Unusual Formation of Cyclopenta[b]indoles from 3‑Indolylmethanols and Alkynes

Acid-promoted synthesis of cyclopenta­[b]­indole frameworks from 3-indolylmethanols and alkynes has been reported. The overall transformation represents a formal [3 + 2] annulation via rearrangement. This protocol showed good generality for the carbinol substrates as well as alkynes and allowed the generation of structurally diverse cyclopenta­[b]­indoles. Terminal alkynes, dialkyl-substituted internal alkynes, and alkynes with electron-deficient substituents were found to be not suitable for this transformation. Similarly, N-Ts and N-Boc groups were compatible with reaction conditions, whereas N-Ac and N-Tf failed to undergo this reaction. Isolation of vinyl chloride intermediate suggested the involvement of a vinylic carbocation intermediate. A mechanism has been proposed involving a ring-opening–ring-closing cascade followed by a 1,3-indole migration process via a spirocyclobutene intermediate.