Unexpected one-pot synthesis of new polycyclic coumarin[4,3-c]pyridine derivatives via a tandem hetero-Diels-Alder and 1,3-dipolar cycloaddition reaction

O-Methyl-4-coumarincarbaldehyde oxime reacted as an azadiene with electron-deficient and electron-rich dienophiles to give, via one-step hetero-Diels-Alder cycloaddition reactions, the corresponding 5H-coumarin4,3-c]pyridin-5-ones. When excess of the dienophile was used, fused azatetracyclo derivatives were also formed via a tandem Diels-Alder and 1,3-dipolar cycloaddition reaction of the dienophile to an azomethine ylide formed by the intermediate 2,3-dihydro-5H-coumarin4,3-c]pyridine-5-one. The regio- and stereoselectivities of the new compounds correspond well with spectroscopic (2D NMR) and theoretical data. A possible mechanistic scheme is provided.