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-γ-carbolines bearing multiple stereocenters. In the
presence of copper­(I)/Ph-Phosferrox complex, an array of chiral tetrahydro-γ-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.]