Pericoannosin A, a Polyketide Synthase–Nonribosomal Peptide Synthetase Hybrid Metabolite with New Carbon Skeleton from the Endophytic Fungus Periconia sp

Sucurchalasins A and B, Sulfur-Containing Heterodimers of a Cytochalasan and a Macrolide from the Endophytic Fungus Aspergillus spelaeus GDGJ-286

Two sulfur-containing heterodimers of a cytochalasan and a macrolide, sucurchalasins A and B (1 and 2), and four known cytochalasan monomers (3-6), as well as four known macrolide derivatives (7-10), were obtained from the endophytic fungus Aspergillus spelaeus GDGJ-286. Sucurchalasins A and B (1 and 2) are the first cytochalasan heterodimers formed via a thioether bridge between cytochalasan and curvularin macrolide units. Their structures were elucidated by detailed analysis of NMR, LC-MS/MS, and X-ray crystallography. In bioassays, 1 and 2 exhibited cytotoxic effects on A2780 cells, with IC50 values of 3.9 and 8.3 μM, respectively. They also showed antibacterial activities against E. faecalis and B. subtilis with MIC values of 3.1 and 6.3 μg/mL, respectively.

Biosynthesis, biological activities, and structure-activity relationships of decalin-containing tetramic acid derivatives isolated from fungi

Covering: up to December 2023Decalin-containing tetramic acid derivatives, especially 3-decalinoyltetramic acids (3-DTAs), are commonly found as fungal secondary metabolites. Numerous biological activities of this class of compounds, such as antibiotic, antiviral, antifungal, antiplasmodial, and antiprotozoal properties, have been the subject of ongoing research. For this reason, these molecules have attracted a lot of interest from the scientific community and various efforts including semi-synthesis, co-culturing with bacteria and biosynthetic gene sequencing have been made to obtain more derivatives. In this review, 3-DTAs are classified into four major groups based on the absolute configuration of the bicyclic decalin ring. Their biosynthetic pathways, various biological activities, and structure-activity relationship are then introduced.

Biological activities of endophytic fungi isolated from Annona muricata Linnaeus: a systematic review

Abstract This systematic review integrates the data available in the literature regarding the biological activities of the extracts of endophytic fungi isolated from Annona muricata and their secondary metabolites. The search was performed using four electronic databases, and studies' quality was evaluated using an adapted assessment tool. The initial database search yielded 436 results; ten studies were selected for inclusion. The leaf was the most studied part of the plant (in nine studies); Periconia sp. was the most tested fungus (n = 4); the most evaluated biological activity was anticancer (n = 6), followed by antiviral (n = 3). Antibacterial, antifungal, and antioxidant activities were also tested. Terpenoids or terpenoid hybrid compounds were the most abundant chemical metabolites. Phenolic compounds, esters, alkaloids, saturated and unsaturated fatty acids, aromatic compounds, and peptides were also reported. The selected studies highlighted the biotechnological potentiality of the endophytic fungi extracts from A. muricata. Consequently, it can be considered a promising source of biological compounds with antioxidant effects and active against different microorganisms and cancer cells. Further research is needed involving different plant tissues, other microorganisms, such as SARS-CoV-2, and different cancer cells.

Endophytic fungal community of Rosa damascena Mill. as a promising source of indigenous biostimulants: Elucidating its spatial distribution, chemical diversity, and ecological functions

The role of endophytes in maintaining healthy plant ecosystems and holding promise for agriculture and food security is deeply appreciated. In the current study, we determine the community structure, spatial distribution, chemical diversity, and ecological functions of fungal endophytes of Rosa damascena growing in the North-Western Himalayas. Culture-dependent methods revealed that R. damascena supported a rich endophyte diversity comprising 32 genera and 68 OTUs. The diversity was governed by climate, altitude, and tissue type. Species of Aspergillus, Cladosporium, Penicillium, and Diaporthe were the core endophytes of the host plant consisting of 48.8% of the endophytes collectively. The predominant pathogen of the host was Alternaria spp., especially A. alternata. GC-MS analyses affirmed the production of diverse arrays of volatile organic compounds (VOC) by individual endophytes. Among the primary rose oil components, Diaporthe melonis RDE257, and Periconia verrucosa RDE85 produced phenyl ethyl alcohol (PEA) and benzyl alcohol (BA). The endophytes displayed varied levels of plant growth-promoting, colonization, and anti-pathogenic traits. Between the selected endophytes, P. verrucosa and D. melonis significantly potentiated plant growth and the flavonoids and chlorophyll content in the host. The potential of these two endophytes and their metabolites PEA and BA was confirmed on Nicotiana tabacum. The treatments of the metabolites and individual endophytes enhanced the growth parameters in the model plant significantly. The results imply that P. verrucosa and D. melonis are potential plant growth enhancers and their activity may be partially due to the production of PEA and BA. Thus, R. damascena harbors diverse endophytes with potential applications in disease suppression and host growth promotion. Further investigations at the molecular level are warranted to develop green endophytic agents for sustainable cultivation of R. damascena and biocontrol of leaf spot disease.

Cytochalasans and Their Impact on Actin Filament Remodeling

The eukaryotic actin cytoskeleton comprises the protein itself in its monomeric and filamentous forms, G- and F-actin, as well as multiple interaction partners (actin-binding proteins, ABPs). This gives rise to a temporally and spatially controlled, dynamic network, eliciting a plethora of motility-associated processes. To interfere with the complex inter- and intracellular interactions the actin cytoskeleton confers, small molecular inhibitors have been used, foremost of all to study the relevance of actin filaments and their turnover for various cellular processes. The most prominent inhibitors act by, e.g., sequestering monomers or by interfering with the polymerization of new filaments and the elongation of existing filaments. Among these inhibitors used as tool compounds are the cytochalasans, fungal secondary metabolites known for decades and exploited for their F-actin polymerization inhibitory capabilities. In spite of their application as tool compounds for decades, comprehensive data are lacking that explain (i) how the structural deviances of the more than 400 cytochalasans described to date influence their bioactivity mechanistically and (ii) how the intricate network of ABPs reacts (or adapts) to cytochalasan binding. This review thus aims to summarize the information available concerning the structural features of cytochalasans and their influence on the described activities on cell morphology and actin cytoskeleton organization in eukaryotic cells.

Fungal Endophytes: Microfactories of Novel Bioactive Compounds with Therapeutic Interventions; A Comprehensive Review on the Biotechnological Developments in the Field of Fungal Endophytic Biology over the Last Decade

The seminal discovery of paclitaxel from endophytic fungus Taxomyces andreanae was a milestone in recognizing the immense potential of endophytic fungi as prolific producers of bioactive secondary metabolites of use in medicine, agriculture, and food industries. Following the discovery of paclitaxel, the research community has intensified efforts to harness endophytic fungi as putative producers of lead molecules with anticancer, anti-inflammatory, antimicrobial, antioxidant, cardio-protective, and immunomodulatory properties. Endophytic fungi have been a valuable source of bioactive compounds over the last three decades. Compounds such as taxol, podophyllotoxin, huperzine, camptothecin, and resveratrol have been effectively isolated and characterized after extraction from endophytic fungi. These findings have expanded the applications of endophytic fungi in medicine and related fields. In the present review, we systematically compile and analyze several important compounds derived from endophytic fungi, encompassing the period from 2011 to 2022. Our systematic approach focuses on elucidating the origins of endophytic fungi, exploring the structural diversity and biological activities exhibited by these compounds, and giving special emphasis to the pharmacological activities and mechanism of action of certain compounds. We highlight the tremendous potential of endophytic fungi as alternate sources of bioactive metabolites, with implications for combating major global diseases. This underscores the significant role that fungi can play in the discovery and development of novel therapeutic agents that address the challenges posed by prevalent diseases worldwide.

Two new lanostane-type triterpenoids from the fungus Periconia sp. TJ403-rc01

The endophytic fungus Periconia sp. TJ403-rc01 (Dematiaceae) isolated from the leaves of Rosa chinensis Jacq. (Rosaceae) was cultivated on rice medium and chemically investigated, affording two new lanostane-type triterpenoids, namely pericinones A and B (1 and 2). Their structures were determined mainly by 1 D and 2 D NMR and HRESIMS data. Notably, it is the first report of lanostane-type triterpenoids from species of Periconia. Compounds 1 and 2 showed moderate anti-inflammatory activity against the NO production with IC₅₀ values of 24.12 ± 0.73 and 11.38 ± 1.56 μM, respectively.

Secondary metabolites from the Endophytic fungi Fusarium decemcellulare F25 and their antifungal activities

Seven new compounds, including three isocoumarins (1-3), three pyrrolidinone derivatives (8-10), and one pentaene diacid (15), together with 13 known compounds, were isolated from the rice culture of the endophytic fungus Fusarium decemcellulare F25. Their structures and stereochemistry were established using HRESIMS, NMR, electronic circular dichroism (ECD) calculations, and single-crystal X-ray diffraction. The possible biosynthetic pathways for compounds 1-3 and 8-10 were proposed. The antifungal efficacies of compounds 1 - 20 were evaluated against Colletotrichum musae, and compounds 13, 14, and 17 exhibited inhibitory activities against C. musae with MIC values of 256, 64 and 128 μg/mL, respectively.

Structural diversification of bioactive bibenzyls through modular co-culture leading to the discovery of a novel neuroprotective agent

Bibenzyls, a kind of important plant polyphenols, have attracted growing attention for their broad and remarkable pharmacological activities. However, due to the low abundance in nature, uncontrollable and environmentally unfriendly chemical synthesis processes, these compounds are not readily accessible. Herein, one high-yield bibenzyl backbone-producing Escherichia coli strain was constructed by using a highly active and substrate-promiscuous bibenzyl synthase identified from Dendrobium officinale in combination with starter and extender biosynthetic enzymes. Three types of efficiently post-modifying modular strains were engineered by employing methyltransferases, prenyltransferase, and glycosyltransferase with high activity and substrate tolerance together with their corresponding donor biosynthetic modules. Structurally different bibenzyl derivatives were tandemly and/or divergently synthesized by co-culture engineering in various combination modes. Especially, a prenylated bibenzyl derivative (12) was found to be an antioxidant that exhibited potent neuroprotective activity in the cellular and rat models of ischemia stroke. RNA-seq, quantitative RT-PCR, and Western-blot analysis demonstrated that 12 could up-regulate the expression level of an apoptosis-inducing factor, mitochondria associated 3 (Aifm3), suggesting that Aifm3 might be a new target in ischemic stroke therapy. This study provides a flexible plug-and-play strategy for the easy-to-implement synthesis of structurally diverse bibenzyls through a modular co-culture engineering pipeline for drug discovery.

New dimeric chromanone derivatives from the mutant strains of Penicillium oxalicum and their bioactivities

Three new chromanone dimer derivatives, paecilins F-H (1-3) and ten known compounds (4-13), were obtained from the mutant strains of Penicillium oxalicum 114-2. Their structures were elucidated by extensive analysis of spectroscopic data and comparison with reported data, and the configurations of 1-3 were resolved by quantum chemical calculations of NMR shifts and ECD spectra. Compounds 5 and 11 showed significant anti-influenza A virus activities with IC50 values of 5.6 and 6.9 μM, respectively. Compounds 8 and 9 displayed cytotoxic activities against the MIA-PaCa-2 cell line with IC50 values of 2.6 and 2.1 μM, respectively. Compound 10 exhibited antibacterial activities against Bacillus cereus with a MIC value of 4 μg mL-1.

Chlorinated Cyclopentene Derivatives and Antifungal Activities from Periconia sp. Induced by the One Strain Many Compounds Strategy and Host Plant Culture

Eleven new chlorinated cyclopentene derivatives, periconsins A-K, and a new diketopiperazine, periconzin, were found from Periconia sp. cultured in three different media by the one strain many compounds strategy. Additionally, the C-1 methyl hydroxylation of chlorinated cyclopentene was found for the first time in the host plant culture. The structures were identified by extensive spectroscopic analyses, electronic circular dichroism (ECD) and ¹³C NMR calculations, and single-crystal X-ray diffraction. Compounds 3, 5, 7-11, 15, and 17 showed significant antifungal activities against the plant pathogens Periconia sp., Altemaria sp., and Nigrospora oryzae with MICs ≤2 μg/mL. Other compounds had antifungal activities with MICs ≤8 μg/mL. The antifungal structure-activity relationship of these metabolites indicated that the chlorine at C-5 can increase the activity, but the hydroxy group at C-1 lowered the activity.

Two new 5/6/6 polyketide-amino acid hybrids from a genetic mutant of Periconia sp. F-31

Peridecalins C and D (1 and 2), one decalin and one oxygen-decalin containing polyketide-amino acid hybrids with 5/6/6 ring system, was isolated from a genetic mutant of Periconia sp. F-31. Their structures were elucidated through extensive spectroscopic data analysis, including 1 D/2D NMR and HR-MS spectra. Biosynthetically, two proposed Diels-Alder reactions are supposed to be involved in the skeleton construction of 1 and 2.

Sesquiterpenes from the endophytic fungus Periconia sp. F-31

One new eremophilane sesquiterpene periconianone L (1), together with four known guaiane-type sesquiterpenes 4,10,11-trihydroxyguaiane (2), (-)-guai-1(10)-ene-4α,11-diolhydroxymecuration (3), guaidiol A (4), and epi-guaidiol A (5) were isolated from the endophytic fungus Periconia sp. F-31. The structure of the new compound was established by spectroscopic methods, including UV, IR, HRESIMS, and extensive NMR techniques. Compound 3 was isolated as natural product for the first time.

Taxonomic Reappraisal of Periconiaceae with the Description of Three New Periconia Species from China

As a result of an ongoing research survey of microfungi in Yunnan, China, several saprobic ascomycetes were collected from various host substrates. Preliminary morphological analyses identified a few of these taxa as Periconia species. We obtained DNA sequence data of the Periconia species from pure cultures and investigated their phylogenetic affinities. Phylogenetic analyses of a combined LSU, ITS, SSU and tef1-α sequence dataset demonstrated that five isolates of Periconia formed well-resolved subclades within Periconiaceae. Accordingly, three new Periconia species are introduced viz. P. artemisiae, P. chimonanthi and P. thysanolaenae, and new host and geographical records of P. byssoides and P. pseudobyssoides, are also reported from dead branches of Prunus armeniaca and Scrophularia ningpoensis. Periconia celtidis formed a monophyletic clade with P. byssoides in the present phylogenetic analyses. Results of the pairwise homoplasy index (PHI) test indicated significant recombination between P. byssoides and P. celtidis. Therefore, P. celtidis has been synonymized under P. byssoides. In addition, we re-illustrated and studied the type specimen of the sexual genus Bambusistroma. As a type species of Bambusistroma, B. didymosporum features similar morphology to the sexual morph of Periconia homothallica and P. pseudodigitata. We therefore synonymize Bambusistroma under Periconia based on morphological and phylogenetic evidence. Furthermore, our new isolates produced brown conidia of asexual morph in agar media typical of the genus Noosia. Based on morphological comparison with Periconia in vitro and phylogenetic status of Noosia, we also treat Noosia as a synonym of Periconia. Detailed descriptions and illustrations of three novel taxa and two new records of Periconia byssoides and P. pseudobyssoides as well as the illustration of P. didymosporum comb. nov. are provided. An updated phylogenetic tree of Periconiaceae using maximum likelihood and Bayesian inference analyses is constructed. Generic circumscription of Periconia is amended.

Biomimetic Diels-Alder Reactions in Natural Product Synthesis: A Personal Retrospect

Nature has been recognized for her super capability of constructing complex molecules with remarkable efficiency and elegancy. Among nature’s versatile synthetic toolkits, Diels-Alder reaction is particularly attractive since it allows for rapid generation of molecular complexity from simple precursors. For natural products biosynthetically formed through Diels-Alder reactions, the most straightforward way to access them should build on biomimetic Diels-Alder reactions. However, the implementation of biomimetic Diels-Alder reactions in a laboratory setting may encounter considerable challenges, particularly for those suffering from complicated reactivity and selectivity issues. Indeed, the translation of a biosynthetic hypothesis into a real biomimetic synthesis entails the orchestrated combination of nature’s inspiration and chemist’s rational design. In this account, we will briefly summarize our recent progress on the application of biomimetic Diels-Alder reactions in natural product synthesis. As shown in the discussed stories, rational manipulation of the structures of biosynthetic precursors plays a crucial role for the successful implementation of biomimetic Diels-Alder reactions. 1 Introduction 2 Biomimetic Synthesis of Rossinone B 3 Biomimetic Synthesis of Homodimericin A 4 Biomimetic Synthesis of Polycyclic and Dimeric Xanthanolides 5 Biomimetic Synthesis of Periconiasins and Pericoannosins 6 Biomimetic Synthesis of Merocyctochalasans 7 Conclusion and outlook

Asperflavipines C–E and aspermichalasine A: three cytochalasan heterotetramers and an unusual cytochalasan monomer from Aspergillus micronesiensis

Asperflavipines C–E, three new cytochalasin heterotetramers, possessing a highly complex tetradecacyclic ring system with continuous bridged ring systems, and aspermichalasine A, possessing an unusual 5/6/5/8 ring system, were isolated.

Biomimetic Synthesis of Natural Products: A Journey To Learn, To Mimic, and To Be Better

Total synthesis of natural products has been one of the most exciting and dynamic areas in synthetic organic chemistry. Nowadays, the major challenge in this field is not whether a given target of interest can be synthesized but how to make it with commendable efficiency and practicality. To meet this grand challenge, a wise way is to learn from Mother Nature who is recognized for her superb capability of forging complicated and sometimes beyond-imagination molecules in her own delicate way. Indeed, since Sir Robert Robinson published his groundbreaking synthesis of tropinone in 1917, biomimetic synthesis of natural products, a process of imitating nature's way to make molecules, has evolved into one of the most popular research directions in organic synthesis.Our group has been engaging in biomimetic synthesis of natural products in the past decade. During this time, we have come to realize that the successful implementation of a biomimetic synthesis entails the orchestrated combination of bioinspiration and rational design. On the one hand, we prefer to utilize some elegant bioinspired transformations (e.g., Diels-Alder dimerization, 6π-electrocyclization, and [2 + 2]-photocycloaddition) as the key steps of our synthesis, which enable rapid construction of the core skeletons of the chased targets with high efficiency; on the other hand, various powerful reactions (e.g., dyotropic rearrangement of β-lactone, tandem aldol condensation/Grob fragmentation reaction, and organocatalytic asymmetric Mukaiyama-Michael addition) are rationally designed by us, which allow for facile access to the requisite precursors for attempting biomimetic transformations. In some cases, the proposed biomimetic transformation may fail to give a satisfactory result in practice, and thus we opt to develop creative tactics (e.g., hydrogen atom transfer-triggered vinyl cyclobutane ring opening/oxygen insertion/cyclization cascade) that can meet the challenge. Guided by this synthesis concept, we have achieved the total syntheses of multiple families of natural products of great importance in both chemistry and biology, representatives of which include xanthanolides, cytochalasans, and plakortin-type polyketides. Of note, most of these targets could be accessed in a concise, efficient, and scalable manner, which paves the way for further exploration of their biological functions and medicinal potential. Moreover, owing to their biomimetic nature, our syntheses provide valuable information for deciphering the underlying biosynthetic pathways of the chased targets, which could not be attained by other synthetic modes.

The grass root endophytic fungus Flavomyces fulophazii: An abundant source of tetramic acid and chlorinated azaphilone derivatives

Fungal endophytes are remarkable sources of biologically active metabolites of ecological and pharmacological significance. In this study, fungal isolates producing yellow pigments and originating from grass roots, were identified as the recently described grass root colonizing dark septate endophyte (DSE), Flavomyces fulophazii (Periconiaceae, Pleosporales). While analyzing the metabolite composition of 17 isolates of this fungus, 11 previously undescribed compounds, including four tetramic acids (dihydroxyvermelhotin, hydroxyvermelhotin, methoxyvermelhotin, oxovermelhotin), and seven chlorinated azaphilones (flavochlorines A-G), together with the known tetramic acid vermelhotin, were tentatively identified by high performance liquid chromatography (HPLC)-tandem mass spectrometry (MS/MS). Among them, flavochlorine A, flavochlorine G, hydroxyvermelhotin and vermelhotin could be isolated by preparative HPLC, thus their structures were also confirmed by nuclear magnetic resonance (NMR) spectroscopy. Vermelhotin was found to be the main compound, reaching its maximum level of 5.5 mg/g in the in vitro cultures of a selected F. fulophazii isolate. A significant amount of vermelhotin was isolated by preparative HPLC from these cultures (4.8 mg from 1.0 g lyophilized culture), confirming the practical utility of F. fulophazii in high-yield vermelhotin production. The main compounds of this endophyte expressed no activity in standardized plant bioassays (i.e., in the Lactuca sativa seed germination and Lemna minor growth tests). An antiproliferative study of the isolated compounds confirmed moderate activity of vermelhotin against a panel of twelve cancer cell lines, with IC₅₀ ranges of 10.1-37.0 μM, without inhibiting the non-cancer Vero cells, suggesting its selectivity towards cancers.

Microbial Metabolites: The Emerging Hotspot of Antiviral Compounds as Potential Candidates to Avert Viral Pandemic Alike COVID-19

The present global COVID-19 pandemic caused by the noble pleomorphic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a vulnerable situation in the global healthcare and economy. In this pandemic situation, researchers all around the world are trying their level best to find suitable therapeutics from various sources to combat against the SARS-CoV-2. To date, numerous bioactive compounds from different sources have been tested to control many viral diseases. However, microbial metabolites are advantageous for drug development over metabolites from other sources. We herein retrieved and reviewed literatures from PubMed, Scopus and Google relevant to antiviral microbial metabolites by searching with the keywords "antiviral microbial metabolites," "microbial metabolite against virus," "microorganism with antiviral activity," "antiviral medicine from microbial metabolite," "antiviral bacterial metabolites," "antiviral fungal metabolites," "antiviral metabolites from microscopic algae' and so on. For the same purpose, the keywords "microbial metabolites against COVID-19 and SARS-CoV-2" and "plant metabolites against COVID-19 and SARS-CoV-2" were used. Only the full text literatures available in English and pertinent to the topic have been included and those which are not available as full text in English and pertinent to antiviral or anti-SARS-CoV-2 activity were excluded. In this review, we have accumulated microbial metabolites that can be used as antiviral agents against a broad range of viruses including SARS-CoV-2. Based on this concept, we have included 330 antiviral microbial metabolites so far available to date in the data bases and were previously isolated from fungi, bacteria and microalgae. The microbial source, chemical nature, targeted viruses, mechanism of actions and IC50/EC50 values of these metabolites are discussed although mechanisms of actions of many of them are not yet elucidated. Among these antiviral microbial metabolites, some compounds might be very potential against many other viruses including coronaviruses. However, these potential microbial metabolites need further research to be developed as effective antiviral drugs. This paper may provide the scientific community with the possible secret of microbial metabolites that could be an effective source of novel antiviral drugs to fight against many viruses including SARS-CoV-2 as well as the future viral pandemics.

The Chemistry and Pharmacology of Fungal Genus Periconia: A Review

Periconia is filamentous fungi belonging to the Periconiaceae family, and over the last 50 years, the genus has shown interest in natural product exploration for pharmacological purposes. Therefore, this study aims to analyze the different species of Periconia containing natural products such as terpenoids, polyketides, cytochalasan, macrosphelides, cyclopentenes, aromatic compounds, and carbohydrates carbasugar derivates. The isolated compound of this kind, which was reported in 1969, consisted of polyketide derivatives and their structures and was determined by chemical reaction and spectroscopic methods. After some years, 77 compounds isolated from endophytic fungus Periconia were associated with eight plant species, 28 compounds from sea hare Aplysia kurodai, and ten from endolichenic fungi Parmelia sp. The potent pharmacological agents from this genus are periconicin A, which acts as an antimicrobial, pericochlorosin B as an anti-human immunodeficiency virus (HIV), peribysin D, and pericosine A as cytotoxic agents, and periconianone A as an anti-inflammatory agent. Furthermore, information about taxol and piperine from Periconia producing species was also provided. Therefore, this study supports discovering new drugs produced by the Periconia species and compares them for future drug development.

Bioactive Compounds from Mangrove Endophytic Fungus and Their Uses for Microorganism Control

Mangroves are ecosystems with unique characteristics due to the high salinity and amount of organic matter that house a rich biodiversity. Fungi have aroused much interest as they are an important natural source for the discovery of new bioactive compounds, with potential biotechnological and pharmacological interest. This review aims to highlight endophytic fungi isolated from mangrove plant species and the isolated bioactive compounds and their bioactivity against protozoa, bacteria and pathogenic viruses. Knowledge about this type of ecosystem is of great relevance for its preservation and as a source of new molecules for the control of pathogens that may be of importance for human, animal and environmental health.

Applications of the Horner–Wadsworth–Emmons Olefination in Modern Natural Product Synthesis

The Horner–Wadsworth–Emmons (HWE) reaction is one of the most reliable olefination reaction and can be broadly applied in organic chemistry and natural product synthesis with excellent selectivity. Within the last few years HWE reaction conditions have been optimized and new reagents developed to overcome challenges in the total syntheses of natural products. This review highlights the application of HWE olefinations in total syntheses of structurally different natural products covering 2015 to 2020. Applied HWE reagents and reactions conditions are highlighted to support future synthetic approaches and serve as guideline to find the best HWE conditions for the most complicated natural products.

1 Introduction and Historical Background

2 Applications of HWE

2.1 Cyclization by HWE Reactions

2.2.1 Formation of Medium- to Larger-Sized Rings

2.2.2 Formation of Small- to Medium-Sized Rings

2.3 Synthesis of α,β-Unsaturated Carbonyl Groups

2.4 Synthesis of Substituted C=C Bonds

2.5 Late-Stage Modifications by HWE Reactions

2.6 HWE Reactions on Solid Supports

2.7 Synthesis of Poly-Conjugated C=C Bonds

2.8 HWE-Mediated Coupling of Larger Building Blocks

2.9 Miscellaneous

3 Summary and Outlook

Progress in the Chemistry of Cytochalasans

Cytochalasans are a group of fungal-derived natural products characterized by a perhydro-isoindolone core fused with a macrocyclic ring, and they exhibit a high structural diversity and a broad spectrum of bioactivities. Cytochalasans have attracted significant attention from the chemical and pharmacological communities and have been reviewed previously from various perspectives in recent years. However, continued interest in the cytochalasans and the number of laboratory investigations on these compounds are both growing rapidly. This contribution provides a general overview of the isolation, structural determination, biological activities, biosynthesis, and total synthesis of cytochalasans. In total, 477 cytochalasans are covered, including "merocytochalasans" that arise by the dimerization or polymerization of one or more cytochalasan molecules with one or more other natural product units. This contribution provides a comprehensive treatment of the cytochalasans, and it is hoped that it may stimulate further work on these interesting natural products.

Bisaspochalasins D and E: Two Heterocycle-Fused Cytochalasan Homodimers from an Endophytic Aspergillus flavipes

Two heterocycle-fused cytochalasan homodimers, bisaspochalasins D (1) and E (2), were isolated from an endophytic Aspergillus flavipes. Their chemical structures were elucidated using a combination of HRESIMS, NMR, theoretical calculations, and crystallographic techniques. Bisaspochalasin D (1) is dimerized by the first reported naturally occurring triple heterobridged 3,8-dioxa-6-azabicyclo[3.2.1]octane framework, while bisaspochalasin E (2) employs a pyrrole ring as the linking moiety. Possible dimerization mechanisms of bisaspochalasins D and E were proposed. The bioassay screening revealed that bisaspochalasin D showed cytotoxic activities against five cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW-480) with IC₅₀ values ranging from 4.45 to 22.99 μM. Additionally, bisaspochalasin D exhibited neurotrophic activities in a PC12 cell-based assay. At a concentration of 10 μM, bisaspochalasin D can promote neurite growth by inducing a differentiation rate of 12.52% for PC12 cells.

Endophytic Microbial Diversity: A New Hope for the Production of Novel Anti-tumor and Anti-HIV Agents as Future Therapeutics

Cancer is a collective name for a variety of diseases that can begin in virtually every organ or body tissue as abnormal cells develop uncontrollably and ten million new cancer cases are diagnosed all over the world at present. Whereas HIV is a virus that makes people susceptible to infection and contributes to the condition of acquired immune deficiency syndrome (AIDS). Almost 37 million people are currently diagnosed with HIV and 1 million people die every year, which is the worst-case scenario. Potential medicinal compounds have played a crucial role in the production of certain clinically beneficial novel anti-cancer and anti-HIV agents that are produced from natural sources especially from plants. These include Taxol, Vinblastine, Podophyllotoxin, Betulinic acid, Camptothecin, and Vincristine, etc. In the past decades, bioactive compounds were extracted directly from the plant sources which was more time consuming, led to low yield productivity, high cost, and bad impact on biodiversity. Endophytes, the microorganisms that reside inside the host plant by not causing any kind of harm to them and have potential applications in agriculture, medicine, pollution, and food industries. Therefore, by isolating and characterizing novel endophytes from medicinal plants and extracting their secondary metabolites to produce useful bioactive compounds can be beneficial for well-being and society as a future therapeutics. This approach is not harmful to biodiversity economical, timesaving, low cost, and can lead to the discovery of various industrial and commercially important novel anti-tumor and anti-HIV agents in the future. The Himalayas are home to several medicinal plants and the endophytic microbial biodiversity of the Himalayan region is also not much explored yet. However, the effect of compounds from these endophytes on anticancer and antiviral activity, especially anti-HIV has been largely unexplored. Hence, the present review is designed to the exploration of endophytic microbial diversity that can give rise to the discovery of various novel potential industrially valuable bioactive compounds that can lessen the rate of such type of pandemic diseases in the future by providing low-cost future therapeutics in future.

Applications of microbial co-cultures in polyketides production

Polyketides are a large group of natural biomolecules that are normally produced by bacteria, fungi and plants. These molecules have clinical importance due to their anti-cancer, anti-microbial, anti-oxidant and anti-inflammatory properties. Polyketides are biosynthesized from units of acyl-CoA by different polyketide synthases (PKSs), which display wide diversity of functional domains and mechanisms of action between fungi and bacteria. Co-culture of different micro-organisms can produce novel products distinctive from those produced during single cultures. This study compared the new polyketides produced in such co-culture systems and discusses aspects of the cultivation systems, product structures and identification techniques. Current results indicate that the formation of new polyketides may be the result of activation of previously silent PKSs genes induced during co-culture. This review indicated a potential way to produce pure therapeutic polyketides by microbial fermentation and a potential way to develop functional foods and agricultural products using co-co-culture of different micro-organisms. It also pointed out a new perspective for studies on the process of functional foods, especially those involving multiple micro-organisms.

1-Isoindolinone scaffold-based natural products with a promising diverse bioactivity

Isoindolin-1-one or 1-isoindolinone framework is referred to phthalimidines or benzo fused γ-lactams of the corresponding γ-amino carboxylic acids and has been of prime interest for scientists for last several decades. 1-Isoindolinone framework is found in a wide range of naturally occurring compounds with diverse biological activities and therapeutic potential for various chronic diseases. Recent developments in synthetic methods for their procurement have opened a new era of 1-isoindolinone chemistry. This review aims to provide an alphabetical quick reference guide to only 1-isoindolinone based natural products and its variable fused, oxidized and reduced state skeleton with information for advanced chemotaxonomic analyses, cellular targets/pathways and diverse biological activities and future use for medicinal chemistry.

Natural Bioactive Compounds from Fungi as Potential Candidates for Protease Inhibitors and Immunomodulators to Apply for Coronaviruses

The inhibition of viral protease is an important target in antiviral drug discovery and development. To date, protease inhibitor drugs, especially HIV-1 protease inhibitors, have been available for human clinical use in the treatment of coronaviruses. However, these drugs can have adverse side effects and they can become ineffective due to eventual drug resistance. Thus, the search for natural bioactive compounds that were obtained from bio-resources that exert inhibitory capabilities against HIV-1 protease activity is of great interest. Fungi are a source of natural bioactive compounds that offer therapeutic potential in the prevention of viral diseases and for the improvement of human immunomodulation. Here, we made a brief review of the current findings on fungi as producers of protease inhibitors and studies on the relevant candidate fungal bioactive compounds that can offer immunomodulatory activities as potential therapeutic agents of coronaviruses in the future.

Fungi: outstanding source of novel chemical scaffolds

A large number of remarkable studies on the secondary metabolites of fungi have been conducted in recent years. This review gives an overview of one hundred and sixty-seven molecules with novel skeletons and their bioactivities that have been reported in seventy-nine articles published from 2013 to 2017. Our statistical data showed that endophytic fungi and marine-derived fungi are the major sources of novel bioactive secondary metabolites.

Formal Total Synthesis of Pericoannosin A

A concise formal total synthesis of pericoannosin A, by the synthesis of an advanced intermediate of pericoannosin A, was achieved in eight steps from commercially available isoprene in a 21.7% overall yield. Key transformations for this expedited route include an enantioselective organocatalytic Diels-Alder reaction to construct the C ring, a diastereoselective reduction (under Felkin-Ahn control), and a hydroboration/oxidation sequence for chain homologation. This work represents the second synthetic effort toward pericoannosin A, the only reported natural product based on a hexahydro-1H-isochromen-5-isobutylpyrrolidin-2-one core.

First report of chromenyl derivatives from spineless marine cuttlefish Sepiella inermis: Prospective antihyperglycemic agents attenuate serine protease dipeptidyl peptidase-IV

Spineless marine cuttlefish Sepiella inermis has been considered as a popular dietary cephalopod species in Asian and Mediterranean coasts. Bioassay-directed purification of organic extract of S. inermis ensued in the characterization of two chromenyl derivatives. The studied compounds exhibited significantly greater antioxidant potencies (IC₅₀  ≤ 0.5 mg/ml) when compared with α-tocopherol. The substituted 1H-isochromenyloxy-11-hydroxyethyl pentanoate isoform (compound 1) efficiently inhibited the carbolytic enzymes along with key regulator of insulin secretion dipeptidyl peptidase-IV (IC₅₀ 0.16 mg/ml). The molecular docking simulations displayed optimum binding affinity of the compound 1 (-10.01 kcal/mol) with dipeptidyl peptidase-IV and lesser inhibition constant (K_i 46.41 nM), which along with its permissible hydrophobic-hydrophilic balance (log P_ow  ~ 2) appeared to play significant roles in its greater antihyperglycemic activity compared to other studied chromenyl isoform. The greater antioxidant and antidiabetic properties of compound 1 could be utilized as an important natural lead against hyperglycemic-related disorders. PRACTICAL APPLICATIONS: The edible spineless marine cuttlefish Sepiella inermis are ubiquitously available in Asian and Mediterranean coasts. The sequential chromatographic purification of the organic extract of S. inermis led to the identification of two pure chromenyl chemotypes. The metabolites with substituted 1H-isochromenyloxy-11-hydroxyethyl pentanoate isoform (compound 1) displayed potential antioxidative and antihyperglycemic activities compared to the chemotype (2) bearing 3H-isochromen-5-yl moiety. The attenuating potential of chromenyl chemotype 1 against carbohydrate hydrolyzing enzymes and insulin secretion regulator attributed towards its efficiency as an important natural lead against postprandial hyperglycemia and incretin hormone regulation to maintain glucose homeostasis in the biological system. The chromenyl metabolites isolated from S. inermis could be utilized as a functional food ingredient in the nutraceutical formulations against hyperglycemic-related disorders.

Armochaetoglasins A-I: Cytochalasan alkaloids from fermentation broth of Chaetomium globosum TW1-1 by feeding L-tyrosine

By feeding L-tyrosine into the culture medium, nine undescribed compounds, termed as armochaetoglasins A-I, together with three known analogues, namely armochaetoglobin E, chaetoglobosin V, and chaetoglobosin J, were isolated and identified from the medicinal terrestrial arthropod-derived fungus Chaetomium globosum TW1-1. Their structures were elucidated by means of NMR spectroscopy, single-crystal X-ray crystallography, and comparison of their electronic circular dichroism (ECD) spectra. Structurally, armochaetoglasin A represented the first tyrosine-derived cytochalasan alkaloid characterized by a 13-membered carbocyclic ring system; armochaetoglasins B and C possessed a rare 19,20-seco-chaetoglobosin skeleton. Armochaetoglasin B, chaetoglobosin V, and chaetoglobosin J showed weak cytotoxic activity with IC₅₀ values ranging from 19.5 to 34.72 μM.

Multifaceted Study on a Cytochalasin Scaffold: Lessons on Reactivity, Multidentate Catalysis, and Anticancer Properties

An intramolecular Diels-Alder (IMDA) reaction efficiently accelerated by Schreiner's thiourea is reported, to build a functionalized cytochalasin scaffold (periconiasin series) for biological purposes. DFT calculation highlighted a unique multidentate cooperative hydrogen bonding in this catalysis. The deprotection end game afforded a collection of diverse structures and showed the peculiar reactivity of the Diels-Alder cycloadducts upon functionalization. Biological studies revealed strong cytotoxicity of a few compounds on breast cancer cell lines while actin polymerization is preserved.

Antiviral Agents From Fungi: Diversity, Mechanisms and Potential Applications

Viral infections are amongst the most common diseases affecting people worldwide. New viruses emerge all the time and presently we have limited number of vaccines and only few antivirals to combat viral diseases. Fungi represent a vast source of bioactive molecules, which could potentially be used as antivirals in the future. Here, we have summarized the current knowledge of fungi as producers of antiviral compounds and discuss their potential applications. In particular, we have investigated how the antiviral action has been assessed and what is known about the molecular mechanisms and actual targets. Furthermore, we highlight the importance of accurate fungal species identification on antiviral and other natural products studies.

Total Synthesis of Pericoannosin A

The first total synthesis of pericoannosin A (1) containing 15 steps in the longest linear sequence with an overall yield of 5.5% is reported. The hybrid peptide-polyketide was isolated from the endophytic fungus Periconia sp. F-31 and bears a unique tricyclic core structure. The key steps are a glycolate aldol reaction and a Diels-Alder reaction utilizing an Evans auxiliary for controlling the stereochemistry. Furthermore, a late-stage equilibration was employed.

Natural Products Research in China From 2015 to 2016

This review covers the literature published by chemists from China during the 2015-2016 on natural products (NPs), with 1,985 citations referring to 6,944 new compounds isolated from marine or terrestrial microorganisms, plants, and animals. The emphasis is on 730 new compounds with a novel skeleton or/and significant bioactivity, together with their source organism and country of origin.

Structural Diversity and Biological Activities of Novel Secondary Metabolites from Endophytes

Exploration of structurally novel natural products greatly facilitates the discovery of biologically active pharmacophores that are biologically validated starting points for the development of new drugs. Endophytes that colonize the internal tissues of plant species, have been proven to produce a large number of structurally diverse secondary metabolites. These molecules exhibit remarkable biological activities, including antimicrobial, anticancer, anti-inflammatory and antiviral properties, to name but a few. This review surveys the structurally diverse natural products with new carbon skeletons, unusual ring systems, or rare structural moieties that have been isolated from endophytes between 1996 and 2016. It covers their structures and bioactivities. Biosynthesis and/or total syntheses of some important compounds are also highlighted. Some novel secondary metabolites with marked biological activities might deserve more attention from chemists and biologists in further studies.

Enantioselective total synthesis of periconiasin A

A concise, enantioselective total synthesis of periconiasin A, a hybrid natural product, has been achieved.

Antiviral Agents From Fungi: Diversity, Mechanisms and Potential Applications

Viral infections are amongst the most common diseases affecting people worldwide. New viruses emerge all the time and presently we have limited number of vaccines and only few antivirals to combat viral diseases. Fungi represent a vast source of bioactive molecules, which could potentially be used as antivirals in the future. Here, we have summarized the current knowledge of fungi as producers of antiviral compounds and discuss their potential applications. In particular, we have investigated how the antiviral action has been assessed and what is known about the molecular mechanisms and actual targets. Furthermore, we highlight the importance of accurate fungal species identification on antiviral and other natural products studies.

Strategies to diversify natural products for drug discovery

Natural product libraries contain specialized metabolites derived from plants, animals, and microorganisms that play a pivotal role in drug discovery due to their immense structural diversity and wide variety of biological activities. The strategies to greatly extend natural product scaffolds through available biological and chemical approaches offer unique opportunities to access a new series of natural product analogues, enabling the construction of diverse natural product-like libraries. The affordability of these structurally diverse molecules has been a crucial step in accelerating drug discovery. This review provides an overview of various approaches to exploit the diversity of compounds for natural product-based drug development, drawing upon a series of examples to illustrate each strategy.