Monanchocidins B-E: Polycyclic Guanidine Alkaloids with Potent Antileukemic Activities from the Sponge Monanchora pulchra

New unusual polycyclic guanidine alkaloids monanchocidins B-E (2-5) along with monanchocidin A (1), which we recently described, were isolated from the Far-Eastern marine sponge Monanchora pulchra. Their structures were established using spectroscopic data and chemical transformations. Compounds 1-5 show potent cytotoxic activities against HL-60 human leukemia cells with IC(50) values of 540, 200, 110, 830, and 650 nM, respectively.