418
HETEROCYCLES
N
NH 2
H
N
NH 2
H
N
NH 2
N
NH 2
H
H
pK a 9.1
pK a 6.8
N
NH 2
N
NH 2
2-aminopyridine
4-aminopyridine
This may be rationalized from a consideration of resonance in the conjugate acids. The conjugate acids
from ring protonation benefit from charge delocalization, which is greater in 4-aminopyridinium that
in 2-aminopyridinium. This type of delocalization is
not possible in 3-aminopyridinium; 3-aminopyridine
(pK a 6.0) is the weakest base of the three aminopyridines, and has basicity more comparable to that of
pyridine (pK a 5.2).
11.4.4 Pyrylium cation and pyrones
The pyrylium cation is isoelectronic with pyridine: it
has the same number of electrons and, therefore, we
also have aromaticity. Oxygen is normally divalent
and carries two lone pairs. If we insert oxygen into
the benzene ring structure, then it follows that, by
having one electron in a p orbital contributing to the
aromatic sextet, there is a lone pair in an sp
2 orbital,
O
H
H
H
H
H
pyrylium cation
O
and the remaining electron needs to be removed,
hence the pyrylium cation. However, oxygen tolerates
a positive charge less readily than nitrogen, and
aromatic stabilization is less than with pyridine.
Pyrones are oxygen analogues of pyridones, and
potentially aromatic. However, there is little evidence that the dipolar resonance forms of either
2-pyrone or 4-pyrone make any significant contribution. Their chemical behaviour suggests they should
be viewed more as conjugated lactones (2-pyrones)
or vinylogous lactones (4-pyrones) rather than aromatic systems, since many reactions lead to ring
opening.
O
O
2-pyrone
O
O
4-pyrone
O
O
O
O
Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most
prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins
contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly
benzopyran or benzopyrylium systems.
O
O
OH
HO
OH
OH
OH
O
OH
HO
OH
OH
OH
quercetin
cyanidin
4-pyrone ring
pyrylium cation
O
O
coumarin
2-pyrone ring
HETEROCYCLES
N
NH 2
H
N
NH 2
H
N
NH 2
N
NH 2
H
H
pK a 9.1
pK a 6.8
N
NH 2
N
NH 2
2-aminopyridine
4-aminopyridine
This may be rationalized from a consideration of resonance in the conjugate acids. The conjugate acids
from ring protonation benefit from charge delocalization, which is greater in 4-aminopyridinium that
in 2-aminopyridinium. This type of delocalization is
not possible in 3-aminopyridinium; 3-aminopyridine
(pK a 6.0) is the weakest base of the three aminopyridines, and has basicity more comparable to that of
pyridine (pK a 5.2).
11.4.4 Pyrylium cation and pyrones
The pyrylium cation is isoelectronic with pyridine: it
has the same number of electrons and, therefore, we
also have aromaticity. Oxygen is normally divalent
and carries two lone pairs. If we insert oxygen into
the benzene ring structure, then it follows that, by
having one electron in a p orbital contributing to the
aromatic sextet, there is a lone pair in an sp
2 orbital,
O
H
H
H
H
H
pyrylium cation
O
and the remaining electron needs to be removed,
hence the pyrylium cation. However, oxygen tolerates
a positive charge less readily than nitrogen, and
aromatic stabilization is less than with pyridine.
Pyrones are oxygen analogues of pyridones, and
potentially aromatic. However, there is little evidence that the dipolar resonance forms of either
2-pyrone or 4-pyrone make any significant contribution. Their chemical behaviour suggests they should
be viewed more as conjugated lactones (2-pyrones)
or vinylogous lactones (4-pyrones) rather than aromatic systems, since many reactions lead to ring
opening.
O
O
2-pyrone
O
O
4-pyrone
O
O
O
O
Flavonoids are natural phenolic systems containing pyrylium and pyrone rings, and provide the most
prominent examples. We have met some of these systems under antioxidants (see Box 9.2). Coumarins
contain a 2-pyrone system. Note that all of these compounds are fused to a benzene ring and are strictly
benzopyran or benzopyrylium systems.
O
O
OH
HO
OH
OH
OH
O
OH
HO
OH
OH
OH
quercetin
cyanidin
4-pyrone ring
pyrylium cation
O
O
coumarin
2-pyrone ring