1. Introduction

Neonicotinoid insecticides have wide application in the pest control due to their high activity, low toxicity, low residue and high selectivity [1, 2]. However, with the increasing of improper application, they caused many negative effects, such as resistance [3], toxicity to bees [4] and environmental pollution [5]. Therefore, it is necessary to find novel neonicotinoid candidate with novel scaffolds, high-efficiency and low toxicity. The core structure of neonicotinoids consists of four segments, that is, a heterocycle, a linkage, a guanidine/amidine and an electro-withdrawing group. Nitromethylene analogue of imidacloprid (NMI) is a good precursor for structural modifications (Scheme 1).

1, 8-Naphthyridine (NAP) derivatives (Scheme 1) [5] have wide biological activity, such as anticancer [6-9], antibacterial [10-13], anti-inflammatory [14-16], antioxidant activities [17, 18], inhibitors of enzymes [19-21], as well as antagonists or agonists of receptors [22-25]. Previously, our research group developed a new method to synthesis NAP derivatives via three components of heterocyclic ketone aminals (HKAs), malononitrile dimmer, and aldehyde through one-pot method [5]. NMI, a HKA and an important neonicotinoid lead compound with excellent insecticidal activities, has higher insecticidal activity than imidacloprid. But its photo instability impeded its commercialization. Therefore, a plenty of chemical modifications have been conducted to improve its physiochemical properties generating many excellent insecticide candidates. Previous study proved that many derivatizations on NMI maintain its high activity. In combination of structure features of NMI and our newly-developed synthetic methods for NAP construction, we herein described a series of NAP derivatives as novel neonicotinoid insecticides.

2. Results and discussion

Nine 1, 8-naphthyridines were easily synthesized via one-pot method according to our previously-reported procedure. Products 4 was prepared through NMI, various aldehydes and malononitrile dimmer. Malononitrile dimmer 3 and aldehydes 2 underwent Knoevenagel condensation at the beginning of the reaction, then the aza-ene reaction took place between NMI 1 and Knoevenagel adducts of malononitrile dimmer 3 and aldehydes 2. For 4a-4c, part of the products was precipitated from the reaction mixture and were collected by filtration. The remaining 4a-4c in filtrate were subject to column chromatography followed by recrystallization to wipe off trace impurities in the end. The compounds 4d-4i can be easily obtained by filtration and recrystallization. The structures of these compounds were well confirmed by NMR and high-resolution mass spectrometry (HRMS). As previously reported [5], the substituent on the aryl ring had great influences on t"showhe yields of products 4, phenyl aldehyde with electrondonating gave lower yields and the steric hindrance of the substituent on the aryl ring also limited the yields of products 4. In this reaction, product 4a bearing a hydroxyl at the benzene ring exhibited lower yield than other substrates. The electron-donating feature of hydroxyl group might limit the Knoevenagel condensation between aldehyde 2 and malononitrile dimmer 3.

The insecticidal activities of all the prepared compounds (4a-4i) against cowpea aphid were screened. Compounds 4a, 4b, 4e, 4g and 4i had good aphicidal activity at concentration of 100 mg/L. Based on these results, the insecticidal activities of compounds 4a, 4b, 4e, 4g and 4i were further tested to obtain the median lethal concentration (LC₅₀). The LC₅₀ values of compounds 4a, 4b, 4e, 4g and 4i are 0.064, 0.067, 0.011, 0.21 and 0.031 mmol/L, respectively (shown in Table 1).

Compounds 4e and 4i have excellent insecticidal activity against cowpea aphids, while compounds 4a, 4b and 4g have moderate aphicidal activity. It is interesting to found that the compounds with mono-substitution on benzene ring have higher activities than compounds with bis-substituted benzene. In mono-substituted compounds, the substituent on the 4-position of benzene ring was favorable to the insecticidal activity. Compound with fluorine on the 4-position of benzene ring has better aphicidal activities than others. These results showed that introduction of fluorine on the 4-position of benzene has beneficial effects on the aphicidal activities. However, compound with fluorine at 2-position of benzene showed lower insecticidal activity against cowpea aphids.

3. Conclusion

In conclusion, several 1, 8-naphthyridine derivatives were designed and synthesized by one-pot method. Some of the target compounds have excellent insecticidal activities against cowpea aphids, while some compounds exhibited moderate antiaphids activities. The introduction of fluorine on the molecule could improve the insecticidal activity. We anticipate that this example can be potentially helpful in the development of future generation of novel insecticides.

4. Experimental 4.1. Chemicals and instrumentations

Melting points (mp) were recorded on a Büchi B540 apparatus (Büchi Labortechnik AG, Flawil, Switzerland) and are uncorrected. ¹H NMR and ¹³C NMR spectra were recorded on a Bruker AM-400 (400 MHz) spectrometer with CDCl₃ or DMSO-d₆ as the solvent and TMS as the internal standard. Chemical shifts are reported in δ (parts per million) values. High-resolution mass spectrometry (HRMS) data were recorded on a MicroMass GCT CA 055 instrument under electro impact (70 eV) condition. Analytical thin-layer chromatography (TLC) was carried out on precoated plates (silica gel 60 F254), the purity of synthesized compounds were checked by thin-layer chromatography (TLC) and spots were visualized with ultraviolet (UV) light.

4.2. Synthetic routes and experimental data

The general synthetic procedures for compounds 4a-4i are depicted in Scheme 2. Unless other special noted, reactant and solvents were used as received from commercial suppliers.

Synthetic methods of compounds 4a-4i. As previously reported [5], 2-chloro-5-[(2-nitromethylene) imidazolidin-1-yl]methyl] pyridine 1 (1 mmol), benzaldehyde 2 (3 mmol), malononitrile dimmer 3 (3 mmol) and ethanol (10 mL) were placed in a 25 mL round-bottom flask and the reaction mixture were stirred for 36 h at 70 ℃. Completion was monitored by TLC. Then, the mixture was filtrated and the crude product was further purified by column chromatography (CH₂Cl₂:MeOH = 20:1). Specially, in the procedure of purification, products 4d-4i were obtained by filtration and recrystallization in methanol.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(4-hydroxyphenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4a). Yield, 15%; mp, 306.2-307.4 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 9.17 (s, 1H), 8.30 (s, 1H), 7.56 (d, 1H, J = 7.9 Hz), 7.25 (d, 1H, J = 8.2 Hz, ), 6.87 (d, 2H, J = 8.1 Hz), 6.56 (d, 2H, J = 8.1 Hz), 6.38 (d, 4H, J = 19.9 Hz), 5.49 (s, 1H), 4.75 (s, 2H), 4.29-3.63 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 159.57, 155.91, 154.27, 154.13, 149.33, 149.18, 148.38, 138.86, 133.30, 131.15, 127.66, 128.82, 116.68, 114.75, 106.79, 90.10, 70.22, 50.54, 50.05, 43.02, 36.56; HRMS (ES+) calcd. for C₂₃H₁₉³⁵ClN₈O₃Na (M+Na)⁺, 513.1166; found, 513.1165. Calcd for C₂₃H₁₉³⁷ClN₈O₃Na (M+Na)⁺, 515.1137; found, 515.1143.

2, 4-Diamino-5-(4-chlorophenyl)-7-((6-chloropyridin-3-yl) methyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4b). Yield, 23%; mp, 294.2-295.3 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.22 (s, 1H), 7.55 (d, 1H, J = 8.3 Hz), 7.19 (dd, 3H, J = 8.4 Hz), 7.02 (d, 2H, J = 8.3 Hz), 6.46 (d, 4H, J = 6.2 Hz), 5.60 (s, 1H), 4.75 (q, 2H, J = 15.7 Hz), 4.23-3.81 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆) d 159.78, 154.37, 153.64, 149.39, 149.01, 148.55, 141.91, 138.77, 131.08, 130.90, 128.49, 127.87, 123.74, 116.60, 106.18, 92.80, 70.14, 50.81, 50.40, 42.97, 36.72 ppm; HRMS (ES+) calcd for C₂₃H₁₈³⁵Cl₂N₈O₂Na (M+Na)⁺, 513.0827; found, 513.0822. Calcd. for C₂₃H₁₈³⁷Cl₂N₈O₂Na (M+Na)⁺, 533.0798; found, 533.0789.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(2-fluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4c). Yield, 45%; mp, 267.9-279.1 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.27 (s, 1H), 7.65 (d, 1H, J = 8.1 Hz), 7.52 (t, 1H, J = 7.6 Hz), 7.27 (t, 1H, J = 12.3 Hz), 7.20 (dd, 1H, J = 12.8, 6.7 Hz), 6.99 (dt, 2H, J = 19.3, 8.1 Hz), 6.44 (s, 2H), 6.10 (s, 2H), 5.51 (s, 1H), 4.85-4.64 (m, 2H), 4.19 (dd, 1H, J = 9.0 Hz), 4.10-3.88 (m, 3H); 13C NMR (100 MHz, DMSO-d₆): δ 159.58, 154.09, 153.25, 149.34, 149.12, 148.59, 147.97, 140.22, 138.94, 131.05, 129.11, 128.41, 126.66, 123.82, 116.43, 115.60, 115.39, 104.78, 92.34, 70.28, 50.61, 43.06, 35.27; HRMS (ES+) calcd. for C₂₃H₁₈³⁵ClFN₈O₂Na (M+Na)⁺, 515.1123; found, 515.1127. Calcd. for C₂₃H₁₈³⁷ClFN₈O₂Na (M+Na)⁺, 517.1093; found, 517.1105.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(3-fluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4d). Yield, 24%; mp, 269.0-271.1 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.23 (s, 1H), 7.58 (d, 1H, J = 7.8 Hz), 7.18 (dd, 2H, J = 14.9, 7.8 Hz), 6.95 (dd, 2H, J = 23.6, 9.3 Hz), 6.82 (d, J = 7.6 Hz, 1H), 6.48 (s, 4H), 5.61 (s, 1H), 4.75 (q, J = 15.8 Hz, 2H), 4.29-3.81 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 160.94, 159.79, 154.38, 153.69, 149.32, 148.99, 148.54, 145.91, 138.77, 131.01, 129. 94, 123.73, 122.45, 116.59, 113.90, 113.59, 113.20, 106.01, 92.87, 70.13, 50.59, 42.99, 37.15; HRMS (ES+) calcd. for C₂₃H₁₈³⁵ClFN₈O₂Na (M +Na)⁺, 515.1123; found, 515.1124. Calcd. for C₂₃H₁₈³⁷ClFN₈O₂Na (M +Na)⁺, 517.1093; found, 517.1096.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(4-fluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4e). Yield, 47%; mp, 287.3-288.6 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.22 (s, 1H), 7.57 (d, 1H, J = 8.0 Hz), 7.22 (d, 1H, J = 8.2 Hz), 7.14-6.98 (m, 2H), 6.94 (t, 2H, J = 8.8 Hz), 6.45 (d, 4H, J = 10.2 Hz), 5.58 (s, 1H), 4.75 (q, 2H, J = 15.7 Hz), 4.27-3.83 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 159.74, 154.32, 153.66, 149.33, 149.01, 148.48, 139.12, 138.82, 131.10, 128.48, 123.78, 116.62, 114.67, 114.46, 106.50, 93.30, 70.14, 50.76, 50.40, 42.97, 36.60; HRMS (ES+) calcd. for C₂₃H₁₈³⁵ClFN₈O₂Na (M+Na)⁺, 515.1123; found, 515.1125. Calcd. for C₂₃H₁₈³⁷ClFN₈O₂Na (M+Na)⁺, 517.1093; found, 517.1086.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(2, 6-difluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4f). Yield, 53%; mp, 293.4-294.2 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.27 (s, 1H), 7.69 (d, 1H, J = 8.2 Hz), 7.46-7.17 (m, 2H), 6.94 (t, 2H, J = 8.3 Hz), 6.53 (s, 2H), 5.66 (d, 3H, J = 21.6 Hz), 4.80 (q, 2H, J = 15.6 Hz), 4.29-3.90 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 161.53, 159.59, 159.12, 154.04, 152.95, 149.33, 147.90, 139.10, 130.88, 128.73, 123.84, 117.55, 116.14, 111.82, 111.57, 103.70, 90.90, 70.42, 50.73, 42.98, 29.96. HRMS (ES+) calcd. for C₂₃H₁₇³⁵ClF₂N₈O₂Na (M+Na)⁺, 533.1029; found, 533.1028. Calcd. for C₂₃H₁₇³⁷ClF₂N₈O₂Na (M+Na)⁺, 535.0999; found, 535.0994.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(3, 4-difluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4g). Yield, 26%; mp, 284.1-285.9 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.16 (s, 1H), 7.59 (d, 1H, J = 8.0 Hz), 7.30-7.00 (m, 3H), 6.79 (s, 1H), 6.47 (d, 4H, J = 16.2 Hz), 5.55 (s, 1H), 4.75 (dd, 2H, J = 43.6, 15.7 Hz), 4.25-3.90 (m, 4H); ¹³C NMR (101 MHz, DMSO-d₆): δ 159.84, 154.33, 153.29, 149.34, 148.90, 148.46, 140.33, 138.78, 131.06, 123.61, 123.02, 116.87, 116.69, 116.55, 115.88, 110.92, 106.06, 92.63, 70.09, 51.16, 50.37, 42.96, 36.71; HRMS (ES+) calcd. for C₂₃H₁₇³⁵ClF₂N₈O₂Na (M+Na)⁺, 533.1029; found, 533.1031. Calcd. for C₂₃H₁₇³⁷ClF₂N₈O₂Na (M+Na)⁺, 535.0999; found, 535.1004.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-5-(2, 4-difluorophenyl)-6-nitro-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8]naphthyridine-3-carbonitrile (4h). Yield, 25%; mp, 279.9-281.4 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.22 (s, 1H), 7.74-7.45 (m, 2H), 7.30 (d, 1H, J = 8.1 Hz), 7.03-6.73 (m, 2H), 6.45 (s, 2H), 6.15 (s, 2H), 5.50 (s, 1H), 4.78 (dd, 2H, J = 15.5 Hz), 4.13 (dd, 4H, J = 18.3, 11.8 Hz); ¹³C NMR (100 MHz, DMSO-d₆): δ 159.62, 154.11, 152.89, 149.42, 149.08, 148.00, 138.92, 132.15, 131.03, 125.34, 123.78, 116.43, 110.43, 104.67, 104.18, 103.92, 103.64, 91.98, 70.29, 50.92, 50.56, 43.02, 34.99; HRMS (ES+) calcd. for C₂₃H₁₇³⁵ClF2N₈O₂Na (M+Na)⁺, 533.1029; found, 533.1033. Calcd. for C₂₃H₁₇³⁷ClF₂N₈O₂Na (M+Na)⁺, 535.0999; found, 535.1003.

2, 4-Diamino-7-((6-chloropyridin-3-yl)methyl)-6-nitro-5-(4-(trifluoromethyl)phenyl)-5, 7, 8, 9-tetrahydroimidazo[1, 2-a][1, 8] naphthyridine-3-carbonitrile (4i). Yield, 45%; mp, 313.3-314.9 ℃; ¹H NMR (400 MHz, DMSO-d₆): δ 8.21 (s, 1H), 7.52 (dd, 3H, J = 16.8, 8.2 Hz), 7.20 (t, 2H, J = 15.6 Hz), 7.13 (d, 1H, J = 8.2 Hz), 6.49 (s, 4H), 5.71 (s, 1H), 4.76 (dd, 2H, J = 42.2, 15.8 Hz), 4.26-3.85 (m, 4H); ¹³C NMR (100 MHz, DMSO-d₆): δ 159.87, 154.45, 153.56, 149.28, 149.03, 148.69, 147.60, 138.75, 130.99, 127.41, 125.74, 124.86, 123.66, 116.57, 105.84, 99.50, 92.51, 70.17, 50.98, 50.37, 42.97, 37.17; HRMS (ES+) calcd. for C₂₄H₁₈³⁵ClF₃N₈O₂Na (M+Na)⁺, 565.1091; found, 565.1089. Calcd. for C₂₄H₁₈³⁷ClF₃N₈O₂Na (M+Na)⁺, 567.1062; found, 517.1071.

4.3. Biological activity

The insecticidal activities of the title compounds against cowpea aphid were tested by the method of leaf dip bioassay [26]. The aqueous solution of each compound were prepared which plus 0.1% Triton X-100 (0.1 mg/L) as a surfactant and DMSO as solvent in the solution, then diluted it with 0.1% Triton X-100 (0.1 mg/L) to obtain the solution with different concentration (500 mg/L, 100 mg/L, 20 mg/L, 4 mg/L, 0.8 mg/L and others). Several adults of aphid were selected, avoid light, and given a starvation treatment about an hour in the dark. Then let them eat broad bean seedling until their mouthparts pierce the bean sprout (2-3 hours). Then, the leaves of the horsebean plant with 40-60 aphids was dipped for 3 s in each concentration 3 times, repeat this three times in each group. After treatment, the burgeoning shoots were placed in a conditioned room (25 ± 1) ℃, 50% RH) for 48 hours. Water containing Triton X-100 (0.1 mg/L) was used as a control. The state of aphid was observed and aphid mortality was recorded after 48 h. The toxicity experiment of each compound was repeated three times, and the results were calculated by SPSS [27]. Median lethal concentration (LC₅₀) were determined by Probit regression analysis program and expressed in parts per million (ppm) [28].

Acknowledgments

This work was financial supported by National Natural Science Foundation of China (Nos. 21372079, 21472046), Shanghai Pujiang Program (No. 14PJD012).