Heterocyclic pyrrolizinone and indolizinones derived from natural lactam as potential antifungal agents

Luobuma Leaf Spot Disease Caused by Alternaria tenuissima in China

Luobuma (Apocynum venetum and Poacynum hendersonni) is widely cultivated for environmental conservation, medicinal purposes and the textile industry. In 2018, a severe leaf spot disease that attacked the leaves of Luobuma was observed in plants cultivated in Yuzhong County, Gansu Province, China. Symptoms of the disease appeared as white or off-white spots surrounded by brown margins on the leaves of A. venetum. The spots expanded and covered a large area of the leaf, presenting as "cankers" with progression of the disease, leading to leaf death. The initial symptoms of the disease on P. hendersonni were similar to the symptoms of A. venetum, with a larger disease spot than A. venetum, and the spot was black and thicker. The aim of this study was to identify the fungal species and evaluate the effectiveness of fungicides (hymexazol and zhongshengmycin) against the pathogen in vitro. The fungi species that caused the new disease was identified as Alternaria tenuissima based on the morphological characteristics, pathogenicity tests, and phylogenetic analysis of the internal transcribed spacer (ITS) region, glyceraldehyde 3-phosphate dehydrogenase (gpd), translation elongation factor 1-alpha (TEF) and the histone 3 (H3) gene sequences. The findings showed that hymexazol fungicide can be used to control leaf spot disease. This is the first report on Luobuma leaf spot disease caused by A. tenuissima in China.

Discovery of γ-Lactam Alkaloid Derivatives as Potential Fungicidal Agents Targeting Steroid Biosynthesis

Biological control of plant pathogens is considered as one of the green and effective technologies using beneficial microorganisms or microbial secondary metabolites against plant diseases, and so microbial natural products have played important roles in the research and development of new and green agrochemicals. To explore the potential applications for natural γ-lactam alkaloids and their derivatives, 26 γ-lactams that have flexible substituent patterns were synthesized and characterized, and their in vitro antifungal activities against eight kinds of plant pathogens belonging to oomycetes, basidiomycetes, and deuteromycetes were fully evaluated. In addition, the high potential compounds were further tested using an in vivo assay against Phytophthora blight of pepper to verify a practical application for controlling oomycete diseases. The potential modes of action for compound D1 against Phytophthora capsici were also investigated using microscopic technology (optical microscopy, scanning electron microscopy, and transmission electron microscopy) and label-free quantitative proteomics analysis. The results demonstrated that compound D1 may be a potential novel fungicidal agent against oomycete diseases (EC₅₀ = 4.9748 μg·mL^-1 for P. capsici and EC₅₀ = 5.1602 μg·mL^-1 for Pythium aphanidermatum) that can act on steroid biosynthesis, which can provide a certain theoretical basis for the development of natural lactam derivatives as potential antifungal agents.

Natural phenolic derivatives based on piperine scaffold as potential antifungal agents

Piperine is a natural alkaloid with a wide range of biological functions. Natural phenolic compounds existed in many essential oils (EOs) are plant-derived aroma compounds with broad range of biological activities, however, their actions are slow, and they are typically unstable to light or heat, difficult to extract and so on. In order to find high-potential fungicides derived from piperine, a series of piperine-directed essential oil derivatives were designed and synthesized. The structures of all molecules were confirmed by satisfied spectral data, including ¹H NMR, ¹³C NMR and ESIMS. The target compounds were screened for their potential fungicidal activities against six species of plant pathogen fungi, including Rhizoctonia solani, Fusarium graminearum, Phomopsis adianticola, Alternaria tenuis Nees, Phytophthora capsici and Gloeosporium theae-sinensis. Some of target compounds exhibited moderate and broad-spectrum activity against tested fungi compared to the parental piperine. Further studies have shown that some different concentrations of compounds have significant inhibitory activity against Alternaria tenuis Nees and Phytophthora capsici compared to commercial carbendazim, and compound 2b exhibited particularly significant broad-spectrum fungicidal activity.

Discovery of γ-lactam derivatives containing 1,3-benzodioxole unit as potential anti-phytopathogenic fungus agents

A series of γ-lactam analogs containing 1,3-benzodioxole moiety were designed, and these derivatives were synthesized from the lead compound of lactam via a structural diversity-oriented synthesis, their structures were confirmed by ¹HNMR,¹³CNMR, ESI-MS spectrum. Their antifungal activities were evaluated against four serious and typically crop-threatening agricultural fungi, including Rhizoctonia solani, Alternaria tenuis Nees, Gloeosporium theae-sinensis, and Fusarium graminearum. Some of these derivatives exhibited activity against Alternaria tenuis Nees higher than that of commercial fungicides carbendazim, such as compounds 7a, 7b, and 7i, compared with the blank control, some of these derivatives showed good antifungal activities against Gloeosporium theae-sinensis and Fusarium graminearum. The systematic study provides evidences for further structural modification and application of lactam analogues as antifungal agents for agriculture.

Heterocyclic lactam derivatives containing piperonyl moiety as potential antifungal agents

To study the novel functionalized heterocyclic molecules with highly potential biological activity, two series of heterocyclic lactam derivatives containing the piperonyl moiety were designed and synthesized. The newly obtained compounds have been identified on the basis of analytical spectral data, including ¹H NMR, ¹³C NMR, and ESI-MS. The target compounds were evaluated for their potential antifungal activities in vitro against twelve species of the plant pathogen fungi (Sclerotinia sclerotiorum, Rhizoctonia solani, Rap Sclerotinia stemrot, Fusarium graminearum, Phomopsis adianticola, Pestallozzia theae, Pestalotiopsis guepinii, Alternaria tenuis Nees, Monilinia fructicola, Colletotrichum gloeosporioides, Phytophthora capsici, Magnaporthe oryzae). Preliminary bioassays suggested that all prepared compounds I1-14 displayed broad-spectrum and moderate antifungal activities compared with the positive control hymexazol, especially for Sclerotinia sclerotiorum, Rap Sclerotinia stemrot, and Monilinia fructicola. In particular, the inhibition rate of compound I9 exhibited good inhibition activity reached 95.16% against Sclerotinia sclerotiorum, and compounds I5, I12 against Phytophthora capsici were 93.44%, 91.25%. Further studies revealed that compounds I5 (IC₅₀ = 19.13 µM) and I12 (IC₅₀ = 9.12 µM) exhibited obviously antifungal activities against Phytophthora capsici, which were better than that of commercial agricultural fungicide hymexazol (IC₅₀ = 325.45 µM). Therefore, these target compounds could be further studied and explored as a lead skeleton for discovery of novel antifungal agents.