Evidence for the non-concerted [4+2]-cycloaddition of N-aryl imines when acting as both dienophiles and dienes under Lewis acid-catalysed conditions

The ytterbium(III) triflate-catalysed reaction of a para-methoxyaniline, ethyl glyoxalate-derived imine, with a series of different dienes has resulted in products which initially suggest the operation of two possible modes of aza-Diels–Alder reaction, however, a more likely explanation is that a common reaction mechanism is operating, involving a stepwise Lewis acid-catalysed process and a common type of zwitterionic ytterbium complex.