An unusual mercapto-containing and three known diketopiperazines from the marine sponge Haliclona sp

A new mercapto-containing diketopiperazine 3-(1-mercaptoethyl) hexahydropyrrolo[1, 2-a]-pyrazine-1, 4-dione, (1) named Haliclomercaptodipeptide, and three known diketopiperazines cyclo(Pro-Leu) (2), cyclo(Pro-Ala) (3), cyclo(Pro-Tyr) (4) were isolated from marine sponge Haliclona sp. The structures were deduced on the basis of spectroscopic methods.