Synthesis of 6H-Dibenzo[b,d]pyran-6-ones Using the Inverse Electron Demand Diels–Alder Reaction

A set of coumarin-fused electron-deficient 1,3-dienes was synthesized, which differ in the nature of the electron-withdrawing group (EWG) at the terminus of the diene unit and (when EWG = CO₂Me) the nature and position of substituents. These dienes reacted with the enamine derived from cyclopentanone and pyrrolidine to afford the corresponding cyclopenteno-fused 6H-dibenzo­[b,d]­pyran-6-ones, most likely via a domino inverse electron demand Diels–Alder (IEDDA)/elimination/transfer hydrogenation sequence. The parent diene (EWG = CO₂Me, no substituents) was reacted with a range of electron-rich dienophiles (mostly enamines) to afford the corresponding 6H-dibenzo­[b,d]­pyran-6-ones or their nondehydrogenated precursors, which were aromatized upon treatment with a suitable oxidant. The enamines could either be synthesized prior to the reaction or generated in situ. The syntheses of 30 dibenzopyranones are reported.