(6 R ,9′ Z )‐Neoxanthin: Synthese, Schmelzverhalten, Spektren und Konformationsberechnungen

(6 R ,9′ Z )‐Neoxanthin: Synthesis, Physical Properties, Spectra, and Calculations of Its Conformation in Solution The synthesis of pure and crystalline (9′ Z )‐neoxanthin ( 6 ) is described. MnO 2 Oxidation of (9 Z )‐C 15 ‐alcohol 7 at room temperature produces a mixture 8/9 of (9 Z )‐ and (9 E )‐aldehydes. Predominant formation of the required (9 Z )‐aldehyde 8 is achieved by performing the oxidation at − 10°. Condensation of 8 with the mono‐Li salt of the symmetrical C 10 ‐diphosphonate 10 gave the (9 Z )‐C 25 ‐monophosphonate 11 . The Wittig‐Horner condensation of 10 with the allenic C 15 ‐aldehyde 1b , under selected conditions allows the preparation of pure and crystalline (9′ Z )‐15,15′‐didehydroneoxanthin ( 12 ) and, after subsequent semireduction, of crystalline (15 Z ,9′ Z )‐neoxanthin ( 13 ). Thermal isomerisation of a AcOEt solution of 13 at 95° yields preferentially (9′ Z )‐neoxanthin ( 6 ). Our crystalline sample shows the highest ϵ‐values in the UV/VIS spectra ever recorded. The CD spectra display a pronounced similarity with those of corresponding violaxanthin isomers. In contrast to the (all‐ E )‐isomer 5 , (9′ Z )‐neoxanthin undergoes very little isomerisation when heated to its melting point. For comparison purposes, a crystalline probe of 6 is also isolated from lawn mowings. Extensive 1 H‐and 13 C‐NMR investigations at 600 MHz of a (D 6 )benzene solution using 2D‐experiments such as COSY, TOCSY, ROESY, HMBC, and HMQC techniques permit the unambiguous assignment of all signals. Force‐field calculations of a model system of 6 indicate the presence of several interconverting conformers of the violaxanthin end group, 66% of which possess a pseudoequatorial and 34% a pseudoaxial OHC(3′). The torsion angle (ω 1 ) around the C(6′)C(7′) bond, known to be of prime importance for the shape of the CD spectra, varies with values of 87° for 55% and 263° for 45% of the molecules. Therefore, the molecules clearly display a preference for the ‘ syn ’‐position of the C(7′)C(8′) bond and the epoxy group. Unexpectedly, the double bonds of C(7′)C(8′) and C(9′)C(10′) are not coplanar. The deviation amounts to ± 20°, both in the ‘ syn ’ ‐ and the ‘ anti ’‐conformation.