Crystallographic and NMR studies of antiinfective tricyclic guanidine alkaloids from the sponge Monanchora unguifera.

Three tricyclic guanidine alkaloids, including 1,8a;8b,3a-didehydro-8beta-hydroxyptilocaulin (1), 1,8a;8b,3a-didehydro-8alpha-hydroxyptilocaulin (2) and mirabilin B (3), were identified from the marine sponge Monanchora unguifera. 1,8a;8b,3a-Didehydro-8alpha-hydroxyptilocaulin (2) is a new stereoisomer of 1, the structure of which was elucidated by spectroscopic analysis, comparison of its spectral data with those of 1, and confirmed by X-ray analysis. Compounds 1 and 2 co-crystallized in an unusual perfect order and packed around an approximate inversion center. A mixture of 1 and 2 is active against the malaria parasite Plasmodium falciparum with an IC50 value of 3.8 microg/mL while mirabilin B (3) exhibited antifungal activity against Cryptococcus neoformans with an IC50 value of 7.0 microg/mL and antiprotozoal activity against Leishmania donovani with an IC50 value of 17 microg/mL.