Efficient Synthesis of 1',4'-Dihydro-2'H-spiro[indoline-3,3'-quinoline] Derivatives for the Identification of New Spirocyclic Herbicidal Leads

Spiro scaffolds are ubiquitous in natural products and drugs. Their special three-dimensional structure and biological activities have aroused wide interest. Among various spiro compounds, spiro-fused indoline derivatives represent a particularly interesting subset but have attracted less attention for drug discovery, especially in the field of pesticides. In this study, twenty-nine 1',4'-dihydro-2'H-spiro[indoline-3,3'-quinoline] derivatives were designed and efficiently synthesized via one-pot synthesis methods; their herbicidal activity was also evaluated. Most of the synthesized spiro-fused indoline derivatives exhibited considerable herbicidal activity against the growth of Echinochloa crusgalli seedlings. Notably, compounds 8a, 8d, and 8f exhibited root and stem inhibition rates exceeding 95% and 85% at a dosage of 50 mg/L, respectively. Furthermore, a greenhouse experiment revealed that compound 8a demonstrated postemergence herbicidal activities against E. crusgalli with a fresh-weight inhibition rate of 73.8% at a dosage of 400 g ai/ha, demonstrating superior activity compared with glyphosate (58.4%). Importantly, compound 8a exhibited good crop selectivity against wheat, maize, and oilseed rape. Therefore, 8a is a promising herbicidal candidate that is worthy of further development.