Electrophilic substitution in indoles. Part 18. Cyclisation of N-acyltryptamines

Abstract

Cyclisation of N_b-acetyltryptamines 8 with trifluoroacetic anhydride, or pentafluoropropionic anhydride, affords spirocyclic indolines of types 14 and 15 in virtually quantitative yields. The mechanism of the reactions involves Cyclisation by ipso-attack at the 3-position of the indole nucleus, to form spirocyclic 3H-indoles 12 and 13, which subsequently undergo addition of the anhydride to the 1,2-double bond of the 3H-indole. The generality of the latter reaction has been established by converting the 3H-indole-3-spirocyclopentane 16 and benzylideneaniline 18 into the anhydride adducts 17a and 19 respectively. The spirocyclic indoline adducts 14a, 14b, 15a, 15b and 17a are rapidly hydrolysed by dilute aqueoust ammonia to the hydroxy spirocyclic indolines 20a, 20b, 21a, 21b and 17b respectively.