12-MEMBERED O-BRIDGED CYCLIC ETHERS OF RED SEAWEEDS IN THE GENUS LAURENCIA EXIST IN SOLUTION AS SLOWLY INTERCONVERTING CONFORMERS

The twelve-membered O-bridged cyclic ether obtusallene IV (1), a new i solate from the red seaweed Laurencia obtusa from Kas in the Turkish M editerranean, revealed temperature-dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180 degr ees flipping of the trans olefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10-bromoobtusallene I (3), obt usallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic der ivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for 2-3 and at low temperature f or 4. Marked cross-saturation-transfer effects between the couples of conformers confirmed these observations. Activation energies for proce sses involving 10-membered subunit rings (2-3) are higher than for 11- membered (4-5) analogues, where faster conformational motion occurs to o resulting in mediated vicinal J couplings. 1,3-Dihydroxy substituent s in 5 form an intramolecular hydrogen bond in low-polarity, non-H-bon ding solvents; this results in the existence of two further conformers , giving more complex NMR spectra. Descriptions in the literature of s ingle conformers for obtusallenes 2-5 must have resulted from overlook ing minor NMR signals or attributing them to impurities.