N-Substituted 5–Chloro-6-phenylpyridazin-3(2H)-ones: Synthesis, Insecticidal Activity Against Plutella xylostella (L.) and SAR Study

A series of N-substituted 5–chloro-6-phenylpyridazin-3(2H)-one derivatives were synthesized based on our previous work; all compounds were characterized by spectral data and tested for in vitro insecticidal activity against Plutella xylostella. The results showed that the synthesized pyridazin-3(2H)-one compounds possessed good insecticidal activities, especially the compounds 4b, 4d, and 4h which showed >90% activity at 100 mg/L. The structure-activity relationships (SAR) for these compounds were also discussed.


Introduction
The diamondback moth (Plutella xylostella L.) is a serious pest insect in many parts of the World [1,2]. Serious yield losses to crucifers (such as cabbage, cauliflower, broccoli, brussels sprouts and turnip) from the diamondback moth have become more common in recent years. Currently, insecticide application is the primary method for controlling this pest [2,3], but particularly severe diamondback moth resistance to insecticides has been resulting from the indiscriminate use of pesticides in many OPEN ACCESS tropical and subtropical countries [4][5][6] so controlling the diamondback moth has become more and more difficult, and the development of novel insecticides for this insect has attracted more and more attention.

Chemistry
The synthetic protocols of the title compounds was depicted in Scheme 1. Friedel-Crafts alkylation of benzene with mucochloric acid leads to γ-phenyldichlorocrotonolactone (7), then 7 reacts in a complex manner with hydrazine hydrate, with elimination of one atom of chlorine [23], to afford a high yields of 5-chloro-6-phenylpyridazin-3(2H)-one (8). Compounds 4a-s were then obtained in excellent yields by reaction of 8 with different halides based on our previous work [22]. The structures of the synthesized compounds were confirmed by 1 H-NMR, 13 C-NMR, IR and elemental analysis. All spectral and analytical data were consistent with the assigned structures. Scheme 1. Synthetic route to N-substituted 5-chloro-6-phenylpyridazin-3(2H)-ones 4a-s.

Structure-Activity Relationship (SAR) Study
The preliminary SAR analysis indicated that a big (in bulk) group with strong electronegativity at the 2-postion on the benzene (in a -CH 2 R group) has a positive influence enhancing the insecticidal activity of the synthesized compounds, that's why the compounds 4b (-CH 2 R = 2-NO 2 -C 6 H 4 CH 2 -) and 4h (-CH 2 R = 2-CH 3 O-C 6 H 4 CH 2 -) showed higher activity than 4e (-CH 2 R = 2-F-C 6 H 4 CH 2 -) and 4m (-CH 2 R = 2-CH 3 -C 6 H 4 CH 2 -), therefore, we can speculate that both the bulk and electronegativity of the substituent group at the 2-position on benzene play important roles in the insecticidal activity against P. xylostella. In addition, the position of the group on benzene also a key factor for the activity, as we can see that compound 4b showed excellent activity, while compound 4c (-CH 2 R= 3-NO 2 -C 6 H 4 CH 2 -) and 4i (-CH 2 R=4-NO 2 -C 6 H 4 CH 2 -) showed little (or no) activity (4b > 4c > 4i); a similar case can be found when the group was CH 3 O-(4h > 4j > 4k). Moreover, the introduction of 2-Cl-substituted pyridine and thiazole in group of -CH 2 R can also enhance insecticidal activity, e.g., compound 4b (-CH 2 R = 2-Cl-pyridine-CH 2 -) and 4t (-CH 2 R = 2-Cl-thiazole-CH 2 -) also displayed good insecticidal activity against P. xylostella.

Chemistry
Melting points were determined by using a XT-4 binocular microscope (Beijing Tech Instrument Co., Beijing, China) and are uncorrected. 1 H and 13 C-NMR spectra were recorded on a JEOL ECX 500 NMR spectrometer operating at room temperature and 500 MHz using acetone-d 6 or CDCl 3 as solvent and TMS as an internal standard. Infrared spectra were recorded by KBr using a Bruker VECTOR 22 spectrometer. Elemental analysis was performed using an Elemental Vario-III CHN analyzer. The course of the reactions was monitored by TLC; analytical TLC was performed on silica gel GF254. All reagents were of analytical grade or chemically pure. All anhydrous solvents were dried and purified according to standard techniques just before use. All the intermediates and title compounds were prepared according to the literature [22]. The properties for compounds 4a-h were reported in our previous work [22]. The properties for 4i-s are listed as follows.        : 158.36, 156.45, 144.16, 135.65, 134.27, 128.76, 128.56, 126.56, 126.35, 126.13

Insecticidal Bioassays
The insecticidal activities for the synthesized compounds against P. xylostella were evaluated using previously reported procedures [24][25][26]. Fresh cabbage discs (diameter 2 cm) were dipped into the prepared solutions containing compounds 4a to 4s for 10 s, dried in air and placed in a Petri dish (diameter 9 cm) lined with filter paper. Ten larvae of second-instar P. xylostella were carefully transferred to the Petri dish. Avermectin and chlorpyrifos were used as controls; three replicates were performed for each experiment. Mortalities were determined after 72 h. The results were summarized in Table 1.

Conclusions
In the present study, a series of N-substituted 5-chloro-6-phenylpyridazin-3(2H)-one derivatives were synthesized by employing mucochloric acid and benzene as the starting materials. The synthesized compounds were characterized by spectral data ( 1 H-NMR, 13 C-NMR, IR) and elemental analysis. The compounds were tested for insecticidal activity in vitro against P. xylostella. The results showed that the synthesized pyridazin-3(2H)-one compounds possessed weak to good insecticidal activities, especially the compounds 4b, 4d, and 4h which showed >90% activities at 100 mg/L. The preliminary SAR analysis indicated that a big (in bulk) group with strong electronegativity at the 2-postion on the benzene ring (in a -CH 2 R group) had a positive influence enhancing the insecticidal activity of the synthesized compounds; moreover, the introduction of a 2-Cl-substituted pyridine and thiazole in the -CH 2 R group can also enhance the insecticidal activity. Further studies are currently underway to optimize the structure to obtain better insecticidal activity in these N-substituted pyridazin-3(2H)-one derivatives.