Semisynthesis of some matrine ether derivatives as insecticidal agents

Abstract

In continuation of our program to discover new potential natural-product-based crop protection agents, we synthesized a series of 14-formyl-15-aryloxy/methoxymatrine and 14-aryloxymethylidenylmatrine derivatives as pesticidal agents by structural modification of matrine, a biorenewable quinolizidine alkaloid isolated from Sophora flavescens. The structural assignment was based on spectroscopic and X-ray analysis data. Their pesticidal activities were carried out against two typically crop-threatening agricultural insect pests, Mythimna separata Walker and Plutella xylostella Linnaeus. Compounds 4i and 4k exhibited more potent oral toxicity than matrine against 3rd-instar larvae of P. xylostella. As compared with matrine, all derivatives displayed a growth inhibitory property against early 3rd-instar larvae of M. separata, and in particular compounds 4i–k displayed more promising insecticidal activity than toosendanin. Some interesting results of structure–activity relationships were also observed.

---

Introduction

Mythimna separata Walker (oriental armyworm) and Plutella xylostella Linnaeus (diamondback moth) are two widely distributed and serious typical lepidopteran insect pests. Their infestations are very troublesome and hard to control.^1,2 In 2012, intermittent outbreaks of M. separata occurred widely in China, and about 4 million hectares of crops were completely lost.³ Although lots of chemical pesticides have been extensively applied to deal with insect pest outbreaks, insect pest resistance and negative impacts on human health and environmental safety have emerged.^4–7 Therefore, the development of new potential alternatives to effectively and selectively control insect pests is highly urgent.

Matrine (1, Fig. 1) is a biorenewable quinolizidine alkaloid isolated from the roots of Sophora flavescens (Kushen), which is widely distributed in Asia, Oceanica, and the Pacific islands.^8,9 Matrine and its derivatives exhibited a broad scope of biological properties such as anticancer, anti-inflammatory, and antiviral activities.^10–16 Meanwhile, compound 1 also exhibited potent insecticidal activity in the agricultural field.^17–19 On the other hand, it was found that the introduction of an acrylic aldehyde scaffold (fragment A, Fig. 1) into dehydroepiandrosterone could produce derivatives I–IV^20–22 (Fig. 1), which showed potent antiproliferative effects. Moreover, their lipophilicity, which improves their ability to cross through cell membranes, was increased, whereas their side effects and toxicity were decreased.²¹ Meanwhile, to the best of our knowledge, little attention has been paid to the structural modifications of compound 1 as pesticidal agents. Based upon the above interesting results, to discover biorenewable matrine-based pesticides, herein an acrylic aldehyde scaffold was introduced into compound 1, followed by transformation to 14-formyl-15-aryloxy/methoxymatrine (4, Fig. 1). Their pesticidal activities were evaluated against two typically crop-threatening insect pests, M. separata and P. xylostella.

Fig. 1 Chemical structures of matrine (1), its derivatives (4), and other dehydroepiandrosterone derivatives (I–IV).

Materials and methods

General

All chemical reagents were purchased and utilized without further purification. Compound 1 was purchased from Baoji Haoxiang Bio-technology Co. Ltd. Melting point (mp) was determined using the XT-4 digital mp apparatus. Optical rotation was measured using an Autopol III automatic polarimeter. Infrared (IR) spectra were measured by a TENSOR 27 spectrometer. Proton nuclear magnetic resonance spectra (¹H NMR) were measured with Avance III 500 MHz equipment. High-resolution mass spectra (HRMS) were carried out with an LTQ FT Ultra instrument. Microwave irradiation was performed in a CEM Discover Synthesis Unit.

Synthesis of 14-formyl-15-chloromatrine (2)

POCl₃ (15 mmol) was added slowly to a solution of DMF (1.2 mL) in dry CH₂Cl₂ (5 mL) at 0 °C. After addition, the mixture was stirred for 1 h at 0 °C. Then, a solution of 1 (5 mmol) in dry CH₂Cl₂ (15 mL) was added dropwise to the above mixture. After addition, the reaction temperature was slowly raised to room temperature, and the reaction process was checked by TLC analysis. After 9 h, the mixture was concentrated in vacuo, and 5 mL of ice water was added. The pH value of the mixture was adjusted with 40% aq. NaOH to 8–9. Subsequently, hydrolysis of the mixture was conducted at 60 °C for 2 h. After cooling, the pale yellow precipitate was filtered and washed with water. Finally, the solid was purified by silica gel column chromatography eluted with dichloromethane/methanol to give 2 (92% yield) as a pale yellow solid.

Data for 2

Mp 106–108 °C; [α]²⁰_D = −83 (c 2.6 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 2776, 1635, 1564; ¹H NMR (500 MHz, CDCl₃) δ: 9.67 (s, 1H), 4.06–4.10 (m, 1H), 4.01 (dd, J = 13.0, 4.0 Hz, 1H), 3.59 (t, J = 13.0 Hz, 1H), 2.79–2.86 (m, 2H), 2.39–2.44 (m, 1H), 2.27–2.33 (m, 1H), 2.18 (s, 1H), 1.96–2.03 (m, 2H), 1.80–1.92 (m, 4H), 1.55–1.71 (m, 5H), 1.43–1.49 (m, 2H), 1.32–1.40 (m, 1H); HRMS (ESI): calcd for C₁₆H₂₄ON₂Cl ([M + H]⁺), 295.1572; found, 295.1571.

General procedure for synthesis of 4a–e, k, l

A mixture of 2 (0.3 mmol), KOH (0.6 mmol), and different phenols (3a–d, k, l, 0.9 mmol) in DMF (5 mL) were stirred at 120 °C for 2–9 h; or a mixture of 2 (0.3 mmol) and KOH (1.5 mmol) in MeOH (3e, 5 mL) was refluxed for 14 h. Then the reaction mixture was diluted with water (10 mL), and extracted with ethyl acetate (15 mL × 3). Subsequently, the combined organic phase was washed by saturated aq. Na₂CO₃ (20 mL × 2), dried over anhydrous Na₂SO₄, concentrated in vacuo, and purified by PTLC eluted with dichloromethane/methanol to afford target compounds 4a–e, k, l.

General procedure for synthesis of 4f–j

Compound 2 (0.3 mmol) reacted with different phenols (3f–j, 0.9 mmol) in the presence of K₂CO₃ (0.66 mmol) and DMF (0.5 mL) under microwave irradiation at 120 °C for 20 min. After cooling to room temperature, the reaction mixture was diluted with CH₂Cl₂ (10 mL), and washed by saturated aq. Na₂CO₃ (5 mL × 2), dried over anhydrous Na₂SO₄, concentrated in vacuo, and purified by PTLC eluted with dichloromethane/methanol to afford target compounds 4f–j.

Data for 4a

Yield: 65%, pale yellow solid, mp 97–99 °C; [α]²⁰_D = −16 (c 3.3 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3051, 2759, 1635, 1576; ¹H NMR (500 MHz, CDCl₃) δ: 9.39 (s, 1H), 7.29–7.32 (m, 2H), 7.04 (t, J = 7.5 Hz, 1H), 7.01 (d, J = 8.0 Hz, 2H), 3.95–4.00 (m, 1H), 3.47 (dd, J = 13.0, 3.5 Hz, 1H), 3.32 (t, J = 13.0 Hz, 1H), 2.74–2.84 (m, 2H), 2.44–2.50 (m, 1H), 2.35–2.41 (m, 1H), 2.11 (s, 1H), 1.89–2.05 (m, 4H), 1.73–1.79 (m, 1H), 1.55–1.67 (m, 4H), 1.36–1.46 (m, 5H); HRMS (ESI): calcd for C₂₂H₂₉O₂N₂ ([M + H]⁺), 353.2224; found, 353.2221.

Data for 4b

Yield: 77%, pale yellow solid, mp 134–136 °C; [α]²⁰_D = −18 (c 3.2 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3052, 2757, 1637, 1564, 1577, 1505; ¹H NMR (500 MHz, CDCl₃) δ: 9.33 (s, 1H), 7.18 (d, J = 7.0 Hz, 1H), 7.08 (t, J = 7.5 Hz, 1H), 6.95 (t, J = 7.5 Hz, 1H), 6.84 (d, J = 8.0 Hz, 1H), 3.97–3.99 (m, 1H), 3.34 (d, J = 8.0 Hz, 2H), 2.74–2.84 (m, 2H), 2.41–2.45 (m, 2H), 2.34 (s, 3H), 2.12 (s, 1H), 1.90–2.03 (m, 4H), 1.78 (s, 1H), 1.56–1.70 (m, 4H), 1.37–1.46 (m, 5H); HRMS (ESI): calcd for C₂₃H₃₁O₂N₂ ([M + H]⁺), 367.2380; found, 367.2380.

Data for 4c

Yield: 64%, pale yellow solid, mp 159–161 °C; [α]²⁰_D = −24 (c 3.0 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3052, 2762, 1635, 1564, 1505; ¹H NMR (500 MHz, CDCl₃) δ: 9.38 (s, 1H), 7.29 (d, J = 9.0 Hz, 2H), 6.92 (d, J = 8.5 Hz, 2H), 3.95–4.00 (m, 1H), 3.51 (dd, J = 13.0, 4.5 Hz, 1H), 3.32 (t, J = 13.0 Hz, 1H), 2.74–2.84 (m, 2H), 2.35–2.47 (m, 2H), 2.12 (s, 1H), 1.90–2.03 (m, 4H), 1.75–1.80 (m, 1H), 1.59–1.69 (m, 4H), 1.37–1.46 (m, 5H), 1.29 (s, 9H); HRMS (ESI): calcd for C₂₆H₃₇O₂N₂ ([M + H]⁺), 409.2850; found, 409.2849.

Data for 4d

Yield: 52%, pale yellow solid, mp 101–103 °C; [α]²⁰_D = −19 (c 2.0 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3059, 2764, 1635, 1571, 1485; ¹H NMR (500 MHz, CDCl₃) δ: 9.29 (s, 1H), 7.19 (d, J = 11.0 Hz, 2H), 6.89 (d, J = 11.0 Hz, ArH, 2H), 3.87–3.93 (m, 1H), 3.24–3.37 (m, 2H), 2.67–2.78 (m, 2H), 2.27–2.42 (m, 2H), 2.05 (s, 1H), 1.83–1.99 (m, 4H), 1.64–1.72 (m, 2H), 1.49–1.60 (m, 4H), 1.33–1.40 (m, 4H); HRMS (ESI): calcd for C₂₆H₂₈O₂N₂Cl ([M + H]⁺), 387.1834; found, 387.1833.

Data for 4e

Yield: 51%, pale yellow solid, mp 84–86 °C; [α]²⁰_D = −40 (c 2.8 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 2757, 1623, 1555;¹H NMR (500 MHz, CDCl₃) δ: 9.50 (s, CHO, 1H), 3.86–3.90 (m, 1H), 3.83 (s, 3H), 3.57 (dd, J = 12.5, 4.5 Hz, 1H), 3.38 (t, J = 12.5 Hz, 1H), 2.78–2.85 (m, 2H), 2.25–2.37 (m, 2H), 2.15 (s, 1H), 1.85–2.01 (m, 4H), 1.72–1.75 (m, 2H), 1.56–1.67 (m, 5H), 1.41–1.48 (m, 3H); HRMS (ESI): calcd for C₁₇H₂₇O₂N₂ ([M + H]⁺), 291.2067; found, 291.2067.

Data for 4f

Yield: 74%, pale yellow solid, mp 100–102 °C; [α]²⁰_D = −30 (c 2.5 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3043, 2761, 1632, 1574; ¹H NMR (500 MHz, CDCl₃) δ: 9.34 (s, 1H), 7.16 (t, J = 7.5 Hz, 1H), 6.86 (d, J = 7.5 Hz, 1H), 6.79–6.82 (m, 2H), 3.97–4.01 (m, 1H), 3.49 (dd, J = 13.0, 4.0 Hz, 1H), 3.32 (t, J = 12.5 Hz, 1H), 2.75–2.84 (m, 2H), 2.38–2.50 (m, 2H), 2.33 (s, 3H), 2.12 (s, 1H), 1.92–2.05 (m, 4H), 1.74–1.80 (m, 1H), 1.58–1.68 (m, 4H), 1.37–1.45 (m, 5H); HRMS (ESI): calcd for C₂₃H₃₁O₂N₂ ([M + H]⁺), 367.2380; found, 367.2380.

Data for 4g

Yield: 72%, pale yellow solid, mp 90–92 °C; [α]²⁰_D = −15 (c 2.7 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3002, 2743, 1628, 1562, 1507; ¹H NMR (500 MHz, CDCl₃) δ: 9.35 (s, 1H), 7.08 (d, J = 8.0 Hz, 2H), 6.89 (d, J = 8.5 Hz, 2H), 3.95–3.99 (m, 1H), 3.48 (dd, J = 13.0, 4.5 Hz, 1H), 3.31 (t, J = 13.0 Hz, 1H), 2.84–2.74 (m, 2H), 2.35–2.50 (m, 2H), 2.30 (s, 3H), 2.11 (s, 1H), 1.89–2.04 (m, 4H), 1.72–1.78 (m, 1H), 1.55–1.67 (m, 4H), 1.39–1.45 (m, 5H); HRMS (ESI): calcd for C₂₃H₃₁O₂N₂ ([M + H]⁺), 367.2380; found, 367.2379.

Data for 4h

Yield: 70%, pale yellow solid, mp 87–89 °C; [α]²⁰_D = −18 (c 2.6 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3057, 2762, 1635, 1579, 1501; ¹H NMR (500 MHz, CDCl₃) δ: 9.40 (s, 1H), 6.94 (d, J = 9.0 Hz, 2H), 6.82 (d, J = 9.0 Hz, 2H), 3.93–3.97 (m, 1H), 3.77 (s, 3H), 3.48 (dd, J = 14.0, 5.0 Hz, 1H), 3.31 (t, J = 12.5 Hz, 1H), 2.74–2.84 (m, 2H), 2.33–2.50 (m, 2H), 2.10 (s, 1H), 1.91–2.03 (m, 4H), 1.71–1.76 (m, 1H), 1.52–1.64 (m, 4H), 1.37–1.45 (m, 5H); HRMS (ESI): calcd for C₂₃H₃₁O₃N₂ ([M + H]⁺), 383.2329; found, 383.2326.

Data for 4i

Yield: 65%, white solid, mp 100–102 °C; [α]²⁰_D = −30 (c 2.6 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3055, 2774, 1628, 1562, 1505; ¹H NMR (500 MHz, CDCl₃) δ: 9.45 (s, 1H), 7.79 (t, J = 8.0 Hz, 2H), 7.71 (d, J = 8.5 Hz, 1H), 7.46 (t, J = 8.0 Hz, 1H), 7.39 (d, J = 7.5 Hz, 1H), 7.33 (s, 1H), 7.26–7.28 (m, 1H), 4.01–4.03 (m, 1H), 3.51 (dd, J = 13.0, 4.0 Hz, 1H), 3.34 (t, J = 12.5 Hz, 1H), 2.73–2.84 (m, 2H), 2.41–2.54 (m, 2H), 2.09–2.11 (m, 2H), 1.88–2.00 (m, 3H), 1.80–1.84 (m, 1H), 1.58–1.71 (m, 4H), 1.34–1.47 (m, 5H); HRMS (ESI): calcd for C₂₆H₃₁O₂N₂ ([M + H]⁺), 403.2380; found, 403.2377.

Data for 4j

Yield: 58%, white solid, mp 103–105 °C; [α]²⁰_D = −11 (c 2.8 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3057, 2757, 1635, 1595, 1576, 1507; ¹H NMR (500 MHz, CDCl₃) δ: 9.41 (s, 1H), 8.28–8.30 (m, 1H), 7.85–7.87 (m, 1H), 7.54–7.57 (m, 3H), 7.32 (t, J = 8.0 Hz, 1H), 6.97 (d, J = 7.5 Hz, 1H), 4.03 (s, 1H), 3.41 (dd, J = 13.0, 4.5 Hz, 1H), 3.32 (t, J = 13.0 Hz, 1H), 2.82 (d, J = 10.5 Hz, 1H), 2.72 (d, J = 11.5 Hz, 1H), 2.47 (s, 2H), 1.87–2.13 (m, 6H), 1.58–1.75 (m, 4H), 1.39–1.56 (m, 5H); HRMS (ESI): calcd for C₂₆H₃₁O₂N₂ ([M + H]⁺), 403.2380; found, 403.2377.

Data for 4k

Yield: 49%, pale yellow solid, mp 162–164 °C; [α]²⁰_D = 32 (c 3.0 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3111, 3029, 2771, 1675, 1650, 1613, 1588, 1553, 1513, 1494; ¹H NMR (500 MHz, CDCl₃) δ: 8.21 (d, J = 8.5 Hz, 2H), 7.16 (d, J = 8.5 Hz, 2H), 6.60 (s, 1H), 4.46 (dd, J = 12.5, 3.5 Hz, 1H), 3.89–3.94 (m, 1H), 3.09 (t, J = 13.0 Hz, 1H), 2.78–2.86 (m, 2H), 2.27–2.41 (m, 3H), 2.10 (s, 1H), 1.87–2.01 (m, 3H), 1.62–1.75 (m, 4H), 1.39–1.55 (m, 6H); HRMS (ESI): calcd for C₂₂H₂₈O₄N₃ ([M + H]⁺), 398.2074; found, 398.2073.

Data for 4l

Yield: 33%, pale yellow solid, mp 152–154 °C; [α]²⁰_D = 18 (c 3.2 mg mL⁻¹, CHCl₃); IR cm⁻¹ (KBr): 3070, 3026, 2774, 1697, 1672, 1596, 1505; ¹H NMR (500 MHz, CDCl₃) δ: 9.92 (s, 1H), 7.85 (d, J = 8.5 Hz, 2H), 7.20 (d, J = 8.5 Hz, 2H), 6.63 (s, 1H), 4.48 (dd, J = 12.5, 3.0 Hz, 1H), 3.89–3.92 (m, 1H), 3.08 (t, J = 12.5 Hz, 1H), 2.78–2.86 (m, 2H), 2.26–2.40 (m, 3H), 2.10 (s, 1H), 1.87–2.01 (m, 3H), 1.63–1.74 (m, 4H), 1.42–1.55 (m, 6H); HRMS (ESI): calcd for C₂₃H₂₉O₃N₂ ([M + H]⁺), 381.2173; found, 381.2172.

Biological assay

Oral toxicity of 1 and 4a, d, e, g, i, k against Plutella xylostella. Thirty 3rd-instar larvae of P. xylostella were chosen as the test insects for each compound. Solutions of 1; 4a, d, e, g, i, k; and toosendanin (a positive control) were prepared in acetone at 20 mg mL⁻¹. The corresponding solution (1 μL) was added to a fresh Brassica oleracea leaf disc (0.5 × 0.5 cm), and dried. A fresh Brassica oleracea leaf disc was treated by acetone alone as the blank control group (CK). One piece of the above discs was offered to and consumed by each insect, which was raised in each well of 12- or 24-well culture plates for 48 h (temperature: 25 ± 2 °C; relative humidity (RH): 65–80%; photoperiod: light/dark (L/D) = 16/8 h). Their corrected mortality rate values were calculated as follows: corrected mortality rate (%) = (T − C) × 100/(100% − C); C is the mortality rate of CK, and T is the mortality rate of the treated P. xylostella.^23,24

Growth inhibitory activity of 1, 2, and 4a–l against Mythimna separata. Thirty early 3rd-instar larvae of M. separata were chosen as the test insects for each compound. Solutions of 1, 2, 4a–l, and toosendanin (a positive control) were prepared in acetone at 1 mg mL⁻¹. After dipping into the corresponding solution for 3 s, wheat leaf discs (1 × 1 cm) were taken out, and dried. Wheat leaf discs were treated by acetone alone as the blank control group (CK). Several of the above discs were added to each culture dish (ten insects per dish). Once the discs had been consumed, additional ones were added. After 48 h, the rest of the compound-soaked discs were cleaned out, and untreated ones were added until the end of pupation (temperature: 25 ± 2 °C; RH: 65–80%; photoperiod: L/D = 12/12 h). Their corrected mortality rate values were calculated as follows: corrected mortality rate (%) = (T − C) × 100/(100% − C); C is the mortality rate of CK, and T is the mortality rate of the treated M. separata.^25,26

Results and discussion

Synthesis

As shown in Scheme 1, first, the key intermediate, 14-formyl-15-chloromatrine (2), was smoothly synthesized by the Vilsmeier–Haack reaction of matrine (1) with DMF in the presence of POCl₃. Then, 14-formyl-15-aryloxy/methoxymatrine derivatives (4a–e) were easily prepared by the reaction of 2 with corresponding phenols (3a–d) or methanol (3e) in the presence of KOH at 120 °C or under reflux.²⁷ However, when 2 reacted with 3f–j under the above reaction conditions, it was very difficult to separate 14-formyl-15-aryloxymatrine derivatives (4f–j) from their by-products. It is noteworthy that when 2 reacted with 3f–j under microwave irradiation at 120 °C for 20 min, compounds 4f–j were smoothly obtained. Interestingly, when 2 reacted with 3k, l in the presence of KOH at 120 °C, only 14-aryloxymethylidenylmatrines (4k, l) were produced. Their structures were well determined by melting points, optical rotation, IR, ¹H NMR, and HRMS. In particular, three-dimensional structures of compounds 2, 4d, 4g and 4k were determined by X-ray crystallography (Fig. 2–5). Crystallographic data (excluding structure factors) of 2, 4d, 4g and 4k were deposited at the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers of 1522361, 1524882, 1524886 and 1524887, respectively. Assignment of the chemical structure for 4l was based on the chemical shift of H-18. As shown in Fig. 6, the chemical shifts of H-18 of 2, 4d, and 4g were at 9.67, 9.29, and 9.35 ppm, respectively. Whereas the chemical shift of H-18 of 4k was at 6.60 ppm. Here the chemical shift of H-18 of 4l was at 6.63 ppm, so the steric structure for 4l was the same as that of 4k.

Scheme 1 Synthesis of 14-formyl-15-aryloxy/methoxymatrines (4a–j) and 14-aryloxymethylidenylmatrines (4k, l) from matrine.

Fig. 2 X-ray crystal structure of compound 2.

Fig. 3 X-ray crystal structure of compound 4d.

Fig. 4 X-ray crystal structure of compound 4g.

Fig. 5 X-ray crystal structure of compound 4k.

Fig. 6 Comparison of partial ¹H NMR spectra of compounds 2, 4d, 4g, 4k, and 4l.

The possible mechanism for 3a–l reacting with 2 is described in Fig. 7. Compounds 3a–j reacted with 2 via the intermediates 5a–j to afford 14-formyl-15-aryloxy/methoxymatrine derivatives (4a–j) by the 1,4-addition–elimination reaction (eqn (1)). However, for compounds 3k, l, first, the hydroxyl ion reacted with 2 via the intermediate 6 to afford 14-formyl-15-hydroxylmatrine (7) by the 1,4-addition–elimination reaction. Then, compound 7 was converted into intermediate 8. Finally, compound 8 reacted with 3k, l, followed by removal of a molecule of water to give 14-aryloxymethylidenylmatrines (4k, l) (eqn (2)).

Fig. 7 Possible mechanism for 3a–l reacting with 2.

Insecticidal activity

The oral toxicity of compounds 1 and 4a, d, e, g, i, k against P. xylostella treated at 20 μg/larvae is described in Table 1. The mortality rates after 48 h of 1, 4a, 4d, 4e, 4g, 4i, and 4k were 28.5%, 32.1%, 35.7%, 35.7%, 32.1%, 39.3%, and 42.8%, respectively. Among them, compounds 4d, 4e, 4i, and 4k exhibited potent oral toxicity when compared with toosendanin. The growth inhibitory activity of compounds 1, 2, and 4a–l against M. separata was tested at 1 mg mL⁻¹. As shown in Table 2, compounds 4i, 4j and 4k showed higher insecticidal activity than toosendanin. For example, the final mortality rates (FMRs) of 4i, 4j and 4k were 62.1%, 58.6%, and 55.2%, respectively; whereas the FMR of 1 was only 24.2%. In particular, compound 4i exhibited the most potent insecticidal activity. The symptoms for the treated M. separata during the larval, pupation and adult periods were observed. Fig. 8 shows the dead larvae with thin and wrinkled bodies at the larval stage; Fig. 9 shows some malformed and dead pupae during the pupation stage; Fig. 10 shows some malformed moths during the adult emergence stage. This demonstrates that matrine derivatives probably affected the insect molting hormone. On the other hand, the times for three growth periods of M. separata treated with 1, 2, and 4a–l are shown in Fig. 11. This suggests that the times from the larvae to the adult in the treated groups were generally prolonged (33–35 days vs. 32 days for CK). Finally, the percentages of FMRs at three different growth stages of compounds 4d, 4e, 4i, 4j, 4k and toosendanin were investigated, as displayed in Fig. 12. More than/equal to half of FMRs for compounds 4d, 4e, 4i, 4j, 4k and toosendanin were at the larval stage. This result was the same with those of esters of fraxinellone C4/10-oxime.²⁴

Table 1 Oral toxicity of compounds 1 and 4a, d, e, g, i, k against P. xylostella treated at 20 μg/Larvae

Compound	Corrected mortality rate (mean ± SD, %)
	24 h	48 h
a Multiple range test using Duncan's test (p < 0.05). The same letters denote treatments not significantly different from each other.
1	10.4 ± 0	28.5 ± 3.3 c^a
4a	13.8 ± 3.3	32.1 ± 3.3 bc
4d	10.4 ± 3.3	35.7 ± 5.8 abc
4e	13.8 ± 3.3	35.7 ± 0 abc
4g	10.4 ± 3.3	32.1 ± 6.7 bc
4i	10.4 ± 3.3	39.3 ± 3.3 abc
4k	17.3 ± 0	42.8 ± 3.3 ab
Toosendanin	0 ± 3.3	46.4 ± 0 a

---

Table 2 Growth inhibitory activity of compounds 1, 2, and 4a–l against M. separata on leaves treated with a concentration of 1 mg mL⁻¹

Compound	Corrected mortality rate (mean ± SD, %)
	10 days	25 days	35 days
a Multiple range test using Duncan's test (p < 0.05). The same letters denote treatments not significantly different from each other.
1	3.3 ± 3.3	16.7 ± 3.3	24.2 ± 3.3 e^a
2	6.7 ± 3.3	23.3 ± 3.3	31.1 ± 6.7 de
4a	10.0 ± 5.8	26.7 ± 6.7	41.4 ± 3.3 cd
4b	6.7 ± 6.7	23.3 ± 6.7	38.0 ± 5.8 cd
4c	10.0 ± 5.8	23.3 ± 3.3	31.1 ± 3.3 de
4d	13.3 ± 6.7	30.0 ± 5.8	48.3 ± 0 bc
4e	20.0 ± 0	33.3 ± 3.3	48.3 ± 5.8 bc
4f	13.3 ± 6.7	33.3 ± 3.3	41.4 ± 3.3 cd
4g	10.0 ± 0	26.7 ± 3.3	38.0 ± 5.8 cd
4h	10.0 ± 5.8	30.0 ± 5.8	41.4 ± 3.3 cd
4i	13.3 ± 3.3	40.0 ± 0	62.1 ± 3.3 a
4j	13.3 ± 3.3	36.7 ± 3.3	58.6 ± 0 ab
4k	20.0 ± 0	33.3 ± 6.7	55.2 ± 3.3 ab
4l	6.7 ± 3.3	23.3 ± 3.3	38.0 ± 5.8 cd
Toosendanin	16.7 ± 6.7	33.3 ± 3.3	48.3 ± 0 bc

---

Fig. 8 The representative abnormal larvae pictures of 4d (HJL-39), 4k (HJL-40), 4i (HJL-62), 4j (HJL-63), and 4e (HJL-72) during the larval period (CK: blank control group).

Fig. 9 The representative malformed pupae pictures of 4d (HJL-39), 4k (HJL-40), 4i (HJL-62), 4j (HJL-63), and 4e (HJL-72) during the pupation period (CK: blank control group).

Fig. 10 The representative malformed moth pictures of 4a (HJL-32), 4b (HJL-45), 4f (HJL-67), 4h (HJL-68), and 4e (HJL-72) during the emergence period (CK: blank control group).

Fig. 11 Times for different developmental stages of M. separata treated with 1, 2, and 4a–l.

Fig. 12 The percentages of FMRs at three different growth stages of compounds 4d, 4e, 4i, 4j, 4k and toosendanin.

Additionally, some interesting results of structure–activity relationships of the tested compounds were also obtained. When the chlorine atom of 2 was substituted by other aryloxy/methoxy groups such as 4-chlorophenyloxy, methoxy, 2-naphthyloxy, and 1-naphthyloxy, the corresponding products 4d (FMR: 48.3%), 4e (FMR: 48.3%), 4i (FMR: 62.1%), and 4j (FMR: 58.6%) showed more potent insecticidal activity than 1 (FMR: 24.2%) and 2 (FMR: 31.1%). Interestingly, one 14-formyl-15-alkyloxymatrine, 4e (containing a methoxy group), exhibited promising insecticidal activity, so in the future other alkyloxy groups could be considered for introduction at the C-15 position of 2. In general, the introduction of an electron-withdrawing group on the phenyl ring of 4a (FMR: 41.4%) could result in a more potent compound. For example, the FMR of 4d (containing 4-chlorophenyl) was 48.3%; whereas the FMRs of 4b (containing 2-methylphenyl), 4c (containing 4-t-butylphenyl), 4f (containing 3-methylphenyl), 4g (containing 4-methylphenyl), and 4h (containing 4-methoxyphenyl), were 38.0%, 31.1%, 41.4%, 38.0%, and 41.4%, respectively. The introduction of a polycyclic aromatic hydrocarbon oxy at the C-15 position of 2 led to more potent products 4i and 4j. For example, the FMR of 4a was 41.4%; whereas the FMRs of 4i and 4j were 62.1%, and 58.6%, respectively.

Conclusions

In summary, we prepared a series of 14-formyl-15-aryloxy/methoxymatrine and 14-aryloxymethylidenylmatrine derivatives as pesticidal agents by structural modification of matrine. A possible reaction mechanism for 3a–l reacting with 2 was also proposed. Their pesticidal activities were evaluated against two crop-threatening agricultural insect pests, M. separata and P. xylostella. In particular, compounds 4i and 4k exhibited more promising pesticidal activities than matrine against M. separata and P. xylostella. This demonstrated that the introduction of an electron-withdrawing group on the phenyl ring of 14-formyl-15-phenyloxymatrine, and the introduction of an alkyloxy or polycyclic aromatic hydrocarbon oxy at the C-15 position of 14-formyl-15-chloromatrine could result in more potent compounds. This will lay the foundations for further structural modification and application of matrines as biorenewable pesticidal agents for agriculture.

Acknowledgements

The present research was partly supported by National Natural Science Foundation of China (No. 31672071), and Special Funds of Central Colleges Basic Scientific Research Operating Expenses (No. 2452015096) to H. X.

Notes and references

1. X. F. Jiang, L. Z. Luo, L. Zhang, T. W. Sappington and Y. Hu, Environ. Entomol., 2011, 40, 516–533.
2. J. Y. Sun, P. Liang and X. W. Gao, Pest Manage. Sci., 2012, 68, 285–289.
3. J. Zeng, Y. Jiang and J. Liu, Plant Prot., 2013, 39, 117–121.
4. H. Wei, J. Liu, B. Li, Z. X. Zhan, Y. X. Chen, H. J. Tian, S. Lin and X. J. Gu, Crop Prot., 2015, 76, 68–74.
5. D. G. Heckel, Science, 2012, 337, 1612–1614.
6. K. Etebari, M. J. Furlong and S. Asgari, Sci. Rep., 2015, 5, 14642.
7. J. K. Kim, S. R. Choi, J. Lee, S. Y. Park, S. Y. Song, J. Na, S. W. Kim, S. J. Kim, I. S. Nou and Y. H. Lee, J. Agric. Food Chem., 2013, 61, 11222–11230.
8. Q. Fu, Q. Fang, B. L. Feng, S. J. Sun, W. Du, A. Enijian, A. P. Xiao and C. Chang, Journal of Chromatography B, 2011, 879, 894–900.
9. Z. J. Wu, D. M. Sun, D. M. Fang, J. Z. Chen, P. Cheng and G. L. Zhang, Int. J. Mass Spectrom., 2013, 341–342, 28–33.
10. Y. Liu, Y. Xu, W. D. Ji, X. Y. Li, B. Sun, Q. G. Gao and C. Q. Su, Tumor Biol., 2014, 35, 5111–5119.
11. B. Zhang, Z. Y. Liu, Y. Y. Li, Y. Luo, M. L. Liu, H. Y. Dong, Y. X. Wang, Y. Liu, P. T. Zhao, F. G. Jin and Z. C. Li, Eur. J. Pharm. Sci., 2011, 44, 573–579.
12. H. G. Hu, S. Z. Wang, C. M. Zhang, L. Wang, L. Ding, J. P. Zhang and Q. Y. Wu, Bioorg. Med. Chem. Lett., 2010, 20, 7537–7539.
13. L. M. Gao, Y. X. Han, Y. P. Wang, Y. H. Li, Y. Q. Shan, X. Li, Z. G. Peng, C. W. Bi, T. A. Zhang, N. N. Du, J. D. Jiang and D. Q. Song, J. Med. Chem., 2011, 54, 869–876.
14. L. Q. He, H. X. Gu, D. K. Yin, Y. H. Zhang and X. S. Wang, Chemical Journal of Chinese Universities, 2010, 31, 1541–1547.
15. L. S. Wang, Y. J. You, S. Q. Wang, X. Liu, B. M. Liu, J. N. Wang, X. Lin, M. S. Chen, G. Liang and H. Yang, Bioorg. Med. Chem. Lett., 2012, 22, 4100–4102.
16. J. L. Huang and H. Xu, Curr. Top. Med. Chem., 2016, 16, 3365–3378.
17. Z. Z. Odimar, D. P. R. Leandro, F. A. Thiago, S. S. Monica, P. B. Gabriela, T. Y. Pedro and D. V. Jose, Crop Prot., 2015, 67, 160–167.
18. B. Z. Zhong, C. J. Lu, X. D. Sun, W. Q. Qin and Z. Q. Peng, Agrochemicals, 2010, 49, 924–926.
19. J. Yuan, L. Z. Lu, B. Cong, Z. J. Zhang and F. Y. Wang, Chin. J. Pestic., 2016, 16, 3365–3378.
20. M. Garrido, M. Cabeza, F. Cortes, J. Gutierrez and E. Bratoeff, Eur. J. Med. Chem., 2013, 68, 301–311.
21. A. Sanchez-Marquez, Y. Arellano, E. Bratoeff, Y. Heuze, K. Cordova, G. Nieves, J. Soriano and M. Cabeza, J. Enzyme Inhib. Med. Chem., 2016, 31, 1170–1176.
22. E. Bratoeff, M. Garrido, T. Ramirez-Apan, Y. Heuze, A. Sanchez, J. Soriano and M. Cabeza, Bioorg. Med. Chem., 2014, 22, 6233–6241.
23. M. Lv, W. J. Wu and H. X. Liu, Pestic. Biochem. Physiol., 2008, 90, 114–118.
24. Q. Li, X. B. Huang, S. C. Li, J. C. Ma, M. Lv and H. Xu, J. Agric. Food Chem., 2016, 64, 5472–5478.
25. R. Wang, X. Y. Zhi, J. Li and H. Xu, J. Agric. Food Chem., 2015, 63, 6668–6674.
26. X. Yu, D. Shi, X. Zhi, Q. Li, X. Yao and H. Xu, RSC Adv., 2015, 5, 31700–31707.
27. A. L. LaFrate, J. R. Gunther, K. E. Carlson and J. A. Katzenellenbogen, Bioorg. Med. Chem., 2008, 16, 10075–10084.

---

Footnotes

† CCDC 1522361, 1524882, 1524886 and 1524887. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ra00954b

‡ These authors contributed equally to this work.

---