Isolation and Identification of the Polyenes Formed During the Thermal Degradation of β,β-Carotene

Article abstract of DOI:10.1021/jo00157a026

It has been proposed that carotenoid natural products are the source of part of the aromatic fraction of petroleum.In order to understand the mechanisms by which carotenoids are converted to aromatic products, an investigation of the polyene intermediates formed in the thermal degradation of β,β-carotene was initiated.As a result of this investigation, four polyene intermediates have been isolated and identified: 1,12-bis(2,6,6-trimethylcyclohex-1-enyl)-3,6,10-trimethyldodeca-1,3,5,7,9,11-hexaene, 1,12-bis(2,6,6-trimethylcyclohex-1-enyl)-3,7-dimethyldodeca-1,3,5,7,9, 11-hexaene, 1,6-bis(2,6,6-trimethylcyclohex-1-enyl)-3-methylhexa-1,3,5-triene, and 1,6-bis(2,6,6-trimethylcyclohex-1-enyl)hexa-1,3,5-triene.Independent syntheses confirmed the structures of the polyene intermediates. 1H NMR established the type and number of methyl substituents.Mass spectra of the saturated analogues confirmed the positions of the in-chain methyl substituents.The structures of the polyene intermediates are consistent with proposals that β,β-carotene thermally degrades by a series of symmetry-allowed electrocyclic processes followed by a thermal elimination.However, not all of the degradation products arise from electrocyclic-type processes.The presence of 1,1,3-trimethylcyclohexane and long chain aromatics indicates that disproportionation reactions are occurring in the complex degradation reaction.