Lineariifolianoids A–D, rare unsymmetrical sesquiterpenoid dimers comprised of xanthane and guaiane framework units from Inula lineariifolia

Lineariifolianoids A–D (1–4), a rare class of unsymmetrical sesquiterpenoid dimers with xanthane and guaiane frameworks have been isolated for the first time from Inula lineariifolia. Notably, lineariifolianoid D (4) features the first exo-adduct of such dimers through a plausible Diels–Alderase catalyzed biogenetic pathway. Moreover, the absolute configuration of japonicone A (5) has been revised by the Flack method. The inhibitory effects of the new isolates on TNF-α-mediated L929 cytotoxicity have also been evaluated and only the exo-adduct lineariifolianoid D (4) displays the inhibitory property.