New Cytotoxic Thymol Derivatives from Inula helianthus-aquatica (Compositae)

A new thymol derivative, 8-hydroxy-9,10-dioxyisopropylidene-thymol (1), together with three known thymol derivatives 10-hydroxy-8, 9-dioxyisopropylidene-thymol (2), 8-hydroxy-9, 10-diisobutyryloxy-thymol (3) and 8, 10-dihydroxy-9-isobutyryloxy-thymol (4), was isolated from 95% EtOH extract of the flowers of Inula helianthus-aquatica. Their structures were elucidated on the basis of detailed spectroscopic analysis. These compounds showed cytotoxicities on six cancer cell lines (K562, HT-29, SGC-7901, DU145, MDA-MB-231, U251), and compound 2 is the strongest one with IC50s of 4.20-33.12 μmol/L. This is the first time that the cytotoxic thymol derivatives were isolated from this titled plant.