The chromens (2) and (3), at 110 °C in pyridine, are converted into citrans by a hitherto unexpected rearrangement involving thermal electrocyclic opeing, (E)/(Z) isomerisation and tautomerism of the intermediate dienones, and reclosure to the alternative chromens; citran formation then proceeds from either of the equilibrating chromens.
L. Crombie, D. A. Slack and D. A. Whiting, J. Chem. Soc., Chem. Commun., 1976, 139 DOI: 10.1039/C39760000139
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