Ca(II)-Catalyzed Cascade Reaction of Tryptamines with Propargylic Alcohols: Temperature-Driven Ring Opening and Closing via the Allene Migration Pathway for the Synthesis of Pyrrolo[1,2-<i>a</i>]indoles
Ashok Kale, Su Jeong Kwon, Joohan Lee, Jae Kyun Lee, Kyeong Lee
Abstract
A temperature-dependent cascade of reactions between tryptamines and propargylic alcohols was developed to achieve selective formation of pyrroloindoline and pyrrolo[1,2- a ]indole heterocycles by Ca(II) catalysis. The cascade consists of electrophilic addition of allene at the C3 carbon of indole followed by intramolecular cyclization at 60 °C to yield pyrroloindolines. Furthermore, simultaneous 1,2-allene migration and pyrrolidine ring opening were followed by intramolecular cyclization via C–N bond formation at reflux temperature to obtain pyrrolo[1,2- a ]indole scaffolds. A wide range of substrates, a clean reaction profile, scalability, and good to excellent yields are the advantages of this protocol.