The Application of N-Protected 3-Vinylindoles in Chiral Phosphoric Acid-Catalyzed [3+2] Cyclization with 3-Indolylmethanols: Monoactivation of the Catalyst to Vinyliminium

A highly enantioselective [3+2] cyclization of 3-vinylindoles with isatin-derived 3-indolylmethanols has been established in the presence of chiral phosphoric acid, which constructed the enantioenriched cyclopenta[b]indole scaffold linking a 3-vinylindole moiety in generally good yields, considerable diastereoselectivities and high enantioselectivities (up to 93%, >95:5 dr, >99.5:0.5 er). More importantly, this reaction utilized N-protected 3-vinylindoles as competent reactants, which confronted the challenge in realizing a successful monoactivation mode of chiral phosphoric acid to vinyliminium in 3-indolylmethanol-involved enantioselective transformations. Besides, this protocol also settled the challenge in realizing a highly enantioselective [3+2] cyclization of 3-vinylindoles with 3-indolylmethanols, which would serve as a useful strategy for constructing an enantioenriched cyclopenta[b]indole scaffold linking a 3-vinylindole moiety.