Design, Synthesis, and Insecticidal Activity of Some Novel Diacylhydrazine and Acylhydrazone Derivatives

In this study a series of diacylhydrazine and acylhydrazone derivatives were designed and synthesized according to the method of active group combination and the principles of aromatic group bioisosterism. The structures of the novel derivatives were determined on the basis on 1H-NMR, IR and ESI-MS spectral data. All of the compounds were evaluated for their in vivo insecticidal activity against the third instar larvae of Spodoptera exigua Hiibner, Helicoverpa armigera Hubner, Plutella xyllostella Linnaeus and Pieris rapae Linne, respectively, at a concentration of 10 mg/L. The results showed that all of the derivatives displayed high insecticidal activity. Most of the compounds presented higher insecticidal activity against S. exigua than the reference compounds tebufenozide, metaflumizone and tolfenpyrad, and approximately identical insecticidal activity against H. armigera, P. xyllostella and P. rapae as the references metaflumizone and tolfenpyrad.


Introduction
Synthetic pesticides have performed major functions in feed, food and fiber production for many years. To a considerable extent, these pesticides may be expected to be used well into the future,

OPEN ACCESS
although the type of pesticides used may shift toward materials with novel modes of action and lower risk to humans and other non-target organisms [1,2].
The phthalic diamide flubendiamide and the anthranilic diamides chlorantraniliprole and cyantraniliprole were successively designed and synthesized by Japanese pesticide companies in 1998 and DuPont in 2001; these novel insecticides act on the ryanodine receptor (RyR) [3][4][5][6][7]. Bayer and Syngenta have also developed the meta-amino benzamides A and B (Figure 1), both of which are considered RyR pesticides [8][9][10]. Compounds A and B present excellent insecticidal activities against pests of different orders, such as Lepidoptera, Diptera, Coleoptera, Hemiptera, and Isoptera, and feature high selectivity, low mammalian toxicity, and environmental friendliness. As determined through electrophysiological and Ca 2+ -release studies these insecticides act by activating insect RyR, a non-voltage-gated calcium channel, to affect calcium release from intracellular stores by locking channels in a partially opened state [3,11,12]. All of the RyR insecticides described above contain two amide structures that are important to their insecticidal activity.  Diacylhydrazines have been identified as one of the most important types of insect regulators since the discovery of the N-tert-butyl-N,N'-diacylhydrazines in the mid-1980s by Rohm and Haas [13][14][15]. Several commercial compounds, such as tebufenozide, methoxyfenozide, chromafenozide, and halofenozide, are all classified as diacylhydrazines, and all of these insecticides affect the ecdysone receptor complex, leading to precocious lethal molting, especially in caterpillars [16,17]. Diacylhydrazines have attracted significant attention because of their high insecticidal selectivity, simple structure, and low toxicity to vertebrates [15].
Metaflumizone, a semicarbazone compound with a structure containing an acylhydrazone moiety, is a novel sodium channel blocker insecticide recently introduced to the Chinese market in 2010 by BASF. It provides excellent control of most economically important pests belonging to the orders Lepidoptera, Coleoptera, Hymenoptera, Hemiptera, Isoptera, Diptera, and Siphonaptera. Metaflumizone presents low risk to pollinators and beneficial insects, as well as humans and the environment. Insect strains that are resistant to organophosphates, carbamates, and imidacloprid do not display cross-resistance to metaflumizone. Thus, this insecticide demonstrates great potential use in Integrated Pest Management (IPM) and resistance management strategies [2,18,19]. Moreover many other acylhydrazone compounds also show strong insecticidal activity [20,21].
Plant pests, such as the beet armyworm (Spodoptera exigua Hiibner), diamondback moth (Helicoverpa armigera Hubner), cotton bollworm (Plutella xyllostella Linnaeus), and cabbage caterpillar (Pieris rapae Linne), are harmful to crops all over the world. Unfortunately, pest control of these species has become increasingly difficult because of development of resistance to traditional insecticides. Poor pest control leads to enormous losses of crop production because of long-term use of conventional pesticides [3,22].
In the present work, we sought to incorporate an aromatic diamide unit into an aromatic diacylhydrazine or acylhydrazone moiety according to the method of active group combination and the principle of aromatic group bioisosterism. A total of four diacylhydrazine derivatives and 12 acylhydrazone derivatives were designed; the structures of these compounds combined a meta-amino benzamide with diacylhydrazine or acylhydrazone active groups (Scheme 1) and they were identified by IR, ESI-MS, and 1 H-NMR spectroscopy. The insecticidal activities of the resulting compounds against the third instarlarvae of beet armyworm, diamondback moth, cotton bollworm, and cabbage caterpillar were screened. Combination of critical components is expected to improve the biological activities of these pesticides.

Chemistry
Scheme 1 shows the route used successfully for the preparation of the four diacylhydrazine derivatives 3a-3d and 12 acylhydrazone derivatives 4a-4l. The raw materials 3-aminoethyl benzoate and acyl chlorides R1COCl (I) were dissolved in chloroform and heated to reflux to prepare the 3-acylamino ethyl benzoates 1a-1g. Using alcohol as a solvent, 3-acylaminobenzoyl hydrazines 2a-2g were obtained by reaction of compounds 1a-1g with hydrazine hydrate. Then, compounds 2a-2g were reacted with acyl chlorides R2COCl (II) in tetrahydrofuran at room temperature to prepare the diacylhydrazine derivatives 3a-3d. Again using alcohol as a solvent and trifluoroacetic acid as a catalyst, compounds 2a-2g were reacted with 2-(4-nitrilo)benzyl-1-substituedphenyl ketones II to form acylhydrazone derivatives 4a-4l. The structures of all of the diacylhydrazine and acylhydrazone derivatives were effectively determined through 1 H-NMR, ESI-MS, and IR spectroscopy.
Ref  Table 1 shows that all of the diacylhydrazine and acylhydrazone derivatives 3a-3d, 4a-4l display strong insecticidal activity against the third instar larvae of beet armyworm (S. exigua) at a concentration of 10 mg/L. Most of the synthesized compounds indicated higher insecticidal activity than the reference compounds tebufenozide, metaflumizone, and tolfenpyrad. Among the synthesized compounds, the mortality caused by compounds 3b, 4b, 4c, 4d, 4f and 4l exceeded 95% at the 72 h time point. Moreover the third instar larvae of beet armyworm showed 100% mortality within 72 h when treated with compounds 4b, 4d, and 4l. Table 1 further demonstrates that insect mortality presents a positive relationship with administration time. Table 2 presents the mortality data of cotton bollworm (H. armigera), diamondback moth (P. xyllostella), and cabbage worm (P. rapae) exposed to acylhydrazone derivatives 4a-4l at a concentration of 10 mg/L for 72 h. Compounds 4a-4l revealed strong insecticidal activity against the third instar larvae of these species. Insect mortalities from exposure to these compounds were approximately identical to those observed from exposure to the reference compounds metaflumizone and tolfenpyrad. Among the synthesized acylhydrazone derivatives, 4b, 4c, 4d, 4f and 4l showed 100% mortalities against the third instar larvae of P. rapae. Compound 4f in particular displayed broad spectrum insecticidal activity.

Insecticidal Activities
According to the data in Tables 1 and 2, it could be found that the presence of fluorine was important for the insecticidal activity of the synthesized compounds. Comparing the differences between the substituent groups and the position of the substituents on the benzene ring, it could be presumed that relatively weak electron-withdrawing effects could strengthen the insecticidal activities of the acylhydrazone derivatives, however electron-donating effects (such as seen in 4j) and strong electron-withdrawing effects (such as in 4h and 4i) could not do so.

General Procedures
All reagents were chemically pure and solvents were dried according to standard methods. 1 H-NMR spectra were obtained on an AM-500 spectrometer (Bruker, Karlsruhe, Germany) with DMSO-d6 as the solvent. IR spectra were recorded on an IR-200 spectrophotometer (Nicolet, Thermo Electron, Madison, WI, USA) using KBr disks. Mass spectra were recorded under ESI conditions on a Q-TOF spectrometer (Micromass, Waters Corp., Manchester, UK). Melting points were measured on a WRS-1A-type melting point apparatus (Shanghai, China) and are reported uncorrected. Analytical TLC was carried out on pre-coated silica gel plates, and spots were visualized through UV illumination (254 nm).

General Procedure for the Preparation of 1a-1g
3-Aminoethyl benzoate (30 mmol) was dissolved in chloroform (25 mL) in an ice-water bath. Then, an acyl chloride R1COCl (I, 30 mmol) was dissolved in chloroform (25 mL) and added dropwise to this solution. The mixture was reacted at 25 °C in a water bath for 5 h and then refluxed for 2 h. The reaction mixture was subsequently cooled to room temperature and filtered under vacuum to obtain the 3-acylaminoethyl benzoates 1a-1g with yields of 85%-95%.

General Procedure for the Preparation of 2a-2g
A mixture of 3-acylaminoethyl benzoates 1a-1g (20 mmol), 80% hydrazine hydrate (100 mmol), and ethanol (100 mL) was stirred and heated under reflux for 4 h. The reaction mixture was cooled to room temperature and filtered under vacuum. The solid obtained was washed with water (50 mL) to provide the 3-acylaminobenzoyl hydrazines 2a-2g in yields of 80%-95%.

General Procedure for Preparation of 3a-3d and 4a-4l
3-Acylaminobenzoyl hydrazines 2a-2g (5 mmol) and NaOH (5.7 mmol) were dissolved in dry tetrahydrofuran (50 mL). Acyl chlorides R2COCl (II, 5.7 mmol) dissolved in dry tetrahydrofuran (30 mL) were then dropped slowly into the above solution at 0 °C in an ice-water bath. The solution was reacted at room temperature for 15 h. After completion of the reaction, the reaction mixture was concentrated in vacuo and mixed with saturated NaHCO3 solution (25 mL). The mixture was stirred for 30 min and then filtered under vacuum. The filtrate was recrystallized with DMF and water (volume ratio, 2:1) to afford the target compounds 3a-3d.

Insecticidal Activity Bioassays
Wheat leaf discs measuring 0.5 cm × 0.5 cm were treated with 5 μL of 1.0 mg of test samples dissolved in 100 mL of acetone. Acetone was used as a negative control, whereas tebufenozide, metaflumizone, and tolfenpyrad were used as positive controls. The third instar larvae of S. exigua were allowed to feed on the discs. Cohorts of 24 beet armyworms were treated each time and bioassays were replicated three times. After 24, 48, and 72 h, the numbers of knocked-down larvae (indications: the larvae were narcotized, their bodies were very soft and immobile, and responses to stimuli disappeared completely) were recorded [23,24]. Insecticidal activity results are listed in Table 1.
Bioassays for insecticidal activity against cotton bollworm, diamondback moth, and cabbage worm was performed according to the method described above, except that the third instar larvae of S. exigua were replaced by the third instar larvae of H. armigera, P. xyllostella, and P. rapae. Metaflumizone and tolfenpyrad were used as positive controls, and the numbers of knocked-down larvae were recorded 72 h after exposure. Bioassay results are listed in Table 2.

Conclusions
In conclusion, four novel diacylhydrazine 3a-3d and 12 acylhydrazone derivatives 4a-4l were designed and synthesized according to the method of active group combination and the principle of aromatic group bioisosterism. The resultant analogs were evaluated (concentration, 10 mg/L) for their insecticidal activity against the third instar larvae of S. exigua, H. armigera, P. xyllostella, and P. rapae in vivo. Bioassays of these analogs showed high insecticidal activity. Most of the synthesized compounds presented higher insecticidal activity against S. exigua than the reference compounds tebufenozide, metaflumizone, and tolfenpyrad. Insecticidal activities against H. armigera, P. xyllostella, and P. rapae similar to that of the reference compounds metaflumizone and tolfenpyrad were also obtained. The results above motivate us to further explore novel diacylhydrazine and acylhydrazone derivatives as insecticidal agents; new findings will be reported in future work.