Brasilane-type sesquiterpenoids from the mediterranean red alga Laurencia obtusa

Three new irregular sesquiterpenoids were isolated and characterized, along with epi-brasilenol, from the Mediterranean red alga Laurencia obtusa. They possess the unusual skeleton of brasilane and one of them contains the unprecedented 5, 6-double bond (instead of a 1,6-double bond). Their structures were determined by detailed spectra analysis. The relative configurations were assigned by NOESY spectroscopy, molecular mechanics calculation (MM2) and, in one case, by a quantitative computer simulation of the lanthanide induced shifts in the 1H NMR spectrum. © 1991.