Iodide/H₂O₂ Catalyzed Intramolecular Oxidative Amination for the Synthesis of 3,2'-Pyrrolidinyl Spirooxindoles

An ammonium iodide/hydrogen peroxide-mediated intramolecular oxidative amination of 3-aminoalkyl-2-oxindoles was achieved, affording the corresponding 3,2'-pyrrolidinyl spirooxindoles and their 6- or 7-membered analogous in moderate to high yields. This metal-free procedure features very mild reaction conditions, non-toxicity and easily handled hydrogen peroxide as a clean oxidant.

A catalytic N-deacylative alkylation approach to hexahydropyrrolo[2,3-b]indole alkaloids

A versatile unprecedented strategy to diversely functionalized hexahydropyrrolo[2,3-b]indole alkaloids is described in high chemical yields. The synthesis features a key Pd(0)-catalyzed deacylative alkylation of N-acyl 3-substituted indoles using only 1 mol% of Pd(PPh3)4. The scope of this methodology is further defined in the asymmetric synthesis of pyrroloindolines using a diastereoselective approach.

Asperversiamides, Linearly Fused Prenylated Indole Alkaloids from the Marine-Derived Fungus Aspergillus versicolor

Asperversiamides A-H (1-8), eight linearly fused prenylated indole alkaloids featuring an unusual pyrano[3,2- f]indole unit, were isolated from the marine-derived fungus Aspergillus versicolor. The structures and absolute configurations of these compounds were elucidated by extensive spectroscopic analyses, single-crystal X-ray diffraction, electronic circular dichroism (ECD) calculations, and optical rotation (OR) calculations. The relative configuration of C-21 of iso-notoamide B was herein revised, and a new methodology for preliminarily determining if the relative configuration of the bicyclo[2.2.2]diazaoctane moiety of a spiro-bicyclo[2.2.2]diazaoctane-type indole alkaloid is syn or anti was developed. The anti-inflammatory activities of the isolated compounds were all tested, and of these compounds, 7 exhibited a potent inhibitory effect against iNOS with an IC₅₀ value of 5.39 μM.

An endophytic Fungi of Ginkgo biloba L. produces antimicrobial metabolites as potential inhibitors of FtsZ of Staphylococcus aureus

A total of 58 fungal isolates, belonging to 24 genera, were obtained from the leaves, stems and roots of Ginkgo biloba L.. Among them, one endophytic fungal strain, Penicillium cataractum SYPF 7131, displayed the strongest antibacterial activity. Four new compounds (1-4) were isolated from the strain fermentation broth together with four known compounds (5-8). These structures were determined on the basis of 1D and 2D NMR and [Rh₂(OCOCF₃)₄]-induced electronic circular dichroism (ECD) spectroscopic analyses. All the isolated compounds were screened for their in vitro antimicrobial activities. Compound 3 and 4 showed moderate inhibitory activity against Staphylococcus aureus. Compound 7 exhibited significant inhibitory activity against S. aureus with MIC value of 10 μg/mL. Further, the in silico molecular docking studies of the active compounds was used to explore the binding interactions with the active site of filamentous temperature-sensitive protein Z (FtsZ) from Staphylococcus aureus. The docking results revealed that compounds 3, 4 and 7 showed high binding energies, strong H-bond interactions and hydrophobic interactions with FtsZ from S. aureus validating the observed antimicrobial activity. Based on antimicrobial activities and docking studies, compounds 3, 4 and 7 were identified as promising antimicrobial lead molecules.

Asymmetric Arylation of 2,2,2-Trifluoroacetophenones Catalyzed by Chiral Electrostatically-Enhanced Phosphoric Acids

A series of highly reactive metal-free chiral phosphoric acids possessing positively charged phosphonium ion substituents are reported and have been applied to Friedel-Crafts alkylations of indoles and 2,2,2-trifluoromethyl aryl ketones. These catalysts are orders-of-magnitude more active and have similar or better enantioselectivities than their noncharged analogues. High tolerance to a range of substrates with electron-withdrawing and electron-donating substituents was also observed.

Eurotiumins A⁻E, Five New Alkaloids from the Marine-Derived Fungus Eurotium sp. SCSIO F452

Three new prenylated indole 2,5-diketopiperazine alkaloids (1–3) with nine known ones (5–13), one new indole alkaloid (4), and one new bis-benzyl pyrimidine derivative (14) were isolated and characterized from the marine-derived fungus Eurotium sp. SCSIO F452. 1 and 2, occurring as a pair of diastereomers, both presented a hexahydropyrrolo[2,3-b]indole skeleton. Their chemical structures, including absolute configurations, were elucidated by 1D and 2D NMR, HRESIMS, quantum chemical calculations of electronic circular dichroism, and single crystal X-ray diffraction experiments. Most isolated compounds were screened for antioxidative potency. Compounds 3, 5, 6, 7, 9, 10, and 12 showed significant radical scavenging activities against DPPH with IC₅₀ values of 13, 19, 4, 3, 24, 13, and 18 µM, respectively. Five new compounds were evaluated for cytotoxic activities.

Indole diterpenoids from the endophytic fungus Drechmeria sp. as natural antimicrobial agents

A fungal strain, Drechmeria sp., was isolated from the root of Panax notoginseng. Totally, seven new indole diterpenoids, drechmerins A-G (1-7), were isolated from the fermentation broth of Drechmeria sp. together with four known analogues (8-11). Their structures were determined on the basis of 1D and 2D NMR and electronic circular dichroism (ECD) spectroscopic analyses as well as theoretical calculations. All the isolated compounds were evaluated for their antimicrobial activities against Candida albicans, Staphylococcus aureus, Bacillus cereus, B. subtillis, Pseudomonas aeruginosa, and Klebsiella pneumonia, respectively. Drechmerin B (2) displayed antimicrobial activity against C. albicans with an MIC value of 12.5 μg/mL. Molecular docking was used to investigate interactions of peptide deformylase with compounds 1-3, 5-7, 9, and 10.

Expanding tryptophan-containing cyclodipeptide synthase spectrum by identification of nine members from Streptomyces strains

Cyclodipeptide synthases (CDPSs) comprise normally 200-300 amino acid residues and are mainly found in bacteria. They hijack aminoacyl-tRNAs from the ribosomal machinery for cyclodipeptide formation. In this study, nine new CDPS genes from eight Streptomyces strains were cloned into pET28a vector and expressed in Escherichia coli. Structural elucidation of the isolated products led to the identification of one cyclo-L-Trp-L-Leu, two cyclo-L-Trp-L-Pro, and three cyclo-L-Trp-L-Trp synthases. Other three CDPSs produce cyclo-L-Trp-L-Ala or cyclo-L-Trp-L-Tyr as the major cyclodipeptide. Total product yields of 46 to 211 mg/L E. coli culture were obtained. Our findings represent rare examples of CDPS family derived from actinobacteria that form various tryptophan-containing cyclodipeptides. Furthermore, this study highlights the potential of the microbial machinery for tryptophan-containing cyclodipeptide biosynthesis and provides valid experimental basis for further combination of these CDPS genes with other modification genes in synthetic biology.

Biosynthesis and Structure-Activity Relationship Studies of Okaramines That Target Insect Glutamate-Gated Chloride Channels

Prenylated indole alkaloid okaramines selectively target insect glutamate-gated chloride channels (GluCls). Because of their highly complex structures, including azocine and azetidine rings, total synthesis of okaramine A or B has not been achieved, preventing evaluation of the biological activities of okaramines. Biosynthetic approaches provide alternatives to accessing structurally diverse derivatives and enabling the elucidation of structure-activity relationships. To explore the biosynthetic potential of okaramines, gene knockout experiments of an okaramine-producer fungus were performed. The deletion mutants of the oxygenase genes okaB, okaD, okaE, and okaG provided analogues that were unlikely to be accumulated in the normal biosynthetic process of the wild-type strain. Analysis of the structure-activity relationships of okaramines collected from the fungal cultures revealed that 1,4-dihydroazocine and N-aliphatic group attached to the indole were crucial for GluCl-activating activity. This provided insights into further derivatization of the complex structure of okaramines in order to facilitate the development of new insecticides.

A Comprehensive Overview of the Cyclodipeptide Synthase Family Enriched with the Characterization of 32 New Enzymes

Cyclodipeptide synthases (CDPSs) use as substrates two amino acids activated as aminoacyl-tRNAs to synthesize cyclodipeptides in secondary metabolites biosynthetic pathways. Since the first description of a CDPS in 2002, the number of putative CDPSs in databases has increased exponentially, reaching around 800 in June 2017. They are likely to be involved in numerous biosynthetic pathways but the diversity of their products is still under-explored. Here, we describe the activity of 32 new CDPSs, bringing the number of experimentally characterized CDPSs to about 100. We detect 16 new cyclodipeptides, one of which containing an arginine which has never been observed previously. This brings to 75 the number of cyclodipeptides formed by CDPSs out of the possible 210 natural ones. We also identify several consensus sequences related to the synthesis of a specific cyclodipeptide, improving the predictive model of CDPS specificity. The improved prediction method enables to propose the main product synthesized for about 80% of the CDPS sequences available in databases and opens the way for the deciphering of CDPS-dependent pathways. Analysis of phylum distribution and predicted activity for all CDPSs identified in databases shows that the experimentally characterized set is representative of the whole family. Our work also demonstrates that some cyclodipeptides, precursors of diketopiperazines with interesting pharmacological properties and previously described as being synthesized by fungal non-ribosomal peptide synthetases, can also be produced by CDPSs in bacteria.

Convenient synthetic approach for tri- and tetraprenylated cyclodipeptides by consecutive enzymatic prenylations

The prenyltransferases EchPT1 and EchPT2 from Aspergillus ruber are responsible for the consecutive prenylations of cyclo-L-Trp-L-Ala, leading to the formation of the triprenylated echinulin as the predominant product. In this study, we demonstrate that EchPT1 also accepts all stereoisomers of cyclo-Trp-Ala and cyclo-Trp-Pro and catalyses regiospecific reverse C2-prenylation at the indole nucleus. EchPT1 products were well accepted by EchPT2 for multiple consecutive prenylations, with conversion yields of 84 to 98% for six of the eight substrates. C2-, C5- and C7-triprenylated derivatives are identified as major enzyme products, with product yields of 40 to 86% in seven cases. High product yields of 25-36%, i.e. approximate 30% of the total enzyme products, were observed for tetraprenylated derivatives in the four reaction mixtures with one D- and one L-configured amino acid residues. To the best of our knowledge, enzymatic preparation of tetraprenylated cyclodipeptides with such high efficacy has not been reported prior to this study.

The synthesis of polysubstituted indoles from 3-bromo-2-indolyl phosphates

A novel methodology for the synthesis of functionalised indoles based on the cross-coupling reactions of 3-bromo-2-indolyl phosphates is described. The preparation involves the conversion of easily available 2-oxindoles to 3,3-dibromo-2-oxindoles followed by the Perkow reaction affording 3-bromo-2-indolyl phosphates. Then bromine atom is substituted regioselectively by the Suzuki coupling reaction. We observed that aluminum chloride promoted the reaction of 3-substituted-2-indolyl phosphates with organozinc reagents furnishing 2,3-disubstituted indoles as final products. The overall diversity and efficiency of the methodology was demonstrated by the synthesis of bioactive molecule from easily available substances.

Catalyst-free [3 + 2] cyclization of dihydroisoquinoline imines and isatin-derived Morita–Baylis–Hillman carbonates via 1,5-electrocyclization: synthesis of tetrahydroisoquinoline-fused spirooxindoles

The incorporation of tetrahydroisoquinoline and spirooxindole frameworks has been achieved.

Structure-based protein engineering enables prenyl donor switching of a fungal aromatic prenyltransferase

Structure-guided molecular modelling and site-directed mutagenesis of the tryptophan dimethylallyl transferase FgaPT2 led to creation of mutants with strongly enhanced activities towards geranyl and farnesyl diphosphates.

Total synthesis of natural productsviairidium catalysis

An overview of the highlights in total synthesis of natural products using iridium as a catalyst is given.

Recent Advances in the Synthesis of Spiroheterocycles via N-Heterocyclic Carbene Organocatalysis

Spiroheterocycles are regarded as a privileged framework because of their wide distribution in various natural products and synthetic molecules and promising bioactivities. This review focuses on the recent advances in the synthesis of spiroheterocycles by using the strategy of N-heterocyclic carbene (NHC) organocatalysis, and is organized based on the stereoselectivity and the reactive intermediates. According to the stereochemistry, this review was divided into two main parts, covering racemic and enantioselective versions. In each part, we firstly describe the synthetic transformations using nucleophilic Breslow intermediates, and then discuss the reactions that employ electrophilic acylazolium or radical cation intermediates. With those distinct catalytic activation modes of NHC organocatlysis, we expect this synthetic protocol will possibly produce new molecules with structural novelty and complexity, which may warrant further research in the field of drug discovery.

Dual nickel and Lewis acid catalysis for cross-electrophile coupling: the allylation of aryl halides with allylic alcohols

Controlling the selectivity in cross-electrophile coupling reactions is a significant challenge, particularly when one electrophile is much more reactive. We report a general and practical strategy to address this problem in the reaction between reactive and unreactive electrophiles by a combination of nickel and Lewis acid catalysis. This strategy is used for the coupling of aryl halides with allylic alcohols to form linear allylarenes selectively. The reaction tolerates a wide range of functional groups (e.g. silanes, boronates, anilines, esters, alcohols, and various heterocycles) and works with various allylic alcohols. Complementary to most current routes for the C3 allylation of an unprotected indole, this method provides access to C2 and C4-C7 allylated indoles. Preliminary mechanistic experiments reveal that the reaction might start with an aryl nickel intermediate, which then reacts with Lewis acid activated allylic alcohols in the presence of Mn.

Bioinspired chemical synthesis of monomeric and dimeric stephacidin A congeners

Stephacidin A and its congeners are a collection of secondary metabolites that possess intriguing structural motifs. They stem from unusual biosynthetic sequences that lead to the incorporation of a prenyl or reverse-prenyl group into a bicyclo[2.2.2]diazaoctane framework, a chromene unit or the vestige thereof. To complement biosynthetic studies, which normally play a significant role in unveiling the biosynthetic pathways of natural products, here we demonstrate that chemical synthesis can provide important insights into biosynthesis. We identify a short total synthesis of congeners in the reverse-prenylated indole alkaloid family related to stephacidin A by taking advantage of a direct indole C6 halogenation of the related ketopremalbrancheamide. This novel strategic approach has now made possible the syntheses of several natural products, including malbrancheamides B and C, notoamides F, I and R, aspergamide B, and waikialoid A, which is a heterodimer of avrainvillamide and aspergamide B. Our approach to the preparation of these prenylated and reverse-prenylated indole alkaloids is bioinspired, and may also inform the as-yet undetermined biosynthesis of several congeners.

Antibacterial and antibiofilm activities of the metabolites isolated from the culture of the mangrove-derived endophytic fungus Eurotium chevalieri KUFA 0006

Five previously undescribed metabolites, including acetylquestinol, two prenylated indole 3-carbaldehyde derivatives, an anthranilic acid derivative and an isochromone derivative, were isolated, in addition to eleven known compounds: palmitic acid, ergosterol 5,8-endoperoxide, emodin, physcion, questin, questinol, (11S, 14R)-cyclo(tryptophylvalyl), preechinulin, neoechinulin E, echinulin and eurocristatine, from the culture of the endophytic fungus Eurotium chevalieri KUFA 0006. The structures of the previously undescribed compounds were established based on an extensive 1D and 2D NMR spectral analysis as well as HRMS and IR data. In case of 2-(2, 2-dimethylcyclopropyl)-1H-indole-3-carbaldehyde and 6, 8-dihydroxy-3-(2S-hydroxypropyl)-7-methylisochromone, the absolute configurations of their stereogenic carbons were established based on comparison of their experimental and calculated ECD spectra. All the compounds, except for palmitic acid and ergosterol 5, 8-endoperoxide, were evaluated for their antibacterial and antibiofilm activities against two Gram-positive and two Gram-negative bacteria, as well as multidrug-resistant isolates from the environment. Emodin not only exhibited moderate antibacterial activity against the Gram-positive bacteria but also showed strong synergistic association with oxacillin against MRSA Staphylococcus aureus.

Total synthesis of architecturally complex indole terpenoids: strategic and tactical evolution

Indole terpenes have attracted the interests of synthetic chemists due to their complex architectures and potent biological activities. Examples of total syntheses of several indole terpenes were reviewed in this article to honor Professor KC Nicolaou.

Bioinspired Indole Prenylation Reactions in Water

Isoprene units derived from dimethylallyl diphosphate (DMAPP) are an important motif in many natural products including terpenoids, carotenoids, steroids, and natural rubber. Understanding the chemical characteristics of DMAPP is an important topic in natural products chemistry, organic chemistry, and biochemistry. We have developed a direct bioinspired indole prenylation reaction using DMAPP or its equivalents as the electrophile in homogeneous aqueous acidic media in the absence of enzyme to provide prenylated indole products. After establishing the bioinspired indole prenylation reaction, this was then used to achieve the synthesis of a series of natural products, namely, N-prenylcyclo-l-tryptophyl-l-proline, tryprostatins, rhinocladins, and terezine D.

Alkyne Hydroheteroarylation: Enantioselective Coupling of Indoles and Alkynes via Rh-Hydride Catalysis

We report an enantioselective coupling between alkynes and indoles. A Rh-hydride catalyst isomerizes alkynes to generate a metal-allyl species that can be trapped with both aromatic and heteroaromatic nucleophiles.

Manipulation of the Precursor Supply in Yeast Significantly Enhances the Accumulation of Prenylated β-Carbolines

The tryptophan derivative 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) is present in many plants and foods including fermentation products of the baker's yeast Saccharomyces cerevisiae. MTCA is formed from tryptophan and acetaldehyde via a Pictet-Spengler reaction. In this study, up to 9 mg/L of MTCA were detected as a mixture of (1S,3S) and (1R,3S) isomers in a ratio of 2.2:1 in Saccharomyces cerevisiae cultures. To the best of our knowledge, this is the first report on the presence of MTCA in laboratory baker's yeast cultures. Expression of three fungal tryptophan prenyltransferase genes, fgaPT2, 5-dmats, and 7-dmats in S. cerevisiae resulted in the formation of MTCA derivatives with prenyl moieties at different positions of the indole ring. Expression of these genes in dimethylallyl diphosphate and tryptophan overproducing strains led to generation of up to 400 mg/L of prenylated MTCAs as mixtures of (1S,3S) and (1R,3S) diastereomers in ratios similar to that of unprenylated MTCA. The structures of the described substances including their stereochemistry were unequivocally elucidated by mass spectrometry as well as one- and two-dimensional NMR spectroscopy. The results of this study provide a convenient system for the production of high amounts of designed prenylated MTCAs in S. cerevisiae. Furthermore, our work can be considered as an excellent example for the construction of more complex molecules by introducing just one key gene.

Development of the Regiodivergent Asymmetric Prenylation of 3-Substituted Oxindoles

This paper describes our efforts to design a Pd-catalyzed asymmetric prenylation of 3-substituted oxindoles that affords access to both the linear and reverse-prenylated products. Both 3-alkyl- and 3-aryloxindoles performed well under our optimized reaction conditions. The regiodivergent alkylation of monoterpene-derived electrophiles using this methodology was also investigated. The utility of this methodology in natural product synthesis was demonstrated through the efficient total syntheses of four Flustra alkaloids, which also allowed the absolute stereochemistry of the prenylated oxindole products to be assigned. Surprisingly, the same enantiomer of ligand produced linear and branched regioisomers of opposite chirality.

Mass Spectrometric Characteristics of Prenylated Indole Derivatives from Marine-Derived Penicillium sp. NH-SL

Two prenylated indole alkaloids were isolated from the ethyl acetate extracts of a marine-derived fungus Penicillium sp. NH-SL and one of them exhibited potent cytotoxic activity against mouse hepa 1c1c7 cells. In order to detect other bioactive analogs, we used liquid chromatogram tandem mass spectrometry (LC-MS/MS) to analyze the mass spectrometric characteristics of the isolated compounds as well as the crude extracts. As a result, three other analogs were detected, and their structures were deduced according to the similar fragmentation patterns. This is the first systematic report on the mass spectrometric characteristics of prenylated indole derivatives.

The Enzymology of Organic Transformations: A Survey of Name Reactions in Biological Systems

Chemical reactions that are named in honor of their true, or at least perceived, discoverers are known as "name reactions". This Review is a collection of biological representatives of named chemical reactions. Emphasis is placed on reaction types and catalytic mechanisms that showcase both the chemical diversity in natural product biosynthesis as well as the parallels with synthetic organic chemistry. An attempt has been made, whenever possible, to describe the enzymatic mechanisms of catalysis within the context of their synthetic counterparts and to discuss the mechanistic hypotheses for those reactions that are currently active areas of investigation. This Review has been categorized by reaction type, for example condensation, nucleophilic addition, reduction and oxidation, substitution, carboxylation, radical-mediated, and rearrangements, which are subdivided by name reactions.

PrenDB, a Substrate Prediction Database to Enable Biocatalytic Use of Prenyltransferases

Prenyltransferases of the dimethylallyltryptophan synthase (DMATS) superfamily catalyze the attachment of prenyl or prenyl-like moieties to diverse acceptor compounds. These acceptor molecules are generally aromatic in nature and mostly indole or indole-like. Their catalytic transformation represents a major skeletal diversification step in the biosynthesis of secondary metabolites, including the indole alkaloids. DMATS enzymes thus contribute significantly to the biological and pharmacological diversity of small molecule metabolites. Understanding the substrate specificity of these enzymes could create opportunities for their biocatalytic use in preparing complex synthetic scaffolds. However, there has been no framework to achieve this in a rational way. Here, we report a chemoinformatic pipeline to enable prenyltransferase substrate prediction. We systematically catalogued 32 unique prenyltransferases and 167 unique substrates to create possible reaction matrices and compiled these data into a browsable database named PrenDB. We then used a newly developed algorithm based on molecular fragmentation to automatically extract reactive chemical epitopes. The analysis of the collected data sheds light on the thus far explored substrate space of DMATS enzymes. To assess the predictive performance of our virtual reaction extraction tool, 38 potential substrates were tested as prenyl acceptors in assays with three prenyltransferases, and we were able to detect turnover in >55% of the cases. The database, PrenDB (www.kolblab.org/prendb.php), enables the prediction of potential substrates for chemoenzymatic synthesis through substructure similarity and virtual chemical transformation techniques. It aims at making prenyltransferases and their highly regio- and stereoselective reactions accessible to the research community for integration in synthetic work flows.

Griseofulvin Derivative and Indole Alkaloids from Penicillium griseofulvum CPCC 400528

A new griseofulvin derivative, 4'-demethoxy-4'-N-isopentylisogriseofulvin (1), three new indole alkaloids, 2-demethylcyclopiamide E (2), 2-demethylsperadine F (3), and clopiamine C (4), and five known metabolites (5-9) were isolated from Penicillium griseofulvum CPCC 400528. Compound 1 is the first reported griseofulvin analogue with an N-isopentane group and the first example of a naturally occurring N-containing griseofulvin analogue. Their structures and absolute configurations were elucidated through extensive spectroscopic analyses, calculated ECD, and single-crystal X-ray diffraction (Cu Kα). The possible biogenetic pathway of 1-3 was proposed. Compounds 1, 2, and 5 exhibited anti-HIV activities with IC₅₀ values of 33.2, 20.5, and 12.6 μM, respectively.

Palladium-catalyzed oxidative arylacetoxylation of alkenes: synthesis of indole and indoline derivatives

A method for the oxidative arylacetoxylation of alkenes has been developed to synthesize indole and indoline derivatives from readily accessible substrates.