First preparation and characterization of the allenic carotenoids (all-E,6S)- and (9 ' Z,6S)-neoxanthin

Conditions for obtaining optimum allenic photoisomerization of (6R)-neoxant hin in benzene solution have been studied using diphenyl diselenide, diphen yl 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 n eoxanthin (UVA, 2 x molar excess diphenyl diselenide:neoxanthin, presence o f Hunig's base). These conditions were superior to iodine catalyzed isomeri zation. (All-E,6S)- and (9'Z,6S)-neoxanthin were prepared for the first time and ch aracterized by chromatographic (HPLC) and spectroscopic (VIS, MS, 2D H-1 NM R, CD) techniques. Other geometrical isomers of the allenic (6S)-series wer e obtained by (E)/(Z) isomerization of (6S)-neoxanthin and characterized by HPLC and VIS data. At comparable conditions the ease of allenic (R)/(S) is omerization was peridinin > fucoxanthin > neoxanthin.