Rhodium catalysed intramolecular benzannulation for the formation of tetracyclic carbazoles

Abstract

Rhodium catalyzed alkyne-tethered intramolecular annulation has been demonstrated for the synthesis of tetracyclic carbazole skeletons and successfully attained a series of five to eight-membered pyrido[3,2,1-jk]carbazoles. The present atom economic annulation...

Update on novel synthetic approaches towards the construction of carbazole nuclei: a review

“… 66 Kumar et al reported the synthesis of tetracyclic carbazoles via an intramolecular benzannulation of alkyne-tethered indoles catalyzed with rhodium acetate in the presence of an oxidant and solvent. 67 The reaction conditions were optimized by screening different catalysts and solvents by altering their concentration. The intramolecular benzannulation of ethyl 3-(indol-3-yl)acrylate tethered with alkyne 42 was performed by employing 5 mol% of rhodium acetate, 2 mol% of copper bromide in dimethyl acetate to obtain the annulated product 43.…”

“…In the final step, the target compound was synthesized by reductive elimination of intermediate C ( Scheme 11 ). 67 The exploitation of stable bisalkynol 44, obtained from N , N -bis(2- bromoallyl)amines, for a gold-catalyzed back-to-front approach to access carbazoles 45 via a double benzannulation strategy was proposed by Muñoz-Torres et al in 2021. 68 The reaction proceeded in the presence of 5 mol% of NaAuCl 4 ·2H 2 O in DCM which induced double benzannulation to give substituted carbazoles in 60–85% yields ( Scheme 12 ).…”