Dihydrocarbazoles

Benzidine, N,N -diethyl-, 36, 21 Benzidine dihydrochloride, 36, 22 Bcnzil, 34, 42 Benzil dihydrazone, 34, 42 Benzilic acid, 33, 37 2-Benzimidazolethiol, 30, 56 l,2-Benzo-3,4-dihydrocarbazole, 30, 91 Benzofuran, 3-methyl, 33, 43 Benzofurazan oxide, 31,14, 15 37,1 Benzoguanamine, 33,13 Benzoic acid, 32, 94 37, 21 Benzoic acid, -acetyl-, methyl ester, 32, 81... 

N-Cyanoguanidine, 35, 69 Cyanohydrin formation, 33, 7 3-Cyano-6-isobutyl-2(l)-pyridone, 32, 34 3-Cyano-6-methyl-2(1)-pyridone, 32, 32 l-Cyano-3-a-naphthylurea, 36, 11 1-Cyano-3-phenylurea, 36, 8 Cyclic acyloins, 36, 82 Cyclization, (3-aminoethylsulfuric acid to ethylenimine, 30, 38 1,2-benzo-3,4-dihydrocarbazole from phenylhydrazine and a-tetralone,... 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

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... 

Carbazoles, too, can be reduced partially to dihydrocarbazoles by sodium in liquid ammonia [460], or to tetrahydrocarbazoles by sodium in liquid ammonia and ethanol [460] or by sodium borohydride [457]. Carbazole was converted by catalytic hydrogenation over Raney nickel or copper chromite to 1,2,3,4-tetrahydrocarbazole, 1,2,3,4,10,11-hexahydrocarbazole, and do-decahydrocarbazole in good yields [430]. 

Diels-Alder reaction of vinylindoles with dienophiles has been established as a versatile and flexible methodology for the synthesis of carbazole alkaloids. Among the two different vinylindoles, 3-vinylindoles were the first to be explored for the Diels-Alder cycloaddition methodology with a range of dienophiles to give polyfunctionalized carbazole derivatives. This reaction is catalyzed by tiifluoroacetic acid, and the yield in the absence of the acidic catalyst is very low. The reaction of substituted 3-vinylindoles 550 and 553 with ethylenic dienophiles 551 and acetylenic dienophiles 535 leads, via a tetrahydrocarbazole and a dihydrocarbazole, to the corresponding carbazoles (552 and 554), respectively (530,531) (Scheme 5.18). 

Using a modification of Sakamoto s indole benzannulation protocol (see Scheme 5.59), Beccalli et al. reported the synthesis of the 3-methoxycarbazole alkaloids hyellazole (245) and 6-chlorohyellazole (246) (540) (Scheme 5.61). Unlike Sakamoto s methodology, this method requires a good leaving group at the 2-position of the indole moiety of the 3-(l,3-butadienyl)indoles 692 to facilitate the aromatization of the intermediate dihydrocarbazole by eliminating the dehydrogenation step. 

Electrophilic aromatic substitution of 708 with the iron-coordinated cation 602 afforded the iron-complex 714 quantitatively. The iron-mediated quinone imine cyclization of complex 714, by sequential application of two, differently activated, manganese dioxide reagents, provided the iron-coordinated 4b,8a-dihydrocarbazole-3-one 716. Demetalation of the iron complex 716 with concomitant... 

Electrophilic substitution at the arylamine 709 using the complex salt 602, provided the iron complex 725 quantitatively. Sequential, highly chemoselective oxidation of the iron complex 725 with two, differently activated, manganese dioxide reagents provided the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727) via the non-cyclized quinone imine 726. Demetalation of the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727), followed by selective O-methylation, provided hyellazole (245) (599,600) (Scheme 5.70). 

The construction of the carbazole framework was achieved by slightly modifying the reaction conditions previously reported for the racemic synthesis (614). Reaction of the iron complex salt 602 with the fully functionalized arylamine 814 in air provided the tricarbonyliron-coordinated 4b,8a-dihydrocarbazole complex 819 via sequential C-C and C-N bond formation. This one-pot annulation is the result of an electrophilic aromatic substitution and a subsequent iron-mediated oxidative cyclization by air as the oxidizing agent. The aromatization with concomitant demetalation of complex 819 using NBS under basic reaction conditions, led to the carbazole. Using the same reagent under acidic reaction conditions the carbazole was... 

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. ... 

Cyclization, of -aminoethylsulfuric acid to ethylenimine, 38 l,2-benzo-3,4-dihydrocarbazole from phenylhydrazine and a-tetralone, 91... 

Diaryl amines, ethers or sulphides, or their aryl vinyl analogues, provide another 6-electron system related to stilbene, but for these a pair of electrons is provided by the single heteroatom (ArXAr or ArXC=C). With dtaryl compounds the initial photocydized product is a zwitterion that undergoes a proton shift to give, for example, N-methyl-4a,4b-dihydrocarbazole from methyldiphenylamine, with subsequent oxidation to N-methylcarbazole (3.74). With the aryl vinyl analogues the product after the proton shift can normally be isolated (3.751. An especially useful variation of this reaction employs... 

Analogous [ 4 + 2] cycloaddition reactions of DMAD with indoles would disrupt the aromatic character of the fused benzene ring. It is not unexpected, therefore, that indole reacts with two molecules of DMAD to give the dihydrocarbazole derivative (234), via an initial Michael addition reaction, followed by cycloaddition of DMAD to the vinylindole. Aromatization of the 1 2 adduct (234) normally yields the carbazoletetracarboxylic ester... 

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