Efficient and mild deamination procedure for 1-aminoanthraquinones yielding a diverse library of novel derivatives with potential biological activity

Article abstract of DOI:10.1016/j.tetlet.2012.09.011

A convenient in situ method is described for reductive removal of the amino group in position 1 of the anthraquinone (AQ) moiety. The reaction proceeds smoothly within a few minutes yielding novel AQ derivatives in excellent yields. Diazonium salt formation is followed by reduction with zinc in ethanol. The method has been applied to a variety of 1-amino-AQ derivatives. It allows access to a large library of new AQ derivatives which possess potential as pharmacological tools for studying purinergic signaling, and as potential drugs, for example, for the treatment of cancer and cardiovascular diseases.

Full text of DOI:10.1016/j.tetlet.2012.09.011

Tetrahedron Letters 53 (2012) 6739–6742
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient and mild deamination procedure for 1-aminoanthraquinones yielding
a diverse library of novel derivatives with potential biological activity
Younis Baqi a,b, , Christa E. Müller a
⇑
a PharmaCenter Bonn, Pharmaceutical Institute, Pharmaceutical Chemistry I, Pharmaceutical Sciences Bonn (PSB), University of Bonn, An der Immenburg 4, D-53121 Bonn, Germany
b Department of Chemistry, Faculty of Science, Sultan Qaboos University, PO Box 36, Postal Code 123, Muscat, Oman
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2012
Revised 4 September 2012
Accepted 5 September 2012
Available online 11 October 2012
A convenient in situ method is described for reductive removal of the amino group in position 1 of the
anthraquinone (AQ) moiety. The reaction proceeds smoothly within a few minutes yielding novel AQ
derivatives in excellent yields. Diazonium salt formation is followed by reduction with zinc in ethanol.
The method has been applied to a variety of 1-amino-AQ derivatives. It allows access to a large library
of new AQ derivatives which possess potential as pharmacological tools for studying purinergic signaling,
and as potential drugs, for example, for the treatment of cancer and cardiovascular diseases.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Anthraquinone
Deamination
Cancer
Cardiovascular
1-Aminoanthraquinone (1-amino-AQ) derivatives (1–5, Fig. 1)
are an important class of dyestuffs used for the coloring of textiles,
including natural and synthetic fibers, and there is a continuous
interest in optimizing this class of compounds.1–4 A wide range
of blue colors can be obtained depending on the substitution pat-
tern of the 1-amino-AQ scaffold. 1-Amino-4-bromo-2-sulfoanthr-
aquinone (6, bromaminic acid) is a key intermediate for the
preparation of 1-amino-AQ derivatives. The presence of an amino
group in the 1-position of the anthraquinone moiety along with
a p-substituted amino function appears to be essential for obtain-
ing the blue color. Syntheses of derivatives of the blue dyes which
are lacking the amino group in the 1-position of the anthraquinone
moiety have not been investigated yet.
In recent years, the AQ dye Reactive Blue 2 (1, RB-2 Fig. 1) as
well as other related AQ derivatives, including Acid Blue 25 (4)
and Acid Blue 129 (5) have been found to interact with nucleo-
tide-binding proteins.5,6 Reactive Blue 2 (1) is the classical non-
selective purine P2 receptor antagonist.5,6 Furthermore, RB-2
inhibits ectoenzymes, for example, ecto-50-nucleotidase (eN,
CD73),7 ecto-nucleoside triphosphate diphosphohydrolase (E-NTP-
Dase1,2,3)8 and nucleotide pyrophosphatases (NPP1 and NPP3).9
These ectoenzymes are hydrolyzing extracellular nucleotides (P2
receptor ligands) eventually forming the respective nucleosides,
particularly the P1 receptor ligand adenosine.10–12 Recently we
have developed new synthetic methodologies to easily access 1-
amino-substituted 4-alkyl-, 4-aryl-, and 4-aralkyl-AQ deriva-
tives.13–15 Within this class of compounds, we identified and
successfully optimized potent and selective P2 receptor antago-
nists,16–19 and ecto-nucleotidase inhibitors.7,8 We found that a
negatively charged group, for example, sulfonate or carboxylate,
in the 2-position of the anthraquinone moiety is essential for the
activity of the compounds, while replacing it by a neutral group
such as methyl abolished the activity toward all nucleotide-
binding targets that have been tested so far.7,8,16–19 The potency
and target-selectivity of the synthesized compounds were found
to be highly dependent on the substituent present in the 4-position
of the AQ core.5–8,13,14,16–19
So far, synthetic efforts in the field have been mainly directed
toward 4-aryl- or 4-(ar)alkyl-amino-substituted 1-amino-AQ
derivatives. While the 1-amino group appears to be required for
obtaining blue colored compounds, its role for interaction with po-
tential drug targets is yet unknown. 1-Amino-AQ derivatives are
easily accessible in large amounts and high yields by microwave-
assisted Ullmann coupling reaction of the commercially available
1-amino-AQ derivative bromaminic acid (BAA, 6, Fig. 1), or its bio-
isosteric analog 7, respectively, with the appropriate amines.13,14 In
the present study we developed a method for obtaining a library of
AQ derivatives lacking the 1-amino function in order to allow for
investigating the role of the amino group with regard to the biolog-
ical activity of the AQ derivatives.
In the course of our investigations on the structure–activity
relationships of AQ derivatives as P2 receptor antagonists and ecto-
nucleotidase inhibitors we observed that the significance of the
amino group in the 1-position on their biological activity was un-
known. The amino group is present in all commercially available
⇑
Corresponding author. Tel.: +968 2414 2347; fax: +968 2414 1469.
E-mail addresses: younis.baqi@uni-bonn.de, baqi@squ.edu.om (Y. Baqi).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.011

6740
Y. Baqi, C. E. Müller / Tetrahedron Letters 53 (2012) 6739–6742
O
O
NH2
O
NH2
O
O
NH2
SO3Na
SO3Na
SO3Na
1
2
3
4
HN
SO3Na
NH
O
HN
HN
SO3Na
N
SO3Na
N Cl
HN
Cl
HN
N
N
N
N
N
N
Cl
N
NH
Cl
HN
SO3Na
SO3Na
m,p-sulfonate
1, Reactive Blue 2 (RB 2)
2
3
, Reactive Blue 4 (RB 4)
, Reactive Blue 5 (RB 5)
O
O
NH2
O
NH2
O
O
NH2
R
SO3Na
SO3Na
CH3
HN
O
HN
Br
H3C
5
, Acid Blue 129 (AB-129)
CH3
6, R = SO3Na: bromaminic acid (BAA)
7, R = CO2H : 1-amino-4-bromo-2-carboxyanthraquinone
4
, Acid Blue 25 (AB-25)
Figure 1. Structures of some commercially available anthraquinone dyes (1–5) and 4-bromoanthraquinone derivatives (6–7) used for Ullmann coupling reactions.
AQs that have been discovered to antagonize P2 receptors and/or
inhibit ectonucleotidases (for examples see Fig. 1). It might be re-
quired as a hydrogen bond donor for interacting with the target
proteins.
addition of zinc, ice, and ethanol, and the mixture was left stirring
for 2 h at 110 °C.22 However, only ca. 1% of the desired product was
detected by HPLC-MS. The main product was 2-sulfoanthraqui-
none (80%). Two isomers of hydroxy-2-sulfoanthraquinone were
also detected, probably the 1-hydroxy- and the 4-hydroxy-2-
sulfoanthraquinone (for more details see Supplementary data).
After unsuccessful attempts to synthesize de-amino-BAA (6) and
the de-amino derivatives of its bioisosteric analog (7) we decided
to deaminate the final products, the 4-(ar)alkylamino- and anili-
no-substituted anthraquinone derivatives. At first we explored
the effects of some non-noble metals, namely nickel (Ni), copper
(Cu), and zinc (Zn), on the reductive deamination of 1-amino-4-
(2-methoxyphenyl)-2-sulfoanthraquinone (8r, PSB-0716, Table 1),
a potent P2Y2 receptor antagonist.16 Ten equivalents of the applied
Bromaminic acid (6) and 1-amino-4-bromo-2-carboxyanthr-
aquinone (7) are key intermediates in the preparation of important
AQ derivatives, and patents have been filed describing their prep-
aration even on a large scale. However, the syntheses of BAA ana-
logs or RB-2 analogs that are lacking the amino group have so far
not been described in the literature. This may be due to difficulties
in the synthesis including regioselective bromination and/or sulfo-
nation of the anthraquinone core. We tried to synthesize a deriva-
tive of compound 7 which lacks the amino group. Carboxylate 7
was selected as a synthetic target rather than sulfonate 6 since in
medicinal chemistry a carboxylate group is a bioisosteric replace-
ment for a sulfonate group. Both functional groups share similar
physicochemical properties resulting in potentially similar biolog-
ical activities, but a carboxylate function confers more drug-like
properties, for example, improved bioavailability, to the compound
due to its higher pKa value as compared to the sulfonate group.20,21
We started our synthesis from commercially available 2-carbo-
xyanthraquinone and tried to selectively brominate the 4-position
by applying several bromination reagents, but all of our attempts
failed, mainly due to solubility problems. We therefore synthesized
the ethyl ester of 2-carboxyanthraquinone, thereby solving the sol-
ubility problem. However, bromination in position 4 of the anthra-
quinone moiety could not be achieved in a regioselective manner
(for more details see Supplementary data). It is much easier to bro-
minate 1-amino-2-sulfo- (or 2-carboxy) anthraquinone to obtain 6
(or 7) in excellent yields because the presence of the amino group
in position 1 of the anthraquinone core will direct the bromination
toward the 4-position, while sulfonate or carboxylate functions
deactivate the aromatic system because of their electron-
withdrawing properties.
Table 1
Exploring the impact of non-noble metals on the reductive deamination of PSB-0716
(8r)a
O
NH2
O
SO3Na
OMe
SO3H
OMe
1. NaNO2, HCl (1M)
0-5 °C, 5 min
2. metal, ethanol, rt
O
HN
O
HN
8r
10r
Entry
Metal
Metal equiv
Reaction time (step 2)
Yieldb
1
2
3
4
5
Ni
Cu
Zn
Zn
—
10
10
10
5
30 min
15 min
30 s
3 min
7 days
75
77
86
81
0
—
a
Reaction conditions: PSB-716 (45 mg, 0.1 mmol) was dissolved in 5 mL of 1 M
HCl then cooled down to 0-5 °C in an ice bath. Then portion-wise addition of NaNO2
(14 mg, 0.2 mmol) dissolved in water (0.5 mL) was added and the mixture was
stirred for 5 min. It was then warmed up to rt followed by the addition of the metal
and ethanol (5 mL) and left stirring at rt.
We subsequently aimed at deaminating compound 6 (BAA,
Fig. 1) to obtain the desired 4-bromo-2-sulfoanthraquinone. First
we diazotized the amino function using concentrated sulfuric acid
and sodium nitrite in the presence of water, followed by the
b
Isolated yield.

Y. Baqi, C. E. Müller / Tetrahedron Letters 53 (2012) 6739–6742
6741
N
N
Cl
O
NH2
O
O
O
O
SO3H
SO3Na
SO3H
NaNO2
Zn, EtOH
rt, 30 sec
HCl (1M)
0-5°C
5 min
O
HN
HN
HN
R
R
R
8a-z
9a-z
10a-z
yield up to 100%
(26 examples)
Scheme 1. Reductive deamination of 1-aminoanthraquinone derivatives.
metal were required to reduce the diazonium salt indicated by the
evolution of N2 gas from the intermediate product, and color
change of the solution from red to purple. The required reaction
time increased in the following sequence: Zn < Cu < Ni (Table 1, en-
tries 1–3).
Thus, Zn was superior to the other metals, and the reaction was
completed within less than one minute (ca. 30 s), while Cu and Ni
required 15, or 30 min, respectively, for complete conversion. Entry
4 in Table 1 shows 5 equiv of Zn to be sufficient for converting the
intermediate into compound 10r within only 3 min. No reaction
was observed in the absence of any metal even after 7 days (Ta-
ble 1, entry 5). Interestingly the intermediate compound of 8r
(the diazonium salt, 9r, Scheme 1) appeared to be stable for such
a long period of time in the presence of ethanol. After 7 days we
added Zn (10 equiv) to the mixture and within less than 1 min
the reaction was completed with very high isolated yield (80%).
We found that at least 5 equiv of Zn are essential for completion
of the reaction, since 1–4 equiv were not sufficient. Even when the
reaction was performed for longer times, stirring the reaction mix-
ture at room temperature for up to 10 h, the intermediate diazo-
nium salt (9r), detected by a red spot on the RP-TLC plate, was
still observed.
The newly developed method (10 equiv Zn/ethanol system) was
then applied to a diverse range of 1-amino-AQ derivatives bearing
Table 2
Synthesis of products 10a-z via a new reductive deamination protocol using a zinc/ethanol systema
O
NH2
O
O
SO3Na
SO3H
1. NaNO2, HCl [1M]
0-5°C, 5 min
2. Zn (10 equiv.)
EtOH, rt, 30 sec
O
HN
HN
R
R
8a-z
10a-z
Entry
R
Products [10a–z]
Yieldb (%)
Purityc (%)
Colord
1
2
3
4
Propyl
Isobutyl
Cyclohexyl
p-Chlorophenethyl
20-Hydroxyphenethyl
Phenyl
p-Fluorophenyl
o-Chlorophenyl
m-Chlorophenyl
p-Chlorophenyl
p-Bromophenyl
m-Methylphenyl
p-Methylphenyl
2,4,6-Trimethylphenyl
o-Ethylphenyl
o-Hydroxyphenyl
p-Hydroxyphenyl
o-Methoxyphenyl
p-Acetaminophenyl
10a
10b
10c
10d
10e
10f
10g
10h
10i
96
73
100
79
88
79
93
85
81
85
77
84
100
60
87
76
89
86
75
100
75
57
89
89
74
87
98.8
98.9
98.6
97.8
99.6
100
Dark red
Brown
dark brown
Red
Dark red
Purple
Dark purple
Dark red
Brown
Purple
Purple
Dark brown
Purple
Dark red
Purple
Dark purple
Dark red
Purple
Purple
Dark Purple
Purple
Dark purple
Dark purple
Purple
Purple
Dark purple
5
6, Acid Blue 25
7
8
9
10
11
12
13
99.1
100
96.7
96.9
96.5
95.8
100
10j
10k
10l
10m
10n
10o
10p
10q
10r
10s
10t
10u
10v
10w
10x
10y
10z
14, Acid Blue 129
15
99.4
99.7
97.5
98.5
95.3
95.0
99.7
97.7
96.1
96.6
97.4
98.7
97.8
16
17
18, PSB-071616
19
20
40-Chloro-20-methylphenyl
20-Carboxy-40-fluorophenyl
20-Carboxy-40-hydroxyphenyl
1-Naphthyl
2-Naphthyl
400-Fluoro-40-phenoxyphenyl
40-Anilino-30-sulfophenyl
21, PSB-095219
22
23
24
25
26, PSB-073917
a
Reaction conditions: 26starting material (8a–z, 0.1 mmol) was dissolved in 5 mL of 1 M HCl then cooled down to 0–5 °C in an ice bath. Subsequently NaNO2 (14 mg,
0.2 mmole) dissolved in water (0.5 mL) was added portion-wise, and the mixture was stirred for 5 min. It was then warmed up to rt followed by the addition of ethanol (5 mL)
and zinc (65 mg, 1 mmol, 10 equiv) and left stirring at rt for <1 min.
b
Isolated yield.
c
Purity measured by HPLC-MS-UV using a diode array detector (DAD) as previously described.14
d
Color of the solid isolated compound.

6742
Y. Baqi, C. E. Müller / Tetrahedron Letters 53 (2012) 6739–6742
2. Lauk, U.; Nowack, P. US Patent 2,005/015,0061 A1 filed 13 Mar. 2003, and
issued 14 Jul. 2005.
3. Prinz, H.; Böhm, K. J.; Müller, K. J. Org. Chem. 2010, 75, 3867–3870.
4. Kukowska-Kaszuba, M.; Dzierzbicka, K.; Serocki, M.; Skladanowski, A. J. Med.
Chem. 2011, 54, 2447–2454.
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6. Glänzel, M.; Bültmann, R.; Starke, K.; Frahm, A. W. Eur. J. Med. Chem. 2005, 40,
1262–1276.
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H.; Bajorath, J.; Müller, C. E. J. Med. Chem. 2010, 53, 2076–2086.
8. Baqi, Y.; Weyler, S.; Iqbal, J.; Zimmermann, H.; Müller, C. E. Purinergic Signalling
2009, 5, 91–106.
9. Iqbal, J.; Lévesque, S. A.; Sévigny, J.; Müller, C. E. Electrophoresis 2008, 29, 3685–
3693.
10. Kukulski, F.; Lévesque, S. A.; Lavoie, E. G.; Lecka, J.; Bigonnesse, F.; Knowles, A.
F.; Robson, S. C.; Kirley, T. L.; Sévigny, J. Purinergic Signalling 2005, 1, 193–204.
11. Zimmermann, H. Biochem. J. 1992, 285, 345–365.
12. Zimmermann, H. Naunyn Schmiedebergs Arch. Pharmacol. 2000, 362, 299–309.
13. Baqi, Y.; Müller, C. E. Org. Lett. 2007, 9, 1271–1274.
different substituents in the 4-position of the anthraquinone core
including (ar)alkylamino residues (Table 2, entries 1–5) and anilino
substituents (Table 2ß entry 6–26) with mono-, di-, or tri-
substituted phenyl groups (Table 2, entries 6–22), a- or b-naphthyl
residues (Table 2,entries 23, 24,) and derivatives with large 4-
substituent containing an additional aromatic ring (Table 2, entries
25, 26). Good to excellent isolated yields were obtained (57–100%,
Table 2). The reaction can easily be followed by visual detection. All
three stages of the reaction have different colors (Scheme 1): the
starting materials (8a–z) are blue, the intermediate diazonium
salts (9a–z) are red, while the products show a purple color
(10a–z).
In conclusion we have developed a novel, fast, mild, and conve-
nient one-pot two-step protocol for the reductive removal of the
aromatically bound amino-groups present in the 1-position of
2,4-substituted AQ derivatives using
a zinc/ethanol system.
14. Baqi, Y.; Müller, C. E. Nat. Protoc. 2010, 5, 945–953.
15. Baqi, Y.; Müller, C. E. J. Org. Chem. 2007, 72, 5908–5911.
16. Weyler, S.; Baqi, Y.; Hillmann, P.; Kaulich, M.; Hunder, A. M.; Müller, I. A.;
Müller, C. E. Bioorg. Med. Chem. Lett. 2008, 18, 223–227.
17. Baqi, Y.; Atzler, K.; Köse, M.; Glänzel, M.; Müller, C. E. J. Med. Chem. 2009, 52,
3784–3793.
18. Hoffmann, K.; Baqi, Y.; Morena, M. S.; Glänzel, M.; Müller, C. E.; von Kügelgen, I.
J. Pharmacol. Exp. Ther. 2009, 331, 648–655.
19. Baqi, Y.; Hausmann, R.; Schmalzing, G.; Müller, C. E. J. Med. Chem. 2010, 54,
817–830.
20. Yan, Y.; Müller, C. E. J. Med. Chem. 2004, 47, 1031–1043.
21. El-Tayeb, A.; Michael, S.; Abdelrahman, A.; Behrenswerth, A.; Gollos, S.; Nieber,
K.; Müller, C. E. ACS Med. Chem. Lett. 2011, 2, 890–895.
22. Bruice, T. C.; Sayigh, A. B. J. Am. Chem. Soc. 1959, 81, 3416–3420.
23. Eisele, J.; Federkiel, W.; Gehm, R.; Rohland, W.; Schuster, C.; Stockl, E.; Tartter,
A. DE Patent 1,066,682 filed 31 Oct. 1957, and issued 22 Jun. 1961.
24. Hoechst AG (Farberke Hoechst AG, Meister Lucius and Bruning). GB Patent
7,613,82 filed 30 Nov. 1953, and issued 14 Nov. 1956.
Twenty-three new AQ derivatives (Table 2, entries 1, 2, 4, 5, 7–
11, 13–26,) were synthesized. Two compounds (Table 2, entries 3
and 12) have been described in one patent reference with neither
synthetic procedure nor spectral data provided,23,24 while one
compound of the present series (Table 2, entry 6) had been isolated
as a by-product (yield of 1%) in the synthesis of Acid Blue 25 (8f,
Table 2).25 The new compounds were obtained in excellent isolated
yields (up to 100%). The intermediate diazonium salt (9r) was
found to be stable for at least several days in ethanol at rt in the
absence of metal. The newly developed method has been applied
to a series of 1-amino-AQ (26 examples) including pharmacologi-
cally active derivatives, such as the potent ecto-50-NT inhibitor
PSB-0952 (8u, Table 2), and the P2Y12 antagonist PSB-0739 (8z,
Table 2).17,18 The new AQ derivatives are now available for biolog-
ical testing and will be investigated in due course.
25. Vinokurov, Yu. V.; Nazarova, N. E.; Solodova, K. V.; Shein, S. M. Zh. Org. Khim.
1989, 25, 1926–1931.
26. General deamination procedure for the synthesis of the products 10a–z
To a 50 mL round-bottomed flask equipped with a magnetic stirring bar was
added 1-amino-anthraquinone (1-amino-AQ) derivative (8a–z, 0.1 mmol)
dissolved in 1 M HCl (5.0 mL) then cooled to 0–5 °C in an ice bath, followed
by the drop wise addition of NaNO2 (0.2 mmol, 2 equiv) dissolved in 0.5 mL
distilled water while the temperature was kept between 0–5 °C for 5 min. The
mixture was then allowed to warm up to rt, and zinc powder (10 equiv) was
added together with 5 mL of ethanol, and the reaction was stirred at rt for ca.
30 s. The reaction can easily be followed due to the color change that is
observed during the course of the reaction: the starting blue material turns red
after diazotization while the final product has a purple color. The reaction was
also followed by RP-TLC using acetone: water (2:3) as the eluent. Then the
product was purified by flash column chromatography using reversed phase
silica gel (RP-18) and water. The polarity of the eluent was then gradually
decreased by the addition of acetone in the following steps: 5%, 20%, 40%, and
60%. Fractions containing purple product were collected. For some compounds
the last step of purification (RP-18 flash chromatography) had to be repeated
twice to obtain pure product (P95% purity as determined by LC–MS/UV and
elemental analysis, to an accuracy of within 0.4%). The pooled fractions
containing a purple color were collected and evaporated under vacuum until
all acetone and most of the water phase were removed. The remaining water
was then removed by a freeze dryer to yield products 10a–z.
Acknowledgments
The authors thank the Deutsche Forschungsgemeinschaft (DFG,
GRK804) for support. Y.B. is indebted to the DAAD for receiving a
fellowship.
Supplementary data
Supplementary (experimental procedures and NMR spectra for
all compounds (10a–z)) data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.
2012.09.011.
References and notes
1. Schmiedl, J.; Damien, S.; Klaus, K. WO Patent 2,004,050,769 A2 filed 24 Nov.
2003, and issued 17 Jun. 2004.