Molecular Simulations Bring New Insights into Protoporphyrinogen IX Oxidase/Protoporphyrinogen IX Interaction Modes

Design, Synthesis, and Herbicidal Activity of Novel Tetrahydrophthalimide Derivatives Containing Oxadiazole/Thiadiazole Moieties

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) has a high status in the development of new inhibitors. To develop novel and highly effective PPO inhibitors, active substructure linking and bioisosterism replacement strategies were used to design and synthesize novel tetrahydrophthalimide derivatives containing oxadiazole/thiadiazole moieties, and their inhibitory effects on Nicotiana tobacco PPO (NtPPO) and herbicidal activity were evaluated. Among them, compounds B11 (Ki = 9.05 nM) and B20 (Ki = 10.23 nM) showed significantly better inhibitory activity against NtPPO than that against flumiclorac-pentyl (Ki = 46.02 nM). Meanwhile, compounds A20 and B20 were 100% effective against three weeds (Abutilon theophrasti, Amaranthus retroflexus, and Portulaca oleracea) at 37.5 g a.i./ha. It was worth observing that compound B11 was more than 90% effective against three weeds (Abutilon theophrasti, Amaranthus retroflexus, and Portulaca oleracea) at 18.75 and 9.375 g a.i./ha. It was also safer to rice, maize, and wheat than flumiclorac-pentyl at 150 g a.i./ha. In addition, the molecular docking results showed that compound B11 could stably bind to NtPPO and it had a stronger hydrogen bond with Arg98 (2.9 Å) than that of flumiclorac-pentyl (3.2 Å). This research suggests that compound B11 could be used as a new PPO inhibitor, and it could help control weeds in agricultural production.

Discovery of Novel N-Phenyltriazinone Derivatives Containing Oxime Ether or Oxime Ester Moieties as Promising Protoporphyrinogen IX Oxidase Inhibitors

Protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) is one of the most important targets for the discovery of green herbicides. In order to find novel PPO inhibitors with a higher herbicidal activity, a series of novel N-phenyltriazinone derivatives containing oxime ether and oxime ester groups were designed and synthesized based on the strategy of pharmacophore and scaffold hopping. Bioassay results revealed that some compounds showed herbicidal activities; especially, compound B16 exhibited broad-spectrum and excellent 100% herbicidal effects to Echinochloa crusgalli, Digitaria sanguinalis, Setaria faberii, Abutilon juncea, Amaranthus retroflexus, and Portulaca oleracea at a concentration of 37.5 g a.i./ha, which were comparable to trifludimoxazin. Nicotiana tabacum PPO (NtPPO) enzyme inhibitory assay indicated that B16 showed an excellent enzyme inhibitory activity with a value of 32.14 nM, which was similar to that of trifludimoxazin (31.33 nM). Meanwhile, compound B16 revealed more safety for crops (rice, maize, wheat, peanut, soybean, and cotton) than trifludimoxazin at a dose of 150 g a.i./ha. Moreover, molecular docking and molecular dynamics simulation further showed that B16 has a very strong and stable binding to NtPPO. It indicated that B16 can be used as a potential PPO inhibitor and herbicide candidate for application in the field.

Understanding the Effects of Ligand Configuration on Protoporphyrinogen IX Oxidase with Rationally Designed 3-(N-Phenyluracil)but-2-enoates

Protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) is a promising target for green herbicide discovery. However, the ligand configuration effects on PPO activity were still poorly understood. Herein, we designed 3-(N-phenyluracil)but-2-enoates using our previously developed active fragments exchange and link (AFEL) approach and synthesized a series of novel compounds with nanomolar ranges of Nicotiana tabacum PPO (NtPPO) inhibitory potency and promising herbicidal potency. Our systematic structure-activity relationship investigations showed that the E isomers of 3-(N-phenyluracil)but-2-enoates displayed improved bioactivity than their corresponding Z isomers. Using molecular simulation studies, we found that the E isomers showed a relatively lower entropy change and could sample more stable binding conformation to the receptor than the Z isomers. Our density functional theory (DFT) calculations showed that the E isomers showed higher chemical reactivity and lower electronic chemical potential than their corresponding Z isomers. Compound E-Ic emerged as the optimal compound with a Ki value of 3.0 nM against NtPPO, exhibiting a broader spectrum of weed control than saflufenacil at 37.5-75 g ai/ha and also safe to maize at 75 g ai/ha, which could be considered as a promising lead herbicide for further development.

Discovery of Pyridyl-Benzothiazol Hybrids as Novel Protoporphyrinogen Oxidase Inhibitors via Scaffold Hopping

In order to discover novel protoporphyrinogen oxidase (PPO) inhibitors with excellent herbicidal activity, a series of structurally novel 6-(pyridin-2-yl) benzothiazole derivatives were designed based on the scaffold hopping strategy. The in vitro experiments demonstrated that the newly synthesized compounds exhibited noteworthy inhibitory activity against Arabidopsis thaliana PPO (AtPPO), with IC50 values ranging from 0.06 to 1.36 μM. Preliminary postemergence herbicidal activity tests and crop safety studies indicated that some of our compounds exhibited excellent herbicidal activity and crop safety. For instance, compound (rac)-7as exhibited superior herbicidal activities to commercially available flumioxazin (FLU) and saflufenacil (SAF) at all the tested concentrations and showed effective herbicidal activities even at a dosage as low as 18.75 g ai/ha. Meanwhile, compound (rac)-7as showed good crop safety for wheat at a dosage as high as 150 g of ai/ha. Although the absolute configuration of compound 7as has no obvious effect on its herbicidal activity, compound (R)-7as showed a slightly higher crop safety than compound (S)-7as. Molecular simulation studies of Nicotiana tabacum PPO (NtPPO) and our candidate compounds showed that the benzothiazole moiety of compounds (R)-7as or (S)-7as formed multiple π-π stacking interactions with FAD, and the pyridine ring generated π-π stacking with Phe-392. Our finding proved that the pyridyl-benzothiazol hybrids are promising scaffolds for the development of PPO-inhibiting herbicides.

Design, Synthesis, and Herbicidal Activity of Substituted 3-(Pyridin-2-yl)Phenylamino Derivatives

To discover protoporphyrinogen oxidase (PPO) inhibitors with robust herbicidal activity and crop safety, three types of substituted 3-(pyridin-2-yl)phenylamino derivatives bearing amide, urea, or thiourea as side chain were designed via structure splicing strategy. Postemergence herbicidal activity assessment of 33 newly prepared compounds revealed that many of our compounds such as 6a, 7b, and 8d exhibited superior herbicidal activities against broadleaf and monocotyledon weeds to commercial acifluorfen. In particular, compound 8d exhibited excellent herbicidal activities and high crop safety at a dosage range of 37.5-150 g ai/ha. PPO inhibitory studies supported our compounds as typical PPO inhibitors. Molecular docking studies revealed that compound 8d provided effective interactions with Nicotiana tabacum PPO (NtPPO) via diverse interaction models, such as π-π stacking and hydrogen bonds. Molecular dynamics (MD) simulation studies and degradation studies were also conducted to gain insight into the inhibitory mechanism. Our study indicates that compound 8d may be a candidate molecule for the development of novel herbicides.

Insight into the Role of an α-Helix Cluster in Protoporphyrinogen IX Oxidase

Protoporphyrinogen IX oxidase (PPO) is the last common enzyme in chlorophyll and heme biosynthesis pathways. In humans, point mutations on PPO are responsible for the dominantly inherited disorder disease variegate porphyria (VP). It is found that several VP-causing mutation sites are located on an α-helix cluster (consisting of α-5, α-6, and α-7 helix, named the G169 helix cluster) of human PPO, although these mutation sites are outside the active site of the human PPO. In this work, we investigated the role of the G169 helix cluster via site-directed mutagenesis, enzymatic kinetics, and computational studies. Kinetic studies showed that mutations on the G169 helix cluster affect the activity of PPO. The MD simulation showed that mutations on the G169 helix cluster reduced the activity of PPO by affecting the proper orientation of substrate protoporphyrinogen within the active site of PPO and possibly the dipole moment of the G169 helix cluster. Moreover, the mutation abolished the interaction between the mutated site and other residues, thus affecting the secondary structure and hydrogen bond interactions within the G169 helix cluster. These results indicated that the integrity of the G169 helix cluster is important for the stabilization of protoporphyrinogen within the active site of PPO to facilitate the interaction between protoporphyrinogen and cofactor FAD and provide a proper electrostatic environment for the activity of PPO. Our result provides new insight into understanding the relationship between the structure and function of PPO.

Design and synthesis of novel PPO-inhibiting pyrimidinedione derivatives safed towards cotton

Developing innovative and effective herbicides is of utmost importance since weed management has become a worldwide agricultural production concern, resulting in severe economic losses every year. In this study, a series of new pyrimidinedione compounds were developed via combination of pyrimidinediones with N-phenylacetamide moiety. The herbicidal activity test (37.5-150 g of ai/ha) indicated that most of the new derivatives exhibited excellent herbicidal activity against dicotyledonous weeds, but less against grasses. Among them, compound 34 was identified as the best postemergence herbicidal activities against six species of weeds (Amaranthus retrof lexus, AR; Abutilon theophrasti, AT; Veronica polita, VP; Echinochloa crusgalli, EC; Digitaria sanguinalis, DS; Setaria viridis, SV), which were comparable to the commercial control agent saflufenacil (≥90%). The protoporphyrinogen oxidase (PPO; EC. 1.3.3.4) activity experiment suggested that compound 34 could significantly reduce the PPO content in weeds, the relative expression levels of the PPO gene were verified by real-time quantitative polymerase chain reaction (RT-qPCR), and the results were consistent with the trend of the enzyme activity data. Molecular docking showed that compound 34 could occupy the PPO enzyme catalytic substrate pocket, which played an excellent inhibitory effect on the activity of receptor protein. Meanwhile, the tolerance of compound 34 to cotton was better than that of the commercial agent saflufenacil at 150 g of ai/ha. Thus, compound 34 exhibits the potential to be a new PPO herbicide for weed control in cotton fields. This study provided a basis for the subsequent structural modification and mechanism research of pyrimidinedione derivatives.

Design, Synthesis, and Metabolism Studies of N-1,4-Diketophenyltriazinones as Protoporphyrinogen IX Oxidase Inhibitors

Protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) is an established site for green herbicide discovery. In this work, based on structural analysis, we develop an active fragment exchange and link (AFEL) approach to designing a new class of N-1,4-diketophenyltriazinones I-III as potent Nicotiana tabacum PPO (PPO) inhibitors. After systematic structure-activity relationship optimizations, a series of new compounds with Ki values in the single-digit nanomolar range toward NtPPO and promising herbicidal activity were discovered. Among them, Ii (Ki = 0.11 nM) displays 284- and 90-fold improvement in NtPPO inhibitory activity over trifludimoxazin (Ki = 31 nM) and saflufenacil (Ki = 10 nM), respectively. In addition, Ip (Ki = 2.14 nM) not only exhibited good herbicidal activity at 9.375-37.5 g ai/ha but also showed high crop safety to rice at 75 g ai/ha by the postemergence application, indicating that Ip could be developed as a potential herbicide for weed control in rice fields. Additionally, our molecular dynamic simulation clarified the molecular basis for the interactions of these molecules with NtPPO. The metabolism studies in planta showed that IIIc could be converted to Ic, which displayed higher herbicidal activity than IIIc. The density functional theory analysis showed that due to the effect of two sulfur atoms at the triazinone moiety, IIIc is more reactive than Ic, making it more easily degraded in planta. Our work indicates that the AFEL strategy could be used to design new molecules with improved bioactivity.

Diazabicyclo derivatives as safeners protect cotton from injury caused by flumioxazin

Flumioxazin, a protoporphyrinogen oxidase (PPO; EC 1.3.3.4) inhibitor, has been used in soybean, cotton, grapes, and many other crops to control broad leaf weeds. Unfortunately, it can cause damage to cotton. To ameliorate phytotoxicity of flumioxazin to cotton, this work assessed the protective effects of diazabicyclo derivatives as potential safeners in cotton. A bioactivity assay proved that the phytotoxicity of flumioxazin on cotton was alleviated by some of the compounds. In particular, the activity of glutathione S-transferases (GSTs) was significantly enhanced by Compound 32, which showed good safening activity against flumioxazin injury. The physicochemical properties and absorption, distribution, metabolism, excretion and toxicity (ADMET) predictions proved that the pharmacokinetic properties of Compound 32 are similar to those of the commercial safener BAS 145138. The present work demonstrated that diazabicyclo derivatives are potentially efficacious as herbicide safeners, meriting further investigation.

Discovery of a Potent Thieno[2,3-d]pyrimidine-2,4-dione-Based Protoporphyrinogen IX Oxidase Inhibitor through an In Silico Structure-Guided Optimization Approach

A key objective for herbicide research is to develop new compounds with improved bioactivity. Protoporphyrinogen IX oxidase (PPO) is an essential target for herbicide discovery. Here, we report using an in silico structure-guided optimization approach of our previous lead compound 1 and designed and synthesized a new series of compounds 2-6. Systematic bioassays led to the discovery of a highly potent compound 6g, 1-methyl-3-(2,2,7-trifluoro-3-oxo-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-benzo[b][1,4]oxazin-6-yl)thieno[2,3-d]pyrimidine-2,4(1H,3H)-dione, which exhibited an excellent and wide spectrum of weed control at the rates of 30-75 g ai/ha by the postemergence application and is relatively safe on maize at 75 g ai/ha. Additionally, the K_i value of 6g to Nicotiana tabacum PPO (NtPPO) was found to be 2.5 nM, showing 3-, 12-, and 18-fold higher potency relative to compound 1 (K_i = 7.4 nM), trifludimoxazin (K_i = 31 nM), and flumioxazin (K_i = 46 nM), respectively. Furthermore, molecular simulations further suggested that the thieno[2,3-d]pyrimidine-2,4-dione moiety of 6g could form a more favorable π-π stacking interaction with the Phe392 of NtPPO than the heterocyclic moiety of compound 1. This study provides an effective strategy to obtain enzyme inhibitors with improved performance through molecular simulation and structure-guided optimization.

Design, synthesis, and molecular simulation studies of N-phenyltetrahydroquinazolinones as protoporphyrinogen IX oxidase inhibitors

Discovering new protoporphyrinogen oxidase (PPO, EC 1.3.3.4) inhibitors is a promising direction for agrochemical research. Herein, we reported the discovery and in silico structure-guided optimization of N-phenyltetrahydroquinazolinones 1 and 2 as new PPO inhibitors. Most of the obtained compounds 1 and 2 exhibited significantly enhanced Nicotiana tabacum PPO (NtPPO) inhibitory potency than that of flumioxazin. Promisingly, 1-(tert-butoxy)-1-oxopropan-2-yl 2-chloro-4-fluoro-5-(4-oxo-5,6,7,8-tetrahydroquinazolin-3(4H)-yl)benzoate, 2o, with a K_i value of 4 nM, showed ten folds more enhanced NtPPO-inhibiting potency than flumioxazin. Additionally, compounds 2b and 2i showed a broad spectrum of broadleaf weeds control at 37.5-150 g ai/ha, and selective for wheat at 150 g ai/ha in the post-emergent application. The molecular simulation studies revealed the vital basis between N-phenyltetrahydroquinazolinones and NtPPO. The present work indicated that the N-phenyltetrahydroquinazolinone motif might be a potential scaffold for herbicide discovery.

Discovery of N-Phenylaminomethylthioacetylpyrimidine-2,4-diones as Protoporphyrinogen IX Oxidase Inhibitors through a Reaction Intermediate Derivation Approach

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is an effective target for green herbicide discovery. In this work, we reported the unexpected discovery of a novel series of N-phenylaminomethylthioacetylpyrimidine-2,4-diones (2-6) as promising PPO inhibitors based on investigating the reaction intermediates of our initially designed N-phenyluracil thiazolidinone (1). An efficient one-pot procedure that gave 41 target compounds in good to high yields was developed. Systematic Nicotiana tabacum PPO (NtPPO) inhibitory and herbicidal activity evaluations led to identifying some compounds with improved NtPPO inhibition potency than saflufenacil and good post-emergence herbicidal activity at 37.5-150 g of ai/ha. Among these analogues, ethyl 2-((((2-chloro-4-fluoro-5-(3-methyl-2,6-dioxo-4-(trifluoromethyl)-3,6-dihydropyrimidin-1(2H)-yl)phenyl)amino)methyl)thio)acetate (2c) (Ki = 11 nM), exhibited excellent weed control at 37.5-150 g of ai/ha and was safe for rice at 150 g of ai/ha, indicating that compound 2c has the potential to be developed as a new herbicide for weed management in paddy fields. Additionally, our molecular simulation and metabolism studies showed that the side chains of compound 2c could form a hydrogen-bond-mediated seven-membered ring system; substituting a methyl group at R1 could reinforce the hydrogen bond of the ring system and reduce the metabolic rate of target compounds in planta.

Design, Synthesis, and Molecular Mechanism Studies of N-Phenylisoxazoline-thiadiazolo[3,4-a]pyridazine Hybrids as Protoporphyrinogen IX Oxidase Inhibitors

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is an important target for green agrochemical discovery. Herein, a novel N-phenylisoxazoline-thiadiazolo[3,4-a]pyridazine herbicidal active scaffold was designed by the scaffold hybridization strategy. Systematic structural optimization enabled the discovery of a series of derivatives with excellent weed control at 9.375-150 g ai/ha by the post-emergent application. Some derivatives exhibited improved Nicotiana tabacum PPO (NtPPO)-inhibitory activity than fluthiacet-methyl. Of these, 2b, with K_i = 21.8 nM, displayed higher weed control than fluthiacet-methyl at the rate of 12-75 g ai/ha, and selective to maize at 75 g ai/ha. In planta, 2b was converted into a bioactive metabolite 5 (K_i = 4.6 nM), which exhibited 4.6-fold more potency than 2b in inhibiting the activity of NtPPO. Molecular dynamics simulation explained that 5 formed stronger π-π interaction with Phe392 than that of 2b. This work not only provides a promising lead compound for weed control in maize fields but is also helpful to understand the molecular mechanism and basis of the designed hybrids.

Design, Synthesis, and Herbicidal Activity of Novel Diphenyl Ether Derivatives Containing Fast Degrading Tetrahydrophthalimide

To seek new protoporphyrinogen oxidase (PPO) inhibitors with better biological activity, a series of novel diphenyl ether derivatives containing tetrahydrophthalimide were designed based on the principle of substructure splicing and bioisomerization. PPO inhibition experiments exhibited that 6c is the most potential compound, with the half-maximal inhibitory concentration (IC₅₀) value of 0.00667 mg/L, showing 7 times higher activity than Oxyfluorfen (IC₅₀ = 0.0426 mg/L) against maize PPO and similar herbicidal activities to Oxyfluorfen in weeding experiments in greenhouses and field weeding experiments. In view of the inspected bioactivities, the structure-activity relationship (SAR) of this series of compounds was also discussed. Crop selection experiments demonstrate that compound 6c is safe for soybeans, maize, rice, peanuts, and cotton at a dose of 300 g ai/ha. Accumulation analysis experiments showed that the accumulation of 6c in some crops (soybeans, peanuts, and cotton) was significantly lower than Oxyfluorfen. Current work suggests that compound 6c may be developed as a new herbicide candidate in fields.

Design and Synthesis of N-phenyl Phthalimides as Potent Protoporphyrinogen Oxidase Inhibitors

Protoporphyrinogen oxidase (PPO) has been identified as one of the most promising targets for herbicide discovery. A series of novel phthalimide derivatives were designed by molecular docking studies targeting the crystal structure of mitochondrial PPO from tobacco (mtPPO, PDB: 1SEZ) by using Flumioxazin as a lead, after which the derivatives were synthesized and characterized, and their herbicidal activities were subsequently evaluated. The herbicidal bioassay results showed that compounds such as 3a (2-(4-bromo-2,6-difluorophenyl) isoindoline-1,3-dione), 3d (methyl 2-(4-chloro-1,3-dioxoisoindolin-2-yl)-5-fluorobenzoate), 3g (4-chloro-2-(5-methylisoxazol-3-yl) isoindoline-1,3-dione), 3j (4-chloro-2-(thiophen-2-ylmethyl) isoindoline-1,3-dione) and 3r (2-(4-bromo-2,6-difluorophenyl)-4-fluoroisoindoline-1,3-dione) had good herbicidal activities; among them, 3a showed excellent herbicidal efficacy against A. retroflexus and B. campestris via the small cup method and via pre-emergence and post-emergence spray treatments. The efficacy was comparable to that of the commercial herbicides Flumioxazin, Atrazine, and Chlortoluron. Further, the enzyme activity assay results suggest that the mode of action of compound 3a involves the inhibition of the PPO enzyme, and 3a showed better inhibitory activity against PPO than did Flumioxazin. These results indicate that our molecular design strategy contributes to the development of novel promising PPO inhibitors.

Discovery of Novel N-Isoxazolinylphenyltriazinones as Promising Protoporphyrinogen IX Oxidase Inhibitors

Protoporphyrinogen oxidase (PPO, EC 1.3.3.4) is a promising target for herbicide discovery. Search for new compounds with novel chemotypes is a key objective for agrochemists. Here, we describe the discovery and systematic SAR-based structure optimization of novel N-isoxazolinylphenyltriazinones 5-9 as PPO inhibitors. The in vivo herbicidal activity and in vitro Nicotiana tabacum PPO (NtPPO) inhibitory activity were explored in detail. A number of the new synthetic compounds displayed strong PPO inhibitory activity with Ki values in the nanomolar range. Some compounds exhibited excellent and broad-spectrum weed control at the rate of 9.375-37.5 g ai/ha by postemergence application and showed improved monocotyledonous weed control compared to saflufenacil. Most promisingly, ethyl 3-(2-chloro-5-(3,5-dimethyl-2,6-dioxo-4-thioxo-1,3,5-triazinan-1-yl)-4-fluorophenyl)-5-methyl-4,5-dihydroisoxazole-5-carboxylate, 5a, with a Ki value of 4.9 nM, displayed over 2- and 6-fold higher potency than saflufenacil (Ki = 10 nM) and trifludimoxazin (Ki = 31 nM), respectively. Moreover, 5a showed excellent and broad-spectrum weed control against 32 kinds of weeds at 37.5-75 g ai/ha. Rice exhibited relative tolerance to 5a at 150 g ai/ha by postemergence application, indicating that 5a could be a potential herbicide candidate for weed control in paddy fields.

Novel Thiazole Phenoxypyridine Derivatives Protect Maize from Residual Pesticide Injury Caused by PPO-Inhibitor Fomesafen

The herbicide fomesafen has the advantages of low toxicity and high selectivity, and the target of this compound is protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4). However, this herbicide has a long residual period and can have phytotoxic effects on succeeding crops. To protect maize from fomesafen, a series of thiazole phenoxypyridines were designed based on structure–activity relationships, active substructure combinations, and bioisosterism. Bioassays showed that thiazole phenoxypyridines could improve maize tolerance under fomesafen toxicity stress to varying degrees at a dose of 10 mg·kg⁻¹. Compound 4i exhibited the best effects. After being treated by compound 4i, average recovery rates of growth index exceeded 72%, glutathione content markedly increased by 167% and glutathione S-transferase activity was almost 163% of fomesafen-treated group. More importantly, after being treated by compound 4i, the activity of PPO, the main target enzyme of fomesafen, recovered to 93% of the control level. The molecular docking result exhibited that the compound 4i could compete with fomesafen to bind with the herbicide target enzyme, which consequently attained the herbicide detoxification. The present work suggests that compound 4i could be developed as a potential safener to protect maize from fomesafen.

Design, Herbicidal Activity, and QSAR Analysis of Cycloalka[d]quinazoline-2,4-dione-Benzoxazinones as Protoporphyrinogen IX Oxidase Inhibitors

In continuation of our search for potent protoporphyrinogen IX oxidase (PPO, EC 1.3.3.4) inhibitors, we designed and synthesized a series of novel herbicidal cycloalka[d]quinazoline-2,4-dione-benzoxazinones. The bioassay results of these synthesized compounds indicated that most of the compounds exhibited very strong Nicotiana tabacum PPO (NtPPO) inhibition activity. More than half of the 37 synthesized compounds displayed over 80% control of all three tested broadleaf weeds at 37.5-150 g ai/ha by postemergent application, and a majority of them showed no phytotoxicity toward at least one kind of crop at 150 g ai/ha. Promisingly, 17i (Ki = 6.7 nM) was 6 and 4 times more potent than flumioxazin (Ki = 46 nM) and trifludimoxazin (Ki = 31 nM), respectively. Moreover, 17i displayed excellent, broad-spectrum herbicidal activity, even at levels as low as 37.5 g ai/ha, and it was determined to be safe for wheat at 150 g ai/ha in postemergent application, indicating the great potential for 17i development as a herbicide for weed control in wheat fields.

Synthesis and Herbicidal Activity of Pyrido[2,3-d]pyrimidine-2,4-dione-Benzoxazinone Hybrids as Protoporphyrinogen Oxidase Inhibitors

To search for new protoporphyrinogen oxidase (PPO, EC 1.3.3.4) inhibitors with improved bioactivity, a series of novel pyrido[2,3-d]pyrimidine-2,4-dione-benzoxazinone hybrids, 9-13, were designed and synthesized. Several compounds with improved tobacco PPO (mtPPO)-inhibiting and promising herbicidal activities were found. Among them, the most potent compound, 3-(7-fluoro-3-oxo-4-(prop-2-yn-1-yl)-3,4-dihydro-2H-benzo[b][1,4] oxazin-6-yl)-1-methylpyrido[2,3-d]pyrimidine-2,4(1H,3H)-dione, 11q, with a Ki value of 0.0074 μM, showed six times more activity than flumioxazin (Ki = 0.046 μM) against mtPPO. Compound 11q displayed a strong and broad spectrum of weed control at 37.5-150 g of active ingredient (ai)/ha by both post- and pre-emergence application, which was comparable to that of flumioxazin. 11q was safe to maize, soybean, peanut, and cotton at 150 g ai/ha, and selective to rice and wheat at 75 g ai/ha by pre-emergence application, indicating potential applicability in these fields.