Synthesis and Evaluation O-Benzyl Oxime-ether Derivatives Containing β-Methoxyacrylate Moiety for Insecticidal and Fungicidal Activities

Abstract

In attempt to lead compounds exhibiting both insecticidal and fungicidal activities, a series of O-benzyl oximeether derivatives were designed and synthesized by introducing ${\beta}$-methoxyacrylate pharmacophore into a scaffold. The insecticidal activity against Aphis fabae and the fungicidal activity against Erysiphe graminis were screened. The title compounds exhibited remarkable insecticidal and fungicidal activities. The most potent compound 6d was identified. Its insecticidal $LC_{50}$ against A. fabae is 6.4 mg/L, which is lower than that of chlorfenapyr (19.4 mg/L) and even close to the level of imidacloprid (4.8 mg/L). Its fungicidal $EC_{90}$ in preventive and curative treatment against E. graminis are 2.2 and 4.8 mg/L, respectively, which are lower than azoxystrobin (7.0 and 5.9 mg/L). These results indicate that compound 6d can be considered as a lead for further developing new O-benzyl oxime-ether typed candidates with both fungicidal and insecticidal activities.

Introduction

Oxime ether/ester derivatives with broad-spectrum of biological activities have received increasing attention recently. 12 Among them, O-benzyl oxime-ether is recognized as one of the most promising scaffolds. In the field of pesticides, O-benzyl oxime-ethers such as flucycloxuron as an insect growth regulator3 and trifloxystrobin as a fungicide have been used successfully (Figure 1).

Strobilurins and oudemansins are naturally-existing β╶ methoxyacrylate fungicides. These are two important classes of agricultural fungicides, because of their high efficacy, broad spectrum, low toxicity to mammalian cells, and environment friendly profile.4-7 Their primary action mechanism is the inhibition of mitochondrial respiration. So far, thousands of analogues have been synthesized,8-13 leading to more than 10 commercial products, such as including metominostrobin, azoxystrobin and so on (Figure 1). However, their derivatives rarely have insecticidal activity. Therefore, new types of strobilurins should be developed to overcome this problem.

A series of O-benzyl oxime-ethers (Figure 1) possessing remarkable insecticidal activity have been identified in our earlier work, but their fungicidal activity is weak.14-18 Later a series of O-benzyl oxime-ether compounds containing β╶ methoxyacrylate moiety with high fungicidal activity and certain insecticidal activity were synthsized.19 This achievement encourages us to search for novel lead compounds with both insecticidal and fungicidal activities. To continue investigation on the design and synthesis of bioactive compounds (Figure 2), the target compounds 6a-6w were designed by introducing the essential pharmacophore from the strobilurin fungicides into the O-benzyl oxime-ether scaffold to possess both insecticidal and fungicidal activities.

Figure 1.Structures of flucycloxuron, azoxystrobin, trifloxystrobin and O-benzyl oxime-ethers in our previous work.

Figure 2.Design strategy of the title compounds.

So this paper mainly reports the synthesis, fungicidal acitivity, insecticidal activity, and the structure╶activity relationship of O╶benzyl Oxime-ether containing β╶methoxyacrylate

 

Experimental

Materials and Instruments. Unless otherwise noted, reagents and solvents were purchased from commercial suppliers. 1H-NMR spectra were obtained with a Varian INOVA- 300 spectrometer using tetramethylsilane (TMS) as the internal standard and deuterated chloroform (CDCl3) as the solvent. Mass spectra (MS) were obtained with both Hewlett- Packard 6890-5973 GC/MS and Agilent 1100 Series LC/ MS. Uncorrected melting points were taken on a WRS-1A digital melting point apparatus.

General Procedure for Synthesis of Target Compounds 6a-6g, 6j-6w. Intermediates 2, 3, 4 and 6 were prepared according to the reported methods.19

A solution of compound 4 (0.01 mol) in N,N-dimethylformamide (DMF) (3 mL) was added dropwise over a period of 0.5 h to a solution of KOH (0.84 g, 0.015 mol) in DMF (20mL) at −5 to 0 ℃. The mixture was stirred at −5 to 0 ℃ for 0.5 h and then a solution of compound 5 (0.01 mol) in DMF (3 mL) was added dropwise. The new mixture was stirred at 20-25 ℃ for 10-12 h, and then poured into ice-water and extracted with ethyl ether. The combined ether extracts were washed with water, dried (anhydrous magnesium sulfate) and filtered, and the solvent was removed. The residue was separated by silica gel column chromatography with petroleum ether-ethyl acetate (12:1, v/v) as eluent, and the yields of the target compounds were 22-53%.

6h and 6i could be synthesized by the method described above from compound 2 instead of compound 4.

Structures of 6a-6w were supported by spectroscopic data shown in the supporting information.

Insecticidal Activity against Aphis fabae (Bean Aphids). The activity of compounds 6 against A. fabae (Bean Aphids) was evaluated according to the reported procedure.20 A. fabae was dipped according to a slightly modified Food and Agriculture Organization (FAO) dip test. The tender shoots of soybean with healthy apterous third-instar nymphae were dipped into the diluted solutions of the compounds for 10 s, and the superfluous fluid was removed and the nymphae were placed in an air-conditioned room. Mortality was calculated 48 h after treatment. Each treatment was performed three times. Imidacloprid and chlorfenapyr were used as standards. The data for the mortality-regression lines of the compounds were used in probit analysis. The results of the median lethal concentrations (LC50) of the derivative compounds, chlorfenapyr and imidacloprid against A. fabae are listed in Table 1.

Table 1.aYield is determined after purification by column chromatography on silica gel. bNot tested.

Fungicidal Activity against Erysiphe graminis. The fungicidal activity of compounds 6 against E. graminis was evaluated according to the reported procedure.19

 

Results and Discussion

Scheme 1.Synthetic pathways for the title compounds.

Synthesis. The synthetic route was shown in Scheme 1. Compund 2 was prepared starting from the substituted benzaldehyde 1 and hydroxylamine hydrochloride in methanol at reflux temperature in excellent yield, and then 2 reacted with tert-butyl hypochlorite in methanol at −5 to 0 ℃ to give 3. The reaction of 3 with sodium alkoxide afforded corresponding intermediates 4. Finally, the reaction of 4 with 5 in the presence of potassium hydroxide produced strobilurin derivatives 6a-6g and 6j-6w. Strobilurin derivatives of 6h and 6i were synthesized from 2 and 5 by the similar method. Their structures were confirmed by spectroscopy and LCMS. Table 1 summarizes the chemical structures and yields of compounds 6a-6w. Physical properties, MS and 1H-NMR data are shown in the supporting information.

Insecticidal Activity. The activity of compounds 6 against A. fabae is shown in Table 1. The commercial insecticide chlorfenapyr and imidacloprid were used as standards.

Most of the target compounds 6 exhibited remarkable activity against A. fabae, some of them showed high insecticidal activity. For example, compounds 6a, 6d, 6f, 6p, 6q, and 6r possessed insecticidal activity 90% at 100 mg/L. Four compounds 6a, 6d, 6p and 6r exhibited promising lethal activity against A. fabae, with LC50 lower than 15 mg/ L, which were better than chlorfenapyr (19.4 mg/L). In particular, compound 6a owned the optimal LC50 of 4.4 mg/ L, which was about equal to imidacloprid (4.48 mg/L).

Structure-Activity Relationship of Insecticidal Activity. Structural optimization of compounds 6 was carried out by modification of three primary substructures: R1, R2 and Y moieties. Variations of R1, R2 and Y moieties can significantly affect the insecticidal activity against A. fabae (Table 1).

Table 2.Preventive activities and curative actives of compounds 6d, 6s and azoxystrobin against E. graminis (14 d after treatment)

When R2 and Y were invariable, the insecticidal activity of the synthesized compounds 6 was influenced by the nature of group R1. The modification of R1 from electron-withdrawing group (CN, 6g) to a hydrogen atom (6h) and then to electron-donating groups (e.g. SCH3, 6a; OCH3, 6d; NCH3, 6f) improved the insecticidal activity. When the methyl substituents in the heteroatoms of the R1 group was changed to ethyl or isopropyl, the insecticidal activity of the corresponding compound decreased, for example, 6a > 6b, 6d > 6e, 6j > 6k, 6l > 6m and 6w > 6v (but 6u < 6t). The insecticidal activity of the corresponding compound decreased as 6a > 6d > 6f and 6l > 6n when Y was kept as CH and R2 was a moiety of p-substitute, but R1 was changed from SCH3 to OCH3 and then to NCH3. However, the insecticidal activity of 6s was equal to 6w‘s when Y was kept as CH, R2 was a moiety of m-substitute, and R1 was changed from OCH3 to SCH3.

When R1 and R2 were kept unchangeably, the insecticidal activity of the synthesized compounds was influenced by group Y. When Y was varied from CH to N, the insecticidal activity of the corresponding compound decreased. It can be shown that 6a > 6j, 6p > 6o, 6s > 6t.

Similarily, the insecticidal activity of the synthesized compounds were also influenced by the nature of group R2, when R1 and Y were kept constant. The modification of R2 from m-substitute to p-substitute increased the insecticidal activity, such as 6r > 6s. Additionally, the insecticidal activity of the corresponding compound decreased, When R2 was changed from 4-OCF3 to 4-CF3, 4-Cl, or 4-F.

Overall, when R1 was OCH3 or SCH3, most of the compounds had excellent insecticidal activity, particularly, 6a and 6d (Y is CH, R2 is 4-OCF3) possessed the optimal results.

Fungicidal Activity. Not only insecticidal activity but also fungicidal activity is disscussed. Table 1 shows the preventive activity of compounds 6 against E. graminis. The commercial strobilurin fungicides azoxystrobin was used as standard.

As shown in Table 1, all the compounds 6 exhibited high fungicidal activity against E. graminis. In particular, 6a, 6d, 6e, 6r, 6s, 6u and 6w showed fungicidal activity 80% at 6.25 mg/L, and 6d and 6s even had fungicidal activity 90% at 1.56 mg/L, while azoxystrobin only exhibited fungicidal activity of 65% at the same dose.

To further explore the more active compounds 6d and 6s, more accurate preventive and curative activities of 6d, 6s and azoxystrobin against were evaluated (Table 2). Clearly, 6d and 6s had higher preventive and curative activities than that of azoxystrobin.

Structure-Activity Relationship of Fungicidal Activity. Structural optimization of compounds 6 was also carried out by modification of three primary substructures: R1, R2 and Y moieties. Variations among R1, R2 and Y groups might greatly affect the fungicidal activity against E. graminis (Table 1).

Surprisingly, fungicidal activity and insecticidal activity have many similarities in structure-activity relationship: (1) When R2 and Y were invariable, the modification of R1 from electron-withdrawing group (CN, 6g) to a hydrogen atom (6h) and then to electron-donating groups (e.g. SCH3, 6a; OCH3, 6d; NCH3, 6f) enhanced the fungicidal activity; (2) When the methyl substituents in the heteroatoms of the R1 group was changed from methyl to ethyl or isopropyl, the fungicidal activity of the corresponding compound decreased; (3) When R1 and R2 were kept constant and Y was changed from CH to N, the fungicidal activity of the corresponding compound decreased.

However, fungicidal activity also had its peculiarity in structure-activity relationship: When R2 and Y were kept constant, and R1 was changed from OCH3 to SCH3 and then to NCH3, the fungicidal activity of the corresponding compound decreased, for example, 6d > 6a > 6f, 6q > 6o, 6s > 6w.

On the whole, when Y was CH and R1 was OCH3, most of the compounds also had good fungicidal activity, and especially, 6d (R2 is 4-OCF3) and 6s (R2 is 3-CF3) possessed the optimal results

 

Conclusions

We successfully found that introduction of the β-methoxyacrylate pharmacophore lead O-benzyl oxime-ethers derivatives to exhibit both insecticidal and fungicidal activities. The insecticidal LC50 of the compounds 6a, 6d, 6p and 6r against A. fabae were 4.4, 6.4, 14.8 and 14.6 mg L−1 respectively, which were all lower than that of chlorfenapyr (19.4 mg/L). In particular, compound 6a possessed the optimal LC50 of 4.4 mg/L, which was about equal to imidacloprid (4.8 mg/L). Meanwhile, compound 6d displayed high fungicidal activities in preventive and curative treatment against E. graminis with EC90 values of 2.2 and 4.8 mg/L, respectively, which were even better than that of azoxystrobin. These results indicate that 6d can be used as a lead compound for further developing new O-benzyl oximeether type candidates with both fungicidal and insecticidal activities.