Synthesis, Crystal Structure, and Insecticidal Activities of Highly Congested Hexahydroimidazo[1,2-<i>a</i>]pyridine Derivatives: Effect of Conformation on Activities

A series of hexahydroimidazo[1,2-a]pyridine derivatives were designed and synthesized through aza-Diels−Alder reactions and evaluated for insecticidal activities. Compounds 6a−d with endo-conformation were endowed with excellent insecticidal activities against cowpea aphid (Aphis craccivora) and armyworm (Pseudaletia separata Walker), whereas exo-compounds 7a−d showed only low activities against cowpea aphid. The difference in activities between the endo- and exo-conformations indicated that conformation was the determinant of life or death of the insects for these compounds.