sialosides, and their inhibition is a prime target for viral infection treatment. The connectivity and type of sialic acid influence the recognition and hydrolysis activity of the many different neuraminidases. The common strategies to evaluate neuraminidase activity, recognition, and inhibition rely on extensive labeling and require a large amount of sialylated glycans. The above limitations make the effort of finding viral inhibitors extremely difficult. We used synthetic sialylated glycans and developed a label-free electrochemical method to show that sialoside structural features lead to selective neuraminidase biosensing. We compared Neu5Ac to Neu5Gc sialosides to evaluate the organism-dependent neuraminidase selectivity-sensitivity relationship. We demonstrated that the type of surface and the glycan monolayer density direct the response to either binding or enzymatic activity. We proved that while the hydrophobic glassy carbon surface increases the interaction with the enzyme hydrophobic interface, the negatively charged interface of the lipoic acid monolayer on gold repels the protein and enables biocatalysis. We showed that the sialoside monolayers can serve as tools to evaluate the inhibition of neuraminidases both by biocatalysis and molecular recognition.

Multivalent sialic acid materials for biomedical applications

Sialic acid is a kind of monosaccharide expressed on the non-reducing end of glycoproteins or glycolipids. It acts as a signal molecule combining with its natural receptors such as selectins and siglecs (sialic acid-binding immunoglobulin-like lectins) in intercellular interactions like immunological surveillance and leukocyte infiltration. The last few decades have witnessed the exploration of the roles that sialic acid plays in different physiological and pathological processes and the use of sialic acid-modified materials as therapeutics for related diseases like immune dysregulation and virus infection. In this review, we will briefly introduce the biomedical function of sialic acids in organisms and the utilization of multivalent sialic acid materials for targeted drug delivery as well as therapeutic applications including anti-inflammation and anti-virus.

Chemical Synthesis of Gangliosides

Synthetic methodologies for gangliosides have evolved over the past three decades. The strategies for constructing ganglioside skeletons can generally be classified as late-stage ceramide coupling, the glucosyl ceramide cassette strategy, or late-stage sialylation. Using these synthetic strategies, numerous natural gangliosides and their structural analogs, including functional probes, have been synthesized. This chapter describes the synthetic strategies for gangliosides and provides examples of the total synthesis of several gangliosides using each strategy.

Total synthesis of Lentinus giganteus glycans with antitumor activities via stereoselective α-glycosylation and orthogonal one-pot glycosylation strategies

The accessibility to long, branched and complex glycans containing many 1,2-cis glycosidic linkages with precise structures remains a challenging task in chemical synthesis. Reported here is an efficient, stereoselective and orthogonal one-pot synthesis of a tetradecasaccharide and shorter sequences from Lentinus giganteus polysaccharides with antitumor activities. The synthetic strategy consists of: (1) newly developed merging reagent modulation and remote anchimeric assistance (RMRAA) α-(1→6)-galactosylation in a highly stereoselective manner, (2) DMF-modulated stereoselective α-(1→3)-glucosylation, (3) RMRAA stereoselective α-(1→6)-glucosylation, (4) several orthogonal one-pot glycosylations on the basis of N-phenyltrifluoroacetimidate (PTFAI) glycosylation, Yu glycosylation and ortho-(1-phenylvinyl)benzoate (PVB) glycosylation to streamline oligosaccharide synthesis, and (5) convergent [7 + 7] glycosylation for the final assembly of the target tetradecasaccharide. In particular, this new RMRAA α-galactosylation method has mild reaction conditions, broad substrate scopes and significantly shortened step counts for the heptasaccharide synthesis in comparison with 4,6-di-tert-butylsilyene (DTBS) directed α-galactosylation. Furthermore, DFT calculations shed light on the origins of remote anchimeric assistance effects (3,4-OBz > 3,4-OAc > 4-OBz > 3-OBz) of acyl groups.

Stereoselective Access to Iminosugar C,C-Glycosides from 6-Azidoketopyranoses

We report the synthesis of iminosugar C,C-glycosides starting from 6-azidoketopyranoses. Their Staudinger-azaWittig-mediated cyclization provided bicyclic N,O-acetals, which were stereoselectively opened with AllMgBr to afford β-hydroxyazepanes with a quaternary carbon α to the nitrogen. Their ring contraction via a β-aminoalcohol rearrangement produced the six-membered l-iminosugars with two functional handles at the pseudoanomeric position. Inversion of the free OH at the azepane level furnished the d-iminosugars.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Stereoselective gem-C,B-Glycosylation via 1,2-Boronate Migration

A novel protocol is established for the long-standing challenge of stereoselective geminal bisglycosylations of saccharides. The merger of PPh₃ as a traceless glycosidic leaving group and 1,2-boronate migration enables the simultaneous introduction of C-C and C-B bonds at the anomeric stereogenic center of furanoses and pyranoses. The power of this method is showcased by a set of site-selective modifications of glycosylation products for the construction of bioactive conjugates and skeletons. A scarce metal-free 1,1-difunctionalization process of alkenes is also concomitantly demonstrated.

Analysis of biochemical features of ST8 α-N-acetyl-neuraminide α2,8-sialyltransferase (St8sia) 5 isoforms

Gangliosides are important components of the membrane and are involved in many biological activities. St8sia5 is an α2,8-sialyltransferase involved in ganglioside synthesis, and has three isoforms. In this study, we analyzed the features of three isoforms, St8sia5-S, -M, and -L that had not been analyzed, and found that only St8sia5-L was localized in the Golgi, while the majority of St8sia5-M and -S were localized in the ER. The localization of Golgi of St8sia5 depended on the stem region. In addition, the incorporation of exogenous GD3 was upregulated only in St8sia5-L expressing cells. Taken together, the localization of St8sia5 is important for the activity of the enzyme.

Development of lacto-series ganglioside fluorescent probe using late-stage sialylation and behavior analysis with single-molecule imaging

Gangliosides are a family of sialic-acid-containing glycosphingolipids that form dynamic domains (lipid rafts) with proteins in cell plasma membranes (PMs), and are involved in various biological processes. The dynamic behavior...

Chemical Synthesis of Sialyl N-Glycans and Analysis of Their Recognition by Neuraminidase

The chemical synthesis of a fully sialylated tetraantennary N-glycan has been achieved for the first time by using the diacetyl strategy, in which NHAc is protected as NAc₂ to improve reactivity by preventing intermolecular hydrogen bonds. Another key was the glycosylation to the branched mannose in an ether solvent, which promoted the desired glycosylation by stabilizing the oxocarbenium ion intermediate. Furthermore, high α-selectivity of these glycosylation reactions was realized by utilizing remote participation. Two asymmetrically deuterium labeled sialyl N-glycans were also synthesized by the same strategy. The synthesized N-glycans were used to probe the molecular basis of H1N1 neuraminidase recognition. The asymmetrically deuterated N-glycans revealed a difference in the recognition of sialic acid on each branch. Meanwhile, the tetraantennary N-glycan was used to evaluate the effects of multivalency and steric hinderance by forming branching structures.

Chemoenzymatic synthesis and biological evaluation of ganglioside GM3 and lyso-GM3 as potential agents for cancer therapy

A highly efficient chemoenzymatic method for synthesizing ganglioside GM3 and lyso-GM3 was reported here. Enzymatic extension of the chemically synthesized lactosyl sphingosine using efficient one-pot multienzyme (OPME) reaction allowed glycosylation to be carried out in aqueous solutions realizing the greening of reactions. Ganglioside GM3 was synthesized through 10 steps with a total yield of 22%. Lyso-GM3 was very useful for kinds of derivatization. The anti-proliferation activity studies demonstrated that these compounds 14-16 with sphingosine exhibited more potency than the corresponding lyso-GM3 with ceramide. All ganglioside GM3 and lyso-GM3 can effectively inhibit the migration of melanoma B16-F10 cells. These chemoenzymaticlly synthesized GM3 and lyso-GM3 exhibited antitumor activities, which can provide valuable sights to search new antitumor agents for cancer therapy.

Efficient synthesis of diverse C-3 monodesmosidic saponins by a continuous microfluidic glycosylation/batch deprotection method

Triterpene and steroid saponins have various pharmacological activities but the synthesis of C-3 monodesmosidic saponins remains challenging. Herein, a series of C-3 glycosyl monodesmosidic saponins was synthesized via the microfluidic glycosylation of triterpenoids or steroids at the C-3 position, without the formation of orthoester byproducts, and subsequent deprotection of the benzoyl (Bz) group. This microfluidic glycosylation/batch deprotection sequence enabled the efficient synthesis of C-3 saponins with fewer purification steps and a shorter reaction time than conventional batch synthesis and stepwise microfluidic glycosylation. Furthermore, this system minimized the consumption of the imidate donor. Using this reaction system, 18 different C-3 saponins and 13 different C-28-benzyl-C-3 saponins, including 8 new compounds, were synthesized from various sugars and triterpenes or steroids. Our synthetic approach is expected to be suitable for further expanding the C-3 saponin library for pharmacological studies.

A FRET-based detection of N-acetylneuraminic acid using CdSe/ZnS quantum dot and exonuclease III-assisted recycling amplification strategy

N-acetylneuraminic acid (Neu5Ac) is widely spread in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. It is of great significance to develop analytical techniques for detection of Neu5Ac. Herein, a high-sensitive fluorescent biosensor for Neu5Ac has been developed based on FRET between CdSe/ZnS quantum dots (QDs) and BHQ₂, as well as exonuclease III (Exo III)-assisted recycling amplification strategy. Employing the specially designed three-level FRET systems and fluorescent signal recovery mechanism, together with five-step recycling signal amplification chain reactions, an ultralow detection limit of 24 fM was achieved. Meanwhile, good linear response ranges within 0.2-12.5 pM and 12.5-1000 pM were founded. The assay has excellent performance in real sample detection, and thus offers great potential for detection of sialic acids modified glycans/lipids in the fields of medical diagnosis and food testing.

Conformation-Controlled Hydrogen-Bond-Mediated Aglycone Delivery Method for α-Xylosylation

α-Xylosylated glycans and xylosyl derivatives are biomedically important molecules which show numerous bioactivities against infection, cancer, inflammation, and so on. Lacking an efficient α-xylosylation method, the synthesis of α-xyloside-containing molecules was full of challenges. Herein, a robust method is presented for selective α-xylosylation via combination of a rare conformation-controlled strategy and the hydrogen-bond-mediated aglycone delivery method. Various native branched α-xyloside structures necessitate an orthogonally protected xyloside, and a three-pot preparation method of the xylosyl donor was developed for this novel α-xylosylation method, which was further applied in the first synthesis of the side chain N of xyloglucan. This work provides an efficient α-xylosylation method which would make various α-xyloside structures achievable. The conformation-controlled strategy also has important reference to the chemistry of five-carbon pyranose.

Diversity-Oriented Chemoenzymatic Synthesis of Sulfated and Nonsulfated Core 2 O-GalNAc Glycans

A diversity-oriented chemoenzymatic approach for the collective preparation of sulfated core 2 O-GalNAc glycans and their nonsulfated counterparts was described. A sulfated trisaccharide and a nonsulfated trisaccharide were chemically synthesized by combining flexible protected group manipulations and sequential one-pot glycosylations. The divergent enzymatic extension of these two trisaccharides, using a panel of robust glycosyltransferases that can recognize sulfated substrates and differentiating the branches with specifically designed glycosylation sequences to achieve regioselective sialylation, provided 36 structurally well-defined O-GalNAc glycans.

Enzymatic Synthesis of Glycosphingolipids: A Review

Glycosphingolipids (GSLs) are the major vertebrate glycolipids, which contain two distinctive moieties, a glycan and a ceramide, stitched together by a β-glycosidic linkage. The hydrophobic lipid chains of ceramide can insert into the cell membrane to form ‘lipid rafts’ and anchor the hydrophilic glycan onto the cell surface to generate microdomains and function as signaling molecules. GSLs mediate signal transduction, cell interactions, and many other biological activities, and are also related to many diseases. To meet the need of biological studies, chemists have developed various synthetic methodologies to access GSLs. Among them, the application of enzymes to GSL synthesis has witnessed significant advancements in the past decades. This short review briefly summarizes the history and progress of enzymatic GSL synthesis.

1 Introduction

1.1 The Glycosphingolipid Structure

1.2 GSL Biosynthesis

1.3 Functions and Biological Significance

1.4 Overview of GSL Synthesis

1.5 Scope of the Review

2 Glycotransferases for GSL Synthesis

3 Glycosynthases for GSL Synthesis

4 Enzymatic Synthesis of Ceramide

5 Conclusion

Advanced Chemical Methods for Stereoselective Sialylation and Their Applications in Sialoglycan Syntheses

Sialic acid is an important component of cell surface glycans, which are responsible for many vital body functions and should therefore be thoroughly studied to understand their biological roles and association with disorders. The difficulty of isolating large quantities of homogenous-state sialoglycans from natural sources has inspired the development of the corresponding chemical synthesis methods affording acceptable purities, yields, and amounts. However, the related syntheses are challenging because of the difficulties in α-glycosylation of sialic acid, which arises from its certain structural features such as the absence of a stereodirecting group at the C3 position and presence of carboxyl group at the anomeric position. Moreover, the structural complexities of sialoglycans with diverse numbers and locations of sialic acid on the glycan chains pose additional barriers. Thus, efficient α-stereoselective routes to sialosides remain highly sought after, although various types of sialyl donors/acceptors have been developed for the straightforward synthesis of α-sialosides. Herein, we review the latest progress in the α-stereoselective synthesis of sialosides and their applications in the preparation of gangliosides and other sialoglycans.

Synthetic O-acetylated sialosides facilitate functional receptor identification for human respiratory viruses

The transmission of viruses from animal reservoirs to humans poses major threats to public health. Preparedness for future zoonotic outbreaks requires a fundamental understanding of how viruses of animal origin have adapted to binding to a cell surface component and/or receptor of the new host. Here we report on the specificities of human and animal viruses that engage with O-acetylated sialic acid, which include betacoronaviruses, toroviruses and influenza C and D viruses. Key to these studies was the development of a chemoenzymatic methodology that can provide almost any sialate-acetylation pattern. A collection of O-acetylated sialoglycans was printed as a microarray for the determination of receptor specificity. These studies showed host-specific patterns of receptor recognition and revealed that three distinct human respiratory viruses uniquely bind 9-O-acetylated α2,8-linked disialoside. Immunofluorescence and cell entry studies support that such a glycotope as part of a ganglioside is a functional receptor for human coronaviruses.

BF3·OEt2-Catalyzed Unexpected Stereoselective Formation of 2,4-trans-Diallyl-2-methyl-6-aryltetrahydro-2H-pyrans with Quaternary Stereocenters

The present manuscript describes a convenient, mild, and highly stereoselective method for the allylation of δ-hydroxy-α,β-unsaturated ketones having a benzylic hydroxyl group at the δ-position using allyltrimethylsilane mediated by BF₃·OEt₂, leading to 2,4-diallyl-2-methyl-6-aryltetrahydro-2H-pyran ring systems with quaternary carbon stereogenic centers. This represents the first example of a tandem isomerization followed by one C-O and two C-C bond-forming reactions in one pot. The isolation of TMS-protected lactol as an intermediate from the reaction strongly supports the proposed mechanistic pathway.

Merging Reagent Modulation and Remote Anchimeric Assistance for Glycosylation: Highly Stereoselective Synthesis of α-Glycans up to a 30-mer

The efficient synthesis of long, branched, and complex carbohydrates containing multiple 1,2-cis glycosidic linkages is a long-standing challenge. Here, we report a merging reagent modulation and 6-O-levulinoyl remote anchimeric assistance glycosylation strategy, which is successfully applied to the first highly stereoselective synthesis of the branched Dendrobium Huoshanense glycans and the linear Longan glycans containing up to 30 contiguous 1,2-cis glucosidic bonds. DFT calculations shed light on the origin of the much higher stereoselectivities of 1,2-cis glucosylation with 6-O-levulinoyl group than 6-O-acetyl or 6-O-benzoyl groups. Orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates and ortho-(1-phenylvinyl)benzoates has been demonstrated in the efficient synthesis of complex glycans, precluding such issues as aglycon transfer inherent to orthogonal one-pot synthesis based on thioglycosides.

Synthesis of the sialylated pentasaccharide repeating unit of the capsular polysaccharide of Streptococcus group B type VI

An efficient synthetic strategy has been developed for the synthesis of the sialic acid containing pentasaccharide repeating unit of the cell wall O-antigen of Streptococcus group B type VI strain involving stereoselective α-glycosylation of sialic acid thioglycoside derivative. Stereoselective glycosylation of glycosyl trichloroacetimidate derivatives and thioglycosides were carried out using perchloric acid supported over silica (HClO₄-SiO₂) as a solid acid catalyst. A panel of sialic acid donors has been screened for achieving satisfactory yield and stereochemical outcome of the glycosylation reaction.

β-Selective Glycosylation Using Axial-Rich and 2-O-Rhamnosylated Glucosyl Donors Controlled by the Protecting Pattern of the Second Sugar

Herein, we describe two counterexamples of the previously reported β/α-selectivity of 96/4 for glycosylation using ethyl 2-O-[2,3,4-tris-O-tert-butyldimethylsilyl (TBS)-α-L-rhamnopyranosyl]-3,4,6-tris-O-TBS-thio-β-D-glucopyranoside as the glycosyl donor. Furthermore, we investigated the effects of protecting group on the rhamnose moieties in the glycosylation with cholestanol and revealed that β-selectivity originated from the two TBS groups at the 3-O and 4-O positions of rhamnose. In contrast, the TBS group at the 2-O position of rhamnose hampered the β-selectivity. Finally, the β/α-selectivity during the glycosylation was enhanced to ≥99/1. The results obtained herein suggest that the protecting groups on the sugar connected to the 2-O of a glycosyl donor with axial-rich conformation can control the stereoselectivity of glycosylation.

A Modular Chemoenzymatic Synthesis of Disialosyl Globopentaosylceramide (DSGb5Cer) Glycan

The total synthesis of the oligosaccharide moiety of disialosyl globopentaosylceramide (DSGb5 Cer), a dominant ganglioside isolated from malignant renal cell carcinoma tissues, is reported. The synthetic strategy relies on a chemical α(2,6)-sialylation at the internal GalNAc unit of a Gb5 pentasaccharide backbone that furnishes a Neu5Acα(2,6)GalNAc-linked hexasaccharide, suitable for an enzymatic α(2,3)-sialylation of the terminal Gal residue to construct a heptasaccharide glycan. Convergent access to this key α(2,6)-sialylated hexasaccharide was also achieved through a [3+3] glycosylation building upon a Galβ(1,3)[Neu5Acα(2,6)]GalNAc-based trisaccharide donor and a Gb3 acceptor. The synthetic DSGb5 glycan bearing a 6-azidohexyl aglycon at the reducing end could undergo further regioselective functionalization. This approach represents a viable chemoenzymatic method for accessing complex ganglioside glycans and should be useful for the synthesis and biological investigation of DSGb5 derivatives.

A Siloxane-Bridged Glycosyl Donor Enables Highly Stereoselective β-Xylulofuranosylation

We report a siloxane-protected donor (7) for the highly stereoselective formation of β-(2,3-cis)-xylulofuranosyl bonds. Glycosylation reactions with 7 gave >80% yields, and only β-xylulofuranosides were isolated in all cases. The utility of 7 for the synthesis of complex glycans was shown by its successful application to the preparation of the repeating unit from the lipopolysaccharide O-antigen of Yersinia enterocolitica serovars O:5/O:5,27. This structure is a pentasaccharide with two β-xylulofuranose residues; using 7, both were introduced simultaneously with excellent stereocontrol.

Improved α-Sialylation through the Synergy of 5-N,4-O-Oxazolidinone Protection and Exocyclic C-1 Neighboring Group Participation

Stereoselective construction of α-sialyl linkages is one of the most significant challenges in carbohydrate chemistry. In this research, we developed a novel strategy for stereoselective synthesis of α-linked sialosides by protecting the 5-N,4-O-positions of a sialyl donor with an oxazolidinone group and its C-1 carboxylic functionality with a cyanoethyl ester to promote α-glycosylation. We also adopted the more electrophilic N-bromosuccinimide as a promoter to readily activate p-tolyl thiosialoside at -78 °C. The sialylation using this sialyl donor gave excellent yields and α-selectivity. The new synthetic method was used to successfully construct naturally occurring α-sialosides having sialic acid linked to the 6-O- or 3-O-position of galactoside, or 8-O-position of another sialic acid, respectively, as well as other α-linked sialosides.

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.