Synthesis and spectral characterization of selective pyridine compounds as bioactive agents

)-thione compound N -(4- Compound (2) underwent cyclization upon heating in ethanolic sodium ethoxide solution to give the corresponding cyclized form 3-amino- N -(4-chlorophenyl)-4,6-distyrylthieno[2,3- b ]pyridine-2-carboxamide (3). The elemental analyses and spectroscopic data of compounds (2) and (3) are in agreement with their proposed structures. Their insecticidal activity against cowpea aphid, Aphis craccivora Koch, was studied. The results of insecticidal activity for compounds (2) and (3) against the nymphs and the adults of the tested insects exhibited that compounds (2) and (3) have a higher insecticidal activity than that of acetamiprid, a reference insecticide, after 24 h of treatment.


Introduction
The chemistry of functionalised pyridine compounds has been extensively developed, which is reflected in many papers that have been published. [1][2][3][4][5][6][7][8] This is associated with both a theoretical interest in this class of organic compounds and a very broad spectrum of the practical uses of pyridine derivatives. For example, more than 10% of the most commonly used modern medicines contain pyridine or the hydrogenated pyridine ring. 9 A lot of neonicotinoids insecticides contain pyridine ring in their structure and at the present time the use of these insecticides is increased in the field of crop protection as a result of their different advantages. [10][11][12][13][14][15] As an Expanding our work that focused on the synthesis of heterocyclic compounds and continuing our investigations on the chemistry of some pyridine derivatives neonicotinoid analogs, [1][2][3][4][5][6] we report herein the synthesis of two new heterocyclic compounds containing pyridine ring and screened their insecticidal activity against cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae) in the presence of acetamiprid insecticide as a reference.
IR spectrum of compound 2 showed absorption bands at 3292, 2216 and 1662 cm -1 characteristics for (NH), (C≡N) and (C=O) groups respectively. The absorption band of (C≡N) of compound 2 was disappeared when cyclised to give the thienopyridine 3 and was replaced by 3462 and 3408 cm -1 for NH2. 1 H NMR spectrum (DMSO-d6, 400 MHz) of compound 2 showed singlet signals at 10.51 and 4.27 for (NH) and (CH2) groups respectively. The signal of (CH2) group of compound 2 was disappeared when cyclised to give compound 3. DEPT 135 (DMSO-d6, 100 MHz) spectrum of compound 2 showed characteristic signal at 35.36 for (CH2) group which disappeared when cyclised to give compound 3.

Insecticidal activity test for the cowpea aphid nymphs.
Compounds 2 and 3 were tested for their insecticidal activity against the nymphs of the collected aphids and the results are presented in Table 1. After 24 h, insecticidal activity data showed that compounds 2 and 3 exhibited strong to weak insecticidal activity against nymphs of cowpea aphid and the LC50 values were 0.029 and 0.040 ppm, respectively, whereas the LC50 value of acetamiprid was 0.045 ppm. After 48 h of test, it is found that the insecticidal activity of compounds 2 and 3 against nymphs of cowpea aphid, Aphis craccivora Koch ranged from good to moderate and LC50 values were 0.006 and 0.007 ppm, respectively, whilst the LC50 value of acetamiprid 0.006 ppm. These results indicate that compounds 2 and 3 have excellent insecticidal activity compared with the insecticidal activity of acetamiprid insecticide because the toxic ratio of compounds 2 and 3 is more than that of acetamiprid after 24 h of test.

Insecticidal activity test for the cowpea aphid adults
Compounds 2 and 3 were tested for their insecticidal activity against the adults of the gathered aphids and the results are in Table 2. The results showed that after 24 h of test, compounds 2 and 3 have strong to weak activity and LC50 values were 0.149 and 0.183 ppm, respectively, whilst the LC50 value of acetamiprid was 0.225 ppm. After 48 h of insecticidal activity test, the insecticidal activity of compounds 2 and 3 varied from high to low and LC50 values were 0.017 and 0.022 ppm, respectively, against cowpea aphid adults, whilst 0.023 ppm was the LC50 value of acetamiprid. The above results revealed that the insecticidal activity of compounds 2 and 3 against adults of cowpea aphid was more than that of acetamiprid after 24 and 48 h of treatment.

Structure-action relationship
According to the general framework structure of the used compounds 2 and 3, it appears that the compound N-(4-chlorophenyl)-2-((3-cyano-4,6-distyrylpyridin-2-yl)thio)acetamide (2) is more active than the compound 3-amino-N-(4-chlorophenyl)-4,6-distyrylthieno[2,3-b]pyridine-2-carboxamide (3) against the cowpea aphids. The high activity associated with compound 2 may be due to the presence of the opened form structure in compound 2 and the presence of cyano group, but compound 3 was found in the cyclized form and cyano group is absent in its structure. (3), have been prepared in pure state and their insecticidal activity against cowpea aphid, Aphis craccivora Koch was evaluated. The results of this insecticidal activity illustrated that these compounds have a higher activity against cowpea aphid than that of acetamiprid insecticide and this emphasizes the importance of pyridine compounds in the agricultural uses.

Materials and methods
Melting points were determined by using a Fisher-Johns apparatus for the compounds 2 and 3.
Elemental analysis (C, H, N, and S) was determined by a Vario EL C, H, N, S analyzer. DEPT 135, 1 H NMR and 13 C NMR spectra measurement were accomplished via a Bruker 400 MHz spectrometer in the presence of tetramethylsilane (TMS) as an internal reference. Infrared (IR) spectra were determined by a Pye-Unicam SP3-100 spectrophotometer using the KBr disk technique. Thin-layer chromatography (TLC) was used for the purity check of the synthesized compounds and  (ppm) is the unit of chemical shifts. The field strain of cowpea aphids was collected from faba bean, Vicia faba L., fields of the experimental farm of Assiut University. Compound 1 was prepared according to the reported method 6 and the acetamiprid insecticide was purchased from Sigma-Aldrich (France). Acetamiprid insecticide and compounds (2 and 3) were tested against the collected cowpea aphid, Aphis craccivora Koch.

Laboratory bioassay
The insecticidal activity for the aforementioned compounds 2 and 3 was checked via leaf dip bioassay method. 16 The concentration of these chemical compounds that is required to kill 50% (LC50) of cowpea aphid, Aphis craccivora Koch was reported here. In this experiment, six concentrations of compounds 2 and 3 plus 0.1% Triton X-100 as a surfactant were used and a total of 20 adults and 20 nymphs, approximately of the same size, were dipped for 10 seconds in each concentration three times. The treated aphids were allowed to dry at room temperature for about 0.5 h. Control batches of aphids were similarly dipped in a solution of distilled water plus 0.1% Triton X-100. Then, after drying of the treated batches of cowpea aphids, they were transferred to Petri dishes (9 cm diameter) and held for 24 and 48 h at 22 + 2 °C, 60 + 5% relative humidity, and photoperiod of 12:12 (light/dark). Recording aphid mortality was 24 and 48 h after treatment by using a binocular microscope. The aphid that was unable to coordinate forward movement was considered dead. Insecticidal activity test of compounds 2 and 3 was repeated twice and the results were corrected using Abbott's formula. 17 Median lethal concentrations (LC50) and slope values of compounds 2 and 3 were determined by the Probit regression analysis program and expressed in parts per million (ppm). 18