An amidation/cyclization approach to the synthesis of N-hydroxyquinolinones and their biological evaluation as potential anti-plasmodial, anti-bacterial, and iron(II)-chelating agents

Article abstract of DOI:10.1016/j.bmcl.2014.12.014

A 26-member library of novel N-hydroxyquinolinone derivatives was synthesized by a one-pot Buchwald-type palladium catalyzed amidation and condensation sequence. The design of these rare scaffolds was inspired from N-hydroxypyridones and 2-quinolinones classes of compounds which have been shown to have rich biological activities. The synthesized compounds were evaluated for their anti-plasmodial and anti-bacterial properties. In addition, these compounds were screened for their iron(II)-chelation properties. Notably, four of these compounds exhibited anti-plasmodial activities comparable to that of the natural product cordypyridone B.

Full text of DOI:10.1016/j.bmcl.2014.12.014

Bioorganic & Medicinal Chemistry Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
An amidation/cyclization approach to the synthesis
of N-hydroxyquinolinones and their biological evaluation
as potential anti-plasmodial, anti-bacterial,
and iron(II)-chelating agents
Yanbo Teng a, Rossarin Suwanarusk b, Mun Hong Ngai a, Rajavel Srinivasan c, Alice Soh Meoy Ong b,
Bow Ho d, Laurent Rénia b, Christina L. L. Chai a,c,
⇑
a Department of Pharmacy, National University of Singapore, 18 Science Drive 4, Singapore 117543, Singapore
b Singapore Immunology Network (SIgN), Agency for Science Technology and Research, (A⁄STAR), 8A Biomedical Grove, Immunos Building, Singapore 138648, Singapore
c Institute of Chemical and Engineering Sciences (ICES), Agency for Science Technology and Research, (A⁄STAR), 8 Biomedical Grove, Singapore 138665, Singapore
d Department of Microbiology, National University of Singapore, 5 Science Drive 2, Singapore 117545, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A
26-member library of novel N-hydroxyquinolinone derivatives was synthesized by a one-pot
Received 15 August 2014
Revised 22 November 2014
Accepted 5 December 2014
Available online xxxx
Buchwald-type palladium catalyzed amidation and condensation sequence. The design of these rare
scaffolds was inspired from N-hydroxypyridones and 2-quinolinones classes of compounds which have
been shown to have rich biological activities. The synthesized compounds were evaluated for their
anti-plasmodial and anti-bacterial properties. In addition, these compounds were screened for their
iron(II)-chelation properties. Notably, four of these compounds exhibited anti-plasmodial activities
comparable to that of the natural product cordypyridone B.
Keywords:
N-Hydroxyquinolinone
Anti-plasmodial activity
Anti-bacterial activity
Iron chelation
Ó 2014 Elsevier Ltd. All rights reserved.
Molecular hybridization, a strategy of combining pharmaco-
phoric elements of two or more biologically active small molecules,
results in hybrid compounds that may retain the biological
characteristics of the parent compounds or lead to new or
enhanced biological activities.1 These hybridized entities are typi-
cally synthetically more accessible than the parent compounds as
only the key pharmacologically active groups are necessary. To this
Akanthomycin gracilis.4 Moreover, compounds with the N-hydroxy-
pyridone motif are widely considered as siderophores and their
biological activities have been recognized to originate from their
ability to sequester metal ions.5 For example, pyridoxatin–Fe
complex (named terricolin) has been isolated and the absolute
structure was elucidated by X-ray crystallographic methods.6
On the other hand, the 4-quinolone class of compounds has
long been used as broad-spectrum antibiotics since its discovery
about 40 years ago.7 4-quinolone drugs (such as sitafloxacin) act
as anti-bacterials by inhibiting the topoisomerase ligase domain
of the bacteria leading to DNA fragmentation and ultimately cell
death.8 Recently, a 4-quinolone compound, ELQ-300 with an IC50
value of 14.9 nM against the drug resistant Plasmodium falciparum
was identified as a lead candidate for the treatment of malaria
(Fig. 1).9 Despite these developments, there is a continual urgent
need to develop new and effective anti-malarial and anti-bacterial
therapies as the current suite of marketed drugs are becoming
ineffective at an alarming rate due to resistance. To date, it is
known that the malaria parasite has developed resistance towards
the last-line drug artemisinin and threatens malaria control.10
Similarly, the emergence of bacterial resistance to all the four
generations of quinolone antibiotics necessitates the development
of new therapies.8
end, we envisioned N-hydroxyquinolinones,
a scarce and a
relatively unexplored scaffold in medicinal chemistry as a blend
of N-hydroxypyridone and quinolone scaffolds and anticipate these
to possess the combined biological properties of the parent
compounds.
There is a myriad of known N-hydroxypyridone-based natural
products possessing anti-plasmodial, anti-bacterial, and anti-can-
cer properties.2 For example, cordypyridone B (Fig. 1) was reported
to display potent anti-plasmodial activity against Plasmodium
falciparum (K1 strain) with an IC50 value of 37 ng/mL3 as well as
display anti-bacterial properties against Staphylococcus aureus.4
Other examples of naturally occurring N-hydroxypyridones
include akanthomycin, an antibiotic isolated from the fungus
⇑
Corresponding author.
http://dx.doi.org/10.1016/j.bmcl.2014.12.014
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Teng, Y.; et al. Bioorg. Med. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.bmcl.2014.12.014

2
Y. Teng et al. / Bioorg. Med. Chem. Lett. xxx (2014) xxx–xxx
OH
N
OH
N
H
O
N
O
O
R3
R2
R1
OH
OH
OH
Quinolones
-approved drugs/lead compounds
N-hydroxypyridones
-established biological
properties
N-hydroxyquinolinones
-under explored scaffold
HO
O
HO
N
O
N
H
F
COOH
O
O
OH
O
OH
H
Cl
H3C
CH3
OCF3
N
N
OH
CH3
Cl
CH3
N
H
H3CO
H2N
F
H3C
CH3
Cordypyridone B
Akanthomycin
Sitafloxacin
ELQ-300
Figure 1. Hybrid approach to N-hydroxyquinolinones inspired from N-hydroxypyridones and quinolones.
Although the N-hydroxypyridone-based natural products are
68–88% yield. Gratifyingly, the coupling of the protected N-hydrox-
yamides 4 with the benzaldehydes or benzoates under palladium
catalyzed Buchwald-type C–N bond formation followed by a tan-
dem cyclodehydration furnished the quinolinones (5a–z) in good
yields. Various substituents on 4 including 2-methyl phenyl, 2-tri-
fluromethyl phenyl, 2-methoxy phenyl, 1-naphthyl, 2-naphthyl
were tolerated under the reaction conditions above. In these stud-
ies, Pd2(dba)3 (0.25% equiv) was used as a palladium source in the
presence of Cs2CO3 as a base and Xantphos (5% equiv) as a ligand.
Although this kind of coupling–cyclization sequence with
amides has been documented,14 to the best of our knowledge this
is the first method wherein hydroxylamine-derived amides were
shown to be amenable for the C–N bond formation reaction, thus
opening up an easy and direct access to N-hydroxyquinolinone
scaffolds. Following the above procedure compounds 5a–z were
synthesized. Subsequently, the final compounds 6a–z were
obtained by treating compounds 5a–z with trifluoroacetic acid in
dichloromethane.15 Additionally, the quinolinone 7 was synthe-
sized by adopting the reported protocol14 and used as a reference
to understand the importance of the –N-OH functionality during
biological evaluation. All of these compounds were evaluated for
their biological activities.
promising anti-plasmodial candidates, their abundance in nature
is low and their molecular complexity and presence of multiple
stereo-centers make them less attractive to be considered as drug
candidates. There are few reports on the total syntheses of these
natural products but the crucial N-hydroxylation step relies on
the use of Vedejs reagent11 (containing highly carcinogenic
hexa-methylphosphoramide coordinated to molybdenum) and
the tedious purification procedures associated with the removal
of residual molybdenum contamination makes those routes unde-
sirable particularly on a production scale. Hence, we sought to
develop a new route to N-hydroxyquinolinones, which could avoid
the use of Vedejs reagent and at the same time be amenable for
library synthesis. Herein, we report a one-pot palladium catalyzed
cascade of amidation/cyclization sequence to construct
a
26-member library of N-hydroxyquinolinone derivatives and
report their in vitro anti-plasmodial, anti-bacterial and iron-
chelation properties.
The detailed synthetic route to N-hydroxyquinolinones is
depicted in Scheme 1. Our strategy commenced with the synthesis
of protected N-hydroxyphthalimide 2 by treating p-methoxyben-
zyl chloride with N-hydroxyphthalimide (1).12 Treatment of 2 with
hydrazine monohydrate furnished 87% of O-(4-methoxybenzyl)
hydroxylamine13 (3), which was subsequently treated with 6
The 27 synthesized compounds were screened for their in vitro
anti-plasmodial activities. Their MBC (minimum bactericidal con-
centration) values were also determined. The results are summa-
rized in Table 1.
different commercially available
a-arylacetyl chlorides to afford
6 different N-((4-methoxybenzyl)oxy)-2-arylacetamides (4a–f) in
The anti-plasmodial screening was conducted with chloroquine
sensitive 3D7 strain using a well established in vitro maturation
assay.16 Most of these compounds displayed moderate anti-plas-
modial activities. It is noteworthy that compounds 6n, 6q, 6r,
O
O
a
b
O
NH2
N
OPMB
N
OH
and 6u show potent anti-plasmodial activity (1.1–1.4
lM) as com-
O
e
O
2
O
pared to that of the natural product cordypyridone B (0.8
l
M). The
3
1
X
N
X
quinolinone 7 is not active within the range tested suggesting the
importance of the –N-OH functionality for the biological activity, as
compared to 6h. Compounds with a hydroxyl group at the 4-posi-
tion of the heterocyclic ring were less active as compared to the
ones without a –OH group (compounds 6a–f vs compounds 6g–l)
suggesting that the –OH group is not necessary for the activity.
The presence of the methoxy group at the 7-position of the quinoli-
none ring significantly decreased the anti-plasmodial activities, as
shown by compounds 6m, 6n, 6o and 6t. Electron-withdrawing or
electron-donating groups have subtle effects on the activities when
present on C-5 or C-6 of the fused aryl ring as shown by com-
pounds 6n and 6p, 6q and 6r.
R1
O
R1
O
H
N
c
d
R2
PMBO
R1
R2
O
N
OH
OPMB
5a-z
4a-f
6a-z
1
1
4d:
4a:
R =
R =
F3C
4e: R1=
4b: R1=
7:
N
H
O
H3C
1
1
4f:
4c:
R =
R =
O
Scheme 1. Reagents and conditions: (a) PMBCl, DMF, Et3N, 90 °C; (b) hydrazine
monohydrate, DMF, MeOH, 60 °C; (c) aryl acetyl chloride, DCM, Et3N, rt; (d)
substituted 2-bromobenzaldehyde or methyl 2-bromobenzoate, Cs2CO3, Pd2(dba)3,
Xantphos, toluene, 110 °C; (e) TFA, anisole, DCM, rt.
The MBC values of these compounds were determined against
Escherichia coli and Staphylococcus aureus. Compound 6b is not
active towards E. coli but displayed mild activity towards S. aureus,
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Y. Teng et al. / Bioorg. Med. Chem. Lett. xxx (2014) xxx–xxx
3
Table 1
In vitro anti-plasmodial and anti-bacterial activities of targeted compounds
X
5
8
R1
O
6
7
R2
N
OH
Compounds
X
R1
R2
MBC (mM)
Anti-plasmodial
IC50 (lM)
E. coli
S. aureus
6a
6b
OH
OH
H
H
4.0
NA
NA
NA
NA
NA
1.7
NA
6c
OH
OH
H
H
NA
NA
6d
NA
NA
NA
NA
F3C
H3C
O
6e
OH
H
NA
6f
OH
H
H
H
H
NA
NA
NA
NA
NA
NA
NA
14
6g
6h
H
2.6
6i
6j
H
H
H
H
NA
NA
3.6
3.4
2.9
5.9
F3C
H3C
O
6k
H
H
H
NA
4.1
3.0
6l
H
H
H
H
H
H
H
H
1.9
NA
NA
NA
NA
NA
NA
3.4
3.8
NA
NA
NA
3.1
NA
NA
NA
7.6
4.2
1.3
5.0
1.9
1.1
1.4
2.5
6m
6n
6o
6p
6q
6r
6,7-OCH2O–
6-OCH3
6-OCH3
7-OCH3
5,6-OCH2O–
6-F
5-F
6s
6-OH
6t
H
H
H
H
H
H
6,7-OCH2O–
6-OCH3
NA
NA
NA
NA
NA
NA
NA
3.6
NA
NA
NA
NA
4.6
1.3
5.5
3.6
4.8
5.0
H3C
H3C
H3C
H3C
H3C
H3C
6u
6v
6w
6x
6y
6-OCH3
7-OCH3
5,6-OCH2O–
6-F
5-F
(continued on next page)
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Y. Teng et al. / Bioorg. Med. Chem. Lett. xxx (2014) xxx–xxx
Table 1 (continued)
Compounds
X
R1
R2
MBC (mM)
Anti-plasmodial
IC50
(lM)
E. coli
S. aureus
6z
H
6-OH
1.9
3.8
3.5
H3C
7
NA
NT
NA
NT
NA
0.8
Cordypyridone B
NA: not active up to the maximum concentration tested (for MBC: up to 1024 lg/ml, anti-plasmodial against 3D7 strain: up to 5092 ng/ml); NT: not tested.
Table 2
cordypyridone B. These compounds have the advantages that they
are simple to synthesize and scale-up. Their anti-bacterial as well
as the iron(II)-chelating abilities were evaluated and discussed.
Chelation ability of selected N-hydroxyquinolinones
Compounds
50% CA (mM)
Compounds
50% CA (mM)
6b
6n
6q
6r
0.28
0.25
0.24
0.24
0.13
6u
6x
6y
7
0.28
0.29
0.28
NA
Acknowledgments
We greatly appreciate the Ministry of Education (MOE) for a
research scholarship for Mr Teng Yanbo and the National Univer-
sity of Singapore and the Agency for Science Technology and
Research (A⁄STAR) for financial support of this project (JCO-10/
03/FG/06/02).
EDTA
CA: chelation ability; NA: no chelation observed up to the maximum concentration
tested.
Supplementary data
indicating that gram positive bacterial may be susceptible to com-
pound 6b. Four compounds (6i, 6j, 6k, 6l) indicate effective killing
against Staphylococcus aureus, whereas for compound 6z, no differ-
ence in the activities of these compounds towards gram negative
or gram positive bacteria was observed. Although these com-
pounds are not potent anti-bacterials, the results are encouraging
and provide further clues towards further discovery of new
antibiotics.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2014.12.
014.
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7 was not able to chelate iron within the range tested. The N-
hydroxyquinolinones have a narrow range of chelation abilities
as reported by the 50% CA values, that is, a range from 0.24 mM
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to 0.29 lM. EDTA was used as control and its 50% chelation ability
was determined to be 0.13 mM. Thus the iron chelation assay
shows that N-hydroxyquinolinones are promising iron chelating
agents, though this ability is relatively insensitive to the nature
of the substituents studied.
In summary, we have implemented a new route for the facile
synthesis of a library of N-hydroxyquinolinones based on a one-
pot palladium catalyzed Buchwald-type C–N amidation/dehydro-
cyclization sequence. The design of these compounds was inspired
from the naturally occurring cordypyridones and therapeutically
valuable quinolones. These new set of compounds exhibited prom-
ising anti-plasmodial activities, some of which are as potent as
Please cite this article in press as: Teng, Y.; et al. Bioorg. Med. Chem. Lett. (2014), http://dx.doi.org/10.1016/j.bmcl.2014.12.014