ABSTRACT
The pyrylium cation presents an intriguing dichotomy – it is both ‘aromatic’, and therefore, the beginning student would be tempted to understand, ‘stable’, yet it is very reactive – the tropylium cation and the cyclopentadienyl anion can also be described in this way. Pyrylium ions readily add nucleophilic reagents, at an a-position, generating 2H-pyrans which then often ring open. Virtually all the known reactions of pyrylium salts fall into this general category. Turning to benzopyrylium systems, one finds exactly comparable behaviour – a readiness to add nucleophiles, adjacent to the positively charged oxygen, in the heterocyclic ring. Pyrylium is more reactive in such nucleophilic additions than pyridinium – oxygen tolerates a positive charge less well than nitrogen. It is worth pointing out again the analogy with carbonyl chemistry – the nucleophilic additions which characterise pyrylium systems are nothing more nor less than those which occur frequently in acid-catalysed chemistry of carbonyl groups.
