74 Pyryliums, Benzopyryliums, Pyrones and Benzopyrones
ammonia or amines – clearly, in these transformations the ammonia/amine must attack initially at C-2, not at the
carbonyl carbon.
The reactivity of 2-pyrones as dienes in Diels–Alder reactions is notable and is a measure of the diene, (rather than
aromatic) character of 2-pyrones. There is a tendency for the cycloadducts to lose carbon dioxide in a cycloreversion
on stronger heating. The elimination of carbon dioxide can be discouraged by operating at a lower temperature and
using very high pressure, or by using a catalyst.
The pericyclic reactivity of 3- and 5-bromo- and 3,5-dibromo-2-pyrones is special since they act as effi cient dienes
towards both electron-poor dienophiles (normal electron demand) and electron-rich dienophiles (inverse electron
demand). 3,5-Dibromo-2-pyrone undergoes palladium(0)-catalysed cross-coupling selectively at C-3 and the resulting
C-3-substituted 5-bromo-2-pyrones can also be used with both types of dienophile, as the scheme illustrates (only the
majorendo isomer shown in each case).
Ring synthesis of pyryliums from 1,5-diketones (1,2-bond made)
The synthesis of pyrylium cations centres on the use of a 1,5-dicarbonyl compound (or its synthesis in situ). As an
example, mono-enolisation of heptane-2,6-dione, intramolecular hemi-acetal formation, loss of water and fi nally an
oxidation produces the six-membered aromatic salt. Various oxidants can be used – in the example shown, the species