1D and 2D NMR study of some allenic carotenoids of the fucoxanthin series

Abstract

AbstractThe allenic carotenoid all‐trans‐(3S,5R,6S,3′S,5′R,6′R)‐19′‐butanoyloxyfucoxanthin, isolated from the marine chrysophyte Pelagococcus subviridis, and its 9′‐cis isomer, obtained in 45% yield on iodine‐catalysed stereomutation, were submitted to a detailed 1H and 13C NMR analysis. For comparison, all‐trans‐fucoxanthin [(3S,5R,6S3′S,5′R,6′R)‐5,6‐epoxy‐3,3′,5′‐trihydroxy‐6′,7′‐didehydro‐5,6,7,8,5′,6′‐hexahydro‐β,β‐caroten‐8 ‐one‐3′‐acetate] was included in the study. The complete assignments of the 400 MHz 1H NMR spectra and of the 100.6 MHz 13C NMR spectra, including the stereochemistry, were possible by application of two‐dimensional 1H,1H chemical shift correlated spectroscopy (2D COSY), 1D and 2D total correlation spectroscopy (TOCSY, HOHAHA), rotating‐frame nuclear Overhauser spectroscopy (ROESY, CAMELSPIN) and 1H‐detected one‐bond (with and without 13C decoupling) and multiple‐bond 1H, 13C‐correlated 2D spectroscopy. The 9′‐trans to 9′‐cis stereomutation was unequivocally proved for 19′‐butanoyloxyfucoxanthin, mainly by 1D and 2D ROESY experiments; this is in contrast to a previously claimed 6′R → 6′S stereomutation in related allenic carotenoids.