Pyrido carbazole synthesis

An improved approach109 which allows the direct synthesis of ring-A-substituted pyrido-carbazoles [e.g. 9-methoxyellipticine (196)] applies the Borsche method for the preparation of the intermediate carbazole (195) (Scheme 19) the synthesis is then completed by application of the modified110 Cranwell-Saxton route. 

Lescot and co-workers (100) have described two routes to the 11//-pyrido[a]carbazoles. In the first (Scheme 36), a Fischer indole cyclization of the naphthylhydrazone 213, followed by dehydrogenation, gave the fully aromatic pyrido[a]carbazoles 215-217. Methylation or demethylation completed the preparation of the desired target compounds. A different route was used to synthesize the 5-methyl-2-aza derivatives 226 and 227 (Scheme 37) (100). Condensation of aldehyde 224 with 4-ethylpyridine gave the vinylindole 225. Deacylation and a variation of the Snieckus pyrido[c]carbazole synthesis (101) gave the desired compounds 226 and 227. 

Pyrazolopyridines, general review 84AHC(36)343. 6H-Pyrido[4,3-h]carbazoles, synthesis of 84S289. 

When 3-phenyl-3//-triazolo[4,5-/]quinoline was heated at 390 00°C, IH-pyrido[2,3-c]carbazole 149 originated. Its structure could be confirmed by unambiguous synthesis from the 8,9,10,1 l-tetrahydro-7//-pyrido[2,3-c]carbazole (52CJC711). 

The yellow colored, sparcely soluble 5-ethyl-2-methyl-l l/f-pyrido[3,4-u] carbazolium 347 isolated from Aspidosperma gilbertii exists as a hydroxide after filtration of the corresponding iodide over basic aluminum oxide. A short synthesis was described (80CB3245). The Pyrido[3,4-a]carbazole ring system is present in the alkaloid AG-1, whereas Cryptolepine (348) possesses the indolo[3,2-b]quinoline moiety (65MI1). 

The antitumor alkaloid ellipticine (5,1 l-dimethyl-6//-pyrido[4,3-b]carbazole) has been isolated from many species of Ochrosia and Aspidosperma. Ellipticine and its derivatives are highly active versus several experimental neoplasms, and the compound has been widely subjected to studies devoted to its total synthesis, the preparation of derivatives, and metabolism. Metabolic transformation studies with ellipticines have been conducted, using microorganisms, in vivo and in vitro... 

This indole synthesis has been extended to P-tetrahydrocarbolines (300) [371], azaketotetrahydrocarbazoles [372], carbolines, carbazoles, and pyrido[l,2-a]benzimidazoles [373]. Examples of the former two reaction types are illustrated. An early Heck cyclization of 2-carboxy-2 -iododiphenylamine to 1-carbazolecarboxylie acid (73% yield) [374] has been generally overlooked by subsequent investigators. 

The strong interest in the synthesis of pyrido[4,3-h]carbazole alkaloids started in the late 1960s with the disclosure of the antitumor activity of ellipticine (228) and 9-methoxyellipticine (229) (see Scheme 2.56) in several animal and human tumor systems. This discovery made these alkaloids to important synthetic targets and induced extensive studies of structure modification. These synthetic efforts have... 

Two years later, the same group reported a formal synthesis of ellipticine (228) using 6-benzyl-6H-pyrido[4,3-f>]carbazole-5,ll-quinone (6-benzylellipticine quinone) (1241) as intermediate (716). The optimized conditions, reaction of 1.2 equivalents of 3-bromo-4-lithiopyridine (1238) with M-benzylindole-2,3-dicarboxylic anhydride (852) at —96°C, led regioselectively to the 2-acylindole-3-carboxylic acid 1233 in 42% yield. Compound 1233 was converted to the corresponding amide 1239 by treatment with oxalyl chloride, followed by diethylamine. The ketone 1239 was reduced to the corresponding alcohol 1240 by reaction with sodium borohydride. Reaction of the alcohol 1240 with f-butyllithium led to the desired 6-benzylellipticine quinone (1241), along with a debrominated alcohol 1242, in 40% and 19% yield, respectively. 6-Benzylellipticine quinone (1241) was transformed to 6-benzylellipticine (1243) in 38% yield by treatment with methyllithium, then hydroiodic acid, followed... 

A versatile and convenient method for the synthesis of substituted benzo[o]carbazoles and pyrido[2,3-a]carbazoles has recently been developed [59]. Treatment of 2-(o-tolyl)- or 2-(3-methyl-2-pyridyl)-substituted indole-3-carbaldehydes (obtained by Suzuki reaction) with potassium tert-butoxide in DMF at 70-80 °C under irradiation by a 400 W high-pressure mercury lamp afforded benzo[a] carbazoles and pyrido] 2,3-o] carbazoles, respectively, in good yields (Eq. (30)). 

Fused carbazoles related to pyrido[4,3-6]carbazole alkaloids were prepared by a Diels-Alder route, and a 3-aza bioisostere of the antitumor alkaloid olivacine was synthesized <02CPB1479>. Indoloid [3.3]cyclophane 40a gave the pentacyclic indoloid 41a upon heating <02OL127>. This led to a concise formal total synthesis of ( )-strychnine in 12 facile steps from tryptamine when a similar transannular inverse-electron-demand Diels-Alder reaction of indoloid [3.3]cyclophane 40b gave 41b <02AG(E)3261>. 

The extension of the above regioselective 6-endo cyclization to appropriately substituted substrates provided a novel synthetic entry to the pyrido[4,3-b]carbazole skeleton of the indole alkaloid olivacine, which resulted in a concise total synthesis of its tetrahydro derivative ( )-guatambuine <6 IT 160 67CJC89>. 

Miyake and co-workers (40) have published a synthesis of ellipticine that features a novel reductive phenylation of nitroarenes (41) (Scheme 4). Nitration of 5,8-dimethyl-l, 2,3,4-tetrahydroisoquinoline (22) gave an inseparable mixture of nitro compounds 23. Treatment of this mixture with iron pentacarbonyl and triflic acid in the presence of benzene gave a 2 1 mixture of amines 24 and 25. Separation of these isomers and diazotization of each with nitrous acid, conversion to the azide, and thermolysis yielded ellipticine (1) and isoellipticine (27) (5,11-dimethyl-10f/-pyrido[3,4- )]carbazole), respectively, following Pd/C dehydrogenation of the initially formed nitrene insertion product (e.g., 26). The overall yield of ellipticine is 9%. 

In two papers, Miller and co-workers (45,46) have extended their intramolecular ring B cyclization strategy (47) to the use of aryl nitrenes in the synthesis of pyridocarbazoles. Thus, in the first paper, isoquinoline azide 39 was heated at 180-200°C to aiford ellipticine (1) as the minor product (20%) (Scheme 7). The major product was the isomeric pyrido[3,4-a]carbazole 40 (60%). This result is consistent with the relative nucleophilicities of C-6 and C-8 of isoquinoline. The isomeric azido isoquinoline 41 exhibited comparable re-gioselectivity in the cyclization of the corresponding nitrene to yield isoellipticine (27) as the minor product (Scheme 7). 

Archer and co-workers (84) have used the original Stillwell ellipticine synthesis (87), as later exploited by Gouyette et al. (88) to prepare the simple 9-hydroxy-6//-pyrido[4,3-fe]carbazole (158) (Scheme 27). V-Benzyl-4-piperidone was converted via enamine 154 to the enone 155. Hydrogenation gave a mixture of cis- and trawj-ketones 156 which were separately converted to indole 157 by Fischer indolization. Some of the nonlinear pyrido[3,4-c]carbazole (17%) was formed from the cis-ketone. Dehydrogenation and demethylation gave the desired 158. 

Scheme 27. Archer et al. synthesis of 9-hydroxy-6/f-pyrido[4,3-ft]carbazole (158) (84).

Scheme 31. Viossat et al. synthesis of 4-hydroxy-2,5,ll-trimethyl-6/f-pyrido[3,2-h]carbazole (183) (93).

Scheme 32. Gribble et al. synthesis of the 10//-pyrido[2,3-b]carbazole ring system (e.g., 187) (94).

Scheme 37. Lescot et al. synthesis of the 5-methyl-1 l//-pyrido[3,4-a]carbazole ring system (226, 111) (700).

Scheme 38. Synthesis of the 5-methyl-7//-pyrido[4,3-c]carbazole ring system (e.g., 231, 233) by Roques and co-workers (102).

Archer and co-workers (103) have also employed the Snieckus oxidative pho-tocyclization in the key step of their synthesis of the 7//-pyrido[4,3-c]carbazole ring system (Scheme 40). Thus, a Wittig condensation between pyruvate 242 and pyridine 243 gave the unsaturated ester 244. Photocyclization gave the tetracyclic ester 245. Reduction and reaction with methyl isocyanate led to carbamate 247. Oxidation of alcohol 246 to aldehyde 248, followed by a standard one-carbon homologation, gave the desired ethyl derivative 249. These chemists also synthesized the 10-methoxyl derivative of each compound. 

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