New Methods for the Synthesis of Glycosides and Oligosaccharides?Are There Alternatives to the Koenigs-Knorr Method? [New Synthetic Methods (56)]

Zinc Tetrafluoroborate Catalyzed Stereo- and Regioselective O-Glycosylation for the Direct Synthesis of β-Glycosides from Armed O-Glycosyl Trichloroacetimidates

Efficient stereo- and regioselective O-glycosylation methods remain essential to capacitate the studies of sugars and sugar derivatives in various disciplines. In this work, we demonstrated an operationally simple and cost-effective strategy for the synthesis of 1,2-trans glycosides by the activation of armed O-glycosyl trichloroacetimidates donor using zinc tetrafluoroborate. This mild, transition metal-free, and scalable approach allowed stereo- and regioselective synthesis of β-glycosides with a wide range of acceptors containing various protecting groups/functionalities. This method is exemplified by synthesizing a branched trisaccharide fragment related to the cell wall O-polysaccharide of E. Coli O27.

Chemoenzymatic synthesis of sialyl-α2,3-lactoside-functionalized BSA conjugate inhibits influenza infection

Influenza remains a global public health threat, and the development of new antivirals is crucial to combat emerging drug-resistant influenza strains. In this study, we report the synthesis and evaluation of a sialyl lactosyl (TS)-bovine serum albumin (BSA) conjugate as a potential multivalent inhibitor of the influenza virus. The key trisaccharide component, TS, was efficiently prepared via a chemoenzymatic approach, followed by conjugation to dibenzocyclooctyne-modified BSA via a strain-promoted azide-alkyne cycloaddition reaction. Biophysical and biochemical assays, including surface plasmon resonance, isothermal titration calorimetry, hemagglutination inhibition, and neuraminidase inhibition, demonstrated the strong binding affinity of TS-BSA to the hemagglutinin (HA) and neuraminidase (NA) proteins of the influenza virus as well as intact virion particles. Notably, TS-BSA exhibited potent inhibitory activity against viral entry and release, preventing cytopathic effects in cell culture. This multivalent presentation strategy highlights the potential of glycocluster-based antivirals for combating influenza and other drug-resistant viral strains.

Expeditious Synthesis of Gwanakoside A and the Chloronaphthol Glycoside Congeners

The synthesis of gwanakoside A, a chlorinated naphthol bis-glycoside, and its analogues was achieved through stepwise chlorination and donor-equivalent controlled regioselective phenol glycosylation with glycosyl N-phenyltrifluoroacetimidates as donors. Gwanakoside A displayed considerable inhibitory effects against various cancer cells and Staphylococcus aureus strains.

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.

BODIPY-Based Analogue of the TREM2-Binding Molecular Adjuvant Sulfavant A, a Chemical Tool for Imaging and Tracking Biological Systems

Recently, we described synthetic sulfolipids named Sulfavants as a novel class of molecular adjuvants based on the sulfoquinovosyl-diacylglycerol skeleton. The members of this family, Sulfavant A (1), Sulfavant R (2), and Sulfavant S (3), showed important effects on triggering receptor expressed on myeloid cells 2 (TREM2)-induced differentiation and maturation of human dendritic cells (hDC), through a novel cell mechanism underlying the regulation of the immune response. As these molecules are involved in biological TREM2-mediated processes crucial for cell survival, here, we report the synthesis and application of a fluorescent analogue of Sulfavant A bearing the 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3a,4a-diaza-s-indacene moiety (Me4-BODIPY). The fluorescent derivative, named PB-SULF A (4), preserving the biological activity of Sulfavants, opens the way to chemical biology and cell biology experiments to better understand the interactions with cellular and in vivo organ targets and to improve our comprehension of complex molecular mechanisms underlying the not fully understood ligand-induced TREM2 activity.

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.

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.

Urea-catalyzed N-Glycosylation of Amides/Azacycles with Glycosyl Halides

The efficient synthesis of N-glycosides via direct N-glycosylation of amides/azacycles has been reported. The glycosylation of amides/azacycles with glycosyl halides in the presence of a catalytic amount of urea proceeded smoothly to provide the corresponding N-glycosylated amides or nucleosides in good to excellent yields with 1,2-trans-stereoselectivity. Moreover, by the addition of terpyridine, the 1,2-cis-stereoselectivity was achieved.

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.

Dynamic Kinetic Stereoselective Glycosylation via RhII and Chiral Phosphoric Acid-Cocatalyzed Carbenoid Insertion to the Anomeric OH Bond for the Synthesis of Glycoconjugates

Chemical synthesis of glycoconjugates is essential for studying the biological functions of carbohydrates. We herein report an efficient approach for the stereoselective synthesis of challenging α-linked glycoconjugates via a RhII /chiral phosphoric acid (CPA)-cocatalyzed dynamic kinetic anomeric O-alkylation of sugar-derived lactols via carbenoid insertion to the anomeric OH bond. Notably, we observed excellent anomeric selectivity, excellent diastereoselectivity, broad substrate scope, and high efficiency for this glycosylation reaction by exploring various parameters of the cocatalytic system. DFT calculations suggested that the anomeric selectivity was mainly determined by steric interactions between the C2-carbon of the carbohydrate and the phenyl group of the metal carbenoid, while π/π interactions with the C2-OBn substituent on the carbohydrate substrate play a significant role for diastereoselectivity at the newly generated stereogenic center.

Acetylation in Ionic Liquids Dramatically Increases Yield in the Glycosyl Composition and Linkage Analysis of Insoluble and Acidic Polysaccharides

Glycosyl composition and linkage analyses are important first steps toward understanding the structural diversity and biological importance of polysaccharides. Failure to fully solubilize samples prior to analysis results in the generation of incomplete and poor-quality composition and linkage data by gas chromatography-mass spectrometry (GC-MS). Acidic polysaccharides also do not give accurate linkage results, because they are poorly soluble in DMSO and tend to undergo β-elimination during permethylation. Ionic liquids can solubilize polysaccharides, improving their derivatization and extraction for analysis. We show that water-insoluble polysaccharides become much more amenable to chemical analysis by first acetylating them in an ionic liquid. Once acetylated, these polysaccharides, having been deprived of their intermolecular hydrogen bonds, are hydrolyzed more readily for glycosyl composition analysis or methylated more efficiently for glycosyl linkage analysis. Acetylation in an ionic liquid greatly improves composition analysis of insoluble polysaccharides when compared to analysis without acetylation, enabling complete composition determination of normally recalcitrant polysaccharides. We also present a protocol for uronic acid linkage analysis that incorporates this preacetylation step. This protocol produces partially methylated alditol acetate derivatives in high yield with minimal β-elimination and gives sensitive linkage results for acidic polysaccharides that more accurately reflect the structures being analyzed. We use important plant polysaccharides to show that the preacetylation step leads to superior results compared to traditional methodologies.

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 the Hexasaccharide Related to the Exopolysaccharide from Lactobacillus mucosae VG1 through Regioselective Glycosylation

Chemical synthesis of the hexasaccharide repeating unit associated with the exopolysaccharide of Lactobacillus mucosae VG1 is reported. The total synthesis is accomplished through a convergent [2 + 2 + 2] strategy using rationally protected monosaccharide derivatives. Chemoselective activation of the glycosyl donors and regioselective nucleophilicity of the acceptors were successfully employed throughout the chemical synthesis.

Potassium carbonate-mediated β-selective anomeric O-alkylation with primary electrophiles: Application to the synthesis of glycosphingolipids

Stereoselective construction of a variety of β-glycosides can be achieved using abundant and inexpensive K₂CO₃-mediated stereoselective anomeric O-alkylation of sugar lactols with primary electrophiles. In addition, application of this methodology to the synthesis of various azido-modified glycosphingolipids has been accomplished in good yields and excellent anomeric selectivity using sphingosine-derived primary triflate.

ortho-Methoxycarbonylethynylphenyl Thioglycosides (MCEPTs): Versatile Glycosyl Donors Enabled by Electron-Withdrawing Substituents and Catalyzed by Gold(I) or Cu(II) Complexes

With easily accessible and operator-friendly reagents, shelf-stable ortho-methoxycarbonylethynylphenyl thioglycosides were efficiently prepared. Based on these MCEPT glycoside donors, a novel glycosylation protocol featuring mild and catalytic promotion conditions with Au(I) or Cu(II) complexes, expanded substrate scope encompassing challenging donors and acceptors and clinically used pharmaceuticals, and versatility in various strategies for highly efficient synthesis of glycosides has been established. The practicality of the MCEPT glycosylation protocol was fully exhibited by highly efficient and scalable synthesis of surface polysaccharide subunits of Acinetobacter baumannii via latent-active, reagent-controlled divergent orthogonal one-pot and orthogonal one-pot strategies. The underlying reaction mechanism was investigated systematically through control reactions, leading to the isolation and characterization of the vital catalyst species in MCEPT glycosylation, the benzothiophen-3-yl-gold(I) complex. Based on the results obtained both from control reactions and from studies leading to the glycosylation protocol establishment, an operative mechanism was proposed and the effect of the vital catalyst species reactivity on the results of metal-catalyzed alkyne-containing donor-involved glycosylation was disclosed. Moreover, the mechanism for C-glycosylation side product formation from ortho-(substituted)ethynylphenyl thioglycoside donors with electron-donating substituents was also illuminated.

Enantioselective Total Synthesis of (+)-Sieboldine A and Analogues Thereof

An 11-step enantioselective total synthesis of (+)-sieboldine A (1) has been accomplished from (5R)-methylcyclohex-2-en-1-one (16), in which an intramolecular ketone/ester reductive coupling followed by one-pot acidic treatment to quickly construct the unique oxa-spiroacetal and a TsOH-catalyzed displacement to directly form the characteristic N-hydroxyazacyclononane ring successfully served as the key methodologies. Moreover, several full-skeleton analogues of 1 were also synthesized on the basis of the advanced intermediates, and their inhibitory effects on electric eel acetylcholinesterase were examined.

Ni(II)-Catalyzed Regio- and Stereoselective O-Alkylation for the Construction of 1,2-cis-Glycosidic Linkages

A transition-metal-catalyzed O-alkylation for the regio- and stereoselective construction of 1,2-cis-glycosidic linkages is presented. With nonprecious and readily available Ni(II) as a catalyst, 1,2-cis-glycosides were obtained via O-alkylation of 1,2-carbohydrate diols that can be accessed in a small number of steps. The tedious design of protecting groups or anomeric leaving groups could be avoided with this method. The strategy was applied for the efficient preparation of an important commercialized glycosidic compatible solute GG, its derivative MGG, and a branched α-glucan.

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

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

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.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Like Visiting an Old Friend: Fischer Glycosylation in the Twenty-First Century: Modern Methods and Techniques

Fischer glycosylation is typically the chemical reaction of a monosaccharide and an alcohol in presence of an acidic catalyst to afford glycosides in pyranosidic and furanosidic forms. This reaction is still applied today for the synthesis of specialized glycosides, and optimization and modification of the method have continued since its discovery by Emil Fischer in the 1890s. This review presents advancements in Fischer glycosylation described in literature of the past 15 years and its implementation in modern chemical methods.

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.

Synthesis of Type-I and Type-II LacNAc-Repeating Oligosaccharides as the Backbones of Tumor-Associated Lewis Antigens

Type-I and Type-II LacNAc are Gal-GlcNAc disaccharides bearing a β1,3- or β1,4-linkage respectively. They exist as the backbones of Lewis antigens that are highly expressed in several cancers. Owing to the promise of developing carbohydrate-based anti-cancer vaccines, glycan synthesis at a large scale is indeed an important task. Synthesis of Type-I and Type-II tandem repeat oligomers has been hampered by the presence of GlcNAc residues. Particularly, N-protecting group plays a determining role in affecting glycosyl donor’s reactivity and acceptor’s nucleophilicity. This review discusses several representative studies that assembled desirable glycans in an efficient manner, such as chemoselective one-pot synthesis and chemoenzymatic methods. Additionally, we also highlight solutions that have been offered to tackle long-lasting problems, e.g., prevention of the oxazoline formation and change of donor/acceptor reactivity. In retrospect of scientific achievements, we present the current restrictions and remaining challenges in this less explored frontier.

Chemo-Enzymatic Production of 4-Nitrophenyl-2-acetamido-2-deoxy-α-D-galactopyranoside Using Immobilized β-N-Acetylhexosaminidase

α-Nitrophenyl derivatives of glycosides are convenient substrates used to detect and characterize α-N-acetylgalactosaminidase. A new procedure combining chemical and biocatalytic steps was developed to prepare 4-nitrophenyl-2-acetamido-2-deoxy-α-D-galactopyranoside (4NP-α-GalNAc). The α-anomer was prepared through chemical synthesis of an anomeric mixture followed by selective removal of the β-anomer using specific enzymatic hydrolysis. Fungal β-N-acetylhexosaminidase (Hex) from Penicillium oxalicum CCF 1959 served this purpose owing to its high chemo-and regioselectivity towards the β-anomeric N-acetylgalactosamine (GalNAc) derivative. The kinetic measurements of the hydrolytic reaction showed that the enzyme was not inhibited by the substrate or reaction products. The immobilization of Hex in lens-shaped polyvinyl alcohol hydrogel capsules provided a biocatalyst with very good storage and operational stability. The immobilized Hex retained 97% of the initial activity after ten repeated uses and 90% of the initial activity after 18 months of storage at 4 °C. Immobilization inactivated 65% of the enzyme activity. However, the effectiveness factor and kinetic and mass transfer phenomena approached unity indicating negligible mass transfer limitations.

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation

Deoxy sugars represent an important class of carbohydrates, present in a large number of biomolecules involved in multiple biological processes. In various antibiotics, antimicrobials, and therapeutic agents the presence of deoxygenated units has been recognized as responsible for biological roles, such as adhesion or great affinity to receptors, or improved efficacy. The characterization of glycosidases and glycosyltranferases requires substrates, inhibitors and analogous compounds. Deoxygenated sugars are useful for carrying out specific studies for these enzymes. Deoxy sugars, analogs of natural substrates, may behave as substrates or inhibitors, or may not interact with the enzyme. They are also important for glycodiversification studies of bioactive natural products and glycobiological processes, which could contribute to discovering new therapeutic agents with greater efficacy by modification or replacement of sugar units. Deoxygenation of carbohydrates is, thus, of great interest and numerous efforts have been dedicated to the development of methods for the reduction of sugar hydroxyl groups. Given that carbohydrates are the most important renewable chemicals and are more oxidized than fossil raw materials, it is also important to have methods to selectively remove oxygen from certain atoms of these renewable raw materials. The different methods for removal of OH groups of carbohydrates and representative or recent applications of them are presented in this chapter. Glycosidic bonds in general, and 2-deoxy glycosidic linkages, are included. It is not the scope of this survey to cover all reports for each specific technique.

Silane promoted glycosylation and its applications for synthesis of sugar compounds and active pharmaceutical ingredients (APIs)

Silane promoted glycosylation and its applications for preparation of active pharmaceutical ingredients.

Total Synthesis of Mycinamicin IV as Integral Part of a Collective Approach to Macrolide Antibiotics

The total synthesis of the 16‐membered macrolide mycinamicin IV is outlined, which complements our previously disclosed, largely catalysis‐based route to the aglycone. This work must also be seen in the context of our recent conquest of aldgamycin N, a related antibiotic featuring a similar core but a distinctly different functionalization pattern. Taken together, these projects prove that the underlying blueprint is integrative and hence qualifies for a collective approach to this prominent class of natural products. In both cases, the final glycosylation phase mandated close attention and was accomplished only after robust de novo syntheses of the (di)deoxy sugars of the desosamine, chalcose, mycinose and aldgarose types had been established. Systematic screening of the glycosidation promoter was also critically important for success.

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.

Continuous synthesis of bromoalkyl glycosides by Fischer glycosylation in a microreactor

In this study, bromoalkyl glycosides were successfully synthesized in microreactor scale by Fischer glycosylation. Yields between 24 and 40% after purification were achieved using various acidic catalysts and conditions. In some experiments, yields 180% higher than with previously known methods could be achieved. This study showed also that reversed-phase flash chromatography is more successful than normal-phase flash chromatography for the purification of bromoalkyl glycosides. Furthermore, longer bromoalcohols were shown to be more compatible than shorter bromoalcohols under these reaction conditions.

Chemical synthesis of the rare D-Fuc3NAc containing tetrasaccharide repeating unit of the O-antigenic polysaccharide from E. coli O74

Chemical synthesis of the tetrasaccharide repeating unit of the O-antigen from E. coli O74 is accomplished by a convergent [2 + 2] block synthesis strategy. The challenging rare D-Fuc3NAc has been prepared using DTBP and TIPST mediated deoxygenation reaction. Other monosaccharide synthons are prepared through rational protecting group manipulations and the stereoselective glycosylations are achieved either by the activation of thioglycoside or glycosyl trichloroacetimidate. The target tetrasaccharide is made in the form of its 2-aminoethyl glycoside to facilitate further glycoconjugate formation without affecting the anomeric stereochemistry.

Scalable De Novo Synthesis of Aldgarose and Total Synthesis of Aldgamycin N

Since the accompanying study had shown that the introduction of the eponymous aldgarose sugar to the C5-OH group of the macrocyclic aglycone of aldgamycin N is most difficult, if not even impossible, the synthesis route was revised and the glycosidation performed at an earlier stage. To mitigate the "cost" of this strategic amendment, a practical and scalable de novo synthesis of this branched octose was developed. The glycoside formation required mild conditions; it commenced with the reaction of the aglycone with the trichloroacetimidate donor to give a transient orthoester, which slowly rearranged to the desired aldgaropyranoside. The presence of the polar peripheral groups in the product did not impede the selective late-stage functionalization of the macrolide ring itself: the contained propargylic alcohol entity was readily transformed into the characteristic acyloin motif of the target by a ruthenium-catalyzed trans-hydrostannation followed by a modified Chan-Lam-type coupling.

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.

Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.

A novel synergistic anticancer effect of fungal cholestanol glucoside and paclitaxel: Apoptosis induced by an intrinsic pathway through ROS generation in cervical cancer cell line (HeLa)

In the search for efficient therapeutics with economically viable for cancer treatment, combination therapy has developed as a keystone in the pursuit of novel approaches for drug discovery. In this regard, we confirmed the presence of cholestanol glucoside (CG) in Lasiodiplodia theobromae culture filtrate and its production was estimated to be 20.01 mg/l. The purified fungal CG was obtained with a molecular mass of 550.18 m/z. The combination of CG and paclitaxel (PTX) was found to have potent cytotoxicity against HeLa cells. We revealed that the synergistic effect of CG and PTX induced apoptosis through the formation of nuclear fragments, DNA fragmentation and sub G1 cell cycle arrest. Further, it was proven that apoptosis took place by loss of the mitochondrial membrane potential (MMP) through reactive oxygen species (ROS) production and caspase 3/7 activity. Moreover, the data suggests that the synergistic effect of CG and PTX played a role in a mitochondrial intrinsic pathway through the apoptotic gene expression of Bax, caspase-9 and caspase-3. In addition, the down-regulation of Bcl-2 strongly described the induced apoptosis through an intrinsic pathway using the Western blot analysis. The conclusion of this study is that a combination of CG and PTX has synergistic apoptotic effects in HeLa cells, which provides a possible therapeutic strategy for cancer therapy in the future.

Exploring the Biochemical Foundations of a Successful GLUT1-Targeting Strategy to BNCT: Chemical Synthesis and In Vitro Evaluation of the Entire Positional Isomer Library of ortho-Carboranylmethyl-Bearing Glucoconjugates

Boron neutron capture therapy (BNCT) is a noninvasive binary therapeutic modality applicable to the treatment of cancers. While BNCT offers a tumor-targeting selectivity that is difficult to match by other means, the last obstacles preventing the full harness of this potential come in the form of the suboptimal boron delivery strategies presently used in the clinics. To address these challenges, we have developed delivery agents that target the glucose transporter GLUT1. Here, we present the chemical synthesis of a number of ortho-carboranylmethyl-substituted glucoconjugates and the biological assessment of all positional isomers. Altogether, the study provides protocols for the synthesis and structural characterization of such glucoconjugates and insights into their essential properties, for example, cytotoxicity, GLUT1-affinity, metabolism, and boron delivery capacity. In addition to solidifying the biochemical foundations of a successful GLUT1-targeting approach to BNCT, we identify the most promising modification sites in d-glucose, which are critical in order to further develop this strategy toward clinical use.

Total Synthesis of Sieboldine A

Previously, we have finished the total synthesis of lycojaponicumin A (2) via development of an efficient synthetic strategy using semipinacol rearrangement as a key step. In order to further demonstrate the generality of this synthetic route, herein, we report the total synthesis of another fawcettimine-type alkaloid sieboldine A (1) from the same intermediate, which possesses an A/B/D tricyclic ring system and vicinal quaternary centers of 1. The synthesis features late-stage site-selective redox reactions, Schmidt glycosylation cyclization, and highly selective transformations.

One-pot construction of carbohydrate scaffolds mediated by metal catalysts

Owing to the environmental concern worldwide and also due to cost, time and labour issues, use of one-pot reactions [domino/cascade/tandem/multi-component (MC) or sequential] has gained much attention among the scientific and industrial communities for the generation of compound libraries having different scaffolds. Inclusion of sugars in such compounds is expected to increase the pharmacological efficacy because of the possibility of better interactions with the receptors of such unnatural glycoconjugates. In many of the one-pot transformations, the presence of a metal salt/complex can improve the reaction/change the course of reaction with remarkable increase in chemo-/regio-/stereo-selectivity. On the other hand because of the importance of natural polymeric glycoconjugates in life processes, the development and efficient synthesis of related oligosaccharides, particularly utilising one-pot MC-glycosylation techniques are necessary. The present review is an endeavour to discuss one-pot transformations involving carbohydrates catalysed by a metal salt/complex.