ALGAL CAROTENOIDS .48. STRUCTURAL ASSIGNMENTS OF GEOMETRICAL-ISOMERS OF FUCOXANTHIN

Isolated, crystallized all-trans (3S,5R,6S,3'S,5'R,6'R)-fucoxanthin has been submitted to iodine-catalyzed stereomutation. HPLC systems are given for the semipreparative separation of the quasi-equilibrium mixture, which consisted of all-trans (42% of total), 9'-cis (20%), 13'-cis (14%), 13-cis (11%), 9',13'-dicis (6%), 13,9'-dicis (4%) and, tentatively, of 13,13'-dicis (3%), and 15-cis (1%). Deduction of the structures of the individual isomers (except 15-cis) was based on the assignment of the H-1 NMR spectra by 2D H-1-H-1 correlated spectroscopy (COSY) and by comparison with the known data of the all-trans compound. The structure of 9'-cis was additionally supported by rotating-frame nuclear Overhauser effect 2D spectroscopy (ROESY). The results are consistent with the visible absorption data (lambda(max) shifts and cis-peak intensities) as well as CD data. Mono-cis isomers exhibited inverted Cotton effects relative to the all-trans isomer, a feature previously associated with conservative CD spectra. The results confirm our recent finding obtained for related compounds, that the 9'-momo-cis isomer had been previously misidentified as the all-trans allenic (6'S)-isomer.