Biogenesis of Carbazole Alkaloids

A decaketide (188) cyclising to a benzoanthraquinone (189) has been shown to be precursor of the kinamycin group of alkaloids 111, 112). The benzoanthraquinone undergoes an unusual process involving oxida-lion/nitrogen insertion/ring contraction to yield prekinamycin (190). 

Perhaps more direct proof for this hypothesis will be forthcoming. It is interesting to note that the first natural product with a cyanamide functionality, i.e. N-cyano-s c.—pseudostrychnine isolated from Stry-chnos wallichina steud. ex DC, occurs (14) in the leaves of the plant together with its N-methyl analogue. 

Tubingensins A (20) and B (21) are considered to arise from indolic diterpenes such as nominine (191) which occur in related species of Aspergillus. 

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Several hypotheses have been proposed for the biogenesis of carbazole alkaloids (see Chapter 3 of this review). However, there is no deep experimental knowledge of the biosynthesis of this class of alkaloids. A comparison of the structural features of carbazole alkaloids isolated from higher plants suggests that 3-methylcarbazole (2) may represent the key intermediate in their biosynthesis (Scheme 1.1). 

In the present review, we summarize the occurrence, biogenesis, biological activity, and the chemistry of carbazole alkaloids, which have been classified based on their natural sources, ring system, and substitution pattern. While a comprehensive overview is given on all carbazole alkaloids isolated from natural sources, only their total syntheses published since 1990 are discussed. 

Multicomponent reactions in alkaloid-based drug discovery 12KGS38. Muscarine, imidazole, oxazole, and thiazole alkaloids 13NPR869. Occurrence, biogenesis, and synthesis of biologically active carbazole alkaloids 12CRV3193. 

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