Synthesis, characterization and radical scavenging activity of aromatic amine conjugates of 5-Aminosalicylic acid

Article abstract of DOI:10.4314/bcse.v28i3.17

5-Aminosalicylic acid is an anti-inflammatory drug used in the treatment of inflammatory bowel diseases including ulcerative colitis and Crohn's disease. Due to its rapid and extensive absorption in the upper gastro-intestinal tract, substantial amount of 5-aminosalicylic acid is already lost before reaching the site of action, i.e. the colon. In order to prevent this loss of drug, carrier linked prodrug approach has been used and azo prodrugs have been synthesized with a purpose of colon targeting. The present research describes the synthesis and characterization of azo prodrugs of 5-aminosalicylic acid with aromatic amines. The synthesized prodrugs were tested for antioxidant activity using DPPH (2,2-diphenyl-1-picrylhydrazy1) free radical scavenging activity. All the synthesized compounds were found to possess mild to moderate radical scavenging activity.

Full text of DOI:10.4314/bcse.v28i3.17

Bull. Chem. Soc. Ethiop. 2014, 28(3), 475-480.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v28i3.17
ISSN 1011-3924
 2014 Chemical Society of Ethiopia
SHORT COMMUNICATION
SYNTHESIS, CHARACTERIZATION AND RADICAL SCAVENGING ACTIVITY OF
AROMATIC AMINE CONJUGATES OF 5-AMINOSALICYLIC ACID
Kaur Harveer and Sidana Jasmeen*
School of Pharmaceutical Sciences, Lovely Faculty of Applied Medical Sciences, Lovely
Professional University, Punjab-144411
(Received October 8, 2013; revised August 27, 2014)
ABSTRACT. 5-Aminosalicylic acid is an anti-inflammatory drug used in the treatment of inflammatory bowel
diseases including ulcerative colitis and Crohn’s disease. Due to its rapid and extensive absorption in the upper
gastro-intestinal tract, substantial amount of 5-aminosalicylic acid is already lost before reaching the site of action,
i.e. the colon. In order to prevent this loss of drug, carrier linked prodrug approach has been used and azo prodrugs
have been synthesized with a purpose of colon targeting. The present research describes the synthesis and
characterization of azo prodrugs of 5-aminosalicylic acid with aromatic amines. The synthesized prodrugs were
tested for antioxidant activity using DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging activity. All
the synthesized compounds were found to possess mild to moderate radical scavenging activity.
KEY WORDS: 5-Aminosalicylic acid, Antioxidant, Inflammatory bowel disease, Prodrugs
INTRODUCTION
Inflammatory bowel diseases (IBDs) including ulcerative colitis and Crohn’s disease are serious
conditions of the large bowel leading to impairment of intestinal functions [1-3]. The unknown
etiology of IBDs hampers the development of new therapies. Also, the inability of conventional
therapies in preventing relapse and achieving remission advocates the requirement of more
efficient approaches [4, 5].
The knowledge of biochemical, molecular and inflammatory events has explained the
involvement of oxidative stress in IBDs [6]. Hence, the use of antioxidants is an important
approach used for mucosal protection against reactive oxygen metabolites including free
radicals [7]. The antioxidant activity of allupurinol, a xanthine oxidase inhibitor, showed
beneficial effects in prolonging the time to relapse in ulcerative colitis. Also, the enzyme
superoxide dismutase showed encouraging effects in Crohn’s disease [8, 9].
Normally, most of the tissues possess significant amounts of enzymes like superoxide
dismutase [10], catalase [11], GSH peroxidase [12] and non-enzymatic (thiols, ascorbates), etc
[13]. Owing to their antioxidant activity, these enzymes neutralize most of the oxidative agents.
But, in case of IBDs, the overproduction of reactive oxygen species overwhelms the action of
natural antioxidants [14]. Thus, the use of antioxidants along with conventional regimen (anti-
inflammatory, immunomodulators, immunosuppressive agents and corticosteroids) may help to
prevent the remission episodes in IBDs.
5-Aminosalicylic acid (5-ASA), an aminosalicylate, used for anti-inflammatory action also
possesses antioxidant activity. However, the effectiveness of this drug is limited by its extensive
systemic absorption leading to gastrointestinal side effects as well. Thus, an attempt was made
to synthesize new prodrugs of 5-ASA to overcome the systemic side effects, achieve colon
targeting as well as maintain the antioxidant activity of the active moiety. Considering that
aromatic amines can be used as inert carriers for the active drug, the present work was designed
__________
*Corresponding author. E-mail: jasmeen1410@gmail.com

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Kaur Harveer and Sidana Jasmeen
to synthesize the aromatic amine conjugates of 5-ASA. The synthesized conjugates were
evaluated for their antioxidant activity using DPPH radical scavenging assay.
EXPERIMENTAL
General. The reactants used in the synthesis were of AR grade and were purchased from
Qualikem Fine Chem. and Molychem. Pvt. Ltd., India. The solvents used were purchased from
Merck Inc. The IR spectra of the synthesized compounds were recorded on Shimadzu, 530 FTIR
1
in potassium bromide pellets. H and 13C NMR spectra of the synthesized compounds were
recorded in DMSO-d6 on Bruker Advance II 400 MHz instrument using TMS as internal
standard.
Synthetic procedures. Aromatic amine (1a-i) (0.01 mol) was dissolved in 1:1 ratio of conc. HCl
and water. Sodium nitrite solution (0.02 mol/mL) was added to the reaction mixture dropwise
with continuous stirring. The reaction mixture was stirred for 45 min. at a temperature of 0-5 oC.
The diazotization step was followed by coupling of the diazotized amines by drop wise addition
to the solution of sodium salicylate (0.01 mole of salicylic acid dissolved in 0.02 mol/mL
o
solution of sodium hydroxide). The reaction was stirred at 0-5 C for 2 hours under alkaline
conditions. The conjugates of salicylic acid and aromatic amines (3a-i) were obtained as orange
to red powders after the pH was neutralized to 6-7.
2-Hydroxy-5-phenylazo-benzoic acid (3a). Yield 56%, orange to brownish crystals, m.p. 218-
o
220 C. Rf 0.31 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3425 (phenolic -OH str),
3063 (aromatic CH str), 1585 (C=O str), 1483 (N=N str), 1286 (CH bend aromatic ring), 1251
(C–O str), 1174 cm-1 (C–N str). 1H-NMR (400 MHz, DMSO-d6): δ (ppm) 16.6 (s, 1H), 8.42 (d,
J = 2.4 Hz, 1H), 7.8 (m, 3H), 7.46 (t, J = 7.6 Hz, 2H), 7.38 (t, J = 7.2 Hz, 1H), 6.78 (d, J = 8.80
Hz, 1H), 3.82 (s, 3H). 13C-NMR (100 MHz, DMSO-d6): δ (ppm) 171.8, 166.9, 152.17, 143.14,
129.85, 129, 126.6, 126, 121.9, 119, 117.2, 78.8, 39.4.
2-Hydroxy-5-o-tolyl benzoic acid (3b). Yield 59%, red crystals, m.p. 202-204 oC. Rf 0.38
(CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3628 (phenolic -OH str), 3064 (aromatic CH
str), 1670 (C=O str), 1485 (N=N str), 1286 (C–O str), 1193 cm-1 (C–N str). 1H NMR (400 MHz,
DMSO-d6): δ (ppm) 11.3 (s, 1H), 8.38 (d, J = 2.40 Hz, 1H), 8.04 (dd, J = 8.84, 2.48 Hz, 1H),
7.57 (d, J = 7.60 Hz, 1H), 7.36 (d, J = 4.04 Hz, 2H), 7.27 (m, 1H), 7.09 (d, J = 8.80 Hz, 1H),
2.68 (s, 3H), 13C-NMR (100 MHz, DMSO-d6): δ (ppm) 164.3, 149.8, 144.3, 137, 131.1, 130.5,
129.1, 128.1, 127.6, 126.4, 122.2, 118, 115, 78.8, 39.5, 17.1.
2-Hydroxy-5-m-tolyl benzoic acid (3c). Yield 61%, orange amorphous solid, m.p. 199-201oC. Rf
0.42 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3333 (phenolic O-H str), 1599 (C=O
str), 1483 (N=N str), 1257 cm-1 (C–O str). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.40 (d, J =
2.56 Hz, 1H), 7.83 (dd, J = 8.76, 2.56 Hz, 1H), 7.6 (d, J = 8.00 Hz, 2H), 7.38 (t, J = 8.00 Hz,
1H), 7.24 (d, J = 8.00 Hz, 1H), 6.82 (d, J = 8.72 Hz, 1H). 13C-NMR (100 MHz, DMSO-d6): δ
(ppm) 171.9, 166.2, 152.2, 143.4, 138.3, 130.5, 128.7, 126.6, 126, 122.1, 119.4, 118.9, 117.1,
78.6, 39.3, 20.9.
o
2-Hydroxy-5-p-tolyl benzoic acid (3d). Yield 58%, red crystals, m.p. 195-197 C. Rf 0.40
(CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3383 (phenolic -OH str), 1585 (C=O str),
1
1485 (N=N str), 1288 (C–O str), 831 cm-1 (aromatic out of plane bend). H NMR (400 MHz,
DMSO-d6): 17.0 (s, 1H), 8.32 (d, J = 2.52 Hz, 1H), 7.78 (dd, J = 4.72, 2.56 Hz, 2H), 7.69 (d, J =
8.16 Hz, 2H), 7.29 (d, J = 8.16 Hz, 1H), 6.78 (d, J = 8.76 Hz, 1H), 2.50 (t, J = 1.60 Hz, 3H).
13C-NMR (100 MHz, DMSO-d6): δ (ppm) 171.3, 167.5, 150.3, 142.7, 139.5, 129.5, 125.9,
121.8, 119.4, 117, 78.8, 39.5, 20.9.
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Short Communication
477
2-Hydroxy-5-(o-nitro-phenylazo)-benzoic acid (3e). Yield 66%, orange colored amorphous
o
solid, m.p. 219-221 C., Rf 0.40 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3479
(phenolic -OH str), 3101 (aromatic CH str), 1589 (C=O str), 1494 (N=N str), 1303 (CH bend
1
aromatic ring), 1249 (C–O str), 1130 (C–N str). H NMR (400 Hz, DMSO-d6) δ (ppm) 17.6 (s,
1H), 8.35 (d, J = 2.64 Hz, 1H), 7.93 (m, 1H), 7.58 (m, 1H), 6.79 (t, J = 4.40 Hz, 1H). 13C-NMR
(100 MHz, DMSO-d6): δ (ppm) 170.6, 146.8, 144.5, 142.4, 132.8, 129.4, 128, 126.5, 123.5,
118.2, 78.7, 39.5.
2-Hydroxy-5-(p-nitro-phenylazo)-benzoic acid (3f). Yield 60%, orange crystals, m.p. 216-218
oC. Rf 0.37 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3425 (phenolic -OH str), 3063
(aromatic CH str), 1585 (C=O str), 1483 (N=N str), 1251 (C–O str), 1174 (C–N str), 831 cm-1
(aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6): δ (ppm) 17.5 (s, 1H), 8.41 (d, J =
2.36 Hz, 1H), 8.33 (d, J = 8.92 Hz, 2H), 7.94 (d, J = 8.88 Hz, 2H), 7.85 (dd, J = 9.16, 2.48 Hz,
1H), 6.78 (d, J = 8.84 Hz, 1H). 13C-NMR (100 MHz, DMSO-d6): δ (ppm) 170.6, 155.9, 147,
142.6, 127.9, 126.8, 124.6, 122.5, 118.9, 118.1, 78.7, 39.5.
2-Hydroxy-5-(p-methoxy-phenylazo)-benzoic acid (3g). Yield 70%, yellowish green amorphous
solid, m.p. 222 oC (decomp.). Rf 0.35 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3502
(phenolic -OH str), 3333 (aromatic CH str), 1641 and 1484 (aromatic C=C) 1583 (C=O str),
1
1494 (N=N str), 1251 (C–O str), 1176 (C–N str), 833 cm-1 (aromatic out of plane bend). H
NMR (400 MHz, DMSO-d6): δ (ppm) 16.6 (s, 1H), 8.33 (d, J = 2.56 Hz, 1H), 7.76 (m, 3H), 7
(m, 2H), 6.79 (d, J = 8.68 Hz, 1H), 3.82 (s, 3H), 13C-NMR (100 MHz, DMSO-d6): δ (ppm)
171.5, 166.7,160.7, 146.3, 142.8, 125.7, 123.5, 119.4, 116.9, 114.1, 78.8, 55.3, 39.5.
5-(p-Chloro-phenylazo)-2-hydroxy-benzoic acid (3h). Yield 64%, orange crystals, m.p. 192-194
oC. Rf 0.42 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3446 (phenolic -OH str), 3063
(aromatic CH str), 1587 (C=O str), 1483 (N=N str), 1288 (CH bend aromatic ring), 1172 cm-1
1
(C–N str). H NMR (400 MHz, DMSO-d6): δ (ppm) 17.0 (s, 1H), 8.41 (d, J = 2.52 Hz, 1H),
7.81 (m, 3H), 7.49 (dd, J = 15.48, 8.64 Hz, 2H), 6.82 (d, J = 8.76 Hz, 1H). 13C-NMR (100 MHz,
DMSO-d6): δ (ppm) 171.0, 168.8, 151, 142.5, 134.1, 128.9, 126.3, 119.4, 117.4, 78.8, 39.5
o
2-Hydroxy-5-(1-naphthaylazo)-benzoic acid (3i). Yield 66%, orange crystals, m.p. 188-190 C
(decomp.). Rf 0.40 (CHCl3/CH3OH/CH3COOH 8:1:1). IR (KBr): νmax 3470 (phenolic -OH str),
3047 (aromatic CH str), 1585 (C=O str), 1491 (N=N str), 1288 (CH bend aromatic ring), 763
1
cm-1 (aromatic out of plane bend). H NMR (400 MHz, DMSO-d6): δ (ppm) 17.2 (s, 1H), 8.87
(d, J = 8.28 Hz, 1H), 8.52 (d, J = 1.96 Hz, 1H), 7.97 (m, 3H), 7.74 (d, J = 7.36 Hz, 1H), 7.65
(m, 3H), 6.87 (d, J = 8.72 Hz, 1H). 13C-NMR (100 MHz, DMSO-d6): δ (ppm) 171.5, 167.8, 147,
143.7, 133.8, 130.3, 129.7, 127.8, 126.5, 125.7, 122.9, 117.4, 111.1, 78.8, 39.4.
Antioxidant activity. The 0.1 mM solution of 2,2-diphenyl-1-picrylhydrazyl (DPPH) in methanol
was prepared by dissolving 3.94 mg of DPPH in methanol and making the volume up to 100
mL. This solution (1 mL) was added to 3 mL of synthesized compound solution in methanol at
different concentrations (10-100 µg/mL). The solution was then incubated at room temperature
for 45 min. in dark and the absorbance was measured at 517 nm against blank solution [15, 16].
Percentage inhibition of DPPH free radical was calculated based on the control reading, which
contains DPPH solution without any compound using the following equation
[DPPH scavenging activity = (A control- A test/A control ) x 100]
where, Acontrol is absorbance of DPPH solution, Atest is absorbance of DPPH solution in presence
of synthesized compounds.
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Kaur Harveer and Sidana Jasmeen
RESULTS AND DISCUSSION
Commercially available aniline (1a) was treated with sodium nitrite in an acidic medium (35%
HCl) to give the diazonium salt 2a (Scheme 1). Since the diazonium salts are unstable moieties
and readily convert back to amines or to corresponding phenols, 2a was generated in situ and
reaction was continued for coupling with salicylic acid in alkaline medium. The product thus
obtained was a sodium salt that was acidified to pH 6-7 to yield 3a. The other derivatives 3b-3i
were synthesized following the same procedure (Scheme 1).
N2Cl
NH2
HO
(a)
(b)
N
R
R
NaOOC
N
R
1(a-i)
2(a-i)
(c)
HO
HOOC
HO
HOOC
N
N
N
N
R
3i
3(a-h)
where R = H (a), o-CH3 (b), m-CH3 (c), p-CH3 (d),
o-NO2 (e), p-NO2 (f), p-OCH3 (g) and p-Cl (h)
Scheme 1. Reagents and conditions: (a) NaNO2, 35% HCl, 0-5 oC, (b) aq. NaOH (0.02 M), 0-5
oC, salicylic acid, and (c) H+, pH 6-7.
The electrophilic aromatic substitution takes place at para position since phenols direct
ortho and para position for the substituent to the hydroxyl group. However, as ortho position is
substituted here with -COOH group and also the ortho- substituted product is unstable, thus this
electrophilic substitution of aromatic diazonium salt occurs at para position of salicylic acid
yielding the required conjugates (3a-3i). Different aromatic amines were used as reactants (1a-
1i) to get corresponding conjugates with same reaction mechanism.
Biological activity. The synthesized compounds were screened for in vitro antioxidant activity
using DPPH (1,1 diphenyl-2-picrylhydrazyl) radical. Ascorbic acid was used as the positive
control in the assay.
Antioxidants react with the stable DPPH free radical reducing it to the DPPH-H and as
consequence the absorbance decreases. The degree of discoloration indicates the scavenging
potential of the antioxidant compounds in terms of hydrogen donating ability. The scavenging
reaction between (DPPH) and an antioxidant (H-A) can be written as:
(DPPH) + (H-A)
(Yellow)
DPPH-H + (A)
(Purple)
The hydrogen donating activity measured at 540 nm showed a significant relationship
between concentration of novel molecules and percentage inhibition. Table 1 presents the results
of DPPH free radical scavenging activity bioassay.
Bull. Chem. Soc. Ethiop. 2014, 28(3)

Short Communication
Table 1. Results of the DPPH radical scavenging activity.
479
Entry
3a
Mean % inhibition DPPH
Entry
3g
Mean % inhibition DPPH
47.44
44.84
46.66
47.60
48.50
49.66
52.14
51.38
41.96
67.20
53.76
3b
3c
3h
3i
3d
3e
Ascorbic acid (positive control)
5-ASA (standard drug)
3f
The antioxidant activity of the compounds 3g and 3h is comparable to that of the standard
drug 5-ASA. The activity of all the synthesized prodrugs is expected to increase in vivo because
of the release of the active moiety, i.e. 5-ASA into the system. The synthesized prodrugs bear
structural resemblance with prontosil, a prodrug of antibacterial sulphanilamide. Prontosil is
metabolized in the body to give sulphanilamide [17]. Therefore, it is expected that 5-ASA will
be liberated from the prodrugs in an analogous manner (Scheme 2).
Scheme 2. Metabolic pathway of the synthesized prodrugs.
This metabolism is brought about by the enzyme azo-reductases secreted by colonic
microflora [18]. Therefore, aromatic amines may serve as carriers in the synthesis of colon-
targeted azo-prodrugs of 5-ASA. Further, 5-ASA has anti-inflammatory as well as antioxidant
activities. Hence, it protects the cells from the inflammatory damage and the oxidative stress in
the IBDs. The synthesized prodrugs may further be tested for their anti-inflammatory potential.
CONCLUSION
Nine novel conjugates of salicylic acid with aromatic amines were synthesized. Aromatic
amines (such as aniline, o-toluidine, m-toluidine, p-toluidine, o-nitroaniline, p-nitroaniline, p-
chloroaniline, p-anisidine and naphthylamine) were conjugated as inert carriers to the active
moiety through azo linkage making it colon specific. All the synthesized conjugates showed
moderate inhibition of DPPH radical. Compound 3g was found to have most potent antioxidant
activity amongst the synthesized conjugates.
ACKNOWLEDGEMENTS
Authors are grateful to Prof. Monica Gulati and Dr. Amit Mittal, School of Pharmaceutical
Sciences, Lovely Professional University for support.
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