Mechanisms of cyclisation of indolo oxime ethers. Part 2: Formation of ethyl 6,8-dimethoxypyrazolo[4,5,1-hi]indole-5-carboxylates

The cyclisation of a series of ethyl 3′-phenyl-4′,6′-dimethoxyindol-7′-yl-2-(hydroxyimino)acetates was investigated using ¹H NMR spectroscopy to determine the mechanism of formation of the corresponding ethyl 6,8-dimethoxypyrazolo[4,5,1-hi]indole-5-carboxylates. The electronic requirements of the reaction were determined and used, along with the effect of removing the ester functionality, to determine that the reaction proceeds through a concerted intramolecular substitution.