Synthesis, crystal structure, and insecticidal activities of highly congested hexahydroimidazo[1,2-a]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.