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

Abstract

The cyclohexanone 3 and the cyclohexenone 13 have been prepared, converted to their phenylhydrazones, and subjected to the Fischer indole synthesis under conditions ranging from 7% to 60% sulfuric acid in methanol. The tetrahydrocarbazoles 4 and 5 were isolated in a 2:1 ratio in the sequence starting from 3 and no significant variation in the ratio was observed through the range of conditions used. In the sequence starting with 13, the dihydrocarbazoles 14 and 15 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 observed regioselectivity of these reactions is not predicted from mechanistic considerations, and no mechanistic explanation for the results is apparent. As part of the proof of structure of 4 and 14, their N-benzyl derivatives were prepared from 1-benzyl-2-vinylindole by Diels–Alder reactions.