Synthesis of Furo[3,2‑b]quinolines and Furo[2,3‑b:4,5-b′]diquinolines through [4 + 2] Cycloaddition of Aza‑o‑Quinone Methides and Furans

An approach for the construction of furo­[3,2-b]­quinolines and furo­[2,3-b:4,5-b′]­diquinolines is developed through a metal-free [4 + 2] cycloaddition of easily available in situ generated aza-o-quinone methides and furans. The reaction tolerates a wide range of aza-o-quinone methides and substituted furans to afford the corresponding dihydro- or tetrahydrofuroquinolines in good to excellent yields. Mechanistic studies reveal that the reaction involves a concerted [4 + 2] cycloaddition pathway and shows a high regioselectivity of cycloaddition for a furan ring. The present method features mild reaction conditions, dearomatization of furans, high regio- and diastereoselectivity, gram-scalable preparations, and diversity of furoquinolines.