Acid-promoted synthesis and photophysical properties of substituted acridine derivatives

Article abstract of DOI:10.1039/d0ob01824d

A simple and efficient synthetic protocol for the preparation of acridinium esters and amides through the cyclization and esterification or amidation of isatins with alcohols or amines as nucleophiles in the presence of CF3SO3H is established. A series of polycyclic acridine derivatives bearing large π-conjugated systems were obtained in high yields, including some key intermediates for the synthesis of biologically active molecules. The photophysical properties of these synthesized acridines were investigated, demonstrating that the sulfur heterocyclic acridine 9w was obtained in a high quantum yield. This journal is

Full text of DOI:10.1039/d0ob01824d

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Biomolecular Chemistry
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Acid-promoted synthesis and photophysical
properties of substituted acridine derivatives†
Cite this: DOI: 10.1039/d0ob01824d
Zi-Long Bian, Xin-Xin Lv, Ya-Lan Li, Wen-Wu Sun, * Ji-Kai Liu* and Bin Wu
*
A simple and efficient synthetic protocol for the preparation of acridinium esters and amides through the
cyclization and esterification or amidation of isatins with alcohols or amines as nucleophiles in the pres-
ence of CF3SO3H is established. A series of polycyclic acridine derivatives bearing large π-conjugated
systems were obtained in high yields, including some key intermediates for the synthesis of biologically
active molecules. The photophysical properties of these synthesized acridines were investigated, demon-
strating that the sulfur heterocyclic acridine 9w was obtained in a high quantum yield.
Received 3rd September 2020,
Accepted 24th September 2020
DOI: 10.1039/d0ob01824d
rsc.li/obc
applications in materials science, the photophysical properties
of these synthesized acridines were investigated.
Introduction
Acridine derivatives exhibit many biological activities, exempli-
fied by compounds 1–6 in Fig. 1, and have therefore been
attracting the attention of organic chemists.1,2 Due to their
importance in human health care and many other appli-
cations, significant effort has been devoted to developing
efficient synthetic methodologies for the preparation of acri-
dine derivatives over the past few decades.3 Most methods for
the synthesis of acridines involve several types of ring closure
reactions, including the reaction of arynes with a number of
partners, such as 2-aminoaryl ketones and hydrazones.4 The
cross-coupling reactions of aromatic amines or azide com-
pounds with suitable partners have provided various substi-
tuted acridines under catalysis by Pd or Rh.5 The intra-
molecular electrophilic cyclization of diphenylamine-2-carbo-
nyls or 2,6-diaryl-3,5-dialkynylpyridines has also led to the
formation of acridines.6 A thermal rearrangement reaction of
indazolium salts afforded 9-amino-substituted acridines.7
Although there are some reports on the synthesis of acridine-9-
carboxylic acids by the treatment of isatin with a base, both
substrate scope and functional group tolerance are very
limited.8 Here, we report an efficient, one-step synthetic
method to synthesize various 9-ester or amide substituted
acridines from commercially available isatins in the presence
of an acid promoter. Furthermore, to explore their potential
Results and discussion
We first examined the reaction of 1-phenylisatin 7a in the pres-
ence of 2.0 equivalents of BF3·Et2O in MeOH at 100 °C for
36 h. To our delight, the methyl acridine-9-carboxylate 9a was
obtained in 10% yield (Table 1, entry 1), and its structure was
unambiguously confirmed by X-ray diffraction analysis.9 We
then investigated the solvent effect on the reaction and found
that 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was the best
solvent (Table 1, entries 2–5). When the amount of BF3·Et2O
was increased to 5.0 equivalents, the yield of product 9a
increased from 40% to 54% (Table 1, entries 5 and 6). Next, we
tested some other Lewis acids in the reaction. All three Lewis
acids (Al(OTf)3, p-TsOH·H2O and CF3SO3H) promoted the reac-
tion. The reaction in the presence of 7.0 equivalents of
p-TsOH·H2O or 2.0 equivalents of CF3SO3H afforded the acri-
School of Pharmaceutical Sciences, South-Central University for Nationalities,
Wuhan 430074, China. E-mail: 2016027@mail.scuec.edu.cn, jkliu@mail.kib.ac.cn,
2015084@mail.scuec.edu.cn
†Electronic supplementary information (ESI) available: All experimental details,
details of chemical synthesis, additional figures, tables and crystallographic
data. CCDC 1960437 and 1968988. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/d0ob01824d
Fig. 1 Biologically active compounds with acridinium ester and amide
derivatives.
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Table 1 Optimization of the reaction conditionsa
Table 2 Substrate scopea
Lewis acid
(equiv.)
T
(°C)
Time
(h)
Yield of 9a
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
MeOH
TCE
BF3·Et2O/2.0
BF3·Et2O/2.0
BF3·Et2O/2.0
BF3·Et2O/2.0
BF3·Et2O/2.0
BF3·Et2O/5.0
Al(OTf)3/5.0
p-TsOH·H2O/5.0
p-TsOH·H2O/7.0
CF3SO3H/5.0
CF3SO3H/2.0
CF3SO3H/1.0
CF3SO3H/2.0
None
100
100
100
100
100
100
100
100
100
100
100
100
80
36
36
36
36
36
36
36
24
36
24
12
24
36
36
10
13
5
MeCN
Toluene
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
16
40
54
65
81
94
85
84
62
51
0
9
10
11
12
13
14
100
a Reaction conditions: 7a (0.1 mmol) and MeOH (10.0 equiv.) in
solvent (1.0 mL). Isolated yield.
a Reaction conditions: 7 (0.10 mmol), 8a (1.00 mmol), CF3SO3H
(0.20 mmol), and HFIP (1.0 mL), 100 °C, 12 h. Isolated yield. b The
reaction time was 48 h.
dine 9a in 94% and 84% yields, respectively (Table 1, entries 9
and 11).10 The yield of product 9a decreased with lower temp-
erature (Table 1, entry 13). The reaction did not occur in the
absence of an acid (Table 1, entry 14). Considering the
different efficiencies of p-TsOH.H2O and CF3SO3H, 2.0 equiva-
lents of CF3SO3H at 100 °C for 12 h were chosen as the
optimal conditions.
Table 3 Substrate scopea
With the optimized conditions in hand, we investigated the
substrate scope of the reaction. Various electron-withdrawing
or electron-donating groups were introduced into the benzene
ring of the isatin scaffold (Table 2). The products 9b–9d
bearing Me or OMe groups were obtained in 49–79% yields. A
series of halogen substituted products 9e–9k were readily pre-
pared in good to excellent yields, which provided a good basis
for the synthesis of more complex acridine esters by transition-
metal catalyzed cross-coupling reactions. The reaction of the
isatin compounds 7l and 7m with NO2 and CO2Me groups sub-
stituted at the 5-position afforded the products 9l and 9m in
86% and 87% yields, respectively.
N-Aryl substituted isatins 7n–7w were synthesized via
copper-mediated cross coupling of isatin with various aryl-
boron reagents. The reaction of the starting materials 7n–7w
under the standard conditions proceeded well to afford the
acridines 9n–9w bearing a large π-conjugated system in good
yields up to 87%. Interestingly, we used two routes to prepare
the acridine product 9g in 79% and 53% yields, respectively
(Table 3).
a Reaction conditions: 7 (0.10 mmol), 8a (1.00 mmol), CF3SO3H
(0.20 mmol), and HFIP (1.0 mL), 100 °C, 12 h. Isolated yield.
city of isopropanol, product 9ad was obtained in moderate
To expand the utility of this reaction, the scope of nucleo- yield (Table 4, entries 1–3). Compound 7a reacted with chlor-
philes including alcohols and alkylamines was explored oalcohol 8g under the standard conditions to afford acridine-
(Table 4). The reaction of 7a with primary alcohols 8b and 8c 9-ester 9ag, which was the key precursor for the preparation of
proceeded smoothly to afford the corresponding products 9ab bioactive molecule 2 (Scheme 1). Furthermore, glycol 8h, with
and 9ac in good yields. Probably due to the lower nucleophili- two hydroxyl groups, worked well to provide product 9ah,
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Table 4 Substrate scopea
Entry
1
8
9
Yield (%)
63
Scheme 2 Formal synthesis of platinum(II) complexes 4 and 6a.
2
3
70
39b
4
5
66c
57
Scheme 3 Transformations of acridinium ester 9a.
10 in 76% yield.11 Compound 9a was totally oxidized to 11 in
quantitative yield.12 The reaction of 9a with Lewis acid
BF3·Et2O led to 12 in 90% yield.13 In addition, 9a was reduced
to 13 in excellent yield.
In recent years, acridine derivatives emerge as one of the
important fluorescent probes for metal ions and anions.14
Although the biotinylated fluorophore (2-biotinyloxyethyl)acri-
dine-9-carboxylate (6a) and its analogue (2-biotinylamidoethyl)
acridine-9-carboxamide (6b) were synthesized as fluorescent
materials and labels in bioanalytical applications, little has
been studied on their fluorescence properties based on acri-
dine skeletons.2f,g Thus, the photophysical properties of com-
pounds 9a–9ah, 10 and 11 were investigated by UV/Vis absorp-
tion and fluorescence spectroscopic techniques. The typical
absorption and emission spectral data of these compounds
were recorded in dichloromethane, as shown in Table 5, and
the fluorescence spectra of 9c, 9d, 9q, 9s, 9u–9w and 10 are
shown in Fig. 2. In the absorption spectra, the acridinium
ester and amide derivatives show an absorption band at
355–460 nm, where the position of the absorption depends on
the substitution pattern on the benzene ring. The absorption
a Reaction conditions: 7a (0.10 mmol), 8 (1.00 mmol), CF3SO3H
(0.20 mmol), and HFIP (1.0 mL), 100 °C, 12 h. Isolated yield. b The
reaction time was 24 h. c In the case of 9ae, 7a (0.10 mmol), 8e
(1.00 mmol), TsOH·H2O (0.70 mmol), and HFIP (1.0 mL), 100 °C, 24 h.
Scheme 1 Formal synthesis of
a
competitive inhibitor of
acetylcholinesterase.
bearing a free hydroxyl group at its terminal end. Compound bands of the acridine-9-carboxylic acid derivatives increase
9ah could be used to prepare biologically active platinum(II) with the increase of the extended π-conjugated system. The
complex 4 and 6a (Scheme 2). A primary alkylamine was also emission wavelength and fluorescence quantum yield of all
suitable as a nucleophile for this reaction. For example, the acridine-9-carboxylic acid derivatives were tested to explain the
reaction of 7a with benzylamine 8e and aminoalcohol 8f in the substituent effects. Most substituent groups at different posi-
presence of CF3SO3H produced 9ae and 9af in 66% and 57% tions of the acridine ring lead to a certain redshift of emission
yields, respectively (Table 4, entries 4 and 5). To further illus- wavelengths. Alkyl, halogen, nitro and other substituent
trate the applications of this type of acridine compound, four groups have little influence on the emission wavelength and
other derivative reactions starting from 9a were conducted fluorescence quantum yield. With the further enhancement of
(Scheme 3). An N-methylation reaction was achieved to provide the π-conjugate system and rigid plane, an obvious redshift of
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Table 5 Photophysical properties of acridinium ester and amide
derivativesa
Absorption
Fluorescence
ε c/cm−1 M−1
λFLmax c/nm
ФFL
d,e
Compd
λmax b/nm
9a
9b
9c
9d
9e
9f
9g
9h
9i
9j
9k
9l
9m
9n
9o
9p
9q
9r
9s
9t
9u
9v
9w
9ab
9ac
9ad
9ae
9af
9ag
9ah
10
11
362
364
356
355
361
366
367
364
368
366
371
367
366
362
364
376
370
391
392
380
460
397
392
362
362
362
361
361
362
362
367
446
9797
4850
6801
7377
5167
10 954
8709
4051
6601
5864
13 832
5188
4919
3088
5359
14 495
6040
446
453
483
475
444
449
446
440
447
448
444
476
455
468
455
475
507
450
449
457
561
470
522
445
444
445
444
446
446
446
520
457
0.013
0.037
0.254
0.131
0.025
0.013
0.016
0.013
0.013
0.001
0.050
0.007
0.014
0.038
0.039
0.082
0.451f
0.033
0.144
0.041
0.205f
0.245
0.813f
0.016
0.018
0.026
0.017
0.015
0.049
0.002
0.600
0.031
Scheme 4 Plausible mechanism.
yields of 13–81%. Notably, sulfur heterocyclic acridine 9w
shows the highest quantum yield among these compounds.
Based on these primary investigations and results from
the literature,6b a mechanism can be proposed, as shown in
Scheme 4. Intermediate B was formed by protonation of A, fol-
lowed by an intramolecular nucleophilic attack of benzene on
the ketone moiety at the 3-position of isatin to afford inter-
mediate C. C could be converted to D by esterification with
MeOH, followed by a proton transfer and dehydration reaction
to give methyl acridine-9-carboxylate F.
5219
10 182
11 735
10 154
8500
10 331
5611
9853
15 626
9680
9843
12 186
6897
11 668
18 885
Experimental
General procedure for the preparation of substituted isatin 7 15
A mixture of the substrate (1.0 equiv.), arylboronic acid
(2–3 equiv.), anhydrous Cu(OAc)2 (1–2 equiv.), and triethyl-
amine or pyridine (2–3 equiv.) in dichloromethane (10–12 mL/
0.5 g of substrate) was stirred at room temperature for 24–72 h.
The progress of the reaction was monitored by TLC. The pro-
ducts were isolated by direct flash column chromatography of
the crude reaction product.
a In CH2Cl2. b Longest wavelength absorption maximum is shown.
c Excited at the wavelength of the absorption maximum. d Fluorescence
quantum yield. e Determined with quinine sulfate as a standard.
f Determined with Rh6G as a standard.
Typical procedure for the products 9a–w and 9ab–ah
Substituted isatin 7 (0.1 mmol), CF3SO3H (30.0 mg, 0.2 mmol),
alcohol or amine (1.0 mmol) and HFIP (1 mL) were added in
turn into a 10 mL glass sealed tube. The tube was tightly
capped. The reaction mixture was stirred at 100 °C for 12 h.
After cooling down to room temperature, the reaction mixture
was diluted with ethyl acetate, washed with saturated NaHCO3,
dried over anhydrous Na2SO4, and concentrated. Purification
by thin layer chromatography afforded the products 9a–w and
9ab–ah.
Typical procedure for the products 11
To a stirred solution of methyl acridine-9-carboxylate (23.7 mg,
0.1 mmol) in CHCl3 (1 mL) was added 70% m-CPBA (17.3 mg,
0.1 mmol) portionwise at 0 °C. The mixture was stirred at
room temperature for 12 h. After complete consumption of the
starting material was observed by TLC, the reaction mixture
was diluted with CHCl3, and solid K2CO3 (4.0 equiv.) was
Fig. 2 Fluorescence spectra of 9c, 9d, 9q, 9s, 9u–9w and 10 in CH2Cl2.
the emission band and a significant increase of quantum yield added. The resulting mixture was stirred for an additional
can be observed. The fluorescence spectra of 9c, 9d, 9q, 9s, 10 min and then washed with water three times. The organic
9u–9w and 10 show emissions at 440–561 nm with quantum layer was separated and dried with anhydrous Na2SO4, filtered,
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and concentrated. The crude product was purified by flash
chromatography to afford product 11 (26.1 mg, 100% yield).
Notes and references
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and M. Sharma, Medicinal chemistry of acridine and its
analogues, MedChemComm, 2018, 9, 1589–1618.
Typical procedure for the products 12
To a stirred solution of methyl acridine-9-carboxylate (23.7 mg,
0.1 mmol) in Et2O (1 mL) was added BF3·OEt2 (42.6 mg,
0.3 mmol) portionwise at room temperature. Product 12
(27.5 mg, 90% yield) was obtained after filtration.
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Typical procedure for the products 13
Methyl acridine-9-carboxylate (47.4 mg, 0.2 mmol) was dis-
solved in anhydrous THF (3 mL), and LiAlH4 (1 M in THF,
0.25 mL) was added dropwise under an inert atmosphere. The
reaction mixture was slowly heated to 80 °C and maintained at
80 °C for 3 h. The reaction was then quenched by the addition
of 15% aq. NaOH. The resulting precipitate was filtered off.
Then the filtrate was diluted with EtOAc, washed with water
and saturated aq. NaHCO3, and dried with anhydrous Na2SO4.
Purification of the crude product by column chromatography
(ethyl acetate/hexane, 1 : 4) yielded the alcohol as a pale, yellow
solid (41.0 mg, 97%).
Conclusions
In summary, we developed an efficient CF3SO3H-promoted
synthesis of substituted acridinium ester and amide deriva-
tives in good to excellent yields and with high functional
group tolerance. Large π-conjugated systems with polycyclic
aromatic hydrocarbons were obtained. This method was
applied to the formal synthesis of biologically active com-
pounds with 9-ester acridines as the key intermediates. The
photophysical properties of these acridine compounds were
investigated, indicating that the sulfur heterocyclic acridine 9w
was obtained at a high quantum yield, which may provide
some useful information for exploring potential applications
in materials science in the future.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (no. 21772236). We thank Prof. Dan Zhao (South-
Central University for Nationalities) for her useful discussion
about the spectral data analysis. We thank the Analytical &
Measuring Center, School of Pharmaceutical Sciences (South-
Central University for Nationalities) for collection of the spec-
tral data.
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