Photodegradation of the isoxazolidine fungicide SYP-Z048 in aqueous solution: kinetics and photoproducts

Mapping pesticide residues in soil for China: Characteristics and risks

The widespread application of pesticides in China has led to the accumulation of residues in soil. However, few regional studies have fully elucidated the characteristics of pesticide residues in soil (PRS) and the associated risks to the ecosystem and human health on a national level. Therefore, this study aims to compile a dataset on PRS in China from 2006 to 2020 and analyze the interactions and impacts between PRS and the environment. The average concentration of PRS in China was 243.96 μg/kg which was lower than the levels reported in Euro-Americans and other nations. This study revealed PRS in China predominantly originates from organochlorine pesticide residues, with DDTs and HCHs being significant contributors. Despite the high intensity of pesticide application in the Southeast China, PRS concentrations were comparable to those in the Northeast, due to environmental factors that favor pesticide degradation in the Southeast. Both legacy and in-use pesticides were transported by surface runoff or air current, resulting in their accumulation in soil of the lower Yangtze River basin or the piedmont soil of Qinling Mountains, respectively. The average soil environment carrying capacity of PRS in China was -69.5 kg. The ecological risk contributed by PRS in China was mainly at a negligible level. Carcinogenic risks of PRS to adults (4.6 ×10-4) and children (6 ×10-4) exceeded the tolerable thresholds (10-5) by a small margin.

Photodegradation of the Novel Herbicide Pyraquinate in Aqueous Solution: Kinetics, Photoproducts, Mechanisms, and Toxicity Assessment

Pyraquinate, a newly developed 4-hydroxyphenylpyruvate dioxygenase class herbicide, has shown excellent control of resistant weeds in paddy fields. However, its environmental degradation products and corresponding ecotoxicological risks after field application remain ambiguous. In this study, we systematically investigate the photolytic behaviors of pyraquinate in aqueous solutions and in response to xenon lamp irradiation. The degradation follows first-order kinetics, and its rate depends on pH and the amount of organic matter. No vulnerability to light radiation is indicated. Ultrahigh-performance liquid chromatography with quadrupole-time-of-flight mass spectrometry and UNIFI software analysis reveals six photoproducts generated by methyl oxidation, demethylation, oxidative dechlorination, and ester hydrolysis. Gaussian calculation suggests that activities due to hydroxyl radicals or aquatic oxygen atoms caused these reactions on the premise of obeying thermodynamic criteria. Practical toxicity test results show that the toxicity of pyraquinate to zebrafish embryos is low but increases when the compound is combined with its photoproducts.

Identification and ecotoxicity prediction of pyrisoxazole transformation products formed in soil and water using an effective HRMS workflow

Pyrisoxazole, an isoxazoline-class fungicide, has been registered and used for approximately 19 years. However, its environmental transformation products (TPs) and corresponding ecotoxicological effects remain ambiguous. In this study, the photolysis, hydrolysis, and soil transformation behavior of pyrisoxazole were systematically investigated by indoor simulation experiments and analyzed by liquid chromatography quadrupole-time-of-flight mass spectrometry (LC-QTOF-MS) and UNIFI software. Transformation products in different environemnts were effectively identfied by a proposed workflow, which organically combined suspect and non-target screening strategies. In total, 17 TPs were screened out. Eight TPs were confirmed using the corresponding reference standards. Structures of another 9 compounds were tentatively proposed based on diagnostic evidence. Among them, 14 products were reported for the first time. The transformation pathways of pyrisoxazole in soil and water were proposed. Pathway analysis demonstrated that the different pH of aqueous solutions had little effect on the pathways, while the influence of different soil types and oxygen conditions was evident. Finally, the toxicity of the proposed TPs to fish and daphnids was predicted using ECOSAR software. These proposed TPs in soil and water, transformation pathways, and predicted ecotoxicity information could provide systematic insight into the fate and environmental risks of pyrisoxazole.

Development of RS-pyrisoxazole for reduction of pesticide inputs: A new insight from systemic evaluation of pyrisoxazole at the stereoisomeric level

Pyrisoxazole is a chiral fungicide that is routinely applied to agricultural plant protection, but the potential environmental risk may be under- or over-estimated because the risk induced by stereoisomers have never been evaluated individually. Thus, we carried out a systemic evaluation of pyrisoxazole at the stereoisomeric level, including absolute configuration, stereoselective bioactivity, acute toxicity, and stereoselective dissipation behavior. There were 99.0-3545.3 fold difference in bioactivity toward six target pathogens (e.g., Alternaria solani) and 1.3-4.0 times difference in toxicity against aquatic organisms (Selenastrum capricornutum and Daphnia magna) between the best and worst stereoisomer. There appeared to be no significant stereoselective dissipation in all three kinds of soil under aerobic and anaerobic conditions. Stereoselective dissipation in buffer solution and river water only observed between diastereomers rather than between enantiomers. In addition, photolysis played a central role in the dissipation of pyrisoxazole in river water. RS-pyrisoxazole was 2.2- to 6.9-times more bioactive and 1.2- to 2.1-times more toxic than Rac-pyrisoxazole, and what is more, RS-pyrisoxazole degraded faster than other stereoisomers in river water. The result implicated that developing pure RS-pyrisoxazole as commercial product could reduce the input of inactive isomer on the basis of guaranteeing the efficacy against the target pathogens.

Design, Synthesis and Bioactivities of Novel Isoxazole-Containing Pyrazole Oxime Derivatives

In this study, in order to find novel biologically active pyrazole oxime derivatives, twenty-eight new pyrazole oxime compounds containing a substituted isoxazole ring were synthesized and evaluated for their acaricidaland insecticidal activities. Bioassays exhibited that some target compounds indicated good acaricidal and insecticidal activities against Tetranychus cinnabarinus, Aphis medicaginis, Mythimna separata, and Nilaparvata lugens. Especially, compounds 9c, 9h, 9u, and 9v showed 100.00%, 90.56%, 90.78%, and 90.62% insecticidal activities against A. medicaginis at the concentration of 20 μg/mL, respectively, compounds 9k and 9u had 70.86% and 100.00% insecticidal activities against M. separata at 20 μg/mL, respectively.

Isoxazolidine: A Privileged Scaffold for Organic and Medicinal Chemistry

The isoxazolidine ring represents one of the privileged structures in medicinal chemistry, and there have been an increasing number of studies on isoxazolidine and isoxazolidine-containing compounds. Optimization of the 1,3-dipolar cycloaddition (1,3-DC), original methods including electrophilic or palladium-mediated cyclization of unsaturated hydroxylamine, has been developed to obtain isoxazolidines. Novel reactions involving the isoxazolidine ring have been highlighted to accomplish total synthesis or to obtain bioactive compounds, one of the most significant examples being probably the thermic ring contraction applied to the total synthesis of (±)-Gelsemoxonine. The unique isoxazolidine scaffold also exhibits an impressive potential as a mimic of nucleosides, carbohydrates, PNA, amino acids, and steroid analogs. This review aims to be a comprehensive and general summary of the different isoxazolidine syntheses, their use as starting building blocks for the preparation of natural compounds, and their main biological activities.

Novel isoxazolidine analogues of homonucleosides and homonucleotides

Isoxazolidine analogues of homonucleos(t)ides were synthesized from nucleobase-derived nitrones 20a-20e (uracil, 5-fluorouracil, 5-bromouracil, thymine, adenine) employing 1,3-dipolar cycloadditions with allyl alcohol as well as with alkenylphosphonates (allyl-, allyloxymethyl- and vinyloxymethyl- and vinylphosphonate). Besides reactions with vinylphosphonate the additions proceeded regioselectively to produce mixtures of major cis and minor trans 3,5-disubstituted isoxazolidines (d.e. 28–82%). From vinylphosphonate up to 10% of 3,4-disubstituted isoxazolidines was additionally produced. Vicinal couplings, shielding effects and 2D NOE correlations were employed in configurational assignments as well as in conformational analysis to find out preferred conformations for several isoxazolidines and to observe anomeric effects (pseudoaxial orientation of phosphonylmethoxy groups) for those obtained from vinyloxymethylphosphonate. None of the tested compounds were endowed in vitro with antiviral activity against a variety of DNA and RNA viruses at subtoxic concentrations (up to 250 μM) nor exhibited antiproliferative activity towards L1210, CEM, and HeLa cells (IC₅₀ = ≥100 μM).

Photodegradation of the novel fungicide fluopyram in aqueous solution: kinetics, transformation products, and toxicity evolvement

The aqueous photodegradation of fluopyram was investigated under UV light (λ ≥ 200 nm) and simulated sunlight irradiation (λ ≥ 290 nm). The effect of solution pH, fulvic acids (FA), nitrate (NO3 (-)), Fe (III) ions, and titanium dioxide (TiO2) on direct photolysis of fluopyram was explored. The results showed that fluopyram photodegradation was faster in neutral solution than that in acidic and alkaline solutions. The presence of FA, NO3 (-), Fe (III), and TiO2 slightly affected the photodegradation of fluopyram under UV irradiation, whereas the photodegradation rates of fluopyram with 5 mg L(-1) Fe (III) and 500 mg L(-1) TiO2 were about 7-fold and 13-fold faster than that without Fe (III) and TiO2 under simulated sunlight irradiation, respectively. Three typical products for direct photolysis of fluopyram have been isolated and characterized by liquid chromatography tandem mass spectrometry. These products resulted from the intramolecular elimination of HCl, hydroxyl-substitution, and hydrogen extraction. Based on the identified transformation products and evolution profile, a plausible degradation pathway for the direct photolysis of fluopyram in aqueous solution was proposed. In addition, acute toxicity assays using the Vibrio fischeri bacteria test indicated that the transformation products were more toxic than the parent compound.

Stereoselective analysis of novel chiral fungicide pyrisoxazole in cucumber, tomato and soil under different application methods with supercritical fluid chromatography/tandem mass spectrometry

Various new chiral pesticides have been registered and used in crop yields. However, few studies have focused on the environmental behavior of such new registered chiral compounds on the stereoisomer level. In this study, an effective and sensitive chiral analytical method was first developed to detect pyrisoxazole stereoisomers and then further applied to investigate the stereoselective dissipation in vegetables and soil using supercritical fluid chromatography/tandem triple quadrupole mass spectrometry. Optimal separation condition was achieved with IA column using CO2/MeOH (75:25) as mobile phase at 2.0 mL/min in 5 min, 35 °C and 2400 psi. The average recoveries in all of the matrices at four spiking levels ranged from 84.0% to 105.6%. Significant stereoselective dissipation was observed in cucumber and tomato under both application modes. (-) Pyrisoxazole A and (-) pyrisoxazole B were preferentially degraded in cucumber under foliar spraying mode. In contrast, (+) pyrisoxazole A and (-) pyrisoxazole B were preferentially degraded in cucumber under soil irrigation mode. (-) Pyrisoxazole A and (-) pyrisoxazole B were degraded faster than their antipodes in tomato under both application modes. However, no significant stereoselectivity was observed in soil. The results of this study could help facilitate more accurate risk assessments of pyrisoxazole.

Heterologous expression of the Monilinia fructicola CYP51 (MfCYP51) gene in Pichia pastoris confirms the mode of action of the novel fungicide, SYP-Z048

The novel agricultural fungicide 3-[5-(4-chlorophenyl)-2,3-dimethyl-3-isoxazolidinyl] pyridine (SYP-Z048) developed by China Shenyang Research Institute of Chemical Industry has been confirmed to be an ergosterol biosynthesis inhibitor (EBI). Previous studies have shown that EBIs target the proteins from a range of genes, including CYP51, ERG2 and/or ERG24, and ERG27, which are involved in the ergosterol biosynthesis pathway. In the current study the ERG2, ERG24, and ERG27 genes were cloned from wild type and resistant mutants of Monilinia fructicola in an attempt to clarify the target site of SYP-Z048. Comparative analysis of the deduced aa sequence of these genes, as well as CYP51, revealed several point mutations that resulted in amino acid variation among the sensitive and resistant isolates. However, sensitivity assays indicated that only one, the substitution of phenylalanine (F) for the tyrosine (Y) at 136 in CYP51, was correlated with reduced sensitivity to SYP-Z048. Heterologous expression of MfCYP51-136Y (MfCYP136Y) and MfCYP51-136F (MfCYP136F) in Pichia pastoris revealed that MfCYP136F significantly reduced sensitivity to SYP-Z048, increasing the average EC50 of the transformants 11-fold relative to those carrying MfCYP136Y. However, neither the additional copy of MfCYP136Y nor multiple copies of MfCYP136F were found to reduce sensitivity relative to the empty vector control or single copy transformants, respectively. Molecular docking experiments using SYP-Z048 with HsCYP145Y and the mutated version HsCYP145F as substitutes for MfCYP136Y and MfCYP136F, respectively, indicated that the reduced affinity of HsCYP145F for SYP-Z048 resulted from the loss of a hydrogen bond between the fungicide and the active site. Taken together these results indicate that MfCYP51 is the major target site of SYP-Z048 in M. fructicola, which has important implications for the resistance management of this fungicide in the field.