Synthesis of spiro[cycloalkane‐1,3′‐[3H]indoles] from cycloalkanecarbaldehydes. Acid‐catalyzed rearrangement to cycloalkano[b]indoles

Abstract Spiro[cycloalkane‐1,3′‐[3 H ]indoles] 2 can be obtained from the cycloalkanecarbaldehydes 1 by the Fischer reaction of their phenylhydrazones. These cyclizations are sensitive to the acid catalyst, solvent and temperature employed. Rearrangement of the 2 to the homologous cycloalkane derivatives 3 can occur by an acid catalyst or by thermal treatment of 2 in ethyleneglycol.