Synthese und Chiralität von (5S, 6R)‐5,6‐Epoxy‐5,6‐dihydro‐β,β‐carotin und (5R, 6R)‐5,6‐Dihydro‐β,β‐carotin‐5,6‐diol, einem Carotinoid mit ungewöhnlichen Eigenschaften

Synthesis and Chirality of (5S,6R)‐5,6‐Epoxy‐5,6‐dihydro‐β,β‐carotene and (5R,6R)‐5,6‐Dihydro‐β,β‐carotene‐5,6‐diol, a Compound with Unexpected Solubility Characteristics Wittig‐condensation of azafrinal (1e) with the phosphorane derived from 7 leads to a (1:3)‐mixture of (E)‐9′‐ and (Z)‐9′‐β,β‐carotene‐diol 3, from which pure and optically active 3 ((5R,6R)‐5,6‐dihydro‐β,β‐carotene‐5,6‐diol) has been isolated as bright violet leaflets, m.p. 168°. Due to the trans‐configuration of the diol moiety and to severe steric hindrance, hydrogen bonding is reduced to such an extent, that 3 behaves much more as a hydrocarbon than as a diol. There is good evidence that the so‐called ‘β‐oxycarotin’ obtained by Kuhn & Brockmann [15] by chromic acid oxidation of β, β‐carotene is the corresponding racemic cis‐diol. 3 has been converted into (5S, 6R)‐5,6‐epoxy‐5.6‐dihydro‐β,β‐carotene (4), m.p. 156°. This transformation establishes for the first time the chirality of a caroteneepoxide (without other O‐functions). Full spectral and chiroptical data including a complete assignement of 13C‐chemical shifts for azafrin methyl ester and 3 are presented.