Mesoionic pyrido[1,2-a]pyrimidinones: Discovery of triflumezopyrim as a potent hopper insecticide1

The discovery of indazapyroxamet: a novel 3-pyridinyl insecticide targeting piercing/sucking insectsa

In recent years, the registrations for a number of commercial insecticides utilized for piercing/sucking insects have been cancelled or restricted. To meet this growing need for new hemipteran controlling agrochemicals, we discovered a 2-(pyridin-3-yl)-thiazole compound, with limited insecticidal activity against cotton/melon aphid (Aphis gossypii). The 2-(pyridin-3-yl)-thiazole moiety offered us a basis to pursue the bicyclic 2-(pyridin-3-yl)-2H-indazole carboxamides. Evaluation of such 2-(pyridin-3-yl)-2H-indazole carboxamides revealed that even analogs containing only simple alkyl amides attached at the 4 or 5 positions possess promising insecticidal activity. Extensive optimization of this novel class of 2-(pyridin-3-yl)-2H-indazole carboxamides led to identifying indazapyroxamet for commercial development. © 2024 Society of Chemical Industry.

Discovery of Pyrido[1,2-α] Pyrimidinone Mesoionic Compounds as Potential Control Agents Against Potato Virus Y

Potato virus Y (PVY) relies on aphids and tubers to spread in the field and causes serious economic losses in the potato industry. Here, we found that pyrido[1,2-α] pyrimidinone mesoionic compounds with insecticidal activity against aphids possessed a good inhibitory effect on PVY. Among them, compound 35 had the best inhibitory activity against PVY (EC50 = 104 μg/mL), even superior to that of ningnanmycin (125 μg/mL). The fluorescence and qPCR results confirmed that compound 35 could inhibit the proliferation of PVY in Nicotiana benthamiana. Preliminary experiments on the mechanism of action indicated that compound 35 had good binding affinity with the coat protein (CP), which plays an essential role in aphid-PVY interactions. Molecular docking revealed that compound 35 could bind to the pocket of CP formed by Ser52, Glu204, and Arg208. Compound 35 had substantially lower binding affinity (Kd) values with CPS52A (219 μM), CPE204A (231 μM), and CPR208A (189 μM) than those with CPWT (5.80 μM). A luciferase assay confirmed that mutating Ser52, Glu204, and Arg208 significantly affected the expression level of CP and further reduced virus proliferation. Therefore, the broad-spectrum activity of compound 35 provides a unique strategy for the prevention and treatment of PVY.

Design, Synthesis, and Insecticidal Activity of Pyridino[1,2-a]pyrimidines Containing Indole Moeites at the 1-Position

Research on mesoionic structures in pesticide design has gained significant attention in recent years. However, the 1-position of pyridino[1,2-a]pyrimidine is usually designed with 2-chlorothiazole, 2-chloropyridine, or cyano moieties commonly found in neonicotinoid insecticides. In order to enrich the available pharmacophore library, here, we disclose a series of new pyridino[1,2-a]pyrimidine mesoionics bearing indole-containing substituents at the 1-position. Most of these target compounds are confirmed to have good insecticidal activity against aphids through bioevaluation. In addition, a three-dimensional structure-activity relationship model is established to allow access to optimal compound F45 with an LC50 value of 2.97 mg/L. This value is comparable to the property achieved by the positive control triflumezopyrim (LC50 = 2.94 mg/L). Proteomics and molecular docking analysis suggest that compound F45 has the potential to modulate the functioning of the aphid nervous system through its interaction with neuronal nicotinic acetylcholine receptors. This study expands the existing pharmacophore library for the future development of new mesoionic insecticides based on 1-position modifications of the pyridino[1,2-a]pyrimidine scaffold.

Synthesis, insecticidal activity, and mode of action of novel imidazopyridine mesoionic derivatives containing an amido group

BACKGROUND

In our previous work, we applied a new synthetic strategy to design and synthesize a series of imidazopyridine mesoionic derivatives with an ester group. The newly synthesized compounds had excellent insecticidal activity against aphids; however, insecticidal activity against planthoppers was less than satisfactory. In the present study, we designed and synthesized a series of novel imidazopyridine mesoionic compounds, containing an amido group, and these compounds were found to have improved insecticidal activity against planthoppers.

RESULTS

The bioassay results demonstrated that most of the target compounds had moderate-to-good insecticidal activity against Sogatella furcifera, and some exhibited good-to-excellent insecticidal activity against Aphis craccivora. Among them, compound C6 had the highest insecticidal activity against S. furcifera and A. craccivora, with LC₅₀ values of 10.5 and 2.09 μg mL^-1 , respectively. Proteomic results suggested that the differentially expressed proteins mainly were enriched in the nervous system-related pathways after compound C6 treatment. Enzymatic assay results showed that compound C6 and triflumezopyrim had a certain inhibitory effect on acetylcholinesterase. Molecular docking and real-time quantitative PCR results indicated that compound C6 not only may act on the nicotinic acetylcholine receptor, but also may interact with the α4 and β1 subunits of this receptor.

CONCLUSION

The results reported here contribute to the development of new mesoionic insecticides and further our understanding of the mode-of-action of imidazopyridine mesoionic derivatives. © 2022 Society of Chemical Industry.

Applying a Bioisosteric Replacement Strategy in the Discovery and Optimization of Mesoionic Pyrido[1,2-a]pyrimidinone Insecticides: A Review

Mesoionic pyrido[1,2-a]pyrimidinones are a unique class of heterocyclic compounds. Compounds from this class with a n-propyl group substituted at the 1 position of the mesoionic core were discovered with interesting insecticidal activity in our screen. In this overview, we showcase how a bioisosteric replacement strategy was applied during the discovery and optimization of this class of compounds. Through exploring various substituents at the 1 position, evaluating a variety of mesoionic bicyclic ring scaffolds, and examining substituents on the phenyl group at the 3 position of the mesoionic core as well as substituents on the mesoionic ring skeleton, many compounds were discovered with excellent hopper activity or potent activity against a wide range of Lepidoptera. Ultimately, dicloromezotiaz was identified for commercial development to control a broad spectrum of lepidopteran pests.

Risk Assessment of Triflumezopyrim and Imidacloprid in Rice through an Evaluation of Residual Data

Triflumezopyrim, a novel mesoionic insecticide used to control planthoppers, is a potential substitute for imidacloprid. In this study, triflumezopyrim and imidacloprid residues in rice were determined using a quick, easy, cheap, effective, rugged, and safe procedure combined with ultra-high-performance liquid chromatography–tandem mass spectrometry. The limit of quantification of both triflumezopyrim and imidacloprid was 0.01 mg kg⁻¹, and the average recovery values were 94–104% and 91–106%, with relative standard deviations (RSDs) of 1.1–1.4% and 2.1–3.4% (n = 5), respectively. The consumer protection level was assessed by calculating the theoretical maximum daily intake using the reported maximum residue limits of triflumezopyrim and imidacloprid. The established method was successfully applied to 200 commercial rice samples collected from four provinces in China, and their potential public health risks were assessed using triflumezopyrim and imidacloprid residues. The risk associated with triflumezopyrim and imidacloprid dietary intake was assessed by calculating the national estimated short-term intake and the acute reference dose percentage (%ARfD). The results show that the theoretical maximum daily intake (NEDI) values of triflumezopyrim and imidacloprid in different age and gender groups were 0.219–0.543 and 0.377–0.935 μg kg⁻¹ d⁻¹ bw, and the risk quotient (RQ) values were 0.188–0.467% and 0.365–0.906%, respectively. The acute reference dose (%ARfD) of triflumezopyrim and imidaclopridin ranged from 0.615 to 0.998% and from 0.481 to 0.780%, respectively.

Pyrido [1,2-a] Pyrimidinone Mesoionic Compounds Containing Vanillin Moiety: Design, Synthesis, Antibacterial Activity, and Mechanism

Xanthomonas oryzae pv. oryzicola (Xoo) is a plant pathogen responsible for rice bacterial blight disease that remains challenging for prevention and cure. To discover innovative and extremely potent antibacterial agents, vanillin moiety was introduced to develop a series of novel mesoionic derivatives. Compound 15 demonstrated excellent in vitro antibacterial activity against Xoo, with a 50% effective concentration value (EC₅₀) of 27.5 μg/mL, which was superior to that of the positive control agent thiodiazole copper (97.1 μg/mL) and comparable to that of compound "A11" (17.4 μg/mL). The greenhouse pot experiment also revealed that compound 15 had 38.5% curative and 36.8% protective efficacy against rice bacterial leaf blight in vivo at 100 μg/mL, which was higher than those of thiodiazole copper (31.2 and 32.6%, respectively) and compound "A11" (29.6 and 33.2%, respectively). Compound 15 enhanced the activities of related defense enzymes, increased chlorophyll content, and promoted the resistance of rice to bacterial infection by modulating the photosynthetic pathway. This study provides a basis for the subsequent structural modification and mechanism research of mesoionic derivatives.

Discovery of Pyrido[1,2-a]pyrimidine Mesoionic Compounds Containing Benzo[b]thiophene Moiety as Potential Pesticide Candidates

The increasing evolution of insect resistance has made it challenging for traditional insecticides to control the bean aphid (Aphis craccivora Koch). To address this pending issue, a range of pyrido[1,2-a]pyrimidine mesoionic compounds containing benzo[b]thiophene were designed and synthesized. The biological activity test results of the target compounds indicated that they had moderate to outstanding insecticidal activity against the bean aphid (Aphis craccivora) and moderate insecticidal activity against the white-backed planthopper (Sogatella furcifera). Compound L14 exhibited significant insecticidal activity against A. craccivora, with an LC₅₀ value of 1.82 μg/mL, which was superior to triflumezopyrim (LC₅₀ = 4.76 μg/mL). The results of enzyme activity assay showed that compound L14 had a definite inhibitory effect on ATPase. Moreover, the proteomics and docking findings of compound L14 suggested that it may act on the central nervous system of aphids and interact with nicotinic acetylcholine receptors. Therefore, compound L14 is a potentially novel insecticide candidate for further utilization.

First Discovery of Imidazo[1,2-a]pyridine Mesoionic Compounds Incorporating a Sulfonamide Moiety as Antiviral Agents

The applications of mesoionic compounds and their analogues as agents against plant viruses remain unexplored. This was the first evaluation of the antiviral activities of mesoionic compounds on this issue. Our study involved the design and synthesis of a series of novel imidazo[1,2-a]pyridine mesoionic compounds containing a sulfonamide moiety and the assessment of their antiviral activities against potato virus Y (PVY). Compound A33 was assessed on the basis of three-dimensional quantitative structure-activity relationship (3D-QSAR) model analysis and displayed good curative, protective, and inactivating activity effects against PVY at 500 mg/L, up to 51.0, 62.0, and 82.1%, respectively, which were higher than those of commercial ningnanmycin (NNM, at 47.2, 50.1, and 81.4%). Significantly, defensive enzyme activities and proteomics results showed that compound A33 could enhance the defense response by activating the activity of defense enzymes, inducing the glycolysis/gluconeogenesis pathway of tobacco to resist PVY infection. Therefore, our study indicates that compound A33 could be applied as a potential viral inhibitor.

Discovery of Mesoionic Derivatives Containing a Dithioacetal Skeleton as Novel Potential Antibacterial Agents and Mechanism Research

In this study, the design and synthesis of novel pyrido[1,2-a]pyrimidinone mesoionic derivatives incorporating dithioacetal structures were carried out. The three-dimensional quantitative structure-activity relationship (3D-QSAR) model was built according to the EC₅₀ values and directed the synthesis of compound A32. The biological activity test against Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc) indicated that compound A32 showed good antibacterial activity with EC₅₀ values of 10.9 and 17.5 mg/L, which were lower than the EC₅₀ values of bismerthiazol (29.3 and 39.8 mg/L) and thiodiazole copper (64.8 and 78.1 mg/L). Furthermore, the in vivo antibacterial activity against bacterial leaf blight (BLB) and bacterial leaf streak (BLS) revealed that the protective activity of compound A32 was 43.9 and 41.7%, respectively, which was better than the protective activity of thiodiazole copper (40.6 and 35.0%). In addition, the protective activity against bacterial leaf blight of compound A32 was associated with the increasing rice defensive enzyme activity and the upregulation of proteins involved in oxidative phosphorylation. Moreover, compound A32 could upregulate the expression of complex I (nicotinamide adenine dinucleotide hydrogen (NADH) dehydrogenase) in the oxidative phosphorylation pathway, which was verified by complex I activity evaluation.

Design, Synthesis, and Insecticidal Activity of Novel Pyrido[1, 2-a]pyrimidinone Mesoionic Compounds Containing an Indole Moiety as Potential Acetylcholine Receptor Insecticides

In this study, a series of novel mesoionic pyrido[1,2-a]pyrimidinone compounds containing a natural skeleton indole were designed and synthesized, and the insecticidal activities of the target compounds were tested. The results showed that the target compounds had good to excellent insecticidal activities against white-backed planthoppers (Sogatella furcifera) and bean aphids (Aphis craccivora). Among them, compound 7 showed outstanding insecticidal activities against both S. furcifera and A. craccivora, with LC₅₀ values of 0.86 and 0.85 μg/mL, respectively. The insecticidal activity against bean aphids (A. craccivora) was superior to that of triflumezopyrim (LC₅₀ = 3.67 μg/mL). Proteomics and quantitative real-time polymerase chain reaction (qRT-PCR) results revealed that compound 7 may interact with α1 and α7 nAChR subunits of S. furcifera. The results of enzyme activities indicated that compound 7 was an inhibitor of AChE in S. furcifera. This study provides new ideas for the discovery of new mesoionic pyrido[1,2-a]pyrimidinone insecticides.

New Synthetic Method and Insecticidal Activities of Novel Imidazopyridine Mesoionic Derivatives Containing an Ester Group

To develop novel insecticides with high efficiency, a new mode of action, and safety to nontarget organisms and the environment, a series of imidazopyridine mesoionic compounds containing an ester group have been designed and synthesized via a new synthetic method discovered by our group. The bioactivity results showed that most of the target compounds exhibited significant insecticidal activities against Aphis craccivora, and some of them showed moderate insecticidal activities against Sogatella furcifera. Among them, compounds C2, C4-C11, and D3 showed excellent insecticidal activities against A. craccivora (LC₅₀ values were lower than 4.5 μg/mL), which were superior to those of pymetrozine (LC₅₀ = 6.19 μg/mL) and triflumezopyrim (LC₅₀ = 4.68 μg/mL). Remarkably, the insecticidal activity of compound C9 was 5.9-fold greater than that of triflumezopyrim with an LC₅₀ value of 0.8 μg/mL. Proteomics and molecular docking results indicated that compound C9 may affect the nervous system of A. craccivora and act on nicotinic acetylcholine receptors like triflumezopyrim.

Discovery of Novel Benzo[4,5]thiazolo(oxazolo)[3,2-a]pyrimidinone Mesoionic Derivatives as Potential Antibacterial Agents and Mechanism Research

A series of benzo[4,5]thiazole(oxazole)[3,2-a]pyrimidine mesoionic compounds were designed and synthesized. Antibacterial activity tests revealed that compound A23 showed good in vitro activities against Xanthomonas oryzae pv. Oryzicola (Xoc) and Xanthomonas oryzae pv. oryzae (Xoo), with half-maximal effective concentration (EC₅₀) values of 47.6 and 36.8 μM, respectively, which were better than positive control agents thiodiazole copper (281 and 259 μM) and bismerthiazol (245 and 220 μM). The protective activities of compound A23 anti-Xoc and anti-Xoo were 39.7% and 49.2%, respectively, which were better than those of bismerthiazol (31.5% and 40.7%). Compound A23 improved defensive enzyme activities in rice. In addition, compound A23 could upregulate the expression of succinate dehydrogenase (SDH) in the oxidative phosphorylation (OXPHOS) pathway through proteomics analysis, which was consistent with the result of the SDH activity test. Thus, compound A23 is a novel potential antibacterial agent that can be further developed.

Structural Biology-Guided Design, Synthesis, and Biological Evaluation of Novel Insect Nicotinic Acetylcholine Receptor Orthosteric Modulators

The development of novel and safe insecticides remains an important need for a growing world population to protect crops and animal and human health. New chemotypes modulating the insect nicotinic acetylcholine receptors have been recently brought to the agricultural market, yet with limited understanding of their molecular interactions at their target receptor. Herein, we disclose the first crystal structures of these insecticides, namely, sulfoxaflor, flupyradifurone, triflumezopyrim, flupyrimin, and the experimental compound, dicloromezotiaz, in a double-mutated acetylcholine-binding protein which mimics the insect-ion-channel orthosteric site. Enabled by these findings, we discovered novel pharmacophores with a related mode of action, and we describe herein their design, synthesis, and biological evaluation.

Discovery of Pyrido[1,2-a]pyrimidinone Mesoionic Compounds Incorporating a Dithioacetal Moiety as Novel Potential Insecticidal Agents

A series of novel mesoionic pyrido[1,2-a]pyrimidinone compounds incorporating a dithioacetal skeleton were designed and synthesized for use as insecticidal agents. The biological activity of the title compounds indicated good to excellent insecticidal activities against bean aphids (Aphis craccivora) and white-backed planthoppers (Sogatella furcifera). Compound 34 showed excellent insecticidal activity against bean aphids (A. craccivora) with an LC₅₀ value of 2.80 μg/mL, exceeding the insecticidal activity of trifluoropyrimidine (LC₅₀ = 4.20 μg/mL). Proteomics and molecular docking results indicated that compound 34 could act on nicotinic acetylcholine receptors. This study provides support for the application of mesoionic pyrido[1,2-a]pyrimidinone compounds containing dithioacetal as novel insecticidal agents.

Discovery of novel iminosydnone compounds with insecticidal activities based on the binding mode of triflumezopyrim

Triflumezopyrim (TFM) is a new mesoionic insecticide developed by DuPont. Like other neonicotinoid insecticides, it binds to the orthosteric site of the nicotinic acetylcholine receptor (nAChR), but the binding mode has not been reported. Nicotinic acetylcholine binding proteins (nAChBPs) are ideal alternative structure for nAChRs. In this study, molecular docking, molecular dynamics (MD) simulations, binding free energy calculation, and per-residue binding free energy decomposition were used to study the binding modes of TFM and other 12 mesoionic insecticides. By comparing the binding free energy and the insecticidal activity, it was found that the sub-pocket around the benzyl group of the mesoionic insecticide is the key area for maintaining its activity, which is composed of A: Val116, A: Met124, A: Ile126, B: Trp155 and B: Val156. In order to verify the druggability of the sub-pocket, a series of iminosydnone compounds were designed and synthesized based on the structure of the sub-pocket. The lethality rate of compound 1 against Mythimna separata were 100% at 500 mg/L. Our research provides a basis for designing new mesoionic insecticides based on structure.

Selective actions of insecticides on desensitizing and non-desensitizing nicotinic acetylcholine receptors in cockroach (Periplaneta americana) neurons

BACKGROUND

Insect desensitizing nicotinic acetylcholine (nAChD) receptors are desensitized by low concentrations of agonists, including neonicotinoid insecticides, but are essentially insensitive to spinosyns, while non-desensitizing nicotinic acetylcholine (nAChN) receptors are selectively activated by spinosyns and relatively insensitive to neonicotinoids.

RESULTS

The single-electrode voltage-clamp technique was used to measure the actions of newer nicotinic insecticides dinotefuran, sulfoxaflor, triflumezopyrim, spinetoram and GS-ω/k-hexatoxin-Hv1a on cockroach neuronal nAChD and nAChN currents. Like imidacloprid and clothianidin, newer orthosteric nicotinic agonist insecticides dinotefuran and sulfoxaflor act by desensitizing nAChD receptors. The mesoionic insecticide triflumezopyrim selectively inhibited nAChD current with an half maximal inhibitory concentration (IC₅₀ ) of 1.2 nmol L^-1 , with no activation. Unlike other Group 4 insecticides, it did not activate nAChN current, but inhibited it with an IC₅₀ of 3.8 μmol L^-1 , indicating that the compound is a true antagonist. Spinosad and the spinosyn-derived insecticide spinetoram potently and selectively activated nAChN receptors. GS-ω/k-hexatoxin-Hv1a had no effect on nAChN currents and it had a complex action on nAChD currents, inhibiting at sub-nanomolar concentrations and causing some activation and enhancement of ACh-evoked currents at 30 nmol L^-1 and above. Some cells express GS-ω/k-hexatoxin-Hv1a-resistant nAChD receptors.

CONCLUSIONS

Nicotinic acetylcholine receptor competitive modulators (IRAC Group 4) and nicotinic acetylcholine receptor allosteric modulators, site II (hexatoxins, IRAC Group 32) are selective for nAChD receptors, while nicotinic acetylcholine receptor allosteric modulators, site I (spinosyns, IRAC Group 5) are selective for nAChN receptors. It is proposed that IRAC Groups 5 and 32 be re-named non-desensitizing nicotinic acetylcholine receptor allosteric modulators and desensitizing nicotinic acetylcholine receptor allosteric modulators, respectively. © 2021 Society of Chemical Industry.

Novel trifluoromethyl sydnone derivatives: Design, synthesis and fungicidal activity

Crop pathogens reduce the yield and quality of agricultural production. The development of new fungicides will help to sustain this protection and overcome fungicide resistance. Sydnone is a kind of mesoionic, which has a wide range of biological activities. The application of sydnones in agriculture is less, and the study of these compounds will lead to the discovery of new active compounds. In this study, we designed and synthesized a series of noval sydnone mesoionic derivatives by active substructure splicing. All compounds were characterized using ¹H and ¹³C NMR spectroscopy. Among them, trifluoromethyl compound D17 showed good bioactivity against Pseudoperonospora cubensis (EC₅₀ = 49 mg L^-1) in vivo, the activity was similar to that of the control Kresoxim-methyl (EC₅₀ = 44 mg L^-1). However, the target of these compounds should not only be tyrosinase, and the mode of action needs to be further studied. In addition, the structure-activity relationship indicated that the trifluoromethyl group was more beneficial for antifungal activity. This is the first report that fluorine-containing N(3)-benzyl sydnone compounds have good fungicidal activity. These results will provide a basis for the development of sydnone mesoionic as new lead fungicidal agents.

Molecular Docking and Biophysical Studies for Antiproliferative Assessment of Synthetic Pyrazolo-Pyrimidinones Tethered with Hydrazide-Hydrazones

Chemotherapy represents the most applied approach to cancer treatment. Owing to the frequent onset of chemoresistance and tumor relapses, there is an urgent need to discover novel and more effective anticancer drugs. In the search for therapeutic alternatives to treat the cancer disease, a series of hybrid pyrazolo[3,4-d]pyrimidin-4(5H)-ones tethered with hydrazide-hydrazones, 5a-h, was synthesized from condensation reaction of pyrazolopyrimidinone-hydrazide 4 with a series of arylaldehydes in ethanol, in acid catalysis. In vitro assessment of antiproliferative effects against MCF-7 breast cancer cells, unveiled that 5a, 5e, 5g, and 5h were the most effective compounds of the series and exerted their cytotoxic activity through apoptosis induction and G0/G1 phase cell-cycle arrest. To explore their mechanism at a molecular level, 5a, 5e, 5g, and 5h were evaluated for their binding interactions with two well-known anticancer targets, namely the epidermal growth factor receptor (EGFR) and the G-quadruplex DNA structures. Molecular docking simulations highlighted high binding affinity of 5a, 5e, 5g, and 5h towards EGFR. Circular dichroism (CD) experiments suggested 5a as a stabilizer agent of the G-quadruplex from the Kirsten ras (KRAS) oncogene promoter. In the light of these findings, we propose the pyrazolo-pyrimidinone scaffold bearing a hydrazide-hydrazone moiety as a lead skeleton for designing novel anticancer compounds.

Oxidative stress and DNA damage in earthworm (Eisenia fetida) induced by triflumezopyrim exposure

Triflumezopyrim is an excellent pesticide for preventing and controlling rice planthoppers. It is widely used in the production of field rice and mainly through spraying with some inadvertent loss of pesticide to the soil. The future may bring development and utilization of seeds containing triflumezopyrim which will certainly impact earthworms. To evaluate the toxic effects of triflumezopyrim on earthworms, reactive oxygen species (ROS), superoxide dismutase (SOD) and catalase (CAT) activities, malondialdehyde content (MDA), glutathione S-transferase (GST), and DNA oxidative damage (8-hydroxy-2'-deoxyguanosine, 8-OHdG) were measured after 7, 14, 21, and 28 d analyzing the effects of 6 concentrations (0, 0.5, 1, 2.5, 5 and 10 mg/kg) of triflumezopyrim, respectively. ROS content did not change significantly in the early stages but showed a significant dose-effect relationship in the late stages. Antioxidant enzymes were activated in most treatment groups, and catalase activity was the same as that in the control group at 28 d. GST activity showed a trend of increasing first and then decreasing whereas MDA content had no obvious change in trend. 8-OHdG showed significant positive correlation with the concentration of triflumezopyrim at 28 d, indicating that the degree of DNA oxidative damage increased accordingly. The results showed that chronic exposure of triflumezopyrim may cause both oxidative stress and DNA damage in earthworms and alter the activity of antioxidant enzymes.

Neonicotinoid Insecticides: Molecular Targets, Resistance, and Toxicity

Neonicotinoids have been used to protect crops and animals from insect pests since the 1990s, but there are concerns regarding their adverse effects on nontarget organisms, notably on bees. Enhanced resistance to neonicotinoids in pests is becoming well documented. We address the current understanding of neonicotinoid target site interactions, selectivity, and metabolism not only in pests but also in beneficial insects such as bees. The findings are relevant to the management of both neonicotinoids and the new generation of pesticides targeting insect nicotinic acetylcholine receptors.

Recent Research Progress in and Perspectives of Mesoionic Insecticides: Nicotinic Acetylcholine Receptor Inhibitors

Triflumezopyrim exemplifies a new class of mesoionic insecticides and has attracted increasing attention as a result of its unique structure, high level of insecticidal activity, new mechanisms of action, low toxicity toward non-target organisms, and environmental friendliness. It inhibits the nicotinic acetylcholine receptor and has high potency against sucking pests, including the brown planthopper (Nilaparvata lugens), which has developed serious resistance to conventional neonicotinoids and low cross-resistance to some newly developed neonicotinoids. This review focuses on the discovery, synthesis, structure-activity relationships, and mechanism of action of mesoionic insecticides. Finally, potential directions for the development of mesoionic insecticides are discussed.

Toxicity and Sublethal Effect of Triflumezopyrim Against Red Imported Fire Ant (Hymenoptera: Formicidae)

The use of insecticide remains the frontline method in controlling red imported fire ant (Solenopsis invicta Buren) (Hymenoptera: Formicidae), one of the worst invasive ants in the world. Neonicotinoids are effective ingredients in toxic baits for suppressing S. invicta population. To search for new and effective insecticides, the toxicity and sublethal effects of triflumezopyrim, a novel neonicotinoid analog, were evaluated against S. invicta. No high mortality of ants was observed after they fed on sugar water containing 120 μg/ml triflumezopyrim for 72 h; however, 100% mortality was achieved after ants fed on sugar water containing 10 μg/ml triflumezopyrim for 2 wk. Furthermore, at 10 μg/ml, triflumezopyrim did not inhibit ant food consumption within the 7-d treatment period. These results indicate that triflumezopyrim is a slow acting toxin and may be qualified as bait toxin for managing red imported fire ants. At 1 μg/ml, triflumezopyrim did not cause any significant effect on colony growth within 56 d and did not inhibit the food consumption during the whole trial period. At 10 μg/ml, triflumezopyrim displayed a significant reduction of aggressiveness during confrontation with native ants, resulting in higher mortality than the ants in the control. However, at 1 μg/ml, triflumezopyrim did not show any significant impact on both aggressiveness and mortality of the red imported fire ants.

Effects of maximum residue limit of triflumezopyrim exposure on fitness of the red imported fire ant Solenopsis invicta

The impact of exposure to free feeding concentrations of triflumezopyrim to the red imported fire ant, Solenopsis invicta, in maximum residue tolerances for 56 days was investigated to understand whether triflumezopyrim, a novel neonicotinoid, poses unacceptable risks to the environment. Our results demonstrated that neither 0.5 μg/ml nor 0.2 μg/ml triflumezopyrim have a significant impact on the growth of the S. invicta colony and their food consumption (sugar water and locusts) during the length of treatment. While both 0.5 μg/ml and 0.2 μg/ml triflumezopyrim improved the grasping ability of S. invicta, and 0.5 μg/ml not 0.2 μg/ml triflumezopyrim increased their rate of locomotion. In addition, although 0.5 μg/ml and 0.2 μg/ml triflumezopyrim increased their individual aggressiveness index, the probability of the survival of S. invicta was not impacted by triflumezopyrim treatments in aggressive group encounters. This study suggests that triflumezopyrim did not have a negative impact on the fitness of S. invicta at 0.5 μg/ml and 0.2 μg/ml exposures.

Crystal structure of 3-methyl-N-(pyrimidin-5-ylmethyl)pyridin-2-amine, C11H12N4

[C11H12N4], triclinic, P1̄, a = 6.6939(10) Å, b = 8.8034(13) Å, c = 9.1140(13) Å, α = 97.749(2)°, β = 99.285(2)%, γ = 95.917(2)°, V = 520.90(13) Å3, Z = 2, Rgt(F) = 0.0381, wRref(F2) = 0.1030, T = 296(2) K.

First Discovery of Novel Pyrido[1,2-a]pyrimidinone Mesoionic Compounds as Antibacterial Agents

Plant bacterial diseases cause tremendous decreases in crop yield and quality, and there is a lack of highly effective and low-risk antibacterial agents. A series of novel pyrido[1,2-a]pyrimidinone mesoionic compounds containing vanillin moieties were synthesized, and the application of these mesoionic compounds as plant antibacterial agents was reported here for the first time. The bioassay results revealed that the mesoionic compounds had good antibacterial activity. Of these compounds, compound 11 showed excellent in vitro activity against Xanthomonas oryzae pv. oryzae, with an EC₅₀ value of 1.1 μg/mL, which was substantially better than that of bismerthiazol (92.7 μg/mL) and thiodiazole copper (105.4 μg/mL). Moreover, greenhouse condition trials indicated that the protective and curative activities of compound 11 against rice bacterial leaf blight were 75.12 and 72.04%, respectively, which were better than those of bismerthiazol (62.24 and 50.83%, respectively) and thiodiazole copper (53.35 and 65.04%, respectively). These results provide a basis for the application of mesoionic vanillin moieties as new antibacterial agents.

Synthesis and Insecticidal Activity of Mesoionic Pyrido[1,2-α]pyrimidinone Derivatives Containing a Neonicotinoid Moiety

Mesoionic pyrido[1,2-α]pyrimidinone derivatives containing a neonicotinoid moiety were designed, synthesized, and evaluated for their insecticidal activity. Some of the title compounds showed remarkable insecticidal properties against Aphis craccivora. Compound I13 exhibited satisfactory insecticidal activity against A. craccivora. Meanwhile, label-free proteomics analysis of compound I13 treatment identified a total of 821 proteins. Of these, 35 proteins were up-regulated, whereas 108 proteins were down-regulated. Differential expressions of these proteins reflected a change in cellular structure and metabolism.

Mesoionic Pyrido[1,2-a]pyrimidinone Insecticides: From Discovery to Triflumezopyrim and Dicloromezotiaz

One of the greatest global challenges is to feed the ever-increasing world population. The agrochemical tools growers currently utilize are also under continuous pressure, due to a number of factors that contribute to the loss of existing products. Mesoionic pyrido[1,2-a]pyrimidinones are an unusual yet very intriguing class of compounds. Known for several decades, this class of compounds had not been systemically studied until we started our insecticide discovery program. This Account provides an overview of the efforts on mesoionic pyrido[1,2-a]pyridinone insecticide discovery, beginning from the initial high throughput screen (HTS) discovery to ultimate identification of triflumezopyrim (4, DuPont Pyraxalt) and dicloromezotiaz (5) for commercialization as novel insecticides. Mesoionic pyrido[1,2-a]pyrimidinones with a n-propyl group at the 1-position, such as compound 1, were initially isolated as undesired byproducts from reactions for a fungicide discovery program at DuPont Crop Protection. Such compounds showed interesting insecticidal activity in a follow-up screen and against an expanded insect species list. The area became an insecticide hit for exploration and then a lead area for optimization. At the lead optimization stage, variations at three regions of compound 1, i.e., side-chain (n-propyl group), substituents on the 3-phenyl group, and substitutions on the pyrido- moiety, were explored with many analogues prepared and evaluated. Breakthrough discoveries included replacing the n-propyl group with a 2,2,2-trifluoroethyl group to generate compound 2, and then with a 2-chlorothiazol-5-ylmethyl group to form compound 3. 3 possesses potent insecticidal activity not only against a group of hopper species, including corn planthopper (Peregrinus maidis (Ashmead), CPH) and potato leafhopper (Empoasca fabae (Harris), PLH), as well as two key rice hopper species, namely, brown planthopper (Nilaparvata lugens (Stål), BPH) and rice green leafhopper (Nephotettix virescens (Distant), GLH), but also against representative lepidoptera species Diamondback moth (Plutella xylostella (Linnaeus), DBM) and fall armyworm (Spodoptera frugiperda (J.E. Smith), FAW). Further optimization based on 3 led to discovery of triflumezopyrim (4), with a 5-pyrimidinylmethyl group, as a potent hopper insecticide for rice usage. Optimization of the substituents on the pyrido- moiety of 3 resulted in discovery of dicloromezotiaz (5) as a lepidoptera insecticide. In this Account, we present the discovery and optimization of mesoionic pyrido[1,2-a]pyrimidinone insecticides toward the identification of triflumezopyrim (4) and dicloromezotiaz (5). We hope that knowledge and lessons derived from this discovery program will provide valuable information for future agrochemical and drug discovery. Our successful discovery and commercialization development of two novel insecticides based on meosoionic pyrido[1,2-a]pyridiminones may also stimulate interests of scientists from other disciplines to adopt this uncommon yet intriguing heterocycle ring system in pharmaceutical and other material science discovery research.

Splice Variants of pH-Sensitive Chloride Channel Identify a Key Determinant of Ivermectin Sensitivity in the Larvae of the Silkworm Bombyx mori

The pH-sensitive chloride channels (pHCls) are broadly expressed in insects, but little is known about their physiologic role, diversity, and sensitivity to insecticides acting on relevant chloride channels. Here we have sequenced 50 transcripts of the pHCl-1 gene from the brain, third thoracic ganglion (T3G), and midgut of larvae of silkworm Bombyx mori It was found that >50 variants were expressed with distinct splicing in the T3G compared with the brain and midgut. Of the variants detected, variant 9, which was expressed most abundantly in the larvae, was reconstituted in Xenopus laevis oocytes to characterize its pH and ivermectin sensitivity. Variant 9 formed a functional pHCl with half-maximal activation at a pH of 7.87, and was activated by ivermectin irrespective of the extracellular pH. This was in contrast to variant 1, which was activated more profoundly at acidic rather than basic pH. To identify a key determinant for such differential ivermectin sensitivity, different amino acids in variants 1 and 9 were swapped, and the effects of the mutations on ivermectin sensitivity were investigated. The V275S mutation of variant 1 enhanced ivermectin sensitivity, whereas the S275V mutation of variant 9 caused a reduction in sensitivity. In homology models of the Bombyx pHCls, Val275 of variant 1 interacted more strongly with Ala273 than Ser275 of variant 9 at the channel gate. This interaction is likely to prevent ivermectin-induced opening of the channel, accounting, at least partially, for the differential macrolide action on the two variants.