Conservation of the chair-like transition state by base-catalysed epimerization of meso-tetramethyl hexa-1,5-diene-2,3,4,5-tetracarboxylate prior to its Cope rearrangement
Abstract
meso-Tetramethyl hexa-1,5-diene-2,3,4,5-tetracarboxylate (5) rearranges at 20–50 °C in the presence of sodium carbonate to (E,E)-tetramethyl hexa-1,5-diene-1,2,5,6-tetracarboxylate (7), by base-catalysed epimerization followed by Cope rearrangement of the resulting (±)-tetraester (6)via the expected four-centred, chair-like transition state. In the absence of base the meso-tetraester (5) undergoes [3,3] sigmatropic rearrangement to produce (E,Z)-tetramethyl hexa-1,5-diene-1,2,5,6-tetracarboxylate (8) exclusively.