Three new imidazole alkaloids, leucettamines A [1] and B [2] and leucettamidine [31, have been isolated from the Palauan sponge Leucetta microraphis. Their structures were established on the basis of extensive spectral analyses. Leucettamine A showed potent leukotriene B4 receptor binding activity (K¡ =1.3 µ ), while leucettamine B was essentially inactive (K¡ = 100 µ ) and leucettamidine showed significant activity (K¡ = 5.3 µ ). With leucettamine A identified as a pure LTB4 receptor antagonist, a new structure lead is presented to inflammation therapy.

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During the large-scale isolation of the batzelladines A and B, which are inhibitors of gp120-CD4 binding, we isolated 19 minor guanidine alkaloids. Three new tricyclic guanidine alkaloids isolated from this group were named 8a,8b-dehydroptilocaulin (2), 8a,8b-dehydro-8-hydroxyptilocaulin (3), and 1,8a;8b,3a-didehydro-8-hydroxyptilocaulin (5). The recently described mirabilin B (4) was also isolated. The structures and relative stereochemistry of these compounds were determined by spectral analysis and by comparison with spectral data of ptilocaulin (1).

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