Carotenoids chirality

Zsila F, Deli J, Bikadi Z, and Simonyi M. 2001. Supramolecular assemblies of carotenoids. Chirality 13(10) 739-744. 

Chiroptical Methods. The c.d. spectra of some mono-cw-carotenoids have been studied. All showed opposite Cotton effects relative to the all-frans-isomers. The intensity of c.d. bands in the cw-region was enhanced in the cis-isomers, especially if the cis-double bond was near to the centre of the chromophore. It was confirmed that in c-ring carotenoids, chiral centres at C-2 and C-3 do not contribute significantly to the c.d. spectra, but a 19-hydroxy-group does influence chiroptical properties. The c.d. properties of squid and octopus rhodopsin and metarhodopsin have been discussed in relation to the conformation of the retinal chromophore.The o.r.d. spectra of retinal and retinylidene phos-phorylethanolamine in a micelle have been obtained. 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

A major trend in organic synthesis, however, is the move towards complex systems. It may happen that one needs to combine a steroid and a sugar molecule, a porphyrin and a carotenoid, a penicillin and a peptide. Also the specialists in a field have developed reactions and concepts that may, with or without modifications, be applied in other fields. If one needs to protect an amino group in a steroid, it is advisable not only to search the steroid literature but also to look into publications on peptide synthesis. In the synthesis of corrin chromophores with chiral centres, special knowledge of steroid, porphyrin, and alkaloid chemistry has been very helpful (R.B. Woodward, 1967 A. Eschenmoser, 1970). 

The determination of the absolute configuration of a carotenoid is only possible by circular dichroism (CD) measurement. The spectrum interpretation can only be done by comparison with reference or model compounds with known chiralities. The sample requirement is as low as 5 to 50 pg, but CD facilities are not so commonly available. Buchecker and Noack reported experimental aspects and discussion of the relationships of carotenoid structures and CD spectra. 

Simonyi M, Bikadi Z, Zsila F, and Deli J. 2003. Supramolecular exciton chirality of carotenoid aggregates. Chirality 15(8) 680-698. 

Spano, F. C. 2009. Analysis of the UV/Vis and CD spectral line shapes of carotenoid assemblies Spectral signatures of chiral H-aggregates. J. Am. Soc. 131 4267-4278. 

Zsila, F., Z. Bikadi, J. Deli, and M. Simonyi. 2001c. Chiral detection of carotenoid assembhes. Chirality 13 446-453. 

The two terminal groups, R and R, are in most cases totally responsible for the chirality of carotenoids, which can thus be further classified into the three categories homodichiral, with two identical end groups, of symmetry C2 heterodichiral, with two different end groups or monochiral, with one chiral and one achiral terminal group. Naturally, the chiral terminal groups are responsible for the CD of these molecules, a subject which... 

The carotenoid family have chiral centres which enable the use of circular dichroism. However, the chirality of carotenoids is not sufficiently characteristic so that the chiroptical properties do not serve as a good analytical tool. 

Hydrogenation of 2,4,4-trimethyl-2-cyclohexenone with rrans-RuCl2(tolbinap)(dpen) and (CH3)3COK under 8 atm of hydrogen gives 2,4,4-trimethyl-2-cyclohexenol quantitatively with 96% ee (Scheme 1.70) [256,275,276]. In this case, unlike in the reaction of aromatic ketones, the combination of the R diphosphine and S,S diamine most effectively discriminates the enantiofaces. The chiral allylic alcohol is a versatile intermediate in the synthesis of carotenoid-derived odorants and other bioactive terpens such as a-damascone and dihydroactinidiolide [277]. 

According to the list of natural carotenoids by O. Straub 38), more than half of the over 400 natural carotenoids described are chiral. The asymmetric optically active terpene phosphonium salts which have recently become known, and which can be employed for the synthesis of chiral carotenoids, are contained in a review article by H. Mayer 47). 

Leuenberger, H. G., Boguth, W., Widmer, E., and Zell, R. 1976. Synthesis of optically active natural carotenoids and structurally related compounds. I. Synthesis of chiral key compound (4A 6A)-4-hydioxy-2,2,6-trimethylcyclohexanone. Helvetica Chimica... 

Stereochemistry.—Carotenoids. The absolute configuration of astaxanthin [3,3 -dihydroxy-/3,/3-carotene-4,4 -dione (33)] has been determined21 as (3S,3 S) by c.d. correlation of the tetrol (34) obtained by LiAlH4 reduction of astaxanthin diester (from lobster) with (37 ,3. R)-zeaxanthin (7). The astaxanthin thus cannot exist in vivo as a bis-dianion, e.g. (35), bound to protein, since chirality could not be introduced by solvent extraction. Actinioerythrin [3,3 -dihydroxy-2,2 -dinor-/3,/3-carotene-4,4 -dione 3,3 -diacylate (36)] is also optically active, with the two end-groups having the same (undetermined) chirality. The (3S,3 S) chirality has also been demonstrated for astaxanthin from the spider mite Schizonobia sycophanta.22... 

Lancaster, C. R. D., and Michel, H., 1999, Refined crystal structures of reaction centres from Rhodopseudomonas viridis in complexes with the herbicide atrazine and two chiral atrazine derivatives also lead to a new model of the bound carotenoid. J. Mol. Biol., 286 883n898. 

Two key chiral building blocks used in the total synthesis of a-tocopherol were prepared via microbial reduction of unsaturated carbonyl compounds with baker s yeast and with Geotrichum candidum Similarly, a key intermediate in the total synthesis of optically active natural carotenoids was prepared by microbial reduction of oxoisophorone with baker s yeast. An alternative approach to the synthesis of a-tocopherol employs a chiral building block that was obtained by baker s yeast reduction of 2-methyl-5-phenylpentadienal. ... 

Lactone. Various fungi, including Bakers yeast and Geotrichum candi-dum, have been shown to produce optically active lactones, useful chiral synthon, via the stereoselective reduction of suitable unsaturated precursors (42). Figure 15 shows the production scheme for a chiral, substituted diketone, a synthon for optically active carotenoids. 

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