Synthesis and Bioactivities Study of Novel Pyridylpyrazol Amide Derivatives Containing Pyrimidine Motifs

Introduction

Plant fungal and insect diseases have posed serious threats to crops in the world and caused a severe loss throughout the world (Strange and Scott, 2005; Yang et al., 2015). Nowadays, some of the available traditional fungicides and insecticides, such as Kresoxim-methyl, Pyrimethanil, Chlorantraniliprole, etc., are widely used to prevent plant harmful fungal and insect diseases. However, prolonged use of traditional pesticides can not only lead to drug resistance, but also have a harmful influence on the safety of the plants and the environment. Therefore, the development of novel and promising fungicides and insecticides is still an urgent task.

Pyrimidine derivatives, which play an important role in synthesis of various active molecules, have versatile properties in modern life, such as antifungal (Chen et al., 2008; Zhang et al., 2016), antibacterial (Triloknadh et al., 2018; Fang et al., 2019), insecticidal (Liu et al., 2017; Shen et al., 2018), herbicidal (Chen et al., 2015; Li et al., 2018), and antiviral (Xu et al., 2015; Wang Y. Y. et al., 2018) activities. In the previous work, some of the pyrimidine derivatives (for example, Mepanipyrim, Pyrimethanil, Diflumetorim, Azoxystrobin, and so on), which were known for their abilities to control severe fungal diseases, have been marketed as commercial pesticides worldwide. Meanwhile, in our preliminary work, several series of pyrimidine derivatives containing 1,3,4-oxadiazole (Figures 1A,E), 1,3,4-thiadiazole (Figures 1B,E), 1,2,4-triazole (Figure 1C) or amide (Figure 1D) moiety, as shown in Figure 1, were reported and revealed better antiviral, antifungal, and antibacterial activities (Wu et al., 2015, 2016a,b, 2019a,b).

FIGURE 1

Figure 1. The structures of the pyrimidine derivatives containing 1,3,4-oxadiazole (A,E), 1,3,4-thiadiazole (B,E), 1,2,4-triazole (C) or amide (D) moiety reported by our research group.

In recent years, pyridylpyrazole amide derivatives have attracted more and more considerable attention owing to their broad class of biological activities in pesticide chemistry, such as antifungal (Yan et al., 2012; Wang B. L. et al., 2018) and insecticidal (Wang et al., 2013, 2019; Shi et al., 2017; Wang B. L. et al., 2018) activity. In the past few years, some representative examples of pyridylpyrazole amide derivatives (for example, chlorantraniliprole and cyantraniliprole) were commercialized as pesticides, as shown in Figure 2.

FIGURE 2

Figure 2. Structures of chlorantraniliprole and cyantraniliprole.

To develop effective pesticide agents, we aim to introduce the pyrimidine ring to the pyridiylpyrazol amide skeleton to design a series of novel pyridiylpyrazol amide derivatives containing a pyrimidine moiety (Figure 3). As far as we know, it is the first report on the antifungal and insecticidal activities of pyridylpyrazol amide derivatives containing a pyrimidine moiety.

FIGURE 3

Figure 3. Design strategy for title compounds 6a–6m.

Experimental and Methods

General Information

JEOL-ECX 500 NMR spectrometer (JEOL, Tokyo, Japan) was used to analyse the NMR spectral (¹H NMR and ¹³C NMR) at room temperature using TMS as an internal standard and DMSO-d₆ as the solvent. Elemental analysis was performed on the Elementar Vario-III CHN analyser (Elementar, Hanau, Germany). Mass spectral were conducted on the Agilent 5973 organic mass spectrometer (Agilent Technologies, Palo Alto, CA, USA). Melting points were determined on the XT-4 binocular microscope (Beijing Tech Instrument Co., China). All commercial reagents and solvents were used as they did not require any purification before use.

Synthesis

Preparation Procedure of the Key Intermediate 5

The synthetic procedure for the key intermediate 5 is shown in Scheme 1. To a mixture of 2,3-dichloropyridine (0.1 mol) dissolved in anhydrous ethanol (120 mL), 80% hydrazine hydrate (80 mL) was added dropwise and reacted under reflux. Upon completion of the reaction, the reaction solution was cooled to room temperature and the solvent was removed under reduced pressure. The residue was washed with water and recrystallized with ethanol to gain intermediate 1. Intermediate 1 (90 mmol) and diethyl maleate (90 mmol) were added to the mixture of sodium ethoxide (90 mmol) and ethanol (100 mL), then a moderate amount of glacial acetic acid was added when the temperature was below 60°C. Upon completion of the reaction, the reaction solution was poured into 100 mL distilled water to precipitate the solid, then the solid was recrystallized with ethanol to obtain intermediate 2. Then, a mixture of intermediate 2 (45 mmol), phosphorus oxychloride (POCl₃, 50 mmol) or phosphorus oxybromide (POBr₃, 50 mmol), and acetonitrile (CH₃CN, 50 mL) was reacted under reflux. After ending the reaction, the reaction mixture was poured into 30 mL distilled water, extracted with dichloromethane (CH₂Cl₂), and dried with anhydrous sodium sulfate (Na₂SO₄) to give intermediate 3 (Wang B. L. et al., 2018). After that, a mixture of intermediate 3 (40 mmol), potassium persulfate (K₂S₂O₈, 44 mmol), and CH₃CN (50 mL) reacted under reflux. Upon completion of the reaction, the reaction mixture was poured into 100 mL of distilled water. The residue was washed with water and recrystallized with ethanol to obtain intermediate 4 (Wang B. L. et al., 2018). Finally, sodium hydroxide (NaOH, 30 mmol) dissolved in 10 mL of water was added to the mixture of intermediate 4 and methanol (20 mL) and reacted under reflux. When the reaction was completed, the reaction mixture was poured into 100 mL of distilled water and acidified the mixture to pH 5–6 using concentrated hydrochloric acid. The key intermediate 5 was attained after recrystallization with ethanol. ¹H NMR spectral data for intermediates 1–5 are reported in the Supplementary Data.

SCHEME 1

Scheme 1. Synthetic route of the title compounds 6a–6m.

General Procedure for the Preparation of the Target Compounds 6a−6m

As shown in Scheme 1, oxalyl chloride (2 mL) and 4 drops of N, N-dimethylformamide (DMF) were added to a mixture of intermediate 5 dissolved in 10 mL CH₂Cl₂, and reacted at room temperature. When the reaction was completed, the CH₂Cl₂ and excess oxaloyl chloride were removed under reduced pressure. Then, 5-substituted-4-amino-2-methylpyrimidine was added to the residue dissolved in 5 mL tetrahydrofuran (THF) and reacted under reflux. At the end of the reaction, the reaction mixture was poured into 20 mL of water, extracted with ethyl acetate, and recrystallized with ethanol to obtain the title compounds 6a–6m.

Methyl4-(3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-2-methyl-pyrimidine-5-carboxylate (6a). White solid; yield 45.1%; m.p. 220–222°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.85 (s, 1H, CONH), 8.84 (s, 1H, Pyrimidine-H), 8.52 (d, 1H, J = 4.6 Hz, Pyridine-H), 8.23 (d, 1H, J = 6.9 Hz, pyridine-H), 7.67 (dd, 1H, J₁ = 4.55 Hz, J₂ = 8.0 Hz,pyridine-H), 7.58 (s, 1H, Pyrazole-H), 3.61 (s, 3H), 2.62 (s, 3H);¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.51, 165.33, 159.38, 156.62, 154.98, 148.73, 147.82, 140.40, 140.06, 138.27,1 128.50, 127.55, 115.07, 109.32, 52.83, 26.00; MS (ESI) m/z: 451.1 ([M+H]⁺); Anal. Calcd. for C₁₆H₁₂BrClN₆O₃: C 42.55, H 2.68, N 18.61; found: C 42.60, H 2.70, N 18.66.

Ethyl4-(3-bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-2-methyl-pyrimidine-5-carboxylate (6b). White crystals; yield 50.4%; m.p. 176–177°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.88 (s, 1H, CONH), 8.84 (s, 1H, Pyrimidine-H), 8.51 (d, 1H, J = 4.0 Hz, Pyridine-H), 8.24 (d, 1H, J = 8.0 Hz, pyridine-H), 7.66 (dd, 1H, J₁ = 4.55 Hz, J₂ = 8.0 Hz, pyridine-H), 7.59 (s, 1H, Pyrazole-H), 4.09 (q, 2H, J = 6.9 Hz), 2.64 (s, 3H), 1.08 (t, 3H, J = 7.45 Hz); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.51, 164.80, 159.38, 156.64, 148.73, 147.80, 140.33, 140.07, 138.27, 128.46, 127.52, 115.38, 109.34, 61.68, 25.97, 14.32; MS (ESI) m/z: 465.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₄BrClN₆O₃: C 43.85, H 3.03, N 18.05; found: C 43.90, H 3.00, N 18.06.

Methyl4-(3-chloro-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-2-methyl-pyrimidine-5-carboxylate (6c). White solid; yield 48.2%; m.p. 213–215°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.87 (s, 1H, CONH), 8.85 (s, 1H, Pyrimidine-H), 8.53 (d, 1H, J = 2.85 Hz, Pyridine-H), 8.25 (d, 1H, J = 7.65 Hz, pyridine-H), 7.68 (dd, 1H, J₁ = 2.85 Hz, J₂ = 8.0 Hz, pyridine-H), 7.55 (s, 1H, Pyrazole-H), 3.62 (s, 3H), 2.64 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.54, 165.32, 159.41, 156.62, 155.01, 148.71, 147.85, 140.48, 140.01, 138.30, 128.51, 127.54, 115.05, 109.30, 52.82, 26.01; MS (ESI) m/z: 407.1 ([M+H]⁺); Anal. Calcd. for C₁₆H₁₂Cl₂N₆O₃: C 47.19, H 2.97, N 20.64; found: C 47.20, H 3.00, N 20.62.

Ethyl4-(3-chloro-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-2-methyl-pyrimidine-5-carboxylate (6d). Yellow crystals; yield 38.5%; m.p. 191–192°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.88 (s, 1H, CONH), 8.84 (s, 1H, Pyrimidine-H), 8.51 (d, 1H, J = 2.85Hz, Pyridine-H), 8.23 (d, 1H, J = 7.65 Hz, pyridine-H), 7.66 (dd, 1H, J₁ = 2.85 Hz, J₂ = 8.00 Hz, pyridine-H), 7.53 (s, 1H, Pyrazole-H), 4.06 (q, 2H, J = 6.90 Hz), 2.64 (s, 3H), 1.08 (t, 3H, J = 6.85 Hz); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.52, 164.80, 159.38, 156.64, 148.73, 147.80, 140.33, 140.07, 138.27, 128.46, 127.52, 115.38, 109.34,61.68, 25.97,14.32; MS (ESI) m/z: 422.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₄Cl₂N₆O₃: C 48.47, H 3.35, N 19.95; found: C 48.50, H 3.40, N 20.02.

4-(3-Bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-N-2-dimethylpyrimidine-5-carboxamide (6e). White solid; yield 52.6%; m.p. 228–233°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 12.33 (s, 1H, CONH), 8.78 (s, 1H, Pyrimidine-H), 8.74 (d, 1H, J = 4.55 Hz, CONH), 8.48 (d, 1H, J = 4.00 Hz, Pyridine-H), 8.19 (d, 1H, J = 8.05Hz, Pyridine-H), 7.62 (dd, 1H, J₁ = 4.60 Hz, J₂ = 8.00 Hz, Pyridine-H), 7.32 (s, 1H, Pyrazole-H), 2.67 (d, 3H, J = 4.6 Hz), 2.45 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz)δ: 169.38, 165.98, 157.28, 156.02, 154.80, 148.58, 147.82, 140.09, 139.38, 128.40, 127.68, 127.48, 114.27, 111.72, 26.67, 26.07; MS (ESI) m/z: 450.1 ([M+H]⁺); Anal. Calcd. for C₁₆H₁₃BrClN₇O₂: C 42.64, H 2.91, N 21.76; found: C 42.66, H 2.90, N 21.77.

4-(3-Bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-N-ethyl-2-methylpyrimidine-5-carboxamide (6f). White solid; yield 60.1%; m.p. 236–237°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 12.33 (s, 1H, CONH), 8.78 (s, 1H, Pyrimidine-H), 8.72 (s, 1H, CONH), 8.48 (d, 1H, J = 4.00 Hz, Pyridine-H), 8.19 (d, 1H, J = 8.05 Hz, Pyridine-H), 7.62 (dd, 1H, J₁ = 4.60 Hz, J₂ = 8.00 Hz, Pyridine-H), 7.32 (s, 1H, Pyrazole-H), 2.84 (q, 2H, J = 6.85 Hz), 2.45 (s, 3H), 1.04 (t, 3H, J = 6.25 Hz); ¹³C NMR (DMSO-d₆, 125 MHz)δ: 169.35, 166.00, 157.31, 156.04, 154.78, 148.60, 147.81, 140.11, 139.35, 128.42, 127.65, 127.49, 114.27, 111.76, 26.66, 26.03, 15.32; MS (ESI) m/z: 464.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₅BrClN₇O₂: C 42.64, H 2.91, N 21.76; found: C 42.66, H 2.90, N 21.77.

4-(3-Chloro-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-N,2-dimethylpyrimidine-5-carboxamide (6g). White solid; yield 45.9%; m.p. 213–215°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.87 (s, 1H, CONH), 8.85 (s, 1H, Pyrimidine-H), 8.71 (s, 1H, CONH), 8.53 (d, 1H, J= 2.85 Hz, Pyridine-H), 8.25 (d, 1H, J = 7.65 Hz, Pyridine-H), 7.68 (dd, 1H, J₁ = 2.85 Hz, J₂ = 8.0 Hz, Pyridine-H), 7.55 (s, 1H, Pyrazole-H), 2.85 (s, 3H), 2.64 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.51, 165.33, 159.38, 156.62, 154.98, 148.73, 147.82, 140.40, 140.06, 138.27, 128.50, 127.55, 115.07, 109.32, 52.83, 26.62, 26.04; MS (ESI) m/z: 406.1 ([M+H]⁺); Anal. Calcd. for C₁₆H₁₃Cl₂N₇O₂: C 47.31, H 3.23, N 24.14; found: C 47.32, H 3.20, N 24.12.

4-(3-Chloro-1-(3-chloropyridin-2-yl)-1H-pyrazole-5-carboxamido)-N-ethyl-2-methylpyrimidine-5-carboxamide (6h). Yellow crystals; yield 47.8%; m.p. 191–192°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.88 (s, 1H, CONH), 8.84 (s, 1H, Pyrimidine-H), 8.71 (s, 1H, CONH), 8.51 (d, 1H, J = 2.85 Hz, Pyridine-H), 8.23 (d, 1H, J = 7.65 Hz, pyridine-H), 7.66 (dd, 1H, J₁ = 2.85 Hz, J₂ = 8.00 Hz, pyridine-H), 7.53 (s, 1H, Pyrazole-H), 4.06 (q, 2H, J = 6.90 Hz), 2.64 (s, 3H), 1.08 (t, 3H, J = 6.85 Hz); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.52, 164.89, 159.38, 156.64, 148.73, 147.80, 140.33, 140.07, 138.27, 128.46, 127.52, 115.38, 109.34, 61.68, 25.97,15.32; MS (ESI) m/z: 420.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₅Cl₂N₇O₂: C 48.59, H 3.60, N 23.33; found: C 48.61, H 3.62, N 23.30.

3-Bromo-1-(3-chloropyridin-2-yl)-N-(2-methyl-5-(5-methyl-1,2,4-oxadiazol-3-yl)pyrimidin-4-yl)-1H-pyrazole-5-carboxamide (6i). Yellow crystals; yield 36.5%; m.p. 180–181°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.64 (s, 1H, CONH), 8.99 (s, 1H, Pyrimidine-H), 8.49 (d, 1H, J = 4.55 Hz, Pyridine-H), 8.18 (d, 1H, J = 8.05 Hz, Pyridine-H), 7.63 (dd, 1H, J₁ = 4.60 Hz, J₂ = 8.0 Hz, Pyridine-H), 7.54 (s, 1H, Pyrazole-H), 2.67 (s, 3H), 2.61 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz)δ: 177.49, 169.89, 164.89, 158.92, 155.80, 155.08, 148.68, 147.69, 139.94, 138.68, 128.48, 127.52, 127.41, 112.34, 111.50, 25.96, 12.43; MS (ESI) m/z: 475.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₂BrClN₈O₂: C 42.92, H 2.54, N 23.56; found: C 42.91, H 2.50, N 23.60.

3-Chloro-1-(3-chloropyridin-2-yl)-N-(2-methyl-5-(5-methyl-1,2,4-oxadiazol-3-yl)pyrimidin-4-yl)-1H-pyrazole-5-carboxamide (6j). Yellow crystals; yield 41.3%; m.p. 151–152°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.64 (s, 1H, CONH), 8.99 (s, 1H, Pyrimidine-H), 8.49 (d, 1H, J = 4.55 Hz, Pyridine-H), 8.18 (d, 1H, J = 8.05 Hz, Pyridine-H), 7.63 (dd, 1H, J₁ = 4.60 Hz, J₂ = 8.0 Hz, Pyridine-H), 7.54 (s, 1H, Pyrazole-H), 2.67 (s, 3H), 2.61 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz)δ: 177.52, 169.87, 164.92, 158.94, 155.85, 155.11, 148.65, 147.72, 139.99, 138.71, 128.50, 127.53, 127.45, 112.36, 111.32, 26.02, 12.45; MS (ESI) m/z: 431.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₂BrClN₈O₂: C 47.35, H 2.80, N 25.98; found: 47.37, H 2.77, N 25.99.

3-Bromo-1-(3-chloropyridin-2-yl)-N-(2-methyl-5-(5-(methylthio)-1,3,4-oxadiazol-2-yl)pyrimidin-4-yl)-1H-pyrazole-5-carboxamide (6k). Yellow crystals; yield 46.8%; m.p. 166–168°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.84 (s, 1H, CONH), 9.04 (s, 1H, Pyrimidine-H), 8.46 (d, 1H, J = 5.15 Hz, Pyridine-H), 8.18 (d, 1H, J = 8.0 Hz, Pyridine-H), 7.62 (dd, 1H, J₁ = 4.55 Hz, J₂ = 7.7 Hz, Pyridine-H), 7.58 (s, 1H, Pyrazole-H), 2.72 (s, 3H), 2.66 (s, 3H); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.30, 164.72, 162.00, 158.79, 156.05, 148.51, 148.10, 147.81, 140.01, 138.54, 128.35, 127.60, 127.42, 112.61, 108.46, 26.10, 15.62; MS (ESI) m/z: 507.1 ([M+H]⁺); Anal. Calcd. for C₁₇H₁₂BrClN₈O₂S: C 40.21, H 2.38, N22.07; found: C 40.24, H 2.40, N22.05.

3-Bromo-1-(3-chloropyridin-2-yl)-N-(5-(5-(ethylthio)-1,3,4-oxadiazol-2-yl)-2-methylpyrimidin-4-yl)-1H-pyrazole-5-carboxamide (6l). Yellow crystals; yield 42.5%; m.p. 176–177°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.84 (s, 1H, CONH), 9.04 (s, 1H, Pyrimidine-H), 8.46 (d, 1H, J = 5.15 Hz, Pyridine-H), 8.18 (d, 1H, J = 8.0 Hz, Pyridine-H), 7.62 (dd, 1H, J₁ = 4.55 Hz, J₂ = 7.7 Hz, Pyridine-H), 7.58 (s, 1H, Pyrazole-H), 3.12 (q, 2H, J = 6.9 Hz), 2.66 (s, 3H), 1.35 (t, 3H, J = 7.45 Hz); ¹³C NMR (DMSO-d₆, 125 MHz) δ: 170.25, 164.74, 161.95, 158.75, 156.08, 148.25, 148.12, 147.69, 140.07, 138.55, 128.27, 127.60, 127.44, 112.63, 108.56, 27.86, 26.10, 14.23; MS (ESI) m/z: 521.1 ([M+H]⁺); Anal. Calcd. for C₁₈H₁₄BrClN₈O₂S: C 41.43, H 2.70, N 21.48; found: C 41.45, H 2.74, N 21.49.

3-Bromo-1-(3-chloropyridin-2-yl)-N-(2-methyl-5-(5-(propylthio)-1,3,4-oxadiazol-2-yl)pyrimidin-4-yl)-1H-pyrazole-5-carboxamide (6m). Yellow crystals; yield 39.8%; m.p. 197–199°C; ¹H NMR (DMSO-d₆, 500 MHz) δ: 11.84 (s, 1H, CONH), 9.04 (s, 1H, Pyrimidine-H), 8.46 (d, 1H, J=5.15 Hz, Pyridine-H), 8.18 (d, 1H, J = 8.0 Hz, Pyridine-H), 7.62 (dd, 1H, J₁ = 4.55 Hz, J₂ = 7.7 Hz, Pyridine-H), 7.58 (s, 1H, Pyrazole-H), 3.15 (t, 2H, J = 6.9 Hz), 2.66 (s, 3H), 1.68 (m, 2H), 0.95 (t, 3H, J = 7.45 Hz); ¹³C NMR (DMSO-d₆, 125 MHz)δ: 170.31, 164.70, 162.03, 158.75, 156.02, 148.48, 148.07, 147.77, 140.04, 138.50, 128.32, 127.60, 127.44, 112.67, 108.49, 34.43, 26.10, 22.92, 13.33; MS (ESI) m/z: 535.1 ([M+H]⁺); Anal. Calcd. for C₁₉H₁₆BrClN₈O₂S: C 42.59, H 3.01, N 20.91; found: C 42.53, H 3.00, N 20.92.

Antifungal Biological Assay

The antifungal activities of the title compounds 6a–6m against Sclerotinia sclerotiorum (S. sclerotiorum), Phytophthora infestans (P. infestans), Thanatephorus cucumeris (T. cucumeris), Gibberella zeae (G. zeae), Fusarium oxysporum (F. oxysporum), Cytospora mandshurica (C. mandshurica), Botryosphaeria dothidea (B. dothidea), and Phompsis sp. were evaluated at the concentration of 50 μg/mL (Min et al., 2016; Wu et al., 2019a,b). The target compounds 6a–6m (5 mg) were dissolved in dimethyl sulfoxide (1 mL) and sterile water (9 mL) before mixing with 90 mL potato dextrose agar (PDA) to generate a final concentration of 50 μg/mL. Then, 4 mm diameter of the mycelia dishes were cut from a culture medium of pathogenic fungi, then inoculated in the middle of PDA and cultivated at 27 ± 1°C for 4–5 days. DMSO in sterile distilled water served as a negative control, while Kresoxim-methyl and Pyrimethanil acted as positive controls. For each treatment, three replicates were conducted. The radial growth of the fungal colonies was measured and the data were statistically analyzed. The inhibition rate I (%) of the test compounds against eight pathogenic fungi were calculated by the following formula, where C represents the diameter of fungi growth on untreated PDA, and T represents the diameter of fungi on treated PDA.

$\begin{array}{l}I(\%)=\left[(C-T)/(C-0.4)\right]\times 100\end{array}$

Insecticidal Biological Assay

The insecticidal activities of all synthesized compounds 6a–6m against Spodoptera litura (S. litura), Mythimna separata (M. separata), Pyrausta nubilalis (P. nubilalis), Tetranychus urticae (T. urticae), Rhopalosiphum maidis (R. maidis), and Nilaparvata lugens (N. lugens) were performed according to the reported method (Wang B. L. et al., 2018; Wang et al., 2019). The target compounds 6a–6m were dissolved in NP-10 (0.1 mg/L) solution to generate a final concentration of 200 μg/mL. Then, 15 maize leaves (approximately 5 cm in length) and 5 sweet potato leaves (diameter 3 cm) were dipped in the test compounds solutions for 10 s, dried and placed into a tumbler. After that, 30 larvae of second-instar S. litura, M. separata, P. nubilalis, T. urticae, R. maidis, and N. lugens were transferred to the petri dish. Chlorantraniliprole was used as a control. All bioassays were performed in the laboratory at 27 ± 1°C for 48 h. Three replicates were performed for each treatment. The percentage of mortalities for the target compounds were determined using Abbott's formula.

Results and Discussion

In this study, using 2,3-dichloropyridine as the starting material, the title compounds 6a–6m were synthesized in six steps, including hydrazidation, cyclization, bromination or chlorination, oxidation, hydrolyzation, and condensation. The target compounds structures were confirmed by ¹H NMR, ¹³C NMR, MS, and elemental analysis. In the ¹H NMR spectra of compound 6a, a singlet at 2.45 ppm assigned to CH₃ protons of pyrimidine-CH₃, the doublet signal at 2.67 ppm indicated the presence of CH₃ proton in CONH-CH₃, meanwhile, a singlet at 8.78 and 7.32 ppm indicated the presence of pyrimidine and pyrazole ring. Two proton signals of two -CONH- in amide moiety was observed at 12.33 and 8.74 ppm. The structure of 6b was also confirmed by its mass spectral data. In its mass spectrum, the molecular ion peak was noticed m/z at 450.1 ([M+H]⁺) corresponding to its molecular weight.

The in vitro antifungal activities at 50 μg/mL of the target compounds against eight plant fungi are listed in Table 1. Table 1 showed that, at 50 μg/mL, compounds 6a–6m indicated certain antifungal activities against S. sclerotiorum, P. infestans, T. cucumeris, G. zeae, F. oxysporum, C. mandshurica, B. dothidea, and Phompsis sp. with the inhibition rates of 6.5–54.1%, 8.7–48.6%, 3.2–54.7%, 0–54.9%, 0–66.3%, 10.7–49.4%, 30.1–85.9%, and 36.2–81.2%, respectively. Among the title compounds, compound 6i revealed good in vitro antifungal activities against P. infestans and B. dothidea, with the inhibition rates of 48.6% and 85.9%, respectively, which were equally to those of Kresoxim-methyl or Pyrimethanil. Meanwhile, compound 6j revealed good in vitro antifungal activities against S. sclerotiorum, G. zeae, C. mandshurica, and B. dothidea, with the inhibition rates of 54.1, 54.9, 49.4, and 85.9%, respectively, which were similar with those of Kresoxim-methyl or Pyrimethanil.

TABLE 1

Table 1. The in vitro antifungal activity of the title compounds 6a–6m at 50 μg/mL.

Based on the preliminary antifungal bioassays, the EC₅₀ values of compounds 6i and 6j were also tested and presented in Table 2. Table 2 showed that compounds 6i and 6j showed good activities against B. dothidea, with EC₅₀ values of 56.4 and 65.3 μg/mL, respectively, which were similar to that of Pyrimethanil (57.6 μg/mL).

TABLE 2

Table 2. The EC₅₀ values of compounds 6i and 6j against B. dothidea.

The in vitro insecticidal properties of title compounds against S. litura, M. separata, P. nubilalis, T. urticae, R. maidis, and N. lugens were evaluated and the insecticidal bioassay results are listed in Table 3. Table 3 showed that the target compounds 6a–6m indicated certain insecticidal activities against S. litura, M. separata, P. nubilalis, T. urticae, R. maidis, and N. lugens at 200 μg/mL, with the mortality rates of 10.0–70.2%, 12.0–75.4%, 0–61.3%, 43.0–80.9%, 20.0–88.9%, and 11.0–78.6%, respectively, which were lower than those of Chlorantraniliprole. Especially, compound 6f revealed good insecticidal activities against S. litura, M. separata, P. nubilalis, T. urticae, and N. lugens with mortality rates of 70.2, 75.4, 61.3, 80.9, and 78.6%, respectively. Meanwhile, compound 6c had a good mortality rate of 88.9% against R. maidis.

TABLE 3

Table 3. The insecticidal activities of the title compounds 6a–6m at 200 μg/mL.

Conclusions

In summary, thirteen novel pyridylpyrazolamide derivatives containing pyrimidine motifs were synthesized, and their structures confirmed by ¹H NMR, ¹³C NMR, MS and elemental analysis. Bioassay results showed that some of title compounds revealed good antifungal and insecticidal properties. The results provided strategy for leading the synthesis of novel pyridylpyrazolamide derivatives containing pyrimidine motifs.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

Author Contributions

WW and QF contributed to the synthesis, purification, characterization of all compounds, and prepared the original manuscript. YZ and YG performed the activity research. MC and HC perfected the language and assisted with the structure elucidation and manuscript revision. GO and MY designed and supervised the research and revised the manuscript. All authors discussed, edited, and approved the final version.

Funding

This research was financially supported by China Postdoctoral Science Foundation (No. 2017M623070), the National Natural Science Foundation of China (No. 31701821 and 21762037), the Opening Foundation of the Key Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Guizhou University (No. 2018GDGP0102), the Guizhou Province Biological and Pharmaceutical engineering Research Centerand (No. QJHKY[2019]051), and the special Funding of Guiyang Science and Technology Bureau and Guiyang University [GYU-KYZ(2019~2020)PT10-06].

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor declared a shared affiliation, though no other collaboration, with the authors YZ, MY, and GO.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2020.00522/full#supplementary-material

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