Further eleganolone-derived diterpenes from the brown alga Bifurcaria bifurcata

Four new diterpenes (1-4) together with six known members of this family were isolated from the brown alga Bifurcaria bifurcata collected off the coast near Roscoff, France. The structures of the new compounds were established in a comprehensive study on the basis of 1D and 2D NMR spectroscopic and mass spectrometric (ESI-MS) analyses. Our assignment is supported by a comparison of the 13C NMR chemical shifts to known compounds and predicted values. The new diterpenes are derivatives of the diterpene eleganolone (5). The structural relation and hypothetical metabolic pathway of the new diterpenes (1-4) with the co-isolated known diterpenes are discussed.