Design and characterisation of novel hexadentate 3-hydroxypyridin-4-one ligands

Article abstract of DOI:10.1016/j.tetlet.2004.12.115

Two novel hexadentate 3-hydroxypyridin-4-one ligands have been designed and synthesised. The physico-chemical properties of one of the hexadentate ligands have been determined and the results indicate that the hexadentate ligand possesses high affinity for iron(III).

Full text of DOI:10.1016/j.tetlet.2004.12.115

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 1333–1336
Design and characterisation of novel hexadentate
3-hydroxypyridin-4-one ligands
Sirivipa Piyamongkol, Tao Zhou, Zu D. Liu, Hicham H. Khodr and Robert C. Hider*
Department of Pharmacy, KingÕs College London, Franklin-Wilkins Building, 150Stamford Street, London SE1 9NN, UK
Received 10 November 2004; revised 17 December 2004; accepted 21 December 2004
Available online 12 January 2005
Abstract—Two novel hexadentate 3-hydroxypyridin-4-one ligands have been designed and synthesised. The physico-chemical prop-
erties of one of the hexadentate ligands have been determined and the results indicate that the hexadentate ligand possesses high
affinity for iron(III).
Ó 2005 Elsevier Ltd. All rights reserved.
Currently there is considerable interest in the design of
therapeutically useful iron ligands,1 and naturally occur-
ring siderophores provide excellent models for such mole-
cules. Enterobactin (1), a tricatecholate hexadentate
ligand, possesses an extremely high stability constant
(logK1 = 49)2 and therefore very high affinity for
iron(III) at physiological pH values.3 However, the
effectiveness of this molecule to scavenge iron at low
pH values is limited by its weak acidity and the required
loss of six protons on binding iron(III).4 Furthermore,
catechol-based siderophores are able to bind to sidero-
phore receptors and thereby donate iron(III) to bacteria,
resulting in undesirable side effects.5 Many research
groups have focused on siderophores and their ana-
logues as promising candidates for pharmaceutical
applications.6 Several have focused on the synthesis of
enterobactin analogues by using hydroxypyridinone
subunits (2 and 3) in place of catechol.7 Hydroxypyrid-
inones are stronger acids than catechol and since they
are monoprotic acids, hexadentate ligands formed from
three of these units only need to lose three protons to
form iron complexes.4 Among the three classes of
hydroxypyridinone, namely 1-hydroxypyridin-2-one, 3-
hydroxypyridin-2-one and 3-hydroxypyridin-4-one, the
3-hydroxypyridin-4-one class possesses the highest affin-
ity for iron(III). This can be attributed to the extensive
delocalisation of the lone pair of electrons of the ring
nitrogen. Although there are a number of reports on
the synthesis of hexadentate ligands based on hydroxy-
pyridinone chelating units, none have utilised 3-
hydroxypyridin-4-one units.
OH
OH
HN
O
O
O
O
O
O
OH O
HO
O
N
H
NH
O
HO
OH
NHX'
1
NHX
N
X'HN
XHN
NHX
OH
NHX'
2
3
O
OH
or
or
N
O
X, X' =
N
OH
O
O
O
N
CH3
O
In order for the ligand to adopt the correct geometry for
iron(III) binding, it is essential that the backbone be
connected to the ring at the ortho position relative to
Keywords: 3-Hydroxypyridin-4-one; Hexadentate; Iron chelator.
*
Corresponding author. Tel.: +44 207 848 4882; fax: +44 207 848
4800; e-mail: robert.hider@kcl.ac.uk
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.12.115

1334
S. Piyamongkol et al. / Tetrahedron Letters 46 (2005) 1333–1336
the oxygen anion.8 Therefore an amide linkage was
introduced at the 2-position of the ring. The advantage
of having an amide group adjacent to the 3-hydroxyl
group is that it can form an intramolecular hydrogen
bond, which results in the stability of the iron complex
at neutral pH values.8 In this letter we present a novel
synthetic route to build the 3-hydroxypyridin-4-one with
a carboxylic acid function at the 2-position of the ring
(Scheme 1). The pyridone oxygen of 4 was protected9
before it was allowed to further oxidise to form 6.10
The N-oxide group of 6 was subjected to acetylation
where intramolecular rearrangement led to the forma-
tion of an acetylated alcohol on the 2-methyl group.11
Subsequent saponification with base gave 7, which upon
oxidation yielded the carboxylic acid 8. Activation of 8
to an active ester 9 was achieved in the presence of N-
hydroxysuccinimide (NHS) and DCCI. Ligands 1012
and 1113 were synthesised by the coupling of the active
ester 9 with primary amines including tris(2-amino-
ethyl)amine (TREN) and 1,3,5-tris(aminomethyl)benz-
ene (TRAM) backbones followed by the removal of
the benzyl protecting groups (Scheme 2). Studies have
demonstrated that when there is no N-alkyl substitution
in the ring, the amide NH and the 3-hydroxyl group can
form a coplanar intramolecular hydrogen bond (Fig.
1a). However, such a bond is not so well favoured in
the presence of N-alkyl substitution, as appreciable ste-
ric repulsion exists between the 1-alkyl group and the
amide oxygen atom (Fig. 1b).
The pKa values of ligand 11 were investigated by spec-
trophotometric titration.14 Ligand 11 can be considered
as a trimer of the bidentate ligand 12 and therefore pos-
sesses two sets of intrinsic site pKa values, namely 0.66,
1.88, 3.60 and 6.58, 7.65, 8.10, which are similar to the
corresponding two pKa values of 12, namely 1.99,
6.32. The tertiary amine function has a pKa value of
4.52.
The stability constant (logK1) of 11 was determined
spectrophotometrically by competition with the well
characterised hexadentate ligand N,N0-di(2-hydroxybenz-
yl)ethylenediamine-N,N0-diacetic acid (HBED), at
pH 4.27. The UV/visible spectra of 11, in the presence
of iron(III) and HBED are shown in Figure 2 where
the kmax of the spectra were found to shift appreciably
when 11 was added to the system. The absolute stability
constant of 11 was determined to be 30.7 0.6.
The speciation plot of compound 11–iron(III)
complexes (Fig. 3) demonstrated that the 1:1 ligand–
iron(III) complex is the dominant species over the
O
OBn
OBn
OBn
OBn
OBn
CH3
DEAD, BnOH, TPP
CH3
THF, reflux, 79%
1) (Ac)2O, reflux
MCPBA, CH2Cl2
rt, 77%
N
H
N
O
CH3
N
2) 2M NaOH, reflux
81% in two steps
4
5
6
OBn
OBn
N
OBn
1) DMSO, Py-SO3,
Et3N, rt, 62%
O
O
OBn
OBn
OBn
OH
NHS, DCCI
DMF, rt, 67%
OH
O N
2) NaCl2, H2NSO3,
acetone:H2O (1:1),
rt, 77%
N
N
O
O
7
8
9
Scheme 1. Synthetic route leading to the construction of 3-hydroxypyridin-4-one ligands.
HO
HO
Cl
H
Cl
H
N
N
O
O
OH
HN
NH OH
1) Et3N, TRAM,
DMF, rt, 1 d,
64%
OH
OBn
N
H
O
1) TREN, DMF,
rt, 2 d, 83%
OH
O
O
OBn
Cl
N
N
H
Cl
H
N
H
N
O N
N
H
Cl
2) BCl3, CH2Cl2,
0oC to rt, 3 h,
89%
NH OH
2) BCl3, CH2Cl2,
0oC to rt, 3 h,
85%
OH
O
HN
HO
O
N
O
OH
OH
O
H
N
H
9
Cl
Cl
HO
11
10
1) NH2CH2CH2OH, 72%
2) H2, Pd/C, HCl, 87%
OH
OH
H
N
N
H
OH
Cl
O
12
Scheme 2. Coupling of the active ester with various primary amines resulted in both bi- and hexadentate ligands.

S. Piyamongkol et al. / Tetrahedron Letters 46 (2005) 1333–1336
1335
Figure 1. Energy minimised conformer of the 2-amido-3-hydroxypyridin-4-ones: (a) without N-alkyl substitution, (b) with N-methyl substitution.
1.0
0.8
0.6
0.4
0.2
0.0
than the stability constant, since it takes into account
the effect of ligand basicity, denticity, degree of proton-
ation and difference in metal–ligand complexes.16 The
application of pFe3+ values permits the comparison of
different ligands under specified conditions. Since the
formation constant of a hexadentate ligand has only first
order dependence on the free ligand concentration,
whilst that of bidentate ligands has third order depen-
dence, the pFe3+ values of hexadentate ligands are
proportionally increased when compared to those of
bidentate ligands. The pFe3+ value of 11, obtained by
using measured stability constant and pKa values, is
30.5 at pH 7.45. Thus, even though the stability constant
of 11 is lower than its bidentate counterpart 12
(logb3 = 31.4), when comparing the pFe3+ value the situ-
ation is reversed to 8 log units higher than that of 12
(pFe3+ value at pH 7.45 = 21.97). In fact, 11 gives the
highest ever reported pFe3+ value of a hydroxypyridi-
none hexadentate ligand. The physico-chemical proper-
ties of ligand 10 was not investigated because of its poor
solubility in water.
1:1
1:0
350
400
450
500
550
600
650
Wavelength nm
Figure 2. Visible spectra of an 11–iron(III) complex solution in com-
petition with HBED at pH 4.27. [Fe3+
]total = 0.2 mM, [HBED]total =
0.4 mM, the concentration ratio of Fe3+ to 11 was varied from 1:0 to
1:1.
1.0
FeL
In summary, two novel hexadentate 3-hydroxypyridin-
4-one ligands have been synthesised by conjugating
three protected bidentate carboxyl units on a backbone
amine. The hexadentate ligand 11 possesses a high affin-
ity for iron(III). The potential of 11 as a therapeutic iron
chelator is currently under investigation.
Fe3+ = 30.53
Fe
P
0.8
0.6
0.4
0.2
0.0
Fe(OH)4
Acknowledgements
The authors would like to thank Apotex Research Inc.
Canada and Biomed EC grant BMH4-CT97-2149 for
supporting this research project. S. Piyamongkol would
like to thank The Royal Thai Government and The Fac-
ulty of Pharmacy, Chiang Mai University, Chiang Mai,
Thailand for a studentship.
0
2
4
6
pH
8
10
12
Figure 3. Speciation plot of 11 in the presence of iron(III). pFe3+
values were determined by calculating the equilibrium concentration of
free hexaaquoiron(III) in a solution of pH 7.45 containing 10ꢀ6 M
iron(III) and 10ꢀ5 M ligand.
References and notes
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negative logarithm of concentration of the free iron(III)
in solution, is a more suitable factor for comparison
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