Direct construction of carbazoles from 2-methyl-indole-3-carbaldehydes and enals

The direct and rapid construction of carbazoles was achieved via the reaction of 2-methyl-indole-3-carbaldehydes with enals promoted by LiCl/DBU in a single operation. This mild and green reaction proceeds through a [4 + 2] cycloaddition/dehydration/oxidative aromatization cascade to generate carbazoles in good to excellent yields. The reaction features mild reaction conditions, a broad substrate scope, and excellent functional group tolerance, using O₂ (1 atm) as the sole oxidant and affording H₂O as the only by-product. More importantly, 4-fluoroquinocarbazole, a significant bioactive compound, was generated in 80% yield in only one step from the obtained carbazole.