I2 /DMSO-Promoted Synthesis of Chromeno[4,3- b ]quinolines through an Imine Formation/Aza-Diels–Alder/Aromatization Tandem Reaction under Metal-Catalyst- and Photosensitizer-Free Conditions.

A tandem approach was developed for the efficient synthesis of substituted chromeno[4,3- b ]quinolines from arylamines and O -cinnamyl salicylaldehydes under metal-catalyst- and photosensitizer-free reaction conditions. Our protocol is based on an inexpensive I2 /DMSO system in which molecular iodine first acts as a Lewis acid to promote the formation of the corresponding imine bearing the alkene moiety; then, this species fulfills a second role by catalyzing an intramolecular aza-Diels–Alder cycloaddition to generate the respective tetrahydrochromenoquinoline as an intermediate. Finally, the dual behavior of DMSO as an oxidant and as a solvent proved crucial at this stage, allowing the regeneration of I2 and promoting the aromatization of the tetrahydrochromenoquinoline intermediates to yield the desired 7-aryl-6 H -chromeno[4,3- b ]quinolines. This protocol is mild and easy to perform, features high step-economy (tandem process) and provides a new access to biologically important nitrogen- and oxygen-containing heterocyclic molecules. [ABSTRACT FROM AUTHOR]