Pyridine Derivatives as Insecticides. Part 5. New Thieno[2,3-<i>b</i>]pyridines and Pyrazolo[3,4-<i>b</i>]pyridines Containing Mainly Ethyl Nicotinate Scaffold and Their Insecticidal Activity toward <i>Aphis gossypii</i> (Glover,1887)

Abstract

Ethyl 5-cyano-1,6-dihydro-2-methyl-4-(2'-thienyl)-6-thioxonicotinate (<b>A</b>) was synthesized and reacted with ethyl chloroacetate in the presence of sodium acetate or sodium carbonate to give ethyl 5-cyano-6-((2-ethoxy-2-oxoethyl)thio)-2-methyl-4-(2'-thienyl)nicotinate (<b>1a</b>) or its isomeric thieno[2,3-<i>b</i>]pyridine <b>2a</b>. 3-Aminothieno[2,3-<i>b</i>]pyridine-2-carboxamide <b>2b</b> was also synthesized by the reaction of <b>A</b> with 2-chloroacetamide. The reaction of <b>1a</b> with hydrazine hydrate in boiling ethanol gave acethydrazide <b>3</b>. Heating ester <b>1a</b> with hydrazine hydrate under neat conditions afforded 3-amino-1<i>H</i>-pyrazolo[3,4-<i>b</i>]pyridine <b>10</b>. Compounds <b>2b</b>, <b>3,</b> and <b>10</b> were used as precursors for synthesizing other new thieno[2,3-<i>b</i>]pyridines and pyrazolo[3,4-<i>b</i>]pyridines containing mainly the ethyl nicotinate scaffold. Structures of all new compounds were confirmed by elemental and spectral analyses. Most of the obtained compounds were evaluated for their insecticidal activity toward the nymphs and adults of <i>Aphis gossypii</i> (Glover,1887). Some compounds such as <b>4</b>, <b>9b,</b> and <b>9c</b> showed promising results. The effect of some sublethal concentrations, less than LC₅₀, of compounds <b>4</b>, <b>9b,</b> and <b>9c</b> on the examined <i>Aphis</i> was subjected to a further study. The results demonstrated that exposure of <i>A. gossypii</i> nymphs to sublethal concentrations of compounds <b>4</b>, <b>9b,</b> and <b>9c</b> had noticeable effects on their biological parameters, i.e., nymphal instar duration, generation time, and adult longevity. The highest concentration C1 of all three compounds increased the nymphal instar duration and generation time and decreased adult longevity and vice versa.