Iodine-catalyzed oxidative cyclisation for the synthesis of sarisan analogues containing 1,3,4-oxadiazole as insecticidal agents

Abstract

In continuation of our research aimed at the discovery and development of natural products-based insecticidal agents, a series of novel sarisan analogues containing 1,3,4-oxadiazole through iodine-catalyzed oxidative cyclisation were prepared as insecticidal agents against the pre-third Mythimna separata Walker at 1 mg mL⁻¹ in vivo. Compound 8r, 8t, 8k and 8q exhibited more promising insecticidal activities with final mortality rates of >60%, when compared with sarisan and toosendanin (a commercial insecticide). An efficient way using iodine as the catalyst and K₂CO₃ as base for the synthesis sarisan analogues containing 1,3,4-oxadiazole was developed. The results revealed that introduction of fluorophenyl or 4-cyanophenyl units on the 1,3,4-oxadiazole ring at the C-3 position of sarisan could afford more potent compounds. Moreover, introduction of heteroaromatic fragments on the 1,3,4-oxadiazole ring at the C-3 position of sarisan was very crucial for the insecticidal activity.

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Introduction

Although synthetic chemical insecticides have a longstanding and key role in agriculture and public health with the characteristics of high efficiency, quick-fix, low-price and insecticides spectrum broad, the overuse and improper application of synthetic chemical insecticides over the years have resulted in the development of pest resistance, pesticide residues, environmental problems, threat to non-target organisms, etc.^1–3 Since the publication of Rachel Carson's book Silent Spring, more than 550 pest species have been reported with resistance to one or more existing insecticides.^4,5 Hence the development of the effective, low mammalian toxicity, quick degradation, selective and eco-friendly insecticides for pest management have been encouraged to be necessary in the future. Botanical insecticides are naturally occurring chemicals resulted from the interaction between plants and the environment (life and non life) during the long period of evolution in plants, which are just satisfied with the demand of low insecticide resistance, quick degradation, eco-friendly characteristics and so on mentioned above.^6–8 In light of the advantages of botanical insecticides, the discovery and development of new insecticides from plant secondary metabolites or using them as the lead compounds for further modification have recently been a frequent topic of interest and the periodic subject for researchers to develop novel pesticides.^9–11

Sarisan (I, Fig. 1), bearing a piperonyl unit, is a naturally occurring monolignan and a major constituent of the essential of many species of plants in Umbelliferae, Piperaceae, Lauraceae and Saururaceae, including Ligusticum mutellina,¹² Heteromorpha trifoliata,¹² Piper solmsianum C.DC,¹³ Piper guineense,¹⁴ Piper sarmentosum,¹⁵ Beilschmiedia miersii¹⁶ Saururus chinensis.¹⁷ It is also an isomer of myristicin (II, Fig. 1) with known synergistic activity like piperonyl butoxide (III, Fig. 1), a commercial synergist for pyrethroids and carbamates.^16,18 Sarisan has shown promising activities in the field of medicine such as antimicrobial activity,¹² psychopharmacologic effect¹⁹ and anti-septic activity.²⁰ Additionally, the most noteworthy is its insecticidal activity against armyworms and Aedes aegypti.^21,22 Although sarisan was very easy to synthesize, to the best of our knowledge, little attention has been paid to structural modifications at C-3 or C-2′,3′ position of compound I for use as pesticides. On the other hand, 1,3,4-oxadiazoles are an important class of N-heterocyclic compounds with a wide range of biological activities such as anticancer, antimicrobial, antiviral, analgesic etc.,²³ especially insecticidal²⁴ and herbicidal activities,²⁵ for example, Oxadiazon (IV, Fig. 1) was the first 1,3,4-oxadiazole herbicide discovered by Rhone Poulenc Inc., in 1969, Metoxadiazone (V, Fig. 1), discovered by Sumitomo Chemical Co., Ltd and launched in 1978, has shown an excellent insecticidal activity against medical insects.^26,27 In light of the above-mentioned interesting results, and in continuation of our program aimed at the discovery and development of natural products-based insecticides,^28–31 herein we designed and used iodine as catalyst to successfully prepare a series of sarisan analogues containing 1,3,4-oxadiazole as insecticidal agents.

Fig. 1 Chemical structures of sarisan (I) and related compounds (II–V).

Experimental

Instrument and materials

All reagents and solvents were of reagent grade or purified according to standard methods before use. Analytical thin-layer chromatography (TLC) and preparative thin-layer chromatography (PTLC) were performed with silica gel plates using silica gel 60 GF₂₅₄ (Qingdao Haiyang Chemical Co., Ltd.). Melting points were determined on a digital melting-point apparatus and were uncorrected (Beijing Tech Instrument Co., Ltd.). Infrared spectra (IR) were recorded on a PE-1710 FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA). NMR spectra were carried out 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).

General procedure for the synthesis of compound (2)

A mixture of 1 (8 mmol, 1105 mg), allylbromide (9.6 mmol, 1161 mg) and K₂CO₃ (16 mmol, 2208 mg) in acetone (20 mL) was refluxed. When 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 silica gel column chromatography eluting with petroleum ether/ethyl acetate (10 : 1, v/v) to afford 2 in 93% yield.

General procedure for the synthesis of compound (3)

A solution of 2 (8 mmol, 1425 mg) in N,N-dimethylaniline (10 mL) was heated at 180 °C for 5 h. After cooling the mixture to RT, the mixture was diluted with ethyl acetate and washed with 1 N hydrochloric acid (3 × 20 mL), brine (2 × 20 mL) and dried over anhydrous Na₂SO₄. The crude product was concentrated in vacuo and purified by silica gel column chromatography eluting with petroleum ether/ethyl acetate (10 : 1, v/v) to afford 3 in 90% yield.

General procedure for the synthesis of compound (4 and 5)

To a solution of 3 (12 mmol, 2148 mg) in toluene (20 mL) at RT under N₂, the DBU (14.4 mmol, 2.15 mL) and SnCl₄ (3.6 mmol, 0.42 mL) were added in sequence. The reaction mixture was stirred at RT for 20 min and then paraformaldehyde (26.4 mmol, 1587 mg) was added. The resulting yellow solution was heated to 100 °C for 12 h. After cooling to RT, the reaction was quenched into 2 N HCl (10 mL) and extracted with ethyl acetate (3 × 20 mL). The combined organic layer was dried over anhydrous Na₂SO₄, filtered and evaporated. The residue was purified by silica gel column chromatography eluting with petroleum ether/ethyl acetate (10 : 1, v/v) to afford 4 and 5 in 20% and 58%, respectively.

Data for 4. CAS: 259138-63-1. White solid, yield: 20%, mp 83–85 °C (lit. 82–84 °C);¹⁸ IR cm⁻¹ (KBr): 3396, 3086, 3007, 2980, 2886, 2785, 1640, 1443, 1177, 972; ¹H NMR (400 MHz, CDCl₃) δ: 6.83 (s, 1H, –OH), 6.55 (s, 1H, H-6), 5.91–6.01 (m, 1H, H-2′), 5.85 (s, 2H, –OCH₂O–), 5.06–5.12 (m, 2H, H-3′), 4.85 (s, 2H, –CH₂OH), 3.30 (d, J = 6.4 Hz, 2H, H-1′).

Data for 5. CAS: 259138-64-2. Bright yellow solid, yield: 58%, mp 67–69 °C (lit. 68–70 °C);¹⁸ IR cm⁻¹ (KBr): 3251, 3072, 2971, 2907, 1657, 1459, 1241, 930; ¹H NMR (400 MHz, CDCl₃) δ: 10.68 (s, 1H, –CHO), 10.12 (s, 1H, –OH), 6.90 (s, 1H, H-6), 6.03 (s, 2H, –OCH₂O–), 5.87–5.97 (m, 1H, H-2′), 5.07–5.09 (m, 2H, H-3′), 3.30 (d, J = 6.4 Hz, 2H, H-1′).

General procedure for the synthesis of compounds (6)

A mixture of 5 (3.74 mmol, 771.2 mg), CH₃I (37.4 mmol, 2.3 mL) and K₂CO₃ (7.48 mmol, 1033.8 mg) in acetone (15 mL) was refluxed. 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 silica gel column chromatography eluting with petroleum ether/ethyl acetate (8 : 1, v/v) to afford 6 in 85% yield.

Data for 6. CAS: 316800-07-4. Pale yellow solid, yield: 81%, mp 100–101 °C (lit. 101–102 °C);¹⁸ IR cm⁻¹ (KBr): 3010, 2973, 2872, 2750, 1681, 1466, 1241, 924; ¹H NMR (400 MHz, CDCl₃) δ: 10.28 (s, 1H, –CHO), 6.90 (s, 1H, H-6), 6.10 (s, 2H, –OCH₂O–), 5.87–5.97 (m, 1H, H-2′), 5.06–5.13 (m, 2H, H-3′), 3.81 (s, 3H, –OCH₃), 3.35 (d, J = 6.4 Hz, 2H, H-1′).

General procedure for the synthesis of compounds (8a–u)

A mixture of 6 (0.25 mmol, 55.05 mg) and the corresponding hydrazides (0.275 mmol) in absolute ethanol (10 mL) was refluxed until the reaction was complete according to TLC analysis (1–5 h), and then the solvent was removed under reduced pressure, and the residue was redissolved in DMSO (5 mL), followed by addition of K₂CO₃ (0.75 mmol, 103.7 mg), iodine (0.3 mmol, 76.1 mg) in sequence. The reaction mixture was stirred at 100 °C until the reaction was complete according to TLC analysis (2–12 h). After being cooled to RT, the mixture was treated with saturated Na₂S₂O₃ (20 mL) and extracted with ethyl acetate (3 × 20 mL). The combined organic layer was washed with brine (3 × 20 mL), dried over anhydrous Na₂SO₄, filtered and evaporated. The given residue was purified by PTLC to give the target products 8a–u in 20–89% yield. The example data of 8a–e are shown as follows, whereas data of 8f–u can be found in the ESI.†

Data for 8a. White solid, yield: 52%, mp 86–87 °C; IR cm⁻¹ (KBr): 3076, 2978, 2950, 2923, 1619, 1478, 993, 928; ¹H NMR (400 MHz, CDCl₃) δ: 8.15–8.19 (m, 1H, –Ar), 7.52–7.58 (m, 1H, –Ar), 7.24–7.34 (m, 2H, –Ar), 6.84 (s, 1H, H-6), 6.12 (s, 2H, –OCH₂O–), 5.91–6.01 (m, 1H, H-2′), 5.09–5.13 (m, 2H, H-3′), 3.82 (s, 3H, –OCH₃), 3.42 (d, J = 6.4 Hz, 2H, H-1′). ¹³C NMR (100 MHz, CDCl₃) δ: 161.5, 160.0, 158.8, 150.2, 146.1, 144.4, 136.6, 133.5, 133.4, 130.0, 126.7, 124.7, 124.6, 117.0, 116.8, 116.4, 112.4, 112.3, 102.4, 101.9, 62.7, 33.5. HRMS (ESI): calcd for C₁₉H₁₆O₄N₂F ([M + H]⁺), 355.1100; found, 355.1100.

Data for 8b. White solid, yield: 44%, mp 74–75 °C; IR cm⁻¹ (KBr): 3075, 2999, 2942, 2922, 1636, 1470, 994, 929; ¹H NMR (400 MHz, CDCl₃) δ: 8.05 (dd, J = 8.0, 2.0 Hz, 1H, –Ar), 7.56 (dd, J = 8.0, 0.8 Hz, 1H, –Ar), 7.40–7.50 (m, 2H, –Ar), 6.84 (s, 1H, H-6), 6.12 (s, 2H, –OCH₂O–), 5.91–6.01 (m, 1H, H-2′), 5.09–5.13 (m, 2H, H-3′), 3.80 (s, 3H, –OCH₃), 3.42 (d, J = 6.4 Hz, 2H, H-1′). ¹³C NMR (100 MHz, CDCl₃) δ: 163.0, 160.1, 150.2, 146.1, 144.4, 136.6, 133.3, 132.4, 131.4, 131.2, 127.1, 126.7, 123.3, 116.4, 112.3, 102.4, 101.9, 62.8, 33.5. HRMS (ESI): calcd for C₁₉H₁₆O₄N₂Cl ([M + H]⁺), 371.0791; found, 371.0793.

Data for 8c. Pale yellow solid, yield: 46%, mp 88–89 °C; IR cm⁻¹ (KBr): 3008, 2944, 2914, 1628, 1474, 988, 926; ¹H NMR (400 MHz, CDCl₃) δ: 7.98 (dd, J = 7.6, 1.6 Hz, 1H, –Ar), 7.76 (d, J = 8.0 Hz, 1H, –Ar), 7.38–7.49 (m, 2H, –Ar), 6.84 (s, 1H, H-6), 6.12 (s, 2H, –OCH₂O–), 5.90–6.00 (m, 1H, H-2′), 5.09–5.13 (m, 2H, H-3′), 3.81 (s, 3H, –OCH₃), 3.41 (d, J = 6.4 Hz, 2H, H-1′). ¹³C NMR (100 MHz, CDCl₃) δ: 163.6, 160.1, 150.2, 146.1, 144.4, 136.6, 134.5, 132.4, 131.8, 127.6, 126.7, 125.4, 121.8, 116.4, 112.3, 102.4, 101.9, 62.8, 33.5. HRMS (ESI): calcd for C₁₉H₁₆O₄N₂Br ([M + H]⁺), 415.0288; found, 415.0288.

Data for 8d. White solid, yield: 57%, mp 99–100 °C; IR cm⁻¹ (KBr): 3005, 2955, 2923, 1630, 1465, 994, 931; ¹H NMR (400 MHz, CDCl₃) δ: 8.03 (d, J = 7.6 Hz, 1H, –Ar), 7.41–7.45 (m, 1H, –Ar), 7.32–7.37 (m, 2H, –Ar), 6.83 (s, 1H, H-6), 6.12 (s, 2H, –OCH₂O–), 5.91–6.01 (m, 1H, H-2′), 5.09–5.13 (m, 2H, H-3′), 3.81 (s, 3H, –OCH₃), 3.42 (d, J = 6.4 Hz, 2H, H-1′), 2.78 (s, 3H, –CH₃). ¹³C NMR (100 MHz, CDCl₃) δ: 164.8, 159.1, 150.1, 146.0, 144.4, 138.6, 136.7, 131.7, 131.2, 129.1, 126.7, 126.1, 123.0, 116.4, 112.1, 102.4, 102.1, 62.6, 33.5, 22.1. HRMS (ESI): calcd for C₂₀H₁₉O₄N₂ ([M + H]⁺), 351.1340; found, 351.1339.

Data for 8e. White solid, yield: 62%, mp 118–119 °C; IR cm⁻¹ (KBr): 3073, 2948, 2913, 2847, 1637, 1473, 993, 930; ¹H NMR (400 MHz, CDCl₃) δ: 8.01 (dd, J = 7.6, 1.6 Hz, 1H, –Ar), 7.49–7.54 (m, 1H, –Ar), 7.06–7.11 (m, 2H, –Ar), 6.82 (s, 1H, H-6), 6.11 (s, 2H, –OCH₂O–), 5.91–6.01 (m, 1H, H-2′), 5.09–5.13 (m, 2H, H-3′), 3.98 (s, 3H, –OCH₃), 3.80 (s, 3H, –OCH₃), 3.41 (d, J = 6.4 Hz, 2H, H-1′). ¹³C NMR (100 MHz, CDCl₃) δ: 163.4, 159.3, 157.9, 150.2, 146.0, 144.3, 136.7, 133.0, 130.7, 126.5, 120.8, 116.3, 113.1, 111.9, 102.3, 102.3, 62.6, 56.0, 33.5. HRMS (ESI): calcd for C₂₀H₁₉O₅N₂ ([M + H]⁺), 367.1287; found, 367.1288.

Biological assay

The insecticidal activity of sarisan (I), 4–6 and 8a–u against the pre-third-instar larvae of Mythimna separata Walker was assessed by leaf-dipping method as described previously.^28,30 For each compound, 30 larvae (10 larvae per group) were used. Acetone solutions of sarisan (I), 4, 5, 6, 8a–u and toosendanin (used as a positive control) were prepared at the concentration of 1 mg mL⁻¹. Fresh corn leaf discs (1 × 1 cm) were dipped into the corresponding solution for 3 s, then taken out and dried in RT. Leaf discs treated with acetone alone were used as a blank control group. Several treated leaf discs were kept in each dish, where every 10 larvae were raised. If the treated leaves were consumed, the corresponding ones were added to the dish. The experiment was carried out in a conditioned room (25 ± 2 °C and relative humidity (RH) 65–80%, and on 12 h/12 h (light/dark) photoperiod). After 48 h, untreated fresh leaves were added to all dish until the adult emergence. The corrected mortality rate values of the tested compounds against the pre-third-instar larvae of M. separata Walker was calculated by the following formula:

Corrected mortality rate (%) = (T − C) × 100/(1 − C)

where T is the mortality rate in the treated group expressed as a percentage, and C is the mortality rate in the untreated group expressed as a percentage.

Result and discussion

Synthesis

As shown in Scheme 1, in order to save economic costs, sesamol (1 was purchased from Macklin Biochemical Co., Ltd. Shanghai, China) was used as the starting material for our synthesis instead of using sarisan extracted from plants. When sesamol reacted with allylbromide and K₂CO₃, allyl ether 2 formed in 93% yield, followed by Claisen rearrangement to obtain phenol 3. Formylation of phenol 3 with SnCl₄, paraformaldehyde and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to afford 4 and 5 in 20%, 58% yield, respectively. Subsequently, methylation of compound 5 with CH₃I–K₂CO₃ in acetone was smoothly acquired 3-formylsarisan (6), finally, 3-formylsarisan was first condensed with each different arylhydrazide, after the reaction was complete, the solvent was evaporated under reduced pressure, and then the residue was redissolved in DMSO, followed by the iodine-catalyzed oxidative cyclisation to obtain a series of sarisan analogues containing 1,3,4-oxadiazole (8a–u). The structures of all target compounds were characterized by ¹H NMR, ¹³C NMR, IR, HRMS, and mp.

Scheme 1 Synthetic route for the preparation of sarisan analogues containing 1,3,4-oxadiazole 8a–u.

What's more, taking the synthesis of compound 8p as a model (Table 1), the reaction conditions for the synthesis of sarisan analogues containing 1,3,4-oxadiazole using molecular iodine as the catalyst and different base were investigated in DMSO at 100 °C. The results indicated that the usage of the inorganic base (Na₂CO₃, K₂CO₃, Cs₂CO₃) gave higher yields and shorter reaction time than the organic base such as Et₃N and DBU. Especially, the usage of K₂CO₃ as the base gave the best result (yield: 89%, reaction time: 2 h).

Table 1 Investigation of reaction conditions for the synthesis sarisan analogues containing 1,3,4-oxadiazole 8p from 3-formylsarisan (6) using different base^a

Entry	Base	Time	Isolated yield^b (%)
a Optimal reaction conditions: molar ratio of 6 (0.5 mmol)/7p (0.55 mmol)/I2 (0.6 mmol)/K2CO3 (1.5 mmol) = 1 : 1.1 : 1.2 : 3, DMSO, 100 °C.b Isolated yields after silica gel column chromatography.
1	Na2CO3	3.5	52
2	K2CO3	2	89
3	Cs2CO3	1	71
4	Et3N	6	26
5	DBU	4	45

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Insecticidal activity

The insecticidal activity of sarisan (I), 4–6 and 8a–u against the pre-third-instar larvae of M. separata Walker was evaluated as the mortality rates at 1 mg mL⁻¹. Toosendanin, a commercial insecticide, 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 2, the corresponding mortality rates after 35 days were generally higher than those after 25 and 10 days, which indicated that these compounds showed delayed insecticidal activity.³² This phenomenon was different from traditionally synthetic pesticides possessing the quick-acting, such as organophosphates, organochlorine pesticide etc. In addition, in treated groups, due to feeding too much treated leaves during the first 48 h, some larvae died slowly during the larvae period (Fig. 2). And part of larvae cannot successfully molt to normal pupae, and these malformed pupae will die during the pupation period (Fig. 3). In the last stage of emergence period, many malformed sharply appeared with shrinking or immature wings (Fig. 4). This symptom was also consistent with the reason why the corresponding mortality rates after 35 days were generally higher than those after 25 and 10 days. As described in Table 2, ten compounds, 8a, 8b, 8f, 8k, 8l, 8q, 8r, 8s, 8t and 8u exhibited more potent insecticidal activity than toosendanin. Especially, compound 8r displayed the most outstanding insecticidal activity with a final mortality rate of 69.0%, which was 20.7% points higher than the positive control toosendanin. Meanwhile, we discovered some interesting results of structure–activity relationships of (I) and 8a–u. Introduction of heteroaromatic units on the 1,3,4-oxadiazole ring at the C-3 position of sarisan could lead to the promising compounds. For example, the final mortality rates of 8r (R = pyridin-3-yl), 8s (R = 2-ethoxy-pyridin-3-yl), 8t (R = pyridin-4-yl) and 8u (R = thiophene-2-yl) were 69.0, 55.2, 65.5 and 51.7%, respectively. Moreover, we found that introduction of fluorophenyl or o/p-chlorophenyl fragments on the 1,3,4-oxadiazole ring at the C-3 position of sarisan could also bring about the potent compounds. For example, the final mortality rates of 8a (R = o-fluorophenyl), 8f (R = m-fluorophenyl), 8k (p-fluorophenyl), 8b (R = o-chlorophenyl) and 8l (R = p-chlorophenyl) were 58.6, 55.2, 62.1, 51.7 and 51.7%, respectively. However, when bromophenyl fragments on the 1,3,4-oxadiazole ring were introduced at C-3 position of sarisan could not afford the more potent compounds (e.g., 37.9% for 8c, 41.4% for 8h and 37.9% for 8m). And also when the 1,3,4-oxadiazole ring possessing electron-donating groups on the phenyl ring substituted at C-3 position of sarisan to afford 8d, 8e, 8i, 8j, 8n and 8o, the final mortality rates of them were no more than 44.8%. (e.g., 41.4% for 8d, 8j and 8n, 44.8% for 8e, 8i and 8o). Furthermore, introduction of 4-cyanophenyl on the 1,3,4-oxadiazole ring at the C-3 position of sarisan to obtain 8q, which also showed better insecticidal activity than toosendanin (62.1% for 8q vs. 48.3% for toosendanin).

Table 2 Insecticidal activity of sarisan analogues containing 1,3,4-oxadiazole 8a–u against M. separata Walker on leaves treated at a concentration of 1 mg mL⁻¹

Compd.	Corrected mortality rate (% ± SD)
	10 days	20 days	35 days
Sarisan (I)	13.8(±3.3)	20.7(±3.3)	44.8(±3.3)
4	3.4(±3.3)	20.7(±3.3)	41.4(±3.3)
5	6.9(±5.8)	24.1(±3.3)	37.9(±5.8)
6	3.4(±3.3)	24.1(±6.7)	41.4(±3.3)
8a	24.1(±3.3)	41.4(±3.3)	58.6(±5.8)
8b	10.3(±3.3)	24.1(±3.3)	51.7(±3.3)
8c	10.3(±6.7)	27.6(±0)	37.9(±5.8)
8d	6.9(±0)	24.1(±3.3)	41.4(±3.3)
8e	27.6(±5.8)	34.5(±3.3)	44.8(±3.3)
8f	31.0(±3.3)	37.9(±0)	55.2(±3.3)
8g	27.6(±0)	37.9(±5.8)	44.8(±3.3)
8h	13.8(±3.3)	24.1(±3.3)	41.4(±3.3)
8i	24.1(±3.3)	34.5(±3.3)	44.8(±3.3)
8j	6.9(±0)	24.1(±3.3)	41.4(±3.3)
8k	20.7(±3.3)	31.0(±3.3)	62.1 (±3.3)
8l	3.4(±3.3)	20.7(±3.3)	51.7(±3.3)
8m	24.1(±3.3)	27.6(±0)	37.9(±5.8)
8n	31.0(±3.3)	34.5(±3.3)	41.4(±3.3)
8o	3.4(±3.3)	17.2(±5.8)	44.8(±3.3)
8p	10.3(±3.3)	24.1(±3.3)	44.8(±3.3)
8q	27.6(±5.8)	41.4(±3.3)	62.1(±3.3)
8r	10.3(±3.3)	34.5(±6.7)	69.0(±5.8)
8s	6.9(±0)	27.6(±5.8)	55.2(±3.3)
8t	10.3(±3.3)	27.6(±5.8)	65.5(±3.3)
8u	17.2(±5.8)	24.1(±3.3)	51.7(±3.3)
Toosendanin	10.3(±3.3)	17.2(±0)	48.3(±5.8)
Blank control	3.3(±3.3)	3.3.(±3.3)	3.3(±3.3)

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Fig. 2 The representative abnormal larvae pictures of 8i (QLL-52), 8r (QLL-53), 8u (QLL-57), 8a (QLL-58), 8n (QLL-60), 8s (QLL-66) and 8e (QLL-69) during the larval period (CK: blank control group).

Fig. 3 The representative malformed pupae pictures of 8g (QLL-49), 8r (QLL-53), 8a (QLL-58), 8n (QLL-60), 8d (QLL-61), 8q (QLL-65) and 8e (QLL-69) during the pupation period (CK: blank control group).

Fig. 4 The representative malformed moth pictures of 8m (QLL-45), 8f (QLL-47), 8g (QLL-49), 8t (QLL-54), 8n (QLL-60), 8q (QLL-65) and 8e (QLL-69) during the emergence period (CK: blank control group).

Conclusions

In summary, a series of novel sarisan analogues containing 1,3,4-oxadiazole were prepared and evaluated for their insecticidal activity against the pre-third-instar larvae of M. separata Walker in vivo. Among all of derivatives, compound 8r, 8t, 8k and 8q exhibited more promising insecticidal activities with the final mortality rates of >60%. On the other hand, we have developed an efficient way using iodine as the catalyst and K₂CO₃ as base for the synthesis sarisan analogues containing 1,3,4-oxadiazole. The results suggested that introduction of fluorophenyl, o/p-chlorophenyl or 4-cyanophenyl fragments on the 1,3,4-oxadiazole ring at the C-3 position of sarisan could afford more potent compounds than that possessing bromophenyl or electron-donating groups on the phenyl ring fragments, and introduction of heteroaromatic units on the 1,3,4-oxadiazole ring at the C-3 position of sarisan was very crucial for the insecticidal activity. Now, the development of novel sarisan analogues containing other heterocyclic ring at other position (such as C-2′,3′) of sarisan is ongoing in our lab. This work will provide definitive experimental and theoretical basis for further design, structural modification, and development of sarisan as a novel insecticidal agent.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21502176), the China Postdoctoral Science Foundation (No. 2015M582207), Postdoctoral Research Sponsorship in Henan Province (2015001) and Startup Research Fund of Zhengzhou University (145-51090119). We are deeply thankful to Dr Bing Zhao for the NMR tests and Dr Yi Wang (Shanxi Agricultural University) for help in biological assay.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22343e

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