(6R,9'Z)-Neoxanthin: Synthesis, physical properties, spectra, and calculations of its conformation in solution

Article abstract of DOI:10.1002/hlca.19940770405

The synthesis of pure and crystalline (9'Z)-neoxanthin (6) is described. MnO₂ Oxidation of (9Z)-C₁₅-alcohol 7 at room temperature produces a mixture 8/9 of (9Z)- and (9E)-aldehydes. Predominant formation of the required (9Z)-aldehyde 8 is achieved by performing the oxidation at -10°. Condensation of 8 with the mono-Li salt of the symmetrical C₁₀-diphosphonate 10 gave the (9Z)-C₂₅-monophosphonate 11. The Wittig-Horner condensation of 10 with the allenic C₁₅-aldehyde 1b, under selected conditions allows the preparation of pure and crystalline (9'Z)-15,15'-didehydroneoxanthin (12) and, after subsequent semireduction, of crystalline (15Z,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 ¹H- and ¹³C-NMR investigations at 600 MHz of a (D₆)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 OH-C(3'). The torsion angle (ω₁) 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.