Carotenoid oxonium ions

Whereas carotenoids in general are often considered by non-specialists as unstable compounds, they may easily be studied when particular precautions are made [1], This is also true for most subtypes with charged functional groups. The instability of carotenoid oxonium ions, radical cations, mono- and dications, however, represent special challenges. This review appears to be the first compilation on charged carotenoid species. 

A classical colour test for carotenoid 5,6-epoxides (e.g. violaxanthin (37)) is the formation of a dark blue colour upon treatment with strong acid [78,79], The chemistry behind this reaction was elucidated in our laboratory, as illustrated for neoxanthin (38) in Fig. 16 [80]. Furanoxides such as neochrome (39) are intermediates in this reaction. The elimination of a hydride ion from C-8 of the furanoxide is a key step in the formation of the blue oxonium ion 40 and is effected by strong mineral acid (HC1), CF3COOH or by the hydride acceptor triphenylcarbenium tetrafluoro-borate. 

Haugan, J.-A. and Liaaen-Jensen, S. (1994a) Blue carotenoids. Part 1. Novel oxonium ions derived from fucoxanthin. Acta Chem. Scand., 48, 68-75. 

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