Semisynthesis and insecticidal activity of some novel fraxinellone-based thioethers containing 1,3,4-oxadiazole moiety

Abstract

Two series of novel fraxinellone-based thioethers containing 1,3,4-oxadiazole moiety were prepared as insecticidal agents against the oriental armyworm, Mythimna separata Walker. The structural assignment was based on the spectroscopic and X-ray analysis data. Among all the target compounds, compounds 4b, 4k, 5b, 5j and 5k exhibited more potent insecticidal activity with final mortality rates (FMRs) of more than 65%, especially 4k with the FMR of 75.9%, when compared with toosendanin. Some interesting results of structure–activity relationships are also discussed.

1. Introduction

The oriental armyworm (Mythimna separata Walker, Lepidoptera: Noctuidae), a typical long-distance migratory insect, is one of the most serious pests of cereal crops in countries including China, India, Australia and New Zealand [1,2]. Seasonal outbreaks of this pest can cause significant economic damage to cereal crops in China and other countries [3]. A recent outbreak of M. separata has been reported in northeast and central China during 2012, which caused losses of approximately 10 million acres of crops [4]. Synthetic chemical pesticides play a crucial role in agriculture with the characteristics of high-efficiency, quick-fix and broad-spectrum insecticides. Although they have been extensively used to control insect pest outbreaks, the overuse and improper application of synthetic chemical pesticides over the years has resulted in enhancement of pest resistance, environmental problems and negative impacts on human health [5–7]. Hence the discovery and development of effective, selective and eco-friendly pesticides is necessary in the future.

Fraxinellone (1, figure 1), a naturally occurring degraded limonoid, isolated from Fagaropsis glabra, [8] Dictamnus albus, [9] Melia azadarach [10] and Dictamnus dasycarpus, [11] exhibits a variety of interesting activities both in the fields of medicinal chemistry and agrochemistry, such as anti-inflammatory [12], vascular relaxing activity [13] and insecticidal activity [14–16]. The total synthesis of fraxinellone can be easily achieved, which has been reported early in 1972 [17]. Previously, we have studied the insecticidal activity of some fraxinellone-based hydrazones and esters [18,19] (I–IV, figure 1) modified at the C-4 or C-10 position in the A ring of fraxinellone, and N-phenylpyrazole fraxinellone hybrid compounds [20] (V, figure 1), and found some compounds against M. separata displayed higher insecticidal activity than positive control toosendanin. To the best of our knowledge, little attention has been paid to the introduction of active N-heterocyclic moieties on the furyl-ring of fraxinellone as insecticidal agents. 1,3,4-Oxadiazoles are an important class of N-heterocyclic compounds with a wide range of biological activities [21] including antimicrobial, analgesic, anticancer activities, especially insecticidal and herbicidal activities [22,23]. In a continuation of our programme aimed at the development of fraxinellone-based insecticidal agents, herein we prepared two series of novel fraxiellone-based thioethers containing 1,3,4-oxadiazole moiety (VI and VII, figure 1) as insecticidal agents against M. separata.

Figure 1.

Chemical structures of fraxinellone (1) and its derivatives (I–VII).

2. Experimental

2.1. Instrument and materials

The intermediate 2-mercapto-5-aryl-1,3,4-oxadiazoles a–k (scheme 1) were synthesized as previously reported [24]. Other reagents were of analytically grade and purchased from commercial resources. Fraxinellone (1) was isolated from Dictamnus dasycarpus and its purity was more than 99% as measured with reverse phase high-performance liquid chromatography (RP-HPLC). Analytical thin-layer chromatography (TLC) and preparative thin-layer chromatography (PTLC) were prepared by silica gel plates using silica gel GF₂₅₄ (Qingdao Haiyang Chemical Co., Ltd, Qingdao, China). Melting points were determined on a digital melting-point apparatus and were uncorrected (Beijing Tech Instrument Co., Ltd). Optical rotation was measured using an Autopol III automatic polarimeter (Rudolph Research Analytical, NJ, USA). Infrared spectra (IR) were recorded on a PE-1710 FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA). NMR spectra were obtained in CDCl₃ on a Bruker Avance (400 MHz) spectrometer using tetramethylsilane (TMS) as the internal standard (Bruker, Bremerhaven, Germany). High-resolution mass spectra (HR-MS) were carried out with LTQ FT Ultra instrument (Thermo Fisher Scientific Inc., Waltham, MA, USA).

Scheme 1.

Synthetic route for the preparation of fraxiellone-based thioethers containing 1,3,4-oxadiazole moiety (4a–k and 5a–k).

2.1.1. Data for 1

White solid, m.p. 113–115°C; IR cm^–1: 3148, 2930, 1741, 1671, 1607, 1202, 1022; ¹H NMR (400 MHz, CDCl₃) δ: 7.46 (s, 1H, H-2′), 7.43 (t, J = 1.6 Hz, 1H, H-5′), 6.34 (d, J = 1.2 Hz, 1H, H-4′), 4.88 (s, 1H, H-8), 2.13–2.31 (m, 2H, H-4), 2,11(s, 3H, H-10), 1.71–1.87 (m, 3H, H-5, 6), 1.42–1.48 (m, 1H, H-6), 0.86 (s, 3H, H-9); MS (ESI), m/z (%) 233.05 ([M + H]⁺, 58).

2.2. General procedure for synthesis of compounds 2 and 3

To a stirred suspension solution of AlCl₃ (1.0 mmol) in dry CH₂Cl₂ (5 ml) at RT, chloroacetyl chloride (1.1 mmol) was added. The mixture was then stirred for 10 min, and a solution of compound 1 (1.0 mmol) in dry CH₂Cl₂ (5 ml) was added dropwise to the above mixture. When the reaction was complete according to TLC analysis, the reaction mixture was poured into ice water (15 ml) and extracted with CH₂Cl₂ (40 ml × 3). The combined organic phase was washed with saturated brine (40 ml), dried over anhydrous Na₂SO₄, concentrated in vacuo, and then purified by PTLC to give the pure products 2 (35% yield) and 3 (45% yield).

2.2.1. Data for 2

White solid, yield: 35%, m.p. 96–98°C; [α]²⁰_D = 45 (c 4.0 mg ml^–1, acetone); IR cm^–1: 2946, 2918, 2872, 1757, 1687, 1472, 1203, 975; ¹H NMR (400 MHz, CDCl₃) δ: 7.55 (d, J = 1.6 Hz, 1H, H-5′), 6.79 (d, J = 2.0 Hz, 1H, H-4′), 5.66 (s, 1H, H-8), 4.60–4.72 (m, 2H, –CH₂Cl), 2.19–2.26 (m, 2H, H-4), 2.13 (s, 3H, H-10), 1.77–1.83 (m, 2H, H-5, 6), 1.58–1.63 (m, 2H, H-5, 6), 0.84 (s, 3H, H-11); HRMS (ESI): Calcd for C₁₆H₁₈O₄Cl ([M+ H]⁺), 309.0888; found, 309.0888.

2.2.2. Data for 3

White solid, yield: 45%, m.p. 106–108°C; [α]²⁰_D = –20 (c 3.5 mg ml^–1, acetone); IR cm^–1: 2956, 2923, 2870, 1737, 1671, 1496, 1235, 908; ¹H NMR (400 MHz, CDCl₃) δ: 7.68 (s, 1H, H-2′), 7.27 (s, 1H, H-4′), 4.89 (s, 1H, H-8), 4.59 (s, 2H, –CH₂Cl), 2.23–2.34 (m, 2H, H-4), 2.14 (s, 3H, H-10), 1.72–1.86 (m, 3H, H-5, 6), 1.47–1.53 (m, 1H, H-6), 0.84 (s, 3H, H-11); HRMS (ESI): Calcd for C₁₆H₁₈O₄Cl ([M + H]⁺), 309.0888; found, 309.0888.

2.3. General procedure for synthesis of compounds 4a–k and 5a–k

A mixture of the corresponding 2-mercapto-5-aryl-1,3,4-oxadiazole (0.3 mmol), 2 or 3 (0.2 mmol, 61.6 mg), K₂CO₃ (0.3 mmol, 41.5 mg) and KI (0.05 mmol, 8.3 mg) in acetone (10 ml) was stirred at room temperature. After the reaction was complete according to TLC analysis, the solvent was removed and the residue was dissolved in CH₂Cl₂ and filtered. The filtrate was concentrated in vacuo and purified by PTLC to give pure products 4a–k and 5a–k. The example data of 4a–d and 5a–d are described as follows, whereas the data of other compounds 4e–k and 5e–k are shown in the electronic supplementary material.

2.3.1. Data for 4a

White solid, yield: 54%, m.p. 76–79°C; [α]²⁰_D = 18 (c 3.4 mg ml^–1, acetone); IR cm^–1: 2935, 1750, 1673, 1587, 1473, 1414, 1355, 1204, 1129, 1046; ¹H NMR (400 MHz, CDCl₃) δ: 7.97 (dd, J = 7.6, 1.2 Hz, 2H, –Ph), 7.60 (d, J = 1.6 Hz, 1H, H-5′), 7.47–7.53 (m, 3H, –Ph), 6.83 (d, J = 1.2 Hz, 1H, H-4′), 5.67 (s, 1H, H-8), 4.67–4.87 (m, 2H, –COCH₂S–), 2.13–2.26 (m, 5H, H-4, 10), 1.71–1.77 (m, 2H, H-5, 6), 1.55–1.61 (m, 2H, H-5, 6), 0.86 (s, 3H, H-11).

2.3.2. Data for 4b

White solid, yield: 68%, m.p. 66–68°C; [α]²⁰_D = 11 (c 3.6 mg ml^–1, acetone); IR cm^–1: 2923, 1752, 1674, 1588, 1495, 1472, 1414, 1204, 1129, 1048; ¹H NMR (400 MHz, CDCl₃) δ: 7.97–8.01 (m, 1H, –Ph), 7.60 (d, J = 1.6 Hz, 1H, H-5′), 7.50–7.55 (m, 1H, –Ph), 7.28 (d, J = 8.0 Hz, 1H, –Ph), 7.21 (d, J = 8.4 Hz, 1H, –Ph), 6.83 (d, J = 1.6 Hz, 1H, H-4′), 5.67 (s, 1H, H-8), 4.67–4.88 (m, 2H, –COCH₂S–), 2.13–2.26 (m, 5H, H-4, 10), 1.73–1.77 (m, 2H, H-5, 6), 1.55–1.63 (m, 2H, H-5, 6), 0.86 (s, 3H, H-11).

2.3.3. Data for 4c

White solid, yield: 72%, m.p. 81–83°C; [α]²⁰_D = 10 (c 4.2 mg ml^–1, acetone); IR cm^–1: 2927, 1753, 1673, 1587, 1478, 1414, 1355, 1275, 1204, 1129, 1046; ¹H NMR (400 MHz, CDCl₃) δ: 7.91 (dd, J = 7.6, 1.6 Hz, 1H, –Ph), 7.60 (d, J = 1.6 Hz, 1H, H-5′), 7.52–7.54 (m, 1H, –Ph), 7.44 (td, J = 7.6, 2.0 Hz, 1H, –Ph), 7.37 (td, J = 7.6, 1.2 Hz, 1H, –Ph), 6.83 (d, J = 1.6 Hz, 1H, H-4′), 5.67 (s, 1H, H-8), 4.67–4.88 (m, 2H, –COCH₂S–), 2.15–2.26 (m, 2H, H-4), 2.13 (s, 3H, H-10), 1.71–1.77 (m, 2H, H-5, 6), 1.54–1.58 (m, 2H, H-5, 6), 0.86 (s, 3H, H-11).

2.3.4. Data for 4d

Pale yellow solid, yield: 73%, m.p. 64–66°C; [α]²⁰_D = 5 (c 3.7 mg ml^–1, acetone); IR cm^–1: 2931, 1752, 1673, 1587, 1475, 1413, 1355, 1259, 1203, 1129, 1046; ¹H NMR (400 MHz, CDCl₃) δ: 7.84 (d, J = 8.4 Hz, 2H, –Ph), 7.63 (d, J = 8.4 Hz, 2H, –Ph), 7.60 (d, J = 1.6 Hz, 1H, H-5′), 6.83 (d, J = 1.6 Hz, 1H, H-4′), 5.66 (s, 1H, H-8), 4.67–4.88 (m, 2H, –COCH₂S–), 2.08–2.26 (m, 2H, H-4, 10), 1.71–1.77 (m, 2H, H-5, 6), 1.54–1.58 (m, 2H, H-5, 6), 0.86 (s, 3H, H-11).

2.3.5. Data for 5a

White solid, yield: 65%, m.p. 81–83°C; [α]²⁰_D = –2 (c 3.2 mg ml^–1, acetone); IR cm^–1: 2940, 2920, 1744, 1676, 1474, 1205, 1133, 1048, 1003; ¹H NMR (400 MHz, CDCl₃) δ: 7.98 (d, J = 6.4 Hz, 2H, –Ph), 7.70 (s, 1H, H-2′), 7.47–7.53 (m, 3H, –Ph), 7.29 (s, 1H, H-4′), 4.89 (s, 1H, H-8), 4.73 (s, 2H, –COCH₂S–), 2.17–2.34 (m, 2H, H-4), 2.14 (s, 3H, H-10), 1.83–1.91 (m, 2H, H-5, 6), 1.71–1.78 (m, 1H, H-5), 1.46–1.53 (m, 1H, H-6), 0.85 (s, 3H, H-11).

2.3.6. Data for 5b

White solid, yield: 87%, m.p. 111–112°C; [α]²⁰_D = –19 (c 3.2 mg ml^–1, acetone); IR cm^–1: 2935, 1750, 1682, 1618, 1495, 1472, 1388, 1206, 1161, 1047; ¹H NMR (400 MHz, CDCl₃) δ: 7.97–8.01 (m, 1H, –Ph), 7.71 (s, 1H, H-2′), 7.48–7.55 (m, 1H, –Ph), 7.21–7.30 (m, 3H, H-4′ and –Ph), 4.90 (s, 1H, H-8), 4.73 (s, 2H, –COCH₂S–), 2.17–2.34 (m, 2H, H-4), 2.14 (s, 3H, H-10), 1.84–1.87 (m, 2H, H-5, 6), 1.73–1.77 (m, 1H, H-5), 1.47–1.54 (m, 1H, H-6), 0.85 (s, 3H, H-11).

2.3.7. Data for 5c

White solid, yield: 79%, m.p. 106–108°C; [α]²⁰_D = –12 (c 2.1 mg ml^–1, acetone); IR cm^–1: 2930, 1746, 1675, 1599, 1506, 1474, 1466, 1265, 1208, 1169, 1037; ¹H NMR (400 MHz, CDCl₃) δ: 7.91 (dd, J = 7.6, 1.6 Hz, 1H, –Ph), 7.70 (s, 1H, H-2′), 7.53 (d, J = 8.0 Hz, 1H, –Ph), 7.44 (td, J = 8.0, 2.0 Hz, 1H, –Ph), 7.38–7.41 (m, 1H, –Ph), 7.29 (s, 1H, H-4′), 4.90 (s, 1H, H-8), 4.73 (s, 2H, –COCH₂S–), 2.17–2.34 (m, 2H, H-4), 2.14 (s, 3H, H-10), 1.84–1.87 (m, 2H, H-5, 6), 1.73–1.77 (m, 1H, H-5), 1.46–1.54 (m, 1H, H-6), 0.86 (s, 3H, H-11).

2.3.8. Data for 5d

Pale yellow solid, yield: 89%, m.p. 131–132°C; [α]²⁰_D = –16 (c 3.2 mg ml^–1, acetone); IR cm^–1: 2961, 2930, 2862, 1746, 1675, 1600, 1507, 1466, 1393, 1317, 1207, 1169, 1020; ¹H NMR (400 MHz, CDCl₃) δ: 7.86 (dd, J = 7.6, 1.6 Hz, 1H, –Ph), 7.72 (dd, J = 8.0, 1.2 Hz, 1H, –Ph), 7.70 (s, 1H, H-2′), 7.42 (td, J = 7.6, 1.2 Hz, 1H, –Ph), 7.35 (td, J = 7.6, 1.6 Hz, 1H, –Ph), 7.30 (s, 1H, H-4′), 4.90 (s, 1H, H-8), 4.74 (s, 2H, –COCH₂S–), 2.17–2.34 (m, 2H, H-4), 2.14 (s, 3H, H-10), 1.84–1.87 (m, 2H, H-5, 6), 1.73–1.77 (m, 1H, H-5), 1.46–1.54 (m, 1H, H-6), 0.86 (s, 3H, H-11).

2.4. X-ray crystallography

The structures of compounds 4e and 5d were unambiguously confirmed by X-ray crystallography. Crystallographic data (excluding structure factors) of compounds 4e and 5d were deposited at the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers of CCDC 1552786 and 1552787, respectively.

2.5. Biological assay

Growth inhibitory activity of compounds 1–3, 4a–k and 5a–k against M. separata was evaluated by leaf-dipping method as described previously [19,25]. For each compound, 30 pre-third-instar larvae of same size and level of health (10 larvae per group) were chosen as the tested pests. Solutions of compounds 1–3, 4a–k and 5a–k and toosendanin (used as a positive control) were prepared in acetone at the concentration of 1 mg ml⁻¹. The larvae of tested groups were fed with compound-coated leaves (fresh corn leaf discs (1 × 1 cm) were dipped into the corresponding solution for 3 s, then taken out and dried at RT), whereas the blank control group (CK) was fed with acetone alone. Several treated leaf discs were kept in each dish. Once the treated leaves were consumed, the corresponding ones were added to the dish. The experiment was carried out at 25 ± 2°C; relative humidity (RH) 65–80%, and on 12 h/12 h (light/dark) photoperiod. After 48 h, untreated fresh leaves were added to all dishes until the adult emergence. The corrected mortality rate values of the tested compounds were calculated by the following formula: corrected mortality rate (%) = (T – C) × 100/(1 – C); Where T is the mortality rate in the treated group, and C is the mortality rate of CK.

3. Result and discussion

3.1. Synthesis

As shown in scheme 1, compounds a–k were synthesized based on previously reported literature [24]. Using different hydrazides as starting materials, compounds a–k were prepared by cyclization reaction of different hydrazides with carbon disulfide in the presence of KOH in EtOH at reflux temperature, followed by acidification with 5% HCl. When fraxinellone reacted with chloroacetyl chloride in the presence of AlCl₃, the corresponding 2'-chloroacetylfraxinellone (2) and 5'-chloroacetylfraxinellone (3) were obtained. Subsequently, different 2-mercapto-5-aryl-1,3,4-oxadiazoles (a–k) reacted with compound 2 or 3 in the presence of K₂CO₃/KI in anhydrous acetone at RT to smoothly acquire desired compounds 4a–k and 5a–k, respectively. The structures of all target compounds 4a–k and 5a–k were fully characterized by melting points, IR, optical rotation and ¹H NMR. Additionally, comparison of partial ¹H NMR spectra of compounds 1–3, 4a and 5a was illustrated in figure 2. It is obvious that the chemical shifts of H-4′ and H-8 of 2'-substituted fraxinellone are very different from 5'-substituted fraxinellone. When chloroacetyl substituted on 2'-position of fraxinellone, the proton of H-4′ was shifted from 6.34 [d, J = 1.2 Hz, 1, figure 2 (1)] to 6.79 [d, J = 2.0 Hz, 2, figure 2 (2)] ppm, and the proton of H-8 was shifted from 4.88 [s, 1, figure 2 (1)] to 5.66 [s, 2, figure 2 (2)] ppm; By contrast, when chloroacetyl substituted on 5'-position of fraxinellone, the proton of H-4′ was largely shifted from 6.34 [d, J = 1.2 Hz, 1, figure 2 (1)] to 7.27 [s, 3, figure 2 (4)] ppm, while the proton of H-8 was slightly shifted from 4.88 [s, 1, figure 2 (1)] to 4.89 ppm [s, 3, figure 2 (4)]. The chemical shifts of H-4′ and H-8 of compounds 4a and 5a were similar to compounds 2 and 3, respectively.

Figure 2.

Comparison of partial ¹H NMR spectra of compounds 1–3, 4a and 5a.

The structures of compounds 4e and 5d were further confirmed by X-ray crystallography (figures 3 and 4). As shown in figure 3, to the compound 4e, 2-mercapto-5-(4-fluorophenyl)-1,3,4-oxadiazole (e) linked with chloroacetyl was at the 2'-position on the furyl ring. On the contrary, to the compound 5d (figure 4), 2-mercapto-5-(2-bromophenyl)-1,3,4-oxadiazole (d) linked with chloroacetyl was at the 5'-position on the furyl ring of fraxinellone.

Figure 3.

X-ray crystal structure of compound 4e.

Figure 4.

X-ray crystal structure of compound 5d.

3.2. Insecticidal activity

The growth inhibitory activity of compounds 1–3, 4a–k and 5a–k against M. separata was tested at 1 mg ml⁻¹. Toosendanin, a commercial insecticide derived from Melia azedarach, was used as the positive control at 1 mg ml⁻¹, and corn leaves treated with acetone alone were used as a blank control. As shown in table 1, compounds 4b, 4c, 4j, 4k, 5b–d, 5j, 5k exhibited higher insecticidal activity than toosendanin and their precursor fraxinellone. For example, the final mortality rates (FMRs) of compounds 4b, 4c, 4j, 4k, 5b–d, 5j, 5k were 72.4%, 55.2%, 62.1%, 75.9%, 69.0%, 62.1%, 55.2%, 65.5% and 69.0%, respectively. In particular, compound 4k showed the most potent insecticidal activity, which was about 24% higher than toosendanin. The symptoms for the tested M. separata during the different periods of larval, pupation and adult were recorded by the same methods as our previous reports [19,25]. For example, in treated groups, due to overfeeding of treated leaves in the beginning, some larvae died slowly with thin and wrinkled bodies (figure 5). This phenomenon maybe results from these fraxinellone derivatives effects to nutritional or digestive interference [26]. During the pupation stage, some of the larvae did not successfully moult to normal pupae, and died (figure 6). In the last stage of emergence, many malformed moths appeared with shrunken or immature wings (figure 7). These results suggest that the fraxinellone derivatives containing the 1,3,4-oxadiazole probably affected the insect moulting hormone, which was crucial for the growth of M. separata. On the other hand, as displayed in figure 8, the percentages of FMRs of compounds 4b, 4j, 4k, 5b, 5c, 5j, 5k and toosendanin at three different growth stages of M. separata were investigated. We found that at least 50% of FMRs for compounds 4b, 4k, 5c, 5j, 5k and toosendanin were at the larval period except for compounds 4j and 5b.

Table 1.

Growth inhibitory activity of compounds 1–3, 4a–k and 5a–k against M. separata on leaves treated at a concentration of 1 mg ml^–1.

	corrected mortality rate (% ± s.d.)
compound	10 days	20 days	34 days
1	16.7(±3.3)	24.1(±3.3)	44.8(±3.3)
2	20.0(±5.8)	27.6(±5.8)	41.4(±3.3)
3	23.3(±3.3)	31.0(±3.3)	48.3(±5.8)
4a	13.3(±3.3)	24.1(±3.3)	37.9(±5.8)
4b	30.0(±0)	48.3(±5.8)	72.4(±6.7)
4c	20.0(±0)	37.9(±5.8)	55.2(±3.3)
4d	23.3(±3.3)	41.4(±3.3)	51.7(±3.3)
4e	16.7(±3.3)	31.0(±3.3)	51.7(±3.3)
4f	16.7(±3.3)	31.0(±3.3)	48.3(±0)
4g	23.3(±3.3)	31.0(±6.7)	44.8(±3.3)
4h	16.7(±3.3)	20.7(±3.3)	41.4(±3.3)
4i	13.3(±3.3)	24.1(±3.3)	37.9(±0)
4j	23.3(±6.7)	41.4(±3.3)	62.1(±3.3)
4k	33.3(±3.3)	51.7(±3.3)	75.9(±3.3)
5a	13.3(±3.3)	20.7(±3.3)	37.9(±5.8)
5b	26.7(±3.3)	41.4(±3.3)	69.0(±0)
5c	26.7(±3.3)	37.9(±5.8)	62.1(±3.3)
5d	23.3(±3.3)	34.5(±3.3)	55.2(±3.3)
5e	16.7(±3.3)	27.6(±0)	48.3(±5.8)
5f	20.0(±5.8)	31.0(±6.7)	48.3(±5.8)
5g	16.7(±6.7)	27.6(±0)	41.4(±3.3)
5h	13.3(±3.3)	17.2(±0)	34.5(±3.3)
5i	16.7(±3.3)	24.1(±3.3)	44.8(±3.3)
5j	26.7(±3.3)	41.4(±3.3)	65.5(±3.3)
5k	30.0(±5.8)	55.2(±3.3)	69.0(±5.8)
toosendanin	16.7(±3.3)	34.5(±3.3)	51.7(±3.3)
blank control	0(±0)	3.3(±3.3)	3.3(±3.3)

Figure 5.

The representative abnormal larvae pictures of 5e (XSN-16), 4e (XSN-17), 4b (XSN-21), 5c (YG-158), 5d (YG-160), 5j (YG-163) and 5k (YG-165) during the larval period (CK: blank control group).

Figure 6.

The representative malformed pupae pictures of 4b (XSN-21), 5c (YG-158), 5d (YG-160), 5i (YG-161), 4a (YG-166), 4f (YG-167) and 4d (YG-170) during the pupation period (CK: blank control group).

Figure 7.

The representative malformed moth pictures of 4b (XSN-21), 5b (YG-157), 5j (YG-163), 5k (YG-165), 4d (YG-170), 4j (YG-173) and 4k (YG-174) during the emergence period (CK: blank control group).

Figure 8.

The percentages of FMRs during three different growth periods of compounds 4b, 4j, 4 k, 5b, 5c, 5j, 5k and toosendanin.

Finally, we also discovered some interesting results of structure–activity relationships of the tested compounds.

When 2'-chloroacetylfraxinellone (2) or 5'-chloroacetylfraxinellone (3) linked with 2-mercapto-5-(pyridinyl)-1,3,4-oxadiazoles exhibited more potent insecticidal activity than toosendanin. For example, the FMRs of compounds 4j, 4 k, 5j and 5k were 62.1%, 75.9%, 65.5% and 69.0%, respectively. In other fraxinellone-based thioethers, introduction of electron-donating groups on the phenyl of 4a/5a resulted in less active compounds (e.g. 41.4% for 4h, 37.9% for 4i, 34.5% for 5h and 44.8% for 5i). When halogen atoms were introduced on the para-position of phenyl of 4a/5a, the compounds were not potent (except 4e); whereas halogen atoms at the ortho-position of phenyl of 4a/5a gave potent compounds. For instance, the FMRs of compounds 4b–d and 5b–d were greater than or equal to toosendanin (51.7%), especially the FMR of 4b was 72.4%. In our previous research, we found that introduction of heterocycle, fluorophenyl or o-chlorophenyl fragments on the 1,3,4-oxadiazole ring at the C-3 position of sarisan could afford more potent compounds [27]. In this paper, introduction of 3/4-pyridinyl or o-fluoro/chlorophenyl units on the 1,3,4-oxadiazole ring to the compound 2 or 3 also obtained the promising compounds 4j, 5j, 4k, 5k, 4b, 5b, 4c and 5c, respectively. Hence, this suggested that we could introduce the 2-mercapto-5-(3/4-pyridinyl/o-fluoro/chlorophenyl)-1,3,4-oxadiazoles activity units into other insecticidal lead compounds in the future.

4. Conclusion

In summary, we have prepared two series of novel fraxinellone-based thioethers containing 1,3,4-oxadiazole and evaluated for their insecticidal activity against a cereal crop-threatening agricultural insect pest, M. separata. The structures of key compounds 4e and 5d were assigned by X-ray crystallography. Among all target compounds, compounds 4b, 4k, 5b, 5j and 5k exhibited more potent insecticidal activity with FMRs of more than 65%. The results suggested that the introduction of 3/4-pyridinyl or o-fluoro/chlorophenyl units on the 1,3,4-oxadiazole ring to the compound 2 or 3 could afford more promising compounds. This will lay the foundations for further structural modification and application of fraxinellone as novel pesticidal agents in agriculture.

Ethics

The study was approved by the Research Ethics Committee of Zhengzhou University, Henan Province, PR China.

Data accessibility

Electronic supplementary material is available at the Dryad Digital Repository: https://doi.org/10.5061/dryad.424tb [28]. Crystallographic data (excluding structure factors) of compounds 4e and 5d were deposited at the Cambridge Crystallographic Data Centre (CCDC) with deposition numbers of CCDC 1552786 and 1552787, respectively. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. [fax, +44 (0)1223 336033; or e-mail, deposit@ccdc.cam.ac.uk].

Authors' contributions

Y.G. and Y. Zhang designed the study, and Y.G. performed part of the experiments and wrote the manuscript. X.W., J.F., Q.Z., Y.W., Y. Zhao, M.H. and M.D. carried out the experiments. All authors gave their final approval for publication.

Competing interests

We declare we have no competing interests.

Funding

This work was partly supported by National Natural Science Foundation of China (no. 21502176), the China Postdoctoral Science Foundation (no. 2015M582207), Postdoctoral Research Sponsorship in Henan Province (2015001), Startup Research Fund of Zhengzhou University (145-51090119) and the National Department of Public Benefit Research Foundation (201403031).