Cu(I)-Catalyzed Chemoselective and Stereoselective [3+3] Cycloaddition of Azomethine Ylides with 2-Indolylnitroethylenes: Facile Access to Highly Substituted Tetrahydro-γ-Carbolines

A Cu(I)-catalyzed asymmetric [3 + 3] cycloaddition of azomethine ylides with 2-indolylnitroethylenes is described. Challenges confronted in this reaction include chemoselectivity between [3 + 2] cycloaddition and [3 + 3] cycloaddition, reactivity and stereoselectivity of intramolecular Friedel Crafts reaction, and enantioselective control for constructing tetrahydro-gamma-carbolines bearing multiple stereocenters. In the presence of copper(I)/Ph-Phosferrox complex, an array of chiral tetrahydro-gamma-carboline compounds was generally obtained in moderate to high yields (up to 92%) with moderate to high chemoselectivities (up to 94:6 cr) and a high level of stereoselectivity (up to >98:2 dr, >99% ee in most cases). [Here, cr is the chemoselectivity ratio, dr is the diastereoisomeric ratio, and ee is enantiomeric excess.]