Conditions for obtaining optimum allenic photoisomerization of (6R)-neoxanthin in benzene solution have been studied using diphenyl diselenide, diphenyl ditelluride or iodine as catalyst with various light qualities (sunlight, UVA irradiation or artificial, visible light) and intensities. The effect of adding base in order to prevent furanoid rearrangement was examined. At optimum conditions 77% conversion of (R)- to (S)-allene was observed for neoxanthin (UVA, 2 × molar excess diphenyl diselenide∶neoxanthin, presence of Hünig’s base). These conditions were superior to iodine catalyzed isomerization.
(All-E,6S)- and (9′Z,6S)-neoxanthin were prepared for the first time and characterized by chromatographic (HPLC) and spectroscopic (VIS, MS, 2D 1H NMR, CD) techniques. Other geometrical isomers of the allenic (6S)-series were obtained by (E)/(Z) isomerization of (6S)-neoxanthin and characterized by HPLC and VIS data. At comparable conditions the ease of allenic (R)/(S) isomerization was peridinin > fucoxanthin > neoxanthin.
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