Carotenoids optical isomers

Separation and Assay. Procedures for the separation, purification, and assay of carotenoids and retinoids by h.p.l.c., g.c., and g.c.-m.s. are given in an extensive article." Another, general, review includes information on the h.p.l.c. separation of retinoids.A particularly useful method has been developed for resolution and analysis of some carotenoid optical isomers.For example, (3R,3 R)-, (3S,3 S)-, and (3/ ,3 5)-astaxanthin were converted into the diastereomeric (-)-camphanic acid diesters, which were separated by h.p.l.c. This procedure has been used to analyse the isomeric composition of a natural astaxanthin sample. An h.p.l.c. procedure for separation of a-, P-, and y-carotenes (173)—(175) and lycopene (176) has been described." Several papers describe methods for the h.p.l.c. separation and purification of various retinal and retinol isomers and derivatives.A procedure for the preparative t.l.c. of oxidation products of retinyl acetate has been described,and a competitive protein-binding radioassay for retinoic has been reported. 

Keywords Carotenoids analysis sample preparation UV/Visible spectroscopy geometrical isomers optical isomers HPLC mass spectrometry nuclear magnetic resonance metabolites. 

Foss, P. Storebakken, T. Schiedt, K. Liaaen-Jensen, S. Austreng, E. Streiff, K. 1984. Carotenoids in diets for salmonids I. Pigmentation of rainbow trout with the individual optical isomers of astaxanthin in comparison with canthaxanthin. Aquaculture 41 213-226. 

Most of the naturally occurring carotenoids are chiral and therefore targets for synthesis in enantiomerically pure form. This can sometimes be achieved by preparing the carotenoid in racemic form and then resolving the optical isomers via diastereoisomers or by chromatography on a chiral stationary phase. However, in most cases, the construction of the optically pure end group is the most efficient way and for... 

The 3-hydroxy-p end group is the most abundant chiral end group in carotenoids and is often called the zeaxanthin end group. Zeaxanthin (55) possesses two chiral centres at C(3) and C(3 ), making possible three optical isomers, namely the (3R,3 R)-isomer (most abundant in Nature) and the (3S,3 S)-isomer as well as the (3R,3 S)-isomer which constitutes a meso-form. It is this optically inactive mixture of isomers which is usually obtained by synthesis of the so-called racemate [50]. 

The Roche group extended this work and in 1981, at the 6th International Symposium on Carotenoids in Liverpool, reported the total synthesis of several of the ten optical isomers of e,8-carotene-3,3 -diol (tunaxanthin, 149) and of four diastereoisomers of p,e-carotene-3,3-diol, including the most common (3R,3 / ,6 / )-isomer, lutein (133). The starting material for these syntheses was 6-oxoisophorone, which the Roche scientists went on to use to synthesize a large number of dicyclic carotenoids in optically inactive and active form, as reported at the 7th International Symposium on Carotenoids in Munich in 1984. 

Regarding absolute configuration classical polarimetric studies in the thirties indicated that natural carotenoids occiured as discrete optical isomers 110). Pioneering synthetic work in the fifties by Karrer s school 165, 166) resulted in preparation of the first optically active... 

The key step of the synthesis is the rearrangement of the a-acetylenic alcohol 97 into the unsaturated carbonyl compound 124. This rearrangement was carried out with tris(triphenylsilyl)vanadate, triphenylsilanol and benzoic acid to give a mixture of the isomers 124 and 125. The latter was converted by iodine catalysis into the desired isomer 124. This key intermediate was afterwards transformed into the Cis-phosphonium salt 123 by standard procedures. The Wittig olefination of the Cio-dial 45 first with the fucoxanthin end group 123 and then with the peridinin end group 122 gave, in five steps, the C4o-carotenoid 126. Finally the epoxidation of this compound resulted in optically active fucoxanthin (121) and its (5S,6R)-isomer (Scheme 28). 

The (9Z)> and (9Z,9 Z)-isomers of acetylenic carotenoids have been prepared by synthesis. The total synthesis of the (9Z,9 Z)-isomer of optically inactive alloxanthin (117) [12,13] has been treated in detail in Chapter 3 Part IV, as has the synthesis of (9Z)-mytiloxanthin (353) [33,34]. The (9Z)- and (9Z,9 Z)-isomers, respectively, of 402 and 400, the mono- and diacetylenic analogues of (3.5,3 5)-astaxanthin, have been synthesized [14], and recently also the (9Z)-isomers of (3/ ,3 / )-diatoxanthin (118) and (3/ )-7,8-didehydro-p,p-caroten-3-ol [35]. All these syntheses are based on an acetylenic C 5-phosphonium salt, as discussed in Section C.4, where the Wittig condensation with an appropriate aldehyde leads to stereoselective formation of the thermodynamically stable (9Z)-isomer. 

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