Polycyclic furanobutenolide-derived norditerpenoids from the South China Sea soft corals Sinularia scabra and Sinularia polydactyla with immunosuppressive activity.

One new polycyclic furanobutenolide-derived norcembranoid, xiguscabrolide H (1), together with eleven known related norditerpenoids 2-12 were isolated from South China Sea soft corals Sinularia scabra and S. polydactyla, respectively. Among them, compounds 1, 6, 8, and 12 were discovered from the former species, while compounds 2-5, 7, and 9-11 were obtained from the latter species. The structure of new compound 1 was elucidated by extensive spectroscopic analysis and by the comparison with the reported data. With the assistance of time-dependent density functional theory electronic circular dichroism (TDDFT-ECD) calculations, its absolute configuration was determined. Moreover, the absolute stereostructures of the known compounds 3, 4, and 9-12, of which only relative configurations were assigned, were established for the first time by X-Ray diffraction analysis and TDDFT-ECD calculations, respectively. In bioassay, several isolates exhibited potent inhibitory effects on the ConA-induced T lymphocytes and/or LPS-induced B lymphocytes proliferation.