Omriolide A and B; two new rearranged spongian diterpenes from the marine sponge Dictyodendrilla aff. retiara

Two new rearranged spongian diterpenes designated omriolide A and B were isolated from the southern Kenyan sponge Dictyodendrilla aff. retiara collected at Wasini Is., Shimoni Channel. Omriolide A possesses an octalin linked to a unique trioxatricyclo[5.2.1.04,10]decane ring system, forming a tricyclic ‘cap’. Omriolide B comprised a tricyclo[5.4.0.02,4]undecane linked to a dioxabicyclo[3.3.0]octane. The structures of both diterpenes were elucidated by interpretation of MS results and, mainly, 1D and 2D NMR spectra.