Electrophilic substitution in indoles—IV : The cyclization of indolylbutanol to tetrahydrocarbazole

Abstract The use of tritium labelling has shown that 3,3-spirocyclopentanoindolenine is an intermediate in the BF 3 catalysed cyclization of 4-(3-indolyl)butanol to tetrahydrocarbazole. This confirms our earlier suggestion that electrophilic substitution at the 2-position of a 3-substituted indole occurs by an indirect process involving prior attack at the 3-position followed by rearrangement.