Gabosines P and Q, new carbasugars from Streptomyces sp. and their α-glucosidase inhibitory activity

The limitless endophytes: their role as antifungal agents against top priority pathogens

Multi resistant fungi are on the rise, and our arsenal compounds are limited to few choices in the market such as polyenes, pyrimidine analogs, azoles, allylamines, and echinocandins. Although each of these drugs featured a unique mechanism, antifungal resistant strains did emerge and continued to arise against them worldwide. Moreover, the genetic variation between fungi and their host humans is small, which leads to significant challenges in new antifungal drug discovery. Endophytes are still an underexplored source of bioactive secondary metabolites. Many studies were conducted to isolate and screen endophytic pure compounds with efficacy against resistant yeasts and fungi; especially, Candida albicans, C. auris, Cryptococcus neoformans and Aspergillus fumigatus, which encouraged writing this review to critically analyze the chemical nature, potency, and fungal source of the isolated endophytic compounds as well as their novelty features and SAR when possible. Herein, we report a comprehensive list of around 320 assayed antifungal compounds against Candida albicans, C. auris, Cryptococcus neoformans and Aspergillus fumigatus in the period 1980-2024, the majority of which were isolated from fungi of orders Eurotiales and Hypocreales associated with terrestrial plants, probably due to the ease of laboratory cultivation of these strains. 46% of the reviewed compounds were active against C. albicans, 23% against C. neoformans, 29% against A. fumigatus and only 2% against C. auris. Coculturing was proved to be an effective technique to induce cryptic metabolites absent in other axenic cultures or host extract cultures, with Irperide as the most promising compounds MIC value 1 μg/mL. C. auris was susceptible to only persephacin and rubiginosin C. The latter showed potent inhibition against this recalcitrant strain in a non-fungicide way, which unveils the potential of fungal biofilm inhibition. Further development of culturing techniques and activation of silent metabolic pathways would be favorable to inspire the search for novel bioactive antifungals.

A Synthetic Approach to Several C7-Carbasugars via a Key Intramolecular Morita-Baylis-Hillman Reaction

A new synthetic strategy for C7-carbasugars is developed via an intramolecular Morita-Baylis-Hillman reaction, in which a substituted dial precursor prepared from d-mannose cyclizes smoothly in the presence of DMAP to afford polyhydroxylated cyclohex-1-enecarbaldehyde with good yield. By employment of the cyclization products as key intermediates, the first syntheses of carbasugar ester 1 and epicorepoxydon A, as well as practical syntheses of epoxydines B and C, (-)-MK7607, (-)-streptol, and (-)-gabosine E are achieved.

Purification and characterization of anti-phytopathogenic fungi angucyclinone from soil-derived Streptomyces cellulosae

Actinomycete strain YIM PH20352, isolated from the rhizosphere soil sample of Panax notoginseng collected in WenShang, Yunnan Province, China, exhibited antifungal activity against some phytopathogenic fungi. The structures of bioactive molecules, isolated from the ethyl acetate extract of the fermentation broth of the strain, were identified as rabelomycin (1) and dehydrorabelomycin (2) based on extensive spectroscopic analyses. Compound 1 exhibited antifungal activity against four tested root-rot pathogens of the Panax notoginseng including Plectosphaerella cucumerina, Alternaria panax, Fusarium oxysporum, and Fusarium solani with the MIC values at 32, 64, 128, and 128 μg/mL, respectively. Compound 2 exhibited antifungal activity against F. oxysporum, P. cucumerina, F. solani, and A. panax with the MIC values at 64, 64, 128, and 128 μg/mL, respectively. Based on the phylogenetic analyses, the closest phylogenetic relative of strain YIM PH20352 is Streptomyces cellulosae NBRC 13027 ^T (AB184265) (99.88%), so strain YIM PH20352 was identified as Streptomyces cellulosae. To the best of our knowledge, this is the first report of rabelomycin and rabelomycin-type antibiotics from Streptomyces cellulosae and their antifungal activity against root-rot pathogens of the Panax notoginseng.

Identification of antimicrobial metabolites produced by a potential biocontrol Actinomycete strain A217

AIMS

To extract and identify the metabolites of strain A217 as well as its antifungal spectrum and control effect on various plant pathogens.

METHODS AND RESULTS

Strain A217 was identified as a Streptomyces sp. which was most similar to Streptomyces lienomycini. An antimicrobial spectrum test indicated that strain A217 inhibited several plant pathogenic fungi and strong antibacterial effect such as Phytophthora capsici, Botrytis cinerea, Sclerotinia sclerotiorum, Fusarium oxysporum, Pseudomonas syringae and Xanthomonas campestris. An in vivo tissue test demonstrated that the fermentation broth of strain A217 exerted therapeutic and protective effects of 49·47 and 61·60% respectively, on S. sclerotiorum. Additionally, the fermentation broth of A217 exerted control effects on walnut black spot disease in walnut leaves and branches amounting to 79·33 and 81·52% respectively. In a pot experiment, the fermentation broth exhibited a stronger protective and control effect (68·29%), as well as better bacteriostatic and disease control effects on Phytophthora blight of pepper, compared with Metalaxyl. Compounds possessing antifungal and antibacterial activities were obtained from the fermentation broth of strain A217, using column chromatography and HPLC. Chemical and structural analyses conducted using MS and nuclear magnetic resonance confirmed that these compounds were 1H-pyrrole-2-carboxylic acid and 1H-pyrrole-2-carboxamide. The EC₅₀ values of compound 1H-pyrrole-2-carboxylic acid1 for S. sclerotiorum and P. capsici were 20·13 and 50·36 μg ml^-1 respectively. Compound 1H-pyrrole-2-carboxamide2 showed significant antibacterial activity against different plant pathogenic bacteria. The MIC values of P. syringae, X. campestris and X. campestris pv. jugiandis were 7·5, 30 and 15·0 μg ml^-1 respectively.

CONCLUSIONS

Actinomyces A217 fermentation products have a broad spectrum of bacteriostasis, and have good bacteriostasis activity to many plant pathogenic fungi and bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study revealed a new antimicrobial producing strain of Streptomyces and its potential application as a biological control agent for plant diseases.

Molecular networking-based for the target discovery of potent antiproliferative polycyclic macrolactam ansamycins from Streptomyces cacaoi subsp. asoensis

Molecular networking-based for the target discovery of potent antiproliferative polycyclic macrolactam ansamycins.

A divergent strategy to synthesize gabosines featuring a switchable two-way aldol cyclization

Gabosines and their natural analogues, belonging to C7 carbasugars, have attracted great attention in synthesis due to their rich structural variety and promising biological activities. A new diversity-oriented approach for the gabosine-type carbasugars based on a tunable regioselective aldol cyclization of flexible precursor 2 is explored. Two cyclization modes (A and B) of the precursor can be well controlled by switching promoters to selectively produce two resulting cyclohexa(e)nones 3 and 10, both of which are versatile intermediates for various C7 carbasugars. After the conversion of 3 to eight natural carbasugars, the utility of intermediate 10 is illustrated by the first synthesis of (-)-gabosine L, as well as the new synthesis of (-)-gabosine A, (-)-gabosine B, (-)-gabosine N and (-)-gabosine O. The chemical structure and the absolute configuration of (-)-gabosine L are confirmed by its total synthesis.

Isolation and Characterization of New Phenazine Metabolites with Antifungal Activity against Root-Rot Pathogens of Panax notoginseng from Streptomyces

Three new phenazine metabolites, strepphenazine A-C (1-3), along with a known compound baraphenazine E 4 were isolated from the culture broth of a Streptomyces strain YIM PH20095. The structures were elucidated based on the spectral data. Compounds 1-4 showed different antifungal activity against Fusarium oxysporum, Plectosphaerella cucumerina, Alternaria panax, and Phoma herbarum, which caused root-rot disease of Panax notoginseng with minimal inhibitory concentrations (MICs) of 16-64 μg/mL; compared with compound 4, compounds 1-3 showed better antifungal activity against some of these pathogenic fungi with MICs of 16-32 μg/mL, while compound 4 showed antifungal activity against F. oxysporum, P. cucumerina, and A. panax with the same MICs of 64 μg/mL. Thus, strain YIM PH20095 provides new sources for the development of biological control agents to prevent the infection of pathogenic fungi of P. notoginseng.

Antileishmanial Carbasugars from Geosmithia langdonii

Two new carbasugar-type metabolites, (1 S,2 R,3 R,4 R,5 R)-2,3,4-trihydroxy-5-methylcyclohexyl-2',5'-dihydroxybenzoate (1) and (1 S,2 S,3 S,4 R,5 R)-4-[(2',5'-dihydroxybenzyl)oxy]-5-methylcyclohexane-1,2,3-triol (2), were isolated from the filamentous fungus Geosmithia langdonii isolated from cotton textiles from Assiut, Egypt. The structures of 1 and 2 were elucidated based on comprehensive 1D and 2D NMR and MS data. Compounds 1 and 2 showed antileishmanial activity against Leishmania donovani with IC₅₀ values of 100 and 57 μM, respectively. The (1 S,2 R,3 R,4 R,5 R) absolute configuration of carbasugar 1 was assigned via 2D NMR and experimental and calculated electronic circular dichroism (ECD) data. Similarly, the tentative structure of compound 2 was shown to possess a (1 S,2 S,3 S,4 R,5 R) absolute configuration via comparing its experimental ECD data and the specific rotation with 1 as well as examining the energy-minimized 3D computational models of compounds 1 and 2.

Ansamycins with Antiproliferative and Antineuroinflammatory Activity from Moss-Soil-Derived Streptomyces cacaoi subsp. asoensis H2S5

Three new 21-membered macrocyclic benzenoid ansamycins, trienomycins J-L (1-3), together with seven known analogues, trienomycins A-G (4-10), were isolated from liquid culture of the moss soil-derived actinomycete Streptomyces cacaoi subsp. asoensis H2S5. The structures of the new compounds were elucidated by extensive NMR spectroscopic analysis and HRESIMS data. The absolute configurations of trienomycins were established by Marfey's method. Antiproliferative assays showed that compound 1 had the greatest activity against HepG2 cells, with an IC₅₀ value of 0.1 μM. The induction of apoptosis of HepG2 cells by 1 was investigated by flow cytometry and evaluation of nuclear morphology. In addition, all of the compounds inhibited nitric oxide production with IC₅₀ values of 0.02 to 8.3 μM, and compounds 1, 4, and 7 were the most potent inhibitors. These findings will facilitate the development of new antineuroinflammatory agents.

Chaetolactone, A Skeletally New Cyclopentenone from Chaetomium sp. C521

Chaetolactone, a new polyketide with an unprecedented carbon skeleton, was constructed by Chaetomium sp. C521. Its structure was characterized by a combination of detailed spectroscopic analysis and ECD method, which facilitated the establishment of the absolute configuration for chaetolactone. The new compound was tested for anti-phytopathogenic activity against Botrytis cinerea, Alternaria solani, Gibberella saubinettii, and Magnaporthe oryzae, but exhibited negligible inhibition at a concentration of 20 μM.

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.

Polyoxygenated Cyclohexenoids with Promising α-Glycosidase Inhibitory Activity Produced by Phomopsis sp. YE3250, an Endophytic Fungus Derived from Paeonia delavayi

Seven new polyoxygenated cyclohexenoids, namely, phomopoxides A-G (1-7), were isolated from the fermentation broth extract of an endophytic fungal strain Phomopsis sp. YE3250 from the medicinal plant Paeonia delavayi Franch. The structures of these compounds were established by spectroscopic interpretation. The absolute configurations of compounds 1 and 4 were confirmed by X-ray crystallographic analysis and chemical derivative approach. All isolated compounds showed weak cytotoxic activities toward three human tumor cell lines (Hela, MCF-7, and NCI-H460) and weak antifungal activities against five pathogenic fungi (Candida albicans, Aspergillus niger, Pyricularia oryzae, Fusarium avenaceum, and Hormodendrum compactum). In addition, compounds 1-7 showed a promising α-glycosidase inhibitory activity with IC₅₀ values of 1.47, 1.55, 1.83, 2.76, 2.88, 3.16, and 2.94 mM, respectively, as compared with a positive control of acarbose (IC₅₀ = 1.22 mM).

Total Syntheses of (+)-Gabosine P, (+)-Gabosine Q, (+)-Gabosine E, (-)-Gabosine G, (-)-Gabosine I, (-)-Gabosine K, (+)-Streptol, and (-)-Uvamalol A by a Diversity-Oriented Approach Featuring Tunable Deprotection Manipulation

A new diversity-oriented approach to C7-cyclitols, which possess a broad spectrum of biological activities, is developed. The key polyoxygenated intermediates with different O-protecting groups were accessed by an intramolecular aldol-cyclization of a diketone derived from δ-d-gluconolactone. The versatile intermediates can be easily transformed into structurally different carbasugars based on control of deprotection manipulation. The utility of the robust approach is illustrated by the first syntheses of (+)-gabosines P and Q, as well as the syntheses of several other gabosines and related analogues viz. (+)-gabosine E, (-)-gabosine G, (-)-gabosine I, (-)-gabosine K, (+)-streptol, and (-)-uvamalol A. In addition, the absolute configuration of (-)-uvamalol A is assigned by its total synthesis.