Carbolines from 3-Acylindoles and Acetylene

Interestingly that from N-alkyl-substituted oximes of 3-acylindoles, carbolines are not apparently formed in the similar conditions only the expected O-vinyl oximes and 3-pyrrolylindoles are isolated from the reaction mixture [229], 

Obviously, this reaction (formation of y-carboline) proceeds in insignificant degree. Though the expected doubling of signals in the H NMR spectra of 5-carboline, which could be assigned to impurities of isomeric y-carboline, is really observed, the intensity of these signals is low. 

Certainly, the aforementioned mechanism of 5-carboline formation needs experimental support. For example, radical nature of the reaction assumes a possibility of its inhibition in the presence of typical inhibitors of radical processes. In such a case, intermediate radicals could be detected by ESR techniques. Also, the rearrangonent of O-vinyl oxime to 5-carboline without formation of radicals and ions must not be ruled out. 

SCHEME 1.173 8-Carboline from 3-acetylindole oxime and acetylene. 

SCHEME 1.174 Plausible radical transformations of 3-acetylindole O-vinyl oxime to 5-carboline. 

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