Catalytic and asymmetric Friedel–Crafts alkylation of indoles with nitroacrylates. Application to the synthesis of tryptophan analogues

An asymmetric Friedel–Crafts alkylation of indoles with nitroacrylates catalyzed by chiral (4R,5S)-DiPh-BOX (L1)–Cu(OTf)2 complex (10 mol %) has been developed. The reactions provide tryptophan nitro-precursors in moderate diastereoselectivities (anti/syn up to 72:28) and good to excellent enantioselectivities (up to 99% ee). The alkylation products could be easily reduced to optically active tryptophan analogues with Zn/H+.