Synthesis of Furo[3,2-<i>b</i>]quinolines and Furo[2,3-<i>b</i>:4,5-<i>b′</i>]diquinolines through [4 + 2] Cycloaddition of Aza-<i>o</i>-Quinone Methides and Furans
Lu Lei, Yi-Yun Yao, Lijuan Jiang, Xiuqiang Lu, Cui Liang, Dong‐Liang Mo
Abstract
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.