ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
(6R,9′Z)-Neoxanthin: Synthesis, Physical Properties, Spectra, and Calculations of Its Conformation in SolutionThe 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 ∊-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.
Additional Material:
20 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19940770405
