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Chemical investigation of the MeOH extract from the gorgonian Pseudopterogorgia americana afforded two rare sterols, ameristerenol A (1) and B (2), both 9,11-secosterols possesses a seven-membered cyclic enol-ether in ring C, and ameristerol A (3) is the first example of a naturally occurring 9,11-secosterol containing a gorgosterol side chain with a C-24(28) double bond. Ameristerenol A (1) was converted to the sterol derivatives 4–6 to provide additional chemical diversity and comparison for biological screening. The structures of compounds 1–6, along with three related known analogues 7–9, were determined on the basis of extensive spectroscopic analysis and by comparison with literature data. Compound 6 exhibited slight cytotoxicity activity against human breast cancer cell line MDA-MB-231.