Efficient Synthesis of 1′,4′-Dihydro-2′<i>H</i>-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'<i>H</i>-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 <i>Echinochloa crusgalli</i> seedlings. Notably, compounds <b>8a</b>, <b>8d</b>, and <b>8f</b> 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 <b>8a</b> demonstrated postemergence herbicidal activities against <i>E. crusgalli</i> 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 <b>8a</b> exhibited good crop selectivity against wheat, maize, and oilseed rape. Therefore, <b>8a</b> is a promising herbicidal candidate that is worthy of further development.