Rogioldiol A, a New Obtusane Diterpene, and Rogiolal, a Degraded Derivative, of the Red Seaweed Laurencia Microcladia from Il Rogiolo along the Coast of Tuscany: a Synergism in Structural Elucidation

The structure of rogioldiol A ((-)-1), isolated from the red seaweed Laurencia microcladia, was determined. Employing the exciton-coupling technique for rogioldiol A p-bromobenzoate (2), the absolute configuration at C(9) of (-)-1 was assigned, and, together with extensive NMR experiments, the absolute configuration at C(10) and preferred conformations of (-)-1 were determined. The absolute configuration of the hetero-substituted cyclohexane ring was deduced in analogy from the X-ray structure of 4. a derivative of the aldehyde 3, which was isolated from the same seaweed and is believed to be a degradation product of (-)-1.