Abstract
17-Isovalerate esters of ferrediol and sideritriol (compounds 1 and 5) and 17-(2-methylbutyrate) ferrediol (compound 2), which have not been earlier described, were isolated along with five known ent-15-kaurene diterpenes (siderol, sideridiol, sideroxol, eubotriol, and epi-candicandiol), one ent-beyerene (flavovirol), and one phytosterol (β-sitosterol) from the aerial parts of Sideritis clandestina subsp. peloponnesiaca (Boiss. & Heldr.) Baden that is a mountain tea species endemic to Northern Peloponnese in Greece. Compounds 1-5 exhibited weak antiproliferative activity against triple-negative MDA-MB-231 and Hs578T breast cancer cells. In MCF-7 breast cancer cells, compounds 1, 2, siderol, and sideridiol were equipotent to 5-fluorouracil (moderate antiproliferative activity).