Highly Enantioselective Tandem Michael Addition of Tryptamine‐Derived Oxindoles to Alkynones: Concise Synthesis of Strychnos Alkaloids

A highly enantioselective tandem Michael addition of tryptamine-derived oxindoles to alkynones was developed by taking advantage of a chiral N,N′-dioxide Sc(OTf)3 catalyst. The reaction enables the facile preparation of enantioenriched spiro[pyrrolidine-3,3′-oxindole] compounds, which provides a novel strategy for the synthesis of monoterpenoid indole alkaloids. As a demonstration, the asymmetric synthesis of strychnos alkaloids [(−)-tubifoline, (−)-tubifolidine, (−)-dehydrotubifoline] was achieved in 10–11 steps.