For designation of the geometrical configuration around carbon-carbon double bonds in carotenoids the use of the prefixes (E) and (Z) is now a recommended practice. In most cases (E) corresponds to the trans configuration and (Z) to cis. However, if the polyene chain, including the lateral methyl groups, has oxygen substituents attached, some eis double bonds must be designated (E) and some trans double bonds (Z). In this case the cis/trans nomenclature is still maintained to avoid confusion (see Vol. 1A Chapter 3).KeywordsDouble BondTriple BondTotal SynthesisPhosphonium SaltWittig ReactionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
A series of charge-delocalized carotenoid mono- and dications have been prepared by treatment of selected carotenoids with Brønsted and Lewis acids. The detailed structures of the carbocations were established by NMR studies in the temperature range from −10 to −20 °C. The general strategy for structure elucidation by NMR of several cationic components in a mixture is outlined. Bond type and regions of bond inversion were established, as well as the charge distribution, which was determined from the difference in 13C chemical shift at each carbon. This method gave a more accurate estimate for the partial charges than by using the Spiesecke−Schneider relationship. The resulting charge distribution was used as models for the structure of charged solitons. These carotenoid cations have the most delocalized charge so far determined, and the monocations represent the first experimental structure determination of positively charged solitons. The soliton width determined here is in good agreement with the results of previous AM1 calculations.
The structure of a sponge metabolite from Microciona prolifera, previously considered to be (6S)-2,3-didehydro- or 3,4-didehydro-gamma, chi-carotene, has been further studied. Attempted total synthesis of the 3,4-didehydro derivative provided the hitherto unknown gamma, chi-carotene, the synthesis of which is described. Hydrolysis of lutein methanesulfonate diester (dimesylate) gave elimination products possessing the 3,4-didehydro gamma end-group. 1H NMR data for this gamma end-group were identical with those for the sponge carotenoid. The mesylate elimination reaction described may mimic the metabolic formation of the 3,4-didehydro-gamma-carotenoid end-group. In connection with other investigations on functionalized carotenoids we further report the preparation of zeaxanthin and lutein mesylates and their base-catalyzed elimination reactions. SN2 type substitution reactions of zeaxanthin dimesylate with appropriate nucleophiles did not produce beta, beta-carotene, zeaxanthin diacetate or thiozeaxanthin.
