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Electrophilic Substitution in Indoles. Part 11. The Mechanism of Substitution in 5-Methoxyindoles

DOI: 10.1039/P29820000909

Source and publish data:

Journal of the Chemical Society. Perkin transactions II p. 909 - 916 (1982)

Update date:2022-08-05

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Authors:

Clack, Denis W.Clack, Denis W.

Jackson, Anthony H.Jackson, Anthony H.

Prasitpan, NoojareePrasitpan, Noojaree

Shannon, Patrick V. R.Shannon, Patrick V. R.

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Article abstract of DOI:10.1039/P29820000909

Deuterium labelling experiments show that the boron trifluoride-catalysed cyclisation at 90 deg C of 4-(5-methoxyindole-3-yl)butanol (1e) to 6-methoxytetrahydrocarbazole (11a) occurs by two simultaneous pathways.The main route (83.5percent) involves initial cyclisation at the 3-position of (1e) to give an intermediate spirocyclic indolenine which then rearranges to the tetrahydrocarbazole.The minor pathway (16.5percent) involves direct attack at the 2-position.A similar duality of mechanism of substitution applies to the 6-methoxy-, 4,6-dimethoxy-, and 5,6-dimethoxy-indole analogues for which the extent of substitution at the 2-position can be correlated with the calculated change in ?-electron density at the 2- and 3-positions for a series of methoxy-substituted 3-methyl-indoles.These calculations do not, however, fit the experimental findings for the 5-methoxy-derivative which appears to be anomalous, showing an unexpectedly high percentage of direct substitution at the 2-position.A possible explanation of this result is advanced.

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