New rearrangements involving dienone–chromen equilibration, in the pyridine-catalysed conversion of chromens into citrans
Abstract
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.