Sulfuric acid immobilized on silica: an excellent catalyst for Fischer type glycosylation

Topological analysis and reactivity study of monomeric and dimeric forms of 2-methyl-4(1H)-quinolone: a computational study

CONTEXT

Quinolone derivatives have gathered major attention largely due to their wonderful biological activities. Quinolones are a class of molecules that are derived from quinolines and also extracted from natural sources. Most of these quinolones have significant medicinal properties ranging from antiallergenic and anticancer to antimicrobial activities. Some bacteria produce several 2-alkyl-4(1H)-quinolones. In past years, a variety of methods have been reported for the synthesis of quinolone derivatives. In this present work, structural, wave functional, and electronic properties of monomeric and dimeric forms of 2-methyl-4(1H)-quinolone are investigated. From the calculated binding energies, it was found that the formation of dimers is thermodynamically favorable. The analysis of reactivity parameters confirms that the keto form is more reactive than the enol form and keto-keto dimer is more reactive than compared to all monomeric and dimeric forms of our studied compound.

METHODS

Geometry optimizations of monomers and dimers of studied molecules were carried out using the B3LYP-D3(BJ)/ma-def2-TZVPP level of theory. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were calculated using the B3LYP/def2-TZVP level of theory. All DFT calculations were done with the ORCA 5.0.3 program. The reactivity parameters such as ionization potential, electron affinity, global hardness, global softness, electronegativity, chemical potential, and electrophilicity index were calculated. The nature of intermolecular interactions within the dimers was studied using topological analysis such as atoms in molecule (AIM) and reduced density gradient (RDG) surface analyses. To visualize the electron delocalization in the dimer electron localization function (ELF) and localized orbital locator (LOL) studies were also performed. The analyses such as AIM, RDG, ELF, and LOL were carried out by the multifunctional wavefunction analysis program Multiwfn 3.8.

Lipase-Mediated Mechanoenzymatic Synthesis of Sugar Esters in Dissolved Unconventional and Neat Reaction Systems

Mechanochemical and biocatalytic approaches in modern research are two major assets to develop greener processes. In the present study, these modular tools of sustainability are pointed toward the production of versatile and daily employed compounds such as surfactants. Toward this aim, glycolipids, a class of nonionic surfactants composed of ubiquitous and primary metabolites such as sugar and fatty acid moieties, represent a promising alternative to petroleum-derived surface-active agents. Therefore, the combination of biocatalysis with mechanochemistry aiming at glycolipid synthesis seemed a logical step that was taken in this study for the first time. The monoacylated model compound glucose-6-O-decanoate was synthesized with the help of a bead mill apparatus using two different unconventional dissolved reaction systems, namely, menthol-based hydrophobic deep eutectic solvents and 2-methyl-2-butanol, thus reaching up to 12% yield in the latter based on the conversion of vinyl decanoate, after only 90 min of reaction. In addition, a neat reaction system using an excess of vinylated fatty ester as an adjuvant allowed a 27 mM/h space-time yield. The overall significant increase in productivities, up to 6 times, compared to standard heating and shaking methods, shows the tremendous potential of mechanoenzymatic synthesis.

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.

Selectivity of 1-O-Propargyl-d-Mannose Preparations

Thanks to their ability to bind to specific biological receptors, mannosylated structures are examined in biomedical applications. One of the most common ways of linking a functional moiety to a structure is to use an azide-alkyne click reaction. Therefore, it is necessary to prepare and isolate a propargylated mannose derivative of high purity to maintain its bioactivity. Three known preparations of propargyl-α-mannopyranoside were revisited, and products were analysed by NMR spectroscopy. The preparations were shown to yield by-products that have not been described in the literature yet. Our experiments showed that one-step procedures could not provide pure propargyl-α-mannopyranoside, while a three-step procedure yielded the desired compound of high purity.

Reversible immobilization of laccase onto glycopolymer microspheres via protein-carbohydrate interaction for biodegradation of phenolic compounds

It is challenging to regenerate enzyme carriers when covalently immobilized enzymes suffered from inactivation during continuous operations. Hence, it is urgent to develop a facile strategy to immobilize enzymes reversibly. Herein, the non-covalent interaction between protein and carbohydrate was used to adsorb and desorb enzymes reversibly. Laccase was immobilized onto glycopolymer microspheres via protein-carbohydrate interaction using lectins as the intermediates. The enzyme loading and immobilization yield were up to 49 mg/g and 77.1% with highly expressed activity of 107.9 U/mg. The immobilized laccase exhibited enhanced pH stability and high activity in catalyzing the biodegradation of paracetamol. During ten successive recoveries, the immobilized laccases could be recycled while maintaining relatively high enzyme activity. The glycopolymer microspheres could be efficiently regenerated by elution with an aqueous solution of mannose or acid for further enzyme immobilization. This glycopolymer microspheres has excellent potential to act as reusable carriers for the non-covalent immobilization of different enzymes.

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.

Parameters Influencing Lipase-Catalyzed Glycolipid Synthesis by (Trans-)Esterification Reaction

Glycolipids are biodegradable, non-toxic surfactants with a wide range of applications. Enzymatic esterification or transesterification facilitated in reaction media of low water activity is a reaction strategy for the production of tailor-made glycolipids as a high structural diversity can be achieved. Organic solvents, ionic liquids, and deep eutectic solvents have been applied as reaction media. However, several challenges need to be addressed for efficient (trans-)esterification reactions, especially for the lipophilization of polar substrates. Therefore, crucial parameters in (trans-)esterification reactions in conventional and non-conventional media are discussed and compared in this review with a special focus on glycolipid synthesis.

Synthesis of Non-symmetrical Dispiro-1,2,4,5-Tetraoxanes and Dispiro-1,2,4-Trioxanes Catalyzed by Silica Sulfuric Acid

A novel protocol for the preparation of non-symmetrical 1,2,4,5-tetraoxanes and 1,2,4-trioxanes, promoted by the heterogeneous silica sulfuric acid (SSA) catalyst, is reported. Different ketones react under mild conditions with gem-dihydroperoxides or peroxysilyl alcohols/β-hydroperoxy alcohols to generate the corresponding endoperoxides in good yields. Our mechanistic proposal, assisted by molecular orbital calculations, at the ωB97XD/def2-TZVPP/PCM(DCM)//B3LYP/6-31G(d) level of theory, enhances the role of SSA in the cyclocondensation step. This novel procedure differs from previously reported methods by using readily available and inexpensive reagents, with recyclable properties, thereby establishing a valid alternative approach for the synthesis of new biologically active endoperoxides.

A comparative study of the removal of o-xylene from gas streams using mesoporous silicas and their silica supported sulfuric acids

The three types of silica supported sulfuric acids (SSA), with the same sulfuric acid loading of 9.25 mmol g^-1, were prepared by a wet impregnation method from silica gel (SG), SBA-15 and MCM-41. Characterization of the prepared SSA showed that two anchoring states coexisted for sulfuric acid supported on the surface of the silicas: A physiosorbed (P)-state sulfuric acid; and a chemically bonded (C)-state sulfuric acid. Dynamic adsorption results showed that each SSA had a significant removal capacity for o-xylene gas in the reactive temperature regions. The ranges of the reactive regions were 120-220 °C (SSA/SG), 120-230 °C (SSA/SBA-15) and 120-250 °C (SSA/MCM-41), and this could be attributed to the sulfonation reaction between o-xylene and the anchored sulfuric acid. SSA/MCM-41 showed the highest theoretical breakthrough adsorption capacity (Q_{B, th}, 526.71 mg g^-1) compared with SSA/SBA-15 (363.54 mg g^-1) and SSA/SG (239.15 mg g^-1). Q_{B, th} was closely associated with the amount or proportion of the C-state sulfuric acid on the surface of each SSA. Optimum breakthrough time and Q_{B, th} was obtained by increasing the bed height and decreasing flow rate and inlet concentration. The SSA exhibited excellent recyclability and reuse performance over eight consecutive adsorption/desorption/regeneration cycles. The results suggested that the SSA, especially SSA/MCM-41, might have good potential in applications using adsorbents for the removal of BTEX pollutants.

Hyaluronan (HA)-inspired glycopolymers as molecular tools for studying HA functions

Hyaluronic acid (HA), the only non-sulphated glycosaminoglycan, serves numerous structural and biological functions in the human body, from providing viscoelasticity in tissues to creating hydrated environments for cell migration and proliferation. HA is also involved in the regulation of morphogenesis, inflammation and tumorigenesis through interactions with specific HA-binding proteins. Whilst the physicochemical and biological properties of HA have been widely studied for decades, the exact mechanisms by which HA exerts its multiple functions are not completely understood. Glycopolymers offer a simple and precise synthetic platform for the preparation of glycan analogues, being an alternative to the demanding synthetic chemical glycosylation. A library of homo, statistical and alternating HA glycopolymers were synthesised by reversible addition-fragmentation chain transfer polymerisation and post-modification utilising copper alkyne-azide cycloaddition to graft orthogonal pendant HA monosaccharides (N-acetyl glucosamine: GlcNAc and glucuronic acid: GlcA) onto the polymer. Using surface plasmon resonance, the binding of the glycopolymers to known HA-binding peptides and proteins (CD44, hyaluronidase) was assessed and compared to carbohydrate-binding proteins (lectins). These studies revealed potential structure-binding relationships between HA monosaccharides and HA receptors and novel HA binders, such as Dectin-1 and DEC-205 lectins. The inhibitory effect of HA glycopolymers on hyaluronidase (HAase) activity was also investigated suggesting GlcNAc- and GlcA-based glycopolymers as potential HAase inhibitors.

Hexavalent thiofucosides to probe the role of the Aspergillus fumigatus lectin FleA in fungal pathogenicity

Aspergillus fumigatus is a pathogenic fungus infecting the respiratory system and responsible for a variety of life-threatening lung diseases. A fucose-binding lectin named FleA which has a controversial role in A. fumigatus pathogenesis was recently identified. New chemical probes with high affinity and enzymatic stability are needed to explore the role of FleA in the infection process. In this study, we developed potent FleA antagonists based on optimized and non-hydrolysable thiofucoside ligands. We first synthesized a set of monovalent sugars showing micromolar affinity for FleA by isothermal titration calorimetry. The most potent derivative was co-crystallized with FleA to gain insights into the binding mode in operation. Its chemical multimerization on a cyclodextrin scaffold led to an hexavalent compound with a significantly enhanced binding affinity (K_d = 223 ± 21 nM) thanks to a chelate binding mode. The compound could probe the role of bronchial epithelial cells in a FleA-mediated response to tissue invasion.

Temporary ether protecting groups at the anomeric center in complex carbohydrate synthesis

The synthesis of a carbohydrate building block usually starts with introduction of a temporary protecting group at the anomeric center and ends with its selective cleavage for further transformation. Thus, the choice of the anomeric temporary protecting group must be carefully considered because it should retain intact during the whole synthetic manipulation, and it should be chemoselectively removable without affecting other functional groups at a late stage in the synthesis. Etherate groups are the most widely used temporary protecting groups at the anomeric center, generally including allyl ethers, MP (p-methoxyphenyl) ethers, benzyl ethers, PMB (p-methoxybenzyl) eithers, and silyl ethers. This chapter provides a comprehensive review on their formation, cleavage, and applications in the synthesis of complex carbohydrates.

New class of alkynyl glycoside analogues as tyrosinase inhibitors

A new series of alkynyl glycoside analogues were designed and synthesized from cheap and a commercially available sugar by introduction of various alkynyl and alkyl groups at C-1 and C-6 positions of the sugar ring. The inhibitory abilities of alkynyl glycosides were investigated in vitro on mushroom tyrosinase for the catalysis of l-Tyrosine and l-DOPA as substrates and comparing with arbutin and kojic acid. Non-terminal alkyne compound 2d showed excellent tyrosinase inhibitory activity (IC₅₀ 54.0 μM) against l-Tyrosine comparable to arbutin (IC₅₀ 1.46 mM) while 2b exhibited potent activities (IC₅₀ 34.3 μM) against L-DOPA higher than kojic acid (IC₅₀ 0.11 mM) and arbutin (IC₅₀ 13.3 mM). Kinetic studies revealed that compound 2d was a non-competitive inhibitor with the best Ki value of 21 μM and formed an irreversible receptor complex with mushroom tyrosinase. The SARs results showed that the type of alkyne and alkyl groups at position C-6 on sugar and the stereoisomer played an important role in determining their inhibitory activities. The potent activity of alkynyl glycosides identified in this study highlight the importance of this scaffold and these compounds are very modestly potent to the development of new class for tyrosinase inhibitor.

Nanoparticles Based on Novel Carbohydrate-Functionalized Polymers

Polymeric nanoparticles can be used for drug delivery systems in healthcare. For this purpose poly(lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG) offer an excellent polymeric matrix. In this work, PLGA and PEG polymers were functionalized with coumarin and carbohydrate moieties such as thymidine, glucose, galactose, and mannose that have high biological specificities. Using a single oil in water emulsion methodology, functionalized PLGA nanoparticles were prepared having a smooth surface and sizes ranging between 114–289 nm, a low polydispersity index and a zeta potential from −28.2 to −56.0 mV. However, for the corresponding PEG derivatives the polymers obtained were produced in the form of films due to the small size of the hydrophobic core.

Systematic synthesis and characterization of a series of different bromoalkylglycosides by Fischer glycosylation

In order to investigate the possibilities of Fischer glycosylation towards the synthesis of bromoalkylglycosides we performed a variety of different reactions resulting in a small library of 16 different glycosides. Using standardized reaction conditions we could gain a broad range of results from small to higher yields. Finally we randomly selected three reactions and performed them with higher amounts of bromoalcohol resulting in significantly better yields, showing the optimization potential of these basic research work.

The uncommon strong inhibition of α-glucosidase by multivalent glycoclusters based on cyclodextrin scaffolds

New inhibitors of α-glucosidase based on perglycosylated cyclodextrins were synthesized via click-chemistry and compared to acarbose.

Methyl glycosides via Fischer glycosylation: translation from batch microwave to continuous flow processing

ABSTRACT

A continuous flow procedure for the synthesis of methyl glycosides (Fischer glycosylation) of various monosaccharides using a heterogenous catalyst has been developed. In-depth analysis of the isomeric composition was undertaken and high consistency with corresponding results observed under microwave heating was obtained. Even in cases where addition of water was needed to achieve homogeneity-a prerequisite for the flow experiments-no detrimental effect on the conversion was found. The scalability was demonstrated on a model case (mannose) and as part of the target-oriented synthesis of d-glycero-d-manno heptose, both performed on multigram scale.

GRAPHICAL ABSTRACT

One-Pot Approach for the Synthesis of Bis-indole-1,4-disubstituted-1,2,3-triazoles

A new strategy for the synthesis of bis-indoletriazoles was developed using a sequential one-pot four-step procedure via I₂ and H₂SO₄-SiO₂ catalyzed Friedel-Crafts reactions of indole with aldehyde followed by N-alkylation with propargyl bromide, azidation, and copper(I)-catalyzed azide alkyne cycloaddition (CuAAC). The reaction proceeded smoothly at room temperature in a short time, and a series of bis-indoletriazoles were obtained in good to excellent yields proving the generality of this one-pot methodology.

Chemical synthesis and cytotoxicity of neo-glycolipids; rare sugar-glycerol-lipid compounds

Rare sugars are defined as monosaccharides and their derivatives, which rarely exist in nature and have various beneficial effects on organisms, biomaterials and foods. Glycolipids are composed of sugars and lipids and have been intensively studied in various fields such as environmental engineering, nanotechnology and molecular biology. Here, we synthesise new types of glycolipids composed of rare sugars, glycerol and lipids (RSGLs), using 6 different types of rare sugars by combining the modified Fischer and lipase reverse reactions. We confirm the production of RSGLs by thin layer chromatography (TLC), Fourier-transform infrared (FT-IR) spectroscopy and matrix assisted laser desorption/ionisation time of flight mass spectroscopy (MALDI-TOF-MS) and investigate the cytotoxicity of RSGLs by lactate dehydrogenase (LDH) and alamar blue assays. We successfully synthesise novel RSGLs; i.e., D-ribose-glycorol-lipid, D-allose-glycorol-lipid, L-rhamnose-glycorol-lipid, L-lyxorse-glycorol-lipid, L-gulose-glycorol-lipid and L-fucose-glycorol-lipid. We finally clarify the effect of the concentration of those RSGLs on cytotoxicity, which is of great importance considering the utilisation of RSGLs particularly in the field of biomedicine.

Fucose-Functionalized Precision Glycomacromolecules Targeting Human Norovirus Capsid Protein

Norovirus infection is the major cause of nonbacterial gastroenteritis in humans and has been the subject of numerous studies investigating the virus's biophysical properties and biochemical function with the aim of deriving novel and highly potent entry inhibitors to prevent infection. Recently, it has been shown that the protruding P domain dimer (P-dimer) of a GII.10 Norovirus strain exhibits two new binding sites for l-fucose in addition to the canonical binding sites. Thus, these sites provide a novel target for the design of multivalent fucose ligands as entry inhibitors of norovirus infections. In this current study, a first generation of multivalent fucose-functionalized glycomacromolecules was synthesized and applied as model structures to investigate the potential targeting of fucose binding sites in human norovirus P-dimer. Following previously established solid phase polymer synthesis, eight precision glycomacromolecules varying in number and position of fucose ligands along an oligo(amidoamine) backbone were obtained and then used in a series of binding studies applying native MS, NMR, and X-ray crystallography. We observed only one fucose per glycomacromolecule binding to one P-dimer resulting in similar binding affinities for all fucose-functionalized glycomacromolecules, which based on our current findings we attribute to the overall size of macromolecular ligands and possibly to steric hindrance.

Synthesis and Assembly of Laccase-Polymer Giant Amphiphiles by Self-Catalyzed CuAAC Click Chemistry

Covalent coupling of hydrophobic polymers to the exterior of hydrophilic proteins would mediate unique macroscopic assembly of bioconjugates to generate amphiphilic superstructures as novel nanoreactors or biocompatible drug delivery systems. The main objective of this study was to develop a novel strategy for the synthesis of protein-polymer giant amphiphiles by the combination of copper-mediated living radical polymerization and azide-alkyne cycloaddition reaction (CuAAC). Azide-functionalized succinimidyl ester was first synthesized for the facile introduction of azide groups to proteins such as albumin from bovine serum (BSA) and laccase from Trametes versicolor. Alkyne-terminal polymers with varied hydrophobicity were synthesized by using commercial copper wire as the activators from a trimethylsilyl protected alkyne-functionalized initiator in DMSO under ambient temperature. The conjugation of alkyne-functionalized polymers to the azide-functionalized laccase could be conducted even without additional copper catalyst, which indicated a successful self-catalyzed CuAAC reaction. The synthesized amphiphiles were found to aggregate into spherical nanoparticles in water and showed strong relevance to the hydrophobicity of coupled polymers. The giant amphiphiles showed decreased enzyme activity yet better stability during storage after chemical modification and self-assembly. These findings will deepen our understanding on protein folding, macroscopic self-assembly, and support potential applications in bionanoreactor, enzyme immobilization, and water purification.

Site-Selective Functionalization of Flagellin by Steric Self-Protection: A Strategy To Facilitate Flagellin as a Self-Adjuvanting Carrier in Conjugate Vaccine

Flagellin (FliC) can act as a carrier protein in the preparation of conjugate vaccines to elicit a T-cell-dependent immune response and as an intrinsic adjuvant to activate the toll-like receptor 5 (TLR5) to enhance vaccine potency. To enable the use of FliC as a self-adjuvanting carrier, an effective method for site-selective modification (SSM) of pertinent amino-acid residues in the D2 and D3 domains of FliC is explored without excessive modification of the D0 and D1 domains, which are responsible for activating and binding with TLR5. In highly concentrated Na₂ SO₄ solution, FliC monomers form flagellar filaments, in which the D0 and D1 domains are situated inside the tubular structure. Thus, the lysine residues (K219, K224, K324, and K331) in the D2 and D3 domains of flagellin are selectively modified by a diazo-transfer reaction with imidazole-1-sulfonyl azide. The sites with azido modification are confirmed by MALDI-TOF-MS, ESI-TOF-MS, and LC-MS/MS analyses along with label-free quantitation. The azido-modified filament dissolves to give FliC monomers, which can conjugate with alkyne-hinged saccharides by the click reaction. Transmission electron microscopy imaging, dynamic light scattering measurements, and the secreted embryonic alkaline phosphatase reporter assay indicate that the modified FliC monomers retain the ability either to bind with TLR5 or to reassemble into filaments. Overall, this study establishes a feasible method for the SSM of FliC by steric self-protection of the D0 and D1 domains.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

1,2-cis Alkyl glycosides: straightforward glycosylation from unprotected 1-thioglycosyl donors

A 1,2-cis-alkyl glycosidation protocol that makes use of unprotected phenyl 1-thioglycosyl donors is reported. Glycosylation of various functionalized alcohols was accomplished in moderate to high yield and selectivity to give the 1,2-cis-glycosides. In order to quickly develop optimum glycosylation conditions, an FIA (flow injection analysis)-ESI-TOF-MS method was developed that enabled rapid and quantitative evaluation of yield on small scale. This methodology, coupled with NMR spectroscopy, allowed for rapid evaluation of the overall reactions.

Synthesis and Anticancer Activity of 4β-Triazole-podophyllotoxin Glycosides

A series of novel 4β-triazole-podophyllotoxin glycosides were synthesized by utilizing the Click reaction. Evaluation of cytotoxicity against a panel of five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, SW480) using MTT assay shows that most of these compounds show weak cytotoxicity. It was observed that compound 16 shows the highest activity with IC50 values ranging from 2.85 to 7.28 μM, which is more potent than the control drugs etoposide and cisplatin against four of five cancer cell lines tested. Compound 16 is characterized with an α-D-galactosyl residue directly linked to the triazole ring and a 4'-OH group on the E ring of the podophyllotoxin scaffold. HPLC investigation of representative compound indicates that incorporation of a sugar moiety seems to improve the chemical stability of the podophyllotoxin scaffold.

Design, synthesis, and cytotoxicity of perbutyrylated glycosides of 4β-triazolopodophyllotoxin derivatives

A series of novel perbutyrylated glycosides of 4β-triazolopodophyllotoxin derivatives were synthesized by utilizing the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Evaluation of cytotoxicity against a panel of five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, SW480) using the MTT assay shows that some of these glycosylated derivatives have good anticancer activity. Among the synthesized compounds, compound 21a shows the highest activity, with IC₅₀ values ranging from 0.49 to 6.70 μM, which is more potent than the control drugs etoposide and cisplatin. Compound 21a is characterized by a perbutyrylated α-D(+)-galactosyl residue, the absence of an additional linking spacer between the sugar residue and the triazole ring, as well as a 4'-OH group on the E ring of the podophyllotoxin scaffold.