Tryptanthrins as multi-bioactive agents: discovery, diversity distribution and synthesis

Tryptanthrin and its derivatives, representing a type of alkaloids with indoloquinazoline structures, were first obtained from blue plants and indigo, and then extracted from fungi, marine bacteria and a number of many other natural sources. Various strategies for their chemical synthesis have been reported while tryptanthrin biosynthesis has been less investigated. Tryptanthrin and its derivative products have a broad range of pharmacological and biological functions. In this review, we cover the sources, chemical synthesis and biosynthesis, modes of action and biological activities of tryptanthrin and its derivatives.

Discovery of type II polyketide synthase-like enzymes for the biosynthesis of cispentacin

Type II polyketide synthases (PKSs) normally synthesize polycyclic aromatic compounds in nature, and the potential to elaborate further diverse skeletons was recently revealed by the discovery of a polyene subgroup. Here, we show a type II PKS machinery for the biosynthesis of a five-membered nonaromatic skeleton contained in the nonproteinogenic amino acid cispentacin and the plant toxin coronatine. We successfully produce cispentacin in a heterologous host and reconstruct its biosynthesis using seven recombinant proteins in vitro. Biochemical analyses of each protein reveal the unique enzymatic reactions, indicating that a heterodimer of type II PKS-like enzymes (AmcF-AmcG) catalyzes a single C2 elongation as well as a subsequent cyclization on the acyl carrier protein (AmcB) to form a key intermediate with a five-membered ring. The subsequent reactions, which are catalyzed by a collection of type II PKS-like enzymes, are also peculiar. This work further expands the definition of type II PKS and illuminates an unexplored genetic resource for natural products.

Late-Stage Chemo- and Enantioselective Oxidation of Indoles to C3-Monosubstituted Oxindoles

Catalytic asymmetric preparation of chiral 3-monosubstituted oxindoles represents a significant challenge in synthetic chemistry due to the ease of racemization of the tertiary stereocenter through enolization. Here, we describe a general titanium-catalyzed chemo- and enantioselective indole oxidation to produce a diverse set of chiral 3-monosubstituted oxindoles with up to 96% yield, 99% ee, and with a substrate/catalyst ratio of 10,000 by using the combination of a simple titanium(salan) catalyst with green and atom-economic terminal oxidant H2O2. The mild approach tolerates a broad range of functional groups, enabling late-stage asymmetric diversification of a series of commercial drugs and natural products together with late-stage asymmetric construction of a wide set of enzyme antagonists, all of which are difficult to achieve through existing methods.

Multicomponent Assembly of Complex Oxindoles by Enantioselective Cooperative Catalysis

Chiral oxindoles are important chemical scaffolds found in many natural products, and their enantioselective synthesis thus attracts considerable attention. Highly diastereo- and enantioselective synthetic methods for constructing C3 quaternary oxindoles have been well-developed. However, the efficient synthesis of chiral 3-substituted tertiary oxindoles has been rarely reported due to the ease of racemization of the tertiary stereocenter via enolization. Therefore, we herein report on the multicomponent assembly (from N-aryl diazoamides, aldehydes, and enamines/indoles) of complex oxindoles by enantioselective cooperative catalysis. These reactions proceed under mild conditions and show broad substrate scope, affording the desired coupling products (>90 examples) with good to excellent stereocontrol. Additionally, this research also demonstrates the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of Speradine A.

Unusual Secondary Metabolites from the Mangrove Ecosystems: Structures, Bioactivities, Chemical, and Bio-Syntheses

Mangrove ecosystems are widely distributed in the intertidal zone of tropical and subtropical estuaries or coasts, containing abundant biological communities, for example, mangrove plants and diverse groups of microorganisms, featuring various bioactive secondary metabolites. We surveyed the literature from 2010 to 2022, resulting in a collection of 134 secondary metabolites, and classified them into two major families in terms of the biological sources and 15 subfamilies according to the chemical structures. To highlight the structural diversity and bioactivities of the mangrove ecosystem-associated secondary metabolites, we presented the chemical structures, bioactivities, biosynthesis, and chemical syntheses.

Development of spiro-3-indolin-2-one containing compounds of antiproliferative and anti-SARS-CoV-2 properties

A series of 1″-(alkylsulfonyl)-dispiro[indoline-3,2'-pyrrolidine-3',3″-piperidine]-2,4″-diones 6a‒o has been synthesized through regioselective multi-component azomethine dipolar cycloaddition reaction of 1-(alkylsulfonyl)-3,5-bis(ylidene)-piperidin-4-ones 3a‒h. X-ray diffraction studies (6b‒d,h) confirmed the structures. The majority of the synthesized analogs reveal promising antiproliferation properties against a variety of human cancer cell lines (MCF7, HCT116, A431 and PaCa2) with good selectivity index towards normal cell (RPE1). Some of the synthesized agents exhibit potent inhibitory properties against the tested cell lines with higher efficacies than the standard references (sunitinib and 5-fluorouracil). Compound 6m is the most potent. Multi-targeted inhibitory properties against EGFR and VEGFR-2 have been observed for the synthesized agents. Flow cytometry supports the antiproliferation properties and shows the tested agents as apoptosis and necrosis forming. Vero cell viral infection model demonstrates the anti-SARS-CoV-2 properties of the synthesized agents. Compound 6f is the most promising (about 3.3 and 4.8 times the potency of the standard references, chloroquine and hydroxychloroquine). QSAR models explain and support the observed biological properties.

Pyrazinopyrimidine alkaloids from a mangrove-derived fungus Aspergillus versicolor HDN11-84

Pyrazinopyrimidine-type alkaloids bearing a pyrazino[1,2-a] pyrimidine moiety, often have different functional groups substituted at C-8' or C-2'/C-8', generally further forming unique spiro-/conjugated ring systems. Four undescribed pyrazinopyrimidine-type alkaloids, including three natural products pyrasplorines A-C and an artifact deg-pyrasplorine B, as well as a biogenetically related versicoloid A were discovered from the extract of a mangrove-derived fungus Apergillus verisicolor HDN11-84. Pyrasplorine A contains unique spiral-type skeleton (composed of cyclopentenone ring with the pyrazino[1,2-a] pyrimidine core) which is unprecedented in pyrazinopyrimidine-type alkaloids. The deg-pyrasplorine B could be spontaneously converted from pyrasplorine B in mild conditions. Their structures including absolute configurations were elucidated based on NMR spectroscopic analysis, computational calculations and Marfey's method. The absolute configuration of versicoloid A was re-assigned in this study. All the isolated compounds are non-cytotoxic and deg-pyrasplorine B showed anti-influenza A virus H1N1 activity with the IC₅₀ of 50 μM.

A single-domain small protein Med-ORF10 regulates the production of antitumour agent medermycin in Streptomyces

Med-ORF10, a single-domain protein with unknown function encoded by a gene located in a gene cluster responsible for the biosynthesis of a novel antitumour antibiotic medermycin, shares high homology to a group of small proteins widely distributed in many aromatic polyketide antibiotic pathways. This group of proteins contain a nuclear transport factor-2 (NTF-2) domain and appear to undergo an evolutionary divergence in their functions. Gene knockout and interspecies complementation suggested that Med-ORF10 plays a regulatory role in medermycin biosynthetic pathway. Overexpression of med-ORF10 in its wild-type strain led to significant increase of medermycin production. It was also shown by qRT-PCR and Western blot that Med-ORF10 controls the expression of genes encoding tailoring enzymes involved in medermycin biosynthesis. Transcriptome analysis and qRT-PCR revealed that Med-ORF10 has pleiotropic effects on more targets. However, there is no similar conserved domain available in Med-ORF10 compared to those of mechanistically known regulatory proteins; meanwhile, no direct interaction between Med-ORF10 and its target promoter DNA was detected via gel shift assay. All these studies suggest that Med-ORF10 regulates medermycin biosynthesis probably via an indirect mode.

Pleiotropic effects of ActVI-ORFA as an unusual regulatory factor identified in the biosynthetic pathway of actinorhodin in Streptomyces coelicolor

Regulatory networks play critical roles in controlling the the biosynthesis of natural products in Streptomyces. ActVI-ORFA, a regulatory factor encoded by the actinorhodin biosynthetic gene cluster (act cluster), positively controls the production of actinorhodin (ACT) in Streptomyces coelicolor, although its regulatory mechanism remains obscure. This study aimed to identify the regulatory targets of ActVI-ORFA. Deletion of ActVI-ORFA caused the differential expression of hundreds of proteins, as determined by two-dimensional electrophoresis and peptide mass fingerprinting analysis. qRT-PCR analysis of some genes encoding these differentially expressed proteins, including act genes and non-act genes, confirmed that ActVI-ORFA could control their transcriptional levels. In an electrophoretic mobility shift assay with a promoter region of a target gene located in the act cluster, no binding was detected, consistent with the lack of a recognizable DNA-binding domain in ActVI-ORFA. Overall, our findings suggest that ActVI-ORFA is a pleiotropic regulatory factor that controls multiple physiological pathways, including secondary metabolite production, probably via an indirect mode.

Naphthoquinone-Based Meroterpenoids from Marine-Derived Streptomyces sp. B9173

Naphthoquinone-based meroterpenoids are hybrid polyketide-terpenoid natural products with chemical diversity and a broad range of biological activities. Here, we report the isolation of a group of naphthoquinone-containing compounds from Streptomyces sp. B9173, and their structures were elucidated by using a combination of spectroscopic techniques, including 1D, 2D NMR, and high-resolution mass (HRMS) analysis. Seven flaviogeranin congeners or intermediates, three of which were new, have been derived from common naphthoquinone backbone and subsequent oxidation, methylation, prenylation, and amino group incorporation. Both flaviogeranin B1 (1) and B (2) contain an amino group which was incorporated into the C8 of 1,3,6,8-terhydroxynaphthalene (THN). Flaviogeranin D (3) contains an intact C-geranylgeranyl residue attached to the C2 of THN, while the O-geranylgeranyl group of 2 links with the hydroxyl on the C2 site of THN. Four compounds were selected and tested for antibacterial activity and cytotoxicity, with 3 and flaviogeranin C2 (5) displaying potent activity against selected bacteria and cancer cell lines. In light of the structure features of isolated compounds and the biosynthetic genes, a biosynthetic pathway of naphthoquinone-based flaviogeranins has been proposed. These isolated compounds not only extend the structural diversity but also represent new insights into the biosynthesis of naphthoquinone-based meroterpenoids.

Refactoring the Concise Biosynthetic Pathway of Cyanogramide Unveils Spirooxindole Formation Catalyzed by a P450 Enzyme

Cyanogramide (1) from the marine actinomycete Actinoalloteichus cyanogriseus WH1-2216-6 features a unique spirooxindole skeleton and exhibits significant bioactivity to efficiently reverse drug resistance in tumor cells. The biosynthetic gene cluster of 1 in A. cyanogriseus WH1-2216-6 was identified and refactored by promoter engineering for heterologous expression in Streptomyces coelicolor YF11, thereby enabling the production of 1 and five new derivatives. Interesting, four of them, including 1, were identified as enantiomeric mixtures in different ratios. The functions of tailoring enzymes, including two methyltransferases (CyaEF), and three cytochrome P450 monooxygenases (CyaGHI) were confirmed by gene inactivation and feeding experiments, leading to the elucidation of a concise biosynthetic pathway for 1. Notably, CyaH was biochemically verified to catalyze the formation of the spirooxindole skeleton in 1 through an unusual carbocation-mediated semipinacol-type rearrangement reaction.

Synthesis of N-substituted 3-(2-aryl-2-oxoethyl)-3-hydroxyindolin-2-ones and their conversion to N-substituted (E)-3-(2-aryl-2-oxoethylidene)indolin-2-ones: synthetic sequence, spectroscopic characterization and structures of four 3-hydroxy compounds and five oxoethylidene products

An operationally simple and time-efficient approach has been developed for the synthesis of racemic N-substituted 3-(2-aryl-2-oxoethyl)-3-hydroxyindolin-2-ones by a piperidine-catalysed aldol reaction between aryl methyl ketones and N-alkylisatins. These aldol products were used successfully as strategic intermediates for the preparation of N-substituted (E)-3-(2-hetaryl-2-oxoethylidene)indolin-2-ones by a stereoselective dehydration reaction under acidic conditions. The products have all been fully characterized by 1H and 13C NMR spectroscopy, by mass spectrometry and, for a representative selection, by crystal structure analysis. In each of (RS)-1-benzyl-3-hydroxy-3-[2-(4-methoxyphenyl)-2-oxoethyl]indolin-2-one, C24H21NO4, (Ic), and (RS)-1-benzyl-3-{2-[4-(dimethylamino)phenyl]-2-oxoethyl}-3-hydroxyindolin-2-one, C25H24N2O3, (Id), inversion-related pairs of molecules are linked by O-H...O hydrogen bonds to form R22(10) rings, which are further linked into chains of rings by a combination of C-H...O and C-H...π(arene) hydrogen bonds in (Ic) and by C-H...π(arene) hydrogen bonds in (Id). The molecules of (RS)-1-benzyl-3-hydroxy-3-[2-oxo-2-(pyridin-4-yl)ethyl]indolin-2-one, C22H18N2O3, (Ie), are linked into a three-dimensional framework structure by a combination of O-H...N, C-H...O and C-H...π(arene) hydrogen bonds. (RS)-3-[2-(Benzo[d][1,3]dioxol-5-yl)-2-oxoethyl]-1-benzyl-3-hydroxyindolin-2-one, C24H19NO5, (If), crystallizes with Z' = 2 in the space group P-1 and the molecules are linked into complex sheets by a combination of O-H...O, C-H...O and C-H...π(arene) hydrogen bonds. In each of (E)-1-benzyl-3-[2-(4-fluorophenyl)-2-oxoethylidene]indolin-2-one, C23H16FNO2, (IIa), and (E)-1-benzyl-3-[2-oxo-2-(thiophen-2-yl)ethylidene]indolin-2-one, C21H15NO2S, (IIg), the molecules are linked into simple chains by a single C-H...O hydrogen bond, while those of (E)-1-benzyl-3-[2-oxo-2-(pyridin-4-yl)ethylidene]indolin-2-one, C22H16N2O2, (IIe), are linked by three C-H...O hydrogen bonds to form sheets which are further linked into a three-dimensional structure by C-H...π(arene) hydrogen bonds. There are no hydrogen bonds in the structures of either (E)-1-benzyl-3-[2-(4-methoxyphenyl)-2-oxoethylidene]indolin-2-one, C24H19NO3, (IIc), or (E)-1-benzyl-5-chloro-3-[2-(4-chlorophenyl)-2-oxoethylidene]indolin-2-one, C23H15Cl2NO2, (IIh), but the molecules of (IIh) are linked into chains of π-stacked dimers by a combination of C-Cl...π(arene) and aromatic π-π stacking interactions.

Community Structures and Antifungal Activity of Root-Associated Endophytic Actinobacteria of Healthy and Diseased Soybean

The present study was conducted to examine the influence of a pathogen Sclerotinia sclerotiorum (Lib.) de Bary on the actinobacterial community associated with the soybean roots. A total of 70 endophytic actinobacteria were isolated from the surface-sterilized roots of either healthy or diseased soybeans, and they were distributed under 14 genera. Some rare genera, including Rhodococcus, Kribbella, Glycomyces, Saccharothrix, Streptosporangium and Cellulosimicrobium, were endemic to the diseased samples, and the actinobacterial community was more diverse in the diseased samples compared with that in the heathy samples. Culture-independent analysis of root-associated actinobacterial community using the high-throughput sequencing approach also showed similar results. Four Streptomyces strains that were significantly abundant in the diseased samples exhibited strong antagonistic activity with the inhibition percentage of 54.1-87.6%. A bioactivity-guided approach was then employed to isolate and determine the chemical identity of antifungal constituents derived from the four strains. One new maremycin analogue, together with eight known compounds, were detected. All compounds showed significantly antifungal activity against S. sclerotiorum with the 50% inhibition (EC₅₀) values of 49.14-0.21 mg/L. The higher actinobacterial diversity and more antifungal strains associated with roots of diseased plants indicate a possible role of the root-associated actinobacteria in natural defense against phytopathogens. Furthermore, these results also suggest that the root of diseased plant may be a potential reservoir of actinobacteria producing new agroactive compounds.

Identification of the Amipurimycin Gene Cluster Yields Insight into the Biosynthesis of C9 Sugar Nucleoside Antibiotics

Feeding studies indicate a possible synthetic pattern for the N-terminal cis-aminocyclopentane carboxylic acid (ACPC) and suggest an unusual source of the high-carbon sugar skeleton of amipurimycin (APM). The biosynthetic gene cluster of APM was identified and confirmed by in vivo experiments. A C9 core intermediate was discovered from null mutants of ACPC pathway, and an ATP-grasp enzyme (ApmA8) was reconstituted in vitro for ACPC loading. Our observations allow a first proposal of the APM biosynthetic pathway.