Indolo carbazole alkaloid synthesis

The application of indolo-2,3-quinodimethanes and their cyclic analogs in the synthesis of carbazole alkaloids has attracted wide interest since they could undergo Diels-Alder reactions with a wide variety of dienophiles to afford functionalized carbazole derivatives. This represents the shortest and most elegant method for the preparation of selectively functionalized carbazole derivatives (514). 

In synthesis of the alkaloid tjipanazole E (19b), the required symmetric dichloro-indolo[2,3-fl]carbazole 20a was obtained in a two-step procedure starting fi-om 4-chlorophenylhydrazine hydrochloride and 1,2-cyclohexanedione employing a Fischer indolization. Subsequent attachment of an acetyl-protected glucopyranosyl moiety to one of the nitrogens, followed by cleavage of the protective groups with ammonia in methanol, produced the desired natural product (91X7739). 

Indolo[3,2-fl]pyrrolo[3,4-c]carbazoles 120 have been obtained in one step from indole and the corresponding maleimides in acetic acid, with coformation of the Michael adducts 121 (Scheme 15). This reactitai required careful temperature control in order to obtain the desired product ratios. An alternative independent synthesis of compounds 120 could also be accompKshed from 2,3 -biindolyl (115) andsuitable maleimides in hot acetic acid (99T2363). The system 120 where R = H has also been reported as a minor product during studies toward a synthesis of the alkaloid arcyriaflavin A (95TL2689). 

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). 

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