Synthesis of acridine-quinone systems - A potential electrochemical fluorescent switch

Article abstract of DOI:10.1080/00397910802154261

The synthesis of various acridine-quinones is reported. The fluorescence-quenching quinone moiety has been incorporated into various acridine molecules, each having a different functionality at the ninth position of the acridine ring. It is envisaged that

Full text of DOI:10.1080/00397910802154261

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Synthesis of Acridine–Quinone
Systems—A Potential
Electrochemical Fluorescent
Switch
Verity J. Litchfield a , Robert B. Smith b , Anthony M.
Franklin a & James Davis a
a Chemistry, School of Science and Technology,
Clifton Campus, Nottingham Trent University ,
Nottingham, UK
b Centre for Materials Science, University of Central
Lancashire , Preston, UK
Published online: 29 Sep 2008.
To cite this article: Verity J. Litchfield , Robert B. Smith , Anthony M. Franklin
& James Davis (2008) Synthesis of Acridine–Quinone Systems—A Potential
Electrochemical Fluorescent Switch, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 38:20, 3447-3455,
DOI: 10.1080/00397910802154261
To link to this article: http://dx.doi.org/10.1080/00397910802154261
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Synthetic Communications1, 38: 3447–3455, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802154261
Synthesis of Acridine–Quinone Systems—A
Potential Electrochemical Fluorescent Switch
Verity J. Litchfield,1 Robert B. Smith,2 Anthony M. Franklin,1 and
James Davis1
1Chemistry, School of Science and Technology, Clifton Campus,
Nottingham Trent University, Nottingham, UK
2Centre for Materials Science, University of Central Lancashire,
Preston, UK
Abstract: The synthesis of various acridine-quinones is reported. The
fluorescence-quenching quinone moiety has been incorporated into various
acridine molecules, each having a different functionality at the ninth position
of the acridine ring. It is envisaged that the quenching nature of the quinone will
lead to the fluorescent properties of the acridine being controlled in a way similar
to turning on a light switch.
Keywords: Acridine, fluorescence, oxidation, quinone, reduction
In our investigation into fluorescent electrochemical sensors, we have
synthesized several 1,4-acridinequinones. Our study has been focused
on a replacement for 1,4-napthoquinone (1), which is a well-known elec-
trochemical redox sensor,[1–3] in favor of a 1,4-acridinequinone (2) sys-
tem. The conjugated quinone moiety itself is a well-established
fluorescent quencher.[4,5] It is envisaged that upon Michael addition at
the second or third position of the acridine ring system, with a human
health biomarker such as glutathione, the conjugated system will undergo
a reduction to a 1,4-diol (3) species. Once in this state, fluorescence will
Received April 15, 2008.
Address correspondence to Robert B. Smith, Centre for Materials Science,
University of Central Lancashire, Preston, PR1 2HE, UK. E-mail:
RBSmith@UCLan.ac.uk
3447

3448
V. J. Litchfield et al.
Figure 1. 1,4-Naphthoquinone (1), 1,4-acridinequinone (2) and the formation of
a 1,4-diol system upon attachment of glutathione to 1,4-acridinequinone (3).
occur for a short period until air oxidation forces the 1,4-diol back to the
quinone. The aforementioned compounds are shown in Fig. 1.
The Ullmann–Goldberg condensation in DMF was employed using
1 equivalent of 2,5-dimethoxyaniline (4) to 1.5 equivalents of bromo-
benzoic acid (5) in the presence of potassium carbonate as base and
a catalytic amount of copper bronze. This yielded 70% of 2-(2,5-
dimethoxyphenyl)aminobenzoic acid (6), which was purified by an acid=
base wash. 1,4-Dimethylacrid-9-one (7) was formed in 59% yield upon
the reaction of 6 with polyphosphoric acid (PPA), followed by a careful
workup with ammonium hydroxide as in shown in Fig. 2. The reductive
aromatization of 7 yielded 1,4-dimethoxyacridine (8) at 27% using condi-
tions similar to those recorded by Lu et al.[6] and is shown in Fig. 3.
Chlorination of 7 using oxalyl chloride in the presence of a catalytic
amount of DMF yielded 64% of 1,4-dimethoxy-9-chloroacridine (9).
Heating 9 at 100 ꢀC in DMF in the presence of potassium carbonate
and phenol gave 1,4-dimethoxy-9-phenoxyacridine (10) as an orange
solid in 48% yield.[7] The formation of 1,4-dimethoxy-9-aminoacridine
(11) was recorded upon heating 9 in the presence of phenol and
ammonium carbonate at 80 ꢀC.[8] Oxidation of the 2,5-dimethoxy moi-
eties with cerium ammonium nitrate (CAN) gave the acridine-1,4-
quinones (12a–e).[9] The aforementioned synthetic route can be seen in
Figure 2. The Ullmann–Goldberg condensation to yield 2-(2,5-dimethoxyphenyl)-
aminobenzoic acid (6) followed by ring closure to yield the 1,4-methoxyacrid-
9-one (7).

Acridine–Quinone Systems
3449
Figure 3. Reductive aromatization yields the 1,4-methoxyacridinine (8) and
oxidation with CAN yields 1,4-acridininequinone (12a).
Fig. 4. The structures of compounds 6–12 were determined using NMR
spectroscopy and MS spectrometry data. The 13C spectroscopy data
for the 1,4-acridinequinones were not recorded because the samples were
Figure 4. The stepwise synthetic route to yield the acridinequinones (12b–12e):
(i) Oxalyl chloride=DMF, (ii) phenol=K2CO3, (iii) NH2(CO)3=phenol, (iv)
CAN=MeCN.

3450
V. J. Litchfield et al.
very dilute. As further evidence for the presence of the 1,4-acridinequi-
1
nones, no peaks were observed around 3–4 ppm in the H NMR spec-
trum. Also, there was no evidence of the methoxy moieties being
present when these compounds were studied using infrared spectroscopy.
An expected lack of fluorescence was also noted.
In summary, we have designed a convenient and efficient route for
the synthesis of a series of 1,4-acridine-quinones. This method can be
used if other substrates are required to be attached to the ninth position
of the acridine ring such as amino acids.
EXPERIMENTAL
NMR spectra were measured on a Jeol ECX 400-MHz spectrometer.
Chemical shifts are reported in parts per million (ppm) downfield from
tetramethylsilane (TMS). Mass spectra were recorded on a Micromass
Platform liquid chromatography mass spectrometry-electrospray ioniza-
tion (LCMS-ESI) and an Agilent gas chromatography-mass spectrometry
(GC-MS) (6850NGC and 5975XL-MSD). All chemicals and solvents
were bought from either Sigma Aldrich or Apollo Synthesis and used
without further purification.
2-(2,5-Dimethoxyphenyl)aminobenzoic Acid (6)
A mixture of 2,5-dimethoxyaniline (1.53 g, 10 mmol), 2-bromobenzoic
acid (3.01 g, 15 mmol), copper bronze (0.13 g, 2 mmol), and potassium
carbonate (2.76 g, 20 mmol) were refluxed in DMF (40 ml) for 6 h.
The brown solution was poured over ice and treated with sodium
hydroxide solution until the pH was 10. The brown solution was heated
to 80 ꢀC, activated charcoal was added, and after 5 min of stirring the
solution was filtered and treated with hydrochloric acid untill the pH
was 5. The green precipitate produced was filtered and dried to yield
2-(2,5-dimethoxyphenyl)aminobenzoic acid 6 (1.90 g, 70%) as a green
solid.
1H NMR (d6-DMSO): 3.68 (s, 3H, O-CH3), 3.75 (s, 3H, O-CH3),
6.55 (d, 1H, J ¼ 8.8 Hz, Ar-H), 6.77 (t, 1H, J ¼ 7.6 Hz, Ar-H), 6.96–
6.94 (m, 2H, Ar-H), 7.28 (d, 1H, J ¼ 8.5 Hz, Ar-H), 7.39 (t, 1H,
J ¼ 7.8 Hz, Ar-H), 7.90 (d, 1H, J ¼ 8.0 Hz, Ar-H), 9.64 (brs, 1H, COOH).
13C NMR (d6-DMSO): 55.32, 56.16, 106.17, 106.72, 112.51, 113.25,
114.09, 117.58, 130.34, 131.92, 134.15, 144.89, 146.19, 153.33, 169.76.
IR (ATR): 3319, 2885, 2824, 2640, 1576, 1026, 738. LCMS-ESI (m=z)
274 [M–H].

Acridine–Quinone Systems
3451
1,4-Dimethoxyacridin-9-one (7)
A mixture of 6 (20 g, 73 mmol) and polyphosphoric acid (60 ml) was
stirred at 120 ꢀC for 3 h. Upon cooling to 60 ꢀC, distilled water (100 ml)
was added slowly to yield a brown solution, which upon careful addition
of aqueous ammonium hydroxide produced a yellow precipitate. When
the solution achieved a pH of 8, the yellow solid was filtered to yield
1,4-dimethoxyacridin-9-one 7 (11.05 g, 59%) as a yellow solid, which
was dried under vacuum in a desiccator.
1H NMR (d6-DMSO): 3.77 (s, 3H, O-CH3), 3.94 (s, 3H, O-CH3),
6.55 (d, 1H, J ¼ 8.8 Hz, Ar-H), 7.16–7.20 (m, 2H, Ar-H), 7.61 (t, 1H,
J ¼ 7.7 Hz, Ar-H), 7.77 (d, 1H, J ¼ 8.4 Hz, Ar-H), 8.12 (d, 1H, J ¼ 8.2 Hz,
Ar-H), 10.83 (brs, 1H, NH). 13C NMR (d6-DMSO): 55.88, 56.34, 101.06,
111.72, 112.95, 117.72, 121.11, 122.34, 125.79, 132.57, 133.66, 139.73,
141.04, 153.70, 157.98. IR (ATR): 3319, 2885, 2824, 2640, 1576, 1026,
738. LCMS-ESI (m=z) 273 [M–H].
1,4-Dimethoxyacridine (8)
Zinc dust (13.33 g, 0.2 mol) in was added to a suspension of 7 (2 g,
7.8 mmol) in glacial acetic acid (235 ml) one portion. The reaction mix-
ture was refluxed for 6 h and a color change was observed (yellow to
red). The reaction mixture was cooled to room temperature and extracted
with ethyl acetate, and the organic phase was washed with a mixture of
saturated solution of NaHCO3 and distilled water. The organic layer
was removed, dried over sodium sulphate, and concentrated under
reduced pressure. The crude product was purified by column chromato-
graphy over silica gel with diethyl ether as eluent to yield 1,4-dimethox-
yacridine 8 (0.31 g, 27%) as a yellow solid.
1H NMR (CDCl3): 4.04 (s, 3H, O-CH3), 4.12 (s, 3H, O-CH3), 6.66
(d, 1H, J ¼ 8.2 Hz, Ar-H), 6.92 (d, 1H, J ¼ 8.3 Hz, Ar-H), 7.55 (t, 1H,
J ¼ 7.0 Hz, Ar-H), 7.78 (t, 1H, J ¼ 7.0 Hz, Ar-H), 8.03 (d, 1H, J ¼ 8.4 Hz,
Ar-H), 8.39 (d, 1H, J ¼ 8.8 Hz, Ar-H), 9.17 (s, 1H, Ar-H). 13C NMR
(CDCl3): 55.17, 56.09, 100.82, 105.79, 120.98, 125.09, 126.13, 128.38,
120.02, 130.18, 131.44, 142.06, 148.12, 148.80, 149.21. IR (ATR): 2831,
1733, 1628, 1579, 1531, 797. GCMS-EI (m=z) 239 [M–H].
1,4-Dimethoxy-9-chloroacridine (9)
Into a round-bottomed flask charged with 7 (5.10 g, 20 mmol) in toluene
(200 ml), DMF (20 drops) and oxalyl chloride (5.14 ml, 60 mmol) were

3452
V. J. Litchfield et al.
added. The reaction was heated at 60 ꢀC for 3 h, allowed to cool, and fil-
tered. Ammonium hydroxide (35%) was added to the cooled reaction mix-
ture with slight stirring until the solution went from purple to yellow. The
yellow solution was transferred into a separating funnel and washed with a
minimum amount of water until the organic layer was pH 7. The organic
layer was removed, dried with sodium sulphate, and evaporated to dryness
to yield 1,4-dimethoxy-9-chloroacridine 9 (3.5 g, 64%) as a yellow solid.
1H NMR (CDCl3): 3.98 (s, 3H, O-CH3), 4.10 (s, 3H, O-CH3), 6.80
(d, 1H, J ¼ 8.5 Hz, Ar-H), 6.91 (d, 1H, J ¼ 8.5 Hz, Ar-H), 7.63 (t, 1H,
J ¼ 7.7 Hz, Ar-H), 7.63 (t, 1H, J ¼ 7.7 Hz, Ar-H), 8.34 (d, 1H, J ¼ 8.7 Hz,
Ar-H), 8.58 (d, 1H, J ¼ 8.8 Hz, Ar-H). 13C NMR (CDCl3): 56.25, 56.51,
105.11, 106.03, 118.54, 124.82, 127.17, 128.17, 128.98, 130.24, 130.37,
140.10, 142.92, 147.48, 149.55. IR (ATR): 2960, 2935, 2837, 1623,
1456, 1076, 812, 757 GCMS-EI (m=z) 273 [M–H].
1,4-Dimethoxy-9-phenoxyacridine (10)
Into a round-bottomed flask charged with phenol (1.14 g, 15 mmol) and
potassium carbonate (2.07 g, 15 mmol), dry DMF (25 ml) was added, and
the mixture was stirred for 30 min under nitrogen. Upon the addition of 9
(1.36 g, 5 mmol), the reaction mixture was heated to 100 ꢀC for 6 h. The
reaction mixture was transferred to a separating funnel; dichloromethane
(DCM) (100 ml) was added. The mixture was washed with 1 M NaOH
(aq) (5 ꢁ 100 ml). The organic layer was removed, dried with sodium sul-
phate, and evaporated to dryness, leaving a brown solid. The addition of
ethyl acetate (50 ml) left an orange solid, which was filtered and air dried
to yield 1,4-dimethoxy-9-phenoxyacridine 10 (0.81 g, 48%).
1H NMR (CDCl3): 3.63 (s, 3H, O-CH3), 4.13 (s, 3H, O-CH3), 6.62
(d, 1H, J ¼ 8.4 Hz, Ar-H), 6.74 (d, 2H, J ¼ 8.6 Hz, Ar-H), 6.94 (d, 1H,
J ¼ 8.4 Hz, Ar-H), 6.99 (t, 1H, J ¼ 7.4 Hz, Ar-H), 7.24 (t, 2H, J ¼ 8.1 Hz,
Ar-H), 7.48 (t, 1H, J ¼ 7.6 Hz, Ar-H), 7.74 (t, 1H, J ¼ 7.7 Hz, Ar-H), 8.19
(d, 1H, J ¼ 8.7 Hz, Ar-H), 8.39 (d, 1H, J ¼ 8.8 Hz, Ar-H). NMR (CDCl3):
55.99, 56.19, 103.05, 106.30, 114.82, 121.55, 121.81, 122.73, 126.20,
129.43, 129.90, 130.65, 144.15, 148.87, 149.21, 149.39, 155.29, 159.94.
IR (ATR): 3045, 2832, 1626, 1488, 754. LCMS-ESI (m=z) 332 [M–H].
1,4-Dimethoxy-9-aminoacridine (11)
Into a round-bottomed flask charged with phenol (34.8 g, 0.37 mol) and
ammonium carbonate (0.8 g, 8.5 mmol), 9 (2 g, 7.5 mmol) was added. The
reaction mixture was heated at 80 ꢀC for 2 h. The reaction mixture was

Acridine–Quinone Systems
3453
transferred to a separating funnel; DCM (100 ml) was added. The mixture
was washed with 2M NaOH (aq) (10 ꢁ 100 ml). The organic layer was
removed, dried with sodium sulphate, and evaporated to dryness to
yield 1,4-dimethoxy-9-aminoacridine 11 (1.46 g, 79%) as a feathery yellow
solid.
1H NMR (CDCl3): 3.99 (s, 3H, O-CH3), 4.04 (s, 3H, O-CH3), 6.49
(d, 1H, J ¼ 8.4 Hz, Ar-H), 6.81 (d, 1H, J ¼ 8.4 Hz, Ar-H), 6.96 (br s,
2H, NH2), 7.35 (t, 1H, J ¼ 7.6 Hz, Ar-H), 7.65 (t, 1H, J ¼ 7.7 Hz,
Ar-H), 7.84 (d, 1H, J ¼ 8.5 Hz, Ar-H), 8.16 (d, 1H, J ¼ 8.5 Hz, Ar-H).
NMR (CDCl3): 55.46, 55.82, 98.82, 105.48, 106.90, 113.89, 122.00,
122.74, 128.76, 129.93, 142.70, 147.41, 148.86, 151.09, 151.47. IR
(ATR): 3446, 2831, 1636, 1534, 1374, 786. LCMS-ESI (m=z) 255 [M–H].
Preparation of 1,4-Acridinequinones (12a) (General Procedure)
Into a round-bottom flask charged with 1,4-dimethoxyacridine (8)
(4 mmol) and cerium ammonium nitrate (CAN) (15 mmol), acetonitrile
(40 ml) and water (20 ml) were added. The solution was stirred for
20 min at 0 ꢀC and then poured into brine and extracted with chloroform.
The organic layer was removed, concentrated under reduced pressure,
and purified on silica gel using chloroform as eluent. The first fraction
was collected and evaporated to dryness to yield a bright red solid, which
was the 1,4-acridinequinone.
Data
1,4-Acridinequinone (12a)
Yielded 83% as a red solid. 1H NMR (CDCl3): 7.19 (AB-quart,
1H, J ¼ 10.5 Hz, Ar-H), 7.30 (AB-quart, 1H, J ¼ 11.0 Hz, Ar-H), 7.78
(t, 1H, J ¼ 7.6 Hz, Ar-H), 7.97 (t, 1H, J ¼ 7.5 Hz, Ar-H), 8.10 (d, 1H,
J ¼ 8.1 Hz, Ar-H), 8.47 (d, 1H, J ¼ 8.6 Hz), 9.00 (s, 1H Ar-H). LCMS-
ESI (m=z) 210 [M–H].
Acridine-1,4,9-trione (12b)
1
Yielded 45% as a red solid. H NMR (d6-DMSO): 6.80 (AB-quart, 1H,
J ¼ 9.8 Hz, Ar-H), 7.03 (AB-quart, 1H, J ¼ 9.3 Hz, Ar-H), 7.46 (t, 2H,
J ¼ 7.2 Hz, Ar-H), 7.75 (t, 1H, J ¼ 7.0 Hz, Ar-H), 8.06 (d, 1H, J ¼ 6.7 Hz,
Ar-H), 8.15 (d, 1H, J ¼ 6.4 Hz), 12.36 (brs, 1H, NH). LCMS-ESI (m=z)
226 [M–H].

3454
V. J. Litchfield et al.
9-Chloro-1,4-acridinequinone (12c)
1
Yielded 88% as a red solid. H NMR (CDCl3): 7.15 (AB-quart, 1H,
J ¼ 10.6 Hz, Ar-H), 7.23 (AB-quart, 1H, J ¼ 10.4 Hz, Ar-H), 7.88 (t, 2H,
J ¼ 7.2 Hz, Ar-H), 8.00 (t, 1H, J ¼ 7.0 Hz, Ar-H), 8.47 (d, 1H, J ¼ 8.5 Hz,
Ar-H), 8.62 (d, 1H, J ¼ 8.6 Hz). LCMS-ESI (m=z) 244 [M–H].
9-Phenoxy-1,4-acridinequinone (12d)
Yielded 50% as a red solid. 1H NMR (CDCl3): 6.87 (d, 2H, J ¼
7.8 Hz, Ar-H), 6.94 (AB-quart, 1H, J ¼ 10.4 Hz, Ar-H), 7.09 (t,
1H, J ¼ 7.4 Hz, Ar-H), 7.18 (AB-quart, 1H, J ¼ 10.4 Hz, Ar-H), 7.31 (t,
2H, J ¼ 8.1 Hz, Ar-H), 7.72 (t, 1H, J ¼ 7.1 Hz, Ar-H), 7.98 (t, 1H, J ¼
7.0 Hz, Ar-H), 8.27 (d, 1H, J ¼ 8.5 Hz, Ar-H), 8.50 (d, 1H, J ¼ 8.6 Hz).
LCMS-ESI (m=z) 302 [M–H].
9-Amino-1,4-acridinequinone (12e)
1
Yielded 11% as a brown solid. H NMR (CDCl3): 7.01 (AB-quart, 1H,
J ¼ 10.6 Hz, Ar-H), 7.05 (AB-quart, 1H, J ¼ 10.5 Hz, Ar-H), 7.65 (t, 1H,
J ¼ 7.8 Hz, Ar-H), 7.85 (t, 1H, J ¼ 7.5 Hz, Ar-H), 7.94 (d, 1H, J ¼ 8.2 Hz,
Ar-H), 8.30 (d, 1H, J ¼ 8.3 Hz). LCMS-ESI (m=z) 244 [M–H].
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