Ring synthesis carbazole

CARBAZOLES AND AZAINDOLES 5.2.7.1 Carbazole Ring Synthesis and Annulation... 

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). 

Generally high yields of substituted carbazoles can be obtained by oxidative cyclization of diphenylamines using Pd(OAc)2 to effect cyclization (equation 59) (75JOC1365). This method has been used, for example, to close the carbazole ring in the synthesis of the anti-tumor alkaloid ellipticine (equation 60) (80TL3319). 

Among the heterocyclic derivatives, carbazole ring has emerged as a vital building block in the synthesis of many useful optoelectronic materials.93-96... 

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. 

Bergman and Pelcman (131) have discovered a one-step synthesis of the dimethylbenzo[i ]carbazole ring system (Scheme 57) that is remarkable in its simplicity. An acid-promoted Michael reaction between 2-ethylindole (349) and enone 350, followed by cyclization of the methylene group onto the carbonyl leads to a tertiary alcohol. In situ dehydration and ring C (D) oxidation completes the sequence of events. It remains to be seen if this procedure can be applied to the synthesis of pyridocarbazoles. 

Cycloaddition reactions continue to find application in the synthesis of natural products and related substances containing indole and carbazole rings. Moody has published a summary of syntheses of carbazole alkaloids in which cycloadditions of pyrano[3,4-b]indol-3-ones figure prominently. <94SL681>... 

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