A study of the boron trifluoride complexes of some carotenoid hydrocarbons and the products of their hydrolytic and alcoholytic cleavage

Abstract

Formation of a deeply-colored [alpha]-carotene - boron trifluoride complex, by means of boron trifluoride etherate, and subsequent hydrolysis of the complex result mainly in the formation of 4-hydroxy-[alpha]-carotene, indicating that, under the conditions applied, only the [beta]-ionone end of the complex is reactive; ethanolysis gave the corresponding ethoxy derivative. The structures of these two compounds and some of the other reaction products were clarified. On this basis the structure of the BF3-complex has also been established, and a mechanism for its hydrolysis and alcoholysis was postulated. The length of the resonating system within the proposed complex is consistent with the observed deep blue color. Lycopene and [gamma]-carotene were studied along similar lines. The observed formation of 5,6-dihydroxy-5,6-dihydrolycopene by hydrolysis of the lycopene complex can be explained by means of a structure and a hydrolysis mechanism both similar to those of the [alpha]-carotene complex. The formation of 4-hydroxy-[gamma]-carotene by hydrolysis of the [gamma]-carotene complex is in accordance with an analogous structure and hydrolysis mechanism. The [beta]-ionone end of the [gamma]-carotene complex appears to be more reactive than the [psi]-ionone end. The cis-trans isomerization of 4-hydroxy-, 4-keto-, and 4-ethoxy-[alpha]-carotene as well as 5,6-dihydroxy-5,6-dihydrolycopene was studied.