Green Chemistry: Synthesis of Quinolines

Significance

A modification of the Friedländer reaction (2-aminobenzaldehydes with aliphatic ketones and aldehydes), one of the most widely used methods for the preparation of quinolines, is reported. The iridium-catalyzed modified Friedländer protocol is environmentally advantageous in that it proceeds under solvent-free conditions in the presence of solid KOH to give quinolines in good yields (63-91%). The addition of 4 mol% of Ph₃P slightly increases the yields. The reaction is likely to proceed via ketimine formation followed by an oxidative step and an intramolecular aldol-type condensation. A further feature is the excellent regioselectivity (exception: R² = n-pentyl). An extension to the synthesis of pyrroles is also described.