Biscembranoids formed from an α,β-unsaturated γ-lactone ring as a dienophile: structure revision and establishment of their absolute configurations using theoretical calculations of electronic circular dichroism spectra.

Four new biscembranoids, bislatumlides C-F (1-4), were isolated from the Hainan soft coral Sarcophyton latum . Their structures were elucidated by detailed analysis of spectroscopic data and by comparison with reported data of related derivatives, leading to the structure revision of co-occurring bislatumlides A (5) and B (6) at the C-21 configuration. The absolute configurations of bislatumlides C and E (1 and 3) were determined by TDDFT calculations of their solution ECD spectra, allowing the configurational assignment of the related bislatumlides D and F (2 and 4) and A and B (5 and 6) as well. Bislatumlides A-F (1-6) represent the only biscembranoids formed by the undescribed coupling pattern of Diels-Alder cycloaddition between the Δ(1(2)) double bond involving an α,β-unsaturated γ-lactone ring as a dienophile group and a trisubstituted conjugated Δ(21(34))/Δ(35(36))-butadiene moiety. An endo-cycloaddition gave 1, 2, 5, and 6, whereas an exo-cycloaddition produced 3 and 4. This is the first report of exo-addition dicembranoids from marine sources and from nature. Bislatumlides C and E (1, 3) could be used as ECD reference compounds in the determination of absolute configuration for related derivatives.