Carbazole, synthesis

The possible ambiguities that may arise in ring syntheses based upon nueleophilic reaetions with a-halo ketones are also exemplified by the reactions with a-lithioaldimines leading to pyrrole formation, as shown in Scheme 73a 73TL3517). A simitar problem could well arise in the related carbazole synthesis indicated in Scheme 73b if a less symmetrical substrate was employed 81TL1475). 

Pyrano[3,2-g]benzoxazine, dihydrosynthesis, 3, 714 Pyranobenzoxazines synthesis, 6, 190 Pyranobenzoxazoles mass spectra, 3, 615 Pyrano[2,3-a]carbazoles synthesis, 4, 235 Pyrano[2,3- 6]carbazoles synthesis, 4, 235 Pyrano[3,2-a]carbazoles synthesis, 4, 235 Pyranochromones synthesis, 3, 821 Pyranochromones, dihydrosynthesis, 3, 81 817 Pyranocoumarins crystal data, 3, 623 mass spectra, 3, 610 Pyranodipyranones synthesis, 3, 794 Pyrano[3,2-6]indol-4-ones synthesis, 4, 302 Pyran-2-ol, dihydrodehydration, 3, 762 Pyran-2-ol, 3-methyltetrahydro-synthesis, 3, 775 Pyran-2-ol, tetrahydro-6-substituted synthesis, 3, 775 Pyran-4-ol, tetrahydro-IR spectra, 3, 594 Raman spectra, 3, 594 Pyranols, tetrahydro-bond lengths, 3, 621 synthesis, 3, 777... 

GOMBERG - BACHMANN - GRAEBE - ULLMANN Arylatwn Atykaryl bond formation via diazonium salts Carbazole synthesis by intramolecular aryl-aryl bond formation. 

The Bucherer carbazole synthesis " involves the treatment of a naphthyl alcohol (1 or 4) or a naphthyl amine (2 or 5) with a phenylhydrazine 3 in the presence of aqueous sodium bisulfite to afford, after acidic work-up, either a benzo[a]carbazole 4 or benzo[c]carbazole 6. 

The Bucherer carbazole synthesis was first demonstrated when 7 was heated in the presence of phenylhydrazines 8, sodium hydroxide and sodium bisulfite after acidic work-up, the benzocarbazole product 9 was isolated (-70% yield). When 2-naphthol was used the reaction was significantly slower with the yield of benzocarbazole being only 46% after several days at 130 °C Bucherer and co-workers investigated this reaction extensively concluding, incorrectly, that intermediate products were probably carbazole-Al-sulfonic acids due to the ease with which they lost the sulfonic acid residues to yield benzocarbazoles. 

Whereas the similarity between the Bucherer carbazole synthesis and the Fischer indolisation was noted by Bucherer and Seyde in their pioneering work, it was only later... 

When naphthyl amines e.g. 23) are used in the Bucherer carbazole synthesis, they are converted by the catalytic action of aqueous bisulfite into tetralonesulfonic acid derivative 13 by the Bucherer reaction. Addition of NaHSOs gives an enamine, which tautomerises to the imine 24 24 is hydrolysed to keto form 13 and subsequent Bucherer carbazole synthesis follows to afford the benzocarbazole product 20. ... 

The Bucherer carbazole synthesis is, to a certain extent, limited by the availabilty of the starting naphthol and naphthylamine reagents. However, a variety of substituents have been used and these are illustrated for the conversion of 25 to 27. The use of 2-aminoanthracene in the Bucherer carbazole synthesis has also been demonstrated."... 

The Bucherer carbazole synthesis was pivotal in the preparation of the first hexahelicene 37a. Reaction of 2,7-dihydroxynaphthalene 35 with phenylhydrazine and sodium bisulfite afforded helicene 37a although in low yield. More recently, the synthesis was extended to the preparation of 37b using 2,5-dimethylphenylhydrazine 36b. ... 

The thermal decomposition of triazole 1 to form carbazole 2 is regarded as the Graebe-Ullmann carbazole synthesis. 

The Graebe-Ullman carbazole synthesis is normally carried out in sealed tubes and at high temperature. The carbazoles/substituted carbazoles are obtained in general in poor to moderate yields. ... 

The Graebe-Ullman carbazole synthesis has been enmloyed in the preparation of substituted carbolines, as well as indolo[2,3-i] quinolines, which are often difficult to synthesize via other approaches, for example, the Fischer indole process. 

An oxidative cyclization leads to the C-N bond formation and furnishes the carbazole nucleus. The three modes of the iron-mediated carbazole synthesis differ in the procedures which are used for the oxidative cyclization [77,78]. 

The carbazole construction using iron-mediated arylamine cydization for the C-N bond formation was applied to the synthesis of 4-deoxycarbazomycin B [84]. The synthesis of this model compound demonstrates also the course of the two key steps which are involved in the iron-mediated carbazole synthesis (Scheme 10). 

The procedure is also applicable to acetoxy-substituted arylamines, thus broadening the scope of the carbazole synthesis (Scheme 22). Reaction of the complex salt 6a with the 5-acetoxyarylamines 56 affords the tricarbonyliron-complexed 4a,9a-dihydrocarbazoles 57, which on demetalation and subsequent catalytic dehydrogenation [100] give the 4-acetoxycarbazoles 58. Removal of... 

Despite many applications of the iron-mediated carbazole synthesis, the access to 2-oxygenated tricyclic carbazole alkaloids using this method is limited due to the moderate yields for the oxidative cyclization [88,90]. In this respect, the molybdenum-mediated oxidative coupling of an arylamine and cyclohexene 2a represents a complementary method. The construction of the carbazole framework is achieved by consecutive molybdenum-mediated C-C and C-N bond formation. The cationic molybdenum complex, required for the electrophilic aromatic substitution, is easily prepared (Scheme 23). 

Over the past 15 years, we developed three procedures for the iron-mediated carbazole synthesis, which differ in the mode of oxidative cyclization arylamine cyclization, quinone imine cyclization, and oxidative cyclization by air (8,10,557,558). The one-pot transformation of the arylamine-substituted tricarbonyl(ri -cyclohexadiene) iron complexes 571 to the 9H-carbazoles 573 proceeds via a sequence of cyclization, aromatization, and demetalation. This iron-mediated arylamine cyclization has been widely applied to the total synthesis of a broad range of 1-oxygenated, 3-oxygenated, and 3,4-dioxygenated carbazole alkaloids (Scheme 5.24). 

Table 4 lndolo[3,2-b]carbazoles synthesis by intramolecular cyclization... 

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