Huckel and Mobius aromaticity and trimerous transition state behaviour in the pericyclic reactions of [10], [14], [16] and [18]annulenes

The pericyclic reactions of a series of [n]annulenes (n=10, 14, 16, 18) reveal unusual differences in their behaviour related to their aromaticity. For n=10, 14, synchronous pericyclic bond formation (trimerous behaviour) is possible, whereas the synchronicity is not apparent for n=18. The formally anti-aromatic system n=16 exhibits valence bond isomerism, asynchronous pericyclisation and a novel conformation which appears to be an example of a Heilbronner Mobius aromatic, as indicated by nucleus independent chemical shift calculations.