Scopariusicides D-M, ent-clerodane-based isomeric meroditerpenoids with a cyclobutane-fused γ/δ-lactone core from Isodon scoparius.

Scopariusicides D-M (1-10), ten new ent-clerodane-based meroditerpenoids with a cyclobutane-fused γ/δ-lactone core, were isolated from Isodon scoparius. Their structures were determined by comprehensive analysis of spectroscopic data, single-crystal X-ray diffraction, chemical transformation, and TDDFT ECD calculation. A plausible biosynthetic pathway of 1-10 was proposed in which the asymmetrical cyclobutane ring was formed via a crossed "head-to-tail" intermolecular [2 + 2] cycloaddition in anti/syn facial approaches between an ent-clerodane lactone and a cis-4-hydroxycinnamic acid. Bioactivity evaluation manifested that 5 exhibited significant neuroprotective effect against corticosterone-induced injury in PC12 cells, while 6 and 7 exhibited moderate immunosuppressive activity against human T cell proliferation stimulated by anti-CD3/anti-CD28 mAb.