Palladium-Catalyzed Asymmetric Cycloadditions of Vinylcyclopropanes and in Situ Formed Unsaturated Imines: Construction of Structurally and Optically Enriched Spiroindolenines

A palladium-catalyzed
(3 + 2) cycloaddition of vinyl cyclopropane
and α,β-unsaturated imines generated in situ from aryl
sulfonyl indoles is reported. The reaction proceeds with high diastereoselectivity
to provide the optically enriched spirocyclopentane-1,3′-indolenines
in up to 74% yield and with up to 97% ee, which contains an all-carbon
quaternary center and two tertiary stereocenters. The reaction involves
a first conjugate addition of the carbon anion of zwitterionic π-allylpalladium
complex from vinyl cyclopropane to the in situ formed unsaturated
imine followed by a palladium-catalyzed intramolecular C3-allylation of indole.