Enantioselective Synthesis of Axially Chiral Anthracenes via [4+2] Cycloaddition and Cycloreversion of Cyclopentadienones

Abstract

ABSTRACT The sequential inverse‐electron‐demand [4+2] cycloaddition and cycloreversion of cyclopentadienones with alkynes or alkenes represent one of the most important approaches for the synthesis of polycyclic aromatic hydrocarbons. However, catalytic asymmetric variants of these transformations have not yet been realized. Herein, we report a Cu(II)‐catalyzed enantioselective inverse‐electron‐demand Diels–Alder reaction between cyclopentadienones and strained oxabenzonorbornadiene derivatives. Subsequent aromatization via Brønsted acid‐promoted dehydrative aromatization, followed by spontaneous cheletropic retro‐[4+1] extrusion of carbon monoxide, efficiently delivered anthracene products. Using this ring‐extension strategy, a broad range of axially chiral anthracenes are obtained in good yields and with high enantioselectivities from readily accessible prochiral cyclopentadienones. The synthetic utility of this method is further demonstrated through versatile downstream transformations of the anthracene products. DFT calculations reveal that the inverse‐electron‐demand Diels–Alder reaction proceeds in a concert manner via a dynamic kinetic resolution pathway, and elucidates the mechanism of the aromatization step.