5-Arylideneaminouracils: I. Synthesis and relations between physicochemical parameters and biological activity

Article abstract of DOI:10.1134/S107036320905020X

A number of 5-arylideneaminouracils were synthesized by condensation of 5-aminouracil with substituted aromatic aldehydes. Correlation analysis according to Hantzsch revealed strong effects of the lipophilicity parameter and hydration energy of these comp

Full text of DOI:10.1134/S107036320905020X

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 5, pp. 985–990. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.I. Krutikov, A.V. Erkin, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 5, pp. 813–818.
5-Arylideneaminouracils: I. Synthesis and Relations
between Physicochemical Parameters and Biological Activity
V. I. Krutikov and A. V. Erkin
St. Petersburg State Institute of Technology, Moskovskii pr. 26, St. Petersburg, 190013 Russia
e-mail: kruerk@yandex.ru
Received May 29, 2008
Abstract—A number of 5-arylideneaminouracils were synthesized by condensation of 5-aminouracil with
substituted aromatic aldehydes. Correlation analysis according to Hantzsch revealed strong effects of the
lipophilicity parameter and hydration energy of these compounds on their biological activity.
DOI: 10.1134/S107036320905020X
More than 30 years ago, 5-arylideneaminouracils of
the general formula I were synthesized for the first
time with a view to obtain potential antitumor agents
[1]. It was presumed that hydrolysis of these
compounds under physiological conditions should
release active aldehyde which is capable of acting as
alkylating agent. The other hydrolysis product is 5-
aminouracil which is known as antimetabolite.
However, the goal was not reached: the synthesized
compounds showed no antitumor activity.
5-Arylideneaminouracils Ia–Iw were synthesized
by reaction of 5-aminouracil with the corresponding
substituted benzaldehydes in aqueous ethanol (1:1).
The products separated from the reactant mixture on
heating and were isolated as colorless or brightly
colored crystalline substances whose purity was
checked by thin-layer chromatography. All compounds
Ia–Iw melted above 300°C; their yields, Rf values, and
elemental analyses are collected in Table 1. The
structure of 5-arylideneaminouracils Ia–Iw was con-
firmed by the 1H NMR spectra which characteristically
contained a singlet at δ 9.3–9.7 ppm from the CH=N
proton, a singlet at δ 7.8–8.1 ppm from the 6-H proton
in the pyrimidine ring, and a degenerate signal from
two NH protons (average chemical shift δ 11.1–
11.5 ppm). Uracils Ia–Iw displayed in the IR spectra
an absorption band due to stretching vibrations of the
azomethine CH=N bond (1665 cm–1), a broad band in
the region 3560–3320 cm–1 due to OH stretching
vibrations (in the spectra of salicylaldehyde deriv-
atives), NH band in the region 3200–2965 cm–1,
carbonyl absorption bands at 1650–1630 cm–1, and C–Cl
band at 800–700 cm–1 (in the spectra of chloro-
substituted compounds). In the UV spectra of Ia–Iw,
absorption maxima at λ 280–300 nm were present due
to electron transitions in the uracil and aryl fragments.
In addition, an absorption band with its maximum at λ
400 nm was observed in the visible region; this band
characterizes the entire conjugation system.
O
N
HN
R
O
N
H
I
Cherayath et al. [2] and Enrique et al. [3]
synthesized palladium complexes from Schiff bases I
derived from 5-aminouracil and benzaldehyde and
from 5-aminouracil and salicylaldehyde, which were
found to exhibit germicidal activity. Seitembetov [4]
reported on the synthesis of Schiff bases from
substituted 2-hydroxybenzaldehydes and pronounced
antioxidant activity of some of the obtained
compounds. Despite already known physiological
properties of 5-arylideneaminouracil derivatives and
their accessibility from the preparative viewpoint,
interest in these compounds has not been exhausted. In
1998 we patented structurally related compounds
possessing antimicrobial and antiviral properties [5].
5-Arylideneaminouracils showed in vitro strong
antiviral activity against herpes simplex and various
mycobacteria; they were also found to act as interferon
985

986
KRUTIKOV, ERKIN
Table 1. Yields, Rf values, and elemental analyses of 5-arylideneaminouracils Ia–Iw
Found, %
Calculated, %
H
Comp.
no.
R
Yield, %
Rf
Formula
C
H
N
C
N
H
77
81
82
90
92
66
72
94
85
92
86
92
90
58
60
79
81
80
78
65
71
92
45
0.50
0.48
0.47
0.51
0.51
0.44
0.52
0.53
0.47
0.52
0.51
0.52
0.54
0.48
0.49
0.51
0.52
0.49
0.42
0.51
0.45
0.49
0.56
61.8
52.7
52.8
53.1
44.8
57.3
53.5
42.7
50.0
56.4
46.5
44.3
33.8
58.7
54.9
50.8
50.7
50.7
47.5
63.9
71.8
27.3
60.4
4.21
3.13
3.31
3.25
2.70
3.89
3.62
2.56
3.00
3.39
2.43
2.54
1.82
4.50
4.14
3.19
3.11
3.09
2.95
3.94
4.21
1.42
5.38
19.4
C11H9N3O2
61.4
52.9
52.9
52.9
44.9
57.1
53.4
42.6
49.7
56.7
46.5
44.0
34.0
58.8
55.2
50.8
50.8
50.8
47.8
64.0
72.4
27.4
60.5
4.22
3.23
3.23
3.23
2.74
3.92
3.67
2.60
3.04
3.46
2.48
2.35
1.81
4.52
4.24
3.10
3.10
3.10
2.92
3.94
4.16
1.46
5.46
19.5
16.8
16.8
16.8
14.3
18.2
17.0
13.6
15.8
18.0
14.8
14.0
10.8
17.1
16.1
21.5
21.5
21.5
20.3
14.9
13.3
Ia
Ib
Ic
Id
Ie
If
4-Cl
3-Cl
2-Cl
4-Br
4-OH
16.7
16.4
16.9
14.2
18.1
17.0
13.2
15.6
18.1
14.7
14.2
11.0
17.0
16.0
21.4
21.6
21.7
19.9
14.7
13.2
C11H8ClN3O2
C11H8ClN3O2
C11H8ClN3O2
C11H8BrN3O2
C11H9N3O3
2,4-(OH)2
2-OH-5-Br
2-OH-5-Cl
4-F
C11H9N3O4
Ig
Ih
Ii
C11H8BrN3O3
C11H8ClN3O3
C11H8FN3O2
C11H7Cl2N3O2
C11H7Cl2N3O3
C11H7Br2N3O3
C12H11N3O3
C12H11N3O4
C11H8N4O4
Ij
2,4-Cl2
Ik
Il
2-OH-3,5-Cl2
2-OH-3,5-Br2
4-OCH3
Im
In
Io
Ip
Iq
Ir
Is
4-OH-3-OCH3
4-NO2
3-NO2
C11H8N4O4
2-NO2
C11H8N4O4
2-OH-5-NO2
2-OH-5,6-benzo
2,3:5,6-dibenzo
2-OH-3,5-I2
4-N(CH3)2
C11H8N4O5
C15H11N3O3
C19H13N3O2
It
Iu
Iv
Iw
8.70 C11H7I2N3O3
21.9 C13H14N4O2
8.70
21.7
inductors, i.e., these compounds are capable of en-
hancing immunity of warm-blooded organisms.
substituent, which does not depend on the reaction
series. The set of available parameters, which can be
used to construct an appropriate mathematical model
for the effect of organic compounds on various
biological species, is fairly broad. Theoretically, any
parameter characterizing electronic, steric, or hydro-
phobic effect on the reaction center of a biological
target may be involved.
The azomethine fragment in organic compounds is
highly reactive due to the presence of nearby nucleo-
philic and electrophilic centers. This is especially im-
portant for protonation processes which often occur in
biogenic media. Elucidation of the role of the azo-
methine bond and other structural fragments of 5-aryl-
ideneaminouracils in their antiviral and antimicrobial
activity is necessary for target-oriented synthesis of
compounds with required properties.
With a view to optimize biological activity of 5-
arylideneaminouracils, in the present work we selected
the following parameters: (1) lipophilicity parameter
log P which characterizes the solubility ratio of a
compound in water and octan-1-ol; (2) energy of
hydration Ehydr; (3) molar area S and volume V that
characterize steric effect of a biologically active
molecule on its interaction with biological target; and
(4) the constant ΣσIR proposed in the present work,
which reflects joint inductive and resonance effects of
substituents in the aromatic fragment of 5-arylidene-
aminouracils (Table 2).
Correlation analysis is one of the main tools in
chemical studies, specifically in organic chemistry. It is
used to solve various problems, but the most important
application of correlation analysis is concerned with
studies on intramolecular interactions. Correlation
analysis operates with an extended system of
parameters which may be regarded as real factors
affecting the chemical behavior and physical properties
of compounds. Substituent effect on a reaction center
should be related to some intrinsic parameters of that
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5-ARYLIDENEAMINOURACILS: I.
987
Table 2. Parameters of 5-arylideneaminouracils Ia–Iw for structure–biological activity correlations according to Hantzsch
log (1/C50)
HSV-1b
Comp.
no.
log P
E
hydr, kJ mol–1
S, Å2
V, Å3
ΣσIR
MBa
CAc
Ia
Ib
Ic
Id
Ie
If
Ig
Ih
Ii
Ij
Ik
Il
Im
In
Io
Ip
Iq
Ir
Is
–
3.41
3.48
–
–
–
–0.03
0.68
0.68
0.68
0.95
1.30
2.62
2.28
2.00
0.30
1.38
2.71
3.26
–0.09
1.23
–3.75
–3.75
–3.75
–2.43
1.92
1.21
4.19
0.42
9.46
9.46
9.50
9.33
9.41
13.8
16.0
11.8
11.8
9.50
9.12
11.6
11.5
9.66
11.8
20.7
16.9
12.4
20.7
10.9
7.95
11.4
5.06
247
261
261
250
270
252
253
272
263
250
267
274
292
271
281
263
265
259
269
265
282
305
281
374
397
397
384
409
381
386
414
402
378
414
424
448
416
431
406
401
395
392
426
473
469
438
0
–
–0.061
–0.080
–0.061
–0.053
0.153
3.95
3.98
3.97
3.44
–
–
3.48
3.72
3.62
–
–
–
–
–
0.615
4.09
3.72
–
3.75
3.68
–
–
–0.008
0.006
3.72
–
–0.055
–0.243
–0.302
–0.318
0.062
4.01
4.08
4.15
–
–
–
3.78
4.07
2.99
3.02
3.62
3.32
3.42
3.89
–
4.08
3.89
–
–
–
0.243
4.00
4.00
4.02
4.00
4.03
–
3.72
–
0.121
0.106
3.72
3.74
–
0.121
–0.099
0.169
It
Iu
Iv
Iw
–
3.80
4.19
–
–0.018
–0.259
0.174
4.29
–
–
3.01
Pathogenic mycobacteria M. smegmatis. b Herpes simplex HSV-1. c Yeast-like fungus Candida albicans.
a
Optimization according to the Fletcher–Reeves
that quantitatively characterize inductive and
resonance effects of a substituent on reaction center.
The parameter σIR must be taken into account to
achieve appropriate description of the inductive and
resonance effects both on reaction center and on each
other.
method showed that molecules of 5-arylideneamino-
uracils have almost planar structure (Fig. 1). Therefore,
our choice of the parameters S and V may be regarded
as proper. The use in multiparameter correlation
analysis of so-called cross terms (the parameter σIR
may be regarded as such cross term) is dictated by the
formalism of the theory of interactions.
It should be noted that in our case the cross term is
not the product of similar constants of different
substituents but the product of different constants of
the same substituent. We proceeded from the
undeniable fact that the division of constants σ into
Y = y0 + aσI + bσR + cσIR.
Here, Y is a dependent quantity which determines the
rate of chemical reaction, and σI and σR are constants
Ia
Im
Fig. 1. Optimized structures of the molecules of 5-benzylideneaminouracil (Ia) and 5-(3,5-dibromo-2-hydroxybenzylideneamino)-
uracil (Im) according to quantum-chemical calculations.
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988
KRUTIKOV, ERKIN
inductive and resonance components is quite arbitrary,
for it is impossible to isolate these components of
substituent electronic effect in the pure form in a
chemical reaction. On the whole, the problem related
to orthogonalization of inductive and resonance
parameters is intrinsic to the correlation analysis [6]. The
term σIR may be regarded as a parameter that ensures
(as a first approximation) quantitative estimation of the
overall effect of substituents on the rate of chemical
reactions. The above stated makes it possible to use the
cross term for description of reactions involving
formation of highly ordered transition states [7].
Undoubtedly, reactions of 5-arylideneaminouracils
with biological targets belong just to interactions of
such type.
substituents, respectively; here, σ0m, σ0p, and σ0R are,
respectively, the inductive constants for meta and para
substituents and resonance constant; their values were
taken from [6].
Schiff bases in a biological medium are capable of
forming hydrophilic structures. Hydrolysis of Schiff
bases is a two-step process including addition of water
molecule at the double C=N bond with the formation
of carbinol–amine intermediate. At physiological pH
value, the rate-determining step is just hydration of the
C=N bond. Therefore, the energy of hydration was also
taken as parameter for correlation analysis. We believe
that the parameter Ehydr is very important for simulation
of reactions in biological media; it should quan-
titatively characterize the ability of arylideneamino-
uratcls to react with water molecules.
The ΣσIR values were calculated by the formulas
ΣσIR = Σ(σ0Rσ0m) and ΣσIR = Σ(σ0Rσ0p) for meta and para
H
O
O
O
H
+H2O
N
N
N+
HN
HN
HN
O_
H
R
R
R
OH
O
N
H
O
N
H
O
N
H
I
One of the most important parameters used in
structure–biological activity correlations is the
lipophilicity parameter log P. In fact the ratio of
solubilities of a compound in water and lipids largely
determines the level of its biological activity. In the
present work we examined the relation between log P
values of 5-arylideneaminouracils I and their biolo-
gical activity against yeast-like fungi, mycobacteria,
and herpes simplex (HSV-1) with a view to find
optimal structural parameters ensuring enhanced
biological activity of the examined compounds.
of growth of microorganisms). The experimental data
were satisfactorily described by the classical Hantzsch
formula. It should be noted that the correlation
coefficients and mean-square deviations found for the
activity of substituted aminouracils I with respect to
Candida albicans and HSV-1 are fairly poor. The
correlations log (1/C50)–logP for mycobacteria are more
distinct (Fig. 2).
The obtained correlations log (1/C50) = f [(log P)n]
allowed us to draw unambiguous conclusions only on
logP value for potential antimicrobial and antiviral 5-
arylideneaminouracils I. The minimal activity in vitro
is observed for compounds with a logP value close to
–1, i.e., for those less soluble in lipids than in water
(Fig. 2). On the other hand, both increase and decrease
Table 3 contains the parameters of correlations like
log(1/C50) = f [(log P)n] for different kinds of biological
activity of 5-arylideneaminouracils I (C50 is a minimal
concentration of a compound, inducing 50% inhibition
Table 3. Parameters of correlations log (1/C50) = a0 + a1 log P + a2(log P)2 + a3(log P)3
Microorganism
a0
a1
a2
a3
–
r
s
Candida albicans
3.70
3.74
3.21
3.17
3.96
0.0462
0.0133
0.124
0.0143
0.0112
0.0409
0.0409
0.0107
0.877
0.892
0.784
0.794
0.986
0.100
0.103
0.221
0.227
0.018
0.00256
–
HSV-1
0.202
–0.00725
0.00147
M. smegmatis
0.00748
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5-ARYLIDENEAMINOURACILS: I.
989
in the lipophilicity parameter should lead to enhanced
biological activity.
log (1/C50) = 3.94 + 0.0262log P + 0.0121(log P)2
r 0.973, s 0.023
43
42
41
40
39
The above correlations do not provide exhaustive
description of the effect of physicochemical param-
eters on biological activity of arylideneaminouracils I.
We also tried to draw correlations with account taken
of electronic and steric effects of substituents on the
reaction center and of the energy of hydration of
biologically active molecules.
–4
–2
0
2
4
The following correlations were found for in vitro
inhibition of yeast-like fungus Candida albicans:
log P
Fig. 2. Plot of log (1/C50) (M. smegmatis) of 5-arylidene-
aminouracils I versus lipophilicity parameter logP.
log (1/C50) = 3.82 + 0.0389 log P – 0.135 Σ σIR;
R = 0.784, s = 0.129;
log (1/C50) = 3.65 + 0.0389 log P + 0.0139(log P)2
– 0.00363 Ehydr; R = 0.879, s = 0.109;
log (1/C50) = 3.70 + 0.0448 log P + 0.0130(log P)2
– 0.088 Σ σIR; R = 0.882, s = 0.107;
Antibacterial and antiviral activity of 5-
arylideneaminouracils was studied using standard
strains of microorganisms from the Microbiology
Department, Pavlov St. Petersburg State Medical
University.
Mycobacterium smegmatis:
log (1/C50) = 3.91 + 0.0289 log P + 0.0114(log P)2
+ 0.00298 Ehydr; R = 0.976, s = 0.0228;
log (1/C50) = 3.94 + 0.0251 log P + 0.0120(log P)2
– 0.0267 Σ σIR; R = 0.973, s = 0.0243;
5-(3,5-Dichloro-2-hydroxybenzylideneamino)-
pyrimidine-2,4(1H,3H)-dione (Il). 5-Aminouracil,
1.27 g, was dissolved in 150 ml of water on heating
under stirring, and a solution of 1.91 g of 3,5-di-
chlorosalicylaldehyde in 50 ml of ethanol was added
dropwise. A bright orange solid began to separate
almost immediately. The mixture was heated at the
boiling point for 1 h under stirring, stirred for 1 h at
room temperature, and left overnight. The precipitate
was filtered off, washed with warm water and alcohol,
and dried. Yield 2.76 g (92%). The other 5-arylidene-
aminouracils were synthesized in a similar way.
herpes simplex HSV-1:
log (1/C50) = 0.75 + 0.158 log P – 0.0071(log P)2
– 0.555 Σ σIR + 0.228 Ehydr; R = 0.901, s = 0.595.
These correlations suggest higher biological
activity of compounds with larger log P values and
having electron-withdrawing substituents in the aro-
matic fragment.
EXPERIMENTAL
1
The H NMR spectra were recorded from solutions
in DMSO-d6 on a Bruker spectrometer (200 MHz;
Bruker–Franzen Analytik GmbH Version: 950801.1
software); the chemical shifts were determined relative
to the residual proton signals of the solvent. The UV
spectra were measured on an SF-26 spectrophotometer
(MicroCal Origin Version 3.0 software) from solutions
in 50% ethanol with a concentration of 10–4 M. The IR
spectra were recorded in KBr on a Specord M-80
spectrometer. The purity of the synthesized compounds
was checked by thin-layer chromatography on Silufol
UV-254 using carbon tetrachloride–propan-2-ol (9:1)
as eluent.
ACKNOWLEDGMENTS
The authors thank Prof V.V. Tets and co-workers
(Pavlov St. Petersburg State Medical) for performing
microbiological assays.
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