Syntheses of Spiro[cyclopent[3]ene‐1,3′‐indole]s and Tetrahydrocyclohepta[ b ]indoles from 2,3‐Disubstituted Indoles through Sigmatropic Rearrangement

A NaOH‐mediated protocol was developed for the syntheses of a series of 2′‐arylspiro[cyclopent[3]ene‐1,3′‐indole]s and 6‐alkyl‐5,6,7,10‐tetrahydrocyclohepta[ b ]indoles in good yields by cyclization through a sigmatropic rearrangement of suitable 2,3‐disubstituted indoles. These precursors that contain a 4‐chloro‐2‐butenyl side chain at the 3‐position were obtained as a mixture of ( E ) and ( Z ) isomers through the reactions of o ‐alkynylanilines and 3,4‐dichloro‐1‐butene. The cyclization strategy is general, and both medicinally important scaffolds were achieved by changing the substituent at the 2‐position of methyl 3‐(4‐chlorobut‐2‐enyl)‐1 H ‐indole‐1‐carboxylate. The reaction likely proceeds through [1,3]‐ and [3,3]‐sigmatropic rearrangements of the vinylcyclopropane substituent of the indole intermediate.