Spongiains A-C: Three new spongian diterpenes with ring A rearrangement from the marine sponge Spongia sp.

Spongiains A-C (1-3), the first examples of spongian diterpenes bearing a pentacyclic skeleton composed of a fused 5/5/6/6/5 ring system through ring A rearrangement, together with four new spongian diterpenes, spongiains D-G (4-7), were isolated from the marine sponge Spongia sp.. The structures with absolute configurations of these compounds were elucidated by the methods of NMR, X-ray diffraction and quantum chemical approaches. The hypothetical biogenetic pathways as well as cytotoxic activities of 1-7 were also discussed. (C) 2019 Elsevier Ltd. All rights reserved.