Structure-Based Bioisosterism Design, Synthesis, Biological Activity and Toxicity of 1,2,4-Oxadiazole Substituted Benzamides Analogues Containing Pyrazole Rings

In order to discover pesticidal lead compounds with high activity and low toxicity, a series of novel benzamides substituted with pyrazole-linked 1,2,4-oxadiazole were designed via bioisosterism. The chemical structures of the target compounds were confirmed via 1H NMR, 13C NMR and HRMS analysis. The preliminary bioassay showed that most compounds exhibited good lethal activities against Mythimna separate, Helicoverpa armigera, Ostrinia nubilalis and Spodoptera frugiperda at 500 mg/L. Particularly in the case of Mythimna separate, compound 14q (70%) exhibited obvious insecticidal activity. In addition, compound 14h demonstrated good fungicidal activity against Pyricularia oryae with an inhibition rate of 77.8%, and compounds 14e, 14k, 14n and 14r also showed certain antifungal activities (55.6–66.7%). The zebrafish toxicity test showed that the LC50 of compound 14h was 14.01 mg/L, which indicated that it may be used as a potential leading compound for further structural optimization.

Our previous studies showed that benzamides substituted with pyridine-linked 1,2,4-oxadiazole derivatives have certain insecticidal and fungicidal activities [24,25]. Therefore, changing amine fragments in pyrazoleamide of tebufenpyrad into 1,2,4-oxadiazole, a series of novel pyrazole-linked 1,2,4-oxadiazoles were designed according to the principle of bioisosterism ( Figure 2). The chemical structures of the target compounds were confirmed via 1 H NMR, 13 C NMR and HRMS analysis, and their insecticidal activities and fungicidal activities were studied and a toxicity test with zebrafish embryos was performed.
Regarding the synthesis of intermediate 11, refer to our previous work [24]. Finally, the intermediates 7 and 11 went through cyclization, hydrolysis and condensation reactions to obtain the target compounds 14.
Regarding the synthesis of intermediate 11, refer to our previous work [24]. Finally, the intermediates 7 and 11 went through cyclization, hydrolysis and condensation reactions to obtain the target compounds 14.
Scheme 1. Synthetic route of target compounds 14.
In step 2, the Knorr cyclization reaction was carried out in an ice bath to avoid the formation of isomers (Scheme 2). Scheme 2. The Knorr cyclization reaction of compound 3, performed under high temperature.

Spectral Analysis of Target Compounds
Compound 14q was taken as an example to conduct spectral analysis. In the 1 H NMR spectra of 14q, the -NH-proton signal was found at δ 10.43 ppm. The -CH-signals of the benzene ring were assigned at δ 8.67-7.29 ppm, and the single peak at δ 4.25 ppm was the peak of N-CH3 on the pyrazole ring. The signals at δ 2.65 ppm and δ 1.23 ppm were assigned to -CH2 and -CH3 of the pyrazole ring, respectively. In addition, the signal of -CH3 on the benzene ring was found at δ 2.28 ppm. In the 13 C NMR spectra of compound 14q, the appearances of signals at 167.21 ppm and 165.13 ppm were assigned to the carbons of the 1,2,4-oxadiazole ring. In the HRMS spectrogram, the calculated value of the ion peak of this compound was [ In step 2, the Knorr cyclization reaction was carried out in an ice bath to avoid the formation of isomers (Scheme 2). In step 2, the Knorr cyclization reaction was carried out in an ice bath to avoid the formation of isomers (Scheme 2).

Spectral Analysis of Target Compounds
Compound 14q was taken as an example to conduct spectral analysis. In the 1 H NMR spectra of 14q, the -NH-proton signal was found at δ 10.43 ppm. The -CH-signals of the benzene ring were assigned at δ 8.67-7.29 ppm, and the single peak at δ 4.25 ppm was the peak of N-CH3 on the pyrazole ring. The signals at δ 2.65 ppm and δ 1.23 ppm were assigned to -CH2 and -CH3 of the pyrazole ring, respectively. In addition, the signal of -CH3 on the benzene ring was found at δ 2.28 ppm. In the 13 C NMR spectra of compound 14q, the appearances of signals at 167.21 ppm and 165.13 ppm were assigned to the carbons of the 1,2,4-oxadiazole ring. In the HRMS spectrogram, the calculated value of the ion peak of this compound was [

Spectral Analysis of Target Compounds
Compound 14q was taken as an example to conduct spectral analysis. In the 1 H NMR spectra of 14q, the -NH-proton signal was found at δ 10.43 ppm. The -CH-signals of the benzene ring were assigned at δ 8.67-7.29 ppm, and the single peak at δ 4.25 ppm was the peak of N-CH 3 on the pyrazole ring. The signals at δ 2.65 ppm and δ 1.23 ppm were assigned to -CH 2 and -CH 3 of the pyrazole ring, respectively. In addition, the signal of -CH 3 on the benzene ring was found at δ 2.28 ppm. In the 13 C NMR spectra of compound 14q, the appearances of signals at 167.21 ppm and 165.13 ppm were assigned to the carbons of the 1,2,4-oxadiazole ring. In the HRMS spectrogram, the calculated value of the ion peak of this compound was [M + Na] + 456.0989, and the measured value was [M + Na] + 456.0983. The absolute error was within 0.003.

Biological Activities of Target Compounds
The results of the insecticidal activity tests of the target compounds are shown in Table 1. Overall, all the target compounds 14 were found to exhibit certain insecticidal activities against Mythimna separate, Helicoverpa armigera, Ostrinia nubilalis and Spodoptera frugiperda at 500 mg/L. Specifically, the mortality rate of compound 14q against Mythimna separate (70%) was higher than the control drug, tebufenpyrad (60%). At the same time, compounds 14a and 14f also showed moderate activities (50%). Furthermore, the insecticidal activities of compounds 14 against Helicoverpa armigera and Ostrinia nubilalis were all below 50%. For Spodoptera frugiperda, only compound 14i exhibited obvious lethality (70%). Moreover, the inhibitory activities of compounds 14 were all below 40% against Culex pipiens pallens at 10 mg/L. The structure-activity relationship (SAR) of the target compounds showed that when the substituents of the benzene ring were 4-F and 3-Cl-2-CH 3 , the inhibitory activities against the tested targets were superior to others. Compounds containing F and Cl groups are beneficial to enhance insecticidal activity. Comparing 14b, 14o, 14p and 14q, we can see that Cl was beneficial improving the insecticidal activity of the compound. The results of the fungicidal activity tests of the target compounds are shown in Table 2. All the target compounds 14 were found to exhibit inhibitory activity against the 10 fungi at 50 mg/L. Compounds 14h (77.8%), 14e (55.6%), 14k (66.7%), 14n (66.7%) and 14r (55.6%) showed good inhibitory activities against Pyricularia oryae, which were lower than the control drug bixafen (100%). For Sclerotinia sclerotiorum, compounds 14g, 14n, 14o, 14p and 14q possessed moderately inhibitory activities (45.2%-58.1%). As can be seen, compound 14n exhibited good inhibitory activity against Alternaria solani (50.5%), Gibberella zeae (55.9%), Cercospora arachidicola (65.9%) and Riziocotinia solani (53.3%). From Table 3, we can see that compound 14h had good inhibitory activity against Pyricularia oryae with an EC 50 of 16.95 mg/L. In addition, by comparing the control effects of compounds 14a, 14h, 14k, 14q and 14r on Pyricularia oryae, the aniline-containing substituents at the meta position were generally beneficial to improving the fungicidal activity.

Toxicity to Zebrafish Embryo
According to the fungicidal activity results, we selected compound 14h, which showed good activity, to study the lethal and teratogenic effects of exposure upon zebrafish embryos from 6 to 96 hpf (hours post-fertilization). When the concentration of 14h was lower than 40 mg/L, the mortality increased sharply with the increase in the concentration. At 40 mg/L, the mortality rate reached as high as 90%. The mortality rate of 14h showed concentration-dependent curves ( Figure 3) with an LC 50 value of 14.01 mg/L. At 0-24 hpf, zebrafish embryos showed no obvious developmental delay ( Figure 4). However, a series of malformations appeared at 48-96 hpf, such as delayed yolk absorption, a significantly shortened body, pericardial cysts, bent spine, melanin deficiency and yolk sac. At 48 hpf, the yolk absorption rate of zebrafish in the 14 mg/L-exposed group was significantly inhibited compared with the control group. At 72 hpf, larval zebrafish exposed at 14 mg/L showed pericardial edema and shortened body lengths. At 96 hpf, bent spines were observed for the larval zebrafish exposed at 10 or 14 mg/L. At 0-24 hpf, zebrafish embryos showed no obvious developmental delay ( Figure 4). However, a series of malformations appeared at 48-96 hpf, such as delayed yolk absorption, a significantly shortened body, pericardial cysts, bent spine, melanin deficiency and yolk sac. At 48 hpf, the yolk absorption rate of zebrafish in the 14 mg/Lexposed group was significantly inhibited compared with the control group. At 72 hpf, larval zebrafish exposed at 14 mg/L showed pericardial edema and shortened body lengths. At 96 hpf, bent spines were observed for the larval zebrafish exposed at 10 or 14 mg/L.

General Information
Melting points were determined using an X-4 digital microscopic melting point detector (Taike, Beijing, China) and the thermometer was uncorrected. 1 H NMR and 13 C NMR spectra were measured on a BRUKER Avance 500 MHz spectrometer (Bruker 500 MHz, Fallanden, Switzerland) using CDCl3 or DMSO as the solvent. High-resolution electrospray mass spectra (HR-ESI-MS) were determined using an UPLC H CLASS/QTOF G2 XS mass spectrometer (Waters, Milford, CT, USA). All the reagents were of analytical  At 0-24 hpf, zebrafish embryos showed no obvious developmental delay (Figure 4). However, a series of malformations appeared at 48-96 hpf, such as delayed yolk absorption, a significantly shortened body, pericardial cysts, bent spine, melanin deficiency and yolk sac. At 48 hpf, the yolk absorption rate of zebrafish in the 14 mg/Lexposed group was significantly inhibited compared with the control group. At 72 hpf, larval zebrafish exposed at 14 mg/L showed pericardial edema and shortened body lengths. At 96 hpf, bent spines were observed for the larval zebrafish exposed at 10 or 14 mg/L.

General Information
Melting points were determined using an X-4 digital microscopic melting point detector (Taike, Beijing, China) and the thermometer was uncorrected. 1 H NMR and 13 C NMR spectra were measured on a BRUKER Avance 500 MHz spectrometer (Bruker 500 MHz, Fallanden, Switzerland) using CDCl3 or DMSO as the solvent. High-resolution electrospray mass spectra (HR-ESI-MS) were determined using an UPLC H CLASS/QTOF G2 XS mass spectrometer (Waters, Milford, CT, USA). All the reagents were of analytical

General Information
Melting points were determined using an X-4 digital microscopic melting point detector (Taike, Beijing, China) and the thermometer was uncorrected. 1 H NMR and 13 C NMR spectra were measured on a BRUKER Avance 500 MHz spectrometer (Bruker 500 MHz, Fallanden, Switzerland) using CDCl 3 or DMSO as the solvent. High-resolution electrospray mass spectra (HR-ESI-MS) were determined using an UPLC H CLASS/QTOF G2 XS mass spectrometer (Waters, Milford, CT, USA). All the reagents were of analytical grade or synthesized in our laboratory. The characterization data for all synthetic compounds are provided in the Supplementary Materials.
Ethics statement: The Institutional Animal Care and Use Committee (IACUC) at Wenzhou Medical University (SYXK 2019-0009, 4 April 2019 to 4 April 2024) approved our study plan for the proper use of zebrafish. All studies were carried out in strict accordance with the guidelines of the IACUC. All dissections were performed on ice, and all efforts were made to minimize suffering.

Ethyl 2,4-Dioxohexanoate 3
Sodium (2.50 g), toluene (50 mL) and ethanol absolute (30 mL) were added to a three-necked flask successively. Then, the solution of diethyl oxalate (14.63 g, 0.10 mol) in butanone (7.25 g, 0.10 mol) was added dropwise at 0 • C and reacted for 5-6 h. The solvent was removed under reduced pressure and the pH was then adjusted to 2-3 with HCl. Afterwards, the mixture was extracted using toluene and the extraction was dried with MgSO 4 and filtered. The filtration was concentrated to give 12.70 g yellow liquid. Yield: 73.9%.

Ethyl 3-Ethyl-1H-pyrazole-5-carboxylate 4
N 2 H 4 ·H 2 O (4.40 g, 88.50 mmol) was added dropwise to the mixture of ethanol (60 mL) and compounds 3 (12.70 g, 73.80 mmol) at 0 • C to react for 4 h. The solvent was removed under reduced pressure. Then, the residue was extracted using toluene and separated via column chromatography to give 7.20 g light yellow liquid. Yield: 58.2%.

Ethyl 4-Chloro-3-ethyl-1-methyl-1H-pyrazole-5-carboxylate 6
The mixture of compound 5 (6.81 g) and CHCl 3 (50 mL) was heated to 40 • C. Then, SO 2 Cl 2 (7.60 g, 56.00 mmol) was added dropwise and reacted at 60 • C for 2 h. The mixture was washed with saturated Na 2 CO 3 and extracted with ethyl acetate and dried with MgSO 4 . Next, the solvent was removed to give 7.53 g solid. The crude product was subjected to the next reaction without further purification.

Intermediate 7
To a three-necked flask, we added compound 6 (2.10 g 0.01 mol) and ethanol (30 mL), then stirred it to dissolve it completely. Subsequently, NaOH (5 mL, 30%) was added to reflux for 1 h. The solvent was removed and then the pH was adjusted to 2-3 with HCl to precipitate a white solid. The crude product was recrystallized in ethanol and water to afford the pure product.

Intermediate 11
For the synthesis of intermediate 11, refer to our previous work [24].
The newly prepared 4-chloro-3-ethyl-1-methyl-1H-py-razole-5-carbonyl chloride was added dropwise to react at 0 • C for 3 h, then heated to reflux for 2 h. The mixture was washed with water (150 mL) and a saturated sodium chloride solution, successively. Finally, the organic layer was dried with Na 2 SO 4 and the solvent was removed to give 1.

Target Compounds 14
Compounds 13 (2.00 mmol) and thionyl chloride (10 mL) were added to a three-necked flask and heated to reflux for 3 h. Then, thionyl chloride was removed under reduced pressure, followed by the addition of THF (30 mL). Afterwards, the solution (2.20 mmol substituted aniline, 5.00 mmol triethylamine, 2 mL THF) was added dropwise at 0-5 • C. This was stirred overnight and purified by means of column chromatography to yield the target compounds 14a-14s.          13 13

Conclusions
In conclusion, a series of novel pyrazole-linked 1,2,4-oxadiazoles were designed by means of bioisosterism. The preliminary bioassay showed that most compounds exhibited good lethal activities against Mythimna separate, Helicoverpa armigera, Ostrinia nubilalis and Spodoptera frugiperda at 500 mg/L. Specifically, for Mythimna separate, compound 14q (70%) exhibited obvious insecticidal activity. At 50 mg/L, compound 14h (77.8%) displayed fungicidal activity against Pyricularia oryae. In addition, the acute toxicity of 14h to zebrafish embryos was 14.01 mg/L, and it was thus classified as a low-toxicity compound. Therefore, these compounds could potentially be selected as lead compounds for further studies.