The Mode of Cyclisation of the Arylhydrazones and O-Phenyl Ethers of the Oximes of Asymmetric Ketones Under the Conditions of the Fischer Reaction

Both the Fischer synthesis of indoles and the recently discovered rearrangement of O-phenyl ethers of oximes, which takes place under similar conditions and leads to the formation of benzofurans, are assessed in the review from the same standpoint. A number of studies published recently altered the ideas held for a long time concerning the mode of cyclisation of the arylhydrazones of asymmetric ketones into indoles. It was believed previously that CH₃, CH₂, and CH groups in the α-position in the carbonyl fragment constitute a series in which the capacity for cyclisation increases. However, it has been found that the acidity of the medium plays a very significant role in determining the mode of cyclisation and can actually reverse it. The concepts put forward to account for this finding are examined critically and general ideas are developed which, in the author's view, explain satisfactorily the influence of the pH of the medium and of certain other factors on the mode of cyclisation of both arylhydrazones and O-aryloximes.
The bibliography includes 109 references.