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Indole, a ubiquitous and structurally versatile aromatic compound, has emerged as a key player in the synthesis of diverse heterocyclic frameworks via cycloaddition reactions. These reactions are completely atom-economical and, hence, are considered as green reactions. This review article provides a comprehensive overview of the pivotal role played by indole in the construction of complex and biologically relevant heterocyclic compounds. Here we explore the chemistry of indole-based cycloadditions, highlighting their synthetic utility in accessing a wide array of heterocyclic architectures, including cyclohepta[b]indoles, tetrahydrocarbazoles, tetrahydroindolo[3,2-c]quinoline, and indolines, among others. Additionally, we discuss the mechanistic insights that underpin these transformations, emphasizing the strategic importance of indole as a building block. The content of this article will certainly encourage the readers to explore more work in this area.

Cycloaddition is an essential tool in chemical synthesis. Instead of using light or heat as a driving force, marine sponges promote cycloaddition with a more versatile but poorly understood mechanism in producing pyrrole–imidazole alkaloids sceptrin, massadine, and ageliferin. Through de novo synthesis of sceptrin and massadine, we show that sponges may use single-electron oxidation as a central mechanism to promote three different types of cycloaddition. Additionally, we provide surprising evidence that, in contrast to previous reports, sceptrin, massadine, and ageliferin have mismatched chirality. Therefore, massadine cannot be an oxidative rearrangement product of sceptrin or ageliferin, as is commonly believed. Taken together, our results demonstrate unconventional chemical approaches to achieving cycloaddition reactions in synthesis and uncover enantiodivergence as a new biosynthetic paradigm for natural products.

In the present study, dihydroxyethylated amarine and iso-amarine ionic salts (namely AM-OH and IAM-OH) along with diethylated amarine and iso-amarine ionic salts (namely AM-Et and IAM-Et) are firstly synthesized. Subsequently, their catalytic performance for the cycloaddition of CO 2 to epichlorohydrin is quantitatively compared under identical reaction conditions. As expected, dihydroxyethylated imidazolinium bromide salts exhibit notably higher catalytic activity than that of their diethylated counterparts, whereas the catalytic activity of AM-OH and IAM-OH is almost identical, highlighting the importance of H-bonding interactions. Inspired by this finding, a dihydroxyethyl-containing poly(triphenylimidazolinium) network (namely fPPIm-OH) is synthesized by one-pot reaction of dihydroxyethylated cis -(±)-2,4,5-tris( p -formylphenyl)imidazolinium bromide (PIm) with hexamethyldisilazane (HMDS) in N , N -dimethylformamide followed by treatment with excess 2-bromoethanol in CH 3 CN in the presence of K 2 CO 3 . The as-synthesized fPPIm-OH network with a Br − content of 2.87 mmol g −1 shows high catalytic activity for the cycloaddition of CO 2 to epoxides, yielding almost quantitative conversion (99%) and selectivity (99%) for a wide range of epoxides. Remarkably, the fPPIm-OH also shows relatively high activity for the cycloaddition of simulated flue gas (0.15 bar CO 2  + 0.85 bar N 2 ) to epoxides, in which quantitative selectivity (99%) and high conversion (92 ~ 99%) are observed under relatively harsh conditions (80–120 °C). This study presents a facile approach for the synthesis of novel imidazolinium-containing polymeric networks and may inspire more researches aiming at extending the aromatic multialdehydes and simultaneously enhancing the catalytic activity of the resultant polymeric networks. Graphical Abstract

The electronic structure and the participation of the simplest azomethine imine (AI) in [3+2] cycloaddition (32CA) reactions have been analysed within the Molecular Electron Density Theory (MEDT) using Density Functional Theory (DFT) calculations at the MPWB1K/6-311G(d) level. Topological analysis of the electron localisation function reveals that AI has a pseudoradical structure, while the conceptual DFT reactivity indices characterises this three-atom-component (TAC) as a moderate electrophile and a good nucleophile. The non-polar 32CA reaction of AI with ethylene takes place through a one-step mechanism with moderate activation energy, 8.7 kcal·mol−1. A bonding evolution theory study indicates that this reaction takes place through a non-concerted [2n + 2τ] mechanism in which the C–C bond formation is clearly anticipated prior to the C–N one. On the other hand, the polar 32CA reaction of AI with dicyanoethylene takes place through a two-stage one-step mechanism. Now, the activation energy is only 0.4 kcal·mol−1, in complete agreement with the high polar character of the more favourable regioisomeric transition state structure. The current MEDT study makes it possible to extend Domingo’s classification of 32CA reactions to a new pseudo(mono)radical type (pmr-type) of reactivity.

The copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC) reaction is considered to be the most representative ligation process within the context of the “click chemistry” concept. This CuAAC reaction, which yields compounds containing a 1,2,3-triazole core, has become relevant in the construction of biologically complex systems, bioconjugation strategies, and supramolecular and material sciences. Although many CuAAC reactions are performed under homogenous conditions, heterogenous copper-based catalytic systems are gaining exponential interest, relying on the easy removal, recovery, and reusability of catalytically copper species. The present review covers the most recently developed copper-containing heterogenous solid catalytic systems that use solid inorganic/organic hybrid supports, and which have been used in promoting CuAAC reactions. Due to the demand for 1,2,3-triazoles as non-classical bioisosteres and as framework-based drugs, the CuAAC reaction promoted by solid heterogenous catalysts has greatly improved the recovery and removal of copper species, usually by simple filtration. In so doing, the solving of the toxicity issue regarding copper particles in compounds of biological interest has been achieved. This protocol is also expected to produce a practical chemical process for accessing such compounds on an industrial scale.

A synthesis of new enantiomerically enriched derivatives of (S)-α-aminopropionic acid, containing in the β-position 1,2,3-triazole groups coupled with a o-, m- and p-substituted phenyl residue, was developed based on Cu(I) catalyzed [3 + 2] cycloaddition of azides with alkynes. As the starting materials was used the square-planar Ni(II)complex of the Schiff base of propargylglycine with the chiral auxiliary BPB (Benzylprolylbenzophenone) and 1,4-substituted phenyl azides. The assignment of the (S)-absolute configuration of the α-carbon atom of the amino acid residue of the main diastereomeric complexes of the cycloaddition products was carried out on the basis of positive Cotton effects in the region of 480–580 nm of the circular dichroism spectra. The target amino acids were isolated from acid hydrolysates of diastereomeric complexes using ion-exchange demineralization and crystallization from aqueous ethanol. Additional confirmation of the absolute configuration and determination of the enantiomeric purity of the target amino acids were carried out by chiral HPLC analysis. As a result, seven new non-proteinogenic (S)-α-amino acids, containing in the β-position a 1,2,3-triazole moiety, were synthesized.

The metal-organic frameworks (MOFs) with oxidative and acidic active sites demonstrate promising potential for tandem reactions involving olefin oxidation carboxylation. In this study, M-BTCs were synthesized via a solvothermal method, employing 1,3,5-benzenetricarboxylic acid (H 3 BTC) as a ligand in combination with various metals (Mn, Co, Cu, Ni). The good thermal stability and morphology of M-BTC was verified by various characterization techniques, and its catalytic performance was evaluated for oxidative carboxylation. The catalytic activity of Mn-BTC, with Mn 3+ / Mn 2+ as the primary oxidation site, was found to be superior in both olefin epoxidation and CO 2 cycloaddition. The effects of reaction conditions on both epoxidation of styrene and the cycloaddition reaction were investigated, respectively. Under optimal reaction conditions (epoxidation: 10 wt% Mn-BTC of styrene, 80 ℃ for 12 h; cycloaddition: 100 ℃ for 12 h with a CO 2 flow rate of 15 ml/min and tetrabutylammonium bromide (TBAB) amount of 15 mol%), a 53% yield of styrene carbonate (SC) was obtained in the assisted tandem reactions. Furthermore, cycling experiments as well as XRD and FT-IR spectra of the catalysts after use demonstrated that Mn-BTC maintained its crystal structure and retained a yield of 49% SC after three cycles. Finally, a possible mechanism for assisted tandem catalytic reaction over Mn-BTC was proposed. Graphic Abstract

Context The analysis of the changes in the electronic structure along intrinsic reaction coordinate (IRC) paths for model reactions: (i) ethylene + butadiene cycloaddition, (ii) prototype SN2 reaction Cl − + CH3Cl, (iii) HCN/CNH isomerization assisted by water, (iv) CO + HF → C(O)HF was performed, in terms of changes in the deformation density (Δr) and the deformation of MEP (ΔMEP). The main goal was to further examine the utility of the ΔMEP as a descriptor of chemical bonding, and to compare the pictures resulting from Δr and ΔMEP. Both approaches clearly show that the main changes in the electronic structure occur in the TS region. The ΔMEP picture is fully consistent with that based on Δρ for the reactions of the neutral species leading to the neutral products without large charge transfer between the fragments. In the case of reactions with large electron density displacements, the ΔMEP picture is dominated by charge transfer leading to more clear indication of charge shifts than the analysis of Δr. Methods All the calculations were performed using the ADF package. The Becke–Perdew exchange–correlation functional was used with the Grimme’s dispersion correction (D3 version) with Becke-Johnson damping. The Slater TZP basis sets defined within the ADF program were applied. For the analysed reactions, the stationary points were determined and verified by frequency calculations, and the IRC was determined. Further analysis was performed for the structures of reactants, TS, products, and the points corresponding to the minimum and maximum of the reaction force. For each point, two fragments, A and B, corresponding to the reactants were considered. The deformation density was calculated as the difference between the electron density of the system AB and the sum of densities of A and B, Δ ρ r = ρ AB r - ρ A r - ρ B r , with the same fragment definition as in the ETS-NOCV method. Correspondingly, deformation in MEP was determined as Δ V r = V AB r - V A r - V B r .

In this article, an original method is proposed for address and covalent immobilization of anti-E. coli antibodies on a screen-printed electrode (SPE). The method is based on a copper-catalyzed “click” reaction between a polyvinylbenzylazide (PVBA) film electrochemically deposited on the electrode surface and acetylene fragments of propargyl-N-hydroxysuccinimide ester. The products of electrochemical oxidation of copper particles incorporated in the polymer film on the electrode were first used for catalysis of the click reaction. This approach allowed us to reduce the immobilization time from a few hours for conventional methods to just 30 min, and to prevent denaturation of the immunoreceptor. The modified electrodes were characterized by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Based on the results obtained, a label-free impedimetric immunosensor for E. coli detection was developed. The detection limit of the immunosensor was estimated as 6.3 CFU/ml, with a linear range of 103–106 CFU/ml. The immunosensor demonstrated good stability during 30 days of storage in phosphate buffer solution (PBS, pH 7) and selectivity toward excess Staphylococcus aureus bacteria.

Fluorogenic bioorthogonal reactions enable visualization of biomolecules with excellent signal‐to‐noise ratio. A bicyclononyne−tetrazine ligation that produces fluorescent pyridazine products has been developed. In stark contrast to previous approaches, the formation of the dye is an inherent result of the chemical reaction and no additional fluorophores are needed in the reagents. The crucial structural elements that determine dye formation are electron‐donating groups present in the starting tetrazine unit. The newly formed pyridazine fluorophores show interesting photophysical properties the fluorescence intensity increase in the reaction can reach an excellent 900‐fold. Model imaging experiments demonstrate the application potential of this new fluorogenic bioorthogonal reaction. Turning on: A bicyclononyne dienophile reacts with 1,2,4,5‐tetrazines bearing electron‐donating groups to form fluorescent pyridazine products in an inverse electron‐demand Diels‐Alder reaction (iEDDA). The fluorescence turn‐on properties are preserved in biological systems and can be applied to bioimaging. In combination with the fluorogenic trans‐cyclooctene−tetrazine cycloaddition reaction, the reaction enables one‐step two‐color labeling using a single tetrazine as the activator.