Borsche–Drechsel cyclization

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 Edmund Drechsel in 1888[1] and by Walter Borsche in 1908.[2]

Borsche–Drechsel cyclization
Borsche–Drechsel cyclization
Named after Walter Borsche
Edmund Drechsel
Reaction type Ring forming reaction
Identifiers
RSC ontology ID RXNO:0000532

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[3] as proceeding in a manner similar to the Fischer indole synthesis.

Borsche–Drechsel carbazole synthesis

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.

See also

References
  1. E. Drechsel (1888). "Ueber Elektrolyse des Phenols mit Wechselströmen". J. Prakt. Chem. (in German). 38 (1): 65–74. doi:10.1002/prac.18880380105.
  2. W. Borsche (1908). "Ueber Tetra- und Hexahydrocarbazolverbindungen und eine neue Carbazolsynthese. (Mitbearbeitet von. A. Witte und W. Bothe.)". Justus Liebigs Ann. Chem. (in German). 359 (1–2): 49–80. doi:10.1002/jlac.19083590103.
  3. Lie, J. J. (2009). Name Reactions. Springer. p. 60. ISBN 978-3-642-01052-1.
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