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

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Abstract
Thermal isomerization of synthetic all-(E)-.beta.-carotene yielded a multi-component mixture composed of more than 16 (Z)-.beta.-carotenes and 2 thermo-cyclized isomers. They were separated and purified by high-performance liquid chromatography. Ten of these were characterized by spectroscopic methods as 4 mono-(Z)-, 5 di-(Z)-, and 1 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 6 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 1st report on the formation and identification of the sterically hindered 7-(Z)-isomers as well as the (5 .fwdarw. 10)-thermo-cyclized products in the field of alicyclic carotenoids.