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Four new triterpene glycosides, fallaxosides C1 (1), C2 (2), D1 (3) and D2 (4) along with the known cucumarioside A3-2 (5) and koreoside A (6) have been isolated from the sea cucumber Cucumaria fallax (Cucumariidae, Dendrochirotida). Structures of the glycosides have been elucidated by 2D NMR spectroscopy and mass spectrometry. All of the glycosides are rare non-holostane derivatives having shortened side chains and contain pentasaccharide carbohydrate moieties with two or three sulfate groups. Structures of these triterpene glycosides and their comparison with those earlier isolated from Cucumaria spp. and Pseudocnus dubiosus leoninus allow us to suggest that the present assignment of C. fallax to the genus Pseudocnus is not correct, and this species should be assigned to the genus Cucumaria. Cytotoxic activity of glycosides 1–5 against the ascite form of mouse Ehrlich carcinoma cells and mouse spleen lymphocytes and hemolytic activity against mouse erythrocytes have been studied. The glycosides were expectedly not active in all the tests due to the absence of an 18(20)-lactone in their aglycones and the presence of several sulfate groups. There was one exception, cucumarioside A3-2 (5), which demonstrated a weak cytotoxicity against lymphocytes and moderate hemolytic activity.

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