Nucleophilic reactions of charge delocalised carotenoid mono- and dications

Abstract

In the present study insight was gained on the larger complexity of cationic mixtures of diaryl (ϕ,ϕ-carotene, isorenieratene) and aliphatic (ψ,ψ-carotene, lycopene) carotenes, prepared by reaction with BF₃-etherate, compared with β,β-carotene. Chemical reactions of the mono- and dications prepared in situ from the allylic carotenols β,β-caroten-4-ol (isocryptoxanthin) and β,β-carotene-4,4′-diol (isozeaxanthin), and from isorenieratene and lycopene were investigated using selected O, N and S nucleophiles; water, methanol, azide and thioacetate. In total 22, including 18 new, neutral carotenoid products were isolated and identified by VIS, MS and NMR (in part) spectroscopy. Their structures were compatible with the structures of the cationic intermediates. The formal addition of hydride to the various dications, required to rationalise minor reaction products, is discussed in terms of more likely hydrogen radical or proton transfer in cationic reactions. Extensive E/Z isomerisation was observed for all quenching products. The potential use of carotenoid cations for the synthesis of 4,(4′)-substituted β,β-carotenes and 7-oxabicyclo[2,2,1]heptane derivatives is discussed.