Medicinal Chemistry of Isocyanides

"Isocyanates and isocyanides - life-threatening toxins or essential compounds?"

Isocyanides and isocyanates are some of the most reactive compounds in organic chemistry, making them perceived as compounds with high potential for use in both the laboratory and industry. With their high reactivity also comes several disadvantages, most notably their potentially high toxicity. The following article is a collection of information on the toxic effects of the isocyanide group on the human body and the environment. Information on the mechanism of how these harmful substances affect living tissues and the environment, worldwide information on how to protect against these chemicals, current regulations, and exposure limits for specific countries is compiled. The latest research on the application uses of isocyanates and isocyanides is also outlined, as well as the latest safer and greener methods and techniques to work with these compounds. Additionally, the presented article can serve as a brief guide to the organic toxicity of a group of isocyanates and isocyanates.

Biosynthesis of isonitrile lipopeptides

Isonitrile lipopeptides discovered from Actinobacteria have attracted wide attention due to their fascinating biosynthetic pathways and relevance to the virulence of many human pathogens including Mycobacterium tuberculosis. Specifically, the identification of the new class of isonitrile-forming enzymes that belong to non-heme iron (II) and α-ketoglutarate dependent dioxygenases has intrigued several research groups to investigate their catalytic mechanism. Here we summarize the recent studies on the biosynthesis of isonitrile lipopeptides from Streptomyces and Mycobacterium. The latest research on the core and tailoring enzymes involved in the pathway as well as the isonitrile metabolic enzymes are discussed in this review.

Exploiting the Different Nucleophilicity of the Isocyano Group: A Strategy for the Isocyanide Functionalization

By exploiting the different nucleophilicity of aromatic and aliphatic isocyanides, we selectively react aliphatic isocyano groups while preserving aromatic ones in Passerini and Ugi multicomponent reactions. This simple approach allows the synthesis of α-acyloxy carboxamides or α-acylamino carboxamides possessing one or two isocyanide groups, which are challenging to achieve through traditional formylation and dehydration protocols. These analogues have the potential to serve as valuable building blocks with diverse applications.

Actinobacterial chalkophores: the biosynthesis of hazimycins

Copper is a transition metal element with significant effects on the morphological development and secondary metabolism of actinobacteria. In some microorganisms, copper-binding natural products are employed to modulate copper homeostasis, although their significance in actinobacteria remains largely unknown. Here, we identified the biosynthetic genes of the diisocyanide natural product hazimycin in Kitasatospora purpeofusca HV058, through gene knock-out and heterologous expression. Biochemical analyses revealed that hazimycin A specifically binds to copper, which diminishes its antimicrobial activity. The presence of a set of putative importer/exporter genes surrounding the biosynthetic genes suggested that hazimycin is a chalkophore that modulates the intracellular copper level. A bioinformatic survey of homologous gene cassettes, as well as the identification of two previously unknown hazimycin-producing Streptomyces strains, indicated that the isocyanide-based mechanism of copper homeostasis is prevalent in actinobacteria.

Changes in an enzyme ensemble during catalysis observed by high-resolution XFEL crystallography

Enzymes populate ensembles of structures necessary for catalysis that are difficult to experimentally characterize. We use time-resolved mix-and-inject serial crystallography at an x-ray free electron laser to observe catalysis in a designed mutant isocyanide hydratase (ICH) enzyme that enhances sampling of important minor conformations. The active site exists in a mixture of conformations, and formation of the thioimidate intermediate selects for catalytically competent substates. The influence of cysteine ionization on the ICH ensemble is validated by determining structures of the enzyme at multiple pH values. Large molecular dynamics simulations in crystallo and time-resolved electron density maps show that Asp17 ionizes during catalysis and causes conformational changes that propagate across the dimer, permitting water to enter the active site for intermediate hydrolysis. ICH exhibits a tight coupling between ionization of active site residues and catalysis-activated protein motions, exemplifying a mechanism of electrostatic control of enzyme dynamics.

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro- and Deuterodeamination Reactions

Herein, we report the use of isonitriles as alkyl radical precursors in light-mediated hydro- and deuterodeamination reactions. The reaction is scalable, shows broad functional group compatibility and potential to be used in late-stage functionalization. Importantly, the method is general for Cα -primary, Cα -secondary and Cα -tertiary alkyl isonitriles. For most examples, high yields were obtained through direct visible-light irradiation of the isonitrile in the presence of a silyl radical precursor. Interestingly, in the presence of an organic photocatalyst (4CzIPN) a dramatic acceleration was observed. In-depth mechanistic studies using UV/Vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy suggest that the excited state of 4CzIPN can engage in a single-electron transfer with the isonitrile.

Theoretical investigation of nucleophilic substitution reaction of phenyl carbonyl isothiocyanates with pyridines in gas and polar aprotic solvent

This study focuses on the mutual interaction of substituents in the nucleophile and substrate - cross interaction constant, ρXY, in the uncatalyzed aminolysis by substituting pyridine with phenyl carbonyl isothiocyanate. The mechanism was found to be a stepwise process with a rate-limiting breakdown of the -NCS leaving group. This stepwise reaction mechanism considers the cross-interaction constant (CIC) with rate-limiting breakdown of tetrahedral intermediate in gas and solvent phases. The corresponding Hammett coefficients are related to the substituents associated with (1) the nucleophiles (X), ρX (-1.93 to -6.54 for the gas phase and 10.5 to 18.9 in the solvent model), and with (2) the substituents associated with the phenyl ring of the substrate (Y), ρY (0.41-3.48 for the gas phase and 1.83 to -10.70 for the solvent model). It also includes the Brønsted coefficient with X, βX (0.11-1.52 for the gas phase and -2.57 to 3.96 for the solvent model), and CIC values, ρXY (0.69 for the gas phase and 0.87 for the solvent model). In this work, the NBO analysis, reaction potential, reaction electronic flux (REF), dual descriptor, and the structure-energy relationships were considered in interpreting the mechanistic criteria.

Crystal Clear: Decoding Isocyanide Intermolecular Interactions through Crystallography

The isocyanide group is the chameleon among the functional groups in organic chemistry. Unlike other multiatom functional groups, where the electrophilic and nucleophilic moieties are typically separated, isocyanides combine both functionalities in the terminal carbon. This unique feature can be rationalized using the frontier orbital concept and has significant implications for its intermolecular interactions and the reactivity of the functional group. In this study, we perform a Cambridge Crystallographic Database-supported analysis of isocyanide intramolecular interactions to investigate the intramolecular interactions of isocyanides in the solid state, excluding isocyanide-metal complexes. We discuss examples of different interaction classes, including the isocyanide as a hydrogen bond acceptor (RNC···HX), halogen bonding (RNC···X), and interactions involving the isocyanide and carbon atoms (RNC···C). The latter interaction serves as an intriguing illustration of a Bürgi-Dunitz trajectory and represents a crucial experimental detail in the well-known multicomponent reactions such as the Ugi- and Passerini-type mechanisms. Understanding the spectrum of intramolecular interactions that isocyanides can undergo holds significant implications in fields such as medicinal chemistry, materials science, and asymmetric catalysis.

Recent developments in the total synthesis of natural products using the Ugi multicomponent reactions as the key strategy

The total syntheses of selected natural products using different versions of the Ugi multicomponent reaction is reviewed on a case-by-case basis. The revision covers the period 2008-2023 and includes detailed descriptions of the synthetic sequences, the use of state-of-the-art chemical reagents and strategies, as well as the advantages and limitations of the transformation and some remedial solutions. Relevant data on the isolation and bioactivity of the different natural targets are also briefly provided. The examples clearly evidence the strategic importance of this transformation and its key role in the modern natural products synthetic chemistry toolbox. This methodology proved to be a valuable means for easily building molecular complexity and efficiently delivering step-economic syntheses even of intricate structures, with a promising future.

Protein target highlights in CASP15: Analysis of models by structure providers

We present an in-depth analysis of selected CASP15 targets, focusing on their biological and functional significance. The authors of the structures identify and discuss key protein features and evaluate how effectively these aspects were captured in the submitted predictions. While the overall ability to predict three-dimensional protein structures continues to impress, reproducing uncommon features not previously observed in experimental structures is still a challenge. Furthermore, instances with conformational flexibility and large multimeric complexes highlight the need for novel scoring strategies to better emphasize biologically relevant structural regions. Looking ahead, closer integration of computational and experimental techniques will play a key role in determining the next challenges to be unraveled in the field of structural molecular biology.

A Potent Kalihinol Analogue Disrupts Apicoplast Function and Vesicular Trafficking in P. falciparum Malaria

Here we report the discovery of MED6-189, a new analogue of the kalihinol family of isocyanoterpene (ICT) natural products. MED6-189 is effective against drug-sensitive and -resistant P. falciparum strains blocking both intraerythrocytic asexual replication and sexual differentiation. This compound was also effective against P. knowlesi and P. cynomolgi. In vivo efficacy studies using a humanized mouse model of malaria confirms strong efficacy of the compound in animals with no apparent hemolytic activity or apparent toxicity. Complementary chemical biology, molecular biology, genomics and cell biological analyses revealed that MED6-189 primarily targets the parasite apicoplast and acts by inhibiting lipid biogenesis and cellular trafficking. Genetic analyses in P. falciparum revealed that a mutation in PfSec13, which encodes a component of the parasite secretory machinery, reduced susceptibility to the drug. The high potency of MED6-189 in vitro and in vivo, its broad range of efficacy, excellent therapeutic profile, and unique mode of action make it an excellent addition to the antimalarial drug pipeline.

Metal Free Dötz-Type Aminobenzannulation Reaction via 1,1-Dipoles Cross-Coupling

Aryl amines are of constant interest in organic synthesis owing to their ubiquity in natural products, pharmaceuticals, and organic materials. However, C-H amination or pre-functionalization frequently results in uncontrollable site selectivity, over activation and the generation of inseparable mixtures of regio-isomers. Here we present a novel metal free Dötz-type aminobenzannulation reaction that circumvents the selectivity issues inherent in aromatic chemistry, as well as the use of stoichiometric unstable organolithium reagents and toxic chromium complexes. The concept of utilizing readily available isocyanides and Morita-Baylis-Hillman (MBH) carbonates to achieve 1,1-dipoles cross-coupling to construct ketenimine is the key to success, which has been experimentally and computationally verified. The tandem 6π-electrocyclization/aromatization process offers a versatile method for synthesizing functionalized anilines, fused aryl amines and fused heteroaryl amines.

Surface immobilization strategies for the development of electrochemical nucleic acid sensors

Following the recent pandemic and with the emergence of cell-free nucleic acids in liquid biopsies as promising biomarkers for a broad range of pathologies, there is an increasing demand for a new generation of nucleic acid tests, with a particular focus on cost-effective, highly sensitive and specific biosensors. Easily miniaturized electrochemical sensors show the greatest promise and most typically rely on the chemical functionalization of conductive materials or electrodes with sequence-specific hybridization probes made of standard oligonucleotides (DNA or RNA) or synthetic analogues (e.g. Peptide Nucleic Acids or PNAs). The robustness of such sensors is mostly influenced by the ability to control the density and orientation of the probe at the surface of the electrode, making the chemistry used for this immobilization a key parameter. This exhaustive review will cover the various strategies to immobilize nucleic acid probes onto different solid electrode materials. Both physical and chemical immobilization techniques will be presented. Their applicability to specific electrode materials and surfaces will also be discussed as well as strategies for passivation of the electrode surface as a way of preventing electrode fouling and reducing nonspecific binding.

A Strategy for Controlling the Polymerizations of Thiyl Radical Propagation by RAFT Agents

The ability to extend the polymerizations of thiyl radical propagation to be regulated by existing controlled methods would be highly desirable, yet remained very challenging to achieve because the thiyl radicals still cannot be reversibly controlled by these methods. In this article, we reported a novel strategy that could enable the radical ring-opening polymerization of macrocyclic allylic sulfides, wherein propagating specie is thiyl radical, to be controlled by reversible addition-fragmentation chain transfer (RAFT) agents. The key to the success of this strategy is the propagating thiyl radical can undergo desulfurization with isocyanide and generate a stabilized alkyl radical for reversible control. Systematic optimization of the reaction conditions allowed good control over the polymerization, leading to the formation of polymers with well-defined architectures, exemplified by the radical block copolymerization of macrocyclic allylic sulfides and vinyl monomers and the incorporation of sequence-defined segments into the polymer backbone. This work represents a significant step toward directly enabling the polymerizations of heteroatom-centered radical propagation to be regulated by existing reversible-deactivation radical polymerization techniques.

Selective steroidogenic cytochrome P450 haem iron ligation by steroid-derived isonitriles

Alkyl isonitriles, R-NC, have previously been shown to ligate the heme (haem) iron of cytochromes P450 in both accessible oxidation states (ferrous, Fe2+, and ferric, Fe3+). Herein, the preparation of four steroid-derived isonitriles and their interactions with several P450s, including the steroidogenic CYP17A1 and CYP106A2, as well as the more promiscuous drug metabolizers CYP3A4 and CYP2D6, is described. It was found that successful ligation of the heme iron by the isonitrile functionality for a given P450 depends on both the position and stereochemistry of the isonitrile on the steroid skeleton. Spectral studies indicate that isonitrile ligation of the ferric heme is stable upon reduction to the ferrous form, with reoxidation resulting in the original complex. A crystallographic structure of CYP17A1 with an isonitrile derived from pregnanalone further confirmed the interaction and identified the absolute stereochemistry of the bound species.

Rapid Access to Fused Tetracyclic N-Heterocycles via Amino-to-Alkyl 1,5-Palladium Migration Coupled with Intramolecular C(sp3)-C(sp2) Coupling

An unprecedented route for the preparation of fused tetracyclic N-heterocycles is presented through the palladium-catalyzed cyclization of isocyanides with alkyne-tethered aryl iodides. In this transformation, a novel amino-to-alkyl 1,5-palladium migration/intramolecular C(sp3)-C(sp2) coupling sequence was observed first. More importantly, isocyanide exhibited three roles, serving simultaneously as a C1 synthon, a C1N1 synthon, and the donor of C(sp3) for C(sp3)-C(sp2) coupling, and the reaction was the sole successful example that achieved C(sp3)-H activation of isocyanide.

Palladium-Catalyzed Tandem C(sp3)-H Insertion Cyclization of 2-(2-Vinylarene)acetonitriles with Isocyanides to Access Naphthalen-2-amines

A Pd-catalyzed cyclization reaction of 2-(2-vinylarene)acetonitriles and isocyanides has been documented. Various naphthalen-2-amines were obtained in moderate to good yields under mild conditions. The in vitro cytotoxicity of all products was evaluated by MTT assay against seven human cancer cell lines. The results indicated that compounds 3ea, 3ma, and 3ob exhibited effective anticancer activities against the tested cancer cells.

Accessing indole-isoindole derivatives via palladium-catalyzed [3+2] cyclization of isocyanides with alkynyl imines

A facile protocol for the preparation of indole-isoindole derivatives was developed and proceeds via a palladium-catalyzed [3+2] cyclization of isocyanides with alkynyl imines. In this transformation, the palladium catalyst has a triple role, serving simultaneously as a π acid, a transition-metal catalyst and a hydride ion donor, thus enabling the dual function of isocyanide both as a C1 synthon for cyanation and a C1N1 synthon for imidoylation. Significantly, the reaction is the sole successful example for accessing indole-isoindole derivatives, and will open up new avenues to assemble unique N-heterocycle frameworks. Furthermore, the synthetic value of this protocol is demonstrated in the late-stage modification of physiologically active molecules and in the construction of aggregation-induced emission compounds.

Changes in an Enzyme Ensemble During Catalysis Observed by High Resolution XFEL Crystallography

Enzymes populate ensembles of structures with intrinsically different catalytic proficiencies that are difficult to experimentally characterize. We use time-resolved mix-and-inject serial crystallography (MISC) at an X-ray free electron laser (XFEL) to observe catalysis in a designed mutant (G150T) isocyanide hydratase (ICH) enzyme that enhances sampling of important minor conformations. The active site exists in a mixture of conformations and formation of the thioimidate catalytic intermediate selects for catalytically competent substates. A prior proposal for active site cysteine charge-coupled conformational changes in ICH is validated by determining structures of the enzyme over a range of pH values. A combination of large molecular dynamics simulations of the enzyme in crystallo and time-resolved electron density maps shows that ionization of the general acid Asp17 during catalysis causes additional conformational changes that propagate across the dimer interface, connecting the two active sites. These ionization-linked changes in the ICH conformational ensemble permit water to enter the active site in a location that is poised for intermediate hydrolysis. ICH exhibits a tight coupling between ionization of active site residues and catalysis-activated protein motions, exemplifying a mechanism of electrostatic control of enzyme dynamics.

Mining for a new class of fungal natural products: the evolution, diversity, and distribution of isocyanide synthase biosynthetic gene clusters

The products of non-canonical isocyanide synthase (ICS) biosynthetic gene clusters (BGCs) mediate pathogenesis, microbial competition, and metal-homeostasis through metal-associated chemistry. We sought to enable research into this class of compounds by characterizing the biosynthetic potential and evolutionary history of these BGCs across the Fungal Kingdom. We amalgamated a pipeline of tools to predict BGCs based on shared promoter motifs and located 3800 ICS BGCs in 3300 genomes, making ICS BGCs the fifth largest class of specialized metabolites compared to canonical classes found by antiSMASH. ICS BGCs are not evenly distributed across fungi, with evidence of gene-family expansions in several Ascomycete families. We show that the ICS dit1/2 gene cluster family (GCF), which was prior only studied in yeast, is present in ∼30% of all Ascomycetes. The dit variety ICS exhibits greater similarity to bacterial ICS than other fungal ICS, suggesting a potential convergence of the ICS backbone domain. The evolutionary origins of the dit GCF in Ascomycota are ancient and these genes are diversifying in some lineages. Our results create a roadmap for future research into ICS BGCs. We developed a website (https://isocyanides.fungi.wisc.edu/) that facilitates the exploration and downloading of all identified fungal ICS BGCs and GCFs.

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON]^2- = [O(SiMe₂NDipp)₂]^2-, Dipp = 2,6-iPr₂C₆H₃) with a series of isocyanide substrates (R-NC). In the case of tBu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κC and -κN compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C₃-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents. In contrast, using adamantyl isocyanide Ad-NC allowed both the C₂- and C₃-homologation products to be isolated, allowing a degree of control to be exercised over the chain growth process. These data also show that the reaction proceeds through a stepwise addition, supported in this study by the synthesis of the mixed [(Ad-NC)₂(Dmp-NC)]^2- product. Computational analysis of the bonding within the homologised products confirm a high degree of multiple bond character in the exocyclic ketenimine units of the C₂- and C₃-products. In addition, the mechanism of chain growth was investigated, identifying different possible pathways leading to the observed products, and highlighting the importance of the potassium cation in formation of the initial C₂-chain.

Preparation and Optical Study of 1-Formamido-5-Isocyanonaphthalene, the Hydrolysis Product of the Potent Antifungal 1,5-Diisocyanonaphthalene

Aromatic isocyanides have gained a lot of attention lately as promising antifungal and anticancer drugs, as well as high-performance fluorescent analytical probes for the detection of toxic metals, such as mercury, even in vivo. Since this topic is relatively new and aromatic isocyanides possess unique photophysical properties, the understanding of structure-behavior relationships and the preparation of novel potentially biologically active derivatives are of paramount importance. Here, we report the photophysical characterization of 1,5-diisocyanonaphthalene (DIN) backed by quantum chemical calculations. It was discovered that DIN undergoes hydrolysis in certain solvents in the presence of oxonium ions. By the careful control of the reaction conditions for the first time, the nonsymmetric product 1-formamido-5-isocyanonaphthalene (ICNF) could be prepared. Contrary to expectations, the monoformamido derivative showed a significant solvatochromic behavior with a ~50 nm range from hexane to water. This behavior was explained by the enhanced H-bond-forming ability of the formamide group. The significance of the hydrolysis reaction is that the isocyano group is converted to formamide in living organisms. Therefore, ICNF could be a potential drug (for example, antifungal) and the reaction can be used as a model for the preparation of other nonsymmetric formamido-isocyanoarenes. In contrast to its relative 1-amino-5-iscyanonaphthalene (ICAN), ICNF is highly fluorescent in water, enabling the development of a fluorescent turnoff probe.

Mining for a New Class of Fungal Natural Products: The Evolution, Diversity, and Distribution of Isocyanide Synthase Biosynthetic Gene Clusters

The products of non-canonical isocyanide synthase (ICS) biosynthetic gene clusters (BGCs) have notable bioactivities that mediate pathogenesis, microbial competition, and metal-homeostasis through metal-associated chemistry. We sought to enable research into this class of compounds by characterizing the biosynthetic potential and evolutionary history of these BGCs across the Fungal Kingdom. We developed the first genome-mining pipeline to identify ICS BGCs, locating 3,800 ICS BGCs in 3,300 genomes. Genes in these clusters share promoter motifs and are maintained in contiguous groupings by natural selection. ICS BGCs are not evenly distributed across fungi, with evidence of gene-family expansions in several Ascomycete families. We show that the ICS dit1 / 2 gene cluster family (GCF), which was thought to only exist in yeast, is present in ∼30% of all Ascomycetes, including many filamentous fungi. The evolutionary history of the dit GCF is marked by deep divergences and phylogenetic incompatibilities that raise questions about convergent evolution and suggest selection or horizontal gene transfers have shaped the evolution of this cluster in some yeast and dimorphic fungi. Our results create a roadmap for future research into ICS BGCs. We developed a website ( www.isocyanides.fungi.wisc.edu ) that facilitates the exploration, filtering, and downloading of all identified fungal ICS BGCs and GCFs.

Identification of Isonitrile-Containing Natural Products in Complex Biological Matrices through Ligation with Chlorooximes

Isonitrile-containing natural products have garnered attention for their manifold bioactivities but are difficult to detect and isolate due to the chemical lability of the isonitrile functional group. Here, we used the isonitrile-chlorooxime ligation (INC) in a reactivity-based screening (RBS) protocol for the detection and isolation of alkaloid and terpene isonitriles in the cyanobacterium Fischerella ambigua and a marine sponge of the order Bubarida, respectively. A trifunctional probe bearing a chlorooxime moiety, a UV active aromatic moiety, and a bromine label facilitated the chemoselective reaction with isonitriles, UV-Vis spectroscopic detection, and mass spectrometric analysis. The INC-based RBS allowed for the detection, isolation, and structural elucidation of isonitriles in microgram quantities.

Isocyanides in med chem: A scaffold hopping approach for the identification of novel 4-isocyanophenylamides as potent antibacterial agents against methicillin-resistant Staphylococcusaureus

We describe the rational use of the neglected isocyano moiety as pharmacophoric group for the design of novel 4-isocyanophenylamides as antibacterial agents. This class of novel compounds showed to be highly effective against methicillin resistant Staphylococcus aureus strains. In particular, from an extensive screening, we identified compound 42 as lead compound. It has shown a potent antimicrobial activity, an additive effect with most antibiotics currently in use, the ability not to induce the formation of resistant strains after ten passages, and the ability to block the biofilm formation. A nontoxic profile on mammalian cells and a proper metabolic stability on human liver microsome complete the picture of this new weapon against methicillin resistant Staphylococcus aureus infections.

Recent Advances in the Electrochemical Functionalization of Isocyanides

Isocyanides are well-known as efficient CO surrogates and C1 synthons in modern organic synthesis. Although tremendous efforts have been devoted to fully exploiting the reactivity of isocyanides, these transformations are primarily limited by their utilization of stoichiometric toxic chemical oxidants. With the recent resurgence of organic electrochemistry, which has considerably laid dormant over the past several decades, electrolysis has been identified as a green and powerful tool to enrich structural diversity by solely utilizing electric current as clean and inherently safe redox equivalents of stoichiometric chemical oxidants. In this regard, the unique reactivity of isocyanides has been studied in numerous electrochemical transformations. This review comprehensively highlights the most relevant progress in electrochemical strategies towards the functionalization of isocyanides up until June of 2022, with a focus on reaction outcomes and mechanisms.

Stereoselective Synthesis of (E)- and (Z)-Isocyanoalkenes

(E)- and (Z)-isocyanoalkenes were selectively synthesized via the sequential cross coupling of vinyl iodides with formamide, followed by dehydration. The optimal catalyst, generated in situ from CuII and trans-N,N'-dimethyl-1,2-cyclohexanediamine, rapidly coupled (E)- or (Z)-vinyl iodides with formamide, which minimized the isomerization of the resultant vinyl formamide. The method efficiently provided a range of acyclic, carbocyclic, and heterocyclic isocyanoalkenes; the versatility is illustrated with the selective, stereodivergent syntheses of the diastereomeric isocyanoalkene antibiotics, B371 and E-B371.

Scissor-like Face to Face π-π Stacking: A Surprising Preference Induced by the Isocyano Group in the Self-Assembled Dimer of Phenyl Isocyanide

Phenyl isocyanide has been chosen as a prototype to probe the π-π interaction modulated by the -NC group, which has a chameleonic nature with two main resonance forms showing a triple bond and being carbenoid. The rotational spectroscopic investigation complemented with theoretical analyses indicates that the phenyl isocyanide dimer has a scissor-like configuration controlled by dispersive forces along with the formation of π-π stacking. This is the first rotational spectroscopic evidence, to the best of our knowledge, that the mono-substitution by an -NC group on benzene can activate the meta position in forming noncovalent interactions. This work also provides experimental evidence on the importance of substituent effects in modulating π-π stacked structures, as well as practical proof of a biased interaction behavior of isocyanide-substituted aromatic molecules.

Correlating the orbital overlap area and vibrational frequency shift of an isocyanide moiety adsorbed on Pt and Pd covered Au(111) surfaces

Orbital interactions between adsorbed molecules and the underlying metal surfaces play critical roles in a wide range of surface and interfacial processes. Establishing a correlation between an experimental observable (e.g., vibrational frequency shift of the adsorbed molecule) and the orbital interactions is of vital importance. Herein, theoretical calculations are used to investigate the vibrational frequency shift of phenyl isocyanide molecules as a probe molecule adsorbed on mono- and bi-layer Pt and Pd covered Au(111) surfaces and Pd₂Au₄ and Pt₂Au₄ clusters. By analyzing the density of states (DOS) of the adsorption system, we show that the orbital overlap area of d electronic DOS with a molecular σ or π* orbital, particularly their ratio (R_d-σ/d-π*), can be a meaningful descriptor to explain the frequency shift of the CN moiety. This hypothesis has been verified by simulations for phenyl isocyanide with electron donating NH₂- and withdrawing CF₃- substituent groups, formonitrile and carbon monoxide. Quasi-linear dependence of the frequency shift on R_d-σ/d-π* is observed for both the red and blue shift regions. Our findings build up on previous notions of electronic interactions, which will provide a more quantitative and solid footing to understand and analyze the frequency shift of adsorbed molecules on metal surfaces and the related electronic interactions and catalytic properties.

N-Formylsaccharin: A Sweet(able) Formylating Agent in Mechanochemistry

The acylation of amines has always attracted a deep interest as a synthetic route due to its high versatility in organic chemistry and biochemical processes. The purpose of this article is to present a mechanochemical acylation procedure based on the use of acyl-saccharin derivatives, namely N-formylsaccharin, N-acetylsaccharin, and N-propionylsaccharin. This protocol furnishes a valuable solvent-free alternative to the existing processes and aims to be highly beneficial in multi-step procedures due to its rapid and user-friendly workup.

Anti-inflammatory aromadendrane- and cadinane-type sesquiterpenoids from the South China Sea sponge Acanthella cavernosa

One new aromadendrane-type sesquiterpenoid, namely ximaocavernosin P [(+)-1], and three new cadinane-type sesquiterpenoids, namely (+)-maninsigin D [(+)-4], (+)- and (−)-ximaocavernosin Q [(+)- and (−)-5], together with five related known ones [2, 3, (−)-4, 6, and 7], were isolated from the Hainan sponge Acanthella cavernosa. Compounds 4 and 5 were isolated as racemic forms, which were further separated to the corresponding enantiomers [(+)-4/(−)-4 and (+)-5/(−)-5], respectively, by using chiral-phase HPLC. The structures of new compounds were elucidated by extensive spectroscopic analysis and comparison with the reported data. In addition, the absolute configuration of optically pure (+)-1 and 2 were determined by time-dependent density functional theory/electronic circular dichroism (TDDFT-ECD) calculations or X-ray diffraction analysis. A plausible biosynthetic pathway of these sesquiterpenoids and their internal correlation were proposed and discussed. In an in vitro bioassay, (+)-aristolone (3) exhibited promising anti-inflammatory activity by the inhibition of LPS-induced TNF-α and CCL2 release in RAW 264.7 macrophages.

Probing the Mechanism of Isonitrile Formation by a Non-Heme Iron(II)-Dependent Oxidase/Decarboxylase

The isonitrile moiety is an electron-rich functionality that decorates various bioactive natural products isolated from diverse kingdoms of life. Isonitrile biosynthesis was restricted for over a decade to isonitrile synthases, a family of enzymes catalyzing a condensation reaction between l-Trp/l-Tyr and ribulose-5-phosphate. The discovery of ScoE, a non-heme iron(II) and α-ketoglutarate-dependent dioxygenase, demonstrated an alternative pathway employed by nature for isonitrile installation. Biochemical, crystallographic, and computational investigations of ScoE have previously been reported, yet the isonitrile formation mechanism remains obscure. In the present work, we employed in vitro biochemistry, chemical synthesis, spectroscopy techniques, and computational simulations that enabled us to propose a plausible molecular mechanism for isonitrile formation. Our findings demonstrate that the ScoE reaction initiates with C5 hydroxylation of (R)-3-((carboxymethyl)amino)butanoic acid to generate 1, which undergoes dehydration, presumably mediated by Tyr96 to synthesize 2 in a trans configuration. (R)-3-isocyanobutanoic acid is finally generated through radical-based decarboxylation of 2, instead of the common hydroxylation pathway employed by this enzyme superfamily.

Deciphering the Reaction Pathway of Mononuclear Iron Enzyme-Catalyzed N≡C Triple Bond Formation in Isocyanide Lipopeptide and Polyketide Biosynthesis

Despite the diversity of reactions catalyzed by 2-oxoglutarate-dependent nonheme iron (Fe/2OG) enzymes identified in recent years, only a limited number of these enzymes have been investigated in mechanistic detail. In particular, several Fe/2OG-dependent enzymes capable of catalyzing isocyanide formation have been reported. While the glycine moiety has been identified as a biosynthon for the isocyanide group, how the actual conversion is effected remains obscure. To elucidate the catalytic mechanism, we characterized two previously unidentified (AecA and AmcA) along with two known (ScoE and SfaA) Fe/2OG-dependent enzymes that catalyze N≡C triple bond installation using synthesized substrate analogues and potential intermediates. Our results indicate that isocyanide formation likely entails a two-step sequence involving an imine intermediate that undergoes decarboxylation-assisted desaturation to yield the isocyanide product. Results obtained from the in vitro experiments are further supported by mutagenesis, the product-bound enzyme structure, and in silico analysis.

Catalytic Metal-Enabled Romance of Isocyanides for Use as “C1N1” Synthons in Cyclization: Beyond Radical Chemistry

Cyclizative reactions have attracted considerable attentions in organic synthesis with regard to the high atom economy and synthetic efficiency towards cyclic architectures. Especially, isocyanide-based cyclizations have proven to be a...

Rationally designed conjugated microporous polymers for efficient photocatalytic chemical transformations of isocyanides

Click-based conjugated microporous polymers have been rationally designed and prepared for efficient N–H insertion like reaction of aryl isocyanides and photosynthesis of thiocarbamates.

Hydroarylation of Enamides Enabled by HFIP via a Hexafluoroisopropyl Ether as Iminium Reservoir

Here we describe that HFIP greatly expands the scope with respect to both reaction partners of the Brønsted acid-catalyzed hydroarylation of enamides. The reaction is fast and practical and can...

Can the isonitrile biosynthesis enzyme ScoE assist with the biosynthesis of isonitrile groups in drug molecules? A computational study

Computational studies show that the isonitrile synthesizing enzyme ScoE can catalyse the conversion of γ-Gly substituents in substrates to isonitrile. This enables efficient isonitrile substitution into target molecules such as axisonitrile-1.

Fe(III)-Catalyzed N-Amidomethylation of Secondary and Primary Anilines with TosMIC

A Fe(III)-catalyzed N-amidomethylation of secondary and primary anilines with p-toluenesulfonylmethyl isocyanide (TosMIC) in water is described. TosMIC plays dual roles as the source of methylene as well as an amidating reagent to form α-amino amides in this multicomponent reaction. The combination of TosMIC and other isocyanides was also investigated to give the desired products in acceptable yields. The current protocol features use of iron catalyst and nontoxic media, broad substrate scope, mild conditions, and operational simplicity.

The Dark Side of Isocyanides: Visible-Light Photocatalytic Activity in the Oxidative Functionalization of C(sp3)-H Bonds

The possibility to harness aromatic isocyanides as visible-light photocatalysts in the α-amino C(sp³)-H functionalization is herein presented. Actually, the three-component cross-dehydrogenative coupling of aromatic tertiary amines with isocyanides and water leads to amide products under very mild conditions in high yields and with a good substrate scope. While the reaction with aromatic isocyanides proceeds upon direct photoexcitation, aliphatic isocyanides are able to form a photoactive electron-donor-acceptor complex with aromatic amines. Moreover, the use of a catalytic loading of an aromatic isocyanide promotes the oxidative coupling of N-phenyl-1,2,3,4-tetrahydroisoquinoline with an array of different (pro)nucleophiles in good to excellent yields, thus providing the proof-of-concept for the development of a new highly tunable class of organic visible-light photocatalysts.

Experimental Treatment of Hazardous Ash Waste by Microbial Consortium Aspergillus niger and Chlorella sp.: Decrease of the Ni Content and Identification of Adsorption Sites by Fourier-Transform Infrared Spectroscopy

Despite the negative impact on the environment, incineration is one of the most commonly used methods for dealing with waste. Besides emissions, the production of ash, which usually shows several negative properties, such as a higher content of hazardous elements or strongly alkaline pH, is problematic from an environmental viewpoint as well. The subject of our paper was the assessment of biosorption of Ni from ash material by a microbial consortium of Chlorella sp. and Aspergillus niger. The solid substrate represented a fraction of particles of size <0.63 mm with a Ni content of 417 mg kg-1. We used a biomass consisting of two different organisms as the sorbent: a non-living algae culture of Chlorella sp. (an autotrophic organism) and the microscopic filamentous fungus A. niger (a heterotrophic organism) in the form of pellets. The experiments were conducted under static conditions as well as with the use of shaker (170 rpm) with different modifications: solid substrate, Chlorella sp. and pellets of A. niger; solid substrate and pellets of A. niger. The humidity-temperature conditions were also changed. Sorption took place under dry and also wet conditions (with distilled water in a volume of 30-50 ml), partially under laboratory conditions at a temperature of 25°C as well as in the exterior. The determination of the Ni content was done using inductively coupled plasma optical emission spectrometry (ICP-OES). The removal of Ni ranged from 13.61% efficiency (Chlorella sp., A. niger with the addition of 30 ml of distilled water, outdoors under static conditions after 48 h of the experiment) to 46.28% (Chlorella sp., A. niger with the addition of 30 ml of distilled water, on a shaker under laboratory conditions after 48 h of the experiment). For the purpose of analyzing the representation of functional groups in the microbial biomass and studying their interaction with the ash material, we used Fourier-transform infrared (FTIR) spectroscopy. We observed that the amount of Ni adsorbed positively correlates with absorbance in the spectral bands where we detect the vibrations of several organic functional groups. These groups include hydroxyl, aliphatic, carbonyl, carboxyl and amide structural units. The observed correlations indicate that, aside from polar and negatively charged groups, aliphatic or aromatic structures may also be involved in sorption processes due to electrostatic attraction. The correlation between absorbance and the Ni content reached a maximum in amide II band (r = 0.9; P < 0.001), where vibrations of the C=O, C-N, and N-H groups are detected. The presented results suggest that the simultaneous use of both microorganisms in biosorption represents an effective method for reducing Ni content in a solid substrate, which may be useful as a partial process for waste disposal.