Structure of Zoanthamines

Norzoanthamine (12), zoanthamine (13), and its homologues 14-21 were isolated from the colonial zoanthid Zoanthus sp. [23-26]. The relative stereochemistry of norzoanthamines was determined by X-ray analysis. The absolute stereochemistry of norzoanthamine was established by a modified Mosher's method [27].

Interestingly, equilibration was observed between the lactone structure and iminium structure on norzoanthamine (12). The NMR spectrum of norzoanthamine hydrochloride in CD3OD implied the presence of an iminium structure (22, 5C-10 = 193.3) but not a lactone structure 12 in norzoanthamine (Scheme 4). The zwitter iminium structure was also demonstrated by transformation into methyl ester 23 by the treatment of 12 with CH3I - Ag2O. On the other hand, hydrolysis of 23 with aqueous HCl led to the recovery of 12. Recently, zooxathellamine (21) was isolated from the cultured symbiotic dinoflagellate Symbiodinium sp. [28]. As with norzoanthamine (12), 21 also adopted a zwitter ion structure with carboxylate and iminium moieties in D2O, but had a lactone structure in either CDCl3 or CD3OD.

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