Chromolactol, an Oxygenated Diterpene from the Indo-Pacific Nudibranch Goniobranchus coi: Spectroscopic and Computational Studies

A rearranged spongian diterpene chromolactol was obtained from the mantle extract of the Indo-Pacific nudibranch Goniobranchus coi. The structure of chromolactol, either 1a or 1b, which was investigated by extensive NMR experiments and by data comparison as well as by molecular modelling studies and density functional calculations, has a different relative configuration of the 2,8-dioxabicyclo-[3.3.0]-octane ring compared with the co-metabolite norrisolide (2). A biosynthetic pathway leading to the preferred diastereomer of chromolactol (1a) is presented.