Synthesis, Fungicidal Activity and Plant Protective Properties of 1,2,3-Thiadiazole and Isothiazole-Based N-acyl-N-arylalaninates.

The addition of active groups of known fungicides, or systemic acquired resistance inducers, into novel compound molecules to search for potential antifungal compounds is a popular and effective strategy. In this work, a new series of N-acyl-N-arylalanines was developed and synthesized, in which 1,2,3-thiadiazol-5-ylcarbonyl or 3,4-dichloroisothiazol-5-ylcarbonyl (fragments from synthetic plant resistance activators tiadinil and isotianil, respectively) and a fragment of N-arylalanine, the toxophoric group of acylalanine fungicides. Several new synthesized compounds have shown moderate antifungal activity against fungi in vitro, such as B. cinerea, R. solani and S. sclerotiorum. In vivo tests against A. brassicicola showed that compound 1d was 92% effective at a concentration of 200 µg/mL, similar to level of tiadinil, a known inducer of systemic resistance. Thus, 1d could be considered a new candidate fungicide for further detailed study. The present results will advance research and influence the search for more promising fungicides for disease control in agriculture.

Design, synthesis and antifungal activity of (E)-3-acyl-5-(methoxyimino)-1,5-dihydrobenzo[e][1,2]oxazepin-4(3H)-one analogues.

Nitrogen- or oxygen-containing organic compounds which have significant antifungal activity, twenty one novel nitrogen or oxygen-containing (E)-3-acyl-5-(methoxyimino)-1,5-dihydrobenzo[e][1,2]oxazepin-4(3H)-one analogues were designed and synthesized, and their structures were confirmed by 1H NMR, 13C NMR and HRMS. Preliminary bioassay showed that most of them exhibited certain-to-good antifungal activity. Compounds 5k-2, 5n, 5p and 5r exhibited over 80% inhibitory rate against Sclerotinia sclerotiorum at 50 µg/mL, and 5r exhibited good antifungal activity against S. sclerotiorum with EC50 of 7.21 µg/mL. Compounds 5a and 5r also showed over 90% inhibition against Botrytis cinerea. In particular, 5r showed significant higher activity with the lowest EC50 of 7.92 µg/mL than the positive control trifloxystrobin (21.96 µg/mL) and azoxystrobin (9.43 µg/mL). Providing a practical method for the synthesis of new scaffolds 1,2-Benzoxazepinone and systematically investigate their antifungal activity.

Synthesis of novel N-(2-phenyl-3-pyridyl) thiadiazole/isothiazole carboxamide analogs as potent plant elicitors.

BACKGROUND: Plant elicitors are a class of plant protection agents that can stimulate plant immunity against phytopathogen without a potential resistance problem. In searching for novel plant elicitor candidates, a series of novel N-(2-phenyl-3-pyridyl) thiadiazole/isothiazole carboxamide analogs were designed and synthesized. RESULTS: In vitro bioassay showed that all new compounds exhibited weak direct fungicidal activity. However, compounds 3b, 3g, 3n and 3o showed broad spectrum of in vivo activity against four plant fungi tested. In particularly, 3g showed 80% activity against Rhizoctonia solani in a glasshouse at a concentration of 1 µg mL-1 . For induction activity of tobacco against tobacco mosaic virus (TMV), compounds 3c and 3v showed 67% and 68% inhibitory activity, respectively, which were superior to the positive controls ribavirin and ningnanmycin. Compound 3g showed moderate induction activity (41%). Reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis found that, 3g could up-regulate expression of genes that are related to reactive oxygen species (ROS), pathogenesis-related protein (PRP) and salicylic acid (SA) signalling. CONCLUSION: These results indicated that 3g as an elicitor candidate might act on the SA signalling pathway. According to our findings, N-(2-phenyl-3-pyridyl) thiadiazole/isothiazole carboxamide analogs might be promising lead scaffolds as a novel plant elicitor for further investigation.

Discovery of Novel 3,4-Dichloroisothiazole-Containing Coumarins as Fungicidal Leads.

Natural products are one of the resources for discovering novel fungicidal leads. As a natural fungicide, osthole was used as a coumarin-based lead compound for the development of novel fungicides. Here, a series of 3,4-dichloroisothiazole-containing 7-hydroxycoumarins were rationally designed, synthesized, and characterized by introducing a bioactive substructure, 3,4-dichloroisothiazole, into the coumarin skeleton. In vitro bioassay indicated that compound 7g displayed good activity against Rhizoctonia solani, Physalospora piricola, Sclerotinia sclerotiorum, and Botrytis cinerea. Its median effective concentration (EC50) value against each of these fungi fell between 0.88 and 2.50 µg/mL, which was much lower than that of osthole against the corresponding pathogen (between 7.38 and 74.59 µg/mL). In vivo screening validated that 7k exhibited 100%, 60%, and 20% efficacy against R. solani Kühn at 200, 100, and 50 µg/mL, respectively. RNA sequence analysis implied that growth inhibition of R. solani by 7k might result from potential disruptions of fungal membrane formation and intracellular metabolism. Furthermore, a field experiment with cucumber plants indicated that 7b showed 62.73% and 74.03% efficacy against Pseudoperonospora cubensis (Berk. & Curt.) Rostov. at rates of 12.5 g a.i./ha and 25 g a.i./ha, respectively, which showed no significant difference between 7b and osthole at 30 g a.i./ha. Our studies suggested that 7b, 7g, and 7k might be used as fungicidal leads for further optimization.

Synthesis, Fungicidal Activity, and Molecular Docking of 2-Acylamino and 2-Thioacylamino Derivatives of 1H-benzo[d]imidazoles as Anti-Tubulin Agents.

This work deals with the synthesis and evaluation of fungicidal activity of benzimidazole derivatives, which are structural analogues of commercial anti-tubulin fungicides. A number of N-acyl and N-thioacyl derivatives of 2-amino-1H-benzo[d]imidazole were prepared, and their fungicidal activity against 13 strains of phytopathogenic fungi was studied. The most active compounds against the majority of the studied strains were 3a, 4l, and 4o, and the EC50 values of these compounds were in the range 2.5-20 µg/mL. Compound 3a showed the highest activity against the P. infestans strain, the growth of which is not suppressed by carbendazim. The formation of ligand-receptor complexes of various tautomeric forms of the studied benzimidazoles with homologous models of ß-tubulins of B. cinerea, F. oxysporum, and P. infestans was modeled. Induced fit docking has been used for the simulation. The obtained data suggest the possibility of binding of benzimidazole fungicides to ß-tubulin in the ″nocodazole cavity″ in the tautomeric form bearing a double exocyclic C═N bond. The importance of the formation of hydrogen bonds of benzimidazoles with the amino acid residue Val236 along with the Glu198 residue is also revealed in the present study.

Discovery of Novel Thiazole Carboxamides as Antifungal Succinate Dehydrogenase Inhibitors.

To contribute molecular diversity for novel fungicide development, a series of novel thiazole carboxamides were rationally designed, synthesized, and characterized with the succinate dehydrogenase (SDH) as target. Bioassay indicated that compound 6g showed the similar excellent SDH inhibition as that of Thifluzamide with IC50 of 0.56 mg/L and 0.55 mg/L, respectively. Some derivatives displayed improved in vitro fungicidal activities against Rhizoctonia cerealis and Sclerotinia sclerotiorum with EC50 of 1.2-16.4 mg/L and 0.5-1.9 mg/L. Surprisingly, 6g showed promising in vitro fungicidal activities against R. cerealis and S. sclerotiorum with EC50 of 6.2 and 0.6 mg/L, respectively, which was superior to Thifluzamide with the EC50 of 22.1 and 4.4 mg/L, respectively. Additionally, compounds 6c and 6g displayed excellent in vivo fungicidal activities against S. sclerotiorum on Brassica napus L. leaves with protective activity of 75.4% and 67.3% at 2.0 mg/L, respectively, while Thifluzamide without activity at 5.0 mg/L. Transcriptomic analysis of S. sclerotiorum treated with 6g by RNA sequencing indicated the down-regulation of succinate dehydrogenase gene SDHA and SDHB, and the inhibition of the TCA-cycle.

Synthesis and Biological Activity of Novel Succinate Dehydrogenase Inhibitor Derivatives as Potent Fungicide Candidates.

In searching for novel fungicidal leads, the novel bioactive succinate dehydrogenase inhibitor (SDHI) derivatives were designed and synthesized by the inversion of carbonyl and amide groups. Bioassay indicated that compound 5i stood out with a broad spectrum of in vitro activity against five fungi. Its EC50 value (0.73 µg/mL) was comparable to that of boscalid (EC50 of 0.51 µg/mL) and fluxapyroxad (EC50 of 0.19 µg/mL) against Sclerotinia sclerotiorum. For Rhizoctonia cerealis, 5i and 5p with EC50 values of 4.61 and 6.48 µg/mL, respectively, showed significantly higher activity than fluxapyroxad with the EC50 value of 16.99 µg/mL. In vivo fungicidal activity of 5i exhibited an excellent inhibitory rate (100%) against Puccinia sorghi at 50 µg/mL, while the positive control boscalid showed only a 70% inhibitory rate. Moreover, 5i showed promising fungicidal activity with a 60% inhibitory rate against Rhizoctonia solani at 1 µg/mL, which was better than that of boscalid (30%). Compound 5i possessed better in vivo efficacy against P. sorghi and R. solani than boscalid. Molecular docking showed that even the carbonyl oxygen atom of 5i was far from the pyrazole ring. It could also form hydrogen bonds toward the hydroxyl hydrogen and amino hydrogen of TYR58 and TRP173 on SDH, respectively, which consisted of the positive control fluxapyroxad. Fluorescence quenching analysis and SDH enzymatic inhibition studies also validated its mode of action. Our studies showed that 5i was worthy of further investigation as a promising fungicide candidate.

Discovery of Methyl (5 Z)-[2-(2,4,5-Trioxopyrrolidin-3-ylidene)-4-oxo-1,3-thiazolidin-5-ylidene]acetates as Antifungal Agents against Potato Diseases.

Synthesis, isomerism, and fungicidal activity against potato diseases of new (5 Z)-[2-(2,4,5-trioxopyrrolidin-3-ylidene)-4-oxo-1,3-thiazolidin-5-ylidene]acetate derivatives with 1,3-thiazolidine-4-one and pyrrolidine-2,3,5-trione moieties linked by an exocyclic C═C bond were described. Their structures were clearly confirmed by spectroscopic and spectrometric data (Fourier transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance, and mass spectrometry), elemental analysis, and X-ray diffraction crystallography. A bioassay for antifungal activity in vitro against Phytophthora infestans, Fusariun solani, Alternaria solani, Rhizoctonia solani, and Colletotrichum coccodes demonstrated that 2,4,5-trioxopyrrolidin-1,3-thiazolidine derivatives exhibited a relatively broad spectrum of antifungal activity. One of the compounds showed considerable activity against all of the strains; in the case of F. solani, P. infestans, and A. solani, it possesses comparable or better fungicidal efficacy than the positive control Consento. Consequently, this compound is a promising fungicidal candidate for plant protection.