Physico-chemical and synthetic studies on carotenoids
Abstract A study of the C30 analogue of β,β-carotene at 360 MHz 1H-n.m.r. and 90.5 MHz 13C-n.m.r. confirms all of the signal assignments. Staphylloxanthin is identified as a 6- O -ester of α-D-glucopyranosyl 8′-apo-Φ-carotenoate. Introduction of an extra methyl group at C-ll (and C-ll′) is shown to give a ‘hindered’ 11-trans double bond which is still hindered as the ‘corresponding eis isomer. The structure of the natural carotenoid, prolycopene is shown to be 7,9,7′,9′-tetracis-ϕ,ϕ-carotene by n.m.r. spectroscopy. The position of the two ‘hindered’ eis double bonds is confirmed by synthesis. Two new approaches to the synthesis of astaxanthin have been studied. Preparation of the di-O-methyl ether of astacene is described but after reduction the product gives a complex mixture of compounds when treated with acid. Electrochemical reduction of astacene in the presence of acetic anhydride gives astaxanthin tetra-acetate which is hydrolysed to astaxanthin. The conformation of fucoxanthin in the solid state has been examined by X-ray crystallography.