(S)-13-hydroxygeranylgeraniol-derived furanoditerpenes from Bifurcaria bifurcata.

Two new diterpenes were isolated from a sample of the brown alga Bifurcaria bifurcata collected from Brittany on the Atlantic coast and their structures established by spectroscopic methods. The new diterpenes are derived from (S)-13-hydroxygeranylgeraniol by terminal cyclization and oxidation leading to a furan-3-yl ring or a beta,gamma-epoxy-gamma-lactone. One of them showed a cytotoxic effect to fertilized sea urchin eggs. The chemical shifts of the methyl groups and quaternary carbons in (S)-13-hydroxygeranylgeraniol have been revised to take account of the results obtained in a 2D NMR long-range C-H correlation experiment and the absolute configuration at C-13 determined for the first time. The geographical variation in the diterpenoid composition of B. bifurcata was also studied.