BLUE CAROTENOIDS .1. NOVEL OXONIUM IONS DERIVED FROM FUCOXANTHIN

The reactions of fucoxanthin-derived ketals and hemiketals with dilute acid and of fucoxanthin with weak acid (AcOH) and strong organic (CF3 COOH) or mineral (HCl) acids, have been examined. The blue products ob tained in these reactions were identified as novel carotenoid oxonium ions, characterized by visible absorption spectra and chemical behavio ur, including quantitative conversion into cyclic hemiketals of define d structure. The hemiketal products were identified by VIS, IR and mas s 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 mechanistic ally. Isofucoxanthins are not intermediates in the reaction of fucoxan thin with acids. The blue oxonium ions were obtained in reasonable yie ld in competition with electrophilic addition to the polyene chain.