Protecting-Group-Free Synthesis of ADP-Ribose and Dinucleoside Di-/Triphosphate Derivatives via P(V)-P(V) Coupling Reaction

Biomolecules containing adenosine di- or triphosphate (ADP or ATP) are crucial for diverse biological processes. Synthesis of these biomolecules and development of their chemical probes are important to elucidate their functions. Enabling reproducible and high-yielding access to these ADP- and ATP-containing molecules via conventional P(III)-P(V) and P(V)-P(V) coupling reactions is challenging owing to water content in highly polar phosphate-containing substrates. Herein, we report an efficient and reliable method for protecting-group-free P(V)-P(V) coupling reaction through in situ activation of phosphates using hydrolysis-stable 2-[N-(2-methylimidazoyl)]-1,3-dimethylimidazolinium chloride (2-MeImIm-Cl), providing the corresponding electrophilic P(V) intermediates for subsequent nucleophilic attack using their coupling partners. This P(V)-P(V) coupling reaction proceeded even in a wet reaction medium and showed a broad substrate scope, accommodating protecting-group-free synthesis of ADP-ribose and nicotinamide adenine diphosphate analogs, ATP-containing biomolecules, and ADP-ribosyl peptides.

Convergent synthesis of functionalized derivatives of stage-specific embryonic antigens 3 & 4

Stage-specific embryonic antigens (SSEAs) are carbohydrate markers that have diverse roles in embryonic development. However, the exact roles of SSEAs remain unclear. To obtain mechanistic insights into their roles, we aimed to develop functionalized SSEA glycan analogs via chemical synthesis. Herein, we report a convergent synthetic approach for SSEA-3 and SSEA-4 analogs using readily available versatile building blocks. A key step, namely the stereoselective glycosylation of a common tetrasaccharide acceptor, was successfully achieved using a 4-O-Bn Gal donor for SSEA-3 and a Neu-Gal donor for SSEA-4, which were previously developed by our group. The obtained SSEA-3 and SSEA-4 glycans were further functionalized with biotin and deuterated lipid for applications in biological studies. Thus, the findings of this study will facilitate further research on SSEAs.

Fluorescent GD2 analog for single-molecule imaging

Ganglioside GD2 is associated with the proliferation and migration of breast cancer cells. However, the precise role of GD2 is unclear because its tendency to form dynamic and transient domains in cell plasma membranes (PMs), called lipid rafts, makes it difficult to observe. Previously, we developed fluorescent analogs of gangliosides (e.g., GM3 and GM1), which enabled the observation of lipid raft formation for the first time using single-molecule imaging. In this report, we describe the first chemical synthesis of a fluorescent ganglioside, GD2. A biophysical analysis of the synthesized analog revealed its raft-philic character, suggesting its potential to aid single-molecule imaging-based investigations into raft-associated interactions.

Unprecedented neighboring group participation of C2 N-imidoxy functionalities for 1,2-trans-selective glycosylation

Stereoselective glycosylation reactions are important in carbohydrate chemistry. The most used method for 1,2-trans(β)-selective glycosylation involves the neighboring group participation (NGP) of the 2-O-acyl protecting group; nevertheless, an alternative stereoselective method independent of classical NGP would contribute to carbohydrate chemistry, despite being challenging to achieve. Herein, a β-selective glycosylation reaction employing unprecedented NGP of the C2 N-succinimidoxy and phthalimidoxy functionalities is reported. The C2 functionalities provided the glycosylated products in high yields with β-selectivity. The participation of the functionalities from the α face of the glycosyl oxocarbenium ions gives stable six-membered intermediates and is supported by density functional theory calculations. The applicability of the phthalimidoxy functionality for hydroxyl protection is also demonstrated. This work expands the scope of functionalities tolerated in carbohydrate chemistry to include O-N moieties.

Investigation of the Protection of the C4 Hydroxyl Group in Macrobicyclic Kdo Donors

Chemical synthesis of 3-deoxy-d-manno-2-octulosonic acid (Kdo)-containing glycans, such as bacterial lipopolysaccharides (LPSs) and capsular polysaccharides (CPSs), is in high demand for the development of vaccines against pathogenic bacteria. We have recently achieved the complete α-stereoselective glycosidation of Kdo using a macrobicyclic donor tethered at the C1 and C5 positions. In this study, to expand the scope of Kdo glycosidation, we sought to protect the 4-OH group, thereby shortening the reaction time and ensuring the conversion of the glycosyl acceptor via its selective removal. The protection of the 4-OH group influenced the reactivity of the Kdo donor, and the triisopropylsilyl (TIPS) group acted as a selectively removable booster. The 4-O-TIPS donor allowed the synthesis of the α(2,4)-linked dimeric Kdo sequence, which is widely found in bacterial LPSs.

Straightforward Synthesis of the Poly(ADP-ribose) Branched Core Structure

Poly(ADP-ribosyl)ation is a post-translational modification that produces poly(ADP-ribose) with a branched structure every 20-50 units; such branching structure has been previously suggested to be involved in regulating chromatin remodeling. To elucidate its detailed functions, we developed a straightforward method for the synthesis of the poly(ADP-ribose) branched core structure, α-d-ribofuranosyl-(1‴ → 2″)-α-d-ribofuranosyl-(1″ → 2')-adenosine 5',5'',5‴-trisphosphate 1, from 6-chloropurine ribofuranoside 4 in 10 steps and 6.1% overall yield. The structure poses synthetic challenges for constructing iterative α-1,2-cis-glycosidic bonds in the presence of a purine base and the installation of three phosphate groups at primary hydroxyl groups. Iterative glycosidic bonds were formed by α-1,2-cis-selective ribofuranosylation using 2-O-(2-naphthylmethyl)-protected thioglycoside donor 6 and a thiophilic bismuth promoter. After the construction of diribofuranosyl adenosine 5 had been constructed, it was chemo- and regioselectively phosphorylated at a later stage. Subsequent deprotection provided the synthetic target 1.

Protecting-group- and microwave-free synthesis of β-glycosyl esters and aryl β-glycosides of N-acetyl-d-glucosamine

A protecting-group-free method for synthesis of β-glycosyl esters and aryl β-glycosides was developed by using latent chemical reactivity of N-acetyl-d-glucosamine (GlcNAc) oxazoline. The GlcNAc oxazoline was spontaneously reacted with carboxylic acids and phenol derivatives via the oxazoline ring opening without the use of a catalyst or heating conditions (i.e., microwave irradiation), affording the desired products in moderate to excellent yields with β-selectivity. This simple protecting-group-free method exhibits a wide substrate scope and good functional group tolerance, and it allows the efficient production of a novel class of GlcNAc-conjugated biomaterials and prodrug candidates.

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...

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.

Stereoselective Synthesis of Diglycosyl Diacylglycerols with Glycosyl Donors Bearing a β-Stereodirecting 2,3-Naphthalenedimethyl Protecting Group

Diglycosyl diacylglycerols (DGDGs) are major components of Gram-positive bacterial plasma membranes and are involved in the immune response systems. The chemical synthesis of DGDGs has been highly demanded, as it will allow the elucidation of their biological functions at the molecular level. In this study, we have developed a novel β-stereodirecting 2,3-naphthalenedimethyl (NapDM) protecting group that is orthogonal to protecting groups commonly used in oligosaccharide synthesis. The NapDM group can be easily cleaved under TFA-mediated acidic conditions. Futhermore, we demonstrated the application of this protecting group to an acyl protecting-group-free strategy by utilizing the NapDM group for the synthesis of DGDGs. This strategy features the use of the β-stereodirecting NapDM group as an acid-cleavable permanent protecting group and late-stage glycosylation of monoglycosyl diacylglycerol acceptors, enabling the stereoselective synthesis of three different bacterial DGDGs with unsaturated fatty acid chain(s).

Indirect synthetic route to α-l-fucosides via highly stereoselective construction of α-l-galactosides followed by C6-deoxygenation

We developed an indirect synthetic method for α-l-fucosides. Based on the fact that l-fucose is 6-deoxy-l-galactose, our strategy consists of the stereoselective construction of α-l-galactoside and its conversion to α-l-fucoside via C6-deoxygenation. The formation of α-l-galactoside is strongly directed using 4,6-O-di-tert-butylsilylene(DTBS)-protected l-galactosyl donors. The DTBS-directed α-l-galactosylation showed broad substrate applicability along with excellent coupling yield and α-selectivity. In the C6-deoxygenation of α-l-galactosides, the Barton-McCombie reaction facilitated the conversion to l-fucosides with good yield. To demonstrate the applicability of our method, we synthesized naturally occurring α-l-fucosides.

Efficient diversification of GM3 gangliosides via late-stage sialylation and dynamic glycan structural studies with 19F solid-state NMR

GM3 gangliosides have been synthesized via late-stage α-sialylation using a macro-bicyclic sialyl donor. ¹⁹F solid-state NMR analysis of the C5-NHTFAc GM3 analog on a model membrane revealed the influence of cholesterol on glycan dynamics.

Development of Fluorescently Labeled SSEA-3, SSEA-4, and Globo-H Glycosphingolipids for Elucidating Molecular Interactions in the Cell Membrane

Glycosphingolipids (GSLs), such as the globo-series GSLs stage-specific embryonic antigen 3 (SSEA-3), SSEA-4, and Globo-H, are specifically expressed on pluripotent stem cells and cancer cells, and are known to be associated with various biological processes such as cell recognition, cell adhesion, and signal transduction. However, the behavior and biological roles of these GSLs are still unclear. In our previous study, we observed the interactions between the lipid raft and GSLs in real-time using single-molecule imaging, where we successfully synthesized various fluorescent analogs of GSLs (e.g., GM1 and GM3). Here, we have developed fluorescent analogs of SSEA-3, SSEA-4, and Globo-H using chemical synthesis. The biophysical properties of these analogs as raft markers were examined by partitioning giant plasma membrane vesicles from RBL-2H3 cells into detergent-resistant membrane fractions and liquid-ordered/liquid-disordered phases. The results indicated that the analogs were equivalent to native-type GSLs. The analogs could be used to observe the behavior of globo-series GSLs for detailing the structure and biological roles of lipid rafts and GSL-enriched nanodomains during cell differentiation and cell malignancy.

Synthesis and Glycan-Protein Interaction Studies of Se-Sialosides by 77Se NMR

To expand the potential of Se-carbohydrates for multifunctional mimicry of sugars, herein we addressed the synthesis of the highly challenging and biologically significant Se-glycosides of sialic acid (Se-sialosides). An α-sialyl selenolate anion generated in situ smoothly reacted with electrophiles to give α-Se-sialosides as single stereoisomers. A Se-sialoside was sequentially incorporated with selenium, producing a triseleno-sialoside. This molecule was used as a ⁷⁷Se NMR-active handle for studying glycan-protein interaction, revealing different binding profiles of sialic acid binding proteins.

Chemical synthesis of diglucosyl diacylglycerols utilizing glycosyl donors with stereodirecting cyclic silyl protective groups

Chemical syntheses of the bacterial diglucosyl diacylglycerols 1-heptadecanoyl-2-pentadecanoyl-3-O-[6-O-(β-d-glucopyranosyl)-β-d-glucopyranosyl]-sn-glycerol and 1-(cis-13-octadecenoyl)-2-palmitoyl-3-O-[2-O-(α-d-glucopyranosyl)-α-d-glucopyranosyl]-sn-glycerol are described. The syntheses feature the stereoselective construction of glycosidic linkages in glycosylation reaction by utilizing glycosyl donors with stereodirecting cyclic silyl protective groups. The 1,1,3,3-tetraisopropyldisiloxane-1,3-diyl (TIPDS) group was used for formation of the β-glycosidic linkage, while the di-tert-butylsilylene (DTBS) group was used for α-linkage formation. The silyl protective groups were chemoselectively cleavable without affecting acyl functionalities on the glycerol moiety and proved effective for the synthesis of diacylglycoglycerolipids.

Constrained sialic acid donors enable selective synthesis of α-glycosides

Sialic acid is a sugar residue present in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. The chemical structure of sialic acid, which features an anomeric center with carboxyl and methylene substituents, poses a challenge for synthesis of the α-glycoside, thus impeding biological and therapeutic studies on sialic acid–containing glycans. We present a robust method for the selective α-glycosidation of sialic acid using macrobicyclized sialic acid donors as synthetic equivalents of structurally constrained oxocarbenium ions to impart stereoselectivity. We demonstrate the power of our method by showcasing broad substrate scope and applicability in the preparation of diverse sialic acid–containing architectures.

Synthesis of a 1,2-cis-indoxyl galactoside as a chromogenic glycosidase substrate

A synthetically challenging 1,2-cis-indoxyl galactoside, X-α-galactoside, was first prepared in this study using a cyclic ketone indoxyl acceptor and a glycosyl trichloroacetimidate donor to produce an enol glycoside and a 4,6-O-di-tert-butylsilylene-protected galactosyl donor to complete the synthesis. The target compound shows enzyme activity in the presence of α-galactosidase.

The highly challenging synthesis of a 1,2-cis-indoxyl galactoside was first achieved.

Synthesis of the glycan moiety of ganglioside HPG-7 with an unusual trimer of sialic acid as the inner sugar residue

The glycan moiety of ganglioside HPG-7, isolated from the sea cucumber (Holothuria pervicax), was synthesized for the first time. The characteristic substructure, a trisialic acid sequence embedded in the glycan, was deliberately constructed by utilizing suitably differentiated sialyl units for various synthetic purposes. Finally, a pentasaccharide was successfully delivered as the hexyl glycoside.