Structural Revisions of a Class of Natural Products: Scaffolds of Aglycon Analogues of Fusicoccins and Cotylenins Isolated from Fungi

Identification of three novel P450 enzymes involved in the oxidative modification of a newly discovered fusicoccane diterpene

Fusicoccane (FC)-type diterpenoids are a class of diterpenoids characterized by a unique 5-8-5 ring system and exhibit diverse biological activities. Recently, we identified a novel FC-type diterpene synthase MgMS, which produces a myrothec-15(17)-en-7-ol (1) hydrocarbon skeleton, however, its tailoring congeners have not been elucidated. Here, we discovered two additional gene clusters Bn and Np, each encoding a highly homologous terpene synthase to MgMS but distinct tailoring enzymes. Heterologous expression of the terpene synthases BnMS and NpMS yielded the same product as MgMS. Subsequent introduction of three P450 enzymes MgP450, BnP450 and NpP450 from individual gene clusters resulted in four new FC-type diterpenoids 2-5. Notably, MgP450 serves as the first enzyme responsible for hydroxylation of the C19 methyl group, whereas NpP450 functions as a multifunctional P450 enzyme involved in the oxidations at C5, C6, and C19 positions of the 5-8-5 tricyclic skeleton. C5 oxidation of the hydrocarbon skeleton 1 led to broadening of the NMR signals and incomplete spectra, which was resolved by high-temperature NMR spectral analysis.

Mechanistic characterisation of a fungal fusicoccane-type diterpene synthase involved in the biosynthesis of talaro-7,13-diene

The cyclisation mechanism of the fungal fusicoccane (FC)-type diterpene synthase (DTS) TadA was investigated by extensive isotopic labelling experiments, and the pH-dependency of the product selectivity of this enzyme was explored. These studies provide new insights into the cyclisation mechanisms of FC-type DTSs.

Modular chemoenzymatic synthesis of ten fusicoccane diterpenoids

Fusicoccane diterpenoids display intriguing biological activities, including the ability to act as modulators of 14-3-3 protein-protein interactions. However, their innate structural complexity and diverse oxygenation patterns present enormous synthetic challenges. Here we develop a modular chemoenzymatic approach that combines de novo skeletal construction and late-stage hybrid C-H oxidations to achieve the synthesis of ten complex fusicoccanes in 8-13 steps each. A convergent fragment coupling strategy allowed rapid access to a key tricyclic intermediate, which was subjected to chemical and enzymatic C-H oxidations to modularly prepare five oxidized family members. We also conceived a complementary biomimetic skeletal remodelling strategy to synthetically access five rearranged fusicoccanes with unusual bridgehead double bonds. This work may facilitate future investigation into the biological activities of the fusicoccanes and also inspire the implementation of similar hybrid strategies to provide family-level synthetic solutions to other natural product scaffolds.

Three new amide derivatives from the fungus Alternaria brassicicola

Three new amide derivatives (alteralkaloids A-C, 1-3) and three known alkaloids (4-6) were afforded after phytochemical investigation of fungus Alternaria brassicicola. The structures of these compounds were confirmed by NMR spectroscopic and HRESIMS data. Furthermore, the absolute configuration of 1 was determined using the single-crystal X-ray diffraction analysis. Compounds 1-3 belong to a class of amide derivatives that have not been found in nature before, sharing the same characteristic signals of the butyl moiety and amide group. These isolated compounds mentioned above were tested for the cytotoxic activity.

Structural diversification of fungal natural products by oxidative enzymes

Ascomycota and basidiomycota fungi are prolific producers of biologically active natural products. Fungal natural products exhibit remarkable structural diversity and complexity, which are generated by the enzymes involved in their biosynthesis. After the formation of core skeletons, oxidative enzymes play a critical role in converting them into mature natural products. Besides simple oxidations, more complex transformations, such as multiple oxidations by single enzymes, oxidative cyclization, and skeletal rearrangement, are often observed. Those oxidative enzymes are of significant interest for the identification of new enzyme chemistry and have the potential to be biocatalysts for the synthesis of complex molecules. This review presents selected examples of unique oxidative transformations that have been found in the biosynthesis of fungal natural products. The development of strategies for refactoring the fungal biosynthetic pathways with an efficient genome-editing method is also introduced.

Structurally diverse polycyclic polyprenylated acylphloroglucinols with protective effect on human vein endothelial cells injured by high-glucose from Hypericum acmosepalum N. Robson

Hyperacmotone A, a polycyclic polyprenylated acylphloroglucinol (PPAP) with an unprecedented skeleton, along with five undescribed congeners and eleven reported ones, was isolated from Hypericum acmosepalum. Hyperacmotone A possesses a unique monocyclic ring skeleton based on a cyclopent-4-ene-1,3-dione acylphloroglucinol core. Their structures were elucidated by extensive analysis of HRESIMS, NMR, biogenetic pathway, and quantum-chemical calculations. In addition, hypercohone G exhibited significant protective effects on high-glucose-injured HUVECs.

Unusual Enantiodivergence in Chiral Brønsted Acid‐Catalyzed Asymmetric Allylation with β‐Alkenyl Allylic Boronates

We report herein a rare example of enantiodivergent aldehyde addition with β-alkenyl allylic boronates via chiral Brønsted acid catalysis. 2,6-Di-9-anthracenyl-substituted chiral phosphoric acid-catalyzed asymmetric allylation using β-vinyl substituted allylic boronate gave alcohols with R absolute configuration. The sense of asymmetric induction of the catalyst in these reactions is opposite to those in prior reports. Moreover, in the presence of the same acid catalyst, the reactions with β-2-propenyl substituted allylic boronate generated homoallylic alcohol products with S absolute configuration. Unusual substrate-catalyst C-H⋅⋅⋅π interactions in the favoured reaction transition state were identified as the origins of observed enantiodivergence through DFT computational studies.

Anti-neuroinflammatory Activity of New Naturally Occurring Benzylated Hydroxyacetophenone Analogs from the Endophytic Fungus Alternaria sp. J030

Chemical studies on the culture broth of the endophytic fungus Alternaria sp. J030 led to the identification of three benzylated hydroxyacetophenone derivatives, bauvaroalterins A-C (1-3), and 34 structurally diverse metabolites (4-37). The new structures were elucidated by extensive spectroscopic analyses including UV, IR, 1D and 2D NMR, HRESIMS, and further confirmed using single crystal X-ray diffraction. The in vitro anti-neuroinflammatory effects of the co-isolated metabolites were evaluated in lipopolysaccharide (LPS)-stimulated microglial cells. Compounds 1-3 were shown to significantly reduce LPS-induced NO production by inhibiting the expression of iNOS, as well as inhibiting LPS-induced production of the inflammatory factors TNF-α, IL-1β and IL-6. Further studies revealed that 1-3 were capable of down-regulating the expression of NF-κB subunits p50 and p65, thereby suppressing the activation of NF-κB by inhibiting the LPS-induced phosphorylation of IκB-α. Together these findings demonstrate that bauvaroalterins A-C (1-3) exert anti-neuroinflammatory effects via inhibition of the NF-κB/iNOS signalling pathway in LPS induced BV-2 cells.

A Two-Phase Approach to Fusicoccane Synthesis To Uncover a Compound That Reduces Tumourigenesis in Pancreatic Cancer Cells

Alterbrassicicene D (1) and 3(11)-epoxyhypoestenone (2) were synthesised via a two-phase approach featuring concise construction of the 5-8-5 tricyclic intermediate and a tandem base-mediated epoxide opening-transannular oxa-Michael addition cascade to forge the complex skeleton of 2. The route is scalable and we generated 15 g of the tricyclic intermediate in 8 steps from (R)-limonene and 720 mg of the penultimate bioactive intermediate in a protecting-group-free manner. Our synthesis enabled the structural determination of 2 and provided materials for preliminary anticancer evaluation. The penultimate intermediate showed therapeutic potential in terms of its ability to dramatically reduce the tumourigenic potential of PANC-1 pancreatic cancer cells according to a limiting dilution tumour-initiating assay. Our synthetic approach will facilitate unified access to naturally occurring fusicoccanes and their derivatives for anticancer evaluation.

Diterpenes Specially Produced by Fungi: Structures, Biological Activities, and Biosynthesis (2010–2020)

Fungi have traditionally been a very rewarding source of biologically active natural products, while diterpenoids from fungi, such as the cyathane-type diterpenoids from Cyathus and Hericium sp., the fusicoccane-type diterpenoids from Fusicoccum and Alternaria sp., the guanacastane-type diterpenoids from Coprinus and Cercospora sp., and the harziene-type diterpenoids from Trichoderma sp., often represent unique carbon skeletons as well as diverse biological functions. The abundances of novel skeletons, biological activities, and biosynthetic pathways present new opportunities for drug discovery, genome mining, and enzymology. In addition, diterpenoids peculiar to fungi also reveal the possibility of differing biological evolution, although they have similar biosynthetic pathways. In this review, we provide an overview about the structures, biological activities, evolution, organic synthesis, and biosynthesis of diterpenoids that have been specially produced by fungi from 2010 to 2020. We hope this review provides timely illumination and beneficial guidance for future research works of scholars who are interested in this area.

Hypoestins A−D: highly modified fusicoccane diterpenoids with promising Cav3.1 calcium channel inhibitory activity from Hypoestes purpurea

Hypoestins A–D (1–4), four highly modified fusicoccane diterpenoids with two unreported carbon skeletons, and hypoestins E (5) and F (6), two prviously undescribed fusicoccane diterpenoids, were isolated from aerial parts...

Large-scale culture as a complementary and practical method for discovering natural products with novel skeletons

Covering: up to July 2020Fungal metabolites with diverse and novel scaffolds can be assembled from well-known biosynthetic precursors through various mechanisms. Recent examples of novel alkaloids (e.g., cytochalasans and diketopiperazine derivatives), terpenes (e.g., sesterterpenes and diterpenes) and polyketides produced by fungi are presented through case studies. We show that large-scale culture is a complementary and practical method for genome mining and OSMAC approaches to discover natural products of unprecedented skeletal classes from fungi. We also summarize the discovery strategies and challenges for characterizing these compounds.

Cytochrome P450 enzymes in fungal natural product biosynthesis

Covering: 2015 to the end of 2020 Fungal-derived polyketides, non-ribosomal peptides, terpenoids and their hybrids contribute significantly to the chemical space of total natural products. Cytochrome P450 enzymes play essential roles in fungal natural product biosynthesis with their broad substrate scope, great catalytic versatility and high frequency of involvement. Due to the membrane-bound nature, the functional and mechanistic understandings for fungal P450s have been limited for quite a long time. However, recent technical advances, such as the efficient and precise genome editing techniques and the development of several filamentous fungal strains as heterologous P450 expression hosts, have led to remarkable achievements in fungal P450 studies. Here, we provide a comprehensive review to cover the most recent progresses from 2015 to 2020 on catalytic functions and mechanisms, research methodologies and remaining challenges in the fast-growing field of fungal natural product biosynthetic P450s.

EBC-232 and 323: A Structural Conundrum Necessitating Unification of Five In Silico Prediction and Elucidation Methods

Structurally unique halimanes EBC-232 and EBC-323, isolated from the Australian rainforest plant Croton insularis, proved considerably difficult to elucidate. The two diastereomers, which consist an unusual oxo-6,7-spiro ring system fused to a dihydrofuran, were solved by unification and consultation of five in silico NMR elucidation and prediction methods [i.e., ACDLabs, olefin strain energy (OSE), DP4, DU8+ and TD DFT CD]. Structure elucidation challenges of this nature are prime test case examples for empowering future AI learning in structure elucidation.

Modified Fusicoccane-Type Diterpenoids from Alternaria brassicicola

Seven new modified fusicoccane-type diterpenoids (1-7), together with two known congeners (8 and 9), were obtained from Alternaria brassicicola. Their structures were elucidated from a combination of NMR and HRESIMS data and ¹³C NMR calculation as well as DP4+ probability analyses, and the absolute configurations of 1-5 were determined by ECD calculation and single-crystal X-ray diffraction (Cu Kα). Compounds 1-3 belong to a rare class of 16-nor-dicyclopenta[a,d]cyclooctane diterpenoids, and compounds 2 and 4 represent the first examples of fusicoccane-type diterpenoids featuring two previously undescribed tetracyclic 5/6/6/5 ring systems, while compound 5 features a previously undescribed tetracyclic 5/8/5/3 ring system. Compound 7 was moderately anti-inflammatory, and compounds 2, 3, 5, and 7 were weakly cytotoxic.

Fusicoccane-derived diterpenoids with bridgehead double-bond-containing tricyclo[9.2.1.03,7]tetradecane ring systems from Alternaria brassicicola

Fusicoccane-derived diterpenoids bearing a unique bridgehead double-bond-containing tricyclo[9.2.1.0^3,7]tetradecane (5-9-5 ring system) core skeleton represent a rarely reported class of rearranged terpenoids, which traced back to fusicoccanes with a classical dicyclopenta[a,d]cyclooctane (5-8-5 ring system) core skeleton via a crucial Wagner-Meerwein rearrangement reaction. In this research, alterbrassicenes B-D (1-3), three new rearranged fusicoccane diterpenoids bearing a rare bridgehead double-bond-containing tricyclo[9.2.1.0^3,7]tetradecane core skeleton, together with two known congeners, brassicicenes O and K (4 and 5), were isolated from the modified cultures of fungus Alternaria brassicicola. Their structures were elucidated by comprehensive analyses of the NMR and HRESIMS data, and the absolute configurations of 1 and 4 were further confirmed via a combination of ¹³C NMR and ECD calculations and single-crystal X-ray diffraction analysis (Cu Kα). Interestingly, alterbrassicene B (1) represented the second case of bridgehead C-10-C-11 double-bond-containing natural products with a bicyclo[6.2.1]undecane core skeleton, and also featured an undescribed oxygen bridge between C-6 and C-14 to construct an unprecedentedly caged tetracyclic system. Alterbrassicenes B-D showed moderate cytotoxic activity against certain human tumor cell lines with IC₅₀ values in the range of 15.87-36.85 μM.

Talaronoids A–D: four fusicoccane diterpenoids with an unprecedented tricyclic 5/8/6 ring system from the fungus Talaromyces stipitatus

Talaronoids A–D (1–4), four fusicoccane diterpenoids with an unexpected tricyclic 5/8/6 carbon skeleton from Talaromyces stipitatus, represent the first examples of natural products with a benzo[a]cyclopenta[d]cyclooctane skeleton.

Five New Meroterpenoids from the Fruiting Bodies of the Basidiomycete Clitocybe clavipes with Cytotoxic Activity

Five new meroterpenoids, clavipols A-B (1-2) with a 12-membered ether ring and clavilactones G-I (3-5) having a 10-membered carbocycle connected to a hydroquinone and an α,β-epoxy/unsaturated lactone, were obtained from the fruiting bodies of the basidiomycete Clitocybe clavipes. Their structures were determined by comprehensive analysis of their spectroscopic data, and the absolute configuration of 1 was established by quantum chemical calculations of electronic circular dichroism (ECD). All the isolated compounds (1-5) were tested for their cytotoxic activity against three human tumor cell lines (Hela, SGC-7901, and SHG-44) in vitro after treatment for 48 h. Compound 4 exhibited moderate cytotoxic activity against Hela and SGC-7901 tumor cell lines, with IC₅₀ values of 23.5 and 14.5 µM, respectively.

Bialternacins A-F, Aromatic Polyketide Dimers from an Endophytic Alternaria sp

Six novel aromatic polyketide dimers, bialternacins A-F (1-6), were isolated from a plant endophytic Alternaria sp. The structures of compounds 1-6 were elucidated on the basis of extensive spectroscopic analysis, single-crystal X-ray diffraction, and electronic circular dichroism analysis. Compounds 1, 2, 5, and 6 were characterized as four pairs of racemic mixtures. Compound (+)-5 was demonstrated to show acetylcholinesterase inhibitory activity with an IC₅₀ value of 15.5 μM. A putative biosynthetic pathway for these compounds was proposed.

Recent trends in the structural revision of natural products

Covering: 2012 to 2017 This article reviews recent reports on the structural revision of natural products. Through a critical assessment of the original and revised published structures, the article addresses why each structure was targeted for revision, discusses the techniques and key discrepancies that led to the proposal of the revised structure, and offers measures that may have been taken during the original structure determination to prevent error. With the revised structures in hand, weaknesses of original proposals are assessed, providing a better understanding on the logic behind structure determination.

Dongtingnoids A-G: Fusicoccane Diterpenoids from a Penicillium Species

Five new diterpenoid glycosides, dongtingnoids A-E (1-5), two new diterpenoid aglycones, dongtingnoids F and G (6 and 7), and two known analogues, cotylenins E and J (8 and 9), belonging to the fusicoccane family, were isolated from the fungus Penicillium sp. DT10, which was derived from wetland soil from Dongting Lake. Their structures and absolute configurations were elucidated based on spectroscopic analyses, acid hydrolysis, ECD calculations, and X-ray crystallography. Dongtingnoid C (3) is the first 16-nor-fusicoccane diterpenoid glycoside reported and is proposed to form by oxidative demethylation. Compounds 1, 4, and 5 showed comparable seed-germination-promoting activities to that previously reported for the growth regulator cotylenin E (8).

Neogenkwanine I from the flower buds of Daphne genkwa with its stereostructure confirmation using quantum calculation profiles and antitumor evaluation

Neogenkwanine I (1), a new daphnane-type diterpene with 4,7-ether group, along with four known ones (2-5), were isolated from Daphne genkwa. The structure including absolute configurations of 1 was established on the basis of NMR, ¹³C-NMR and ECD calculations and CD exciton chirality analysis. ¹³C-NMR and ECD calculations of daphnane-type diterpenes were reported here for the first time. All of the diterpenes were screened for their cytotoxic activities against MCF-7 and Hep3B cell lines. The cytotoxicity structure- activity relationship of compounds was illustrated with the absence of ortho-ester group of daphnane-type diterpenes.

Polyketide and Prenylxanthone Derivatives from the Endophytic Fungus Aspergillus sp. TJ23

Eight secondary metabolites, including a new polyketide, named asperetide (1) and a new prenylxanthone derivative, called asperanthone (4), and six known compounds, (S)-3-butyl-7-methoxyphthalide (2), ruguloxanthone C (3), tajixanthone hydrate (5), tajixanthone methanoate (6), salimyxin B (7), and ergosterol (8), were isolated and identified from the medicinal plant-derived fungus, Aspergillus sp. TJ23. The new structures and their absolute configurations were elucidated via multiple methods, including 1D- and 2D-NMR, HR-ESI-MS, UV, IR, and the electronic circular dichroism (ECD) calculations. All of the isolates were characterized from the strain for the first time. The in vitro bioassay showed that compounds 3-5 and 8 exerted inhibitory activities against five cancer cell lines (B16, MDA-MB-231, 4T1, HepG2, and LLC) with IC₅₀ values ranging from 5.13 to 36.8 μm.

Total Biosynthesis of Brassicicenes: Identification of a Key Enzyme for Skeletal Diversification

The biosynthetic pathway of brassicicenes, derived from the phytopathogen Pseudocercospora fijiensis, was fully reconstituted. Heterologous expression of the eight genes highly expressed in infected leaf tissues generated a new brassicicene derivative as a final product. Together with the characterization of P450 from Alternaria brassicicola, a late stage of the biosynthetic pathway, which generates remarkable structural diversity, has been proposed. Notably, a unique P450 that converts 3 to the structurally distinct 4 and 6 was identified.

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.

Anti-inflammatory fusicoccane-type diterpenoids from the phytopathogenic fungus Alternaria brassicicola

Eight undescribed fusicoccane-type diterpenoids (1–8), wherein 1 had a rare 16-nor-dicyclopenta[a,d]cyclooctane skeleton, were isolated from Alternaria brassicicola.

Artemisians A-D, Diseco-guaianolide Involved Heterodimeric [4 + 2] Adducts from Artemisia argyi

Artemisians A-D (1-4), the first examples of [4 + 2] Diels-Alder type adducts presumably biosynthesized from a rare 1, 10-4, 5-diseco-guaianolide and a guaianolide diene, along with their possible precursor 5, were isolated from the traditional Chinese medicine Artemisia argyi. The structures of 1-4 were elucidated by extensive spectroscopic analyses and calculated electronic circular dichroism. Compound 2, with an IC₅₀ value of 3.21 μM, exhibited significant antiproliferative activity via apoptosis induction and G2/M arrest in MDA-MB-468 cells.

Natural terpenoid glycosides with in vitro/vivo antithrombotic profiles from the leaves of Crataegus pinnatifida

Two norditerpenoids (1–2) with unique carbon skeletons, four sesquiterpenoids (3–6) and nine nor-sesquiterpenoids (7–15) were isolated from the leaves of Crataegus pinnatifida and evaluated as possessing antithrombotic activities in vitro/vivo.

Furanmonogones A and B: two rearranged acylphloroglucinols with a 4,5-seco-3(2H)-furanone core from the flowers of Hypericum monogynum

Rearranged acylphloroglucinols with an unprecedented 4,5-seco-3(2H)-furanone skeleton, furanmonogones A (1) and B (2), were isolated from the flowers of Hypericum monogynum.