Oxidized welwitindolinones from terrestrial fischerella spp

Recent Advances in the Synthesis of 3,n-Fused Tricyclic Indole Skeletons via Palladium-Catalyzed Domino Reactions

3,n-fused (n = 4-7) tricyclic indoles are pervasive motifs, embedded in a variety of biologically active molecules and natural products. Thus, numerous catalytic methods have been developed for the synthesis of these skeletons over the past few decades. In particular, palladium-catalyzed transformations have received much attention in recent years. This review summarizes recent developments in the synthesis of these tricyclic indoles with palladium-catalyzed domino reactions and their applications in the total synthesis of representative natural products.

3-Hydroxy-2-oxindole Derivatives Containing Sulfonamide Motif: Synthesis, Antiviral Activity, and Modes of Action

3-Hydroxy-2-oxindole motif constitutes a core structure in numerous natural products and imparts notable biological activities. Here, we describe the design and synthesis of four series of novel 3-substituted-3-hydroxy-2-oxindole derivatives containing sulfonamide moiety along with their antiviral activities against potato virus Y (PVY). Compound 10b displayed optimal antiviral activity and superior anti-PVY activity compared with the lead compound and commercial Ningnanmycin in terms of curative and protective effects. Additionally, 10b considerably inhibited PVY systemic infection in Nicotiana benthamiana. Physiological and biochemical analyses revealed that the activities of the four crucial defense-related enzymes increased in the tobacco plant following treatment with 10b. RNA-sequencing analysis revealed that 10b substantially induced the upregulation of 38 differentially expressed genes, which were enriched in the photosynthesis pathway. These findings suggest that 10b is a promising antiviral agrochemical that can effectively control PVY infection and trigger plant host immunity to develop virus resistance. This study provides novel molecular entities and ideas for developing new pesticides.

A diastereoselective three-component reaction for the assembly of succinimide and isatin hybrid molecules with potent anticancer activities

A Rh₂(OAc)₄ catalyzed three-component reaction of vinyl diazosuccinimides with alcohols and isatins has been reported, which provides a practical assess to the direct assembly of succinimide and isatin hybrid molecules in good-to-high yields with excellent stereoselectivity. The antiproliferation activity of these synthesized succinimide and isatin hybrid products has been tested via the CCK8 assay in different cancer cell lines, and compounds 4g (SJSA-1, IC₅₀ = 3.82 μM) and 4r (HCT-116, IC₅₀ = 9.02 μM) exhibit higher anticancer potency than other tested compounds.

Cu(ii)-thiophene-2,5-bis(amino-alcohol) mediated asymmetric Aldol reaction and Domino Knoevenagel Michael cyclization: a new highly efficient Lewis acid catalyst

The highly efficient Lewis acid-catalytic system Cu(ii)-thiophene-2,5-bis(amino-alcohol) has been developed for enantioselective Aldol reaction of isatin derivatives with ketones. The new catalytic system also proved to be highly enantioselective for the one pot three-component Domino Knoevenagel Michael cyclization reaction of substituted isatin with malononitrile and ethylacetoacetate. The chiral ligand (2S,2′S)-2,2′-((thiophene-2,5-diylbis(methylene))bis(azanediyl))bis(3-phenylpropan-1-ol) (L1) in combination with Cu(OAc)₂·H₂O employed as a new Lewis acid catalyst, furnished 3-substituted-3-hydroxyindolin-2-ones derivatives (3a–s) in good to excellent yields (81–99%) with high enantioselectivities (up to 96% ee) and spiro[4H-pyran-3,3-oxindole] derivatives (6a–l) in excellent yields (89–99%) with high ee (up to 95%). These aldol products and spiro-oxindoles constitute a core structural motif in a large number of pharmaceutically active molecules and natural products.

The highly efficient Lewis acid-catalytic system Cu(ii)-thiophene-2,5-bis(amino-alcohol) has been developed for enantioselective Aldol reaction of isatin derivatives with ketones.

One-pot synthesis of 3-substituted-3-hydroxyindolin-2-ones: three component coupling of N-protected isatin, aryne precursor and 1,3-cyclodione under metal-free conditions

A transition-metal free synthetic strategy for the direct synthesis of 3-substituted-3-hydroxy-indolin-2-ones via three component coupling of N-protected isatin, aryne precursor, and 1,3-cyclodione.

Transition-Metal-Free Alkylation Strategy: A Facile Access of Alkylated Oxindoles via Alkyl Transfer

An efficient transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents was described. A wide variety of valuable oxindoles were constructed in a single step with...

Bioactive Metabolites Produced by Cyanobacteria for Growth Adaptation and Their Pharmacological Properties

Cyanobacteria are the most abundant oxygenic photosynthetic organisms inhabiting various ecosystems on earth. As with all other photosynthetic organisms, cyanobacteria release oxygen as a byproduct during photosynthesis. In fact, some cyanobacterial species are involved in the global nitrogen cycles by fixing atmospheric nitrogen. Environmental factors influence the dynamic, physiological characteristics, and metabolic profiles of cyanobacteria, which results in their great adaptation ability to survive in diverse ecosystems. The evolution of these primitive bacteria resulted from the unique settings of photosynthetic machineries and the production of bioactive compounds. Specifically, bioactive compounds play roles as regulators to provide protection against extrinsic factors and act as intracellular signaling molecules to promote colonization. In addition to the roles of bioactive metabolites as indole alkaloids, terpenoids, mycosporine-like amino acids, non-ribosomal peptides, polyketides, ribosomal peptides, phenolic acid, flavonoids, vitamins, and antimetabolites for cyanobacterial survival in numerous habitats, which is the focus of this review, the bioactivities of these compounds for the treatment of various diseases are also discussed.

Medicinal Chemistry of Isocyanides

In eons of evolution, isocyanides carved out a niche in the ecological systems probably thanks to their metal coordinating properties. In 1859 the first isocyanide was synthesized by humans and in 1950 the first natural isocyanide was discovered. Now, at the beginning of XXI century, hundreds of isocyanides have been isolated both in prokaryotes and eukaryotes and thousands have been synthesized in the laboratory. For some of them their ecological role is known, and their potent biological activity as antibacterial, antifungal, antimalarial, antifouling, and antitumoral compounds has been described. Notwithstanding, the isocyanides have not gained a good reputation among medicinal chemists who have erroneously considered them either too reactive or metabolically unstable, and this has restricted their main use to technical applications as ligands in coordination chemistry. The aim of this review is therefore to show the richness in biological activity of the isocyanide-containing molecules, to support the idea of using the isocyanide functional group as an unconventional pharmacophore especially useful as a metal coordinating warhead. The unhidden hope is to convince the skeptical medicinal chemists of the isocyanide potential in many areas of drug discovery and considering them in the design of future drugs.

Synthesis of Spirotetrahydrofuran Oxindoles via Palladium-Catalyzed [4 + 1] Cycloaddition of Diphenyl 2-Oxoindolin-3-yl Phosphates and 2-Methylidenetrimethylene Carbonate

A novel palladium-catalyzed [4 + 1] cycloaddition to give spirotetrahydrofuran oxindoles has been developed, in which diphenyl 2-oxoindolin-3-yl phosphates were used as the both electrophilic and nucleophilic C₁ synthons at C-3 of the oxindole unit and 2-methylidenetrimethylene carbonate was used as the 1,4-dipole. The cycloannulation was performed at room temperature and provided the corresponding spirotetrahydrofuran oxindoles in good to excellent yields. The present method affords a new strategy for the construction of spirooxindole derivatives with unique three-dimensional structures.

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

Symphyonema bifilamentata sp. nov., the Right Fischerella ambigua 108b: Half a Decade of Research on Taxonomy and Bioactive Compounds in New Light

Since 1965 a cyanobacterial strain termed 'Fischerella ambigua 108b' was the object of several studies investigating its potential as a resource for new bioactive compounds in several European institutes. Over decades these investigations uncovered several unique small molecules and their respective biosynthetic pathways, including the polychlorinated triphenyls of the ambigol family and the tjipanazoles. However, the true taxonomic character of the producing strain remained concealed until now. Applying a polyphasic approach considering the phylogenetic position based on the 16S rRNA and the protein coding gene rbcLX, secondary structures and morphological features, we present the strain 'Fischerella ambigua 108b' as Symphyonema bifilamentata sp. nov. 97.28. Although there is the type species (holotype) S. sinense C.-C. Jao 1944 there is no authentic living strain or material for genetic analyses for the genus Symphyonema available. Thus we suggest and provide an epitypification of S. bifilamentata sp. nov. 97.28 as a valid reference for the genus Symphyonema. Its affiliation to the family Symphyonemataceae sheds not only new light on this rare taxon but also on the classes of bioactive metabolites of these heterocytous and true-branching cyanobacteria which we report here. We show conclusively that the literature on the isolation of bioactive products from this organism provides further support for a clear distinction between the secondary metabolism of Symphyonema bifilamentata sp. nov. 97.28 compared to related and other taxa, pointing to the assignment of this organism into a separate genus.

Mandelic Acid: An Efficient Organo-catalyst for the Synthesis of 3-substituted-3- Hydroxy-indolin-2-ones and Related Derivatives in Aqueous Ethanol at Room Temperature

Background:

Indoles and various indolyl derivatives are very common in naturally occurring biologically active compounds. Many methods are being developed for the synthesis of various bioactive indole derivatives.

Objective:

Synthesis of biologically promising structurally diverse indole derivatives under mild and environmentally benign conditions.

Methods:

Synthesis of 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol-3-yl)indolin-2-one was achieved by the reaction of an equimolar mixture of isatin and 3-(trifluoromethoxy)-1H-indol using 20 mol% of mandelic acid as catalyst in aqueous ethanol at room temperature. Under the same optimized reaction conditions, synthesis of 3-(3-hydroxy-2-oxoindolin-3-yl)chroman-2,4-diones was accomplished via the reactions of substituted isatins and 4-hydroxycoumarin. On the other hand, 2-hydroxy-2-(indol-3-yl)- indene-1,3-diones and 10-hydroxy-10-(5-methoxy-1H-indol-3- yl)phenanthren-9(10H)-one were synthesized from the reactions of indoles and ninhydrin or 9,10-phenanthrenequinone respectively using the same 20 mol% of mandelic acid as an efficient organo-catalyst in aqueous ethanol at room temperature.

Results:

Mild, safe and clean reaction profiles, energy efficiency, high atom-economy, use of naturally occurring non-toxic organo-catalyst, easy isolation procedure by avoiding column chromatographic purification and gram scale production are some the major advantages of this developed protocol.

Conclusion:

A simple, straightforward and eco-friendly protocol has been developed for the efficient synthesis of biologically promising novel 3-hydroxy-3-(5-(trifluoromethoxy)-1H-indol- 3-yl)indolin-2- one, 3-(3-hydroxy-2-oxoindolin-3-yl)chroman-2,4-diones, 2-hydroxy-2-(indol-3- yl)-indene-1,3-diones and 10-hydroxy-10-(5-methoxy-1H-indol-3-yl)phenanthren-9(10H)-one using a catalytic amount of mandelic acid in aqueous ethanol at room temperature.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Toxic compounds produced by cyanobacteria belonging to several species of the order Nostocales: A review

Cyanobacteria are well recognised as producers of a wide range of natural compounds that are in turn recognised as toxins that have potential and useful applications in the future as pharmaceutical agents. The order Nostocales, which is largely overlooked in this regard, has become increasingly recognised as a source of toxin producers including Anabaena, Nostoc, Hapalosiphon, Fischerella, Anabaenopsis, Aphanizomenon, Gloeotrichia, Cylindrospermopsis, Scytonema, Raphidiopsis, Cuspidothrix, Nodularia, Stigonema, Calothrix, Cylindrospermum and Desmonostoc species. The toxin compounds (i.e., microcystins, nodularin, anatoxins, ambiguines, fischerindoles and welwitindolinones) and metabolites are about to have a destructive effect on both inland and aquatic environment aspects. The present review gives an overview of the various toxins that are extracted by the order Nostocales. The current research suggests that these compounds that are produced by cyanobacterial species have promising future considerations as potentially harmful algae and as promising leads for drug discovery.

Synthetic applications of type II intramolecular cycloadditions

Type II intramolecular cycloadditions ([4+2], [4+3], [4+4] and [5+2]) have emerged recently as an efficient and powerful strategy for the construction of bridged ring systems. In general, type II cycloadditions provide access to a wide range of bridged bicyclo[m.n.1] ring systems with high regio- and diastereoselectivity in an easy and straightforward manner. In each section of this review, an overview of the corresponding type II cycloadditions is presented, which is followed by highlights of method development and synthetic applications in natural product synthesis. The goal of this review is to provide a survey of recent advances in the field covering literature up to 2020. The review will serve as a useful reference for organic chemists engaged in the total synthesis of natural products containing bridged bicyclo[m.n.1] ring systems and provide strong stimulus for invention and further advances in this exciting research field.

Halogenation-Guided Chemical Screening Provides Insight into Tjipanazole Biosynthesis by the Cyanobacterium Fischerella ambigua

Halogenated natural products (HNPs) show a wide range of interesting biological activities. Chemistry-guided screening with a software tool dedicated to identifying halogenated compounds in HPLC-MS data indicated the presence of several uncharacterised HNPs in an extract of the cyanobacterium Fischerella ambigua (Näg.) Gomont 108b. Three new natural products, tjipanazoles K, L, and M, were isolated from this strain together with the known tjipanazoles D and I. Taking into account the structures of all tjipanazole derivatives detected in this strain, reanalysis of the tjipanazole biosynthetic gene cluster allowed us to propose a biosynthetic pathway for the tjipanazoles. As the isolated tjipanazoles show structural similarity to arcyriaflavin A, an inhibitor of the clinically relevant multidrug-transporter ABCG2 overexpressed by different cancer cell lines, the isolated compounds were tested for ABCG2 inhibitory activity. Only tjipanazole K showed appreciable transporter inhibition, whereas the compounds lacking the pyrrolo[3,4-c] ring or featuring additional chloro substituents were found to be much less active.

Recent advances in the synthesis of gem-dimethylcyclobutane natural products

Covering: January 2000 to July 2018 gem-Dimethylcyclobutanes are a common motif found in a multitude of natural products, and thus these structures have captivated synthetic chemists for years. However, until the turn of the century, most synthetic efforts relied upon the use of widely available terpenes, such as pinene or caryophyllene, that already contain the gem-dimethylcyclobutane motif. This approach limits the scope of molecules that can be accessed readily. This review highlights recent syntheses in which the gem-dimethylcyclobutane is assembled via de novo approaches. An outlook on the future of this research area is also provided.

Regio- and stereoselective synthesis of spiropyrrolidine-oxindole and bis-spiropyrrolizidine-oxindole grafted macrocycles through [3 + 2] cycloaddition of azomethine ylides

A convenient and efficient method for the regioselective macrocyclization of triazole bridged spiropyrrolidine-oxindole, and bis-spiropyrrolizidine-oxindole derivatives was accomplished through intra and self-intermolecular [3 + 2] cycloaddition of azomethine ylides. The chalcone isatin precursors 9a-i required for the click reaction were obtained from the reaction of N-alkylazidoisatin 4 and propargyloxy chalcone 8a-i which in turn were obtained by the aldol condensation of propargyloxy salicylaldehyde 6 and substituted methyl ketones 7a-i. The regio- and stereochemical outcome of the cycloadducts were assigned based on 2D NMR and confirmed by single crystal XRD analysis. High efficiency, mild reaction conditions, high regio- and stereoselectivity, atom economy and operational simplicity are the exemplary advantages of the employed macrocyclization procedure.

Recent advances in the total synthesis of cyclobutane-containing natural products

Recent developments of strategies on the construction of cyclobutanes and their application in complex natural product synthesis are discussed.

Dereplication of Natural Products with Antimicrobial and Anticancer Activity from Brazilian Cyanobacteria

Cyanobacteria are photosynthetic organisms that produce a large diversity of natural products with interesting bioactivities for biotechnological and pharmaceutical applications. Cyanobacterial extracts exhibit toxicity towards other microorganisms and cancer cells and, therefore, represent a source of potentially novel natural products for drug discovery. We tested 62 cyanobacterial strains isolated from various Brazilian biomes for antileukemic and antimicrobial activities. Extracts from 39 strains induced selective apoptosis in acute myeloid leukemia (AML) cancer cell lines. Five of these extracts also exhibited antifungal and antibacterial activities. Chemical and dereplication analyses revealed the production of nine known natural products. Natural products possibly responsible for the observed bioactivities and five unknown, chemically related chlorinated compounds present only in Brazilian cyanobacteria were illustrated in a molecular network. Our results provide new information on the vast biosynthetic potential of cyanobacteria isolated from Brazilian environments.

Assessment of the cytotoxicity, antioxidant activity and chemical composition of extracts from the cyanobacterium Fischerella major Gomont

Cyanoprokaryotes (Cyanobacteria/Cyanophyta) are ancient photosynthetic prokaryotic organisms with cosmopolitan distribution. They are producers of a number of biologically active substances with antitumor and antifungal activity, vitamins, antibiotics, algaecides, insecticides, repellents, hormones, immunosuppressants and toxins. So far, the cyanobacterium Fischerella major Gomont has not been studied regarding its impact on the environment and human health. In this study, the cytotoxic, antioxidant and antitumor activities of four extracts prepared from Fischerella major were evaluated in vitro. In addition, the total phenolic content and the potential for production of cyanotoxins were also analyzed. The conducted GC/MS analysis identified 45 compounds with different chemical nature and biological activity. Presence of microcystins and saxitoxins was detected in all Fischerella major extracts. In vitro testing on cell cultures showed a significant concentration- and time-dependent cytotoxic effect on all cell lines (HeLa, SK-Hep-1 and FL) treated at three exposure times (24, 48 and 72 h) with four extracts. A selective antitumor effect was not observed. This is the first study demonstrating biological activity of extracts from Fischerella major, which makes it an interesting subject for further research, including environmental risk assessments (as producer of cyanotoxins) or as a potential source of pharmaceuticals.

Total Synthesis of Pentacyclic (-)-Ambiguine P Using Sequential Indole Functionalizations

The first synthesis of a pentacyclic ambiguine (ambiguine P) is reported. The synthesis takes advantage of sequential alkylations of an indole core to rapidly construct the pentacyclic framework of the natural product. Key to the success of the synthesis was the use of a Nicholas reaction to alkylate at C2, crafting a fused seven-membered ring that is characteristic of the pentacyclic ambiguines, as well as the use of an amide-directed functionalization at C12 to set a requisite quaternary center. A versatile late-stage intermediate was prepared that may be applicable to the synthesis of the other pentacyclic ambiguines.

Room-Temperature and Transition-Metal-Free Intramolecular α-Arylation of Ketones: A Mild Access to Tetracyclic Indoles and 7-Azaindoles

A novel approach for the synthesis of tetracyclic indoles and 7-azaindoles is reported. The strategy involves four steps, with a fast rt intramolecular α-arylation of ketones as key step. The reaction was inspected synthetically to achieve the synthesis of 11 novel tetracyclic structures with moderate to very good yields (39-85%). Theoretical combined with experimental studies led us to propose a probable polar mechanism (concerted S_NAr).

Catalytic Enantioselective Synthesis of 3,4-Polyfused Oxindoles with Quaternary All-Carbon Stereocenters: A Rh-Catalyzed C-C Activation Approach

The first Rh-catalyzed enantioselective synthesis of a 3,4-polyfused oxindole ring system enabled by carboacylation of acrylic amides based on C-C activation is reported. This transformation provides a new entry to access 3,4-polyfused oxindoles bearing quaternary stereocenters. Tri- to pentacyclic 3,4-fused oxindoles were asymmetrically generated in good yields (up to 95%) with good to excellent enantioselectivity (88%-97% ee). Application in the first total synthesis of xylanigripones A was completed in 6 steps with a 14% overall yield.

Dye-incorporated coordination polymers for direct photocatalytic trifluoromethylation of aromatics at metabolically susceptible positions

Direct trifluoromethylation of unactivated aromatic rings at metabolically susceptible positions is highly desirable in pharmaceutical applications. By incorporating thiophenes into the backbone of triphenylamine to enlarge its π-system, a new approach for constructing coordination polymers is reported for direct trifluoromethylation without prefunctionalization of the aryl precursors. The improved light-harvesting ability and well-modulated excited state redox potential of the designed polymers endow the generated CF3 radicals with suitable reactivity and enhance radical adduct oxidation in pores. The well-configurated interactions between the organic ligands distort the coordination geometry to create active interaction sites within the coordination polymer; thus, the substrates could be docked near the photoredox-active centres. The synergistic electronic and spatial effects in the confined pores balance the contradictory demands of electronic effects and reaction dynamics, achieving regio- and diastereoselective discrimination among reaction sites with unremarkable electronic/steric differences.

Synthesis of CF3-containing spiro-epoxyoxindoles via the Corey–Chaykovsky reaction of N-alkyl isatins with Ph2S+CH2CF3OTf−

CF₃-containing spiro-epoxyoxindoles were successfully prepared via Corey–Chaykovsky reaction of N-alkyl isatins with the ylide generated from Ph₂S⁺CH₂CF₃OTf⁻ with almost exclusive diastereoselectivity.

Photoredox-catalyzed direct aminoalkylation of isatins: diastereoselective access to 3-hydroxy-3-aminoalkylindolin-2-ones analogues

A practical protocol for the direct aminoalkylation of isatins with tetrahydroisoquinolines and other aminesviaa photoredox catalyzed radical–radical cross-coupling process is described.

Green Method To Preparing Oxindole-Fused Spirotetrahydrofuran Scaffolds through Methanesulfonic Acid-Catalyzed Cyclization Reactions of 3-Allyl-3-hydroxy-2-oxindole in Water

Water is an ideal solvent for chemical transformations in environmentally friendly and sustainable processes. An operationally simple, metal-free, and green approach to the synthesis of oxindole-fused spirotetrahydrofurans from 3-allyl-3-hydroxy-2-oxindoles in water is described. This method requires only cheap methanesulfonic acid as a catalyst and eliminates the need of complicated reagents. This atom- and step-economical transformation is highly efficient, which provides a new approach for the construction of oxindole-fused spirotetrahydrofuran molecules with potential pharmaceutical interest.

An Approach to the Welwistatin Core via a Diazoketone Rearrangement-Ring Expansion Strategy

The rhodium-catalyzed decomposition of fused bicyclic α-diazo-β-hydroxyketone 16 and rearrangement to 17 is featured in an approach to the bridged bicyclic core of welwistatin. The bicyclic [4.3.1] core of 25 is furnished from a subsequent cyclopropanation to generate 23, followed by its ring expansion.