Marine Indole Alkaloids

Photochemical domino reaction driven C-H/S-H functionalization of bioactive molecules to access xanthene scaffolds

A visible-light-induced C(sp2)-H functionalization of indoles by using Schreiner's thiourea as the organocatalyst has been reported. With the aid of a three-component domino reaction between 2-hydroxybenzaldehydes, cyclic-1,3-diketones, and a variety of indoles, the corresponding densely functionalized xanthene scaffolds were isolated in good to excellent yields. Apart from these, a broad range of other bioactive natural products including kojic acid, lawsone, and 4-hydroxycoumarin were also investigated instead of indoles for the present work. All the molecules participated in the photochemical reaction smoothly and provided the desired xanthenes in synthetically valuable yields. Therefore, the present energy-efficient catalytic strategy was also very successful in executing challenging carbon-sulfur bond formation reactions, demonstrating the synthetic potentiality of the work. Notably, this air-stable, transition metal-free approach with broad functional group tolerability provides an alternative to conventional methods.

Photoredox-Catalyzed Cross-Coupling of In Situ Generated Quinoxalinones with Indoles for the Synthesis of Tertiary Alcohols

A visible light-driven photoredox-catalyzed direct C(sp2)-H functionalization of N-H free indoles with quinoxalinones generated in situ from 2,2-dihydroxy-1H-indene-1,3(2H)-dione and phenylene-1,2-diamines has been reported with the aid of Na2-Eosin Y as the photocatalyst and the Hünig base as the sacrificial electron and proton donor. The reaction provides easy access to a variety of quaternary-centered C-3 selective indole-substituted tertiary alcohols in good yields. Mechanistic studies demonstrated the realization of photoredox-catalyzed in situ quinoxalinone formation and their proton-coupled single electron reduction to the corresponding ketyl radicals followed by cross-coupling with indoles. The potential applications of the synthesized tertiary alcohols in photoacid-catalyzed carbon-carbon and carbon-sulfur bond-forming reactions feature the key findings of the present work.

Discovery of indolylacryloyl-derived oxacins as novel potential broad-spectrum antibacterial candidates

The emergence of serious bacterial resistance towards clinical oxacins poses a considerable threat to global public health, necessitating the development of novel structural antibacterial agents. Seven types of novel indolylacryloyl-derived oxacins (IDOs) were designed and synthesized for the first time from commercial 3,4-difluoroaniline via an eight-step procedure. The synthesized compounds were characterized by modern spectroscopic techniques. All target molecules were evaluated for antimicrobial activities. Most of the prepared IDOs showed a broad antibacterial spectrum and strong activities against the tested strains, especially ethoxycarbonyl IDO 10d (0.25-0.5 μg/mL) and hydroxyethyl IDO 10e (0.25-1 μg/mL) exhibited much superior antibacterial efficacies to reference drug norfloxacin. These highly active IDOs also displayed low hemolysis, cytotoxicity and resistance, as well as rapid bactericidal capacity. Further investigations indicated that ethoxycarbonyl IDO 10d and hydroxyethyl IDO 10e could effectively reduce the exopolysaccharide content and eradicate the formed biofilm, which might delay the development of drug resistance. Preliminary exploration of the antibacterial mechanism revealed that active IDOs could not only destroy membrane integrity, resulting in changes in membrane permeability, but also promote the accumulation of reactive oxygen species, leading to the production of malondialdehyde and decreased bacterial metabolism. Moreover, they exhibited the capability to bind with DNA and DNA gyrase, forming supramolecular complexes through various noncovalent interactions, thereby inhibiting DNA replication and causing bacterial death. All the above results suggested that the newly developed indolylacryloyl-derived oxacins should hold great promise as potential multitargeting broad-spectrum antibacterial candidates to overcome drug resistance.

Natural product-derived ferroptosis mediators

It has been 11 years since ferroptosis, a new mode of programmed cell death, was first proposed. Natural products are an important source of drug discovery. In the past five years, natural product-derived ferroptosis regulators have been discovered in an endless stream. Herein, 178 natural products discovered so far to trigger or resist ferroptosis are classified into 6 structural classes based on skeleton type, and the mechanisms of action that have been reported are elaborated upon. If pharmacodynamic data are sufficient, the structure and bioactivity relationship is also presented. This review will provide medicinal chemists with some effective ferroptosis regulators, which will promote the research of natural product-based treatment of ferroptosis-related diseases in the future.

Transition-Metal-Free Synthesis of 2-Substituted Benzo[cd]Indoles via the Reaction of 1-Halo-8-lithionaphthalenes with Nitriles

A simple and effective organolithium approach to the synthesis of 2-substituted benzo[cd]indoles from peri-dihalonaphthalenes and nitriles has been developed. The reaction proceeds via a surprisingly easy intramolecular aromatic nucleophilic substitution facilitated by the "clothespin effect". The discovered transformation provides good isolated yields, allows usage of an extensive range of nitriles, and demonstrates a good substituents tolerance. UV-absorption and NMR spectra of the obtained benzo[cd]indoles and their protonated forms demonstrated exclusive protonation to the indole nitrogen atom even in the presence of two NMe2 groups in positions 5 and 6 (i. e. "proton sponge" moiety).

Marine Streptomyces-Derived Novel Alkaloids Discovered in the Past Decade

Natural alkaloids originating from actinomycetes and synthetic derivatives have always been among the important suppliers of small-molecule drugs. Among their biological sources, Streptomyces is the highest and most extensively researched genus. Marine-derived Streptomyces strains harbor unconventional metabolic pathways and have been demonstrated to be efficient producers of biologically active alkaloids; more than 60% of these compounds exhibit valuable activity such as antibacterial, antitumor, anti-inflammatory activities. This review comprehensively summarizes novel alkaloids produced by marine Streptomyces discovered in the past decade, focusing on their structural features, biological activity, and pharmacological mechanisms. Future perspectives on the discovery and development of novel alkaloids from marine Streptomyces are also provided.

CsF-Mediated Reaction of Trifluorodiazoethane with 3-Nitroindoles Enables Access to Trifluoromethylpyrazolo[4,3-b]indoles

A mild and metal-free strategy for the construction of trifluoromethylated pyrazolo[4,3-b]indoles through the reaction of N-substituted 3-nitroindoles with trifluorodiazoethane is reported. This operationally simple transformation involves a [3 + 2] cycloaddition of trifluorodiazoethane with 3-nitroindole, followed by the elimination of the nitro group to furnish pyrazole-fused indoles. The synthetic utility of this method is further demonstrated by applying it to other heterocycles, such as 3-nitrobenzothiophene and 2-nitrobenzofuran.

Oxygen Dependent Switchable Selectivity during Ruthenium Catalyzed Selective Synthesis of C3-Alkylated Indoles and Bis(indolyl)methanes

Herein, we report a ligand-centered redox-controlled oxygen-dependent switchable selectivity during ruthenium-catalyzed selective synthesis of C3-alkylated indoles and bis(indolyl)methanes (BIMs). A wide variety of C3-alkylated indoles and BIMs were prepared selectively in moderate to good isolated yields by coupling a wide variety of indoles and alcohols, catalyzed by a well-defined, air-stable, and easy-to-prepare Ru(II)-catalyst (1a) bearing a redox-active tridentate pincer (L1a). Catalyst 1a efficiently catalyzed the C3-alkylation of indoles under an argon atmosphere while, under an oxygen environment, exclusively producing the BIMs. A few drug molecules containing BIMs were also synthesized efficiently. 1a exhibited excellent chemoselectivity with alcohols containing internal carbon-carbon double bonds. Mechanistic investigation revealed that the coordinated azo-aromatic ligand actively participates during the catalysis. During the dehydrogenation of alcohols, the azo-moiety of the ligand stores the hydrogen removed from the alcohols and subsequently transfers the hydrogen to the alkylideneindolenine intermediate, forming the C3-alkylated indoles. While under an oxygen environment, the transfer of hydrogen from the ligand scaffold to the molecular oxygen generates H2O2, leaving no scope for hydrogenation of the alkylideneindolenine intermediate, rather than it undergoing 1,4-Michael-type addition forming the BIMs.

A Metagenomic Time-Series Approach to Assess the Ecological Stability of Microbial Mats in a Seasonally Fluctuating Environment

Microbial mats are complex ecological assemblages that have been present in the rock record since the Precambrian and can still be found in extant marginalized environments. These structures are considered highly stable ecosystems. In this study, we evaluate the ecological stability of dome-forming microbial mats in a modern, water-level fluctuating, hypersaline pond located in the Cuatro Ciénegas Basin, Mexico. We conducted metagenomic sampling of the site from 2016 to 2019 and detected 2250 genera of Bacteria and Archaea, with only <20 belonging to the abundant taxa (>1%). The microbial community was dominated by Proteobacteria, Euryarchaeota, Bacteroidetes, Firmicutes, and Cyanobacteria, and was compositionally sensitive to disturbances, leading to high taxonomic replacement even at the phylum level, with a significant increase in Archaea from [Formula: see text]1-4% to [Formula: see text]33% throughout the 2016-2019 study period. Although a core community represented most of the microbial community (>75%), relative abundances shifted significantly between samples, as demonstrated by changes in the abundance of Coleofasciculus from 10.2% in 2017 to 0.05% in 2019. Although functional differences between seasons were subtle, co-occurrence networks suggest differential ecological interactions between the seasons, with the addition of a new module during the rainy season and the potential shift in hub taxa. Functional composition was slightly more similar between samples, but basic processes such as carbohydrate, amino acid, and nucleic acid metabolisms were widely distributed among samples. Major carbon fixation processes included sulfur oxidation, nitrogen fixation, and photosynthesis (both oxygenic and anoxygenic), as well as the Wood-Ljundgahl and Calvin cycles.

Marine indole alkaloid diversity and bioactivity. What do we know and what are we missing?

Covering: marine indole alkaloids (n = 2048) and their reported bioactivities up to the end of 2021Despite increasing numbers of marine natural products (MNPs) reported each year, most have only been examined for cytotoxic, antibacterial, and/or antifungal biological activities with the majority found to be inactive in these assays. In this context, why are natural products continuing to be examined in assays they are unlikely to show significant activity in, and what targets might be more useful for expanding knowledge of their biologically relevant chemical space? We have undertaken a meta-analysis of the biological activities for 2048 marine indole alkaloids (MIAs), a diverse sub-class of MNPs reported up to the end of 2021, and this has highlighted that the bioactivity potentials for up to 86% of published MIAs remains underexplored and/or undefined. Although most published MIAs are not cytotoxic or antimicrobial, there is a continued focus on using these assays to evaluate new structurally related analogues. Using cheminformatics analyses, the chemical diversity of the 2048 MIAs were clustered using fragment based fingerprints and their reported bioactivity potency towards specific disease targets was assessed for structure activity trends. These analyses showed that there are groups of MIAs that possess potent and diverse activities and that many analogues, previously tested only in cellular toxicity assays, could be better exploited to generate structure activity relationships associated with leads to treat emerging diseases. A collection of indole drug and drug-lead structures from non-natural sources were also incorporated into the dataset providing complementary bioactivity profiles that were further used to predict underexplored areas of potential new activity and to better direct future testing of MIAs. Our findings clearly suggest the biological evaluation of MIAs continues to be conducted on a narrow range of bioassays and disease targets, and that shifting the focus to non-toxic disease targets should provide expanded knowledge of biologically relevant chemical space aimed at maximising the potential of MIAs for drug discovery.

Sequential annulation and isomerisation reaction of 3-acylmethylidene oxindoles with Huisgen zwitterions and synthesis of 5-(3-oxindolyl)oxazoles

Herein, we report a facile synthesis of 5-(3-oxindolyl)oxazole derivatives via a sequential annulation and isomerisation reaction of 3-acylmethylidene oxindoles with in situ generated Huisgen zwitterions (HZs) from PPh3 and azodicarboxylates. This reaction exhibits good functional group tolerance with 30 examples of structurally diverse products prepared with moderate to good efficiencies (up to 88% yield), thus providing a generally applicable route to the biologically important 5-(3-indolyl)oxazole structural motifs. Key to the success of this sequential one-pot strategy is the utilization of DBU as a base to promote the isomerisation process of the corresponding intermediate annulation products.

An N-N linked dimeric indole alkaloid from the marine sponge-associated rare actinomycetes Kocuria sp. S42

Marine derived rare actinomycetes is emerging as one of the new sources for various natural products for further drug discovery. Dimeric indole alkaloids represent a group of structurally diverse natural products and N-N linkage is a special dimerization mode. Here, we report the isolation of 1,1'-([1,1'-biindole]-3,3'-diyl) bis (ethane-1,2-diol), a new tryptophan-derived indole alkaloid from the marine sponge-derived Kocuria sp. S42. The structure was established based on extensive spectroscopic analyses, including nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass (HR-ESI-MS) spectrometry. The new dimeric indole alkaloid via N-N linkage exhibits moderate antimicrobial activity.

Indole alkaloids from marine resources: Understandings from therapeutic point of view to treat cancers

Cancer is the leading cause of mortality all over the world. Scientific investigation has demonstrated that disruptions in the process of autophagy are frequently interrelated with the emergence of cancer. Hence, scientists are seeking permanent solutions to counter the deadly disease. Indole alkaloids have been extensively studied and are acknowledged to exhibit several bioactivities. The current state of disease necessitates novel pharmacophores development. In recent decades, indole alkaloids have become increasingly significant in cancer treatment and are also used as adjuvants. A substantial amount of pharmacologically active molecules come from indole alkaloids, which are widely distributed in nature. Indole alkaloids derived from marine organisms show immense potential for therapeutic applications and seem highly effective in cancer treatment. A couple of experiments have been conducted preclinically to investigate the possibility of indole alkaloids in cancer treatment. Marine-derived indole alkaloids possess the ability to exhibit anticancer properties through diverse antiproliferative mechanisms. Certain indole alkaloids, including vincristine and vinblastine, were verified in clinical trials or are presently undergoing clinical assessments for preventing and treating cancer. Indole alkaloids from marine resources hold a significant functionality in identifying new antitumor agents. The current literature highlights recent advancements in indole alkaloids that appear to be anticancer agents and the underlying mechanisms.

Azo Coupling of Indoles Revisited: Synthesis of Biindolyl Photoswitches via the Azo-Coupling/C-H Functionalization Domino Approach

When azo coupling of aryldiazonium salts with indoles was carried out in aprotic nonpolar solvent on air, a pseudo-three-component reaction has been discovered. Azo coupling is followed by a nucleophilic addition of a second indole unit to the indolium intermediate; aromatization and oxidation are achieved under air.

2-Oxindole and related heterocycles: synthetic methodologies for their natural products and related derivatives

Natural goods, medications, and pharmaceutically active substances all contain substituted oxindoles. Generally, the C-3 stereocenter of the substituents of oxindoles and their absolute arrangement have a substantial impact on the bioactivity of these substances. In this case, the desire for contemporary probe and drug-discovery programs for the synthesis of chiral compounds using desirable scaffolds with high structural diversity further drives research in this field. Also, the new synthetic techniques are generally simple to apply for the synthesis of other similar scaffolds. Herein, we review the distinct approaches for the synthesis of diverse useful oxindole scaffolds. Specifically, the research findings on the naturally existing 2-oxindole core and a variety of synthetic compounds having a 2-oxindole core are discussed. We present an overview of the construction of oxindole-based synthetic and natural products. In addition, the chemical reactivity of 2-oxindole and its related derivatives in the presence of chiral and achiral catalysts are thoroughly discussed. The data compiled herein provides broad information related to the bioactive product design, development, and applications of 2-oxindoles and the reported techniques will be helpful for the investigation of novel reactions in the future.

Marine-Derived Lead Fascaplysin: Pharmacological Activity, Total Synthesis, and Structural Modification

Fascaplysin is a planar structure pentacyclic alkaloid isolated from sponges, which can effectively induce the apoptosis of cancer cells. In addition, fascaplysin has diverse biological activities, such as antibacterial, anti-tumor, anti-plasmodium, etc. Unfortunately, the planar structure of fascaplysin can be inserted into DNA and such interaction also limits the further application of fascaplysin, necessitating its structural modification. In this review, the biological activity, total synthesis and structural modification of fascaplysin will be summarized, which will provide useful information for pharmaceutical researchers interested in the exploration of marine alkaloids and for the betterment of fascaplysin in particular.

Virtual and In Vitro Screening of Natural Products Identifies Indole and Benzene Derivatives as Inhibitors of SARS-CoV-2 Main Protease (Mpro)

The rapid spread of the coronavirus disease 2019 (COVID-19) resulted in serious health, social, and economic consequences. While the development of effective vaccines substantially reduced the severity of symptoms and the associated deaths, we still urgently need effective drugs to further reduce the number of casualties associated with SARS-CoV-2 infections. Machine learning methods both improved and sped up all the different stages of the drug discovery processes by performing complex analyses with enormous datasets. Natural products (NPs) have been used for treating diseases and infections for thousands of years and represent a valuable resource for drug discovery when combined with the current computation advancements. Here, a dataset of 406,747 unique NPs was screened against the SARS-CoV-2 main protease (M^pro) crystal structure (6lu7) using a combination of ligand- and structural-based virtual screening. Based on 1) the predicted binding affinities of the NPs to the M^pro, 2) the types and number of interactions with the M^pro amino acids that are critical for its function, and 3) the desirable pharmacokinetic properties of the NPs, we identified the top 20 candidates that could potentially inhibit the M^pro protease function. A total of 7 of the 20 top candidates were subjected to in vitro protease inhibition assay and 4 of them (4/7; 57%), including two beta carbolines, one N-alkyl indole, and one Benzoic acid ester, had significant inhibitory activity against M^pro protease. These four NPs could be developed further for the treatment of COVID-19 symptoms.

Geobarrettin D, a Rare Herbipoline-Containing 6-Bromoindole Alkaloid from Geodia barretti

Geobarrettin D (1), a new bromoindole alkaloid, was isolated from the marine sponge Geodia barretti collected from Icelandic waters. Its structure was elucidated by 1D, and 2D NMR (including 1H-15N HSQC, 1H-15N HMBC spectra), as well as HRESIMS data. Geobarrettin D (1) is a new 6-bromoindole featuring an unusual purinium herbipoline moiety. Geobarrettin D (1) decreased secretion of the pro-inflammatory cytokine IL-12p40 by human monocyte derived dendritic cells, without affecting secretion of the anti-inflammatory cytokine IL-10. Thus, compound 1 shows anti-inflammatory activity.

Heteroaromatic Diazirines Are Essential Building Blocks for Material and Medicinal Chemistry

In materials (polymer) science and medicinal chemistry, heteroaromatic derivatives play the role of the central skeleton in development of novel devices and discovery of new drugs. On the other hand, (3-trifluoromethyl)phenyldiazirine (TPD) is a crucial chemical method for understanding biological processes such as ligand-receptor, nucleic acid-protein, lipid-protein, and protein-protein interactions. In particular, use of TPD has increased in recent materials science to create novel electric and polymer devices with comparative ease and reduced costs. Therefore, a combination of heteroaromatics and (3-trifluoromethyl)diazirine is a promising option for creating better materials and elucidating the unknown mechanisms of action of bioactive heteroaromatic compounds. In this review, a comprehensive synthesis of (3-trifluoromethyl)diazirine-substituted heteroaromatics is described.

Cascade Ring-Opening/Cyclization Reaction of Spiro(nitrocyclopropane)oxindoles with Huisgen Zwitterions and Synthesis of Pyrazolo[3,4-b]indoles

Herein, we report a ring-opening/cyclization cascade reaction of spiro(nitrocyclopropane)oxindoles with in situ generated Huisgen zwitterions (HZs) from PPh₃ and azodicarboxylates. This reaction provides an array of polyfunctionalized pyrazolo[3,4-b]indole derivatives in moderate-to-excellent yields and generally high stereoselectivities with a broad substrate scope. The annulation products obtained from di-tert-butyl azodicarboxylates can be readily transformed into aromatic-substituted pyrazolo[3,4-b]indoles in moderate yields upon treatment with trifluoroacetic acid, thus providing a new entry to this fused heterocycle skeleton. In terms of nitro-substituted donor-acceptor cyclopropane, this work significantly broadens the substrate scope for the annulation reaction of nitrocyclopropanes and HZs. The dual roles of the oxindole moiety in the ring opening of cyclopropane and a plausible mechanism for the cascade reaction are also discussed.

Cellular Stress-Induced Metabolites in Escherichia coli

Escherichia coli isolates commonly inhabit the human microbiota, yet the majority of E. coli's small-molecule repertoire remains uncharacterized. We previously employed erythromycin-induced translational stress to facilitate the characterization of autoinducer-3 (AI-3) and structurally related pyrazinones derived from "abortive" tRNA synthetase reactions in pathogenic, commensal, and probiotic E. coli isolates. In this study, we explored the "missing" tryptophan-derived pyrazinone reaction and characterized two other families of metabolites that were similarly upregulated under erythromycin stress. Strikingly, the abortive tryptophanyl-tRNA synthetase reaction leads to a tetracyclic indole alkaloid metabolite (1) rather than a pyrazinone. Furthermore, erythromycin induced two naphthoquinone-functionalized metabolites (MK-hCys, 2; and MK-Cys, 3) and four lumazines (7-10). Using genetic and metabolite analyses coupled with biomimetic synthesis, we provide support that the naphthoquinones are derived from 4-dihydroxy-2-naphthoic acid (DHNA), an intermediate in the menaquinone biosynthetic pathway, and the amino acids homocysteine and cysteine. In contrast, the lumazines are dependent on a flavin intermediate and α-ketoacids from the aminotransferases AspC and TyrB. We show that one of the lumazine members (9), an indole-functionalized analogue, possesses antioxidant properties, modulates the anti-inflammatory fate of isolated TH17 cells, and serves as an aryl-hydrocarbon receptor (AhR) agonist. These three systems described here serve to illustrate that new metabolic branches could be more commonly derived from well-established primary metabolic pathways.

Benthic exometabolites and their ecological significance on threatened Caribbean coral reefs

Benthic organisms are the architectural framework supporting coral reef ecosystems, but their community composition has recently shifted on many reefs. Little is known about the metabolites released from these benthic organisms and how compositional shifts may influence other reef life, including prolific microorganisms. To investigate the metabolite composition of benthic exudates and their ecological significance for reef microbial communities, we harvested exudates from six species of Caribbean benthic organisms including stony corals, octocorals, and an invasive encrusting alga, and subjected these exudates to untargeted and targeted metabolomics approaches using liquid chromatography-mass spectrometry. Incubations with reef seawater microorganisms were conducted to monitor changes in microbial abundances and community composition using 16 S rRNA gene sequencing in relation to exudate source and three specific metabolites. Exudates were enriched in amino acids, nucleosides, vitamins, and indole-based metabolites, showing that benthic organisms contribute labile organic matter to reefs. Furthermore, exudate compositions were species-specific, and riboflavin and pantothenic acid emerged as significant coral-produced metabolites, while caffeine emerged as a significant invasive algal-produced metabolite. Microbial abundances and individual microbial taxa responded differently to exudates from stony corals and octocorals, demonstrating that exudate mixtures released from different coral species select for specific bacteria. In contrast, microbial communities did not respond to individual additions of riboflavin, pantothenic acid, or caffeine. This work indicates that recent shifts in benthic organisms alter exudate composition and likely impact microbial communities on coral reefs.

Pd(II)/Lewis Acid Catalyzed Intramolecular Annulation of Indolecarboxamides with Dioxygen through Dual C-H Activation

Transition-metal ion catalyzed intramolecular dual C-H activation to construct polycyclic heteroarene skeletons is merited for its step and atom-economic advantages in organic synthesis. However, in most cases, stoichiometric oxidants, elevated temperature, and other harsh conditions were commonly faced for this reaction, which apparently block the synthetic applications. Herein, we report a Pd(II)/LA (LA: Lewis acid) catalyzed intramolecular dual C-H activation to construct indoloquinolinone derivatives under mild conditions with dioxygen as the sole oxidant. It was found that adding LA such as Sc³⁺ to Pd(OAc)₂ sharply improved its catalytic efficiency, whereas Pd(OAc)₂ alone was very sluggish. The activity improvement was attributed to the linkage of the Sc³⁺ cation to the Pd(II) species through a diacetate bridge that significantly enhanced the electrophilic properties of Pd(II) for dual C-H activation.

Total Syntheses of the Structures Assigned to the Marine Natural Products Orthoscuticellines A-E

The readily prepared and vinylated β-carboline 11 has been converted over one or two steps into compounds 1-5, the structures assigned to the recently reported marine natural products orthoscuticellines A-E. The spectral data recorded on the synthetically derived compounds are fully consistent with the assigned structures and, on making allowances for variations in the pH of the medium in which the spectra of the natural products were recorded, it is concluded that the structures assigned to orthoscuticellines A-E are most likely correct. Certainly, the calculated ¹³C NMR spectra of the α-, γ-, and δ-carboline isomers of compounds 1-5 suggest that orthoscuticellines A-E do incorporate the assigned β-carboline core.

Iodine pentoxide-mediated oxidative selenation and seleno/thiocyanation of electron-rich arenes

A simple and efficient method for the regioselective selenation of electron-rich arenes by employing non-metal inorganic iodine pentoxide (I₂O₅) as a reaction promoter under ambient conditions has been developed. The present protocol showed broad functional group tolerance and easy-to-operate and time-economical features. Additionally, this protocol also allows access to 3-seleno and 3-thiocyanoindoles by the use of readily available selenocyanate and thiocyanate salts. A mechanistic study indicated that the transformation operated through selenenyl iodide-induced electrophilic substitution processes.

Cyclopiazonic acid type indole alkaloids from Nicotiana tabacum-derived fungus Aspergillus versicolor and their anti-tobacco mosaic virus activities

Indole alkaloids have attracted widespread attention of chemists and biologists. Therefore, the aim of this study is to screen more bioactivities indole alkaloids from the microorganisms. In this study, five undescribed CPA-type indole alkaloids, aspergillines F-J, and three known CPA-type indole alkaloids, aspergilline A, aspergilline C, and cyclopiamide E, were obtained from the Nicotiana tabacum-derived fungus Aspergillus versicolor. Notably, aspergillines F and G represent the first examples of indole alkaloids with a benzo[cd]indol-2(1H)-one skeleton, and aspergilline J is also the firstly obtained indole alkaloids bearing a N-1-(2-(1H-imidazole-5-yl)ethyl) moiety. Aspergillines F-J and cyclopiamide E were tested for their anti-TMV activities, and the results revealed that aspergillines G and J exhibited obvious anti-TMV activities with inhibition rates of 41.2 and 56.8% at the concentration of 20 μM, respectively. These rates are high than that of positive control (with inhibition rate of 32.5%). In addition, the molecular docking studies for the isolated CPA-type indole alkaloids may also reveal that the benzo[cd]indol-2(1H)-one substructure is the fundamental for anti-TMV activity and the oxygen-containing substituent groups at C-19 also increases the inhibitory activity. This study of structure-activity relationship is helpful to find new anti-TMV activity inhibitors.

A Review on Bioactive Compounds from Marine-Derived Chaetomium Species

Filamentous marine fungi have proven to be a plentiful source of new natural products. Chaetomium, a widely distributed fungal genus in the marine environment, has gained much interest within the scientific community. In the last 20 years, many potential secondary metabolites have been detected from marine-derived Chaetomium. In this review, we attempt to provide a comprehensive summary of the natural products produced by marine-derived Chaetomium species. A total of 122 secondary metabolites that were described from 2001 to 2021 are covered. The structural diversity of the compounds, along with details of the sources and relevant biological properties are also provided, while the relationships between structures and their bioactivities are discussed. It is our expectation that this review will be of benefit to drug development and innovation.

Synthesis of functionalized indoles via cascade benzannulation strategies: a decade's overview

Indoles are one of the most prominent aromatic heterocycles in the organic chemistry field. Due to their widespread presence in various natural products, alkaloids, drugs, approved medicines, etc. the synthesis and functionalization of indoles are of great interest. This review emphasizes recent developments and techniques in the domino cascade cyclization process in the last decade starting from the various building blocks. In particular, this review depicts several intriguing benzannulation methods of creating a benzene ring on a pre-existing pyrrole nucleus in an inter/intramolecular fashion under metal-catalyzed/metal-free approaches. Different subsections focus on gradual timely developments in this complementary area and a detailed analysis of the mechanisms and reactivity patterns. As a complementary method, this review gives a significant incentive to various annulation strategies and also gives an overview of the remaining challenges and upcoming possibilities.

Novel Alkaloids from Marine Actinobacteria: Discovery and Characterization

The marine environment is an excellent resource for natural products with therapeutic potential. Its microbial inhabitants, often associated with other marine organisms, are specialized in the synthesis of bioactive secondary metabolites. Similar to their terrestrial counterparts, marine Actinobacteria are a prevalent source of these natural products. Here, we discuss 77 newly discovered alkaloids produced by such marine Actinobacteria between 2017 and mid-2021, as well as the strategies employed in their elucidation. While 12 different classes of alkaloids were unraveled, indoles, diketopiperazines, glutarimides, indolizidines, and pyrroles were most dominant. Discoveries were mainly based on experimental approaches where microbial extracts were analyzed in relation to novel compounds. Although such experimental procedures have proven useful in the past, the methodologies need adaptations to limit the chance of compound rediscovery. On the other hand, genome mining provides a different angle for natural product discovery. While the technology is still relatively young compared to experimental screening, significant improvement has been made in recent years. Together with synthetic biology tools, both genome mining and extract screening provide excellent opportunities for continued drug discovery from marine Actinobacteria.

Marine-Derived Indole Alkaloids and Their Biological and Pharmacological Activities

Novel secondary metabolites from marine macroorganisms and marine-derived microorganisms have been intensively investigated in the last few decades. Several classes of compounds, especially indole alkaloids, have been a target for evaluating biological and pharmacological activities. As one of the most promising classes of compounds, indole alkaloids possess not only intriguing structural features but also a wide range of biological/pharmacological activities including antimicrobial, anti-inflammatory, anticancer, antidiabetic, and antiparasitic activities. This review reports the indole alkaloids isolated during the period of 2016–2021 and their relevant biological/pharmacological activities. The marine-derived indole alkaloids reported from 2016 to 2021 were collected from various scientific databases. A total of 186 indole alkaloids from various marine organisms including fungi, bacteria, sponges, bryozoans, mangroves, and algae, are described. Despite the described bioactivities, further evaluation including their mechanisms of action and biological targets is needed to determine which of these indole alkaloids are worth studying to obtain lead compounds for the development of new drugs.

Lynamicin B is a Potential Pesticide by Acting as a Lepidoptera-Exclusive Chitinase Inhibitor

Insect group h chitinase is a promising target for designing non-target safe pesticides in that it is exclusively distributed in lepidopteran insects, over 80% of which are agricultural pests. In this work, lynamicin B was discovered to be an inhibitor of OfChi-h, the group h chitinase from the lepidopteran pest Ostrinia furnacalis. Lynamicin B was revealed to competitively inhibit OfChi-h with a K_i value of 8.76 μM and does not significantly inhibit other chitinases. The co-crystal structure of lynamicin B and OfChi-h revealed that the dichloroindolyl group of lynamicin B occupies an unexplored pocket below subsites +1 and +2 of the substrate-binding cleft, which is vital for its selectivity. Feeding experiments demonstrated that lynamicin B exhibited high insecticidal activities against other lepidopteran pests Mythimna separata and Spodoptera frugiperda besides O. furnacalis. Moreover, lynamicin B did not affect Trichogramma ostriniae, a natural enemy of O. furnacalis. This study provides a natural-derived potent pesticide for the control of lepidopteran pests, leaving its natural enemy unaffected.

Marine Indole Alkaloids-Isolation, Structure and Bioactivities

Indole alkaloids are heterocyclic natural products with extensive pharmacological activities. As an important source of lead compounds, many clinical drugs have been derived from natural indole compounds. Marine indole alkaloids, from unique marine environments with high pressure, high salt and low temperature, exhibit structural diversity with various bioactivities, which attracts the attention of drug researchers. This article is a continuation of the previous two comprehensive reviews and covers the literature on marine indole alkaloids published from 2015 to 2021, with 472 new or structure-revised compounds categorized by sources into marine microorganisms, invertebrates, and plant-derived. The structures and bioactivities demonstrated in this article will benefit the synthesis and pharmacological activity study for marine indole alkaloids on their way to clinical drugs.

Untapped Potential of Marine-Associated Cladosporium Species: An Overview on Secondary Metabolites, Biotechnological Relevance, and Biological Activities

The marine environment is an underexplored treasure that hosts huge biodiversity of microorganisms. Marine-derived fungi are a rich source of novel metabolites with unique structural features, bioactivities, and biotechnological applications. Marine-associated Cladosporium species have attracted considerable interest because of their ability to produce a wide array of metabolites, including alkaloids, macrolides, diketopiperazines, pyrones, tetralones, sterols, phenolics, terpenes, lactones, and tetramic acid derivatives that possess versatile bioactivities. Moreover, they produce diverse enzymes with biotechnological and industrial relevance. This review gives an overview on the Cladosporium species derived from marine habitats, including their metabolites and bioactivities, as well as the industrial and biotechnological potential of these species. In the current review, 286 compounds have been listed based on the reported data from 1998 until July 2021. Moreover, more than 175 references have been cited.

Amakusamine from a Psammocinia sp. Sponge: Isolation, Synthesis, and SAR Study on the Inhibition of RANKL-Induced Formation of Multinuclear Osteoclasts

A simple methylenedioxy dibromoindole alkaloid, amakusamine (1), was isolated from a marine sponge of the genus Psammocinia, and its structure was determined from spectroscopic data, time-dependent density-functional theory calculations, and synthesis. Compound 1 inhibited the receptor activator of nuclear factor-κB ligand (RANKL)-induced formation of multinuclear osteoclasts with an IC₅₀ value of 10.5 μM in RAW264 cells. The structure-activity relationship of 1 was also investigated with synthetic derivatives.

A sustainable C-H functionalization of indoles, pyrroles and furans under a blue LED with iodonium ylides

Pyrrole and indole derivatives are functionalized via a green initiative with the dimethyl malonate derived phenyl iodonium ylide 4a in the presence of a blue LED via C-H functionalization of the respective heterocycles in methanol to generate the desired compounds 5-7 in moderate to good yields. Control experiments provide insight into the probable reaction mechanism. Finally, the strategy is successfully applied in the generation of azepino[4,5-b]indole 12a/b.

Marine Pyrrole Alkaloids

Nitrogen heterocycles are essential parts of the chemical machinery of life and often reveal intriguing structures. They are not only widespread in terrestrial habitats but can also frequently be found as natural products in the marine environment. This review highlights the important class of marine pyrrole alkaloids, well-known for their diverse biological activities. A broad overview of the marine pyrrole alkaloids with a focus on their isolation, biological activities, chemical synthesis, and derivatization covering the decade from 2010 to 2020 is provided. With relevant structural subclasses categorized, this review shall provide a clear and timely synopsis of this area.

Wittig Reactions of Maleimide-Derived Stabilized Ylides with Alkyl Pyruvates: Concise Approach to Methyl Ester of (±)-Chaetogline A

A facile synthesis of methyl ester of chaetogline A is reported starting from the corresponding methyl 1-methyltryptophanate-derived­ maleimide. A stereoselective Wittig olefination with a carbonyl function in methyl pyruvate followed by phosphorous pentoxide-induced­ regioselective dehydrative cyclization are the essential reactions. An acid-induced thermodynamically driven stereoselective β- to α-position migration of the exocyclic C=C bond unit in ethyl tetrahydroindolizinoindolylidenepropanoate is described.

Marine Alkaloids: Compounds with In Vivo Activity and Chemical Synthesis

Marine alkaloids comprise a class of compounds with several nitrogenated structures that can be explored as potential natural bioactive compounds. The scientific interest in these compounds has been increasing in the last decades, and many studies have been published elucidating their chemical structure and biological effects in vitro. Following this trend, the number of in vivo studies reporting the health-related properties of marine alkaloids has been increasing and providing more information about the effects in complex organisms. Experiments with animals, especially mice and zebrafish, are revealing the potential health benefits against cancer development, cardiovascular diseases, seizures, Alzheimer's disease, mental health disorders, inflammatory diseases, osteoporosis, cystic fibrosis, oxidative stress, human parasites, and microbial infections in vivo. Although major efforts are still necessary to increase the knowledge, especially about the translation value of the information obtained from in vivo experiments to clinical trials, marine alkaloids are promising candidates for further experiments in drug development.

Exploitation of Marine Molecules to Manage Alzheimer's Disease

Neurodegenerative diseases are sociosanitary challenges of today, as a result of increased average life expectancy, with Alzheimer’s disease being one of the most prevalent. This pathology is characterized by brain impairment linked to a neurodegenerative process culminating in cognitive decline and behavioral disorders. Though the etiology of this pathology is still unknown, it is usually associated with the appearance of senile plaques and neurofibrillary tangles. The most used prophylaxis relies on anticholinesterase drugs and NMDA receptor antagonists, whose main action is to relieve symptoms and not to treat or prevent the disease. Currently, the scientific community is gathering efforts to disclose new natural compounds effective against Alzheimer’s disease and other neurodegenerative pathologies. Marine natural products have been shown to be promising candidates, and some have been proven to exert a high neuroprotection effect, constituting a large reservoir of potential drugs and nutraceutical agents. The present article attempts to describe the processes of extraction and isolation of bioactive compounds derived from sponges, algae, marine bacteria, invertebrates, crustaceans, and tunicates as drug candidates against AD, with a focus on the success of pharmacological activity in the process of finding new and effective drug compounds.

Psammocindoles A-C: Isolation, Synthesis, and Bioactivity of Indole-γ-lactams from the Sponge Psammocinia vermis

Psammocindoles A-C (1-3), a new class of indole alkaloids, were isolated from a Psammocinia vermis sponge. By combined spectroscopic analyses, the structures of these compounds were determined to be the indole-γ-lactams derived from three amino acid residues. In addition, an enantiomer psammocindole D (4), and the N-lactam isomers isopsammocindoles A-D (5-8) were also synthesized. These natural products and synthetic analogues were found to significantly stimulate adiponectin secretion in human bone marrow mesenchymal stem cells.

A one-pot "back-to-front" approach for the synthesis of benzene ring substituted indoles using allylboronic acids

Synthesis of only benzene ring functionalized indoles and poly-substituted carbazoles is reported via a one-pot triple cascade benzannulation protocol. Usage of differently substituted and readily accessible allylboronic acids as a 3-carbon annulating partner enables diverse aliphatic and aromatic substitution patterns, which is still a daunting task. This scalable synthetic protocol tolerates broad scope, thus enabling further downstream modifications. As an application, carbazole based natural products glycozoline and glycozolinol were synthesized.