Algal carotenoids.: Part 64.: Structure and chemistry of 4-keto-19′-hexanoyloxyfucoxanthin with a novel carotenoid end group

The structural elucidation of a new carotenoid 4-keto-19'-hexanoyloxyfucoxanthin 5 from Emiliania huxleyi is documented by chromatographic (HPLC, TLC), spectroscopic (VIS, EIMS, FABMS, FABMSMS, 2D H-1 NMR) and chemical evidence. The novel carotenoid end group exhibits particular spectroscopic and chemical properties. In particular the reactions with base and acid are investigated. Due to a very weak molecular ion upon electron impact and facile cleavage to paracentrone 20 related fragments, the new carotenoid was previously misidentified as 19'-hexanoyloxyparacentrone 3-acetate 8, also found in other prymnesiophytes (haptophytes). This novel carotenoid readily undergoes cleavage to a C-31-skeletal paracentrone 20 related product upon storage, preferably in methanol solution. The new end group represents a plausible precursor for C-31-skeletal methyl ketone apocarotenoid metabolites in animals, and differs from the previously suggested precursor.