The Fischer indole synthesis uses a protic or Lewis acid catalyst to convert a phenylhydrazine and an aldehyde or ketone to an indole. The phenylhydrazine makes an iminium ion with the carbonyl compound, which tautomerizes to the ene-hydrazine. A [3,3]-sigmatropic rearrangement disrupts the aromatic ring, but a subsequent proton transfer restores it. Cyclization (5-exo-trig), followed by loss of ammonia produces the indole system.

• Reagents: Protic or Lewis Acid Catalyst
• Reactant: Phenylhydrazine or Derivative, Aldehyde or Ketone
• Product: Substituted Indole

Mechanism

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Related Compounds