New Sesquiterpene Lactone Dimer, Uvedafolin, Extracted from Eight Yacon Leaf Varieties (<i>Smallanthus sonchifolius</i>): Cytotoxicity in HeLa, HL-60, and Murine B16-F10 Melanoma Cell Lines

Uvedafolin, <b>1</b>, a new sesquiterpene lactone dimer, was isolated from the leaves of <i>Smallanthus sonchifolius</i> with five related compounds, <b>2</b>–<b>6</b>, and their cytotoxicity was assessed against three tumor cell lines (HeLa, HL-60, B16-F10 melanoma). The stereostructure of <b>1</b> was newly elucidated by ESI-TOF-MS, 1D/2D NMR, and single-crystal X-ray diffraction. Dimers <b>1</b> and <b>2</b> had the most effective IC₅₀ values, 0.2–1.9 μM, against the three tumor cell lines when compared with monomers <b>3</b>–<b>6</b> (IC₅₀ values 0.7–9.9 μM) and etoposide (IC₅₀ values 0.8–114 μM). The ester linkages of two sets of monomers, uvedalin, <b>5</b>, and sonchifolin, <b>6</b>, for <b>1</b>, and enhydrin, <b>4</b>, and sonchifolin, <b>6</b>, for <b>2</b>, as well as the acetyl group at the C-9 position, were essential for the high cytotoxicity. Dimers <b>1</b> and <b>2</b> would have potential as anticancer agents