Synthesis, Structure and Insecticidal Activities of Some Novel Amides Containing N-Pyridylpyrazole Moeities

In our search for environmentally benign insecticides with high activity, low toxicity and low residue, a novel series of amides containing N-pyridylpyrazole moieties were designed and synthesized. The structures of the title compounds were characterized and confirmed by 1H-NMR and elemental analysis. Furthermore, the structure of compound 7l was determined by single crystal X-ray diffraction. The preliminary bioassay tests showed that some of them exhibited good insecticidal activities against Mythimna separata Walker, Plutella xylostella (Linnaeus, 1758) and Laphygma exigua Hübner.


Introduction
Development of crop-protection molecules with unique modes of action is necessary to combat widespread insecticide resistance. Calcium channels, in particular, the ryanodine receptor (RyR) represent an attractive biological target for insect control and thus offers excellent promise in integrated pest management strategies [1]. Anthranilic diamides, discovered by DuPont, are a promising novel class of insecticides which exhibit their action by binding to insect ryanodine receptors (RyR) and activating the uncontrolled release of calcium stores [2][3][4]. Anthranilic diamide OPEN ACCESS insecticides are characterized by a three-part chemical structure as shown in Figure 1A, where (X) is an anthraniloyl moiety, (Y) an aromatic acyl moiety and (Z) an aliphatic amide moiety. Notably, anthranilic diamides containing an N-pyridylpyrazole in the second section (Y) showed significantly better activity than other heterocyclic derivatives [5]. Work in this area has led to the discovery of Rynaxypyr TM ( Figure 1B), a highly potent and selective activator of insect ryanodine receptors with exceptional activity on a broad range of Lepidoptera, as the first new insecticide from this class [6]. In our previous work, when the N-pyridylpyrazole ring was replaced with 1,2,3-thiadiazole [7] or triazolopyrimidine [8] ones, the insecticidal activities were completely eliminated. In contrast, the modification of insecticidal anthranilic diamides with an ester group [9] or sulfonamide [10] substituting an amide group in the aliphatic amide moiety (Z) showed similar insecticidal activity, though of a lesser degree. Thus, these results suggest that the N-pyridylpyrazole unit plays an important role in the insecticidal activities of anthranilic diamides, but the aliphatic amide moiety (Z) may not be essential to insecticidal activities.
Encouraged by these reports, we developed an idea to examine whether the modification of the anthraniloyl skeleton by removing the aliphatic amide moiety (Z) could have an effect on potential insecticidal activities. Enlightened by all of the descriptions above, to further explore the comprehensive structure-activity relationships of the insecticidal activity, a series of novel amides containing N-pyridylpyrazoles were synthesized, and their insecticidal activities against Mythimna separata Walker, Culex pipiens pallens, Plutella xylostella (Linnaeus, 1758) and Laphygma exigua Hübner were tested and are discussed in this publication.

Chemistry
The synthetic route to the title compounds 7a-s is shown in Scheme 1. The pyrazole carboxylic acid 6 is a key intermediate to the synthesis of target amides 7 containing N-pyridylpyrazoles. Various synthetic routes have been reported for the synthesis of intermediate pyrazole-5-carboxylic acid 6 [11,12]. Considering the practical application of the synthetic method, an alternate route for the preparation of pyrazole carboxylic acid 6 was developed. Reaction of 2,3-dichloropyridine (1) with hydrazine hydrate at reflux using ethanol as solvent gave 3-chloro-2-hydrazinylpyridine (2). Condensation of diethyl maleate with hydrazine 2 in the presence of sodium ethoxide afforded the pyrazolidinone 3. Subsequent treatment of 3 with phosphorus oxybromide in acetonitrile afforded the pyrazoline 4.
A variety of reagents were explored for oxidation of 4 to the pyrazole 5. We first chose potassium permanganate as oxidant, but pyrazole 5 was obtained in only 32% yield. Subsequently potassium persulfate was used to give 5 in a good yield. The intermediate 6 could be prepared by hydrolysis of 5 with known methods. Finally, pyrazole carboxylic acid 6 was treated with oxalyl chloride at reflux to give the corresponding acid chloride, which was then reacted with commercially available substituted anilined to afford the title compounds 7a-s. The various amides 7a-s containing N-pyridylpyrazoles prepared are listed in Table 1

Crystal Structure
The structure of compound 7l was further confirmed by single crystal X-ray diffraction analysis (Figures 2 and 3). In the molecular structure of title compound, the three ring (benzene ring, pyridine ring and pyrazole ring) are nearly vertically with θ angle of 80.6° (benzene ring vs. pyridine ring), 76.8° (pyrazole ring vs. pyridine) respectively, but the pyrazole ring is planar with the benzene ring (7.8°).  The average bond lengths and bond angles of the phenyl ring [13][14][15][16][17], the pyrazole ring [18], pyridine ring [19] and the amide bond [20][21][22][23][24] are normal. The intermolecular edge-to-face π-π stacking appears between the pyridine ring and the phenyl ring in another adjacent molecule, in which the distance of H13 and the centroid of phenyl ring is 3.725 Å. These interactions can help to further stabilize the crystal structure. The title compound has an extensive network of hydrogen bonding involving the two acceptor N atoms. In the bc plane, they are linked together by N-H···N hydrogen bonds. This hydrogen-bonding sequence is repeated to form a ring.

Insecticidal Activities and Structure-Activity Relationship (SAR)
The insecticidal activities of all target compounds 7a-s were determined in vivo. The results are summarized in Table 2. As shown, 7l showed the most potent insecticidal activity against oriental armyworm (M. separata) in all the tested compounds, the death rate is 80% at 10 µg·mL −1 . Compounds 7c, 7i, 7p, 7q, 7i and 7r also exhibited significant insecticidal activity against oriental armyworm, with death rates of more than 70% at 25 µg·mL −1 . Further, the Structure-Activity Relationships (SAR) for different substitutions on the phenyl ring can be inferred from the results. Compound 7i with an electron-donating methyl group at R 1 exhibited similar insecticidal activity to that of compound 7l with an electron-withdrawing chlorine group at R 1 , but the insecticidal activity decreased significantly when the chlorine atom was replaced with a bromine atom at R 1 (compound 7m). From these results, it appears that steric effects rather than electrostatic effects have substantial effects on the insecticidal activity at the R 1 . This observation appears consistent with the structure-activity of anthranilic diamides reported in the literature [25]. Among compounds 7b-e, the electron-withdrawing group substituted analogues at R 3 (compounds 7b-e) were more active than the electron-donating group substituted analogue (compound 7f). Besides, the electron-withdrawing halide substituted analogues at R 3 (compounds 7p, 7q, 7s) exhibited more insecticidal potency than unsubstituted analogs (compounds 7j, 7k). The electron-donating group substitution (compound 7j) and electron-withdrawing group substitution (compound 7k) showed similar levels of activity, and the presence of a chloro substituent at R 5 (compound 7r) and unsubstituted analog (compound 7i) exhibited similar insecticidal activity. This result indicates that different substitutions at R 5 did not exhibit significant influence on insecticidal activities. In addition, the insecticidal activity decreased significantly when electron-withdrawing chloro substituent was present at R 2 and R 4 , such as 7a, 7g, 7o. As shown in Table 2, the title compounds displayed good larvicidal activities against C. p. pallens, for example, the larvicidal activity of 7d was 100% at 2 µg·mL −1 , as compared with 100% mortality of Rynaxypyr TM at the same concentration, but showed no clear SAR trends. Figure 4 shows the symptoms of larvae affected by the title compounds and commercial Rynaxypyr TM . Insects treated with the title compound 7l showed abnormal symptoms such as body contraction, vomiting, feeding cessation, body thickening and shortening, which are similar to those observed for larvae treated with commercial Rynaxypyr TM . These results suggest that the title compounds exhibit their activity by activating insect RyR. For the compounds 7b, 7c, 7p, 7q, 7s, 7l, 7m, further bioassay was conducted against P. xylostella and L. exigua. The results are summarized in Table 3. At a dose of 25 µg·mL −1 , most of compounds have good insecticidal activity against P. xylostella and L. exigua, which can be compared with that of the control Rynaxypyr TM . This result showed that the title compounds have insecticidal activity on a broad spectrum of Lepidoptera.

General
Melting points were determined on an X-4 binocular microscope melting point apparatus and were uncorrected. 1 H-NMR spectra were obtained on a Bruker AC-P500 spectrometer or Bruker Avance 400 spectrometer using tetramethylsilane (TMS) as an internal standard and CDCl 3 or DMSO-d 6 as solvents. Elemental analyses were performed on a Vario EL elemental analyzer. High-resolution mass (HRMS) were recorded on a 7.0-T (Ionspec, Irvine, CA, USA) Fourier transform ion cyclotron resonance mass spectrometer. Crystallographic data of the compound 7l were collected on a Rigaku Saturn diffractometer. All chemicals or reagents were purchased from standard commercial suppliers. Petroleum ether refers to the fraction of bp 60-90 C.

Chemical Synthesis
50% Hydrazine hydrate (200 mL).was added to a suspension of 2,3-dichloropyridine (1, 73.5 g, 0.5 mol) in anhydrous ethanol (300 mL). The resulting mixture was refluxed for 36 h, and then cooled to room temperature. A white crystalline product precipitated out of solution, which was collected by filtration, washed thoroughly with cold ethanol and dried to give hydrazine 2 (49.

Ethyl 2-(3-Chloro-2-pyridinyl)-5-oxopyrazolidine-3-carboxylate (3)
To absolute ethanol (200 mL) in a 500 mL three-necked round-bottomed flask was added sodium (6.9 g, 0.3 mol) cut into pieces of suitable size. After all the sodium had reacted, the mixture was heated to reflux and 2 (39.82 g, 0.277 mol) was added. The mixture was refluxed for 10 min, then diethyl maleate (51.65 g, 0.3 mol) was added dropwise. The resulting orange-red solution was held at reflux for 30 min. After being cooled to 65 °C, the reaction mixture was treated with glacial acetic acid (30 g, 0.51 mol). The mixture was diluted with water (30 mL). After removal of most of the solvent, the residue was treated with water (300 mL). The slurry formed was dissolved in aqueous ethanol (70%, 200 mL) and stirred thoroughly.  (4) To a solution of 3 (0.1 mol) in acetonitrile (300 mL) was added phosphorus oxybromide (0.12 mmol). The reaction mixture was refluxed for 5 h, then most of the solvent (ca. 250 mL) was removed by distillation. The concentrated reaction mixture was slowly poured into saturated aq. Na 2 CO 3 (250 mL) and stirred vigorously for 30 min. The resulting mixture was extracted with CH 2 Cl 2 (250 mL × 2), the organic extract was separated, dried, filtered, concentrated and purified by silica gel chromatography to afford intermediates 4.  (5) To a solution of 4 (51 mmol) in acetonitrile (250 mL) was added sulfuric acid (98%, 10 g, 102 mmol). After being stirred for several minutes, the reaction mixture was treated with K 2 S 2 O 8 (21 g, 76.5 mmol) and refluxed for 4.5 h. After being cooled to 60 °C, the mixture was filtered, the filter cake was washed with acetonitrile (30 mL). The filtrate was concentrated to 100 mL, then added slowly to water (250 mL) under stirring. The solid was collected by filtration, washed with acetonitrile (25%, 30 mL × 3), water (30 mL), and then dried to give intermediates 5.

General Procedure for the Synthesis of Compounds 7a-s
To a suspension of N-pyridylpyrazole acid 6 (1 mmol) in dichloromethane (20 mL) was added oxalyl chloride (3 mmol), followed by dimethylformamide (2 drops). The solution was stirred at room temperature. After 3 h the mixture was concentrated in vacuo to obtain the crude acid chloride. The crude acid chloride in dichloromethane (10 mL) was added slowly to a stirred solution of substituted aniline 1 (1.2 mmol) in dichloromethane (20 mL) in an ice bath. After 20 min, diisopropylethylamine (1 mmol) was added dropwise. The solution was warmed to room temperature and stirred for 12 h, then diluted with CH 2 Cl 2 (20 mL), and washed with 1 mol·L −1 aq. HCl solution (20 mL), saturated aq. NaHCO 3 (20 mL), and brine (20 mL). The organic extract was separated, dried, filtered, concentrated and purified by silica gel chromatography to afford the desired N-pyridylpyrazole-containing amides 7.

Crystal Structure Determination
The prism-shaped single crystal of the title compound was obtained by recrystallization from EtOH. The crystal with dimensions of 0.20 mm × 0.16 mm × 0.12 mm was mounted on a rigaku saturn diffractometer with a graphite-monochromated MoKα radiation (λ = 0.71073Å) by using a Phi scan modes at 113(2) K in the range of 2.28 ≤ θ ≤ 25.02. The crystals are triclinic, space group P-1 with a = 8.8252 (18) Atomic scattering factors and anomalous dispersion corrections were taken from International Table for X-Ray Crystallography [27]. CCDC-893647 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk.

Biological Assay
All bioassays were performed on representative test organisms reared in the laboratory, which were repeated at 25 ± 1 C according to statistical requirements. Assessments were made on a dead/alive basis and evaluations are based on a percentage scale of 0-100 in which 0 = no activity and 100 = total kill.

Stomach Toxicity against Mythimna separata Walker
The leaf dipping assay method was used for M. separata tests [28,29], in which the corn leaves were dipped into a test solution for 20 s and allowed to dry. The treated diet was placed into a 7 cm diameter Petri dish, and 10 fourth-instar M. separata larvae were released into the dish. The symptoms of affected larvae were observed at 24 h after the application, and percentage mortalities were evaluated 72 h after treatment. For comparative purposes, Rynaxypyr TM was tested under the same conditions. Each treatment was performed three times.