Two Novel Guaiane Sesquiterpenes from the Whole Plant of Youngia japonica

Two new guaiane-type sesquiterpenes (1 and 2) together with eight related sesquiterpenoidal constituents (3–10) were isolated from the whole extract of Youngia japonica. The chemical structures of 1–10 were established by spectroscopic analyses as 4-epi-isolipidiol (1), 4′-p-hydroxyphenylacetyl crepiside E (2), isolipidiol (3), isoambeboin (4), grosheimin (5), annuionone D (6), loliolide (7), youngiajaponicoside A (8), crepiside H (9), and crepiside E (10), respectively. Among the isolated components, 5 exhibited a significant inhibitory effect on the proliferation of cultured human tumor cell lines (A549, SK-OV-3, SK-MEL-2, and HCT15).