Electrophilic substitution in indoles. Part 17. The cyclisation of (dimethoxyphenylacetyl)tryptamines to spiropentacyclic indole derivatives
N-(3,4-Dimethoxyphenylacetyl)tryptamine (1a) cyclises with either phosphoryl chloride or phosphorus trichloride to afford the oxo spirocyclic indoline (10a) in 68% yield together with small amounts of β-carboline derivatives. Trifluoroacetic anhydride-catalysed cyclisation affords a virtually quantitative yield of the bis-N-trifluoroacetyl spirocyclic indoline (12). The latter undergoes hydrolysis with aqueous ammonia, followed by autoxidation, to afford (10a) in 95% yield.