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
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