Geometrical isomerization of carotenoids in dichloromethane

Abstract

In a 3 mmol dm^–3 solution of all-trans-canthaxanthin (I) in HPLC grade dichloromethane cis-isomers are formed. Optical absorptions due to cis-isomers in addition to those due to an intermediate are observed in 3 mmol dm^–3 solutions of all-trans-β-carotene(II) and 8′-apo-β-caroten-8′-al (III), as a result of 0.1–0.2 mmol dm^–3 acid normally found in HPLC grade dichloromethane. The cis-isomers were separated by HPLC and characterized by optical and NMR spectroscopic techniques. AM1 calculations performed on the ground state and protonated I showed that 9-cis and 13-cis-isomers will be formed by acid induced isomerization in greater yield than other cis and cis,cis-isomers. This was observed experimentally. If an excess amount of hydrochloric acid (∼ 1 mol dm^–3) is added to the solution of dichloromethane containing the carotenoids, radical cations of the carotenoids are formed.