Synthesis and antifungal activity of carabrone derivatives

Design, synthesis and antifungal activity of novel α-methylene-γ-butyrolactone derivatives containing benzothiophene moiety

BACKGROUND

The discovery of agricultural fungicide candidates from natural products is one of the key strategies for developing environment friendly agricultural fungicides with high efficiency, high selectivity and unique modes-of-action. Based on previous work, a series of novel α-methylene-γ-butyrolactone (MBL) derivatives containing benzothiophene moiety were designed and synthesized.

RESULTS

The majority of the proposed compounds displayed moderate to considerable antifungal efficacy against the tested pathogenic fungi and oomycetes, some exhibiting broad spectrum antifungal activity. Notably, compounds 2 (3-F-Ph) and 7 (4-Cl-Ph) showed excellent antifungal activity against Rhizoctonia with half maximal effective concentration (EC50) values of 0.94 and 0.99 mg L-1, respectively, comparable to the commercial fungicide tebuconazole (EC50 = 0.96 mg L-1), and also displayed significant inhibitory effects against V alsa mali with EC50 values of 2.26 and 1.67 mg L-1, respectively - better than famoxadone and carabrone. The in vivo protective and curative effects against R. solani of compound 2 were 57.2% and 53.7% at 100 mg L-1, respectively, which were equivalent to tebuconazole (51.6% and 52.4%). Further investigations found that compound 2 altered the ultrastructure of R. solani cell, significantly increased the relative conductivity of the cells, and reduced the activity of complex III in a dose-dependent manner. Molecular docking results showed that compound 2 matched well with the Qo pocket.

CONCLUSION

The results revealed that MBL derivatives containing benzothiophene moiety are promising antifungal candidates and provide a new backbone structure for further optimization of novel fungicides. © 2024 Society of Chemical Industry.

Carpesabrolide A, a novel meroterpenoid with anti-inflammatory activity from Carpesium abrotanoides

Carpesabrolide A (1), featuring an unprecedented fumaric acid-guaiane sesquiterpenoid hybrid, has been isolated from the folk medicinal plant Carpesium abrotanoides. The structure with absolute configuration has been established by spectroscopic methods and single crystal X-ray diffraction analysis. The plausible biosynthetic pathway for 1 is proposed. Compound 1 shows significant anti-inflammatory activity by inhibiting NO production with an IC50 value of 2.7 μM.

Chemical constituents of plants from the genus Carpesium

Carpesium (Family – Compositae) is a genus with 24 species of mainly perennial herbs. Several species of this genus have served as folk medicine in China and Korea for their antipyretic, anti-inflammatory, analgesic, antifungal, antibacterial, and cytotoxic activity properties. Chemical constituents are mostly sesquiterpenes, diterpenes, glycosides, and several other types of compounds. This article summarizes the literature on the isolated and identified constituents from various Carpesium species and their various biological activities.

Ethnobotanical Uses, Phytochemical Composition, Biosynthesis, and Pharmacological Activities of Carpesium abrotanoides L. (Asteraceae)

Carpesium abrotanoides L. (Asteraceae) is a medicinal plant with immense therapeutic importance and bioactivities. It is commonly encountered in various Asian regions. It has numerous ethnomedicinal uses for curing diverse ailments such as toothache, stomach ulcer, boils, tonsillitis, bronchitis, bacterial infection, bruises, swelling, virus infection, fever, and amygdalitis, as well as an anthelmintic versus round-, tape-, hook-, and pinworms. Different classes of phytoconstituents such as sesquiterpenes, sesquiterpene dimers, monoterpenes, and nitrogenous compounds have been reported from this plant. These phytoconstituents have proved to possess anti-inflammatory, cytotoxic, antimicrobial, and insecticidal capacities. The present review aims to summarize all published data on C. abrotanoides including traditional uses, phytoconstituents, bioactivities, and toxicological aspects, as well as the synthesis and biosynthesis of its metabolites through an extensive survey on various databases and various publishers. These reported data could draw the attention of various natural-metabolite-interested researchers and medicinal chemists towards the development of this plant and/or its metabolites into medicine for the prevention and treatment of certain illnesses. Despite the diverse traditional uses of C. abrotanoides, there is a need for scientific evidence to support these claims. Clinical trials are also required to further assure these data and validate this plant utilization in treating several diseases.

A Review of the Pharmacological Activities and Recent Synthetic Advances of γ-Butyrolactones

γ-Butyrolactone, a five-membered lactone moiety, is one of the privileged structures of diverse natural products and biologically active small molecules. Because of their broad spectrum of biological and pharmacological activities, synthetic methods for γ-butyrolactones have received significant attention from synthetic and medicinal chemists for decades. Recently, new developments and improvements in traditional methods have been reported by considering synthetic efficiency, feasibility, and green chemistry. In this review, the pharmacological activities of natural and synthetic γ-butyrolactones are described, including their structures and bioassay methods. Mainly, we summarize recent advances, occurring during the past decade, in the construction of γ-butyrolactone classified based on the bond formation in γ-butyrolactone between (i) C5-O1 bond, (ii) C4-C5 and C2-O1 bonds, (iii) C3-C4 and C2-O1 bonds, (iv) C3-C4 and C5-O1 bonds, (v) C2-C3 and C2-O1 bonds, (vi) C3-C4 bond, and (vii) C2-O1 bond. In addition, the application to the total synthesis of natural products bearing γ-butyrolactone scaffolds is described.

A functional analysis of mitochondrial respiratory chain cytochrome bc1 complex in Gaeumannomyces tritici by RNA silencing as a possible target of carabrone

Gaeumannomyces tritici, an ascomycete soilborne fungus, causes a devastating root disease in wheat. Carabrone, a botanical bicyclic sesquiterpenic lactone, is a promising fungicidal agent that can effectively control G. tritici. However, the mechanism of action of carabrone against G. tritici remains largely unclear. Here, we used immunogold for subcellular localization of carabrone and the results showed that carabrone is subcellularly localized in the mitochondria of G. tritici. We then explored the functional analysis of genes GtCytc1 , GtCytb, and GtIsp of the mitochondrial respiratory chain cytochrome bc1 complex in G. tritici by RNA silencing as a possible target of carabrone. The results showed that the silenced mutant ∆GtIsp is less sensitive to carabrone compared to ∆GtCytc1 and ∆GtCytb. Compared with the control, the activities of complex III in all the strains, except ∆GtIsp and carabrone-resistant isolate 24-HN-1, were significantly decreased following treatment with carabrone at EC20 and EC80 in vitro (40%-50% and 70%-80%, respectively). The activities of mitochondrial respiratory chain complex III and the mitochondrial respiration oxygen consumption rates in all the strains, except ∆GtIsp and 24-HN-1, were higher with respect to the control when treated with carabrone at EC20 in vivo. The rates of mitochondrial respiration of all strains, except ∆GtIsp, were significantly inhibited following treatment with carabrone at EC80 (ranging from 57% to 81%). This study reveals that the targeting of the iron-sulphur protein encoded by GtIsp is highly sensitive to carabrone and provides a direction for the research of carabrone's target.

Mitochondrial Signs and Subcellular Imaging Provide Insight into the Antifungal Mechanism of Carabrone against Gaeumannomyces graminis var. tritici

Carabrone, a botanical bicyclic sesquiterpenic lactone, has broad-spectrum antifungal activity and is particularly efficient against the devastating phytopathogen Gaeumannomyces graminis var. tritici (Ggt). The antifungal mechanism of carabrone against Ggt, however, remains unclear. The main objective of this study was to investigate the subcellular localization of carabrone in Ggt to gain a better understanding of its mechanism of action. When Ggt was exposed to carabrone (EC₅₀ value of 28.45 μg/mL) for 7 days, a decline in mitochondrial concentration together with some obvious alternations in mitochondrial structure, including hazy outlines, medullary transitions, excess accumulation of unclear settlings, and vacuolar degeneration, were observed, indicating that carbrone may act on the mitochondria directly. A fluorescent conjugate (TTY) was thus designed and synthesized as a surrogate of carabrone that possessed comparable antifungal activity against Ggt (EC₅₀ of 33.68 μg/mL). Additionally, a polyclonal antibody specific to carabrone and with a high titer (256 000) was also prepared by immunizing mice. Subsequently, two imaging techniques, the use of the fluorescent conjugate (FC) and immunofluorescence (IF), were applied to determine the subcellular localization of carabrone. Both FC and IF fluorescent signals demonstrated its mitochondrial localization with a Pearson's coefficient of 0.83 for FC and 0.86 for IF. These results imply that carabrone exerts its antifungal activity against Ggt by interfering with mitochondrial function.

Natural α-methylenelactam analogues: Design, synthesis and evaluation of α-alkenyl-γ and δ-lactams as potential antifungal agents against Colletotrichum orbiculare

In our continued efforts to improve the potential utility of the α-methylene-γ-lactone scaffold, 62 new and 59 known natural α-methylenelactam analogues including α-methylene-γ-lactams, α-arylidene-γ and δ-lactams, and 3-arylideneindolin-2-ones were synthesized as the bioisosteric analogues of the α-methylenelactone scaffold. The results of antifungal and cytotoxic activity indicated that among these derivatives compound (E)-1-(2, 6-dichlorobenzyl)-3-(2-fluorobenzylidene) pyrrolidin-2-one (Py51) possessed good selectivity with the highest antifungal activity against Colletotrichum orbiculare with IC₅₀ = 10.4 μM but less cytotoxic activity with IC₅₀ = 141.2 μM (against HepG2 cell line) and 161.2 μM (against human hepatic L02 cell line). Ultrastructural change studies performed by transmission electron microscope showed that Py51 could cause important cell morphological changes in C. orbiculare, such as plasma membrane detached from cell wall, cell wall thickening, mitochondria disruption, a dramatic increase in vacuolation, and eventually a complete loss in the integrity of organelles. Significantly, mitochondria appeared one of the primary targets, as confirmed by their remarkably aberrant morphological changes. Analysis of structure-activity relationships revealed that incorporation of the aryl group into the α-exo-methylene and the N-benzyl substitution increased the activity. Meanwhile, the α-arylidene-γ-lactams have superiority in selectivity over the 3-arylideneindolin-2-ones. Based on the results, the N-benzyl substituted α-(2-fluorophenyl)-γ-lactam was identified as the most promising natural-based scaffold for further discovering and developing improved crop-protection agents.

Synthesis and Antimicrobial Activity of Calycanthaceous Alkaloid Analogues

A series of 24 novel derivatives of the calycanthaceous alkaloids with a tetrahydropyrroloindol-based core structure was synthesized from tryptophan in good yields. Their structures were characterized by IR, 1H NMR, and 13C NMR spectroscopy and ESI-MS. The synthesized compounds were evaluated against a wide variety of plant pathogenic fungi. Compound a9 exhibited a high degree of activity against Curvularia lunata, with 91.0% activity at a concentration of 100 μg mL−1 and with an EC50 of 44.6 μg mL−1. a7, a8, a13, and a17 exhibited high degrees of activity against Sclerotinia sclerotiorum, with a8 being the most effective with an EC50 of 38.4 μg mL−1. Compound a9 illustrated activity against Botrytis cinerea, with an EC50 of 79.5 μg mL−1. Considering the compounds evaluated, the alkyl substituents of the chain may contribute to the significant variations in fungicidal potency. The structure antifungal activity relationships are also discussed. These results will pave the way for further design, structural modification, and development of calycanthaceous alkaloids as antimicrobial agents.

New α-Methylene-γ-Butyrolactone Derivatives as Potential Fungicidal Agents: Design, Synthesis and Antifungal Activities

In consideration of the fact that the α-methylene-γ-butyrolactone moiety is a major bio-functional group in the structure of carabrone and possesses some agricultural biological activity, forty-six new ester and six new ether derivatives containing α-methylene-γ-butyrolactone moieties were synthesized, and their fungicidal activities against Colletotrichum lagenarium and Botrytis cinerea were investigated. Most of the synthesized compounds showed moderate to significant fungicidal activity. Among them, halogen atom-containing derivatives showed better activity than others, especially compounds 6a,d which exhibited excellent fungicidal activity against C. lagenarium, with IC₅₀ values of 7.68 and 8.17 μM. The structure-activity relationship (SAR) analysis indicated that ester derivatives with electron-withdrawing groups on the benzene ring showed better fungicidal activity than those with electron-donating groups. A quantitative structure-activity relationship (QSAR) model (R² = 0.9824, F = 203.01, S² = 0.0083) was obtained through the heuristic method. The built model revealed a strong correlation of fungicidal activity against C. lagenarium with the molecular structures of these compounds. These results are expected to prove helpful in the design and exploration of low toxicity and high efficiency α-methylene-γ-butyrolactone-based fungicides.

Synthesis, antifungal activities and qualitative structure activity relationship of carabrone hydrazone derivatives as potential antifungal agents

Aimed at developing novel fungicides for relieving the ever-increasing pressure of agricultural production caused by phytopathogenic fungi, 28 new hydrazone derivatives of carabrone, a natural bioactive sesquisterpene, in three types were designed, synthesized and their antifungal activities against Botrytis cinerea and Colletotrichum lagenarium were evaluated. The result revealed that all the derivatives synthesized exhibited considerable antifungal activities in vitro and in vivo, which led to the improved activities for carabrone and its analogues and further confirmed their potential as antifungal agents.

New antifungal scaffold derived from a natural pharmacophore: synthesis of α-methylene-γ-butyrolactone derivatives and their antifungal activity against Colletotrichum lagenarium

Thirty new and thirty-four known analogues were designed and synthesized to improve the potential use of the α-methylene-γ-butyrolactone ring, a natural pharmacophore. All structures were confirmed by (1)H and (13)C NMR, MS, and single-crystal X-ray diffraction analyses. The results of antifungal and cytotoxic activity indicated that the synthesized analogues showed significant inhibitory activity and limited selectivity. Compound 45 exhibited the highest antifungal activity with IC₅₀=22.8 μM but moderate cytotoxic activity with IC₅₀=28.5 μM (against BGC823 cell line) and 7.7 μM (against HeLa cell line). Analysis of structure-activity relationships revealed that the incorporation of an aromatic ring into the β, γ positions of the lactone ring improved antifungal activity, and that the introduction of electron-withdrawing groups into the aromatic rings increased the activity compared with electron-donating groups. The above results identified 4-phenyl-3-phenyl-2-methylenebutyrolactone (33) as a lead scaffold for discovering and developing novel and improved crop-protection agents.

Synthesis and antifungal activities of carabrol ester derivatives

Thirty-eight new ester derivatives of carabrol were designed, synthesized, and characterized by (1)H and (13)C NMR and HR-ESI-MS. Their antifungal activities against the fungal pathogen Colletotrichum lagenarium were evaluated using a spore germination assay. Of these 38 ester derivatives, 16 showed higher antifungal activity than that of carabrol and 7 showed higher antifungal activity than that of carabrone. It was found that the C-4 position of carabrol was a key position involving its antifungal activity, which showed the variation of 50% inhibition concentration (IC(50)) from 2.70 to 52.33 μg/mL. When substituted by the phenyl ring, the ester derivatives with electron-attracting groups showed higher activity than those with electron-donating ones. Two ester derivatives, carabryl 4-cynaobenzoate (II-17, IC(50) 2.70 μg/mL) and carabryl 4-isopropylbenzoate (II-27, IC(50) 2.82 μg/mL), showed only slightly lower antifungal activity than that of the positive control chlorothalonil (IC(50) 0.87 μg/mL) and have been identified as promising leads for development of new environmentally friendly fungicides.