Cyclohexanone phenylhydrazone

FISCHER - BORSCHE - DRECHSEL Indole synthesis Indole synlhesis Irom phenylhydrazones ol ketones (Fischer), m the case ol cyclohexanone phenylhydrazones letrahydrocarbazoles are oblained (Borsche-Drechsel). 

Tetrahydrocarbazole synthesis from cyclohexanone phenylhydrazone. Cf. Fisher indole synthesis. 

The cyclohexanone phenylhydrazone (512), obtained by reacting cyclohexanone (510) with phenylhydrazine (511), on indolization, furnished tetrahydrocarbazole 513 which, on dehydrogenation, afforded carbazole 514. The success of the reaction is dependent on the reagent used for indolization and the dehydrogenating agent. The mechanism for the formation of the tetrahydrocarbazole involves a tautomeric equilibrium and the formation of a new C-C bond via a [3,3]-sigmatropic rearrangement followed by elimination of ammonia (495,496,498) (Scheme 5.7). 

Tetrahydrocarbazole has been prepared from cyclohexanone phenylhydrazone, by the direct reaction of cyclohexanone with phenylhydrazine, and by the hydrogenation of carbazole. 1,2-Benzo-3,4-dihydrocarbazole has been prepared from the phenylhydrazone of a-tetralone and by the direct reaction of a-tetralone with phenylhydrazine. ... 

D-2,3,4,6/5-Pentahydroxycyclohexanone and 2L-2,4,5,6/3-Pentahydroxy cyclohexanone Phenylhydrazone Pentaacetate and Pentapropio-nate (DL-epi-Inosose-2 Phenylhydrazone Pentaacetate and Pentapropio-nate). These were prepared by the published procedure (41), mp 134°-135°C (pentaacetate) and mp 90°-91°C (pentapropionate). 

The Borsche synthesis of tetrahydrocarbazoles 21 is a special case of the Fischer indole synthesis in which cyclohexanone phenylhydrazones 20 are used as the starting materials (Scheme 9) (see, e.g., <2009BML4110>). 

Cyclohexanone phenylhydrazone undergoes cyclization to 1,2,3,4-tetrahydro-carbazole with such ease that formation and cyclization of the derivative can be combined into one step with acetic acid as catalyst and solvent. A mixture of 1 mole of cyclohexanone and 360 g. of acetic acid is stirred under reflux and 1 mole of phenylhydrazine is added during 1 hr. After refluxing, the mixture is worked... 

Fischer indole synthesis. PPE is useful for Fischer synthesis of indoles from phenylhydrazones gentle refluxing for 5 minutes on a water bath generally completes the reaction.5 Thus cyclohexanone phenylhydrazone (1) under these conditions gives 1,2,3,4-tetrahydrocarbazole (2) in 86% yield. When the reaction mixture was heated to 160°, ethylation occured as well to give 3-ethyl-2,3-tetramethyleneindolenine (3) in fairly good yield. This alkylation reaction has... 

When a methyl ketone or cyclohexanone phenylhydrazone is treated with trifluoroacetimidoyl iodide 82 in the presence of excess sodium hydride, [1+4] cyclization into 5-trifluoromethylpyrazoles 83 and 84, respectively, takes place (97SL679) (Scheme 84). 

An early, non-shape-selective, application of zeolites in the Fischer indole synthesis was reported by Venuto and Landis [18]. They employed calcium-and rare-earth-exchanged zeolite X as catalysts under continuous-flow conditions at 150 °C. Good yields of indoles were obtained with both acetone and cyclohexanone phenylhydrazones a 72.5% yield of indole was obtained from the latter with CaX catalyst. 

FISCHER BORSCHE DRECHSEL Indole synthesis Indole synthesis from phenylhydrazones of ketones (Fischer) in the case of cyclohexanone phenylhydrazones tetrahydrocarbazoles are obtained (Borsche-Drechsel). 

This reaction was first reported by Drechsel in 1888 and then by Borsche in 1904. It is the reaction involving the cyclization of arylhydrazones of cyclohexanone to tetrahy-drocarbazoles and the subsequent oxidation of tetrahydrocarbazoles into carbazoles. In this reaction, cyclohexanone phenylhydrazone is converted into tetrahydrocarbazole when heated with dilute sulfuric acid, a process that is completely analogous to the Fischer Indole Synthesis. However, Borsche was the first to realize the full scope of this reaction, and had prepared many substituted tetrahydrocarbazoles. Other acids can also be used in this reaction—for example, acetic acid has been found to give cleaner products. Different oxidants have been applied for converting the tetrahydrocarbazoles into carbazoles, such as lead oxide, mercury acetate, palladium chloride, and chloranil. However, only chlo-ranil was found to be a good reagent in this reaction. In addition, a mixture of cinnamic acid and palladium black is another excellent reagent for the preparation of carbazoles, in which cinnamic acid was reduced to hydrocinnamic acid. 

Formation of carbazole by acid-catalyzed rearrangement of cyclohexanone phenylhydrazone to tetrahydrocarbazole followed by oxidation ... 

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