Two new alkaloids, dysideanins A (1) and B (2), along with two known diketopiperazines, cyclo-(Pro-Leu) (3) and cyclo-(Pro-Ile) (4), were isolated from the marine sponge Dysidea sp. The structures were established from NMR and MS analysis. Dysideanin B (2) exhibited antibacterial activity. The thiomethylated imidazolinium unit as found in dysideanin A is very rarely encountered in nature.
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