Catalytic Asymmetric [3+2] Cycloadditions of C-3 Unsubstituted 2-Indolylmethanols: Regio-, Diastereo- and Enantioselective Construction of the Cyclopenta[b]indole Framework

The first catalytic asymmetric [3+2] cycloaddition of 2-indolylmethanols with 3-vinylindoles has been established, which makes use of C-3 unsubstituted 2-indolylmethanol as a C-3 building block in the presence of a chiral phosphoric acid. By using this strategy, biologically important cyclopenta[b]indole frameworks were efficiently constructed with regiospecificity in high yields (up to 99%), excellent diastereo- and enantioselectivities (up to >95:5 dr, 96:4 er). This reaction not only represents the first catalytic asymmetric [3+2] cycloaddition of C-3 unsubstituted 2-indolylmethanols, but also has confronted the great challenges in 2-indolylmethanol-involved enantioselective transformations.