Steric Stability of Acetylenic Carotenoids.

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Abstract
Comparative iodine catalyzed stereomutation studies in light of the diacetylenic all-trans alloxanthin (2) and of its 9,9''-di-cis isomer, employing separation of geometrical isomers by HPLC [high pressure liquid chromatography], are reported. The steric stability of all-trans 7,8-didehydro-and of 7,8,7'',8''-tetradehydroastaxanthin (as diacetates 3'' and 4'') was studied. Considerable steric stability for the monoacetylenic 3'' and somewhat less stability for the diacetylenic 4'' was demonstrated during common isolation procedures. The composition of the photostationary mixtures produced in the presence of iodine showed the following features: in the diacetylenic series (2 and 4'') the all-trans isomer was absent and the 9,9''-di-cis and 9-mono-cis isomers were dominan; in the monacetylenic series the all-trans isomer occurred together with larger amounts of the 9-mono-cis isomer. Factors governing the steric stability of acetylenic carotenoids are considered.

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