Synthesis and insecticidal activity studies of novel phenylpyrazole derivatives containing arylimine or carbimidate moiety

Bioorganic & Medicinal Chemistry 27 (2019) 115092

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Synthesis and insecticidal activity studies of novel phenylpyrazole derivatives containing arylimine or carbimidate moiety

T

Qiqi Zhaoa,c, Ranfeng Sunb, , Yuxiu Liua, Peiqi Chenb, Yongqiang Lia, , Shaoxiang Yangd, , ⁎ Qingmin Wanga, ⁎

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a

State Key Laboratory of Elemento-Organic Chemistry, College of Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, People’s Republic of China b Key Laboratory of Green Prevention and Control of Tropical Plant Diseases and Pests, Ministry of Education, College of Plant Protection, Hainan University, Haikou 570228, People’s Republic of China c Patent Examination Cooperation Center of the Patent Office, SIPO, Beijing 100081, People’s Republic of China d Beijing Key Laboratory of Flavor Chemistry, Beijing Technology and Business University, Beijing 100081, People’s Republic of China

ARTICLE INFO

ABSTRACT

Keywords: Phenylpyrazole Gamma-aminobutyric acid (GABA) receptor Insecticidal activity Arylimine

Phenylpyrazole insecticides are successful for crop protection and public hygiene by blocking gamma-aminobutyric acid (GABA)-gated chloride channels and glutamate-gated chloride (GluCl) channels. A series of novel phenylpyrazoles containing arylimine or 1-methoxyaryl groups were designed and synthesized. The addition reaction of methanol to the imines 1–11 was investigated and the cayno addition products 13–15 were obtained. The compounds 1–15 were confirmed by 1H NMR and elemental analysis. The results of bioassay indicated that some compounds exhibited comparable bioactivity to fipronil against a broad spectrum of insects such as bean aphid (Aphis craccivora), mosquito (Culex pipiens pallens), diamondback moth (Plutella xylostella) and Oriental armyworm (Mythimna separata). Especially, the foliar contact activity against bean aphid of compound 7 at 10 µg mL−1 was 68%, the larvacidal activity against mosquito of compounds 5, 13 and 15 at 0.0025 µg mL−1 was 100%, the larvacidal activity against diamondback moth of compounds 9 and 11 at 0.05 µg mL−1 was 100%, the larvacidal activity against Oriental armyworm of compound 9 at 1 µg mL−1 was 100%. The 3-cayno moiety on pyrazole ring was essential for the high insecticidal activities against bean aphid, diamondback moth and Oriental armyworm, while the 3-carbimidate moiety on pyrazole ring was crucial to the excellent high insecticidal activities against mosquito.

1. Introduction Fipronil (A) and ethiprole (B) (Fig. 1), which disrupt the insect central nervous system by blocking gamma-aminobutyric acid (GABA)gated chloride channels and glutamate-gated chloride (GluCl) channels, are successful phenylpyrazole insecticides for crop protection and public hygiene.1–5 However, their high toxicity on crustaceans,6 fishes5 and bees7 greatly limited their applications, so fipronil has been banned from use on maize and sunflower seeds to protect honey bees in European Union (EU) since the end of 20138 and it has also been forbidden for agricultural use in China since October 1, 2009.9,10 In order to reduce the aquatic toxicity of fipronil, vaniliprole (C),11 pyrafluprole (D),12 pyriprole (E)13 and flufiprole (F)14 (Fig. 1) were developed by agrochemical companies. Phloem-mobile insecticides are efficient to control piercing-sucking insects. Introduction of

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monosaccharide or amino acid to the chemical structure of fipronil to improve its phloem mobility was reported by Professor Xu’s group from South China Agricultural University.15–20 A series of phenylpyrazoles containing a 2,2,2-trichloro-1-alkoxyethyl moiety (G, Fig. 2) were reported by our group and these compounds exhibited excellent activities against a broad spectrum of insects such as bean aphid (Aphis craccivora), mosquito (Culex pipiens pallens) and diamondback moth (Plutella xylostella).21 As a further investigation on the insecticidal activities of the similar phenylpyrazoles, a series of phenylpyrazoles (H, Fig. 2) were designed. Unfortunately, only one desired compound 12 (Fig. 3) was obtained during the synthesis process and compounds 13–15 (Fig. 3) were separated from the reaction solutions unintentionally. Then the insecticidal activities of compounds 1–15 (Fig. 3) against bean aphid (Aphis craccivora), mosquito (Culex pipiens pallens), diamondback moth (Plutella xylostella) and oriental

Corresponding authors. E-mail addresses: [email protected] (R. Sun), [email protected] (Y. Li), [email protected] (S. Yang), [email protected] (Q. Wang).

https://doi.org/10.1016/j.bmc.2019.115092 Received 22 July 2019; Received in revised form 18 August 2019; Accepted 6 September 2019 Available online 09 September 2019 0968-0896/ © 2019 Elsevier Ltd. All rights reserved.

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armyworm (Mythimna separata) were tested and evaluated. 2. Materials and methods 2.1. Instruments 1 H NMR spectra were obtained at 300 MHz using a Bruker AV300 spectrometer or at 400 MHz using a Varian Mercury Plus400 spectrometer in CDCl3 or DMSO‑d6 solution with tetramethylsilane as the internal standard. Chemical shift values (δ) are given in ppm. Elemental analyses were determined on a Yanaca CHN Corder MT-3 elemental analyzer. The melting points were determined on an X-4 binocular microscope melting point apparatus (Beijing Tech Instruments Co., Beijing, China) and are uncorrected. Yields were not optimized.

2.2. Synthesis All anhydrous solvents as well as sulfur chloride were dried and purified by standard techniques just before use. The synthetic route is given in Scheme 1.

Fig. 1. Chemical structures of commercial phenylpyrazole insecticides.

2.2.1. General synthesis of compounds 1–11 Compounds I and J were prepared according to the method in the our previous work as shown in Scheme 1.21 A mixture of compound I or compound J (10 mmol), 60 mL toluene, p-toluenesulfonic acid (PTSA, 0.25 g, 1.3 mmol) and aromatic aldehyde (12 mmol) was heated and refluxed for 5–8 h by using a Dean-Stark trap. Then the reaction was cooled to room temperature and successively washed by saturated aqueous NaHCO3 (30 mL), water (30 mL) and saturated aqueous NaCl (30 mL). The organic layer was dried with anhydrous Na2SO4 and the solvent was removed under vacuum after filtration. Finally the residue was purified by silica gel column chromatography (EtOAc/petroleum ether) to afford compounds 1–11 as yellow or pale yellow solids. 2.2.1.1. (E)-5-(Benzylideneamino)-1-(2,6-dichloro-4-(trifluoromethyl) phenyl)-4-(methylthio)-1H-pyrazole-3-carbonitrile (1). Pale yellow solid, mp 159–161 °C, yield = 71%. 1H NMR (CDCl3) δ 9.07 (s, 1H, CH=N), 7.77 (d, 3JHH = 7.2 Hz, 2H, Ph), 7.75 (s, 2H, Ph), 7.54 (t, 3 JHH = 7.2 Hz, 1H, Ph), 7.45 (t, 3JHH = 7.2 Hz, 2H, Ph), 2.50 (s, 3H, CH3). Anal. Calcd for C19H11Cl2F3N4S: C, 50.12; H, 2.44; N, 12.31. Found: C, 50.12; H, 2.27; N, 12.21.

Fig. 2. Design strategies of target phenylpyrazole derivatives.

2.2.1.2. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((furan-2ylmethylene)amino)-4-(methylthio)-1H-pyrazole-3-carbonitrile (2). Pale yellow solid, mp 140–142 °C, yield = 74%. 1H NMR (CDCl3) δ 8.84 (s, 1H, CH=N), 7.73 (s, 2H, Ph), 7.65 (s, 1H, furan), 7.10 (d, 3 JHH = 3.2 Hz, 1H, furan), 6.61–6.56 (m, 1H, furan), 2.48 (s, 3H, CH3). Anal. Calcd for C17H9Cl2F3N4OS: C, 45.86; H, 2.04; N, 12.58. Found: C, 46.31; H, 1.76; N, 12.13. 2.2.1.3. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-4-(methylthio)-5((pyridin-2-ylmethylene)amino)-1H-pyrazole-3-carbonitrile (3). Pale yellow solid, mp 132–133 °C, yield = 69%. 1H NMR (CDCl3) δ 9.21 (s, 1H, CH=N), 8.74 (d, 3JHH = 4.8 Hz, 1H, pyrdine), 7.87 (d, 3 JHH = 7.6 Hz, 1H, pyridine), 7.79–7.73 (m, 3H, pyridine and Ph), 7.44–7.39 (m, 1H, pyridine), 2.54 (s, 3H, CH3). Anal. Calcd for C18H10Cl2F3N5S: C, 47.38; H, 2.21; N, 15.35. Found: C, 47.42; H, 2.20; N, 15.10. 2.2.1.4. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-4-(methylthio)-5((4-nitrobenzylidene)amino)-1H-pyrazole-3-carbonitrile (4). Yellow solid, mp 188–190 °C, yield = 68%. 1H NMR (CDCl3) δ 9.21 (s, 1H, CH=N), 8.74 (d, 3JHH = 4.8 Hz, 1H, pyrdine), 7.87 (d, 3JHH = 7.6 Hz, 1H, pyridine), 7.79–7.73 (m, 3H, pyridine and Ph), 7.44–7.39 (m, 1H, pyridine), 2.54 (s, 3H, CH3). Anal. Calcd for C18H10Cl2F3N5S: C, 47.38; H, 2.21; N, 15.35. Found: C, 47.42; H, 2.20; N, 15.10.

Fig. 3. Chemical structures of phenylpyrazole derivatives 1–15.

2

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Scheme 1. General synthetic route for compounds 1-15.

pyridine), 7.90 (d, 3JHH = 7.2 Hz, 1H, pyridine), 7.83 (d, 4 JHH = 1.2 Hz, 1H, Ph), 7.80 (td, 3JHH = 7.6 Hz, 4JHH = 1.2 Hz, 1H, pyridine), 7.76 (d, 4JHH = 1.2 Hz, 1H, Ph), 7.50–7.45 (m, 1H, pyridine). Anal. Calcd for C18H7Cl2F6N5OS: C, 41.08; H, 1.34; N, 13.31. Found: C, 41.18; H, 1.39; N, 13.32.

2.2.1.5. (E)-5-(Benzylideneamino)-1-(2,6-dichloro-4-(trifluoromethyl) phenyl)-4-(ethylthio)-1H-pyrazole-3-carbonitrile (5). Pale yellow solid, mp 104–105 °C, yield = 79%. 1H NMR (CDCl3) δ 9.17 (s, 1H, CH=N), 7.79–7.72 (m, 4H, Ph), 7.54 (t, 3JHH = 7.2 Hz, 1H, Ph), 7.45 (t, 3JHH = 7.2 Hz, 2H, Ph), 2.88 (q, 3JHH = 7.2 Hz, 2H, CH2), 1.24 ((t, 3 JHH = 7.2 Hz, 3H, CH3). Anal. Calcd for C20H13Cl2F3N4S: C, 51.18; H, 2.79; N, 11.94. Found: C, 51.19; H, 2.71; N, 11.74.

2.2.1.11. 1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((E)-((E)-3phenylallylidene)amino)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3carbonitrile (11). Pale yellow solid, mp 148–150 °C, yield = 76%. 1H NMR (CDCl3) δ 8.70 (d, 3JHH = 9.2 Hz, 1H, CH=N), 7.81 (s, 1H, Ph), 7.75 (s, 1H, Ph), 7.55 (d, 3JHH = 7.6 Hz, 2H, Ph), 7.48–7.38 (m, 4H, Ph and PhCH=CH), 6.96–6.87 (m, 1H, PhCH=CH). HRMS (ESI) calcd for C21H10Cl2F6N4OS [M + Na]+, 572.9754; found, 572.9720.

2.2.1.6. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-4(methylsulfinyl)-5-((pyridin-2-ylmethylene)amino)-1H-pyrazole-3carbonitrile (6). Pale yellow solid, mp 242–245 °C, yield = 25%. 1H NMR (CDCl3) δ 8.88 (s, 1H, CH=N), 8.76 (d, 3JHH = 4.8 Hz, 1H, pyridine), 7.91 (d, 3JHH = 7.2 Hz, 1H, pyridine), 7.82–7.73 (m, 4H, pyridine and Ph), 7.49–7.43 (m, 1H, Ph), 3.19 (s, 3H, CH3). Anal. Calcd for C18H10Cl2F3N5OS: C, 45.78; H, 2.13; N, 14.83. Found: C, 45.71; H, 2.30; N, 14.69.

2.2.2. General synthesis of compounds 12–15 A solution of compounds 7–11 (1 mmol) in absolute methanol (15 mL) was heated and refluxed. After the reaction was complete (monitored by Thin Layer Chromatography (TLC)), the reaction was cooled to room temperature and the solvent was removed under vacuum. Then the residue was purified by silica gel column chromatography (EtOAc/petroleum ether) to afford compounds 12–15 as white solids.

2.2.1.7. (E)-5-(Benzylideneamino)-1-(2,6-dichloro-4-(trifluoromethyl) phenyl)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3-carbonitrile (7). Pale yellow solid, mp 164–165 °C, yield = 61%. 1H NMR (CDCl3) δ 8.90 (s, 1H, CH=N), 7.83 (d, 4JHH = 1.6 Hz, 1H, Ph), 7.79–7.74 (m, 3H, Ph), 7.61 (t, 3JHH = 7.2 Hz, 1H, Ph), 7.47 (t, 3JHH = 7.2 Hz, 2H, Ph). Anal. Calcd for C19H8Cl2F6N4OS: C, 43.45; H, 1.54; N, 10.67. Found: C, 43.51; H, 1.64; N, 10.61.

2.2.2.1. 1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((methoxy(4nitrophenyl)methyl)amino)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3carbonitrile (12). White solid, mp 143–145 °C, yield = 97%. 1H NMR (CDCl3) δ 8.24 (d, 3JHH = 8.4 Hz, 2H, Ph), 7.82 (s, 1H, Ph), 7.76 (s, 1H, Ph), 7.44 (d, 3JHH = 8.4 Hz, 2H, Ph), 6.73 (d, 3JHH = 8.4 Hz, 1H, CH), 5.05 (d, 3JHH = 8.4 Hz, 1H, NH), 2.94 (s, 3H, CH3). Anal. Calcd for C20H11Cl2F6N5O4S: C, 39.88; H, 1.84; N, 11.63. Found: C, 40.08; H, 2.04; N, 11.47.

2.2.1.8. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((4nitrobenzylidene)amino)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3carbonitrile (8). Yellow solid, mp 146–147 °C, yield = 64%. 1H NMR (CDCl3) δ 9.18 (s, 1H, CH=N), 8.30 (d, 3JHH = 8.4 Hz, 2H, Ph), 7.93 (d, 3 JHH = 8.4 Hz, 2H, Ph), 7.86 (s, 2H, Ph), 7.78 (s, 2H, Ph). Anal. Calcd for C19H7Cl2F6N5O3S: C, 40.02; H, 1.24; N, 12.28. Found: C, 39.92; H, 1.32; N, 12.05.

2.2.2.2. (E)-Methyl 5-(benzylideneamino)-1-(2,6-dichloro-4-(trifluoromethyl) phenyl)-4-((trifluoromethyl) sulfinyl)-1H-pyrazole-3-carbimidate (13). White solid, mp 104–105 °C, yield = 93%. 1H NMR (CDCl3) δ 8.98 (s, 1H, CH), 8.66 (s, 1H, NH), 7.80–7.70 (m, 4H, Ph), 7.54 (t, 3JHH = 7.2 Hz, 1H, Ph), 7.43 (t, 3JHH = 7.2 Hz, 2H, Ph), 4.00 (s, 3H, CH3). Anal. Calcd for C20H12Cl2F6N4O2S: C, 43.10; H, 2.17; N, 10.05. Found: C, 43.04; H, 2.08; N, 10.06.

2.2.1.9. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((furan-2ylmethylene)amino)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3carbonitrile (9). Pale yellow solid, mp 119–120 °C, yield = 64%. 1H NMR (CDCl3) δ 8.70 (s, 1H, CH=N), 7.81 (s, 2H, Ph), 7.74 (s, 1H, Ph), 7.70 (s, 1H, furan), 7.23 (d, 3JHH = 3.6 Hz, 1H, furan), 6.64–6.61 (m, 1H, furan). HRMS (ESI) calcd for C17H6Cl2F6N4O2S [M + Na]+, 536.9390; found, 536.9388.

2.2.2.3. (E)-Methyl 1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-5-((furan2-ylmethylene)amino)-4-((tri fluoromethyl)sulfinyl)-1H-pyrazole-3carbimidate (14). White solid, mp 152–154 °C, yield = 85%. 1H NMR (CDCl3) δ 8.79 (s, 1H, CH), 8.63 (s, 1H, NH), 7.76 (s, 1H, Ph), 7.74 (s, 1H, Ph), 7.63 (s, 1H, furan), 7.12 (d, 3JHH = 3.6 Hz, 1H, furan),

2.2.1.10. (E)-1-(2,6-Dichloro-4-(trifluoromethyl)phenyl)-5-((pyridin-2ylmethylene)amino)-4-((trifluoromethyl)sulfinyl)-1H-pyrazole-3carbonitrile (10). Pale yellow solid, mp 140–141 °C, yield = 59%. 1H NMR (CDCl3) δ 98.85 (s, 1H, CH=N), 8.77 (d, 3JHH = 4.8 Hz, 1H, 3

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disks were placed individually into glass tubes. Each dried treated leaf disk was infested with seven third-instar diamondback moth larvae. Percentage mortalities were evaluated 3 days after treatment. Leaves treated with water and dimethylformamide were provided as controls. Each treatment was performed three times. 2.3.4. Stomach toxicity against oriental armyworm (Mythimna separata) The stomach toxicities of compounds 1–15 and the contrast fipronil against oriental armyworm were evaluated by foliar application using the reported procedure.28 For the foliar armyworm tests, individual corn leaves were placed on moistened pieces of filter paper in Petri dishes. The leaves were then sprayed with the test solution and allowed to dry. The dishes were infested with 10 fourth-instar Oriental armyworm larvae. Percentage mortalities were evaluated 3 days after treatment. Each treatment was performed three times.

Scheme 2. Reduction of sulfinyl group to sulfide.

6.58–6.55 (m, 1H, Ph), 3.99 (s, 3H, CH3). Anal. Calcd for C18H10Cl2F6N4O3S: C, 39.50; H, 1.84; N, 10.24. Found: C, 39.40; H, 1.78; N, 10.31. 2.2.2.4. Methyl 1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-5-((E)-((E)-3phenylallylidene)amino)-4-((tri fluoromethyl)sulfinyl)-1H-pyrazole-3carbimidat (15). White solid, mp 104–107 °C, yield = 64%. 1H NMR (CDCl3) δ 8.75 (d, 3JHH = 8.8 Hz, 1H, CH=N), 8.64 (s, 1H, NH), 7.77 (s, 1H, Ph), 7.75 (s, 1H, Ph), 7.53–7.51 (m, 2H, Ph and PhCH=CH), 7.42–7.33 (m, 4H, Ph), 6.91–6.82 (m, 1H, PhCH=CH). Anal. Calcd for C22H14Cl2F6N4O2S: C, 45.30; H, 2.42; N, 9.60. Found: C, 45.16; H, 2.41; N, 9.46.

3. Results 3.1. Synthesis Different substituted 5-aminopyrazole (compounds I and J) and aromatic aldehydes in toluene are refluxed in the presence of the catalytic p-toluenesulfonic acid to produce a variety of 5-position imine 1phenylpyrazole compounds 1–11 as shown in Scheme 1. We also attempted to use 5-amino-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4(methylsulfinyl)-1Hpyrazole-3-carbonitrile (K) and p-nitrobenzaldehyde to give the corresponding imine, however, the reaction is complex and reduction product 1 was obtained in 20% yield (Scheme 2). Although acetic anhydride29 and acetyl chloride30 have been reported to be used as reductants to finish the reduction of fipronil to fipronil sulfide, the aryl aldehyde used as a redutant has not been discussed. At present the yield of the reduction reaction is low (about 20%), but it is worthy to carry out a further investigation on this new reaction. We have tried to perform the addition reaction of methanol to imine compounds 1–4 (Fig. 2) as shown in Scheme 1, but no desired products were obtained. According to the literature,31,32 the electronwithdrawing groups on the aromatic ring of imine favor the addition reaction. Then we tried to use the imine compound 10 (Fig. 2) to perform the addition reaction with methanol, compound 12 was obtained successfully (Scheme 1). The structure of the compound 12 was confirmed by 1H NMR and elemental analysis. When we attempt to perform the addition reaction of methanol to imine compounds 7, 8 and 11 (Scheme 3), desired products were not obtained, however, the cyano addition products 13–15 which were not reported in the literature were separated from the reaction solution. Although the addition reaction of methanol to the cyano group under acidic33 or alkaline34,35 conditions has been described in the literatures, the addition reaction completed in methanol without other reagents was discovered for the first time. Unfortunately, the addition reaction of methanol to the cyano group did not occur when the other 3-cyano-1phenylpyrazoles was refluxed in methanol such as compounds 1–6 (Fig. 2). It can be seen that the change of a substituent on the pyrazole ring significantly influences reactivity of other substituents, and further

2.3. Biological assay All bioassays were performed on representative test organisms reared in the laboratory. The bioassay was repeated at 25 ± 1 °C according to statistical requirements. Assessments were made on a dead/ alive basis and mortality rates were corrected using Abbott’s formula.22 Evaluations are based on a percentage scale of 0–100 in which 0 = no activity and 100 = total kill. 2.3.1. Foliar contact activity against bean aphid (Aphis craccivora) The foliar contact activities of compounds 1–15 and fipronil against bean aphid were tested according to a reported procedure.23–25 Stock solutions of each test sample was prepared in dimethylformamide at a concentration of 200 µg mL−1 and then diluted to the required concentration with water containing TW-20. Tender shoots of soybean with 60 insects of each species were dipped in the diluted solutions of the chemicals for 5 s, then the superfluous liquorwas removed, and they were kept in the conditioned room for normal cultivation. The mortality was evaluated by the number of live larvae in the treated bottles relative to that in the untreated controls after 24 h. Controls were performed under the same conditions. Each test was performed in triplicate. 2.3.2. Toxicity against mosquito (Culex pipiens pallens) The toxicities of compounds 1–15 against mosquito were evaluated according to the reported procedure.21,26 One milliliter of different concentrated dilutions of each compound was added to 99 mL of water to obtain different concentrations of tested solution. Then 20 fourthinstar mosquito larvae were put into the solution. Percentage mortalities were evaluated 1 day after treatment. For comparative purposes, fipronil was tested under the same conditions, and each test was performs in triplicate. 2.3.3. Stomach toxicity against diamondback moth (Plutella xylostella) The stomach toxicities of compounds 1–15 and the contrast fipronil against diamondback moth were tested by the leafdip method using the reported procedure.27 A stock solution of each test sample was prepared in dimethylformamide at a concentration of 200 µg mL−1 and then diluted to the required concentration with water containing TW-20. Leaf disks (6 cm × 2 cm) were cut from fresh cabbage leaves and then were dipped into the test solution for 3 s. After air-drying, the treated leaf

Scheme 3. Cyano addition reaction of imines 7–11. 4

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the contrast fipronil against mosquito. The bioassay results indicated that most compounds exhibited excellent larvicidal activities against mosquito, especially compounds 5, 13 and 15 which showed 100% mortality even at 0.0025 µg mL−1. The structure–activity relationship of cayno addition products (compounds 13–15) and the corresponding imines (compounds 7, 8 and 11 respectively) against mosquito is just the opposite of that aganist bean aphids, for example, the cayno addition products 13–15 exhibited much better larvicidal activities than that of the corresponding imines (compounds 7, 8 and 11 respectively).

Table 1 Foliar contact activities against bean aphid of compounds 1–15 and fipronil. Compd

Larvicidal activity (%) at concn (µg mL−1) 200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fipronil a

46 ± 100 100 100 100 100 100 84 ± 100 100 100 100 19 ± 15 ± 0 100

100 a

2

– 100 100 100 100 100 100 – 100 100 100 100 – – – 100

2

1 2

50

25

– 100 100 27 ± 69 ± 100 100 – 87 ± 88 ± 71 ± 100 – – – 89 ±

– 71 ± 91 ± – – 28 ± 100 – 73 ± 0 43 ± 68 ± – – – 78 ±

2 2

2 3 1

3

10 3 2 3 1 2 2

2

– – 42 – – – 68 – 24 – – – – – – 31

±2

3.2.3. Stomach toxicity against diamondback moth (Plutella xylostella) Table 3 showed that some compounds exhibited excellent larvicidal activity against diamondback moth, for example, the larvicidal activities of compounds 1 and 2 were 100% at 5 µg mL−1, meanwhile, compounds 7–11 caused 100% mortality at 0.5 µg mL−1. However, the cayno addition products 13–15 exhibited 88–100% morality against diamondback moth at 200 µg mL−1 but the insecticidal activity decreased rapidly (only 27% and 77% mortality for compounds 14 and 15 at 100 µg mL−1). The bioassy results indicated that the 3-position cyano group of phenylpyrazoles was crucial to the larvicidal activity against diamondback moth.

±3 ±2

±2

No test data.

research is still underway.

3.2.4. Stomach toxicity against oriental armyworm (Mythimna separata) The bioactivity results indicated that some compounds exhibited excellent activities against Oriental armyworm as shown in Table 4, for example the activities of compounds 9 and 12 were 100% and 80% at 1 µg mL−1, while the activity of fipronil was 80% at the same concentration. Generally, the sulfenyl compounds 6–11 were more potent than the sulfinyl compounds 1–5, for example, sulfenyl compound 6 and the corresponding sulfinyl compound 3 exhibited 100% and 60% mortality at 20 µg mL−1. Moreover, a small change in structure of compounds could lead to a remarkable change of activity, for instance, the cayno addition products 13–15 exhibited no detectable activity against Oriental armyworm at 100 µg mL−1, while compounds 7, 9 and 11 caused 100% mortality at 2.5 µg mL−1.

3.2. Bioassays 3.2.1. Foliar contact activity against bean aphid (Aphis craccivora) It can be seen from Table 1 that some compounds exhibited high larvicidal activity against bean aphid, for example, the larvicidal activities of compounds 3 and 7 were 91% and 100% respectively at 25 µg mL−1, whereas fipronil caused only 78% mortality at the same concentration. At the same time, electron-withdrawing groups on phenyl ring of imine phenylpyrazole derivatives were unfavorable for the insecticidal activities, for example, compound 7 (phenyl group) displayed better insecticidal activities than that of compound 10 (4nitro-phenyl group). The addition product 12 of imine 8 exhibited 100% mortality at 50 µg mL−1, which was much higher than that of the corresponding imine 8 (only 84% mortality at 200 µg mL−1). However, the larvicidal activities of the cayno addition products 13–15 are much lower than that of the corresponding imines (compounds 7, 8 and 11), which indicated that the 3-position cyano group of phenylpyrazoles was very important to the larvicidal activity against bean aphid.

4. Discussion In summary, a series of novel phenylpyrazole containing imine and its addition products were designed and synthesized. The addition reaction of methanol to the imine compounds 1–11 was attempted and discussed. Unfortunately, only one desired imine addition product 12 was obtained and the cayno addition products 13–15 were prepared by accident. The results of bioassays indicated that some compounds

3.2.2. Toxicity against mosquito (Culex pipiens pallens) Table 2 showed the larvacidal activities of the compounds 1–15 and Table 2 Larvacidal activities against mosquito of compounds 1–15 and fipronil. Compd

Larvicidal activity (%) at concn (µg mL−1) 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fipronil a

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

1 100 100 100 100 100 100 100 100 100 100 100 70 ± 2 100 100 100 100

0.5 100 100 100 30 ± 2 100 100 100 100 100 100 40 ± 2 – 100 100 100 100

0.25 100 100 100 – 100 100 20 ± 4 100 100 100 – – 100 100 100 100

0.1

0.05

100 100 100 – 100 100 – 50 ± 1 100 100 – – 100 100 100 100

100 100 100 – 100 50 ± 2 – – 60 ± 2 100 – – 100 100 100 100

No test data. 5

0.025 30 ± 50 ± 20 ± – 100 – – – – 80 ± – – 100 100 100 100

0.01 2 3 3

5

a

– – – – 100 – – – – – – – 100 70 ± 2 100 50 ± 2

0.005

0.0025

0.001

– – – – 100 – – – – – – – 100 – 100 –

– – – – 100 – – – – – – – 100 – 100 –

– – – – 0 – – – – – – – 20 ± 1 – 30 ± 2 –

Bioorganic & Medicinal Chemistry 27 (2019) 115092

Q. Zhao, et al.

Table 3 Larvacidal activities against diamondback moth of compounds 1–15 and fipronil. Compd

Larvicidal activity (%) at concn (µg mL−1) 200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fipronil a

100

100 100 100 100 100 100 100 100 100 100 100 100 88 ± 3 100 100 100

50

100 100 – 100 100 100 100 100 100 100 100 100 – 26 ± 2 77 ± 2 100

100 100 – 100 72 ± 2 100 100 100 100 100 100 100 – – – 100

25

10

100 100 – 87 ± 2 – 63 ± 3 100 100 100 100 100 100 – – – 100

100 100 – 28 ± 2 – 41 ± 2 100 100 100 100 100 100 – – – 100

5 100 100 – – – – 100 100 100 100 100 100 – – – 100

58 ± 3 75 ± 1 – – – – 100 100 100 100 100 100 – – – 100

0.5 a

– – – – – – 100 100 100 100 100 100 – – – 100

0.25

0.1

0.05

0.025

– – – – – – 100 100 100 100 100 70 ± 2 – – – 100

– – – – – – 78 ± 74 ± 100 86 ± 100 38 ± – – – 100

– – – – – – – 43 ± 3 100 29 ± 2 100 – – – – 100

– – – – – – – – – – – – – – – 53 ± 2

4 2 3 5

No test data.

(kyqd1640) and the Open Project Program of Beijing Key Laboratory of Flavor Chemistry of Beijing Technology and Business University (SPFW2018-YB05).

Table 4 Larvacidal activities against oriental armyworm of compounds 1–15 and fipronil. Compd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Fipronil a

2.5

Larvicidal activity (%) at concn (µg mL−1) 100

50

20

10

5

2.5

1

100 100 100 100 100 100 100 100 100 100 100 100 0 0 0 100

100 60 ± 1 60 ± 1 100 100 100 100 100 100 100 100 100 – – – 100

100 – – 80 ± 1 100 100 100 100 100 100 100 100 – – – 100

0 – – – 0 0 100 100 100 10 ± 1 100 100 – – – 100

–a – – – – – 100 100 100 – 100 100 – – – 100

– – – – – – 100 100 100 – 100 100 – – – 100

– – – – – – 0 0 100 – 0 80 ± 2 – – – 80 ± 2

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No test data.

possessed excellent activities against a broad spectrum of insects such as bean aphid, mosquito, diamondback moth and Oriental armyworm, even much higher than the contrast fipronil. In particular, the foliar contact activity against bean aphid of compound 7 at 10 µg mL−1 was 68%, the larvacidal activity against mosquito of compounds 5, 13 and 15 at 0.0025 µg mL−1 was 100%, the larvacidal activity against diamondback moth of compounds 9 and 11 at 0.05 µg mL−1 was 100%, the larvacidal activity against Oriental armyworm of compound 9 at 1 µg mL−1 was 100%, and all of these activities were as the same as or higher than that of the contrast fipronil. Moreover, compounds 1–15 exhibited the same structure-activity relationship of diamondback moth and oriental armyworm, and both of them are of the order Lepidoptera. The results of insecticidal activities also suggested that the structural requirement varied for different insect species, for instance, compound 13 exhibited excellent activity against mosquito but relatively low activity against bean aphid, diamondback moth and Oriental armyworm. Acknowledgments This work was financially supported by the National Science Foundation of China (21462028, 21672117, 21772104), Startup Foundation for Outstanding Young Scientists of Hainan University 6

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