Thermal stereoisomerization of all-(E)-beta-carotene: (Z)-beta-carotenes and electrocyclized beta-carotenes.

Thermal isomerization of synthetic all-(E)-beta-carotene yielded a multi-component mixture composed of more than sixteen (Z)-beta-carotenes and two thermo-cyclized isomers. They were separated and purified by high-performance liquid chromatography. Ten of these were characterized by spectroscopic methods as four mono-(Z)-, five di-(Z)-, and one tri-(Z)-beta-carotenes; the 7-(Z)-, 9-(Z)-, 13-(Z)-, 15-(Z)-, 9.13-(Z)-, 9.15-(Z)-, 9.13'-(Z)-, 13.15-(Z)-, 7.13'-(Z)-, and 9.9'.13-(Z)-isomers were included. Stereochemistry of six additional (Z)-isomers obtained is, as yet, unknown, though one of them might have the 9.9'-(Z)-geometry in the molecule. This is the first report on the formation and identification of the 'sterically hindered' 7-(Z)-isomers as well as the (5 leads to 10)-thermo-cyclized products in the field of alicyclic carotenoids.