Terrestrial alkaloids, halogenated

Several note vorthy halogenated terrestrial alkaloids are known, with epibatidine (104) at the top of the list [88]. This apparent frog Epipedobates tricolor) metabolite has powerful analgesic activity [89] and an intensive search is underway for a clinically useful drug [90]. A few chlorinated plant alkaloids have also been discovered. Romucosine F (lOS) is present in Annora purpurea, a South American bushy tree [91], and the closely related romucosine B (106) is found in the stems of RolUnia mucosa [92]. The furoquinoline alkaloid chlorodesnkolbisine (107) was isolated from the African folk medicine plant Teclea nobilis [93]. The authors of this latter study provide convincing evidence that 107 is not an isolation artifact (e.g. from the corresponding epoxide with HCl). 

Although very few terrestrial plant alkaloids contain halogen, brominated alkaloids have been reported from the marine environment. From the Okinawan marine sponge Hymemacidon sp., several bromopyrrole alkaloids have been described, e.g., tauroacidins A and B, Fig. (35) [262], konbuacidin A, Fig. (36) [263] and spongiacidins A-D [264]. Several species of sponges contain hymenialdisine, Fig. (37), which has been shown as a potent inhibitor of nuclear factor kappa B and interleukin-8 production in vitro [265,266]. 

The biosynthesis of organohalogens has seen enormous interest since the first survey, and several examples are mentioned earlier in the present review. Space does not allow for full coverage of this topic, but some additional examples are presented here. The reader is also directed to general reviews on the biosynthesis of marine natural products, many of which contain halogen (2398-2401), terrestrial fungal (basidiomycetes) metabolites (2402), and halogenated alkaloids (2403). 

Most of the alkaloids, around 30,000, have been isolated from terrestrial plants, but marine algae contain only 44 alkaloids. The origins of some alkaloids in plants are oxidation products of amino acids, whereas others origins remain unknown, especially halogenated alkaloids in marine algae. 

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