Efficient synthesis of 5-amido-3-hydroxy-4-pyrones as inhibitors of matrix metalloproteinases

Article abstract of DOI:10.1021/ol0707665

3-Hydroxy-4-pyrones are a class of important metal chelators with versatile medicinal applications. An efficient pathway for the preparation of new 5-amido-3-hydroxy-4-pyrone derivatives has been developed. The synthesized 5-amido-3-hydroxy-4-pyrones have

Full text of DOI:10.1021/ol0707665

ORGANIC
LETTERS
2007
Vol. 9, No. 13
2517-2520
Efficient Synthesis of
5-Amido-3-hydroxy-4-pyrones as
Inhibitors of Matrix Metalloproteinases
Yi-Long Yan and Seth M. Cohen*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093-0358
scohen@ucsd.edu
Received March 29, 2007
ABSTRACT
3-Hydroxy-4-pyrones are a class of important metal chelators with versatile medicinal applications. An efficient pathway for the preparation
of new 5-amido-3-hydroxy-4-pyrone derivatives has been developed. The synthesized 5-amido-3-hydroxy-4-pyrones have been evaluated as
inhibitors of matrix metalloproteinases.
3-Hydroxy-4-pyrones (also referred to as hydroxypyrones
hereafter) are an important class of biologically active
compounds. They can be readily converted to analogues such
as hydroxythiopyrones and hydroxypyridinones (3,4-HOPOs)
(Figure 1). As charge delocalization is possible within the
Al(III),8-11 Ga(III),3,7,9,12,13 In(III),3,12,13 Zn(II),14-17 Cu(II),15
Ni(II),15,18 Pb(II),19 Ru(II),20 and [VO]2+.17,21-25 The tight
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Figure 1. Examples of hydroxypyrone, hydroxythiopyrone, and
hydroxypyridinone chelators.
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2677.
heterocyclic ring, the exocyclic keto group, and the ortho
oxyanion derived from the hydroxyl group can efficiently
bind to a variety of di- and trivalent metals by forming a
five-membered chelate ring.1 Many reports demonstrate that
hydroxypyrones and their thiopyrone and pyridinone ana-
logues are efficient chelators for metal ions such as Fe(III),1-7
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Bioorg. Med. Chem. Lett. 2007, 17, 45-48.
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Hendrickson, D. N.; Cohen, S. M. Dalton Trans. 2005, 2588-2596.
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545-556.
10.1021/ol0707665 CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/24/2007

metal binding affinity of these hydroxypyrones and their
analogues, as well as the high bioavailability and favorable
toxicity profile suggested by maltol (3-hydroxy-2-methyl-
4-pyrone)1 and kojic acid (6-hydroxymethyl-3-hydroxy-4-
pyrone),26 has led to their exploration in medicinal inorganic
chemistry. Potential medicinal applications reported in the
literature include iron imbalance in anemia and iron overload
disorder,4-6 aluminum removal in Alzheimer’s disease,8-11
treatment of diabetes,21,23-25 contrast agents for medical
imaging,27 and regulation of metalloenzyme activity.28-30
Previous hydroxypyrone derivatives related to such inves-
tigations were synthesized by structural modification of
commercially available maltol and kojic acid (Figure 1),1,26
which are natural products and can be manufactured by
biosynthetic methods from glucose. Because of the versatile
coordination chemistry and potential chemotherapeutic ap-
plication of hydroxypyrones and their thiopyrone and pyri-
dinone congeners, development of new synthetic strategies
to access diverse hydroxypyrone derivatives is highly desir-
able.
pyrone-based chelators can be achieved by introduction of
a peptidomimetic backbone on the pyrone ring, which leads
to enhanced interactions between the MMP subsites and the
inhibitor.30 However, structural modification of maltol and
kojic acid only provides access to a group of 2- and 6-amido-
substituted hydroxypyrones (Scheme 1). No routes to ma-
Scheme 1. Synthetic Path for Pyrone-Based Inhibitors from
Maltol and Kojic Acid
Our laboratory has a particular interest in the development
of hydroxypyrone-based matrix metalloproteinase (MMP)
inhibitors.29,30 MMPs are a class of hydrolytic zinc-dependent
enzymes that catalyzes peptide bond hydrolysis. They are
involved in tissue remodeling, wound healing, and growth.31
Misregulated activity of these enzymes is also implicated in
a variety of diseases such as cancer, arthritis, atherosclerosis,
and heart diseases.32-34 Thus, development of inhibitors for
regulation of MMP activity has great therapeutic value.32-34
We have found that maltol and thiomaltol are more effective
chelating inhibitors for MMP-3 (stromelysin) than the widely
reported hydroxamate ligands.29,35 Furthermore, significant
improvement of inhibition potency and selectivity of such
nipulating maltol or kojic acid in the remaining 5-position
have been reported in the literature.1,5,30,36 As diverse
substitution patterns on the pyrone ring may modulate the
potency, selectivity, binding mode, and biocompatibility of
the inhibitors, our interest in developing pyrone-based MMP
inhibitors prompted us to synthesize 5-amidohydroxypyrones
to access new potential MMP inhibitors. Analysis of the
impact of substituent patterns on inhibitor activity should
provide structure-activity information and guidance for
further structural optimization of our inhibitor design. Herein,
we report the synthesis of 5-amido-3-hydroxy-4-pyrones and
their activity as MMP inhibitors.
The preparation of 5-amido-3-hydroxy-4-pyrones started
with commercially available 3-bromopyruvic acid 1 (Scheme
2).37 Reaction of 1 with triethyl orthoformate in the presence
(20) Kennedy, D. C.; Wu, A.; Patrick, B. O.; James, B. R. Inorg. Chem.
2005, 44, 6529-6535.
(21) McNeill, J. H.; Yuen, V. G.; Hoveyda, H. R.; Orvig, C. J. Med.
Chem. 1992, 35, 1489-1491.
(22) Rangel, M.; Leite, A.; Amorim, M. J.; Garribba, E.; Micera, G.;
Lodyga-Chruscinska, E. Inorg. Chem. 2006, 45, 8086-8097.
(23) Saatchi, K.; Thompson, K. H.; Patrick, B. O.; Pink, M.; Yuen, V.
G. Inorg. Chem. 2005, 44, 2689-2697.
(24) Song, B.; Saatchi, K.; Rawji, G. H.; Orvig, C. Inorg. Chim. Acta
2002, 339, 393-399.
Scheme 2. Synthesis of 5-Substituted
3,3-Diethoxypyran-4-ones
(25) Thompson, K. H.; Liboiron, B. D.; Sun, Y.; Bellman, K. D. D.;
Setyawati, I. A.; Patrick, B. O.; Karunaratne, V.; Rawji, G.; Wheeler, J.;
Sutton, K.; Bhanot, S.; Cassidy, C.; McNeill, J. H.; Yuen, V. G.; Orvig, C.
J. Biol. Inorg. Chem. 2003, 8, 66-74.
(26) Bentley, R. Nat. Prod. Rep. 2006, 23, 1046-1062.
(27) Puerta, D. T.; Botta, M.; Jocher, C. J.; Werner, E. J.; Avedano, S.;
Raymond, K. N.; Cohen, S. M. J. Am. Chem. Soc. 2006, 128, 2222-2223.
(28) Lewis, J. A.; Mongan, J.; McCammon, J. A.; Cohen, S. M.
ChemMedChem 2006, 1, 694-697.
(29) Puerta, D. T.; Lewis, J. A.; Cohen, S. M. J. Am. Chem. Soc. 2004,
126, 8388-8389.
(30) Puerta, D. T.; Mongan, J.; Tran, B. L.; McCammon, J. A.; Cohen,
S. M. J. Am. Chem. Soc. 2005, 127, 14148-14149.
(31) Page-McCaw, A.; Ewald, A. J.; Werb, Z. Nat. ReV. Mol. Cell Biol.
2007, 8, 221-233.
of concentrated H2SO4 as a catalyst provided 3-bromo-2,2-
diethoxypropionic acid 2 in 85-90% yield. The carboxylic
acid intermediate 2 was then transformed to the activated
(32) Puerta, D. T.; Cohen, S. M. Curr. Top. Med. Chem. 2004, 4, 1551-
1573.
(33) Skiles, J. W.; Gonnella, N. C.; Jeng, A. Y. Curr. Med. Chem. 2004,
11, 2911-2977.
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ReV. 1999, 99, 2735-2776.
(36) Puerta, D. T. Ph.D. Thesis: A Bioinorganic Approach to Matrix
Metalloproteinse Inhibition. University of California, San Diego, 2006.
(37) LaMattina, J. L.; Mularski, C. J. Tetrahedron Lett. 1983, 24, 2059-
2062.
(35) Puerta, D. T.; Griffin, M. O.; Lewis, J. A.; Romero-Perez, D.; Garcia,
R.; Villarreal, F. J.; Cohen, S. M. J. Biol. Inorg. Chem. 2006, 11, 131-
138.
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Org. Lett., Vol. 9, No. 13, 2007

ester 4-nitrophenyl 3-bromo 2,2-diethoxypropionate 3 in 60-
65% yield by treatment with 4-nitrophenyl trifluoroacetate
in the presence of pyridine. 5-Substituted 3,3-diethoxypyran-
4-ones 4a and 4b were obtained by refluxing intermediate 3
with a â-keto ester anion in NaH/THF. The yields for this
cyclization were in the range of 64-76%. Two 3,3-
diethoxypyran-4-ones 4a (R ) Et) and 4b (R ) But) were
prepared as they provide different choices for the subsequent
hydrolysis conditions.
Our interest in the development of 5-amido-3-hydroxy-
4-pyrone MMP inhibitors encouraged us to investigate
cyclization reactions of activated bromo ester 3 with several
â-keto amides 6a-c in an attempt to obtain the correspond-
ing 5-amido 3,3-diethoxypyran-4-ones 7a-c (Scheme 5).
Scheme 5. Cyclization Reactions with â-Keto Amides
The mechanism for the formation of 3,3-diethoxypyran-
4-ones 4 is rationalized in Scheme 3. Ketalization of the
Scheme 3. Formation of 3,3-Diethoxypyran-4-one
Interestingly, the cyclization reaction did not proceed well
compared to that of the â-keto ester substrates (Scheme 2).
It was observed that the amide substrates 6a-c formed
insoluble species under NaH/THF conditions. While reaction
of N-methyl â-keto amide 6a provided the expected
5-amidodihydropyrone 7a in low yield (24%), the reactions
of 3 with â-keto amide 6b and 6c only resulted in the
recovery of starting materials and unidentified compounds
without the expected products 7b and 7c.
2-keto group by two ethoxyl groups results in decreased
nucleophilicity and a steric increase on the 3-bromo carbon
of intermediate 3. Thus, the intermolecular nucleophilic
reaction between compound 3 and the â-keto ester anion first
takes place at the activated ester carbonyl group forming
bromo intermediate I. In the presence of excess base, the
R-carbon on intermediate I is further deprotonated leading
to the formation of intermediate II. An intramolecular
nucleophilic reaction at the bromo carbon is favorable under
refluxing conditions in THF via a six-membered intermediate
III which results in the formation of 3,3-diethoxypyran-4-
one 4.38
Further investigations generated an improved approach for
the synthesis of 5-amido-3-hydroxy-4-pyrones as shown in
Scheme 6. Hydrolysis of the ester group of 4 provided
Scheme 6. Synthesis of Compounds 9a,b
The deprotection of the ketal group in 4 is straightforward
by treatment with formic acid or trifluoroacetic acid (TFA)
in the presence of moisture (Scheme 4). For example,
Scheme 4. Deprotection of 3,3-Diethoxypyran-4-one
carboxylic acid 8 as a versatile intermediate for the prepara-
tion of an amide. The yields for the hydrolysis of ethyl ester
4a under NaOH/H2O conditions varied substantially in the
range of 30-60%. This outcome may have been due to the
sensitivity of the ketal group and the R,â-unsaturated moiety
of 4a under basic reaction conditions and acidic aqueous
workup. In contrast, deprotection of tert-butyl ester 4b by
TFA was very efficient and selective providing carboxylic
acid 8 in 95% yield. Note that the ketal protective group on
the pyrone ring can be deprotected by TFA in refluxing THF/
H2O, but the deprotection of the tert-butyl ester by TFA in
CH2Cl2 at room temperature was selective, keeping the ketal
group intact. The amide backbone is introduced by coupling
the carboxylic acid intermediate 8 with the corresponding
amine, forming amide 7 in 63-82% yields. 5-Amido-3-
refluxing of 4a in THF with formic acid provided hydroxy-
pyrone ester 5 in 67% yield. The 5-ester and the 6-methyl
groups of compound 5 provide synthetic handles for further
functional extension at these positions. When compared with
maltol and kojic acid (Scheme 1), 5 should be a versatile
synthon for the preparation of pyrone derivatives with
unprecedented substitution patterns.
(38) Sato, K.; Inoue, S.; Ohashi, M. Bull. Chem. Soc. Jpn. 1973, 46,
1288-1290.
Org. Lett., Vol. 9, No. 13, 2007
2519

hydroxy-4-pyrone 9 was obtained in 68-82% yield upon
deprotection of the ketal group by refluxing compound 7 in
THF in the presence of formic acid or TFA. A thiopyrone
analogue 10 was also prepared by thionation of the corre-
sponding pyrone 9b with P4S10 in refluxing THF in the
presence of HMDO (Scheme 7).39 Several modified condi-
Compared with the activity of the previously reported
regioisomer AM-2,30 compound 9b was several orders of
magnitude less potent as an MMP inhibitor. This outcome
clearly shows that the substitution pattern on the pyrone ring
dramatically affects the activity of hydroxypyrone inhibitors.
This suggests that the zinc-binding group and not the
hydrophobic “backbone” substituent are dictating the orienta-
tion of inhibitor binding in the MMP active site. This
hypothesis is further supported by the lack of selectivity
observed for compound 9b. AM-2 strongly inhibits MMP-2
and MMP-3, but not MMP-1; this is likely due to the large
biphenyl substituent that can be accommodated in the deep
S1′ pockets of MMP-2 and MMP-3, but not in the shallow
S1′ pocket of MMP-1.30 In contrast, 9b shows no notable
selectivity between these three enzymes, indicative of the
biphenyl group no longer being directed toward the S1′
pocket. Again, this indicates that the zinc-binding group, and
not the biphenyl substituent, is the dominant element
dictating the mode of binding. Last, the significant improve-
ment in inhibition potency of compound 10 over 9b by
simply changing the chelator from an O,O to an O,S ligand
is consistent with our earlier studies29 and demonstrates the
substantial impact of the zinc-binding group in inhibitor
activity.
In summary, the first pathway for the synthesis of 5-amido-
3-hydroxy-4-pyrones has been developed. These pyrone
derivatives provide new opportunities to explore the metal-
lopharmaceutical applications of these chelators. We are
currently exploring the coordination chemistry of these
ligands for a variety of bioinorganic applications. These
investigations, in combination with the MMP inhibition data
from pyrones 9b and 10, will provide helpful guidelines for
future structural optimization of pyrone-based MMP inhibi-
tors.
Scheme 7. Synthesis of Inhibitor 10
tions were pursued for this reaction, including the use of
different solvents (CH2Cl2, benzene, dioxane) with or without
HMDO, and alternative reagents (Lawesson’s reagent);
however, none of these adjustments improved the reaction
yield. The low yield (16%) of this transformation may be
due to a steric effect at the ketone position caused by the
neighboring 3-hydroxy and 5-amido groups.18
The inhibitory activity of biphenyl amides 9b and 10
against MMP-1 (collagenase), MMP-2 (gelatinase A), MMP-3
(stromelysin), and MMP-9 (gelatinase B) was examined
using an established fluorescence-based assay (Table 1).40
Table 1. Inhibitory Activity (%) of Compounds 9b and 10
compd
MMP-1
MMP-2
MMP-3
MMP-9
9ba
10b
21
52
34
94c
34
45
33
54
a Percent inhibition at 100 µM. b Percent inhibition at 50 µM. c IC50
23 ( 2 µM.
)
Acknowledgment. Y.-L.Y. thanks Arpita Agrawal (UCSD)
and Dr. Jana A. Lewis (UCSD) for helpful discussions on
the MMP activity assays. This material is based upon work
supported in part by the NIH (R01 HL080049-01) and the
American Heart Association (0430009N). S.M.C. is a Cottrell
Scholar of the Research Corporation.
It was found that both compounds showed relatively poor
inhibition against the MMPs evaluated. For hydroxypyrone
9b at 100 µM concentration, MMP activity was inhibited
21-34%. The hydroxythiopyrone 10 showed more potent
inhibition than 9b. The percent activity of MMP-1, MMP-
3, and MMP-9 was inhibited by 52%, 45%, and 54%,
respectively, in the presence of 50 µM of 10. Furthermore,
MMP-2 activity was inhibited 94% by compound 10 at a
concentration of 50 µM.
Supporting Information Available: Synthetic proce-
dures, characterization, 1H NMR and 13C NMR spectra, and
high-resolution mass spectra (HRMS) of all new compounds;
MMP assay experimental details. This material is available
free of charge via the Internet at http://pubs.acs.org.
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