Stereoisomerization of allenic carotenoids - Kinetic, thermodynamic and mechanistic aspects

Diphenyl diselenide mediated photoisomerization of the allenic carotenoids peridinin and fucoxanthin in benzene solution have been studied, with the a im of optimizing synthetic yields of (6'S)-allenes. The effects of catalyst concentration and of light quality and intensity have been investigated us ing no catalyst, diphenyl diselenide or iodine as catalysts. Similar quasi-equilibrium mixtures of geometrical isomers were obtained wit h both catalysts, indicating a thermodynamic equilibrium. Considerably higher R to S allenic conversion was obtained with diphenyl di selenide than with iodine. Increased allenic isomerization rate upon UVA ra diation and the effect of radical inhibition support a radical mechanism in itiated by photolysis of diphenyl diselenide. The suggested mechanism for t he diphenyl diselenide mediated isomerization may account for the high yiel d obtained [approximate to 88% (6'S)-allene of recovered carotenoid].