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 Delta(1(2)) double bond involving an alpha,beta-unsaturated gamma-lactone ring as a dienophile group and a trisubstituted conjugated Delta(21(34))/Delta(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.