Tandem Rh(II) and Chiral Squaramide Relay Catalysis: Enantioselective Synthesis of Dihydro-β-carbolines via Insertion to C-H Bond and Aza-Michael Reaction

An efficient tandem rhodium(II)/squaramide relay catalysis of readily accessible indole derivatives and N-sulfonyl-1,2,3-triazoles has been developed for the enantioselective synthesis of dihydro-β-carbolines in good yield and enantioselectivity. The developed reaction involves selective insertion of in situ generated azavinyl rhodium carbene onto the C3-H bond of indole derivatives and subsequent squaramide-catalyzed enantioselective intramolecular aza-Michael reaction. Furthermore, the potential of the strategy was demonstrated through the ready conversion to potent tetrahydro-β-carbolines and the tetracyclic alkaloid core structure.