Blue Carotenoids. Part 1. Novel Oxonium Ions Derived from Fucoxanthin.

Abstract

The reactions of fucoxanthin-derived ketals and hemiketals with dilute acid and of fucoxanthin with weak acid (AcOH) and strong organic (CF3COOH) or mineral (HCl) acids, have been examined. The blue products obtained in these reactions were identified as novel carotenoid oxonium ions, characterized by visible absorption spectra and chemical behaviour, including quantitative conversion into cyclic hemiketals of defined structure. The hemiketal products were identified by VIS, IR and mass spectroscopy in comparison with authentic hemiketals or related 5,8-furanoxides. Diagnostic features in the mass spectra of the hemiketals are rationalized. The reactions observed are rationalized mechanistically. Isofucoxanthins are not intermediates in the reaction of fucoxanthin with acids. The blue oxonium ions were obtained in reasonable yield in competition with electrophilic addition to the polyene chain.