Solvent and Temperature Dependent Selectivity in the [4 + 2] Annulation-Initiated Cascade Reactions of Nitroindoles with Curcumins: Access to Linear and Angular Polycycles

A metal-free dearomatization of nitroindole via Michael addition of curcumin in the presence of tetramethylguanidine (TMG) followed by further solvent-dependent cascade intramolecular Michael additions generates tetrahydrocarbazoles and hexahydropyranocarbazolones with high diastereoselectivity, which was rationalized by DFT calculations. Furthermore, an unprecedented and conceptually novel in situ skeletal rearrangement of the tetrahydrocarbazole at elevated temperature leading to angular heterotriquinanes, which exhibit promising photophysical properties, is disclosed. A non-natural analogue of a carbazole-based alkaloid euchrestifoline has also been synthesized by our methodology.