Optical Superposition of Molar Rotations of Halogenated Terpenoids. Configurational Reassignment of Iodocallophycol E from the Red Alga Callophycus serratus

The red alga, Callophycus serratus, is the source of highly halogenated meroditerpenes, including iodocallophycol E (1e), whose stereostructure was assigned as (2S,6S,7S,10S,14R); however, doubt was raised about the C10 and C14 configuration. Here, we revise the absolute configuration of 1e to (10R,14S) employing van’t Hoff’s principle of optical superposition and comparisons with analogs (proxies) of the ring C of 1e, including β-snyderol (2) and related sesquiterpenes. A brief error analysis shows that the optical superposition method improves the accuracy over single-point comparisons of optical rotations, leading to higher confidence and reliability in stereoassignments.