Twelve-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 isolate from the red seaweed Laurencia obtusa from Kaş in the Turkish Mediterranean, revealed temperature‐dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180° flipping of the trans olefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10‐bromoobtusallene I (3), obtusallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic derivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for 2–3 and at low temperature for 4. Marked cross‐saturation‐transfer effects between the couples of conformers confirmed these observations. Activation energies for processes involving 10‐membered subunit rings (2–3) are higher than for 11‐membered (4–5) analogues, where faster conformational motion occurs too resulting in mediated vicinal J couplings. 1,3‐Dihydroxy substituents in 5 form an intramolecular hydrogen bond in low‐polarity, non‐H‐bonding solvents; this results in the existence of two further conformers, giving more complex NMR spectra. Descriptions in the literature of single conformers for obtusallenes 2–5 must have resulted from overlooking minor NMR signals or attributing them to impurities. Contrary to current belief, there are two conformers of obtusallene 1 (below) observable by NMR in solution; the same applies for related 0‐bridged macrocyclic ethers, the minor conformers of which were previously considered to be impurities. Inter‐conversion is by flipping of the trans olefinic bond.