Salmahyrtisol A, a novel cytotoxic sesterterpene from the Red Sea sponge Hyrtios erecta.

The lipophilic partition of a methanol extract of the Red Sea sponge Hyrtios erecta yielded a novel pentacyclic sesterterpene ester salmahyrtisol A (1), three new scalarane-type sesterterpenes, 3-acetyl sesterstatin 1 (3), 19-acetyl sesterstatin 3 (4), and salmahyrtisol B (5), together with the previously reported sesterterpenes hyrtiosal (2), scalarolide (6), and salmahyrtisol C (7). The structure determination was based on extensive NMR studies and high-resolution mass spectral measurements. In addition, salmahyrtisol A has a previously unknown pentacyclic carbon skeleton. The new compounds show significant cytotoxicity to murine leukemia (P-388), human lung carcinoma (A-549), and human colon carcinoma (HT-29). A biosynthetic relationship between 1 and 2 is briefly discussed.