Solvent-Controlled [4 + 2] Annulation Reaction: Selective Synthesis of 1,4-Dihydroquinolines/Quinolines

A practical strategy was developed for the selective synthesis of 1,4-dihydroquinolines or quinolines through [4 + 2] annulation of aminobenzyl phosphonium salts (ABPS) and enaminones. In the process, solvent control alone enables the highly selective progression of this reaction, omitting the need for any metal catalysts or basic additives, providing an array of 1,4-dihydroquinolines or quinolines by using THF or DMSO, respectively. The reaction was featured with mild reaction conditions, readily accessible reagents, excellent substrate generality, and high yields.