Confirmation of the absolute configuration at C45 of amphidinol 3.

Amphidinol 3 (AM3), a membrane-active agent isolated from the dinoflagellate Amphidinium klebsii, consists of a long carbon chain containing 25 stereogenic centers. Although the absolute configuration of AM3 was determined by extensive NMR analysis and degradation of the natural product, the partial structure corresponding to the tetrahydropyran ring system was found to be antipodal to that of karlotoxin 2, a structurally related compound recently isolated from the dinoflagellate Karlodinium veneficum. By extensive degradation of the natural product and conversion of the resulting alcohol to an MTPA ester, the absolute configuration at C45 of AM3 was confirmed to be R, supporting the originally proposed structure.