Inverse electron demand Diels-Alder reactions of heterodienes catalyzed by potassium hydrogen sulfate: Diastereoselective, one-pot synthesis of pyranobenzopyrans, furanobenzopyrans and tetrahydroquinolines derivatives

Potassium hydrogen sulfate catalyzes the one-pot three components coupling of aldehydes, anilines, and electron rich dienophiles such as dihydropyran, dihydrofuran, ethyl vinylether, and cyclopentadiene. With o-hydroxybenzaldehyde the reaction probably proceeds through the formation of o-quincnemethide intermediate, which subsequently undergoes cycloadcition with cyclic and acyclic enol ethers leading to the formation of respective chromanes. However, in case of benzaldehydes with no o-hydroxy group, the imine formed acts as the heterodiene and leads to the formation of tetrahydroquinolines.