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

•A series of polycyclic furanobutenolide-derived norcembranoids were obtained.•TDDFT-ECD and X-Ray diffraction analysis were employed to determine absolute configurations.•ECD spectra data were in agreement with the proposed empirical ECD helicity rule.•Potent immunosuppressive activities against T and/or B lymphocytes were observed.