Pyryliums, Benzopyryliums, Pyrones and Benzopyrones 73Oxygen substituents – pyrones and benzopyrones
The deprotonation of a hydroxyl group at a pyrylium 3-position generates pyrylium-3-olates; these take part in
cycloadditions as 1,3-dipoles, across the 2- and 6-positions (cf. pyridinium-3-olates, page 40 and pyrazinium-3-olates,
page 55). Most often, such zwitterions are generated by elimination of acetic acid from a 6-acetoxy-2H-pyran-3(6H)-
one, in the presence of an external dipolarophile or, in the case illustrated, having a tethered double bond to serve as
an intramolecular dipolarophile.
TheO-deprotonated forms of - and -hydroxypyryliums are neutral pyrones and their properties are like those of
pyridones (pages 35, 40 and 41), their benzanellated analogues, chromones, coumarins and isocoumarins being analo-
gous to quinolones and isoquinolones (pages 63 and 67). It follows that the pyrones and benzopyrones protonate,
reluctantly, on the carbonyl oxygen giving hydroxypyrylium or hydroxybenzopyrylium salts. Highly reactive alkylating
agents, such as Meerwein salts, will alkylate the carbonyl oxygen, giving alkoxy-pyryliums.
There are only a few examples of electrophilic substitution of pyrones, at C-3/C-5, nitration and bromination being
two. Chromones and coumarins are attacked by electrophiles at C-3 in neutral solution.
Much of the remainder of the chemistry of the pyrones and benzopyrones can be summarised by saying that they
behave like unsaturated ketones and lactones. 2-Pyrones can in principle add nucleophilic reactants either at C-2
(carbonyl carbon), C-4, or C-6: their reactions with ammonia/amines are examples, 2-pyrones are converted into
2-pyridones, probably via initial attack at C-2. 4-Pyrones are similarly converted into 4-pyridones on reaction with