Synthesis of γ‐ and β‐Carbolines via Diphenyl Phosphate‐Mediated Condensation of 3/2‐Carbonyl‐2/3‐Vinylindoles with Phenyl Hydrazine Followed by Electrocyclization

Abstract A facile synthesis of 3‐alkyl‐2‐aryl substituted γ‐carboline derivatives is reported involving diphenyl phosphate catalyzed condensation of 3‐(acetyl/propionyl)‐2‐(nitroaryl)vinyl‐1‐phenylsulfonylindoles with phenyl hydrazine followed by concurrent electrocyclization and aromatization through elimination of aniline. This strategy could be successfully explored with 1‐phenylsulfonylindole‐3/2‐carboxaldehydes containing vinyl unit at 2/3‐position to afford the respective γ‐and β‐carboline derivatives. The domino reaction of easily accessible 2/3‐carbonyl indoles containing a vinyl unit at the adjacent positions with phenyl hydrazine as an amine source afforded a wide variety of carboline derivatives in good to acceptable yields. The synthetic utility of a representative γ‐carboline could be demonstrated via transformation into the corresponding indolo[a]carboline framework.