Boron trifluoride etherate promoted microwave-assisted synthesis of antimalarial acridones

Article abstract of DOI:10.1039/c9ra09478d

A microwave-assisted, rapid and efficient method using boron trifluoride etherate (BF₃.Et₂O) for the synthesis of acridones, via an intramolecular acylation of N-phenylanthranilic acid derivatives, has been developed. The reaction proceeds under solvent-free conditions, tolerates a wide range of functional groups, and provides rapid access to a range of acridones in good to excellent yields. Several of the synthesized acridones exhibited potent antimalarial activities against CQ sensitive and multi-drug resistant (MDR) parasites.

Full text of DOI:10.1039/c9ra09478d

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PAPER
Boron trifluoride etherate promoted microwave-
assisted synthesis of antimalarial acridones†
Cite this: RSC Adv., 2019, 9, 42284
a
ab
Rozalia A. Dodean,ab Yuexin Liab and Jane X. Kelly
*
*
Papireddy Kancharla,
A microwave-assisted, rapid and efficient method using boron trifluoride etherate (BF3.Et2O) for the
synthesis of acridones, via an intramolecular acylation of N-phenylanthranilic acid derivatives, has been
developed. The reaction proceeds under solvent-free conditions, tolerates a wide range of functional
groups, and provides rapid access to a range of acridones in good to excellent yields. Several of the
synthesized acridones exhibited potent antimalarial activities against CQ sensitive and multi-drug
resistant (MDR) parasites.
Received 13th November 2019
Accepted 10th December 2019
DOI: 10.1039/c9ra09478d
rsc.li/rsc-advances
report of synthetic method for acridones under a non-
conventional pathway using microwave irradiation in the
Introduction
presence of BF3.Et2O.
Acridones are one of the important classes of heteroaromatic
compounds and they have ubiquitous structural motifs in
biologically active molecules. The natural and synthetic acri-
done products (Fig. 1) are of worldwide interest because of
their wide range of biological properties, including antitumor,
anticancer, antimalarial, anti-HIV, antiviral, and antifungal
activities.1–13 The intriguing structural features and thera-
peutic importance of acridone scaffold has attracted the great
attention of organic and medicinal chemists to discover new
synthetic routes14–18 for the generation of novel acridones to be
evaluated for biological activities and structure–activity rela-
tionships (SAR).
As part of an ongoing interest in developing new antipar-
asitic agents, we recently discovered a novel acridone chemo-
type that demonstrates efficacy against both liver and blood
stage malaria.11,12 Of the many conventional methods available
for the synthesis of acridones from N-phenylanthranilic acid
and ester derivatives, the most widely used is the strong acid
catalyzed intramolecular Friedel–Cras cyclization in the
presence of either sulfuric acid (H2SO4), polyphosphoric acid
(PPA), or Eaton's acid (Scheme 1).12,19,20 In 2006, Nadaraj et al.
demonstrated the use of several other catalysts (e.g. PTSA,
ZnCl2, AlCl3 and PPA) in the synthesis of acridones under
microwave irradiation.21 Recently, Zhang et al. reported the
intramolecular acylation of N,N-diphenylanthranilic acids in
the presence of BF3.Et2O, under conventional conditions
(Scheme 1, top panel),22 however, they did not explore the
substrate scope of the reaction. To date, there has not been any
The currently available methods for the synthesis of acri-
dones, which lack either the aryl or alkyl moiety at N-10 position
of the middle ring, have some limitations, such as low yields,
long reaction times, functional group intolerance, harsh reac-
tion conditions, usage of larger amount of catalysts and,
solvents and tedious workup procedures. An efficient and
improved synthetic method for the synthesis of acridones to
overcome the aforementioned disadvantages is highly desirable
but remains a challenge for organic chemists. In our ongoing
drug discovery program to produce new antimalarial chemo-
types, our synthetic efforts focused on the development of new
and efficient synthetic methods towards the generation of
a large library of antimalarial acridones. In this work we report
a facile, versatile, eco-friendly, and cost-effective synthetic
method that is microwave-assisted for the synthesis of various
acridones. In addition, we report the potent antimalarial activ-
ities of the newly generated acridones.
aDepartment of Chemistry, Portland State University, Portland, Oregon 97201, USA.
E-mail: papiredd@pdx.edu; kellyja@ohsu.edu; Tel: +1-503-725-2566
bDepartment of Veterans Affairs Medical Center, Portland, Oregon 97239, USA.
Tel: +1-503-220-8262 ext. 54356
† Electronic supplementary information (ESI) available: Copies of 1H and
13C-NMR spectra of the nal products. See DOI: 10.1039/c9ra09478d
Fig. 1 Biologically active natural and synthetic acridones.
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temperature and solvent selection was carried out without the
use of microwave irradiation. No desired acridone 2a formation
was observed at room temperature under solvent free condi-
tions, and in 1,4-dioxane, DMF and acetonitrile (Table 1, entries
1–4). However, the acridone 2a was formed with low yield (10%)
when the reaction was performed in 1,4-dioxane under reux
conditions for 6 h (Table 1, entry 5). Acridone 2a was produced
with 30% yield when the reaction was conducted under solvent
free conditions at 80 ꢀC for 24 h (Table 1, entry 6). The yield of 2a
was enhanced to 65% when the reaction was performed at
ꢀ
130 C for 5 h (Table 1, entry 7).
Encouraged by these results, we next evaluated this reaction
under microwave irradiation using 1,4-dioxane, DMF and
acetonitrile as solvent and solvent free conditions at various
temperatures (Table 1, entries 8–15). Great improvement was
observed as acridone 2a was obtained with 98% yield in the
presence of BF3.Et2O (2.0 eq.), with a short reaction time (1.0
min) (Scheme 1 and Table 1, entry 15). Remarkably, this reac-
tion was very efficient under solvent free conditions and
generated no side products. Having now changed several reac-
tion parameters, we wished to determine whether BF3.Et2O was
still the best reagent for the model reaction, so we then screened
the reaction with other Lewis acids SnCl4, TiCl4, FeCl3,
Scheme 1 Present and previous synthetic methods for the synthesis
of acridones.
Results and discussion
ꢀ
Pd(OAc)2, PdCl2 and I2 under microwave irradiation at 150 C
(Table 1, entries 16–23). All these reactions failed to provide the
desired product 2a even at longer reaction time (3.0 min). A
trace amount of the acridone 2a was observed with SnCl4 and
TiCl4 in 1,4-dioxane (Table 1, entries 17 and 19), however, in
both cases, the reaction mixture turned into a black tar. On the
basis of all of these experiments, the optimal reaction
To evaluate the broad scope of boron triuoride etherate
(BF3.Et2O) as a catalyst/reagent for the synthesis of various
acridones, including (10H)-9-acridones, N-alkyl-9-acridones and
N-aryl-9-acridones, we started the initial investigation with N-
phenylanthranilic acid (1a) as the pilot substrate. First a careful
investigation of the reaction conditions, including reaction
Table 1 Screening of various reaction conditions and reagents of Friedel–Crafts intramolecular cyclization of N-phenylanthranilic acid (1a)
Entry
Reaction conditions
Reagent (no. eq.)
Solvent
Reaction time
Yieldb (%) 2a
1
2
3
4
5
6
7
8
RTa
RT
RT
RT
BF3.Et2O (2.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (2.0)
BF3.Et2O (2.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (1.0)
BF3.Et2O (2.0)
BF3.Et2O (2.0)
BF3.Et2O (2.0)
SnCl4 (2.0)
SnCl4 (2.0)
TiCl4 (2.0)
TiCl4 (2.0)
FeCl3 (2.0)
Pd(OAc)2 (2.0)
PdCl2 (2.0)
I2 (2.0)
—
24 h
24 h
24 h
24 h
6 h
24 h
5 h
1.0 min
1.0 min
1.0 min
1.0 min
1.0 min
1.0 min
1.0 min
1.0 min
3.0 min
3.0 min
3.0 min
3.0 min
3.0 min
3.0 min
3.0 min
3.0 min
NRc
NR
NR
NR
10
30
65
10
20
45
NR
33
25
60
1,4-Dioxane
DMF
Acetonitrile
1,4-Dioxane
—
100 ꢀC
80 ꢀC
130 ꢀC
—
MWd, 100 ꢀ
C
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
DMF
Acetonitrile
—
—
—
—
9
MW, 130 ꢀC
10
11
12
13
14
15
16
17
18
19
20
21
22
23
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 100 ꢀ
MW, 130 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
MW, 150 ꢀ
C
C
C
C
C
C
C
C
C
C
C
C
C
C
98
NR
Tracee
NR
Tracee
NR
NR
NR
NR
1,4-Dioxane
—
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
a
RT; room temperature. b Isolated yield. c NR; no reaction. d MW; microwave. e Formed black tar.
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Table 2 Substrate scope of the reaction
Entry
Substrate
Product
Yielda (%)
1
98
2
3
4
5
95
97
97
96
6
95
7
97
98
97
96
8
9
10
11
95
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Table 2 (Contd.)
Entry
Substrate
Product
Yielda (%)
12
94
13
14
15
16
85
67
69
56
17
18
19
65
60
64
20
21
32
72
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Table 2 (Contd.)
Entry
Substrate
Product
Yielda (%)
22
71
23
24
86
45
25
26
—
—
NRb
NRb
27
Tracec
Isolated yield. b NR; no reaction, recovered the substrate at 150 ꢀC, but decomposed at 200 ꢀC. c Trace amount of 2y was observed along with other
a
side products at 170 ꢀC, and decomposed at 200 ꢀC.
conditions were identied as no added solvent at 150 ꢀC for position of the middle ring also responded well under the
1.0 min under microwave irradiation mediated by BF3.Et2O (2.0 optimized conditions to provide the desired acridone 2l in
eq.).
excellent yield (94%). Together, it was demonstrated that the N-
The scope of the reaction was further explored with the newly phenylanthranilic acid analogues bearing an alkyl, aryl and
optimized conditions. Under the same reaction conditions (as alkyl-amine moieties at N-10 position of the middle ring, and
in Table 1, entry 15), N-methyl-N-phenylanthranilic acid methyl electron-donating and electron-withdrawing groups on ring-A
ester (1b) and N,N-diphenylanthranilic acid methyl ester (1c) are well tolerated producing desired products with great effi-
also provided the N-methyl-9-acridone (2b) and N-phenyl-9- ciency. A decrease in yield was observed in the case of acridone
acridone (2c), respectively, in high yields (Scheme 1 and Table 2m and 2n (Table 2, entries 13 and 14) with N-phenylanthranilic
2, entries 2 and 3). It is noteworthy that this is a unique acid methyl ester bearing multiple uoro substitutions on ring-
synthetic strategy to generate various acridones, such as (10H)- A as a substrate.
9-acridones, N-alkyl-9-acridones and N-aryl-9-acridones.
Next, the effect of substituents at meta-position to the methyl
We next examined the scope of the reaction by verifying the ester on ring-B was studied. N-phenylanthranilic acid analogues
functional groups positioned on both aryl rings (A and B) (Table 1o–1v (Table 2, entries 15–22) bearing an electron-donating
2). N-phenylanthranilic acid analogues 1d–1k (Table 2, entries groups at meta-position to the methyl ester on ring-B and
4–11) bearing an electron-donating methoxy group at the para- various electron-donating and electron-withdrawing groups on
position to the methyl ester on ring-B, and various electron- ring-A were converted to their corresponding acridones with
donating and electron-withdrawing groups on ring-A good yields. However, the yields of acridones 2o–2v were
produced the corresponding acridones 2d–2k in excellent roughly 25% lesser than the corresponding positional acridones
yields (95–98%). Notably, N-phenylanthranilic acid methyl ester 2d–2n. In contrast, the N-phenylanthranilic acid analogue (1w)
1l (Table 2, entry 12) bearing (diethylamino)ethyl moiety at N-10 with the methyl instead of methoxy group provided the
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Table 3 In vitro blood-stage antimalarial activity and cytotoxicity of
acridones
corresponding acridone 2w in higher yield (86%). Of note,
during the reaction with substrates 1o–1w small amount of side
products were observed (not isolated). The extended conjuga-
tion on ring-B as with substrate 1x signicantly reduced the
yield of acridone 2x (45%). Disappointingly, when substrates 1y
and 1z bearing a strong electron-withdrawing nitro group at 6
positon of the ring-B were used, no desired acridones were
obtained even at elevated temperatures (170 ꢀC and 200 ꢀC) and
longer reaction time (3.0 min) (Table 2, entries 25 and 26).
However, a trace amount of the acridone 2y was observed (not
isolated) along with other side products at 170 ꢀC, when
substrate 1aa containing a nitro group at 7 positon of the ring-B,
was used. All these results demonstrated that the electron-
withdrawing groups (EWGs) on the ring-B, are not tolerated in
this methodology.
The possible mechanistic pathway involves an intra-
molecular Friedel–Cras cyclization, and we propose that
initially the BF3.OEt2 coordinates with the oxygen of a carbonyl
of the substrate 3 to form the key intermediate 4. The following
intramolecular Friedel–Cras cyclization proceeds via the
intermediates 5 and 6, and the subsequent hydrolysis provides
the desired acridone 2 (Scheme 2). In fact, it appears that the
electron-donating group (EDG) on the ring-B of 3 signicantly
enhances the nucleophilic nature at the oxygen of the carbonyl
group, which facilitates toward the formation of the BF3
complex 4. Conversely, electron-withdrawing group (EWG) on
the ring-B of 7 diminishes the nucleophilic nature at the oxygen
of the carbonyl group, which prevents the formation of BF3
complex (Scheme 2). It is noteworthy that the formation of key
intermediate 4 is vital in this transformation.
In vitro activitya (IC50 nM)
Compd
D6
Dd2
7G8
Cytotoxicitya (nM) vs. HepG2
2d
2e
2f
2g
2h
2i
0.103
8.84
782
11.6
57.7
100
1.10
11.7
753
11.6
60.7
177
1.51
13.5
1138
22.2
56.6
228
>200000
>200000
>200000
>200000
>200000
>200000
>200000
12 296
2j
2l
84.1
405
98.1
684
35.0
462
2n
2o
2p
2q
2r
257
632
573
1493
983
0.05
>2500
11.6
27.7
63.1
7.03
0.10
15.0
325
425
513
1678
932
0.29
1954
0.91
15.5
49.2
0.563
0.10
163
41.8
437
911
>2500
>2500
0.20
393
1.22
46.3
109
>200000
>200000
>200000
>200000
>200000
>200000
>200000
>200000
139435
2s
2t
2u
2v
2w
2x
ATV
CQ
>200000
>200000
23 160
16.5
0.20
172
37 577
a
IC50 values are the average of at least three determinations, each
carried out in triplicate (ꢂ10%). D6: CQ-sensitive; Dd2: MDR strain
with Old World genetic background; 7G8: MDR strain with New
World genetic background; ATV: atovaquone; CQ: chloroquine.
All newly synthesized acridones were then evaluated for their
in vitro antimalarial blood stage activity against a panel of
Plasmodium falciparum malaria parasites with different
geographic and genetic backgrounds using a SYBR Green based
assay.23 Standard antimalarials chloroquine (CQ) and atova-
quone (ATV) were used as reference drugs. In parallel, the
cytotoxicity of acridones was tested against human hepatic
HepG2 cancer cell line.24 The results are summarized in Table 3.
Majority of the acridones have shown great antimalarial activity
(IC50 < 100 nM). Signicantly, acridones 2d, 2e, 2g, 2s, 2u, 2v
and 2x exhibited excellent activity against all tested strains with
a high therapeutic index (the ratio of cytotoxicity to antimalarial
therapeutic efficacy), as demonstrated in Table 3. In general, the
activity and structure–activity relationship (SAR) analyses
demonstrate that the electron donating groups at 2 position of
ring-A, and 6 position of ring-B are well tolerated. Conversely,
the electron withdrawing groups on ring-A have an adverse
effect on antimalarial potency.
To demonstrate the feasibility of the present protocol,
a larger-scale reaction (5 g ꢁ 5) was performed with 1a. Signif-
icantly, our reaction has shown excellent efficiency even at
a larger scale reaction with >95% yield. This reaction was also
carried out under domestic microwave conditions obtaining the
corresponding acridones with excellent yields.
Conclusions
In conclusion, we developed an efficient, simple and convenient
synthetic method for the synthesis of acridones in good to
excellent yields for the rst time in the presence BF3.Et2O under
microwave irradiation conditions. The advantages of this
method are: simple experimental and workup procedures, short
duration of reaction (1.0 min), proceeds under solvent free
Scheme 2 Possible reaction mechanism for BF3.Et2O mediated Frie-
del–Crafts cyclization.
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conditions, the use of mild and low cost catalyst/reagent, (2C), 127.0 (2C), 122.1 (2C), 121.6 (2C), 116.6 (2C), 34.1; HRMS
replacement of the strong acid catalysts, and tolerance of (ESI) calcd for C14H12N1O1 (M + H)+ 210.0913, found 210.0910.
large variety functional groups. This synthetic method has the
10-Phenylacridin-9(10H)-one (2c).27 1H NMR (CDCl3, 400
potential to be carried out on a large scale and is suited for the MHz) d 8.62 (d, J ¼ 7.9 Hz, 2H), 7.72 (m, 3H), 7.53 (m, 2H), 7.40
generation of an extensive library of acridones and their (d, J ¼ 7.5 Hz, 2H), 7.30 (m, 2H), 6.88 (d, J ¼ 8.6 Hz, 2H); 13C
precursors. Signicantly, a number of newly generated acri- NMR (CDCl3, 100 MHz) d 178.1, 143.2, 139.0, 133.4 (2C), 131.1
dones exhibited potent antimalarial activity with great selec- (2C), 130.0 (2C), 129.7 (2C), 127.3 (2C), 121.7 (2C), 121.6 (2C),
tivity. The preparation of a large library containing structurally 118.8 (2C); HRMS (ESI) calcd for C19H14N1O1 (M + H)+ 272.1070,
diversied second generation acridones is currently underway found 272.1066.
in our laboratory.
6-Methoxy-2-(triuoromethoxy)acridin-9(10H)-one (2d). 1H
NMR (DMSO-d6, 400 MHz) d 11.86 (s, 1H), 8.13 (m, 1H), 8.03 (d, J
¼ 1.3 Hz, 1H), 7.71 (dd, J ¼ 9.0, 2.6 Hz, 1H), 7.61 (d, J ¼ 9.0 Hz,
1H), 6.90 (m, 2H), 3.91 (s, 3H); 13C NMR (DMSO-d6, 100 MHz)
d 175.6, 164.1, 143.3, 142.6, 140.0, 128.4, 127.2, 122.0, 121.1,
120.0, 117.5, 114.9, 112.7, 98.4, 56.0; HRMS (ESI) calcd for
Experimental section
General
NMR spectra were recorded on Bruker AMX-400 spectrometer at
400 MHz (1H) and 100 MHz (13C). Experiments were recorded in
CDCl3 and DMSO-d6 at 25 ꢀC. Chemical shis are given in parts
per million (ppm) downeld from internal standard Me4Si.
High-resolution mass spectrometry (HRMS) (electrospray ioni-
zation (ESI)) were recorded on a high-resolution (140 000) Q
Exactive Orbitrap mass spectrometer. The microwave reactions
were conducted using Biotage® Initiator+ microwave synthe-
sizer and domestic microwave. Unless otherwise stated, all
reagents and solvents were purchased from commercial
suppliers and used without further purication. Reactions that
required the use of anhydrous, inert atmosphere techniques
were carried out under an atmosphere of argon/nitrogen.
Chromatography was executed on CombiFlash instrument,
using silica gel (230–400 mesh) as the stationary phase and
mixtures of ethyl acetate (EtOAc) and hexane or dichloro-
methane (DCM) and methanol as eluents. Substrate 1a was
obtained from commercial sources and 1b was synthesized
from 1a via standard methylation reaction. Substrate 1c was
synthesized from methyl 2-bromobenzoate via a copper-
mediated substitution reaction with diphenylamine.25
Substrates 1d–1z, and 1aa were synthesized using Buchwald–
Hartwig cross-coupling methods.12,26
C
15H11F3N1O3 (M + H)+ 310.0686, found 310.0681.
2-(4-Chlorophenoxy)-6-methoxyacridin-9(10H)-one (2e). 1H
NMR (DMSO-d6, 400 MHz) d 11.75 (s, 1H), 8.10 (d, J ¼ 8.9 Hz,
1H), 7.65 (d, J ¼ 2.8 Hz, 1H), 7.55 (m, 1H), 7.51 (m, 1H), 7.45 (m,
2H), 7.09 (dd, J ¼ 6.8, 2.2 Hz, 2H), 6.87 (m, 2H), 3.90 (s, 3H); 13
C
NMR (DMSO-d6, 100 MHz) d 175.6, 163.8, 156.5, 150.9, 143.2,
138.0, 130.4 (2C), 128.4, 127.7, 126.4, 121.8, 120.6 (2C), 120.0,
114.9, 113.9, 112.3, 98.3, 55.9; HRMS (ESI) calcd for
C
20H15Cl1N1O3 (M + H)+ 352.0735, found 352.0733.
3-Methoxy-5-(triuoromethoxy)acridin-9(10H)-one (2f). 1H
NMR (DMSO-d6, 400 MHz) d 11.32 (s, 1H), 8.22 (d, J ¼ 8.1 Hz,
1H), 8.14 (d, J ¼ 8.9 Hz, 1H), 7.80 (d, J ¼ 7.8 Hz, 1H), 7.39 (d, J ¼
2.1 Hz, 1H), 7.30 (m, 1H), 6.91 (dd, J ¼ 8.9, 2.1 Hz, 1H), 3.91 (s,
3H); 13C NMR (DMSO-d6, 100 MHz) d 175.5, 164.2, 143.4, 136.6,
134.4, 128.3, 125.6, 125.2, 123.0, 122.1, 120.8, 115.4, 113.1, 99.3,
56.0; HRMS (ESI) calcd for C15H11F3N1O3 (M + H)+ 310.0686,
found 310.0679.
6-Methoxy-2-((triuoromethyl)sulfonyl)acridin-9(10H)-one
1
(2g). H NMR (DMSO-d6, 400 MHz) d 12.40 (s, 1H), 8.75 (d, J ¼
2.1 Hz, 1H), 8.20 (m, 2H), 7.78 (d, J ¼ 8.9 Hz, 1H), 7.00 (dd, J ¼
8.9, 2.3 Hz, 1H), 6.95 (d, J ¼ 2.2 Hz, 1H), 3.93 (s, 3H); 13C NMR
(DMSO-d6, 100 MHz) d 175.3, 164.7, 146.5, 143.1, 132.8, 132.0,
128.7, 121.6, 120.7, 120.3, 120.2, 116.1, 113.6, 99.7, 56.2; HRMS
(ESI) calcd for C15H11F3N1O4S1 (M + H)+ 358.0355, found
358.0351.
Representative procedure for the synthesis of acridin-9(10H)-
one (2a)21
6-Methoxy-9-oxo-9,10-dihydroacridin-2-yl-triuoromethane-
BF3.Et2O (580 mL, 4.69 mmol) was added to a microwave reac- sulfonate (2h). 1H NMR (DMSO-d6, 400 MHz) d 11.96 (s, 1H),
tion vial containing N-phenylanthranilic acid (1a) (500 mg, 2.34 8.12 (m, 2H), 7.81 (dd, J ¼ 9.2, 3.0 Hz, 1H), 7.65 (d, J ¼ 9.2 Hz,
mmol). Then the reaction mixture was exposed to microwave 1H), 6.92 (d, J ¼ 2.4 Hz, 1H), 6.90 (s, 1H), 3.91 (s, 3H); 13C NMR
irradiation for 1.0 min at 150 ꢀC. Aer cooling to room (DMSO-d6, 100 MHz) d 175.3, 164.2, 143.5, 143.3, 140.7, 128.4,
temperature, the reaction mixture was poured into water (25 126.9, 121.2, 120.4, 120.3, 118.2, 115.0, 112.9, 98.7, 56.0; HRMS
mL) and allowed to stir for 5 min. The solid material was ltered (ESI) calcd for C15H11F3N1O5S1 (M + H)+ 374.0305, found
by a sintered funnel and washed with water (50 mL) and 374.0300.
1
methanol (5 mL) to afford the desired acridone 2a (448 mg.
6-Methoxy-9-oxo-9,10-dihydroacridine-2-carbonitrile (2i). H
98%). 1H NMR (DMSO-d6, 400 MHz) d 11.74 (s, 1H), 8.23 (dd, J ¼ NMR (DMSO-d6, 400 MHz) d 12.05 (s, 1H), 8.51 (d, J ¼ 1.9 Hz,
8.2, 1.4 Hz, 2H), 7.73 (m, 2H), 7.54 (d, J ¼ 8.2 Hz, 2H), 7.26 (m, 1H), 8.14 (d, J ¼ 8.9 Hz, 1H), 7.99 (dd, J ¼ 8.7, 2.0 Hz, 1H), 7.58
2H); 13C NMR (DMSO-d6, 100 MHz) d 177.2, 141.4 (2C), 133.9 (d, J ¼ 8.7 Hz, 1H), 6.94 (m, 2H), 3.91 (s, 3H); 13C NMR (DMSO-
(2C), 126.5 (2C), 121.5 (2C), 120.9 (2C), 117.8 (2C); HRMS (ESI) d6, 100 MHz) d 175.2, 164.4, 143.6, 143.2, 135.0, 132.3, 128.6,
calcd for C13H10N1O1 (M + H)+ 196.0757, found 196.0754.
10-Methylacridin-9(10H)-one (2b).14 1H NMR (DMSO-d6, 400 calcd for C15H11N2O2 (M + H)+ 251.0815, found 251.0812.
120.7, 119.4, 119.0, 115.9, 113.0, 103.3, 99.1, 56.1; HRMS (ESI)
MHz) d 8.34 (d, J ¼ 8.2 Hz, 2H), 7.84 (m, 4H), 7.33 (m, 2H), 3.93
6-Methoxy-2-nitroacridin-9(10H)-one (2j). 1H NMR (DMSO-
(s, 3H); 13C NMR (DMSO-d6, 100 MHz) d 177.0, 142.8 (2C), 134.5 d6, 400 MHz) d 8.92 (d, J ¼ 2.5 Hz, 1H), 8.42 (dd, J ¼ 9.2, 2.6 Hz,
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1H), 8.13 (d, J ¼ 8.9 Hz, 1H), 7.60 (d, J ¼ 9.2 Hz, 1H), 6.96 (dd, J ¼ 7.55 (d, J ¼ 9.0 Hz, 1H), 7.46 (dd, J ¼ 9.0, 2.9 Hz, 1H), 3.87 (s,
9.0, 2.0 Hz, 1H), 6.92 (d, J ¼ 1.9 Hz, 1H), 3.91 (s, 3H); 13C NMR 3H); 13C NMR (DMSO-d6, 100 MHz) d 175.8, 155.4, 142.8, 135.8,
(DMSO-d6, 100 MHz) d 175.7, 164.5, 145.0, 143.1, 141.2, 128.6, 134.8, 132.6, 125.4, 122.1, 120.2, 119.5, 119.4, 119.3, 105.6,
127.5, 123.3, 119.9, 119.0, 115.6, 113.4, 99.4, 56.1; HRMS (ESI) 103.0, 55.9; HRMS (ESI) calcd for C15H11N2O2 (M + H)+ 251.0815,
calcd for C14H11N2O4 (M + H)+ 271.0713, found 271.0709.
found 251.0812.
3-Methoxy-5-nitroacridin-9(10H)-one (2k).28 1H NMR (DMSO-
1,3-Diuoro-7-methoxyacridin-9(10H)-one (2s). 1H NMR
d6, 400 MHz) d 11.37 (s, 1H), 8.63 (m, 2H), 8.09 (d, J ¼ 8.9 Hz, (DMSO-d6, 400 MHz) d 11.88 (s, 1H), 7.56 (d, J ¼ 2.8 Hz, 1H),
1H), 7.62 (d, J ¼ 2.4 Hz, 1H), 7.39 (t, J ¼ 8.0 Hz, 1H), 6.94 (dd, J ¼ 7.47 (d, J ¼ 9.0 Hz, 1H), 7.40 (dd, J ¼ 9.0, 2.9 Hz, 1H), 7.03 (dd,
8.9, 2.4 Hz, 1H), 3.90 (s, 3H); 13C NMR (DMSO-d6, 100 MHz) J ¼ 10.3, 2.3 Hz, 1H), 6.95 (m, 1H), 3.85 (s, 3H); 13C NMR
d 175.1, 164.6, 142.8, 135.5, 135.4, 135.0, 131.5, 128.2, 123.8, (DMSO-d6, 100 MHz) d 174.5, 155.1, 143.7, 135.3, 124.7,
120.4, 115.4, 114.0, 101.0, 56.3; HRMS (ESI) calcd for 122.7, 119.3, 105.7, 98.8, 98.6, 97.6, 97.4, 97.1, 55.9; HRMS
C
14H11N2O4 (M + H)+ 271.0713, found 271.0710.
(ESI) calcd for C14H10F2N1O2 (M + H)+ 262.0674, found
10-(2-(Diethylamino)ethyl)-6-methoxy-2-nitroacridin-9(10H)- 262.0672.
1
one (2l). H NMR (CDCl3, 400 MHz) d 9.23 (d, J ¼ 2.8 Hz, 1H),
1,2,3,4-Tetrauoro-7-methoxyacridin-9(10H)-one (2t). 1H-
8.37 (m, 2H), 7.53 (d, J ¼ 9.5 Hz, 1H), 6.89 (m, 2H), 4.36 (t, J ¼ NMR (DMSO-d6, 400 MHz) d 11.90 (s, 1H), 7.76 (d, J ¼ 9.1 Hz,
7.3 Hz, 2H), 3.90 (s, 3H). 2.85 (t, J ¼ 7.3 Hz, 2H), 2.62 (q, J ¼ 1H), 7.54 (d, J ¼ 2.6 Hz, 1H), 7.45 (dd, J ¼ 9.1, 2.8 Hz, 1H), 3.86
7.2 Hz, 4H), 1.02 (t, J ¼ 7.2 Hz, 6H); 13C NMR (CDCl3, 100 MHz) (s, 3H); HRMS (ESI) calcd for C14H8F4N1O2 (M + H)+ 298.0486,
d 175.7, 165.0, 145.4, 143.5, 141.2, 130.0, 127.3, 124.2, 121.5, found 298.0484.
116.9, 115.9, 111.2, 98.9, 55.7, 50.0, 47.9 (2C), 46.8, 12.1 (2C);
HRMS (ESI) calcd for C20H24N3O4 (M + H)+ 370.1761, found NMR (DMSO-d6, 400 MHz) d 11.91 (s, 1H), 7.58 (s, 1H), 7.45
370.1758.
7-(2-Chloroethoxy)-1,3-diuoroacridin-9(10H)-one (2u). 1H
(m, 2H), 7.03 (d, J ¼ 10.3 Hz, 1H), 6.95 (t, J ¼ 11.4 Hz, 1H), 4.34
1,3-Diuoro-6-methoxyacridin-9(10H)-one (2m).12 1H-NMR (t, J ¼ 4.5 Hz, 2H), 3.99 (t, J ¼ 4.5 Hz, 2H); 13C NMR (DMSO-d6,
(DMSO-d6, 400 MHz) d 11.78 (s, 1H), 8.06 (d, J ¼ 8.9 Hz, 1H), 100 MHz) d 174.5, 153.7, 143.6, 135.6, 124.9, 122.6, 119.4,
7.02–6.93 (m, 2H), 6.88 (dd, J ¼ 8.9, 2.3 Hz, 1H), 6.82 (d, J ¼ 107.6, 106.9, 98.8, 98.6, 97.7, 97.4, 97.2, 68.9, 43.5; HRMS
2.1 Hz, 1H), 3.89 (s, 3H). HRMS (ESI) calcd for C14H10F2N1O2 (M (ESI) calcd for C15H11Cl1F2N1O2 (M + H)+ 310.0441, found
+ H)+ 262.0674, found 262.0672.
310.0437.
5-Methoxy-2-(triuoromethoxy)acridin-9(10H)-one (2v). 1H
1,2,3,4-Tetrauoro-6-methoxyacridin-9(10H)-one (2n). 1H-
NMR (DMSO-d6, 400 MHz) d 11.75 (s, 1H), 8.07 (d, J ¼ 9.0 Hz, NMR (DMSO-d6, 400 MHz) d 11.50 (s, 1H), 8.08 (t, J ¼ 9.2 Hz,
1H), 7.21 (d, J ¼ 2.2 Hz, 1H), 6.94 (dd, J ¼ 9.0, 2.3 Hz, 1H), 3.89 2H), 7.81 (d, J ¼ 8.2 Hz, 1H), 7.75 (dd, J ¼ 9.1, 2.1 Hz, 1H), 7.37
(s, 3H); HRMS (ESI) calcd for C14H8F4N1O2 (M + H)+ 298.0486, (d, J ¼ 7.7 Hz, 1H), 7.24 (t, J ¼ 8.0 Hz, 1H), 4.06 (s, 3H); 13C NMR
found 298.0484.
(DMSO-d6, 100 MHz) d 176.5, 148.4, 142.9, 139.8, 132.3, 127.4,
2-Methoxy-7-(triuoromethoxy)acridin-9(10H)-one (2o). 1H 122.0, 121.8, 121.4, 121.1, 121.0, 117.4, 117.2, 113.2, 56.7; HRMS
NMR (DMSO-d6, 400 MHz) d 12.00 (s, 1H), 8.06 (s, 1H), 7.72 (dd, (ESI) calcd for C15H11F3N1O3 (M + H)+ 310.0686, found
J ¼ 9.0, 2.5 Hz, 1H), 7.65 (d, J ¼ 9.1 Hz, 1H), 7.61 (d, J ¼ 2.9 Hz, 310.0682.
1H), 7.56 (d, J ¼ 9.0 Hz, 1H), 7.45 (dd, J ¼ 9.1, 2.9 Hz, 1H), 3.87
2-Methyl-7-(triuoromethoxy)acridin-9(10H)-one (2w).27 1H
(s, 3H); 13C NMR (DMSO-d6, 100 MHz) d 176.0, 154.8, 142.5, NMR (DMSO-d6, 400 MHz) d 11.93 (s, 1H), 8.05 (m, 2H), 7.73
139.5, 136.1, 127.3, 125.3, 122.0, 121.0, 120.4, 119.9, 119.4, (dd, J ¼ 6.4, 2.6 Hz, 1H), 7.65 (d, J ¼ 9.1 Hz, 1H), 7.62 (dd, J ¼ 8.5,
117.4, 105.1, 55.8; HRMS (ESI) calcd for C15H11F3N1O3 (M + H)+ 2.0 Hz, 1H), 7.49 (d, J ¼ 8.5 Hz, 1H), 2.43 (s, 3H); 13C NMR
310.0686, found 310.0681.
(DMSO-d6, 100 MHz) d 176.4, 142.5, 139.9, 139.4, 135.9, 131.4,
2-(4-Chlorophenoxy)-7-methoxyacridin-9(10H)-one (2p). 1H 127.5, 125.4, 122.0, 120.7, 120.3, 119.5, 118.0, 117.5, 21.0; HRMS
NMR (DMSO-d6, 400 MHz) d 11.89 (s, 1H), 7.68 (d, J ¼ 2.1 Hz, (ESI) calcd for C15H11F3N1O2 (M + H)+ 294.0736, found
1H), 7.58 (m, 3H), 7.43 (m, 4H), 7.08 (d, J ¼ 8.7 Hz, 2H), 3.84 (s, 294.0732.
3H); 13C NMR (DMSO-d6, 100 MHz) d 176.0, 156.5, 154.6, 150.7,
2-(Triuoromethoxy)benzo[b]acridin-12(5H)-one (2x). 1H
137.6, 136.1, 130.4 (2C), 127.7, 126.8, 125.1, 120.7, 120.6 (2C), NMR (DMSO-d6, 400 MHz) d 11.89 (s, 1H), 8.95 (s, 1H), 8.18 (d, J
120.5, 120.4, 119.8, 113.6, 105.0, 55.8; HRMS (ESI) calcd for ¼ 8.3 Hz, 1H), 8.09 (d, J ¼ 1.7 Hz, 1H), 8.02 (d, J ¼ 8.5 Hz, 1H),
C
20H15Cl1N1O3 (M + H)+ 352.0735, found 352.0731.
7.97 (s, 1H), 7.77 (dd, J ¼ 9.0, 2.7 Hz, 1H), 7.65 (d, J ¼ 9.2 Hz,
2-Methoxy-5-(triuoromethoxy)acridin-9(10H)-one (2q). 1H 1H), 7.60 (t, J ¼ 7.3 Hz, 1H), 7.45 (t, J ¼ 7.3 Hz, 1H); 13C NMR
NMR (DMSO-d6, 400 MHz) d 11.48 (s, 1H), 8.25 (t, J ¼ 7.3 Hz, (DMSO-d6, 100 MHz) d 178.0, 142.0, 141.3, 138.0, 136.4, 130.1,
1H), 7.92 (d, J ¼ 9.2 Hz, 1H), 7.81 (m, 1H), 7.63 (d, J ¼ 3.0 Hz, 129.2, 128.4, 127.8, 127.0, 124.9, 122.0, 121.0, 119.9, 119.5,
1H), 7.46 (dd, J ¼ 9.1, 3.0 Hz, 1H), 7.29 (t, J ¼ 8.0 Hz, 1H), 3.87 (s, 119.2, 118.0, 112.8; HRMS (ESI) calcd for C18H11F3N1O2 (M + H)+
3H); 13C NMR (DMSO-d6, 100 MHz) d 176.0, 155.1, 136.9, 136.0, 330.0736, found 330.0733.
133.9, 125.7, 125.2, 125.1, 122.1, 122.0, 121.6, 120.5, 120.4,
105.2, 55.9; HRMS (ESI) calcd for C15H11F3N1O3 (M + H)+
In vitro blood stage antimalarial activity assay
310.0686, found 310.0680.
In vitro antimalarial activity was determined by the Malaria
1
7-Methoxy-9-oxo-9,10-dihydroacridine-2-carbonitrile (2r). H SYBR Green I-based Fluorescence (MSF) assay described previ-
NMR (DMSO-d6, 400 MHz) d 12.16 (s, 1H), 8.55 (d, J ¼ 1.9 Hz, ously with minor modications, and expressed as the
1H), 7.98 (dd, J ¼ 8.8, 2.0 Hz, 1H), 7.61 (dd, J ¼ 6.6, 3.6 Hz, 2H), compound concentration inhibiting growth by 50% (IC50).
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46, 3109.
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Z. Xiao, S. K. Ballentine, Z. Shen, C. A. Fleener,
K. A. Rouleau, M. Obermeier, Z. Yang, K. W. McIntyre,
D. J. Shuster, M. Witmer, D. Dambach, S. Chao, A. Mathur,
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E. J. Iwanowicz, J. Med. Chem., 2007, 50, 3730.
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HepG2 cytotoxicity assay
The general cytotoxic effect of acridones on host mammalian
cells was assessed by functional assay as described previ-
ously, using hepatic HepG2 cells. Drugs were dissolved in
DMSO to make 10 mM stock solutions. Human hep-
atocarcinoma cells (HepG2) were maintained on RPMI-1640
medium supplemented with 10% fetal bovine serum at
37 ꢀC in a humidied 5% CO2 atmosphere. Cells were seeded
at a density of 2 ꢁ 104 per well in 96-well at-bottom tissue
culture plates containing complete medium in a total volume
ꢀ
of 160 mL per well. The cells were allowed to attach at 37 C
overnight. On the following day, drug solutions (40 mL per
well) were serially diluted with complete culture medium
across the plate. The plates were then incubated at 37 ꢀC and
5% CO2 for another 24–36 h. Aerward, the medium was
aspirated and replaced with complete RPMI medium (200 mL
per well), and the plates were incubated for an additional 24 h
¨
9 A. Stankiewicz-Drogon, B. Dorner, T. Erker and
A. M. Boguszewska-Chachulska, J. Med. Chem., 2010, 53,
3117.
10 I. B. Taraporewala, J. W. Cessac, T. C. Chanh, A. V. Delgado
and R. F. Schinazi, J. Med. Chem., 1992, 35, 2744.
11 J. X. Kelly, M. J. Smilkstein, R. Brun, S. Wittlin, R. A. Cooper,
K. D. Lane, A. Janowsky, R. A. Johnson, R. A. Dodean,
R. Winter, D. J. Hinrichs and M. K. Riscoe, Nature, 2009,
459, 270.
ꢀ
at 37 C and 5% CO2. An aliquot of a stock solution of resa-
zurin (Alamar Blue, prepared in 1 ꢁ PBS) was then added at
20 mL per well (nal concentration 10 mM), and the plates
were returned to the incubator for 3 h. Aer this period,
uorescence in each well, indicative of cellular redox activity
was measured in a Gemini EM plate reader with excitation
wavelength at 560 nm and emission wavelength at 590 nm.
IC50 values were determined by nonlinear regression analysis
of logistic concentration–uorescence intensity curves
(GraphPad Prism soware).
12 R. A. Dodean, P. Kancharla, Y. Li, V. Melendez, L. Read,
C. E. Bane, B. Vesely, M. Kreishman-Deitrick, C. Black,
Q. Li, R. J. Sciotti, R. Olmeda, T. L. Luong, H. Gaona,
B. Potter, J. Sousa, S. Marcsisin, D. Caridha, L. Xie,
C. Vuong, Q. Zeng, J. Zhang, P. Zhang, H. Lin, K. Butler,
N. Roncal, L. Gaynor-Ohnstad, S. E. Leed, C. Nolan,
S. J. Huezo, S. A. Rasmussen, M. T. Stephens, J. C. Tan,
R. A. Cooper, M. J. Smilkstein, S. Pou, R. W. Winter,
M. K. Riscoe and J. X. Kelly, J. Med. Chem., 2019, 62, 4375.
13 D. B. C. de Oliveira, L. B. Silva, B. V. da Silva, T. C. Borges,
B. C. Marques, M. B. Dos Santos, L. F. de Oliveira,
V. S. Bolzani, A. R. A. Rodrigues, L. O. Regasini and
A. A. Andrade, J. Appl. Microbiol., 2019, 127, 1362.
14 X. A. Li, H. L. Wang and S. D. Yang, Org. Lett., 2013, 15, 1794.
15 Z. Zheng, L. Dian, Y. Yuan, D. Zhang-Negrerie, Y. Du and
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
We thank the Oregon Health & Science University Medicinal
Chemistry Core Department (Portland, OR) for the use the
Biotage® Initiator+ microwave synthesizer. This project was
supported by NIH/NIAID (award 1R01AI093784) and DOD/
PRMRP (award PR160693/W81XWH-17-2-0041).
16 J. Wen, S. Tang, F. Zhang, R. Shi and A. Lei, Org. Lett., 2017,
19(1), 94.
17 H. Wu, Z. Zhang, Q. Liu, T. Liu, N. Ma and G. Zhang, Org.
Lett., 2018, 20, 2897.
18 J. Song, K. Ding, W. Sun, S. Wang, H. Sun, K. Xiao, Y. Qian
and C. Liu, Tetrahedron Lett., 2018, 59, 2889.
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