Synthese und Circulardichroismus optisch aktiver Carotinoidmodelle

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Abstract
Synthesis and Circular Dichroism of Optically Active Carotenoid ModelsThe synthesis of the following optically active carotenoidic model compounds are described: (—)‐(3,S,3′S)‐3,3′‐diisopropenyl‐16,17,18,16′,17,18′‐hexanor‐β,β‐carotene (1), (3R,3′R)‐19,20,19′,20′‐Metranor‐zeaxanthin (2) and (6R,6′R)‐19,20, 19′,20′‐tetranor‐ε,ε‐carotene (3). These compounds were synthesized for the following reasons: (1) the presence of methyl groups at C(1), C(1′), C(5), C(5′) of cyclic carotenoids profoundly affects the torsional angle of the C(6), C(7)‐ and C(6′), C(7′)‐bonds. Sign and magnitude of this angle are, according to recent theories [4] [5], responsible for a helical chromophore and for strong conservative [4] Cotton effects. CD. measurements of 1 give experimental support to these state‐ 1 exhibits weaker and less temperature dependent Cotton effects. Of more significance, the shape of the curve is no longer conservative, as expected. This constitutes experimental evidence for the contention that the β‐endgroups and the polyene chain indeed form an inherently dissymmetric chromophore in optically active β, β‐carotene derivatives; (2) the slightly S‐shaped form of the polyene chain of carotenoids, shown by X‐ray analyses [12] [13], is mainly ascribed to the presence of the methyl groups in the chain. Models 2 and 3 therefore are assumed to be linear. CD. studies of these compounds should consequently give information about the influence of deviation from Linearity and planarity of the polyene on the CD. spectra of carotenoids. CD measurements of 2 and 3 show that the lack of methyl groups does not alter the general type of the curve. Only the intensity and to some extent the position of the Cotton effects are influenced. Carotenoids with the ε‐endgroup possess inherently symmetric but asymmetrically distorted chromophores.The assumption that non‐conservative CD. spectra could become conservative upon cooling [4] is experimentally confirmed by model 3.The rule stating that pairs of all‐(E) and mono‐(Z) isomers of carotenoids with only one cyclic endgroup should have CD. spectra with the same sign [5] is disproved by the CD. spectra of four stereoisomeric rubixanthins (s. Fig. 5).The UV./VIS. spectrum of 3, λmax 447 (ε 216000), 418 (ε 189000) exhibits the highest molecular extinction ever reported for a carotenoid.

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