Kurt Bernhard

Synthetic Astaxanthin. The Route of a Carotenoid from Research to Commercialisation

The presently witnessed industrial demand for carotenoids can be mainly attributed to their extraordinary properties as colouring agents and supplements for food and feed. No longer is the importance of colour as an indication of the quality of foodstuffs being ignored.

Some recent advances in the synthesis of natural carotenoids

Abstract The total syntheses of naturally occurring (3R,3′R)-zeaxanthin, (3S,3′S)-astaxanthin, (3S,3′S)-asterinic acid and of some related xanthophylls are discussed. Asymmetric hydroboration has been applied to the total synthesis of all configurational isomers of zeaxanthin. The total synthesis of natural actinioerythrin is described and the absolute configuration confirmed by X-ray crystallographic analysis of an intermediate. The optical resolution of α-hydroxy-carbonyl compounds via (−)-camphanic acid esters appears to be generally applicable and can be used as an analytical method for the determination of the optical purity of α-hydroxy-carbonyl carotenoids. Some technological applications are indicated.

Claimed 6R/6S allene isomerization in carotenoids is geometrical 9-trans/9-cis isomerization

Abstract Previously claimed isolations of ( S ) allene isomers of natural carotenoids have been disproved. Detailed 1H NMR have demonstrated that these isomers are geometrical isomers with the cis -bond adjacent to the allenic bond.

RECENT ADVANCES IN THE SYNTHESIS AND ANALYSIS OF 3,4-OXYGENATED XANTHOPHYLLS

Abstract The progress made within the last three years in the synthesis and analysis of astaxanthin and of other related 3,4-oxygenated xanthophylls is reviewed. This includes preparatively attractive routes to all optical isomers of adonirubin, 3-hydroxy echinenone, and of adonixanthin. (3R,3′R)-, (3S,3′S)-, as well as meso-(3R,3′S)-astaxanthin could be separated by HPLC when (3RS,3′RS)-astaxanthin (a 1:1 mixture of race-mate and meso-form) was first converted into the corresponding diaste-reomeric di-(-)-camphanates. The usefulness of this accurate analytical method for the separation of optical isomers of astaxanthin was demonstrated by the analysis of various astaxanthin samples from different natural sources. Surprisingly, some of these were found to be mixtures of the three optical isomers. The extension of this separation technique to other 3-hydroxy-4-oxo carotenoids provided, in combination with spectroscopic data, an excellent analytical method for the determination of the constitution, chirality and purity of these carotenoids.

Total Synthesis of (Z)-Isomers

For designation of the geometrical configuration around carbon-carbon double bonds in carotenoids the use of the prefixes (E) and (Z) is now a recommended practice. In most cases (E) corresponds to the trans configuration and (Z) to cis. However, if the polyene chain, including the lateral methyl groups, has oxygen substituents attached, some eis double bonds must be designated (E) and some trans double bonds (Z). In this case the cis/trans nomenclature is still maintained to avoid confusion (see Vol. 1A Chapter 3).

Enol Ether Condensation

As an example to the enol ether condensation, the synthesis of C10-dialdehyde (12,12′- diapocarotene-12,12′-dial) (4) was chosen because this Compound is not available commercially, but is a building block of major importance in carotenoid synthesis (Chapter 3 Part I). The procedure described is based on previously published work [1–3] (Scheme 1).