Mechanisms of cyclisation of indolo oxime ethers I. Formation of ethyl 9,11-dimethoxy indolo[2,3-c]quinoline-6-carboxylates

The cyclisation of a series of ethyl 3′-aryl-4′,6′-dimethoxyindol-2′-yl-2-(hydroxyimino)acetates was investigated using 1H NMR spectroscopy to determine the mechanism of formation of the corresponding ethyl 9,11-dimethoxy indolo[2,3-c]quinoline-6-carboxylates. The electronic requirements of the reaction were determined and, along with the observation of an intermediate in the process, indicated that the reaction proceeds through an electrocyclic mechanism. The importance of the ester moiety in such a process is discussed.