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Borsche–Drechsel cyclization

Borsche–Drechsel cyclization

Edmund Drechsel The Borsche–Drechsel cyclization is a chemical reaction used to synthesize tetrahydrocarbazoles by the acid-catalyzed cyclization of cyclohexanone arylhydrazones. The reaction was first described by in 1888 and by in 1908.

Borsche–Drechsel cyclization

Borsche–Drechsel cyclization is the central step in Borsche–Drechsel carbazole synthesis, where in the first step phenylhydrazine is condensed with cyclohexanone to form the cyclohexanone phenylhydrazone, and in the final step the resulting tetrahydrocarbazole is oxidized to carbazole itself.

Mechanism

The reaction has been described in the literature as proceeding in a manner similar to the Fischer indole synthesis.

:[[File:Borsche-Drechsel-Mechanismus_V4.svg|500px]]

Here, the acid-catalyzed proton transfer first converts the cyclohexanone phenylhydrazone 1 to the intermediate 2. Subsequently, a heat-induced sigmatropic reaction occurs to produce 3, which is protonated and cyclizes into 4. Elimination of ammonia then leads to the final product, the tetrahydrocarbazole 5.

References

References

  1. {{Cite Q. Q56441554
  2. {{Cite Q. Q29029226
  3. Lie, J. J.. (2009). "Name Reactions". Springer.
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