The regioselectivity of the formation of dihydro- and tetrahydrocarbazoles by the Fischer indole synthesis

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Abstract
Two cyclohexanones were prepared, converted to their phenylhydrazones and subjected to the Fischer indole synthesis under conditions ranging from 7-60% sulfuric acid in methanol. Tetrahydrocarbazoles were isolated in a 2:1 ratio in the sequence starting from one of the cyclohexanones and no significant variation in the ratio occurred through the range of conditions used. Dihydrocarbazoles were isolated in a 1:1 ratio when 7% or 15% sulfuric acid was used; when more concentrated acid was used, normal Fischer products were not obtained but some transformation products were isolated from the complex mixture of products obtained. The regioselectivity of these reactions is not predicted from mechanistic considerations and no mechanistic explanation for the results is apparent. N-benzyl derivatives were prepared from 1-benzyl-2-vinylindole by Diels-Alder reactions to prove structures.

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