Cu-Catalyzed Azide−Alkyne Cycloaddition

Degrading the key component of the inflammasome: development of an NLRP3 PROTAC

This study explores PROTACs for NLRP3, the key player in innate immunity. We utilised a thiophene analogue of the NLRP3 inhibitor MCC950 and employed CuAAC chemistry for the assembly of PROTACs bearing various linkers and recruiting three different E3 ligases. Compounds were evaluated in bidirectional thermal stability studies with NLRP3 and E3 ligases. IL-1β release and protein abundance of NLRP3 were assessed in cellular assays. PROTAC V2 induces a significant VHL-dependent degradation of NLRP3 and constitutes a valuable tool to decipher the intricate details of the NLRP3 inflammasome.

Leveraging N-exo substituents to tune the donor/acceptor properties of mesoionic imines (MIIs)

In this work, we show two synthetic routes to substitute the Nexo position of mesoionic imines (MIIs). By Buchwald-Hartwig amination, 5-amino-1,2,3-triazoles can be arylated at the said position, showing the versatility of amino-triazoles as building blocks for MIIs. The reaction of MIIs with electrophiles (MeI, fluoro-arenes) highlights the nucleophilic nature of MIIs as even at room temperature aromatic C-F bonds can be activated with MIIs. By combining experimental methods such as Tolman/Huynh-electronic-parameter and crystallographic interpretations with theoretical calculations, we establish that MIIs expand the nucleophilicity scale of N-donors. Contrary to the flanking substituents on the triazole scaffold, the Nexo substituent heavily influences the donating ability of MIIs: electron-withdrawing substituents will dramatically decrease the donor strength of the MII ligand. We have now established ways to functionalise not only the triazole backbone but also the Nexo position. More importantly, we show here how the substitution pattern influences the electronic structure of MIIs. Such electronic tunability should make MIIs suitable for use in various fields of chemistry.

Xanthine Nucleosides with Pyrazolo[3,4-d]pyrimidine Skeleton: Functionalization with Halogen Atoms, Clickable Side Chains, Pyrene, and iEDDA Cycloadducts, and Impact of Ionic Forms on Photophysical Properties

Xanthine nucleosides play a significant role in the expansion of the four-letter genetic code. Herein, 7-functionalized 8-aza-7-deazaxanthine ribo- and 2'-deoxyribonucleosides are described. 2-Amino-6-alkoxy nucleosides were converted to halogenated 8-aza-7-deazaxanthine nucleosides by deamination followed by hydroxy/alkoxy substitution. 8-Aza-7-deaza-7-iodo-2'-deoxyxanthosine served as an intermediate in Suzuki-Miyaura, Sonogashira, and Stille reactions. Alkynyl and vinyl side chains as well as fluorescent tags were introduced. Pyrene conjugates were derived by copper(I)-catalyzed cycloaddition. Inverse-electron-demand Diels-Alder reaction of 8-aza-7-deaza-7-vinyl-2'-deoxyxanthosine with 3,6-dipyridyl-tetrazine proceeded with a second-order rate constant of 0.042 L M-1 s-1. X-ray analysis of 8-aza-7-deaza-7-vinyl-2'-deoxyxanthosine displayed two conformers with a syn conformation. Crystal packing is stabilized by xanthine base pairs. UV spectroscopy confirmed the sensitivity of 7-functionalized 8-aza-7-deazaxanthine nucleosides to pH changes. Halogen and alkynyl substituents decrease pK values, and vinyl, pyrene, or benzofuran leads to an increase. Fluorescence measurements of 8-aza-7-deaza-7-benzofuran-2'-deoxyxanthosine disclosed solvatochromism and enhanced fluorescence when the pH or the viscosity of the solvent is increased. Nucleoside pyrene conjugates connected by a linear linker displayed monomer emission, and two pyrene residues connected by a dendritic linker exhibited excimer emission. According to their fluorescence properties and sensitivity to pH changes, the functionalized 8-aza-7-deazaxanthine nucleosides expand the class of nucleosides applicable to fluorescence detection for diagnostic and therapeutic purposes.

An azide-based sampler for monitoring abiotic reduction of chlorinated solvent contaminants in groundwater

There is significant interest in monitoring abiotic decomposition of chlorinated solvents at contaminated sites due to large uncertainties regarding the rates of abiotic attenuation of trichloroethylene (TCE) and perchloroethylene (PCE) under field conditions. In this study, an innovative passive sampling tool was developed to quantify acetylene, a characteristic product of abiotic reduction of TCE or PCE, in groundwater. The sampling mechanism is based on the highly specific and facile click reaction between acetylene and an azide compound to form a biologically and chemically stable triazole product. In preliminary investigations using static solutions of acetylene, a strong linear relationship was observed between the quantity of azide formed and the concentration of aqueous acetylene. The product yield was stable within a solution pH range of 5-9 and in the presence of common groundwater salts or gaseous hydrocarbon molecules. Based on these results, a prototypical acetylene passive sampler was constructed, which featured a piece of PDMS membrane embedded with an azide compound and Cu(0) catalyst to form a reactive phase for acetylene capture. The sampler was evaluated in microcosm reactors containing TCE and an abiotic reductant (viz., sulfur-treated iron powder). Acetylene produced during abiotic dechlorination of TCE was effectively captured by the sampler, and the quantity of the product recovered from the sampler correlates with the amount of TCE converted. The specificity and sensitivity of the proposed acetylene sampler proffers a new diagnostic tool for monitoring abiotic degradation of chlorinated ethenes at complex contamination sites.

Droplet-Based EPR Spectroscopy for Real-Time Monitoring of Liquid-Phase Catalytic Reactions

In situ monitoring is essential for catalytic process design, offering real-time insights into active structures and reactive intermediates. Electron paramagnetic resonance (EPR) spectroscopy excels at probing geometric and electronic properties of paramagnetic species during reactions. Yet, state-of-the-art liquid-phase EPR methods, like flat cells, require custom resonators, consume large amounts of reagents, and are unsuited for tracking initial kinetics or use with solid catalysts. To overcome these limitations, a droplet-based microfluidics platform is introduced for real-time EPR monitoring of liquid-phase catalytic reactions. By encapsulating solid and dissolved species within nanoliter droplets, this approach enables precise control over mass transport, reduces reagent consumption, and maintains uniform residence times irrespective of acquisition duration, permitting precise analysis of each spectral component under identical conditions. The platform's compatibility with standard resonators facilitates straightforward integration into any EPR spectrometer. Its versatility is demonstrated by monitoring dynamic ligand exchange processes, key for activating homogeneous catalysts, and tracking redox and radical kinetics in ascorbic acid oxidation by Cu(II) catalysts. Importantly, this method captures both supported and dissolved transition metal species, offering comprehensive insights into catalyst deactivation via metal leaching. This microfluidic approach sets a new standard for liquid-phase in situ EPR measurements, advancing studies of homogeneous and heterogeneous catalytic systems.

Polyanionic Receptors for Carboxylates in Water

Receptors for carboxylate anions have many possible biomedical applications, including mimicry of the vancomycin group of antibiotics. However, binding carboxylates in water, the biological solvent, is highly challenging due to the hydrophilicity of these polar anions. Here we report, for the first time, the recognition of simple carboxylates such as acetate and formate in water by synthetic receptors with charge-neutral binding sites. The receptors are solubilised by polyanionic side-chains which, remarkably, do not preclude anion binding. The tricyclic structures feature two identical binding sites linked by polyaromatic bridges, capable of folding into closed, twisted conformations. This folding is hypothesised to preorganise the structures for anion recognition, mimicking the process which generates many protein binding sites. The architecture is suitable for elaboration into enclosed structures with potential for selective recognition of biologically relevant carboxylates.

Sequence-selective pulldown of recognition-encoded melamine oligomers using covalent capture on a solid support

The covalent capture of recognition-encoded melamine oligomers (REMO) with a target attached to a solid support was investigated. Sequence-selective pulldown of complementary oligomers was observed when the target was challenged with a randomised library of oligomers. The approach provides an affinity selection method for the discovery of functional REMO sequences.

Chemoselective, regioselective, and positionally selective fluorogenic stapling of unprotected peptides for cellular uptake and direct cell imaging

Peptide stapling reactions represent powerful methods for structuring native α-helices to improve their bioactivity in targeting protein-protein interactions (PPIs). In light of a growing need for regio- and positionally selective stapling methods involving natural amino acid residues in their unprotected states, we report a rapid, mild, and highly chemoselective three-component stapling reation using a class of molecular linchpins based on 2-arylketobenzaldehydes (ArKBCHOs) that create a fluorescent staple, hereafter referred to as a Fluorescent Isoindole Crosslink (FlICk). This methodology offers positional selectivity favouring i, i + 4 helical staples comprising a lysine and cysteine, in the presence of competing nucleophiles on unprotected peptides. In our efforts to further validate this chemistry, we have successfully shown in vitro cytotoxicity of a FlICk-ed peptide (IC50 = 5.10 ± 1.27 μM), equipotent to an olefin-stapled congener. In harnessing the innate fluorescence of the thiol-isoindole, we report new blue-green fluorophores, which arise as a consequence of stapling, with appreciable quantum yields that enable direct cellular imaging in the assessment of cell permeability, thus bridging therapeutic potential with cytological probe development.

In-situ activation of biomimetic single-site bioorthogonal nanozyme for tumor-specific combination therapy

Copper-catalyzed click chemistry offers creative strategies for activation of therapeutics without disrupting biological processes. Despite tremendous efforts, current copper catalysts face fundamental challenges in achieving high efficiency, atom economy, and tissue-specific selectivity. Herein, we develop a facile "mix-and-match synthetic strategy" to fabricate a biomimetic single-site copper-bipyridine-based cerium metal-organic framework (Cu/Ce-MOF@M) for efficient and tumor cell-specific bioorthogonal catalysis. This elegant methodology achieves isolated single-Cu-site within the MOF architecture, resulting in exceptionally high catalytic performance. Cu/Ce-MOF@M favors a 32.1-fold higher catalytic activity than the widely used MOF-supported copper nanoparticles at single-particle level, as first evidenced by single-molecule fluorescence microscopy. Furthermore, with cancer cell-membrane camouflage, Cu/Ce-MOF@M demonstrates preferential tropism for its parent cells. Simultaneously, the single-site CuII species within Cu/Ce-MOF@M are reduced by upregulated glutathione in cancerous cells to CuI for catalyzing the click reaction, enabling homotypic cancer cell-activated in situ drug synthesis. Additionally, Cu/Ce-MOF@M exhibits oxidase and peroxidase mimicking activities, further enhancing catalytic cancer therapy. This study guides the reasonable design of highly active heterogeneous transition-metal catalysts for targeted bioorthogonal reactions.

High-Throughput Approaches to Engineer Fluorescent Nanosensors

Optical sensors are powerful tools to identify and image (biological) molecules. Because of their optoelectronic properties, nanomaterials are often used as building blocks. To transduce the chemical interaction with the analyte into an optical signal, the interplay between surface chemistry and nanomaterial photophysics has to be optimized. Understanding these aspects promises major opportunities for tailored sensors with optimal performance. However, this requires methods to create and explore the many chemical permutations. Indeed, many current approaches are limited in throughput. This affects the chemical design space that can be studied, the application of machine learning approaches as well as fundamental mechanistic understanding. Here, an overview of selection-limited and synthesis-limited approaches is provided to create and identify molecular nanosensors. Bottlenecks are discussed and opportunities of non-classical recognition strategies are highlighted such as corona phase molecular recognition as well as the requirements for high throughput and scalability. Fluorescent carbon nanotubes are powerful building blocks for sensors and their huge chemical design space makes them an ideal platform for high throughput approaches. Therefore, they are the focus of this article, but the insights are transferable to any nanosensor system. Overall, this perspective aims to provide a fresh perspective to overcome current challenges in the nanosensor field.

Synthesis of 1,2,3-Triazole-Methyl-Menadione Derivatives: Evaluation of Electrochemical and Antiparasitic Properties against two Blood-Dwelling Parasites

This study explores the synthesis and evaluation of novel 1,2,3-triazole-methyl-1,4-naphthoquinone hybrids, focusing on their electrochemical properties and antiparasitic efficacies against two human blood-dwelling parasites Plasmodium falciparum and Schistosoma mansoni. Using copper-catalyzed azide-alkyne cycloaddition (CuAAC), a well-established tool in click chemistry, two synthetic routes were assessed to develop α- and β-[triazole-methyl]-menadione derivatives. By optimizing the CuAAC reaction conditions, yields were significantly improved, reaching up to 94 % for key intermediates and resulting in the formation of a library of approximately 30 compounds. Biological evaluation of the compounds in antiparasitic drug assays demonstrated notable antischistosomal potencies, while no significant activity was observed for the same series against P. falciparum parasites. Electrochemical and 'benzylic' oxidation studies confirmed that the active 'benzoyl' metabolite responsible for the antiplasmodial activity of plasmodione cannot be generated. These findings highlight the potential of triazole-linked menadione hybrids as promising early candidates for antischistosomal drug development, and provides insights into structure-activity relationships crucial for future therapeutic strategies.

Red-light Emitting Orthogonally Trireactive Gold Nanoclusters for the Synthesis of Multifunctionalized Nanomaterials

For the development of highly multifunctionalized nanomaterials, the introduction of functional molecules on gold nanoclusters containing thiols preinstalled with connecting groupsconstitutes a promising approach. However, the uniform introduction of multiple connecting groups while avoiding side reactions is a challenging task. Herein, the synthesis of gold nanoclusters (ca. 1 nm) coated with thiol peptides bearing azido, amino, and aminooxy groups is reported. These nanoclusters emit red-light before and after the functionalization, enabling application to cell imaging. A detailed structure analysis using transmission electron microscope and X-ray absorption spectroscopy reveals the formation of Aun(SR)m nanoclusters as promising motifs for red-light emission. The sequential modification of the trireactive nanoclusters with RGD (Arg-Gly-Asp) peptides, metal chelators, and anticancer drugs via [3+2] cycloaddition, oximation, and amidation reactions at each functional group furnish red-light-emissive nanomaterials exhibiting remarkable toxicity against A549 human lung cancer cells. Integration of the multiligation chemistry and gold nanocluster engineering pave the way toward the development of advanced multifunctional nanomaterials for biological applications.

Hydroquinone-treated Cu3(BTC)2: a mixed-valence Cu(I/II) MOF catalyst for efficient cycloadditions

We present mixed-valence Cu(I)1Cu(II)2(BTC)2 [henceforth Cu(I/II)-HKUST-1], post-synthetically prepared via the hydroquinone (H2Q) treatment of Cu(II)3(BTC)2 (also referred to as HKUST-1) and its subsequent catalytic activity. This Cu(I/II)-HKUST-1 exhibits exceptional structural integrity and superior catalytic performance in the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction between phenylacetylene and benzyl azide. These findings highlight the potential of mixed-valence Cu-based MOFs as effective and sustainable heterogeneous catalysts for organic transformations, paving the way for future advancements in MOF-based catalysis.

Germanyl triazoles as a platform for CuAAC diversification and chemoselective orthogonal cross-coupling

We report the synthesis of germanyl triazoles formed via a copper-catalysed azide-alkyne cycloaddition (CuAAC) of germanyl alkynes. The reaction is often high yielding, functional group tolerant, and compatible with complex molecules. The installation of the Ge moiety enables further diversification of the triazole products, including chemoselective transition metal-catalysed cross-coupling reactions using bifunctional boryl/germyl species.

Bifunctional Azido(thio)ureas from an O-Protected 2-Amino-2-deoxy-d-glucopyranose: Synthesis and Structural Analyses

This publication reports a facile and convenient preparation of tri-O-acetyl-glucopyranoses, derived from the corresponding 2-deoxyaminosugar, where the vicinal anomeric and C2 positions are decorated by azido and (thio)ureido groups, respectively. This double functionalization leads to an inherently chiral core incorporating the versatile azido and (thio)ureido linkages prone to further manipulation. The latter also provides a structural element for hydrogen-bonded donor-acceptor (HB-DA) sites, which are of immense value in organocatalytic pursuits. A computation-aided conformational analysis unveils the landscape of available conformers and their relative stability. N-aryl (thio)ureas bearing substituents at ortho positions exist as mixtures of M- and P-atropisomeric conformers.

Stable homogeneous silver(I) catalysts for the highly efficient cycloaddition of azides onto terminal alkynes in water

To develop stable and efficient silver(I) catalysts and evaluate the role of sulfur and phosphorus donor ligands in the catalytic cycloaddition of selected organic azides and alkynes (AAC), a series of mononuclear Ag(I) and heteronuclear Ag(I)-Fe(II) complexes were synthesized. The synthesis utilized phosphine and sulfur donors from the dialkyldithiophosphate and borate families, specifically Na[S2H2B(mt)2]. These complexes are designated as [(dppf)AgSP(S)(OiPr)2] (1), [S2H2B(mt)2Ag(μ-dppf)AgS2H2B(mt)2] (2), [(dppf)Ag(μ-dppf)Ag(dppf)] (3), and [(dppe)AgS2H2B(mt)2] (5). The complexes 1-5 were characterized using a combination of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, as well as IR spectroscopy techniques. In two instances, single-crystal X-ray diffraction analysis was also employed. Complex 3 exhibited high catalytic activity and regioselectivity in the cycloaddition reaction conducted in water at 40 °C.

Copper nanoclusters: emerging photoredox catalysts for organic bond formations

Advancements in fine chemical synthesis and drug discovery continuously demand the development of new and more efficient catalytic systems. In this regard, numerous transition metal-based catalysts have been developed and successfully applied in industrial processes. However, the need for innovative catalyst systems to further enhance the efficiency of chemical transformations and industrial applications persists. Metal nanoclusters (NCs) represent a distinct class of ultra-small nanoparticles (<3 nm) characterized by a precise number of metal atoms coordinated with a defined number of ligands. This structure confers abundant unsaturated active sites and unique electronic and optical properties, setting them apart from conventional nanoparticles or bulk metals. The well-defined structure and monodisperse nature of NCs make them particularly attractive for catalytic applications. Among these, copper-based nanoclusters have emerged as versatile and sustainable catalysts for challenging organic bond-forming reactions. Their unique properties, including natural abundance, accessible oxidation states, diverse ligand architectures, and strong photophysical characteristics, contribute to their growing prominence in this field. In this review, we discuss the photocatalytic activities of Cu-based nanoclusters, focusing on their applications in cross-coupling reactions (C-C and C-N), click reactions, multicomponent couplings, and oxidation reactions.

Impact of Postsynthetic Modification on the Covalent Organic Framework (COF) Structures

Covalent organic frameworks (COFs) have emerged as a versatile class of materials owing to their well-defined crystalline structures and inherent porosity. In the realm of COFs, their appeal lies in their customizable nature, which can be further enhanced by incorporating diverse functionalities. Postsynthetic modifications (PSMs) emerge as a potent strategy, facilitating the introduction of desired functionalities postsynthesis. A significant challenge in PSM pertains to preserving the crystallinity and porosity of the COFs. In this study, we aim to investigate the intricate interplay between PSM strategies and the resulting crystalline and porous structures of the COFs. The investigation delves into the diverse methodologies employed in PSMs, to elucidate their distinct influences on the crystallinity and porosity of the COFs. Through a comprehensive analysis of recent advancements and case studies, the study highlights the intricate relationships among PSM parameters, including reaction conditions, precursor selection, and functional groups, and their impact on the structural features of COFs. By understanding how PSM strategies can fine-tune the crystalline and porous characteristics of COFs, researchers can harness this knowledge to design COFs with tailored properties for specific applications, contributing to the advancement of functional materials in diverse fields. This work not only deepens our understanding of COFs but also provides valuable insights into the broader realm of PSM strategies for other solid materials.

Substrate-Dependent Stereoselective Synthesis of Pyrrolo[3,4-b]Pyridin-5-Ones and Pyridyl Isoindoline-1-Ones Using Bis(benzotriazol-1-yl) Ligand

The documented work highlighted the synthesis of bis(benzotriazol-1-yl) methane derivatives using silicomolybdic acid (SMA) and successfully implemented in the stereoselective synthesis of diverse pyrrolo[3,4-b]pyridin-5-one and pyridyl isoindolinones derivatives in one-pot. The pyridinamide precursor with diverse alkynes furnished Z-selectivity of pyrrolo[3,4-b]pyridin-5-one across exocyclic C=C bond while the various benzamides on treating with 2-ethynyl pyridine afforded (E)-pyridylisoindoline-1-ones as a major isomer. The single-crystal X-ray diffraction provides strong evidence in favor of the existence and orientation of developed compounds. The broad substrate scope, easy accessibility of substrates, high stereoselectivity, scale-up synthesis, and crystal evidence demonstrate the merits of the current decorum.

Azobenzene-Attached (NHC)Gold(I) and (NHC)Copper(I) Complexes as Photoswitchable Catalysts

Photoswitchable (pre)catalysts, N,N'-bis-azobenzene-based (NHC)gold(I) and N,N'-bis-azobenzene-derived (NHC)copper(I) complexes are reported. Trans to cis isomerization of the attached photoswitchable moieties in the Au(I) complex enables four-fold decrement in the rate of oxazoline formation reaction. Whereas the progress of the copper(I) catalyzed, azide-alkyne cycloaddition reaction gets reduced by at least threefold. Alternate exposure to UV and blue light could easily toggle the rate of reactions remotely. The catalytic activity of thermodynamically stable trans-trans isomers is found to be similar to the common N-aryl substituted NHC-Au/Cu(I) complexes. NHC-Au(I) and -Cu(I) compounds bearing (trans)azobenzene moieties were characterized by X-ray diffraction. Photoswitching, recyclability studies, and the metastable isomer's thermal half-life in both complexes were studied via UV-visible spectroscopy. Whereas the extent of photoswitching and concomitant formation of geometrical isomers were investigated by using 1H-NMR spectroscopic study. Calculated percentage buried volumes of the three geometrical isomers show the trend trans-trans Collapse

Key Words

• Azobenzene
• Photochromic (NHC)gold(I) complex
• Photoswitchable (NHC)copper(I) complex
• Photoswitchable catalyst
• Rate-controlled reaction

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MESH Headings Collapse

Grants

• CRG/2021/001247 Science and Engineering Research Board

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Affiliation(s)

Aminul Islam Sk Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India
Ayan Ghosh Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
Kshama Kundu Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
Indukumari Murugan Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India
Pintu K Kundu Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India

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Discovery of A Synthetic Hub for Regio- and Stereoselective Construction of Triazolyl Vinyl Sulfonyl Fluorides

A new sulfonyl fluoride reagent 1-bromobut-3-ene-1,3-disulfonyl difluoride (BEDF) was developed. This unique reagent possesses two clickable functionalities to be used for both azide-alkyne cycloaddition click and SuFEx click reactions. This new reagent was applied for the regioselective construction of a class of novel triazolyl vinyl sulfonyl fluorides in which the C-4 position 1H-1,2,3-triazoles were functionalized with vinyl sulfonyl fluorides of exclusively E-configuration.

Photo-Accelerated Synthesis of Oligo(triazole amide)s

A photo-assisted process is explored for improving the synthesis of oligo(triazole amide)s, which are prepared by solid phase synthesis using a repeated cycle of two reactions: amine-carboxylic acid coupling and copper-catalyzed azide-alkyne cycloaddition (CuAAC). The improvement of the second reaction is investigated herein. A catalytic system involving Cu(II)Cl2, N,N,N',N″,N″-pentamethyldiethylenetriamine (PMDETA) and a titanocene photoinitiator is explored for reducing the reaction time of CuAAC. This catalyst is first tested on a model reaction involving phenylacetylene and ethyl azidoacetate in DMSO. The kinetics of these model experiments are monitored by 1H NMR in the presence of different concentrations of the photoinitiator. It is found that 30 mol% of photoinitiator leads to quantitative reactions in only 8 min. These conditions are then applied to the solid phase synthesis of oligo(triazole amide)s, performed on a glycine-loaded Wang resin. The backbone of the oligomers is constructed using 6-heptynoic acid and 1-amino-11-azido-3,6,9-trioxaundecane as submonomers. Due to slow reagent diffusion, the CuAAC step required more time in the solid phase than in solution. Yet, one hour only is necessary to achieve quantitative CuAAC on the resin, which is twice as fast as previously-reported conditions. Using these optimized conditions, oligo(triazole amide)s of different length are prepared.

Insulated π-Conjugated Azido Scaffolds for Stepwise Functionalization via Huisgen Cycloaddition on Metal Oxide Surfaces

In organic-inorganic hybrid devices, fine interfacial controls by organic components directly affect the device performance. However, fabrication of uniformed interfaces using π-conjugated molecules remains challenging due to facile aggregation by their strong π-π interaction. In this report, a π-conjugated scaffold insulated by covalently linked permethylated α-cyclodextrin moiety with an azido group is synthesized for surface Huisgen cycloaddition on metal oxides. Fourier-transformed infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy confirm the successful immobilization of the insulated azido scaffold on ZnO nanowire array surfaces. Owing to the highly independent immobilization, the scaffold allows rapid and complete conversion of the surface azido group in Huisgen cycloaddition reactions with ethynyl-terminated molecules, as confirmed by FT-IR spectroscopy monitoring. Cyclic voltammetry analysis of modified indium tin oxide substrates shows the positive effects of cyclic insulation toward suppression of intermolecular interaction between molecules introduced by the surface Huisgen cycloaddition reactions. The utility of the scaffold for heterogeneous catalysis is demonstrated in electrocatalytic selective O2 reduction to H2O2 with cobalt(II) chlorin modified fluorine doped tin oxide electrode and photocatalytic H2 generation with iridium(III) dye-sensitized Pt-loaded TiO2 nanoparticle. These results highlight the potential of the insulated azido scaffold for a stepwise functionalization process, enabling precise and well-defined hybrid interfaces.

Recent advances in triazole synthesis via click chemistry and their pharmacological applications: A review

Click chemistry is a flexible method featuring only the most feasible and efficient chemical reactions. The synthesis of 1,2,3-triazole from azides and terminal acetylenes using copper(I) as a catalyst is an extremely powerful reaction due to the extreme dependability, good selectivity, and biocompatibility of the starting materials. Triazole molecules are more than simple passive linkers; through hydrogen bonding and dipole interactions, they rapidly bind with biological targets. Its applications in drug development are expanding, ranging from target-oriented in situ chemistry and combinatorial mechanisms for lead generation to bioconjugation methods to study proteins and DNA. The click chemistry has frequently been used to speed up drug discovery and optimization processes in the past few years. The click chemistry reaction based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a biochemical process with applications in medicinal chemistry and chemical biology. Thus, click reactions are an essential component of the toolkit for medicinal chemistry and help medicinal chemists overcome the barriers in chemical reactions, increase throughput, and improve the standards of compound libraries. The review highlights the recent advancements in the copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry approach for synthesizing biologically important triazole moieties with a greater emphasis on synthesis methodologies and pharmacological applications. Additionally, the triazole-based FDA-approved drugs are also discussed with their mode of action to highlight the importance of the click chemistry approach in synthesizing the bioactive triazole compounds.

Ordered Transfer from 3D-Oriented MOF Superstructures to Polymeric Films: Microfabrication, Enhanced Chemical Stability, and Anisotropic Fluorescent Patterns

Films and patterns of 3D-oriented metal-organic frameworks (MOFs) afford well-ordered pore structures extending across centimeter-scale areas. These macroscopic domains of aligned pores are pivotal to enhance diffusion along specific pathways and orient functional guests. The anisotropic properties emerging from this alignment are beneficial for applications in ion conductivity and photonics. However, the structure of 3D-oriented MOF films and patterns can rapidly degrade under humid and acidic conditions. Thus, more durable 3D-ordered porous systems are desired for practical applications. Here, oriented porous polymer films and patterns are prepared by using heteroepitaxially oriented N3-functionalized MOF films as precursor materials. The film fabrication protocol utilizes an azide-alkyne cycloaddition on the Cu2(AzBPDC)2DABCO MOF. The micropatterning protocol exploits the X-ray sensitivity of azide groups in Cu2(AzBPDC)2DABCO, enabling selective degradation in the irradiated areas. The masked regions of the MOF film retain their N3-functionality, allowing for subsequent cross-linking through azide-alkyne coupling. Subsequent acidic treatment removes the Cu ions from the MOF, yielding porous polymer micro-patterns. The polymer has high chemical stability and shows an anisotropic fluorescent response. The use of 3D-oriented MOF systems as precursors for the fabrication of oriented porous polymers will facilitate the progress of optical components for photonic applications.

Optimization of Reactive Ink Formulation for Controlled Additive Manufacturing of Copolymer Membrane Functionalization

Multifunctional, nanostructured membranes hold immense promise for overcoming permeability-selectivity trade-offs and enhancing membrane durability in challenging molecule separations. Following the fabrication of copolymer membranes, additive manufacturing technologies can introduce reactive inks onto substrates to modify pore wall chemistries. However, large-scale implementation is hindered by a lack of systematic optimization. This study addresses this challenge by elucidating the membrane functionalization mechanisms and optimal manufacturing conditions using a copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. Leveraging a data science toolkit (e.g., nonlinear regression, uncertainty quantification, identifiability analyses, model selection, and design of experiments), we developed two mathematical models: (1) algebraic equations to predict equilibrium concentrations after preparing reactive inks by mixing copper sulfate, ascorbic acid (AA), and an alkynyl-terminated reactant; and (2) reaction-diffusion partial differential equations (PDEs) to describe the functionalization process. The ink preparation chemistry with side reactions was validated through pH and UV-vis measurements, while the diffusion and kinetic parameters in the PDE model were calibrated using time-series conversion of the azide moieties inferred from Fourier-transform infrared spectroscopy. This modeling framework avoids redundant experimental efforts and offers a functionalization protocol for scaling up designs. Ink optimization problems were proposed to reduce the use of expensive and environmentally insulting ink materials, i.e., Cu(II), while ensuring the desired chemical distributions. With optimal ink formulation Cu(II)/AA/alkyne = 1:1:2 identified, we uncovered trade-offs between Cu(II) usage and functionalization time; for example, in continuous roll-to-roll manufacturing with a conserved functionalization bath setup, our optimal operational conditions to achieve ≥90% functionalization enable at least a 20% reduction in total copper investment compared to previous experimental results. The data science-enabled ink optimization framework is extendable for on-demand multifunctional membranes in numerous future applications such as metal recovery from wastewater and brine.

Spot the difference in reactivity: a comprehensive review of site-selective multicomponent conjugation exploiting multi-azide compounds

Going beyond the conventional approach of pairwise conjugation between two molecules, the integration of multiple components onto a central scaffold molecule is essential for the development of high-performance molecular materials with multifunctionality. This approach also facilitates the creation of functionalized molecular probes applicable in diverse fields ranging from pharmaceuticals to polymeric materials. Among the various click functional groups, the azido group stands out as a representative click functional group due to its steric compactness, high reactivity, handling stability, and easy accessibility in the context of multi-azide scaffolds. However, the azido groups in multi-azide scaffolds have not been well exploited for site-specific use in molecular conjugation. In fact, multi-azide compounds have been well used to conjugate to the same multiple fragments. To circumvent problems of promiscuous and random coupling of multiple different fragments to multiple azido positions, it is imperative to distinguish specific azido positions and use them orthogonally for molecular conjugation. This review outlines methods and strategies to exploit specific azide positions for molecular conjugation in the presence of multiple azido groups. Illustrative examples covering di-, tri- and tetraazide click scaffolds are included.

Pd-, Cu-, and Ni-Catalyzed Reactions: A Comprehensive Review of the Efficient Approaches towards the Synthesis of Antibacterial Molecules

A strong synthetic tool for many naturally occurring chemicals, polymers, and pharmaceutical substances is transition metal-catalyzed synthesis. A serious concern to human health is the emergence of bacterial resistance to a broad spectrum of antibacterial medications. The synthesis of chemical molecules that are potential antibacterial candidates is underway. The main contributions to medicine are found to be effective in transition metal catalysis and heterocyclic chemistry. This review underlines the use of heterocycles and certain effective transition metals (Pd, Cu, and Ni) as catalysts in chemical methods for the synthesis of antibacterial compounds. Pharmaceutical chemists might opt for clinical exploration of these techniques due to their potential.

Dynamically Cross-linked Oligo[2]rotaxane Networks Mediated by Metal-Coordination

Polyrotaxanes (PRs) have attracted significant research attention due to their unique topological structures and high degrees of conformational freedom. Herein, we take advantage of an oligo[2]rotaxane to construct a novel class of dynamically cross-linked rotaxane network (DCRN) mediated by metal-coordination. The oligo[2]rotaxane skeleton offers several distinct advantages: In addition to retaining the merits of traditional polymer backbones, the ordered intramolecular motion of the [2]rotaxane motifs introduced dangling chains into the network, thereby enhancing the stretchability of the DCRN. Additionally, the dissociation of host-guest recognition and subsequent sliding motion, along with the breakage of metal-coordination interactions, represented an integrated energy dissipation pathway to enhance mechanical properties. Moreover, the resulting DCRN demonstrated responsiveness to multiple stimuli and displayed exceptional self-healing capabilities in a gel state. Upon exposure to PPh3, which induced network deconstruction by breaking the coordinated cross-linking points, the oligo[2]rotaxane could be recovered, showcasing good recyclability. These findings demonstrate the untapped potential of the oligo[2]rotaxane as a polymer skeleton to develop DCRN and open the door to extend their advanced applications in intelligent mechanically interlocked materials.

Synthesis, evaluation and modification of heterofunctional polyester dendrimers with internally queued bromide groups

Heterofunctional polyester dendrimers up to the third generation, containing 21 internally queued bromine atoms, have been successfully synthesized for the first time using a divergent growth approach. Direct azidation reactions enabled the conversion of the bromide groups to clickable azide pendant functionalities. Therapeutic and chemical moeities could then be coupled to the internal azide or bromide functionalities and external hydroxyl groups of the heterofunctional dendrimers through CuAAC, thiol-bromo click and esterification reactions, expanding their potential for biomedical applications.

Glucose-sensitive insulin with attenuation of hypoglycaemia

The risk of inducing hypoglycaemia (low blood glucose) constitutes the main challenge associated with insulin therapy for diabetes1,2. Insulin doses must be adjusted to ensure that blood glucose values are within the normal range, but matching insulin doses to fluctuating glucose levels is difficult because even a slightly higher insulin dose than needed can lead to a hypoglycaemic incidence, which can be anything from uncomfortable to life-threatening. It has therefore been a long-standing goal to engineer a glucose-sensitive insulin that can auto-adjust its bioactivity in a reversible manner according to ambient glucose levels to ultimately achieve better glycaemic control while lowering the risk of hypoglycaemia3. Here we report the design and properties of NNC2215, an insulin conjugate with bioactivity that is reversibly responsive to a glucose range relevant for diabetes, as demonstrated in vitro and in vivo. NNC2215 was engineered by conjugating a glucose-binding macrocycle4 and a glucoside to insulin, thereby introducing a switch that can open and close in response to glucose and thereby equilibrate insulin between active and less-active conformations. The insulin receptor affinity for NNC2215 increased 3.2-fold when the glucose concentration was increased from 3 to 20 mM. In animal studies, the glucose-sensitive bioactivity of NNC2215 was demonstrated to lead to protection against hypoglycaemia while partially covering glucose excursions.

Green and recyclable catalyst based on chitosan/CuFe2O4 nanocomposite hydrogel for one-step synthesis of 1,2,3-triazoles

The scope of the heterogeneous catalysts has been greatly expanded in last few decades by the development of various catalysts. In this work a new chitosan-based nanocomposite hydrogel (CS/CuFe2O4 NCH) was synthesized as a high-performance heterogeneous catalyst and then, it was utilized for the green synthesis of substituted 1,2,3-triazoles by a multi-component (azide-alkyne-epoxide) cycloaddition reaction. The synthesized nanocomposite hydrogel was investigated by using various instrumental analyses, including FT-IR, XRD, SEM, EDS, HRTEM, DLS, and TGA. The structure of one of the substituted 1,2,3-triazoles was studied by using single-crystal X-ray diffraction analysis. The nanocomposite hydrogel can be easily regenerate after the catalytic reaction. It can be reused frequently without considerable loss of activity. The high catalytic activity, straightforward reaction, easy recyclability, short reaction time, use of a green solvent, and the simple separation of catalyst are the main advantage of the current method, which offers both financial and environmental benefits.

Synthesis of Novel Benazepril-Derived Trizole Compounds Assisted by Ultrasound: In Vitro and In Silico Analysis for Potential Anticancer Properties

Benazepril-based novel trizole derivatives are being explored as potential anticancer agents, designed with an N-substituted 1,2,3-triazole moiety linked to Benazepril's N-1 position via a methylene bridge. An ultrasound irradiated CuAAC method was used to prepare all these compounds and evaluated their anti-proliferative activities against cancer and drug-resistant cell lines. While some of these compounds demonstrated anti-proliferative activity towards leukemic cancer cell line K562, two of them displayed complete inhibitory activity. Interestingly, the compounds 5n and 5o showed potent activity against imatinib-resistant cell lines suggesting their promise to overcome cancer drug resistance. Furthermore, molecular docking analysis revealed that compounds 5n and 5o have higher predicted sensitivity towards ACE protein when compared to benazepril and lisinopril indicating their value as potential drug lead molecules. This research introduces a distinctive approach by employing ultrasound to facilitate CuAAC reactions in medicinal chemistry.

Smart Galactosidase-Responsive Antimicrobial Dendron: Towards More Biocompatible Membrane-Disruptive Agents

Antimicrobial resistance is a global healthcare challenge that urgently needs the development of new therapeutic agents. Antimicrobial peptides and mimics thereof are promising candidates but mostly suffer from inherent toxicity issues due to the non-selective binding of cationic groups with mammalian cells. To overcome this toxicity issue, this work herein reports the synthesis of a smart antimicrobial dendron with masked cationic groups (Gal-Dendron) that could be uncaged in the presence of β-galactosidase enzyme to form the activated Enz-Dendron and confer antimicrobial activity. Enz-Dendron show bacteriostatic activity toward Gram-negative (P. aeruginosa and E. coli) and Gram-positive (S. aureus) bacteria with minimum inhibitory concentration values of 96 µm and exerted its antimicrobial mechanism via a membrane disruption pathway, as indicated by inner and outer membrane permeabilization assays. Crucially, toxicity studies confirmed that the masked prodrug Gal-Dendron exhibited low hemolysis and is at least 2.4 times less toxic than the uncaged cationic Enz-Dendron, thus demonstrating the advantage of masking the cationic groups with responsive immolative linkers to overcome toxicity and selectivity issues. Overall, this study highlights the potential of designing new membrane-disruptive antimicrobial agents that are more biocompatible via the amine uncaging strategy.

Probing Glycerolipid Metabolism using a Caged Clickable Glycerol-3-Phosphate Probe

In this study, we present the probe SATE-G3P-N3 as a novel tool for metabolic labeling of glycerolipids (GLs) to investigate lipid metabolism in yeast cells. By introducing a clickable azide handle onto the glycerol backbone, this probe enables general labeling of glycerolipids. Additionally, this probe contains a caged phosphate moiety at the glycerol sn-3 position to not only facilitate probe uptake by masking negative charge but also to bypass the phosphorylation step crucial for initiating phospholipid synthesis, thereby enhancing phospholipid labeling. The metabolic labeling activity of the probe was thoroughly assessed through cellular fluorescence microscopy, mass spectrometry (MS), and thin-layer chromatography (TLC) experiments. Fluorescence microscopy analysis demonstrated successful incorporation of the probe into yeast cells, with labeling predominantly localized at the plasma membrane. LCMS analysis confirmed metabolic labeling of various phospholipid species (PC, PS, PA, PI, and PG) and neutral lipids (MAG, DAG, and TAG), and GL labeling was corroborated by TLC. These results showcased the potential of the SATE-G3P-N3 probe in studying GL metabolism, offering a versatile and valuable approach to explore the intricate dynamics of lipids in yeast cells.

Improved deconvolution of natural products' protein targets using diagnostic ions from chemical proteomics linkers

Identification of interactions between proteins and natural products or similar active small molecules is crucial for understanding of their mechanism of action on a molecular level. To search elusive, often labile, and low-abundant conjugates between proteins and active compounds, chemical proteomics introduces a feasible strategy that allows to enrich and detect these conjugates. Recent advances in mass spectrometry techniques and search algorithms provide unprecedented depth of proteome coverage and the possibility to detect desired modified peptides with high sensitivity. The chemical 'linker' connecting an active compound-protein conjugate with a detection tag is the critical component of all chemical proteomic workflows. In this review, we discuss the properties and applications of different chemical proteomics linkers with special focus on their fragmentation releasing diagnostic ions and how these may improve the confidence in identified active compound-peptide conjugates. The application of advanced search options improves the identification rates and may help to identify otherwise difficult to find interactions between active compounds and proteins, which may result from unperturbed conditions, and thus are of high physiological relevance.

Mimicking Mytilus edulis foot protein: A versatile strategy for robust biomedical coatings

Universal coatings with versatile surface adhesion, good mechanochemical robustness, and the capacity for secondary modification are of great scientific interest. However, incorporating these advantages into a system is still a great challenge. Here, we report a series of catechol-decorated polyallylamines (CPAs), denoted as pseudo-Mytilus edulis foot protein 5 (pseudo-Mefp-5), that mimic not only the catechol and amine groups but also the backbone of Mefp-5. CPAs can fabricate highly adhesive, robust, multifunctional polyCPA (PCPA) coatings based on synergetic catechol-polyamine chemistry as universal building blocks. Due to the interpenetrating entangled network architectures, these coatings exhibit high chemical robustness against harsh conditions (HCl, pH 1; NaOH, pH 14; H2O2, 30%), good mechanical robustness, and wear resistance. In addition, PCPA coatings provide abundant grafting sites, enabling the fabrication of various functional surfaces through secondary modification. Furthermore, the versatility, multifaceted robustness, and scalability of PCPA coatings indicate their great potential for surface engineering, especially for withstanding harsh conditions in multipurpose biomedical applications.

Conjugation of primary amine groups in targeted proteomics

Primary amines, in the form of unmodified N-terminus of peptide/protein and unmodified lysine residue, are perhaps the most important functional groups that can serve as the starting points in proteomic analysis, especially via mass spectrometry-based approaches. A variety of multifunctional probes that conjugate primary amine groups through covalent bonds have been developed and employed to facilitate protein/protein complex characterization, including identification, quantification, structure and localization elucidation, protein-protein interaction investigation, and so forth. As an integral part of more accurate peptide quantification in targeted proteomics, isobaric stable isotope-coded primary amine labeling approaches eventually facilitated protein/peptide characterization at the single-cell level, paving the way for single-cell proteomics. The development and advances in the field can be reviewed in terms of key components of a multifunctional probe: functional groups and chemistry for primary amine conjugation; hetero-bifunctional moiety for separation/enrichment of conjugated protein/protein complex; and functionalized linker/spacer. Perspectives are primarily focused on optimizing primary amine conjugation under physiological conditions to improve characterization of native proteins, especially those associated with the surface of living cells/microorganisms.

A CuSO4/Bicinchoninic acid/Reducing sugar based stable and non-ROS catalyst system for the CuAAC reaction in bioanalysis

The limitations of commonly used sodium ascorbate-based catalyst system for copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction include excess production of reactive oxygen species and rapid catalyst deactivation. In this study instead of using a highly active reducing agent, such as, sodium ascorbate, we chose reducing sugar as a mild reducing agent to build up the catalyst system for CuAAC reaction. Interestingly, the bicinchoninic acid (BCA) assay system containing reducing sugar satisfies the essential elements of the catalyst system for CuAAC reaction. We found that CuSO4/BCA/Reducing sugar system can catalyze the CuAAC reaction but with low yield. Rational analyses of various parameters in CuSO4/BCA/Glucose catalyst system suggested storage at room temperature might enhance the catalytic activity, which was proven to be the case. Importantly, the system remains stable at room temperature and minimal H2O2 was detected. Notably, our study showed that the coordination between the slow reduction of Cu(I) by reducing sugar and the selective chelation of Cu(I) by BCA is key to developing this system. The CuSO4/BCA/Reducing sugar catalyst system was successfully applied to various CuAAC reaction based bioanalyses, and it is suitable for the CuAAC reaction based bioanalyses that are sensitive to ROS or request long reaction time.

Click-chemistry-inspired synthesis of new series of 1,2,3-triazole fused chromene with glucose triazole conjugates: Antibacterial activity assessment with molecular docking evaluation

A series of new 1,2,3-triazole fused chromene based glucose triazole conjugates were synthesized from chromene fused 1,2,3-triazolyl extended alkyne and 2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl azide in good to excellent yield by a copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The major advantages include mild reaction conditions, high yield, good substrate scope, and shorter reaction time. The antibacterial efficacy of the compounds were assessed in vitro against human pathogenic Gram-negative E. coli and Gram-positive S. aureus bacteria. Compound 24j was found to be the most potent molecule with zone of inhibition (ZI) of 17 mm and minimum inhibitory concentration (MIC) of 25 μg mL-1 in E. coli and ZI of 16 mm and MIC of 25 μg mL-1 in S. aureus. Also, it significantly inhibited E. coli DNA-gyrase in silico with a binding affinity of -9.4 kcal/mol. Among all the synthesized compounds, 24i, 24d, 24e and 24f showed significant antibacterial activity against both strains and inhibited DNA-gyrase in silico with good binding affinities. Hence, these 1,2,3-triazole fused chromene based glucose triazole conjugates may evolve to be powerful antibacterial agents in recent future, according to structure-activity relationships based on strong antibacterial properties and molecular docking studies.

Metabolically activated proteostasis regulators that protect against erastin-induced ferroptosis

We previously showed that the proteostasis regulator compound AA147 (N-(2-hydroxy-5-methylphenyl)benzenepropanamide) potently protects against neurotoxic insults, such as glutamate-induced oxytosis. Though AA147 is a selective activator of the ATF6 arm of the unfolded protein response in non-neuronal cells, AA147-dependent protection against glutamate toxicity in cells of neuronal origin is primarily mediated through activation of the NRF2 oxidative stress response. AA147 activates NRF2 through a mechanism involving metabolic activation of AA147 by endoplasmic reticulum (ER) oxidases, affording an AA147-based quinone methide that covalently targets the NRF2 repressor protein KEAP1. Previous results show that the 2-amino-p-cresol A-ring of AA147 is required for NRF2 activation, while the phenyl B-ring of AA147 is amenable to modification. Here we explore whether the protease-sensitive amide linker between the A- and B-rings of this molecule can be modified to retain NRF2 activation. We show that replacement of the amide linker of AA147 with a carbamate linker retains NRF2 activation in neuronal cells and improves protection against neurotoxic insults, including glutamate-induced oxytosis and erastin-induced ferroptosis. Moreover, we demonstrate that inclusion of this carbamate linker facilitates identification of next-generation AA147 analogs with improved cellular tolerance and activity in disease-relevant assays.

Bioisosteres at C9 of 2-Deoxy-2,3-didehydro-N-acetyl Neuraminic Acid Identify Selective Inhibitors of NEU3

Human neuraminidases play critical roles in many physiological and pathological processes. Humans have four isoenzymes of NEU, making selective inhibitors important tools to investigate the function of individual isoenzymes. A typical scaffold for NEU inhibitors is 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) where C9 modifications can be critical for potency and selectivity against human NEU. To design improved DANA analogues, we generated a library of compounds with either a short alkyl chain or a biphenyl substituent linked to the C9 position through one of six amide bioisosteres. Bioisostere linkers included triazole, urea, thiourea, carbamate, thiocarbamate, and sulfonamide groups. Within this library, we identified a C9 biphenyl carbamate derivative (963) that showed high selectivity and potency for NEU3 (Ki = 0.12 ± 0.01 μM). In contrast, NEU1 and NEU4 isoenzymes preferred amide and triazole linkers, respectively. Finally, analogues with urea, sulfonamide, and amide linkers showed enhanced inhibitory activity for a bacterial NEU, NanI from Clostridium perfringens.

Synthesis of α-Hydroxy-1,2,3-Triazole-linked Sialyltransferase Inhibitors and Evaluation of Selectivity Towards ST3GAL1, ST6GAL1 and ST8SIA2

Tumour-derived sialoglycans, bearing the charged nonulosonic sugar sialic acid at their termini, play a critical role in tumour cell adhesion and invasion, as well as evading cell death and immune surveillance. Sialyltransferases (ST), the enzymes responsible for the biosynthesis of sialylated glycans, are highly upregulated in cancer, with tumour hypersialylation strongly correlated with tumour growth, metastasis and drug resistance. As a result, desialylation of the tumour cell surface using either targeted delivery of a pan-ST inhibitor (or sialidase) or systemic delivery of a non-toxic selective ST inhibitors are being pursued as potential new anti-metastatic strategies against multiple cancers including pancreatic, ovarian, breast, melanoma and lung cancer. Herein, we have employed molecular modelling to give insights into the selectivity observed in a series of selective ST inhibitors that incorporate a uridyl ring in place of the cytidine of the natural donor (CMP-Neu5Ac) and replace the charged phosphodiester linker of classical ST inhibitors with a neutral α-hydroxy-1,2,3-triazole linker. The inhibitory activities of the nascent compounds were determined against recombinant human ST enzymes (ST3GAL1, ST6GAL1, ST8SIA2) showing promising activity and selectivity towards specific ST sub-types. Our ST inhibitors are non-toxic and show improved synthetic accessibility and drug-likeness compared to earlier nucleoside-based ST inhibitors.

7-Deazapurine and Pyrimidine Nucleoside and Oligonucleotide Cycloadducts Formed by Inverse Diels-Alder Reactions with 3,6-Di(pyrid-2-yl)-1,2,4,5-tetrazine: Ethynylated and Vinylated Nucleobases for Functionalization and Impact of Pyridazine Adducts on DNA Base Pair Stability and Mismatch Discrimination

The manuscript reports on 7-deazapurine and pyrimidine nucleoside and oligonucleotide cycloadducts formed by the inverse electron demand Diels-Alder (iEDDA) reaction with 3,6-di(pyrid-2-yl)-1,2,4,5-tetrazine. Cycloadducts were constructed from ethynylated and vinylated nucleobases. Oligonucleotides were synthesized containing iEDDA modifications, and the impact on duplex stability was investigated. iEDDA reactions were performed on nucleoside triple bond side chains. Oxidation was not required in these cases as dihydropyridazine intermediates are not formed. In contrast, oxidation is necessary for reactions performed on alkenyl compounds. This was verified on 5-vinyl-2'-deoxyuridine. A diastereomeric mixture of 1,2-dihydropyridazine cycloadduct intermediates was isolated, characterized, and later oxidized. 12-mer oligonucleotides containing 1,2-pyridazine inverse Diels-Alder cycloadducts and their precursors were hybridized to short DNA duplexes. For that, a series of phosphoramidites was prepared. DNA duplexes with 7-functionalized 7-deazaadenines and 5-functionalized pyrimidines display high duplex stability when spacer units are present between nucleobases and pyridazine cycloadducts. A direct connectivity of the pyridazine moiety to nucleobases as reported for metabolic labeling of vinyl nucleosides reduced duplex stability strongly. Oligonucleotides bearing linkers with and without pyridazine cycloadducts attached to the 7-deazaadenine nucleobase significantly reduced mismatch formation with dC and dG.

Organosulphur and organoselenium compounds as ligands for catalytic systems in the Sonogashira coupling

Sonogashira coupling is a reaction of aryl/vinyl halides with terminal alkynes. It is used for the synthesis of conjugated enynes. Generally, copper (Cu) is required as a mediator for this reaction. It requires a long reaction time, high catalyst loading, or expensive ligands. Recently, homogeneous, heterogeneous, and nanocatalysts have been developed using organosulphur and organoselenium compounds as building blocks. Preformed complexes of metals with organosulphur and organoselenium ligands are used for homogeneous catalysis. Heterogeneous catalytic systems have also been developed using Cu, Pd, and Ni as metals. The nanocatalytic systems (synthesized using such ligands) include copper selenides and stabilized palladium(0) nanospecies. This article aims to cover the developments in the field of the processes and techniques used so far to generate catalytically relevant organic ligands having sulphur or selenium donor sites, the utility of such ligands in the syntheses of homogeneous, heterogeneous, and nanocatalytic systems, and critical analysis of their application in the catalysis of this coupling reaction. The results of catalysis are analyzed in terms of the effects of the S/Se donor, halogen atom of aryl halide, the effect of the presence/absence of electron-withdrawing or electron-donating groups or substituents on the aromatic ring of haloarenes/substituted phenylacetylenes, as well as the position (ortho or para) of the substitution. Substrate scope is discussed for all the kinds of catalysis. The supremacy of heterogeneous and nanocatalytic systems indicates promising future prospects.

Functionalizing Thiosemicarbazones for Covalent Conjugation

Thiosemicarbazones (TSCs) with their modular character (thiosemicarbazides + carbonyl compound) allow broad variation of up to four substituents on the main R1R2C=N(1)-NH-C(S)-N(4)R3R4 core and are thus interesting tools for the formation of conjugates or the functionalization of nanoparticles (NPs). In this work, di-2-pyridyl ketone was introduced for the coordination of metals and 9-anthraldehyde for luminescence as R1 and R2 to TSCs. R3 and R4 substituents were varied for the formation of conjugates. Amino acids were introduced at the N4 position to produce [R1R2TSC-spacer-amino acid] conjugates. Further, functions such as phosphonic acid (R-P(O)(OH)2), D-glucose, o-hydroquinone, OH, and thiol (SH) were introduced at the N4 position producing [R1R2TSC-spacer-anchor group] conjugates for direct NP anchoring. Phenyl, cyclohexyl, benzyl, ethyl and methyl were used as spacer units. Both phenyl phosphonic acid TSC derivatives were bound on TiO2 NPs as a first example of direct NP anchoring. [R1R2TSC-spacer-end group] conjugates including OH, S-Bn (Bn = benzyl), NH-Boc (Boc = tert-butyloxycarbonyl), COOtBu, C≡CH, or N3 end groups were synthesized for potential covalent binding to functional molecules or functionalized NPs through amide, ester, or triazole functions. The synthesis of the thiosemicarbazides H2NNH-C(S)-NR3R4 starting from amines, including amino acids, SCCl2 or CS2, and hydrazine and their condensation with dipyridyl ketone and anthraldehyde led to 34 new TSC derivatives. They were synthesized in up to six steps with overall yields ranging from 10 to 85% and were characterized by a combination of nuclear magnetic resonance spectroscopy and mass spectrometry. UV-vis absorption and photoluminescence spectroscopy allowed us to easily trace the dipyridyl imine and anthracene chromophores.

One-pot chemo- and photo-enzymatic linear cascade processes

The combination of chemo- and photocatalyses with biocatalysis, which couples the flexible reactivity of the photo- and chemocatalysts with the highly selective and environmentally friendly nature of enzymes in one-pot linear cascades, represents a powerful tool in organic synthesis. However, the combination of photo-, chemo- and biocatalysts in one-pot is challenging because the optimal operating conditions of the involved catalyst types may be rather different, and the different stabilities of catalysts and their mutual deactivation are additional problems often encountered in one-pot cascade processes. This review explores a large number of transformations and approaches adopted for combining enzymes and chemo- and photocatalytic processes in a successful way to achieve valuable chemicals and valorisation of biomass. Moreover, the strategies for solving incompatibility issues in chemo-enzymatic reactions are analysed, introducing recent examples of the application of non-conventional solvents, enzyme-metal hybrid catalysts, and spatial compartmentalization strategies to implement chemo-enzymatic cascade processes.

Facile synthesis and bonding of 4-ferrocenyl-1,2,4-triazol-5-ylidene complexes

Ferrocene-substituted carbenes have emerged as attractive, redox-active ligands. However, among the compounds studied to date, ferrocenylated 1,2,4-triazol-5-ylidenes, which are closely related to the archetypal imidazol-2-ylidenes, are still unknown. Here, we demonstrate that the triazolium salt [CHN(Me)NCHN(Fc)]I (2; Fc = ferrocenyl), obtained by alkylation of 4-ferrocenyl-4H-1,2,4-triazole (1) with MeI, reacts selectively with metal alkoxide/hydroxide precursors [(cod)Rh(OMe)]2 and [(IPr)Au(OH)] (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to produce the ferrocene-substituted 1,2,4-triazol-5-ylidene complexes [(cod)RhI{CN(Me)NCHN(Fc)}] and [(IPr)Au{CN(Me)NCHN(Fc)}]I in good yields. The complexes were characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction analysis. Density function theory (DFT) calculations were used to rationalise the electrochemical behaviour of the carbene complexes and to elucidate the bonding situation in these compounds. An analysis using intrinsic bond orbitals (IBOs) revealed that the 1,2,4-triazol-5-ylidene ligand exerted a strong trans influence and showed a synergistic stabilisation by the negative inductive and positive π-donor effects of the nitrogen atoms adjacent to the carbene carbon atom; these effects were enhanced by conjugation with the CHN bond at the exterior, similar to that in imidazol-2-ylidenes.

Attaching Metal-Containing Moieties to β-Lactam Antibiotics: The Case of Penicillin and Cephalosporin

Procedures for the preparation of transition metal complexes having intact bicyclic cepham or penam systems as ligands have been developed. Starting from readily available 4-azido-2-azetidinones, a synthetic approach has been tuned using a copper-catalyzed azide-alkyne cycloaddition between 3-azido-2-azetinones and alkynes, followed by methylation and transmetalation to Au(I) and Ir(III) complexes from the mesoionic carbene Ag(I) complexes. This methodology was applied to 6-azido penam and 7-azido cepham derivatives to build 6-(1,2,3-triazolyl)penam and 7-(1,2,3-triazolyl)cepham proligands, which upon methylation and metalation with Au(I) and Ir(III) complexes yielded products derived from the coordination of the metal to the penam C6 and cepham C7 positions, preserving intact the bicyclic structure of the penicillin and cephalosporin scaffolds. The crystal structure of complex 28b, which has an Ir atom directly bonded to the intact penicillin bicycle, was determined by X-ray diffraction. This is the first structural report of a penicillin-transition-metal complex having the bicyclic system of these antibiotics intact. The selectivity of the coordination processes was interpreted using DFT calculations.

Target identification of usnic acid in bacterial and human cells

Usnic acid is a natural product with versatile biological activities against various organisms. Here, we utilise a chemical proteomic strategy to gain insights into its target scope in bacterial and human cells. First, we excluded DNA binding as a major reason for its antibacterial activity, and second, we commenced with target profiling, which unravelled several metal cofactor-dependent enzymes in both species indicating a polypharmacological mode of action. Interestingly, our synthetic studies revealed a selectivity switch at usnic acid, which maintains antibacterial activity but lacks strong cytotoxic effects.