New polyhalogenated, twelve-membered. O-bridged cyclic C-15-ethers. having in common oxygenation at C(6) and a bromoallene side chain at C(4) (where C(1) is the bromoallene-chain terminus). were isolated from the red seaweed Laurencia obtusa from the Turkish Mediterranean Sea. i.e. the: 9.12-O-bridged obtusallene VII (5). the 6.9 :9.12-bis-O-bridged obtusallene V (3) and obtusallene VI (4), as well as the 6,9-O-bridged obtusallene VIII (8) and obtusallene IX (9). The behaviour of portions of the macrocycle involved in fast motion and their equilibrium position depend on the particular compound, revealing a sub;lc. conformational behavior of these macrocycles, while 8 and 9 show an unprecedented solvolytic reactivity at the masked furfuryl (-2- furyimethyl) C-atom.