Electrophilic substitution in indoles. Part VIII. The mechanism of electrophilic substitution in 6-methoxyindoles

Abstract

Deuterium and tritium labelling experiments show that the boron trifluoride-catalysed cyclisation of 4-(6-methoxy-indol-3-yl)butanol to 7-methoxytetrahydrocarbazole occurs by two simultaneous pathways. The major route involves initial cyclisation at the 3-position to give an intermediate spirocyclic indolenine salt which then rearranges to the tetrahydrocarbazole; the minor pathway involves direct attack at the 2-position. A similar duality of mechanism occurs in the solvolysis of the corresponding methoxyindolylbutyl tosylate.