Amphidinolide C (1) is a unique 25-membered macrolide isolated from a marine dinoflagellate Amphidinium sp. (strain Y-5). The gross structure of 1 has been elucidated by 2D-NMR data, whereas the stereochemistry remains undefined. During our search for bioactive metabolites from marine dinoflagellates, relatively large amounts of 1 have been recently isolated from two strains (Y-56 and Y-71) of the genus Amphidinium, which allowed us to investigate the stereochemistry of 1 Absolute configurations at C-12, C-13, C-20, C-23, C-24, and C-29 of amphidinolide C (1) were assigned by combination of NMR analyses including J-based configuration analysis and Mosher's method, while absolute configurations at C-7 and C-8 of 1 were assigned by application of Mosher's method for erythro glycol and comparison of ^1H-NMR data of natural specimen with those of synthetic C-1-C-10 segments. Absolute configurations at C-3, C-4, C-6, and C-16 of 1 were elucidated by comparison of NMR data of degradation products of 1 with synthetic C-1-C-7 and C-16-C-18 segments. On the other hand, five new 19-membered macrolides amphidinolides T1-T5 (2-6) have been isolated from the Y-56 and Y-71 strains of the genus Amphdinium. The gross structures were elucidated by 2D-NMR analyses. Absolute configurations at C-2, C-13, C-14, and C-18 of amphidinolide T1 (2) were assigned on the basis of NMR data of the MTPA esters of 2 and those of degradation products of 2, while absolute configurations at C-7, C-8, and C-10 of 2 were elucidated by comparison of ^1H-NMR data of C-1-C-12 segment with those of synthetic model compounds for tetrahydrofuran portion. The absolute stereochemistry of amphidinolides T2-T5 (3-6) were also elucidated by similar methods. The stereostructure of 2 was also confirmed by a single crystal X-ray diffraction analysis.
