Stereoisomerization of allenic carotenoids - Kinetic, thermodynamic and mechanistic aspects

Abstract

Diphenyl diselenide mediated photoisomerization of the allenic carotenoids peridinin and fucoxanthin in benzene solution have been studied, with the aim of optimizing synthetic yields of (6′S)-allenes. The effects of catalyst concentration and of light quality and intensity have been investigated using no catalyst, diphenyl diselenide or iodine as catalysts. Similar quasi-equilibrium mixtures of geometrical isomers were obtained with both catalysts, indicating a thermodynamic equilibrium. Considerably higher R to S allenic conversion was obtained with diphenyl diselenide than with iodine. Increased allenic isomerization rate upon UVA radiation and the effect of radical inhibition support a radical mechanism initiated by photolysis of diphenyl diselenide. The suggested mechanism for the diphenyl diselenide mediated isomerization may account for the high yield obtained [≈88% (6′S)-allene of recovered carotenoid].