Glycosyl ortho-(1-phenylvinyl)benzoates versatile glycosyl donors for highly efficient synthesis of both O-glycosides and nucleosides

Highly Reactive Glycosylation with 1-O-(Methylthio)thiocarbonyl Glycosyl Donors under Acidic to Neutral Reaction Conditions

In this study, we have successfully developed a glycosylation method using 1-O-(methylthio)thiocarbonyl-glycoses as donors. Such xanthate donors are easily accessible and shelf-stable. The glycosylation reaction could be promoted by cations (acidic to neutral conditions) under mild conditions, exhibiting a reactivity intermediate between that with glycosyl trichloroacetimidate as the donor and that with thioglycoside as the donor. This methodology tolerates both "armed" and "disarmed" glycosyl donors, as well as various sugar acceptors, and affords the corresponding glycosides in good to excellent yields. Based on the relative higher reactivity of such xanthate donors than thioglycoside donors under the same glycosylation conditions, a trisaccharide was further synthesized in a one-pot glycosylation strategy.

A Radical Activation Strategy for Versatile and Stereoselective N-Glycosylation

Previous N-glycosylation approaches have predominately involved acidic conditions, facing challenges of low stereoselectivity and limited scope. Herein, we introduce a radical activation strategy that enables versatile and stereoselective N-glycosylation using readily accessible glycosyl sulfinate donors under basic conditions and exhibits exceptional tolerance towards various N-aglycones containing alkyl, aryl, heteroaryl and nucleobase functionalities. Preliminary mechanistic studies indicate a pivotal role of iodide, which orchestrates the formation of a glycosyl radical from the glycosyl sulfinate and subsequent generation of the key intermediate, a configurationally well-defined glycosyl iodide, which is subsequently attacked by an N-aglycone in a stereospecific SN2 manner to give the desired N-glycosides. An alternative route involving the coupling of a glycosyl radical and a nitrogen-centered radical is also proposed, affording the exclusive 1,2-trans product. This novel approach promises to broaden the synthetic landscape of N-glycosides, offering a powerful tool for the construction of complex glycosidic structures under mild conditions.

Highly stereoselective synthesis of α-glycosylated carboxylic acids by phenanthroline catalysis

Carbohydrate molecules with an α-glycosylated carboxylic acid motif provide access to biologically relevant chemical space but are difficult to synthesize with high selectivity. To address this challenge, we report a mild and operationally simple protocol to synthesize a wide range of functionally and structurally diverse α-glycosylated carboxylic acids in good yields with high diastereoselectivity. Although there is no apparent correlation between reaction conversion and pK a of carboxylic acids, we found that carboxylic acids with a pK a of 4-5 provide high selectivity while those of a pK a of 2.5 or lower do not. Our strategy utilizes readily available 2,9-dibutyl-1,10-phenanthroline as an effective nucleophilic catalyst to displace a bromide leaving group from an activated sugar electrophile in a nucleophilic substitution reaction, forming phenanthrolinium intermediates. The attack of the carboxylic acid takes place from the α-face of the more reactive intermediate, resulting in the formation of α-glycosylated carboxylic acid. Previous calculations suggested that the hydroxyl group participates in the hydrogen bond interaction with the basic C2-oxygen of a sugar moiety and serves as a nucleophile to attack the C1-anomeric center. In contrast, our computational studies reveal that the carbonyl oxygen of the carboxylic acid serves as a nucleophile, with the carboxylic acid-OH forming a hydrogen bond with the basic C2-oxygen of the sugar moiety. This strong hydrogen bond (1.65 Å) interaction increases the nucleophilicity of the carbonyl oxygen of carboxylic acid and plays a critical role in the selectivity-determining step. In contrast, when alcohol acts as a nucleophile, this scenario is not possible since the -OH group of the alcohol interacts with the C2-oxygen and attacks the C1-anomeric carbon of the sugar moiety. This is also reflected in alcohol-OH's weak hydrogen bond (1.95 Å) interaction with the C2-oxygen. The O(C2)-HO (carboxylic acid) angle was measured to be 171° while the O(C2)-HO (alcohol) angle at 122° deviates from linearity, resulting in weak hydrogen bonding.

Palladium-Catalyzed Regio- and Stereoselective Glycosylation of Azole Heterocycles Enables Access to Diverse Heterocyclic N-Glycosides

An efficient and practical glycosylation platform for synthesizing N-glycosides by leveraging palladium catalysis is disclosed. This approach enables facile access to diverse heterocyclic N-glycosides with excellent regio- and stereoselectivities and high site selectivity of multiple N atoms. The reaction exhibits a broad substrate scope (65 examples), high functional group tolerance, and easy scalability. Its synthetic utility is demonstrated through late-stage functionalization of pharmaceutically relevant molecules and various diastereoselective transformations of the glycoside products. Overall, our method provides a handy tool for efficient and stereocontrolled synthesis of valuable N-glycosylated heterocycles.

Synthesis of C-Alkyl Glycosides from Alkyl Bromides and Glycosyl Carboxylic Acids via Ni/Photoredox Dual Catalysis

C-Alkyl glycosides, an important class of C-glycosides, are widely found in various drugs and natural products. The synthesis of C-alkyl glycosides has attracted considerable attention. Herein, we developed a Ni/photoredox catalyzed decarboxylative C(sp3)-C(sp3) coupling reaction of stable glycosylcarboxylic acids with simple aliphatic bromides to generate C-alkyl glycosides. The method successfully linked several functional molecular fragments (natural products or drugs) to a sugar moiety, showing the extensive application prospects of this transformation. Controlled experiments and DFT calculations demonstrated that the reaction pathway contains a free radical process, and a possible mechanism is proposed.

Zinc(ii)-mediated stereoselective construction of 1,2-cis 2-azido-2-deoxy glycosidic linkage: assembly of Acinetobacter baumannii K48 capsular pentasaccharide derivative

The capsular polysaccharide (CPS) is a major virulence factor of the pathogenic Acinetobacter baumannii and a promising target for vaccine development. However, the synthesis of the 1,2-cis-2-amino-2-deoxyglycoside core of CPS remains challenging to date. Here we develop a highly α-selective ZnI2-mediated 1,2-cis 2-azido-2-deoxy chemical glycosylation strategy using 2-azido-2-deoxy glucosyl donors equipped with various 4,6-O-tethered groups. Among them the tetraisopropyldisiloxane (TIPDS)-protected 2-azido-2-deoxy-d-glucosyl donor afforded predominantly α-glycoside (α : β = >20 : 1) in maximum yield. This novel approach applies to a wide acceptor substrate scope, including various aliphatic alcohols, sugar alcohols, and natural products. We demonstrated the versatility and effectiveness of this strategy by the synthesis of A. baumannii K48 capsular pentasaccharide repeating fragments, employing the developed reaction as the key step for constructing the 1,2-cis 2-azido-2-deoxy glycosidic linkage. The reaction mechanism was explored with combined experimental variable-temperature NMR (VT-NMR) studies and mass spectroscopy (MS) analysis, and theoretical density functional theory calculations, which suggested the formation of covalent α-C1GlcN-iodide intermediate in equilibrium with separated oxocarbenium-counter ion pair, followed by an SN1-like α-nucleophilic attack most likely from separated ion pairs by the ZnI2-activated acceptor complex under the influence of the 2-azido gauche effect.

Total Synthesis of Nona-decasaccharide Motif from Ganoderma sinense Polysaccharide Enabled by Modular and One-Pot Stereoselective Glycosylation Strategy

Polysaccharides from a medicinal fungus Ganoderma sinense represent important and adjunctive therapeutic agents for treating various diseases, including leucopenia and hematopoietic injury. However, the synthetic accessibility to long, branched, and complicated carbohydrates chains from Ganoderma sinense polysaccharides remains a challenging task in chemical synthesis. Here, we report the modular chemical synthesis of nona-decasaccharide motif from Ganoderma sinense polysaccharide GSPB70-S with diverse biological activities for the first time through one-pot stereoselective glycosylation strategy on the basis of glycosyl ortho-(1-phenyvinyl)benzoates, which not only sped up carbohydrates synthesis but also reduced chemical waste and avoided aglycones transfer issues inherent to one-pot glycosylation on the basis of thioglycosides. The synthetic route also highlights the following key steps: (1) preactivation-based one-pot glycosylation for highly stereoselective constructions of several 1,2-cis-glycosidic linkages, including three α-d-GlcN-(1 → 4) linkages and one α-d-Gal-(1 → 4) bond via the reagent N-methyl-N-phenylformamide modulation; (2) orthogonal one-pot assembly of 1,2-trans-glycosidic linkages in various linear and branched glycans fragments by strategic combinations of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl ortho-(1-phenyvinyl)benzoates; and (3) the final [1 × 4 + 15] Yu glycosylation for efficient assembly of nona-decasaccharide target. Additionally, shorter sequences of 4-mer, 5-mer, and 6-mer are also prepared for structure-activity relationship biological studies. The present work shows that this one-pot stereoselective glycosylation strategy can offer a reliable and effective means to streamline chemical synthesis of long, branched, and complex carbohydrates with many 1,2-cis-glycosidic bonds.

Access to Reverse Glycosyl Azides and Rare Sugar-Based Glycosyl Azides via Radical Decarboxylative Azidation: Divergent Synthesis of 4'-C-Azidonucleosides as Potential Antiviral Agents

The radical decarboxylative azidation of structurally diverse uronic acids has been established as an efficient approach to reverse glycosyl azides and rare sugar-derived glycosyl azides under the action of Ag2CO3, 3-pyridinesulfonyl azide, and K2S2O8. The power of this method has been highlighted by the divergent synthesis of 4'-C-azidonucleosides using Vorbrüggen glycosylation of nucleobases with 4-C-azidofuranosyl acetates. The antiviral assessment of the resulting nucleosides revealed one compound as a potential inhibitor of covalently closed circular DNA.

Highly stereoselective α-glycosylation with GalN3 donors enabled collective synthesis of mucin-related tumor associated carbohydrate antigens

Mucin-related tumor-associated carbohydrate antigens (TACAs) are important and interesting targets for cancer vaccine therapy. However, efficient access to a library of mucin-related TACAs remains a challenging task. One of the key issues is the challenging construction of α-GalNAc linkages. Here, we report highly stereoselective α-glycosylation with GalN3N-phenyl trifluoroacetimidate donors, which features excellent yields, outstanding stereoselectivities, broad substrate scope and mild reaction conditions. This method is successfully applied to highly stereoselective synthesis of GalN3-α-O-Ser, which served as the common intermediate for collective synthesis of a wide range of TACAs including TN antigen, STN antigen, 2,6 STF antigen, 2,3 STF antigen, glycophorin and cores 1-8 mucin-type O-glycans. In particular, the rationale for this highly stereoselective α-glycosylation is provided for the first time using DFT calculations and mechanistic studies, highlighting the crucial roles of reagent combinations (TMSI and Ph3PO) and the H-bonding directing effect of the N3 group.

Dehydroxylative radical N-glycosylation of heterocycles with 1-hydroxycarbohydrates enabled by copper metallaphotoredox catalysis

N-Glycosylated heterocycles play important roles in biological systems and drug development. The synthesis of these compounds heavily relies on ionic N-glycosylation, which is usually constrained by factors such as labile glycosyl donors, precious metal catalysts, and stringent conditions. Herein, we report a dehydroxylative radical method for synthesizing N-glycosides by leveraging copper metallaphotoredox catalysis, in which stable and readily available 1-hydroxy carbohydrates are activated for direct N-glycosylation. Our method employs inexpensive photo- and copper- catalysts and can tolerate some extent of water. The reaction exhibits a broad substrate scope, encompassing 76 examples, and demonstrates high stereoselectivity, favoring 1,2-trans selectivity for furanoses and α-selectivity for pyranoses. It also exhibits high site-selectivity for substrates containing multiple N-atoms. The synthetic utility is showcased through the late-stage functionalization of bioactive compounds and pharmaceuticals like Olaparib, Axitinib, and Metaxalone. Mechanistic studies prove the presence of glycosyl radicals and the importance of copper metallaphotoredox catalysis.

Mild Approach to Nucleoside Analogues via Photoredox/Cu-Catalyzed Decarboxylative C-N Bond Formation. Total Synthesis of Oxetanocin A

The conventional N-glycosylation methods for nucleoside synthesis usually require strongly acidic or basic conditions. Here we report the decarboxylative C(sp3)-N coupling of glycosyl N-hydroxyphthalimide esters with nucleobases via dual photoredox/Cu catalysis, which offered a mild approach to nucleoside analogues. A total synthesis of oxetanocin A, an antiviral natural product containing an oxetanose moiety, has been achieved by using this method.

Iodosylbenzene-Promoted Glycosylation with Selenoglycosides: Application in One-Pot Glycosylation

A novel method for the glycosylation of selenoglycosides activated by iodosylbenzene was developed. The glycosylation reaction conditions were mild, fast, and efficient, with a high tolerance to diverse protecting groups and a wide substrate scope, which is advantageous for synthesizing complex glycosides. In addition, selenoglycosides were shown to be orthogonal to thioglycosides under the promotion of iodosylbenzene. Notably, a high yield of the poorly reactive glucuronidation reaction product was obtained by acetyl-protected selenoglycoside. Finally, the orthogonal one-pot synthesis of β-(1→6) oligoglucans demonstrated the usefulness of this method in oligosaccharide synthesis.

Direct Synthesis of α- and β-2'-Deoxynucleosides with Stereodirecting Phosphine Oxide via Remote Participation

2'-Deoxynucleosides and analogues play a vital role in drug development, but their preparation remains a significant challenge. Previous studies have focused on β-2'-deoxynucleosides with the natural β-configuration. In fact, their isomeric α-2'-deoxynucleosides also exhibit diverse bioactivities and even better metabolic stability. Herein, we report that both α- and β-2'-deoxynucleosides can be prepared with high yields and stereoselectivity using a remote directing diphenylphosphinoyl (DPP) group. It is particularly efficient to prepare α-2'-deoxynucleosides with an easily accessible 3,5-di-ODPP donor. Instead of acting as a H-bond acceptor on a 2-(diphenylphosphinoyl)acetyl (DPPA) group in our previous studies for syn-facial O-glycosylation, the phosphine oxide moiety here acts as a remote participating group to enable highly antifacial N-glycosylation. This proposed remote participation mechanism is supported by our first characterization of an important 1,5-briged P-heterobicyclic intermediate via variable-temperature NMR spectroscopy. Interestingly, antiproliferative assays led to a α-2'-deoxynucleoside with IC50 values in the low micromole range against central nervous system tumor cell lines SH-SY5Y and LN229, whereas its β-anomer exhibited no inhibition at 100 μM. Furthermore, the DPP group significantly enhanced the antitumor activities by 10 times.

Boronic Acid-Catalyzed Regio- and Stereoselective N-Glycosylations of Purines and Other Azole Heterocycles: Access to Nucleoside Analogues

In the presence of an arylboronic acid catalyst, azole-type heterocycles, including purines, tetrazoles, triazoles, indazoles, and benzo-fused congeners, undergo regio- and stereoselective N-glycosylations with furanosyl and pyranosyl trichloroacetimidate donors. The protocol, which does not require stoichiometric activators, specialized leaving groups, or drying agents, provides access to nucleoside analogues and enables late-stage N-glycosylation of azole-containing pharmaceutical agents. A mechanism involving simultaneous activation of the glycosyl donor and acceptor by the organoboron catalyst has been proposed, supported by kinetic analysis and computational modeling.

One-Pot Assembly of Mannose-Capped Lipoarabinomannan Motifs up to 101-Mer from the Mycobacterium tuberculosis Cell Wall

Lipoarabinomannan (LAM) from the Mycobacterium tuberculosis cell envelope represents important targets for the development of new therapeutic agents against tuberculosis, which is a deadly disease that has plagued mankind for a long time. However, the accessibility of long, branched, and complex lipoarabinomannan over 100-mer remains a long-standing challenge. Herein, we report the modular synthesis of mannose-capped lipoarabinomannan 101-mer from the M. tuberculosis cell wall using a one-pot assembly strategy on the basis of glycosyl ortho-(1-phenylvinyl)benzoates (PVB), which not only accelerates the modular synthesis but also precludes the potential problems associated with one-pot glycosylation with thioglycosides. Shorter sequences including 18-mer, 19-mer, and 27-mer are also synthesized for in-depth structure-activity relationship biological studies. Current synthetic routes also highlight the following features: (1) streamlined synthesis of various linear and branched glycans using one-pot orthogonal glycosylation on the combination of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl PVB; (2) highly stereoselective construction of 10 1,2-cis-arabinofuranosyl linkages using 5-O-(2-quinolinecarbonyl)-directing 1,2-cis-arabinofuranosylation via a hydrogen-bond-mediated aglycone delivery strategy; and (3) convergent [(18 + 19) × 2 + 27] one-pot synthesis of the 101-mer LAM polysaccharide. The present work demonstrates that this orthogonal one-pot glycosylation strategy can highly streamline the chemical synthesis of long, branched, and complex polysaccharides.

Photosensitizer-free visible-light-promoted glycosylation enabled by 2-glycosyloxy tropone donors

Photochemical glycosylation has attracted considerable attention in carbohydrate chemistry. However, to the best of our knowledge, visible-light-promoted glycosylation via photoactive glycosyl donor has not been reported. In the study, we report a photosensitizer-free visible-light-mediated glycosylation approach using a photoactive 2-glycosyloxy tropone as the donor. This glycosylation reaction proceeds at ambient temperature to give a wide range of O-glycosides or oligosaccharides with yields up to 99%. This method is further applied in the stereoselective preparation of various functional glycosyl phosphates/phosphosaccharides, the construction of N-glycosides/nucleosides, and the late-stage glycosylation of natural products or pharmaceuticals on gram scales, and the iterative synthesis of hexasaccharide. The protocol features uncomplicated conditions, operational simplicity, wide substrate scope (58 examples), excellent compatibility with functional groups, scalability of products (7 examples), and high yields. It provides an efficient glycosylation method for accessing O/N-glycosides and glycans.

Recent Advances in Chemical Synthesis of Structural Domains of Lipopolysaccharides from the Commensal Gut-Associated Microbiota

Lipopolysaccharides from the commensal gut-associated microbiota are interesting biomolecules for the treatment of various inflammatory diseases. Different from pathogenic lipopolysaccharides, commensal lipopolysaccharides have distinct chemical structures and mediate beneficial homeostasis with the immune system of the host. However, the accessibility issues of homogenous and pure commensal lipopolysaccharides hampered the in-depth studies of their functions. In this concept article, we highlight the recent synthesis of lipopolysaccharides from gut-associated lymphoid-tissue-resident Alcaligenes faecalis and Bacteroides vulgatus, which hopes to inspire the more efforts devoting to these fantastic biomolecules.

Recent chemical synthesis of plant polysaccharides

Here, chemical syntheses of long, branched and complex glycans over 10-mer from plants are summarized, which highlights amylopectin 20-mer from starch, 17-mer from carthamus tinctorius, α-glucan 30-mer from Longan, 19-mer from psidium guajava and 11-mer from dendrobium huoshanense. The glycans assembly strategies, protecting groups utilization and glycosylation methods discussed here will inspire the efficient synthesis of diverse complex glycans with many 1,2-cis glycosidic linkages.

Modular and Stereoselective One-Pot Total Synthesis of Icosasaccharide Motif from Cordyceps sinensis EPS-1A Glycan

The first stereoselective one-pot synthesis of the icosasaccharide motif of EPS-1A glycan from Cordyceps sinensis has been achieved. The synthetic approach highlights the following features: (1) merging reagent modulation and remote anchimeric assistance α-glycosylation strategy for the highly stereoselective formation of five and ten continuous 1,2-cis glucosidic bonds; (2) the strategic employment of glycosyl N-phenyltrifluoroacetimidates and glycosyl o-(1-phenylvinyl)benzoates as the matched pair for efficient orthogonal one-pot synthesis; and (3) [11 + 8 + 1] orthogonal one-pot glycosylation for the efficient assembly of the target icosasaccharide.

One-Pot Assembly of the Highly Branched Tetradecasaccharide from Ganoderma lucidum Glycan GLSWA-1 with Immune-Enhancing Activities

The highly branched tetradecasaccharide repeating unit and shorter sequences of GLSWA-1 with immune-enhancing activities from Ganoderma lucidum have been prepared via a one-pot glycan assembly strategy. The synthetic route features (1) orthogonal one-pot glycosylation on the basis of PVB glycosylation to streamline glycan synthesis avoiding such issues as aglycone transfer, (2) one-pot assembly of oligosaccharides with up to four different glycosyl linkages, and (3) modular and convergent [4+5+5] one-pot assembly of the highly branched tetradecasaccharide.

Orthogonal Glycosylation with Phosphate Acceptors for Expeditious Synthesis of Bacterial Inner Core Oligosaccharides

We report a practical one-pot glycosylation strategy for synthesis of bacterial inner core oligosaccharides that composed of unavailable L-glycero-D-manno and D-glycero-D-manno-heptopyranose components. The glycosylation method features a new orthogonal glycosylation procedure; whereby a phosphate acceptor is coupled with a thioglycosyl donor producing a disaccharide phosphate, which can be engaged in another orthogonal glycosylation procedure to couple with a thioglycosyl acceptor. The phosphate acceptors used in above one-pot procedure are directly prepared from thioglycosyl acceptors via the in-situ phosphorylation. Such phosphate acceptor preparation protocol eliminates the traditional protection and deprotection procedures. Based on the new one-pot glycosylation strategy, two partial inner core structures of Yersinia pestis lipopolysaccharide and Haemophilus ducreyi lipooligosaccharide were acquired.

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.

Synthesis of N-Glycosides by Silver-Assisted Gold Catalysis

The synthesis of N-glycosides from stable glycosyl donors in a catalytic fashion is still challenging, though they exist ubiquitously in DNA, RNA, glycoproteins, and other biological molecules. Herein, silver-assisted gold-catalyzed activation of alkynyl glycosyl carbonate donors is shown to be a versatile approach for the synthesis of purine and pyrimidine nucleosides, asparagine glycosides and quinolin-2-one N-glycosides. Thus synthesized nucleosides were subjected to the oxidation-reduction sequence for the conversion of Ribf- into Araf- nucleosides, giving access to nucleosides that are otherwise difficult to synthesize. Furthermore, the protocol is demonstrated to be suitable for the synthesis of 2'-modified nucleosides in a facile manner. Direct attachment of an asparagine-containing dipeptide to the glucopyranose and subsequent extrapolation to afford the dipeptide disaccharide unit of chloroviruses is yet another facet of this endeavor.

Additive-controlled asymmetric iodocyclization enables enantioselective access to both α- and β-nucleosides

β-Nucleosides and their analogs are dominant clinically-used antiviral and antitumor drugs. α-Nucleosides, the anomers of β-nucleosides, exist in nature and have significant potential as drugs or drug carriers. Currently, the most widely used methods for synthesizing β- and α-nucleosides are via N-glycosylation and pentose aminooxazoline, respectively. However, the stereoselectivities of both methods highly depend on the assisting group at the C2' position. Herein, we report an additive-controlled stereodivergent iodocyclization method for the selective synthesis of α- or β-nucleosides. The stereoselectivity at the anomeric carbon is controlled by the additive (NaI for β-nucleosides; PPh₃S for α-nucleosides). A series of β- and α-nucleosides are prepared in high yields (up to 95%) and stereoselectivities (β:α up to 66:1, α:β up to 70:1). Notably, the introduced iodine at the C2' position of the nucleoside is readily functionalized, leading to multiple structurally diverse nucleoside analogs, including stavudine, an FDA-approved anti-HIV agent, and molnupiravir, an FDA-approved anti-SARS-CoV-2 agent.

Synthesis of Nucleosides and Deoxynucleosides via Gold(I)-Catalyzed N-Glycosylation of Glycosyl (Z)-Ynenoates

Nucleoside analogues are widely used as anticancer and antiviral drugs. Here, we develop a highly efficient gold(I)-catalyzed N-glycosylation approach for versatile synthesis of various types of nucleosides and deoxynucleosides with glycosyl (Z)-ynenoates as donors. The wide scope of the N-glycosylation approach was demonstrated by the synthesis of 31 pyrimidine nucleosides and 8 purine nucleosides. Remarkably, the gold(I)-catalyzed N-glycosylation of pyranosyl (Z)-ynenoates with purines was found to be very effective for regioselective synthesis of pyranosyl N9 purine nucleosides. Based on the catalytic N-glycosylation approach, convenient synthesis of two 5'-deoxynucleosides drugs (capecitabine and galocitabine), four 2'-deoxynucleoside drugs (floxuridine, trifluridine, decitabine and cladribine), four 3',5'-dideoxynucleoside analogues, and four 2',5'-dideoxynucleoside analogues was achieved in a collective manner.

The total synthesis of rhynchosporosides via orthogonal one-pot glycosylation and stereoselective α-glycosylation strategies

The efficient and collective synthesis of rhynchosporosides causing scald diseases has been achieved, which features orthogonal one-pot glycosylation on the basis of PTFAI glycosylation, Yu glycosylation, and PVB glycosylation and merging reagent modulation and remote anchimeric assistance (RMRAA) α-glucosylation strategies. The issues inherent to the thioglycoside-based orthogonal one-pot glycosylation strategy, such as aglycone transfer, have been precluded by this orthogonal one-pot glycosylation strategy, which can streamline glycan chemical synthesis.

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.

Glycosyl o-[1-(p-MeO-Phenyl)vinyl]benzoates (PMPVB) as Easily Accessible, Stable, and Reactive Glycosyl Donors for O-, S-, and C-Glycosylations under Brønsted Acid Catalysis

Methods suitable for the synthesis of both O- and S-glycosylations are relatively rare because commonly used promoters like halonium sources or gold catalysts are incompatible with thiols as nucleophiles. Here, we present (p-MeO)phenylvinylbenzoates (PMPVB) as easily accessible, stable, and reactive alkene-based glycosyl donors that can be activated with catalytic amounts of a Brønsted acid. This activation protocol not only allows us to synthesize O-glycosides but also can successfully provide S- and C-linked glycosides. The armed and disarmed donors lead to product formation in 5 min, showcasing the high reactivity of the donors. Competitive experiments show that the PMPVB donors are much more reactive than the corresponding PVB donors even under NIS/TMSOTf conditions, whereas PVB donors are not reactive enough to be efficiently activated under Brønsted acid conditions. The potential of the catalytic glycosylation protocol has also been showcased by synthesizing trisaccharides. The Brønsted acid activation of PMPVB donors also allows access to C-glycosides in a stereoselective fashion. The easy accessibility of the donor aglycon on a multigram scale in just two steps makes the PMPVB donors highly attractive alternatives.

Glycosyl 3-Phenyl-4-pentenoates as Versatile Glycosyl Donors: Reactivity and Their Application in One-Pot Oligosaccharide Assemblies

Both glycoconjugates and oligosaccharides are important biomolecules having significant roles in several biological processes, and a new strategy for their synthesis is crucial. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf) promoted glycosidation approach with shelf-stable 3-phenyl-4-pentenoate glycosyl as a donor for the efficient synthesis of O/C-glycosides with free alcohols, silylated alcohols, and C-type nucleophile acceptors in good to excellent yields. The mild activation conditions and outstanding reactivity of phenyl substituted pentenoate donors analogous to 4-pentenoate glycosyl donors enhance their applicability to various one-pot strategies for the synthesis of oligosaccharides, such as single-catalyst one-pot and acceptor reactivity-controlled one-pot strategies.

Hydrogen bond activated glycosylation under mild conditions

Herein, we report a new glycosylation system for the highly efficient and stereoselective formation of glycosidic bonds using glycosyl N-phenyl trifluoroacetimidate (PTFAI) donors and a charged thiourea hydrogen-bond-donor catalyst. The glycosylation protocol features broad substrate scope, controllable stereoselectivity, good to excellent yields and exceptionally mild catalysis conditions. Benefitting from the mild reaction conditions, this new hydrogen bond-mediated glycosylation system in combination with a hydrogen bond-mediated aglycon delivery system provides a reliable method for the synthesis of challenging phenolic glycosides. In addition, a chemoselective glycosylation procedure was developed using different imidate donors (trichloroacetimidates, N-phenyl trifluoroacetimidates, N-4-nitrophenyl trifluoroacetimidates, benzoxazolyl imidates and 6-nitro-benzothiazolyl imidates) and it was applied for a trisaccharide synthesis through a novel one-pot single catalyst strategy.

A mild glycosylation system was developed using glycosyl imidate donors and a charge-enhanced thiourea H-bond donor catalyst. The method can be used for the effective synthesis of O-, C-, S- and N-glycosides and chemoselective one-pot glycosylation.

Catalytic Stereoselective Synthesis of 2-Deoxy α-glycosides Using Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) Donors

2-Deoxy Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) donors have been presented as stable, reactive glycosyl donors for the synthesis of 2-deoxy α-glycosides. The donors react under Brønsted acid conditions to provide the 2-deoxy-α-glycosides...

Chemical and chemoenzymatic stereoselective synthesis of β-nucleosides and their analogues

β-Nucleosides are fundamental building blocks of biological systems that are widely used as therapeutic agents for treating cancer and viral infections among others. In the last two years, nucleoside analogues...

A One-Pot Synthesis of Glycans and Nucleosides Based on ortho-(1-Phenylvinyl)benzyl Glycosides

One-pot synthesis of both glycans and nucleosides remains rare and challenging. Herein, we report a one-pot glycosylation strategy for glycans and nucleosides synthesis based on ortho-(1-phenylvinyl)benzyl glycosides, which has several advantages, including no aglycon transfers, no undesired interference of departing species, no unpleasant odor, and up to the construction of four different glycosidic linkages.

Visible-light-promoted 3,5-dimethoxyphenyl glycoside activation and glycosylation

A new glycosylation method promoted by visible light with 3,5-dimethoxyphenyl glycoside as the donor was developed. This protocol delivers both O-glycosides and N-glycosides in moderate to excellent yields using a wide range of O-nucleophiles and nucleobases as the glycosyl acceptors.

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.

NIS/TMSOTf-Promoted Glycosidation of Glycosyl ortho-Hexynylbenzoates for Versatile Synthesis of O-Glycosides and Nucleosides

Glycosidation plays a pivotal role in the synthesis of O-glycosides and nucleosides that mediate a diverse range of biological processes. However, efficient glycosidation approach for the synthesis of both O-glycosides and nucleosides remains challenging in terms of glycosidation yields, mild reaction conditions, readily available glycosyl donors, and cheap promoters. Here, we report a versatile N-iodosuccinimide/trimethylsilyl triflate (NIS/TMSOTf)-promoted glycosidation approach with glycosyl ortho-hexynylbenzoates as donors for the highly efficient synthesis of O-glycosides and nucleosides. The glycosidation approach highlights the merits of mild reaction conditions, cheap promoters, extremely wide substrate scope, and good to excellent yields. Notably, the glycosidation approach performs very well in the construction of a series of challenging O- and N-glycosidic linkages. The glycosidation approach is then applied to the efficient synthesis of oligosaccharides via the one-pot strategy and the stepwise strategy. On the basis of the isolation and characterization of the departure species derived from the leaving group, a plausible mechanism of NIS/TMSOTf-promoted glycosidation of glycosyl ortho-hexynylbenzoates is proposed.

An orthogonal and reactivity-based one-pot glycosylation strategy for both glycan and nucleoside synthesis: access to TMG-chitotriomycin, lipochitooligosaccharides and capuramycin

Both glycans (O-glycosides) and nucleosides (N-glycosides) play important roles in numerous biological processes. Chemical synthesis is a reliable and effective means to solve the attainability issues of these essential biomolecules. However, due to the stereo- and regiochemical issues during glycan assembly, together with problems including the poor solubility and nucleophilicity of nucleobases in nucleoside synthesis, the development of one-pot glycosylation strategies toward efficient synthesis of both glycans and nucleosides remains poor and challenging. Here, we report the first orthogonal and reactivity-based one-pot glycosylation strategy suitable for both glycan and nucleoside synthesis on the basis of glycosyl ortho-(1-phenylvinyl)benzoates. This one-pot glycosylation strategy not only inherits the advantages including no aglycon transfers, no undesired interference of departing species, and no unpleasant odors associated with the previously developed orthogonal one-pot glycosylation strategy based on glycosyl ortho-alkynylbenzoates, but also highly expands the scope (glycans and nucleosides) and increases the number of leaving groups that could be employed for the multistep one-pot synthesis (up to the formation of four different glycosidic bonds). In particular, the current one-pot glycosylation strategy is successfully applied to the total synthesis of a promising tuberculosis drug lead capuramycin and the divergent and formal synthesis of TMG-chitotriomycin with potent and specific inhibition activities toward β-N-acetylglucosaminidases and important endosymbiotic lipochitooligosaccharides including the Nod factor and the Myc factor, which represents one of the most efficient and straightforward synthetic routes toward these biologically salient molecules.

Cascade In Situ Phosphorylation and One-Pot Glycosylation for Rapid Synthesis of Heptose-Containing Oligosaccharides

We report a one-pot glycosylation strategy for achieving rapid syntheses of heptose (Hep)-containing oligosaccharides. The reported procedure was designed to incorporate an in situ phosphorylation step into an orthogonal one-pot glycosylation. Hep-containing oligosaccharides were assembled directly from building blocks with minimal effort expended on manipulation of protecting and aglycone leaving groups. The utility of our one-pot procedure was illustrated by synthesizing partial core oligosaccharide structure present in the lipopolysaccharide of Ralstonia solanacearum.