Organocatalytic [5+1] Annulation of Bisindoles With Aldehydes for Constructing Indole-Fused Six-Membered Rings.

The organocatalytic [5+1] annulation of bisindoles as 1,5-dinucleophiles and aldehydes as 1,1-dielectrophiles was achieved. Using this method, a series of indole-fused six-membered rings were synthesized with generally good yields (up to 97%), providing a green and efficient approach for the construction of indole-fused six-membered rings. Furthermore, this method enabled a three-component reaction by in situ generation of bisindoles from indole and 2-indolylmethanol. This reaction represents the first [5+1] annulation of bisindoles under Brønsted acid catalysis, providing a new strategy for the construction of indole-fused six-membered rings.