Ytterbium triflate catalyzed electrophilic substitution of indoles: the synthesis of unnatural tryptophan derivatives

Benzylamine was combined with ethyl glyoxylate to form the intermediate imine which in the presence of catalytic amount of Yb(OTf)3 underwent electrophilic substitution on the 3-position of a variety of indoles to produce unnatural tryptophan derivatives. Penicillin-G-acylase mediated N-acylation produced optically active tryptophan derivatives.