10.1021/jo00157a026
The research investigates the thermal degradation of β,β-carotene to understand the mechanisms by which carotenoids are converted to aromatic products, with the hypothesis that carotenoid natural products may be a source of the aromatic fraction of petroleum. Four polyene intermediates were isolated and identified during this process: 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-dimethyl-dodeca-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 these polyene intermediates, with 'H NMR establishing the type and number of methyl substituents and mass spectra of the saturated analogues confirming the positions of the in-chain methyl substituents. The study suggests that β,β-carotene thermally degrades by a series of symmetry-allowed electrocyclic processes followed by a thermal elimination, but also indicates that disproportionation reactions occur, as evidenced by the presence of 1,1,3-trimethylcyclohexane and long-chain aromatics. Key chemicals involved in the research include β,β-carotene, the polyene intermediates, diethyl (cyanomethyl)phosphonate, sodium hydride, diisobutylaluminum hydride, methanolic HCl, triphenylphosphine, and retinyltriphenylphosphonium chloride, among others used in the synthesis and identification processes.
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