Facile Preparation of Nucleoside-5'-carboxylic Acids

Discovery of highly potent SARS-CoV-2 nsp14 methyltransferase inhibitors based on adenosine 5'-carboxamides

The emergence of SARS-CoV-2, the causative agent of COVID-19, has highlighted the need for advanced antiviral strategies. Targeting the coronaviral methyltransferase nsp14, which is essential for RNA capping, offers a promising approach for the development of small-molecule inhibitors. We designed and synthesized a series of adenosine 5'-carboxamide derivatives as potential nsp14 inhibitors and identified coumarin analogs to be particularly effective. Structural modifications revealed the importance of the 5'-carboxyl moiety for the inhibitory activity, showing superior efficacy compared to other modifications. Notably, compound 18l (HK370) demonstrated high selectivity and favorable in vitro pharmacokinetic properties and exhibited moderate antiviral activity in cell-based assays. These findings provide a robust foundation for developing targeted nsp14 inhibitors as a potential treatment for COVID-19 and related diseases.

Total Syntheses and Stereochemical Assignment of Acremolides A and B

The absolute stereochemical configurations of acremolides A and B were predicted by a biochemistry-based rule and unambiguously confirmed through their total syntheses. The features of the total syntheses include sequential Krische's Ir-catalyzed crotylation, Brown's borane-mediated crotylation, Mitsunobu esterification reaction, and cross-metathesis reaction. The efficient total synthesis enabled clear validation of the predicted stereochemistry for acremolides A and B.

Chemical Synthesis of Acetobacter pasteurianus Lipid A with a Unique Tetrasaccharide Backbone and Evaluation of Its Immunological Functions

Lipopolysaccharide (LPS), a cell surface component of Gram-negative bacteria, activates innate immunity. Its active principle is the terminal glycolipid lipid A. Acetobacter pasteurianus is a Gram-negative bacterium used in the fermentation of traditional Japanese black rice vinegar (kurozu). In this study, we focused on A. pasteurianus lipid A, which is a potential immunostimulatory component of kurozu. The active principle structure of A. pasteurianus lipid A has not yet been identified. Herein, we first systematically synthesized three types of A. pasteurianus lipid As containing a common and unique tetrasaccharide backbone. We developed an efficient method for constructing the 2-trehalosamine skeleton utilizing borinic acid-catalyzed glycosylation to afford 1,1'-α,α-glycoside in high yield and stereoselectivity. A common tetrasaccharide intermediate with an orthogonal protecting group pattern was constructed via [2+2] glycosylation. After introducing various fatty acids, all protecting groups were removed to achieve the first chemical synthesis of three distinct types of A. pasteurianus lipid As. After evaluating their immunological function using both human and murine cell lines, we identified the active principles of A. pasteurianus LPS. We also found the unique anomeric structure of A. pasteurianus lipid A contributes to its high chemical stability.

Total Synthesis and Determination of Absolute Configuration of Cryptorigidifoliol G

The first asymmetric total synthesis of (1S,5R,7S)-cryptorigidifoliol G and (1S,5R,7R)-cryptorigidifoliol G of the proposed natural product was achieved. The key steps in the synthesis involved Keck-Maruoka allylation, our own developed protocol for the construction of the trans-2,6-disubstituted dihydropyran, iodolactonization, cross-metathesis, Prins cyclization, and cis-Wittig olefination reaction. A comparison of the NMR as well as analytical data and thorough analysis of the 2D NMR suggested that the absolute stereochemistry of the proposed natural product is (1S,5R,7S)-cryptorigidifoliol G.

Total Synthesis of the Purported Structure of Branched Resin Glycosides Merremoside G and H2

The first total synthesis of the purported structure of branched resin glycosides merremoside G and H2 is accomplished. A signature step is represented by the sequential transition-metal-catalyzed coupling of stable trisaccharide homoallylic alcohol and monosaccharide alkoxyallene to afford the pentasaccharide skeleton. This de novo strategy is conducted under mild conditions with no need of preactivation. In addition, it allows for efficient preparation of the target compounds in combination with late-stage functionalization.

Total synthesis of highly oxygenated phomopsol B via acid-induced etherification to construct a bridged structure

The first total synthesis of phomopsol B has been achieved in a racemic form. The synthesis comprises a deacetylative cyclization to construct a bicyclic skeleton followed by primary alcohol-assisted dihydroxylation, ether cyclization to construct a dioxabicyclo [3.2.1] skeleton and γ-lactone formation based on oxidation by TEMPO.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Combretastatins D series and analogues: from isolation, synthetic challenges and biological activities

The combretastatin D series and its analogues, corniculatolides and isocorniculatolides belong to a class of macrocycles called cyclic diaryl ether heptanoids (DAEH). This review is intended to highlight the structure elucidation, biosynthesis, and biological activity of these compounds as well as the use of different strategies for their synthesis.

4-O-Substituted Glucuronic Cyclophellitols are Selective Mechanism-Based Heparanase Inhibitors

Degradation of the extracellular matrix (ECM) supports tissue integrity and homeostasis, but is also a key factor in cancer metastasis. Heparanase (HPSE) is a mammalian ECM-remodeling enzyme with β-D-endo-glucuronidase activity overexpressed in several malignancies, and is thought to facilitate tumor growth and metastasis. By this virtue, HPSE is considered an attractive target for the development of cancer therapies, yet to date no HPSE inhibitors have progressed to the clinic. Here we report on the discovery of glucurono-configured cyclitol derivatives featuring simple substituents at the 4-O-position as irreversible HPSE inhibitors. We show that these compounds, unlike glucurono-cyclophellitol, are selective for HPSE over β-D-exo-glucuronidase (GUSB), also in platelet lysate. The observed selectivity is induced by steric and electrostatic interactions of the substituents at the 4-O-position. Crystallographic analysis supports this rationale for HPSE selectivity, and computer simulations provide insights in the conformational preferences and binding poses of the inhibitors, which we believe are good starting points for the future development of HPSE-targeting antimetastatic cancer drugs.

Chemical Synthesis of a Colanic Acid Hexasaccharide

Chemical synthesis of a hexasaccharide relevant to colanic acid, which bears a high prevalence of polyanionic and acetyl decoration, has been achieved, highlighting stereoselective glycosylation and effective installation of O-acetyl and pyruvate residues.

Facile access to 4'-(N-acylsulfonamide) modified nucleosides and evaluation of their inhibitory activity against SARS-CoV-2 RNA cap N7-guanine-methyltransferase nsp14

N-Acylsulfonamides possess an additional carbonyl function compared to their sulfonamide analogues. Due to their unique physico-chemical properties, interest in molecules containing the N-acylsulfonamide moiety and especially nucleoside derivatives is growing in the field of medicinal chemistry. The recent renewal of interest in antiviral drugs derived from nucleosides containing a sulfonamide function has led us to evaluate the therapeutic potential of N-acylsulfonamide analogues. While these compounds are usually obtained by a difficult acylation of sulfonamides, we report here the easy and efficient synthesis of 20 4'-(N-acylsulfonamide) adenosine derivatives via the sulfo-click reaction. The target compounds were obtained from thioacid and sulfonyl azide synthons in excellent yields and were evaluated as potential inhibitors of the SARS-CoV-2 RNA cap N7-guanine-methyltransferase nsp14.

Metal vs. Metal-Free Catalysts for Oxidation of 5-Hydroxymethylfurfural and Levoglucosenone to Biosourced Chemicals

Lignocellulosic feedstocks, such as forestry biomass and agricultural crop residues, can be utilized to generate biofuels and biochemicals. Converting these organic waste materials into biochemicals is widely regarded as a remedial approach to develop a sustainable, clean, and green energy source. Nevertheless, are these methods sustainable and clean? Prior studies have shown that most such conversions use metals - including heavy metals or noble metals - as catalysts. In addition to the fact that many metals (e. g., aluminum, cobalt, titanium, platinum) have been listed as critical minerals, these methods suffer from high cost, deactivation, and leakage problems and the release of toxic wastes. This Review summarizes catalytic methods using metal and metal-free catalysts for the oxidation of the platform molecules 5-hydroxymethylfurfural and levoglucosenone and demonstrates the potential and effectiveness of metal-free catalysts.

Preparation of glycosyl carboxylic acids via stereoselective synthesis and oxidative cleavage of C-vinyl glycosides

We have developed an improved cyanide-free strategy for the synthesis of glycosyl carboxylic acids, employing stereoselective C-vinyl glycosylation and oxidative cleavage of C-vinyl glycosides as key steps. Compared to our previous work, the amount of NaIO₄ required for the oxidative cleavage step is reduced significantly from 18 equivalents to 4.5 equivalents. This modification not only is advantageous in terms of operation and costs but also avoids the over-oxidation problem, thus greatly expanding the substrate scope, which is evidenced by the fact that 10 out of 21 glycosyl carboxylic acids synthesized are undocumented. With differently O5-protected furanosyl acids in hand, we demonstrate that an electron-rich protecting group is beneficial for the decarboxylative arylation of furanosyl carboxylic acids. This represents a rare example of protecting groups affecting the reaction efficiency in radical C-glycosylation. As C-vinyl glycosides can be prepared stereoselectively and the oxidative step is stereoretentive, the approach provides an effective means to access 1,2-trans or 1,2-cis glycosyl acids, which would be a valuable alternative to the cyanide-based synthesis of glycosyl carboxylic acids.

Structure-Activity Relationships for 5''-Modifications of 4,5-Aminoglycoside Antibiotics

Modification at the 5''-position of 4,5-disubstituted aminoglycoside antibiotics (AGAs) to circumvent inactivation by the APH(3',5'') class of aminoglycoside modifying enzymes (AMEs) has been widely reported.  Such modifications, however, impact activity against wild type bacteria and affect target selectivity in unpredictable ways thereby hindering drug development.  We present a systematic survey of modifications to the 5''-position of the 4,5-AGAs and of the related 5- O -furanosyl apramycin derivatives. In the neomycin and the apralog series, all modifications were well-tolerated, but other 4,5-AGAs require the presence of a hydrogen bonding group at the 5''-position for maintenance of high antibacterial activity. Though the 5''-amino modification resulted in comparable activity to the parent compounds, reduced selectivity against the human cytosolic decoding A site renders this modification generally unfavorable in paromomycin, propylamycin, and ribostamycin. Installation of a 5''-formamido group and, to a lesser degree, a 5''-ureido group resulted in comparable activity to the parents without the selectivity cost of the 5''-amino modification. The lessons learned from this work will aid in the design of next-generation AGAs capable of circumventing susceptibility to AMEs while maintaining high antibacterial activity and target selectivity.

Synthesis and self-assembly of cytidine- and guanosine-based copolymers

The base pairing property and the “melting” behavior of oligonucleotides can take advantage to develop new smart thermoresponsive and programmable materials.

Scalable synthesis and structural characterization of reversible KLK6 inhibitors

Scalable asymmetric syntheses of two kallikrein-related protease 6 (KLK6) inhibitors are reported. The inhibitors are assembled by linking enantiomerically enriched fragments via amide bond formation, followed by conversion of a cyano group to an amidine. One fragment, an amine, was prepared using the Ellman auxiliary, and a lack of clarity in the literature regarding the stereochemical outcome of this reaction was solved via X-ray crystallographic analysis of two derivatives. Complexes of the inhibitors bound to human KLK6 were solved by X-ray crystallography, revealing the binding poses.

We report scalable syntheses of two potent and selective kallikrein related peptidase 6 (KLK6) inhibitors, as well as X-ray crystal structures of both inhibitors as protein-ligand complexes.

Total Synthesis of Euonymine and Euonyminol Octaacetate

Euonymine (1) and euonyminol octaacetate (2) share the core structure of euonyminol (3), the most hydroxylated member of the dihydro-β-agarofuran family. In 2, eight of the nine hydroxy groups of 3 are acetylated, and 1 has six acetyl groups and a 14-membered bislactone comprising a pyridine dicarboxylic acid with two methyl groups. The different acylation patterns provide distinct biological activities: 1 and 2 display anti-HIV and P-glycoprotein inhibitory effects, respectively. The 11 contiguous stereocenters and 9 oxygen functionalities of the ABC-ring system of 1 and 2 represent a formidable challenge, which is further heightened by the macrocyclic structure of 1. Here we disclose an efficient synthetic strategy for enantioselective total synthesis of 1 and 2. Starting from (R)-glycerol acetonide, we constructed the B-ring by an Et₃N-accelerated Diels-Alder reaction, the C-ring by intramolecular iodoetherification, and the A-ring by ring-closing olefin metathesis. The 10 stereocenters were installed through a series of substrate-controlled stereoselective C-C and C-O bond formations by exploiting the three-dimensional structures of judiciously designed substrates. These newly developed reaction sequences led to protected euonyminol 5, which served as a common intermediate for assembling 1 and 2. Global deprotection of 5 and subsequent acetylation produced 2. Alternatively, the discriminative protective groups of 5 allowed for site-selective bis-esterification to generate bislactone. Combining [3 + 2]-cycloaddition and reductive desulfurization introduced the last remaining stereocenters of the two methyl groups on the macrocycle. Finally, deprotection and acetylation gave rise to fully synthetic 1 for the first time.

Chemical Modification of Phage-Displayed Helix-Loop-Helix Peptides to Construct Kinase-Focused Libraries

Conformationally constrained peptides hold promise as molecular tools in chemical biology and as a new modality in drug discovery. The construction and screening of a target-focused library could be a promising approach for the generation of de novo ligands or inhibitors against target proteins. Here, we have prepared a protein kinase-focused library by chemically modifying helix-loop-helix (HLH) peptides displayed on phage and subsequently tethered to adenosine. The library was screened against aurora kinase A (AurA). The selected HLH peptide Bip-3 retained the α-helical structure and bound to AurA with a KD value of 13.7 μM. Bip-3 and the adenosine-tethered peptide Bip-3-Adc provided IC50 values of 103 μM and 7.7 μM, respectively, suggesting that Bip-3-Adc bivalently inhibited AurA. In addition, the selectivity of Bip-3-Adc to several protein kinases was tested, and was highest against AurA. These results demonstrate that chemical modification can enable the construction of a kinase-focused library of phage-displayed HLH peptides.

An Update on Pharmacological Relevance and Chemical Synthesis of Natural Products and Derivatives with Anti SARS-CoV-2 Activity

Natural products recognized traditionally as a vital source of active constituents in pharmacotherapy. The COVID-19 infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is highly transmissible, pathogenic, and considered an ongoing global health emergency. The emergence of COVID-19 globally and the lack of adequate treatment brought attention towards herbal medicines, and scientists across the globe instigated the search for novel drugs from medicinal plants and natural products to tackle this deadly virus. The natural products rich in scaffold diversity and structural complexity are an excellent source for antiviral drug discovery. Recently the investigation of several natural products and their synthetic derivatives resulted in the identification of promising anti SARS-CoV-2 agents. This review article will highlight the pharmacological relevance and chemical synthesis of the recently discovered natural product and their synthetic analogs as SARS-CoV-2 inhibitors. The summarized information will pave the path for the natural product-based drug discovery of safe and potent antiviral agents, particularly against SARS-CoV-2.

Concise syntheses and anti-inflammatory effects of isocorniculatolide B and corniculatolide B and C

The first total syntheses of isocorniculatolide B, corniculatolide B, and corniculatolide C, consisting of isomeric corniculatolide skeletons, have been accomplished in a divergent manner. The key features of the synthesis involve the construction of diaryl ether linkages by nucleophilic aromatic substitution, installation of a C14-substituted alkyl side chain via a sequence of Baeyer-Villiger reaction and Claisen rearrangement, and efficient construction of corniculatolide and isocorniculatolide frameworks, including 17-membered (exterior) macrolactone skeletons from a versatile diaryl ether intermediate by Mitsunobu macrolactonization. Moreover, we prepared the structural congeners of isomeric corniculatolides via diverted total synthesis approach including desmethyl analogues and related dimeric macrolides. The anti-inflammatory activities of the synthesized natural products, analogues and synthetic intermediates were also investigated. In particular, corniculatolide B significantly inhibited the protein expression of COX-2 and the mRNA expressions of TNF-α, IL-1β and IL-6 by inhibiting of NF-κB signaling in intestinal epithelial cells induced by lipopolysaccharide treatment. It also significantly inhibited the promoter activity and the phosphorylation of subunits p50 and p65 of NF-κB to the same extent as Bay 11-7082, a potent IκB kinase inhibitor. These results suggest that corniculatolide B might have therapeutic potential in inflammatory bowel disease via NF-κB signaling pathway.

The synthesis of the very long chain polyunsaturated fatty acid (VLC-PUFA) 32:6 n-3

This article describes the synthesis of VLC-PUFA 32:6 n-3, D2-labeled 32:6 n-3, and the uptake of 32:6 n-3 into mouse retinal tissue.

Double-headed nucleosides: Synthesis and applications

Double-headed nucleoside monomers have immense applications for studying secondary nucleic acid structures. They are also well-known as antimicrobial agents. This review article accounts for the synthetic methodologies and the biological applications of double-headed nucleosides.

Intramolecular Photo-Oxidation as a Potential Source to Probe Biological Electron Damage: A Carboxylated Adenosine Analogue as Case Study

A carboxylated adenosine analog (C-Ado-) has been synthesized and probed via time-resolved photoelectron spectroscopy in order to induce intra-molecular charge transfer from the carboxylic acid moiety to the nucleobase. Intra-molecular charge transfer can be exploited as starting point to probe low-energy electron (LEE) damage in DNA and its derivatives. Time-dependent density functional theory (TD-DFT) calculations at the B3LYP-6311G level of theory have been performed to verify that the highest occupied molecular orbital (HOMO) was located on carboxylic acid and that the lowest occupied molecular orbital (LUMO) was on the nucleobase. Hence, the carboxylic acid could work as electron source, whilst the nucleobase could serve the purpose of electron acceptor. The dynamics following excitation at 4.66 eV (266 nm) were probed using time-resolved photoelectron spectroscopy using probes at 1.55 eV (800 nm) and 3.10 eV (400 nm). The data show rapid decay of the excited state population and, based on the similarity of the overall dynamics to deoxy-adenosine monophosphate (dAMP-), it appears that the dominant decay mechanism is internal conversion following 1ππ* excitation of the nucleobase, rather than charge-transfer from the carboxylic acid to the nucleobase.

Total Synthesis and Structural Reassignment of Laingolide A

The asymmetric total synthesis of four diastereomers of laingolide A was achieved, which led to the unambiguous assignment of the stereochemistry of the natural product. The salient features of the convergent, fully stereocontrolled approach were a copper-catalysed stereospecific Kumada-type coupling, a Julia-Kocienski olefination and an RCM/alkene migration sequence to access the desired macrocyclic enamide.

Turn-on mode diarylethenes for bioconjugation and fluorescence microscopy of cellular structures

In superresolution fluorescence microscopy, employing synthetic dyes that can be reversibly photoswitched between a nonfluorescent (“dark”) and a fluorescent (“bright”) state has been an attractive alternative to using photoswitchable fluorescent proteins. However, employing such synthetic dyes has been elusive because they have defied reliable attachment to proteins and required UV light for photoswitching. Here we prepared “turn-on mode” fluorescent diarylethenes (fDAEs) that are switchable with visible rather than UV light and blink between a bright fluorescent and a dark state in aqueous buffers. Moreover, our thienyl-substituted fDAEs effectively labeled two thiol groups on nanobodies bearing a single maleimide tag. With these small-sized probes, we acquired superresolution images of vimentin filaments in cells by applying just yellow (561 nm) light.

The use of photoswitchable fluorescent diarylethenes (fDAEs) as protein labels in fluorescence microscopy and nanoscopy has been limited by labeling inhomogeneity and the need for ultraviolet light for fluorescence activation (on-switching). To overcome these drawbacks, we prepared “turn-on mode” fDAEs featuring thienyl substituents, multiple polar residues, and a reactive maleimide group in the core structure. Conjugates with antibodies and nanobodies displayed complete on-switching and excitation with violet (405 nm) and yellow-green (<565 nm) light, respectively. Besides, they afforded high signal-to-noise ratios and low unspecific labeling in fluorescence imaging. Irradiation with visible light at 532 nm or 561 nm led to transient on-off switching (“blinking”) of the fDAEs of double-labeled nanobodies so that nanoscale superresolution images were readily attained through switching and localization of individual fluorophores.

Enantioselective Synthesis Muqubilin and Negombatoperoxides B and C/D

Muqubilin, negombatoperoxide B, and negombatoperoxide C/D were synthesized through enantioselective routes, with the quaternary center derived from a peroxy chiral building block of known absolute configuration. The C-2/C-3 stereogenic centers were introduced by asymmetric aldol condensation, and the 1,2-dioxane ring was constructed via an intramolecular alkylation of a hydroperoxide with a mesylate. The synthetic samples showed physical and spectroscopic data consistent with those reported in the literature and thus verified the configurations of the natural products. A potentially more expeditious enantioselective entry to the 1,2-dioxane-aldol moiety (C-1 to C-6) of such cyclic peroxides was also briefly explored, where the C-2/C-3 stereogenic centers were installed through a [2+2] cycloaddition and the 1,2-dioxane ring was closed via an intramolecular alkylation coupled with an alkyl-oxygen cleavage of a β-lactone.

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Predictive Analysis of the Side Chain Conformation of the Higher Carbon Sugars: Application to the Preorganization of the Aminoglycoside Ring 1 Side Chain for Binding to the Bacterial Ribosomal Decoding A Site

With a view to facilitating prediction of the exocyclic bond to the pyranoside ring in higher carbon sugars, a model is advanced that relates the relative configuration of the three stereogenic centers comprised of the branchpoint and of the two flanking centers (C4-C5-C6 in aldoheptoses and higher and C5-C6-C7 in sialic and ulosonic acids) to that of the simple ring-opened pentoses. Assignment of a given stereotriad as arabino, lxyo, ribo, or xylo by inspection of the Fischer projection formulas permits prediction of conformation of the exocyclic bond by comparison with the known solution (= crystal in all cases) conformations of the simple pentitols. More remote stereogenic centers in the side chain, as in the 8-position of N-acetylneuraminic acid, have little impact on the conformation of the exocyclic bond. On the basis of this model the conformation of the exocyclic bond in ring I of 6'-homologated 4,5-disubstituted 2-deoxystreptamine class aminoglycoside antibiotics was predicted and was borne out by NMR analysis of newly synthesized derivatives in D2O at pD5. The antiribosomal and antibacterial activity of these derivatives is briefly presented and discussed in terms of preorganization of the side chain for binding to the ribosomal decoding A site. It is anticipated that this predictive analysis will also find use in the prediction of the conformation of the exocyclic bonds in other 2-(1-hydroxyalkyl)-3-hydroxytetrahydropyrans and tetrahydrofurans.

Chemical Synthesis of Fucosylated Chondroitin Sulfate Oligosaccharides

Fucosylated chondroitin sulfates (FuCSs) are a unique type of polysaccharides occurring in sea cucumber that show a variety of biological activities. In particular, well-defined FuCS oligosaccharides, consisting of a trisaccharide repeating unit of β-d-GalNAc(4,6-diS)-(1→4)-[α-l-Fuc(2,4-diS)-(1→3)]-β-d-GlcUA, display potent anticoagulant activity via selective inhibition of the intrinsic tenase, which could be developed into anticoagulant drugs without bleeding risk. Herein, we report an effective approach to the synthesis of FuCS oligosaccharides, as demonstrated by the successful elaboration of FuCS tri-, hexa-, and nonasaccharides. The syntheses employ an orthogonally protected trisaccharide as a pivotal building block that can be readily converted into the donor and acceptor for glycosidic coupling. In addition, the internal patterns of protecting groups, involving N-trichloroacetyl for N-acetyl group, benzylidene and benzyl groups for sulfonated hydroxyl groups, and benzoyl and methyl esters for free hydroxyl and carboxylic acid, respectively, ensure stereoselective formation of the glycosidic linkages and sequential transformation into the desired FuCS oligosaccharides.

Anti-Tick-Borne Encephalitis Virus Activity of Novel Uridine Glycoconjugates Containing Amide or/and 1,2,3-Triazole Moiety in the Linker Structure

Tick-borne encephalitis virus (TBEV) transmitted by ticks is a pathogen of great medical importance. As still no effective antiviral treatment is available, in the present study, a series of uridine glycoconjugates containing amide or/and 1,2,3-triazole moiety in the linker structure was synthesized and evaluated for the antiviral activity against two strains of TBEV: a highly virulent Hypr strain and less virulent Neudoerfl strain, using standardized previously in vitro assays. Our data have shown that four compounds from the series (18–21) possess strong activity against both TBEV strains. The half maximal inhibitory concentration (IC₅₀) values of compounds 18–21 were between 15.1 and 3.7 μM depending on the virus strain, which along with low cytotoxicity resulted in high values of the selectivity index (SI). The obtained results suggest that these compounds may be promising candidates for further development of new therapies against flaviviruses.

The Sulfo-Click Reaction and Dual Labeling of Nucleosides

This article contains detailed synthetic procedures for the implementation of the sulfo-click reaction to nucleoside derivatives. First, 3'-O-TBDMS-protected nucleosides are converted to their corresponding 4'-thioacid derivatives in three steps. Then, various conjugates are synthetized via a biocompatible and chemoselective coupling procedure using sulfonyl azide partners. Finally, to illustrate the potential of the sulfo-click reaction, a nucleoside bearing two orthogonal azido groups is synthesized and engaged in one-pot dual labeling through a sulfo-click/copper-catalyzed azide-alkyne cycloaddition (CuAAC) cascade. The high efficiency of the sulfo-click reaction as applied to nucleosides opens up new possibilities in the context of bioconjugation. © 2020 Wiley Periodicals LLC. Basic Protocol 1: General protocol for the synthesis of 4'-thioacid-nucleoside derivatives Basic Protocol 2: Implementation of the sulfo-click reaction Basic Protocol 3: Synthesis of 3'-azido-4'-(carboxamido)ethane-sulfonyl azide-3'-deoxythymidine Basic Protocol 4: Detailed synthetic procedure for one-pot double-click conjugations.

Impact of C-Terminal Chemistry on Self-Assembled Morphology of Guanosine Containing Nucleopeptides

Herein, we report the design and characterization of guanosine-containing self-assembling nucleopeptides that form nanosheets and nanofibers. Through spectroscopy and microscopy analysis, we propose that the peptide component of the nucleopeptide drives the assembly into β-sheet structures with hydrogen-bonded guanosine forming additional secondary structures cooperatively within the peptide framework. Interestingly, the distinct supramolecular morphologies are driven not by metal cation responsiveness common to guanine-based materials, but by the C-terminal peptide chemistry. This work highlights the structural diversity of self-assembling nucleopeptides and will help advance the development of applications for these supramolecular guanosine-containing nucleopeptides.

Synthesis of Reverse Glycosyl Fluorides and Rare Glycosyl Fluorides Enabled by Radical Decarboxylative Fluorination of Uronic Acids

An efficient protocol for synthesizing reverse glycosyl fluorides is described, relying on silver-promoted decarboxylative fluorination of structurally diverse pentofuran- and hexopyranuronic acids under the mild reaction conditions. The potential applications of the reaction are further demonstrated by converting readily available d-uronic acid derivatives into uncommon d-/l-glycosyl fluorides through a C1-to-C5 switch strategy. The reaction mechanism is corroborated by 5-exo-trig radical cyclization of allyl α-d-C-glucopyranuronic acid triggered by decarboxylative fluorination.

Design and synthesis of dual active neovibsanin derivatives based on a chemical structure merging method

A hybrid compound consisting of neovibsanin and trans-banglene was designed according to a structure merging method and synthesized via a sequence of key steps including a Diels-Alder cycloaddition, stereoselective alkynylation, and intramolecular oxa-Michael addition reaction. The biological activity of the synthetized acetal compound and its hemiacetal analogue was investigated in PC12 cells. These studies revealed that the designed hybrid compounds displayed neuritogenic activity. Furthermore, a relatively strong neurite outgrowth promoting activity was observed in the presence of NGF. These results suggest that the designed hybrid compound exhibited a dual activity.

Total syntheses of spiroviolene and spirograterpene A: a structural reassignment with biosynthetic implications

The recent natural product isolates spiroviolene and spirograterpene A are two relatively non-functionalized linear triquinane terpenes with a large number of structural homologies. Nevertheless, three significant areas of structural disparity exist based on their original assignments, one of which implies a key stereochemical divergence early in their respective biosyntheses. Herein, using two known bicyclic ketone intermediates, a core Pd-catalyzed Heck cyclization sequence, and several chemoselective transformations, we describe concise total syntheses of both natural product targets and propose that the structure of spiroviolene should be reassigned. As a result, these natural products possess greater homology than previously anticipated.