Facile Access to Indolo[2,3-b]quinoline Alkaloids via DMSO-Mediated Cascade Cyclization: Unveiling Their Photophysical Potential

A streamlined and efficient synthetic strategy has been established for the construction of Indolo[2,3-b]quinolines, utilizing C3-alkylated indole as the starting material and mediated by DMSO and HCl. This methodology enables the direct annulation of the quinoline ring onto a pre-functionalized indole core with high functional group compatibility. The reaction proceeds smoothly across a diverse range of substrates, delivering the desired products in consistently high yields. Its operational simplicity, scalability, and broad substrate scope highlight its practical utility, particularly for large-scale synthesis. Notably, this approach provides a valuable platform for the synthesis of complex natural products featuring the Indolo[2,3-b]quinoline framework. Indolo[2,3-b]quinolines show strong blue to blue-green fluorescence with large Stokes shifts due to excited-state proton transfer. Substituents on the pyrrolic nitrogen and aromatic core significantly affect absorption and emission by altering hydrogen bonding and photobasicity. Electron-donating groups at the 5-position increase fluorescence lifetime and quantum yield.