Geometrical E/Z isomers of (6R)- and (6S)-neoxanthin and biological implications

9'Z-(3S,5R,6R,3'S,5'R,6'S)-Neoxanthin reisolated from spinach (Spinacea ole racea) and characterized by HPLC, VIS, MS, and 2D H-1 NMR, has been submitt ed to photoinduced stereomutation in the presence of iodine or diphenyl dis elenide at conditions not involving isomerization of the allenic bond. The six individual geometrical isomers, all-E, 9Z, 9'Z, 13Z, 13'Z, 15Z and thre e minor di-Z-isomers, presumably 9,9'-di-Z,9', 13-di-Z and 9',13'-di-Z, pre sent in the equilibrium mixture have been characterized by HPLC, VIS data, H-1 NMR and reversibility tests. Judged by the quantitative composition of the equilibrium mixture the naturally occurring 9'Z-isomer is thermodynamic ally less stable than the all-E-isomer. The availability of these isomers f acilitates future search in natural sources. 9'Z-(6R)-Neoxanthin represente d > 90% of total neoxanthin in spinach and broccoli (Brassica oleracea var. italica), consistent with previous findings of its abundance in chloroplas ts. all-E-Violaxanthin, the presumed biosynthetic precursor of neoxanthin, represented > 90% of total violaxanthin in the same sources. It is postulat ed that a neoxanthin Delta 9'-isomerase is present. and involved in the bio synthesis of abscisic acid in higher plants. Allenic S-isomers are of inter est as postulated biosynthetic precursors of R-allenes. All-E-(6S)- and 9'Z -(6S)-neoxanthin were available as semi-synthetic model compounds. The alle nic (6S)-diastereomers could not be detected in spinach or broccoli. (C) 20 00 Elsevier Science Ltd. All rights reserved.