Preparation and (E/Z)-isomerization of the diastereoisomers of violaxanthin

Violaxanthin A (=(all-E,3S,5S,6R,3'S,5'S,6'R)-5,6:5',6'-diepoxy-5,6,5',6'-tetrahydro-beta,beta-carotene-3,3'-dioI =syn,syn-violaxanthin; 5) and violaxanthin B (=(all-E,3S,5S,6R,3'S,5'R,6'S)-5,6:5',6'-diepoxy-5,6,5',6'-tetrahydro-beta,beta-carotene-3,3'-diol = syn,anti-violaxanthin; 6) were prepared by epoxidation of zeaxanthin diacetate (1) with monoperphthalic acid. Violaxanthins 5 and 6 were submitted to thermal isomerization and I-2-catalyzed photoisomerization. The structure of the main products, i.e., (9Z)-5, (13Z)-5, (9Z)-6, (9'Z)-6, (13Z)-6, and (13'Z)-6, was determined by their UV/VIS, CD, H-1-NMR, C-13-NMR, and mass spectra.