Niphateolide A: isolation from the marine sponge Niphates olemda and determination of its absolute configuration by an ECD analysis

A diterpene with a new skeleton, niphateolide A (1), was isolated from the marine sponge, Niphates olemda, as an inhibitor of the p53-Hdm2 interaction. Its structure was elucidated by NMR spectroscopy and its absolute configuration was established as 10R, 11R by ECD at the vacuum-ultraviolet region with a theoretical calculation. Compound 1 was observed as an inseparable stereoisomeric mixture at C-17 and an ECD analysis was subsequently performed by adopting a virtual equilibrium between the simplified two forms, 10R,11R,17R- and 10R,11R,17S-1a, in which the calculated ECD spectra were correctively integrated with the theoretically-derived internal and free energies.