New Esters of Ferrediol and Sideritriol in Sideritis\nclandestina subsp. <i>peloponnesiaca</i>: Characterization and Antiproliferative Activity

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