Protonated astaxanthin

Structure 44, Fig. 19, was recently proposed for protonated astaxanthin in crustacyanin with the delocalisation of the positive charge on the polyene chain mainly close to the rings [9]. Acidic amino acids were at that time suggested to be responsible for the protonation of the carotenoid. 

The (13-Z) isomer of astaxanthin is a noncentrosymmetric carotenoid, thus the proton shifts of both sides of the chain are not equal any longer. For example, this causes proton 15 to have a spectrum of higher order, while it exhibits a doublet in the all-E compound. The largest shift differences... 

The use of 1,2-epoxybutane in these reactions is superior in some respects to the use of alkali metal alcoholates as bases. Thus, for example, in the synthesis of the a-hydroxyketone (3i 5,3 / 5)-astaxanthin [(3RS,yRS)-403] from the Cis-phosphonium salt 3RS)-49 the yields achieved, after thermal isomerization and recrystallization, were 77% with NaOMe as proton acceptor and 83% in boiling 1,2-epoxybutane, based on the dialdehyde 34 used [69] (Scheme 12). 

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