Merging Visible-Light Photoredox and Lewis Acid Catalysis for the Intramolecular Aza-Diels-Alder Reaction: Synthesis of Substituted Chromeno[4,3-b]quinolines and [1,6]Naphthyridines

Substituted chromeno[4,3-b]quinolines and [1,6]naphthyridines were achieved by tandem intramolecular aza-Diels-Alder reaction using a strategy of combination of visible-light photoredox and Lewis-acid-catalysis. This intramolecular aza-Diels-Alder cycloaddition took place between the insitu generated benzylidene anilines derived from arylamines and salicylaldehyde or 2-aminoaryl aldehydes bearing a tethered alkene partner, followed by oxidative aromatization to give the target products. The study on the oxidative aromatization step revealed that it is much faster than the cyclization step; both the combination of imine and Lewis acid, and the combination of photocatalyst and Lewis acid under aerobic condition with the irradiation of visible light are efficient to achieve the transformation. This method provided a new access to the synthesis of important heterocycles under mild reaction conditions.