Isomerization iodine catalysed

Physical Methods.—Separation and Assay. A range of isomers of astaxanthin (8) diacetate (9-cis, 13-cis, 15-cis, 9,9 -di-cis, 9,13-di-cw, 9,13 -di-cw, 13,13 -di-cw, 13,15-di-cw), prepared by thermal and iodine-catalysed isomerization of irans-(S) have been separated by h.p.l.c.126 A procedure has been developed for separation of bean leaf etioplast pigments, including carotenoids,127 by h.p.l.c. H.p.l.c. separations of esters of all-trans-, 9-cis-, 11-cw-, and 13-cw-retinol,128-130 and determinations of retinol in serum,131 retinol and 13-cw-retinoic acid,132 and the aromatic retinoid (195)133 in plasma have been described. A reversed-phase ion-pair... 

Z)-Isomers of trisubstituted double bonds and of sterically unhindered disubstituted double bonds are sufficiently stable in solution to allow their isolation in the pure state. (Z)-Isomers of sterically hindered disubstituted double bonds are not formed during iodine-catalysed isomerization. When prepared by synthesis in a carbon-carbon double-bond forming reaction they readily isomerize to the corresponding ( )-isomer. Their thermodynamic instability therefore requires particular attention. 

Bromine adsorbed on graphite in dichloromethane reacts with diverse acetylenes to give ( )- dibromoalkenes exclusively. No (E)- -> (Z)-isomerization, which is usually catalysed by bromine, occurs in the presence of graphite Alkynes adsorbed on alumina add iodine... 

The phenomenon of (E/Z)-isomerization (or cisitrans isomerization) in carotenoids has long been recognized. Interconversion of the (all- )- and various (mono-Z)- and (di-Z)-isomers occurs readily in solution, a process known to be catalysed by iodine in the presence of light [1]. Pure fZ)-isomers may be isolated by chromatography from such isomerization mixtures. The interest in pure (Z)-isomers of carotenoids has recently increased considerably [2,3], and the alternative preparation of (Z)-isomers by total synthesis has received growing attention. 

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