Structure activity relationship, drug likeness and evaluation of antioxidant activity of some mannich bases of dihydropyrimidinones

Article abstract of DOI:10.14233/ajchem.2019.22008

A series of 21 O- and N-Mannich bases of 3,4-dihydropyrimidinones (2a-j and 3a-k) were synthesized by using microwave irradiation technique by multi-component reaction in two steps. All the compounds were evaluated for their free radical scavenging activi

Full text of DOI:10.14233/ajchem.2019.22008

Asian Journal of Chemistry; Vol. 31, No. 8 (2019), 1767-1773
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.22008
Structure Activity Relationship, Drug Likeness and Evaluation of
Antioxidant Activity of Some Mannich Bases of Dihydropyrimidinones
1,*
2
3
RAVINDER MAMIDALA , SOLOMON RAJ S. BHIMATHATI and APARNA VEMA
1Department of Medicinal Chemistry, St. Peter's Institute of Pharmaceutical Sciences, Warangal-506001, India
2Department of Pharmacology, Siddhartha Institute of Pharmacy, Narapally, Ghatkesar, Hyderabad-501301, India
3Department of Pharmaceutical Chemistry, College of Pharmacy, K L University, Vaddeswaram-520002, India
*Corresponding author: E-mail: mamidalapharmacy@gmail.com
Received: 18 February 2019;
Accepted: 29 March 2019;
Published online: 28 June 2019;
AJC-19449
A series of 21 O- and N-Mannich bases of 3,4-dihydropyrimidinones (2a-j and 3a-k) were synthesized by using microwave irradiation
technique by multi-component reaction in two steps. All the compounds were evaluated for their free radical scavenging activity by four
methods. Structure activity relationship studies revealed that the compounds 2h, 2g, 3h and 3g exhibited profound antioxidant properties
compared to standard ascorbic acid. Among O- and N-Mannich bases, N-Mannich bases were found to be more potent in scavenging free
radicals. The correlation between structure and activities of these compounds with concern to drug likeliness profile and other physico-
chemical parameters are portrayed and verified experimentally.
Keywords: Antioxidant activity, Dihydropyrimidinones, Drug likeness, Mannich bases, Structure activity relationship.
[14]. The biological importance of this pharmacophore has
attracted the interest of chemists in drug discovery and research.
In the present study, the scope of this pharmacophore is
extended by aryl substitution at 4th and 6th positions, which
are further substituted by electron donating and electron with-
drawing groups at various positions.
Mannich bases of some dihydropyrimidinones (DHPMs)
were synthesized by microwave irradiation and screened for
antioxidant activity by 2,2-diphenyl-1-picrylhydrazyl (DPPH)
method, hydrogen peroxide method, nitric oxide scavenging
method and iron chelation method, to establish the structure
activity relationships (SAR).
INTRODUCTION
Excess production of reactive oxygen species (ROS) leads
to oxidative stress and damages cell components [1]. ROS that
carries a single electron in an atomic orbital are capable enough
to interact with various biomolecules [2] thereby leading to
several pathological conditions such as Parkinson's,Alzheimer's,
cancer, CVS disorders, cirrhosis, etc. [3,4].
Antioxidants are stable components that can donate an
electron to highly unstable ROS and can neutralize them, there-
by preventing their harmful effects [5]. Antioxidants breaks
the chain reactions instigated by ROS by donating electron
and can works like single oxygen quencher, hydrogen and electron
donor, metal chelating agents, a radical scavenger, peroxide
decomposer, synergist and as an enzyme inhibitor [6]. There-
fore, the scaffolds that hold antioxidant activity are of tremen-
dous importance in the current drug research targeting multiple
diseases.
EXPERIMENTAL
The titled compounds were characterized by melting point,
IR, 1H NMR, Mass and elemental analyses. The IR spectra were
recorded on Perkin-Elmer FT-IR instrument using KBr discs
(Perkin Elmer, USA). 1H NMR was recorded on a BrukerAvance
II 300 instrument (300 MHz) in CDCl3 solvent and TMS as
internal standard. The mass spectra were obtained by electron
spray ionization (ESI) on Shimadzu LCMS 2010A spectro-
Dihydropyrimidinones (DHPMs) exhibits diverse range
of biological activities including anticancer [7], antimicrobial
[8], anti-inflammatory [9], antidiabetic [10], analgesic [11],
antitubercular [12], antimalarial [13] and antiviral activities
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1768 Mamidala et al.
Asian J. Chem.
meter. Microwave irradiation was carried out using an IFB
domestic microwave oven. Melting points were measured by
melting point apparatus on an open capillary and are uncorrected.
Precoated silica gel TLC plates (Silica gel GF254) were obtained
from E. Merck and the solvent system of 1:1:0.2 ratio of ethyl
acetate:n-hexane:methanol was employed for TLC and was
selected by trial and error method. All the chemicals were
procured from S.D. Fine Chemicals Ltd. (Mumbai) and Sigma-
Aldrich (Merck), India. The solvents were of laboratory grade.
General procedure for the synthesis of 3,4-dihydro-
pyrimidine-2-ones (1): 3,4-Dihydropyrimidinones were synthe-
sized by modification of protocol described byVenkateswarlu
et al. [15], by replacing catalystAlCl3 with p-toulene sulphonic
acid (PTSA).A mixture of 0.01 mol of appropriately substituted
benzaldehydes, acetophenones, urea and PTSA were dissolved
in 30 mL of ethanol. The reaction mixture was irradiated in a
microwave oven at 60 % power. The reaction was monitored
by TLC.After completion of the reaction, contents were poured
in ice cold water with continuous stirring. The product formed
was filtered and dried. The product obtained was purified by
recrystallization from ethanol.
mol of dimethylamine were taken in a flask and stirred for
couple of hours. Then the reaction mixture was irradiated in a
microwave oven at 60 % power. Completion of reaction was
confirmed by TLC and the product formed was refrigerated
for 36 h. The separated product was filtered, washed with cold
water and dried. Recrystallization was done from petroleum
ether:chloroform (1:1 ratio).
Synthesis of N-Mannich bases (3a-k): A mixture of
0.005 mol of DHPM (dissolved in 10 mL of DMSO), 0.01
mol of 37 % formaldehyde and 0.005 mol of dimethylamine
were taken in a flask and stirred for couple of hours. Then the
reaction mixture was irradiated in a microwave oven at 60%
power. Completion of reaction was confirmed by TLC and the
product formed was refrigerated for 36 h. The separated product
was filtered, washed with cold water and dried. Recrystal-
lization was done from petroleum ether:chloroform (1:1 ratio)
(Scheme-I).
Antioxidant activity: All the titled compounds were
dissolved in methanol to make concentrations of 20, 40, 60,
80 and 100 µg/mL. These concentrations were allowed to react
with different free radicals in various methods. Antioxidant
activity was determined by four different methods viz., DPPH
scavenging activity [16], H2O2 scavenging activity [17], NO
scavenging activity [18] and iron chelation activity [19].
Synthesis of O-Mannich bases (2a-j):A mixture of 0.005
mol of DHPM (dissolved in 10 mL of DMSO), 0.01 mol of
37% formaldehyde, 20 mg of anhydrous K2CO3 and 0.005
O
H
O
CH3
R
O
i
+
+
H2N
NH2
R
R1
NH
N
H
O
R1
(1a-m)
R
iii
ii
N
N
N
H
O
R
R1
(3a-k)
N-Mannich bases
N
N
H
O
N
R1
(2a-j)
O-Mannich bases
Scheme-I: Synthesis of O- and N-Mannich bases of DHPMs. Reagents and conditions (i) PTSA, ethanol, MW 60% (ii) Dimethylamine,
formaldehyde, K2CO3, DMSO, MW 60% (iii) Dimethylamine, formaldehyde, DMSO, MW 60 %

Vol. 31, No. 8 (2019)
SAR Drug Likeness and Evaluation of Antioxidant Activity of Some Mannich Bases of Dihydropyrimidinones 1769
Ascorbic acid was used as the standard. Control was prepared
by adding all the contents except test compound. The absor-
bance was measured on UV-visible spectrophotometer (Shimadzu,
UV-1800, Japan). All the experiments were carried out in trip-
licate and the results were expressed as mean standard error
mean (SEM).The free radical scavenging activity (FRSA) of
the tested samples and standard was calculated as % FRSA in
all methods by using following formula:
2.1 (s,6H, 2CH3), 4.4 (s, 2H, CH2), 8.8 (s, 1H, NH), 6.8-7.6
(m, 8H, Ar-H), 5.2 (d, 1H, CH), 5.8 (d, 1H, C=CH, J = 6.2
Hz); ESI-MS: Found [M+2] 379; Elemental analyses for calcd.
(found) %: C 60.63 (60.54), H 5.05 (5.12), N 11.17 (11.24).
4-(Phenyl)-6-(4-hydroxyphenyl)-1,4-dihydropyrimidine-
2-yl-oxy-N,N-dimethylmethanamine (2f): Colour: pale brown-
ish amorphous solid, m.w. 323, m.p. 192-194 ºC, Rf: 0.55; IR
(KBr, νmax, cm-1): 3500, 3399, 1242, 1320, 3120, 2894, 1608,
942, 754; 1H NMR (300 MHz, CDCl3) δ: 2.8 (s, 6H, 2CH3),
4.0 (s, 2H, CH2), 8.2 (s, 1H, NH), 6.4-7.7 (m, 9H, Ar-H), 4.2
(s, 1H, OH), 4.9 (d, 1H, CH), 5.4 (d, 1H, C=CH, J = 8.3 Hz);
ESI-MS: Found [M+] 323; Elemental analyses for calcd.
(found) %: C 70.58 (70.42), H 6.50 (6.44), N 13.00 (12.98).
4-(4-Hydroxyphenyl)-6-(phenyl)-1,4-dihydropyrimidine-
2-yl-oxy-N,N-dimethylmethanamine (2g): Colour: yellowish
amorphous solid, m.w. 323, m.p. 214-216 ºC, Rf: 0.74; IR (KBr,
Ao − At
Ao
FRSA (%) =
×100
where, Ao = Absorbance of control, At = Absorbance of tested
samples. % FRSA was plotted against concentration. IC50
values were obtained from the line equation above. Smaller
IC50 value indicates higher radical scavenging activity.
4-(4-Methoxyphenyl)-6-(3-nitrophenyl)-1,4-dihydro-
pyrimidine-2-yl-oxy-N,N-dimethylmethanamine (2a): Colour:
pale brownish amorphous solid, m.w. 382, m.p. 205-208 ºC,
Rf: 0.58 (n-hexane:ethylacetate:methanol; 1:1:0.2 for all
compounds); IR (KBr, νmax, cm-1): 3294, 1529, 1220, 1427,
2921, 2821, 1590, 952, 804; 1H NMR (300 MHz, CDCl3) δ:
2.56 (s, 6H, 2CH3), 3.8 (s, 3H, OCH3), 5.2 (s, 2H, CH2), 8.6 (s,
1H, NH), 6.8-7.5 (m, 8H, Ar.H), 5.56 (d, 1H, CH), 6.2 (d, 1H,
C=CH, J = 4.2 Hz); ESI-MS: Found [M+] 382; Elemental
analyses for calcd. (found) %: C 62.82 (62.79), H 5.75 (5.72),
N 14.65 (14.71).
ν
max, cm-1): 3422, 3220 , 1212, 1268, 3066, 2921, 1606, 982,
748; 1H NMR (300 MHz, CDCl3) δ: 2.5 (s, 6H, 2CH3), 3.8 (s,
2H, CH2), 9.3 (s, 1H, NH), 4.8 (s,1H, OH), 6.4-7.1 (m, 9H,
Ar-H), 5.1 (d, 1H, CH), 5.6 (d, 1H, C=CH, J = 5.6 Hz); ESI-
MS: Found [M+] 323; Elemental analyses for calcd. (found)
%: 70.58 (70.46), H 6.50 (6.45), N 13.00 (13.18).
4,6-bis-(4-Hydroxyphenyl)-1,4-dihydropyrimidine-2-
yl-oxy-N,N-dimethylmethanamine (2h): Colour: brownish
crystalline solid, m.w. 339, m.p. 219-221 ºC, Rf: 0.44; IR (KBr,
ν
max, cm-1): 3395, 3294 , 1196, 1344, 3053, 2905, 1588, 944,
4-(4-Chlorophenyl)-6-(phenyl)-1,4-dihydropyrimidine-
2-yl-oxy-N,N-dimethylmethanamine (2b): Colour: pale yellow-
ish amorphous solid, m.w. 341, m.p. 218-220 ºC, Rf: 0.65; IR
(KBr, νmax, cm-1): 751, 3231, 1195, 1241, 3024, 2914,
1596,1152, 1082, 816; 1H NMR (300 MHz, CDCl3) δ: 2.3 (s,
6H, 2CH3), 4.8 (s, 2H, CH2), 8.8 (s, 1H, NH), 6.6-7.3 (m, 9H,Ar-H),
4.5 (d, 1H, CH), 5.6 (d, 1H, C=CH, J = 3.0 Hz); ESI-MS: Found
[M+2] 344; Elemental analyses for calcd. (found) %: C 66.66
(66.52), H 5.84 (5.79), N 12.28 (12.34).
786; 1H NMR (300 MHz, CDCl3) δ: 2.9 (s, 6H, 2CH3), 3.6 (s,
2H, CH2), 8.4 (s, 1H, NH), 6.2 (s,1H, OH), 6.6 (s, 1H, OH),
7.1-7.9 (m, 8H, Ar.H), 5.4 (d, 1H, CH), 5.7 (d, 1H, C=CH, J =
4.3 Hz); ESI-MS: Found [M+1] 341; Elemental analyses for
calcd. (found) %: C 67.25 (67.31), H 6.19 (6.25), N 12.38 (12.41).
4-(Phenyl)-6-(3-nitrophenyl)-1,4-dihydropyrimidine-
2-yl-oxy-N,N-dimethylmethanamine (2i): Colour: pale yellow-
ish amorphous solid, m.w. 352, m.p. 244-246 ºC, Rf: 0.39; IR
(KBr, νmax, cm-1): 3600, 1490, 1206, 1392, 3018, 2788, 1596,
968, 754; 1H NMR (300 MHz, CDCl3) δ: 2.6 (s,6H, 2CH3), 3.6
(s, 2H, CH2), 8.4 (s, 1H, NH), 7.1-8.0 (m, 9H, Ar-H), 4.2 (d, 1H,
CH), 4.9 (d, 1H, C=CH, J = 5.2 Hz); ESI-MS: Found [M+]
352; Elemental analyses for calcd. (found) %: C 64.77 (64.72),
H 5.68 (5.73), N 15.90 (15.87).
4-(4-Hydroxyphenyl)-6-(3-nitrophenyl)-1,4-dihydropy-
rimidine-2-yl-oxy-N,N-dimethylmethanamine (2j): Colour:
pale brownish amorphous solid, m.w. 368, m.p. 221-223 ºC,
Rf: 0.77; IR (KBr, νmax, cm-1): 3420, 1512, 3178, 1178, 1248,
3096, 2922, 1602, 843, 736; 1H NMR (300 MHz, CDCl3) δ:
3.1 (s,6H, 2CH3), 3.4 (s, 2H, CH2), 8.2 (s, 1H, NH), 6.4 (s,1H,
OH), 6.9-7.7 (m, 8H,Ar.H), 5.2 (d, 1H, CH), 5.8 (d, 1H, C=CH,
J = 8.1 Hz); ESI-MS: Found [M+1] 369; ; Elemental analyses
for calcd. (found) %: C 61.95 (61.82), H 5.43 (5.38), N 15.21
(15.25).
4-(Phenyl)-6-(4-chlorophenyl)-1,4-dihydropyrimidine-
2-yl-oxy-N,N-dimethylmethanamine (2c): Colour: white
amorphous solid, m.w. 341, m.p. 228-230 ºC, Rf: 0.52; IR (KBr,
ν
max, cm-1): 752, 3213, 1083, 1521, 3050, 2778, 1609, 962,
810; 1H NMR (300 MHz, CDCl3) δ: 2.6 (s, 6H, 2CH3), 4.7 (s,
2H, CH2), 8.2 (s, 1H, NH), 6.4-7.5 (m, 9H, Ar-H), 5.4 (d, 1H,
CH), 6.0 (d, 1H, C=CH, J = 5.4 Hz); ESI-MS: Found [M+1]
343; Elemental analyses for calcd. (found) %: C 64.77 (64.72),
H 5.68 (5.56)), N 15.90 (15.84).
4-(4-Chlorophenyl)-6-(3-nitrophenyl)-1,4-dihydropy-
rimidine-2-yl-oxy-N,N-dimethylmethanamine (2d): Colour:
Yellowish amorphous solid, m.w. 386, m.p. 198-201 ºC, Rf:
0.71; IR (KBr, νmax, cm-1): 759, 1514, 3190, 1298, 1242, 3034,
2884, 1630, 978, 806; 1H NMR (300 MHz, CDCl3) δ:2.4 (s,6H,
2CH3), 4.9 (s, 2H, CH2), 8.0 (s, 1H, NH), 7.0-7.8 (m, 8H, Ar-
H), 5.2 (d, 1H, CH), 6.1 (d, 1H, C=CH, J = 4.7 Hz); ESI-MS:
Found [M+] 387; Elemental analyses for calcd. (found) %: C
58.91 (59.02), H 4.90 (4.96), N 14.47 (14.38).
4-(2-Chlorophenyl)-6-(3-chlorophenyl)-1,4-dihydro-
pyrimidine-2-yl-oxy-N,N-dimethylmethanamine (2e):
Colour: brownish amorphous solid, m.w. 375, m.p. 234-236
ºC, Rf: 0.48; IR (KBr, νmax, cm-1): 730, 752, 3282, 1230, 1354,
3072, 2926, 1636, 954, 778; 1H NMR (300 MHz, CDCl3) δ:
4-(4-Methoxyphenyl)-6-(4-chlorophenyl)-1,4-dihydro-
pyrimidine-2(1H)-one-3-yl-N,N-dimethylmethanamine
(3a): Colour: white amorphous solid, m.w. 371, m.p. 231-233
ºC, Rf: 0.62; IR (KBr, νmax, cm-1): 743, 1670, 3208, 1220, 1285,
3024, 2916, 1592, 944, 780; 1H NMR (300 MHz, CDCl3) δ:
2.3 (s,6H, 2CH3), 4.2 (s, 2H, CH2), 8.1 (s, 1H, NH), 3.3 (O-
CH3), 8.8 (m,1H, OH), 7.1-7.6 (m, 8H,Ar-H), 4.7 (d, 1H, CH),
5.6 (d, 1H, C=CH, J = 4.2 Hz); ESI-MS: Found [M+1] 373;

1770 Mamidala et al.
Asian J. Chem.
Elemental analyses for calcd. (found) %: C 64.51 (64.58), H
5.91 (5.86), N 11.29 (11.22).
pale brownish amorphous solid, m.w. 339, m.p. 204-206 ºC,
Rf: 0.37; IR (KBr, νmax, cm-1): 3390, 3332, 1666, 1224, 3082,
2912, 1594, 980, 726; 1H NMR (300 MHz, CDCl3) δ: 1.8 (s,
6H, 2CH3), 3.7 (s, 2H, CH2), 8.6 (s, 1H, NH), 5.6 (s, 1H, OH),
5.9 (s,1H, OH), 6.8-7.6 (m, 8H, Ar-H), 5.1 (d, 1H, CH), 6.2
(d, 1H, C=CH, J = 6.8 Hz); ESI-MS: Found [M+1] 341;
Elemental analyses for calcd. (found) %: C 67.25 (67.34), H
6.19 (6.15), N 12.38 (12.33).
4-(4-Chlorophenyl)-6-(phenyl)-1,4-dihydropyrimidine-
2(1H)-one-3-yl-N,N-dimethyl methanamine (3b): Colour:
pale yellowish amorphous solid, m.w. 341, m.p. 207-209 ºC,
Rf: 0.68; IR (KBr, νmax, cm-1): 739, 3380, 1687, 1340, 3012,
2842, 1598,1152, 810-982; 1H NMR (300MHz, CDCl3) ?
2.5 (s,6H, 2CH3), 3.8 (s, 2H, CH2), 8.6 (s, 1H, NH), 7.2-7.9
(m, 9H, Ar.H), 4.3 (d, 1H, CH), 5.2 (d, 1H, C=CH, J=7.4 Hz);
ESI-MS: Found [M+] 342; Elemental analyses for calcd.
(found) %: C 66.66 (66.69), H 5.84 (5.88), N 12.28 (12.21).
4-(Phenyl)-6-(4-chlorophenyl)-1,4-dihydropyrimidine-
2(1H)-one-3-yl-N,N-dimethylmethanamine (3c): Colour:
yellowish crystalline solid, m.w. 341, m.p. 240-242 ºC, Rf: 0.46;
IR (KBr, νmax, cm-1): 759, 3388, 1606, 1221, 3096, 2962, 1621,
4-(Phenyl)-6-(3-nitrophenyl)-1,4-dihydropyrimidine-
2(1H)-one-3-yl-N,N-dimethylmethanamine (3i): Colour:
Yellowish amorphous solid, m.w. 352, m.p. 190-192 ºC, Rf:
0.79; IR (KBr, νmax, cm-1): 3510, 1524, 1724, 1222, 3046, 2892,
1
1576, 954, 786; H NMR (300 MHz, CDCl3) δ: 3.1 (s, 6H,
2CH3), 3.6 (s, 2H, CH2), 8.2 (s, 1H, NH), 6.9-7.7 (m, 9H, Ar-
H), 4.4 (d, 1H, CH), 5.6 (d, 1H, C=CH, J = 4.6 Hz); ESI-MS:
Found [M+] 352; Elemental analyses for calcd. (found) %: C
64.77 (64.79), H 5.68 (5.69), N 15.90 (15.89).
1
976, 820; H NMR (300 MHz, CDCl3) δ: 3.2 (s,6H, 2CH3),
3.7 (s, 2H, CH2), 8.9 (s, 1H, NH), 7.0-7.6 (m, 9H, Ar-H), 4.5
(d, 1H, CH), 5.5 (d, 1H, C=CH, J = 8.5 Hz); ESI-MS: Found
[M+1] 343; Elemental analyses for calcd. (found) %: C 66.66
(66.62), H 5.84 (5.81), N 12.28 (12.25).
4-(4-Hydroxyphenyl)-6-(3-nitrophenyl)-1,4-dihydro-
pyrimidine-2(1H)-one-3-yl-N,N-dimethylmethanamine
(3j): Colour: pale brownish crystalline solid, m.w. 368, m.p.
241-243 ºC, m.p. 229-231 ºC, Rf -0.82; IR (KBr, νmax, cm-1):
3395, 1512, 3355, 1706, 1245, 3030, 2972, 1624, 964, 726;
1H NMR (300 MHz, CDCl3) δ: 2.5 (s,6H, 2CH3), 3.4 (s, 2H,
CH2), 8.0 (s, 1H, NH), 4.9 (s,1H, OH), 7.0-7.7 (m, 8H, Ar-H),
5.3 (d, 1H, CH), 6.2 (d, 1H, C=CH, J = 8.2 Hz); ESI-MS:
Found [M+1] 369; Elemental analyses for calcd. (found) %:
C 61.95 (61.91), H 5.43 (5.45), N 15.21 (15.26).
4-(4-Chlorophenyl)-6-(3-nitrophenyl)-1,4-dihydropy-
rimidine-2(1H)-one-3-yl-N,N-dimethyl methanamine (3k):
Colour: yellowish crystalline solid, m.w. 386, m.p. 241-243
ºC, Rf: 0.84; IR (KBr, νmax, cm-1): 730, 1516, 3291, 1714, 1320,
3008, 2946, 1606, 1240, 972, 740; 1H NMR (300 MHz, CDCl3)
δ: 2.6 (s,6H, 2CH3), 4.2 (s, 2H, CH2), 8.3 (s, 1H, NH), 6.6-7.4
(m, 8H,Ar-H), 5.0 (d, 1H, CH), 5.8 (d, 1H, C=CH, J = 5.6 Hz);
ESI-MS: Found [M+1] 388; Elemental analyses for calcd. (found)
%: C 58.91 (58.82), H 4.90 (4.88), N 14.47 (14.42).
4,6-bis-Phenyl-1,4-dihydropyrimidine-2(1H)-one-3-yl-
N,N-dimethylmethanamine (3d): Colour: pale brownish
amorphous solid, m.w. 307, m.p. 215-217 ºC, Rf: 0.41; IR (KBr,
ν
max, cm-1): 3332, 1676,1246, 3116, 2930, 1618, 732-926; 1H
NMR (300 MHz, CDCl3) δ: 2.6 (s,6H, 2CH3), 3.4 (s, 2H, CH2),
7.9 (s, 1H, NH), 6.6-7.2 (m, 10H, Ar-H), 3.9 (d, 1H, CH), 5.6
(d, 1H, C=CH, J = 3.9 Hz); ESI-MS: Found [M+] 307;
Elemental analyses for calcd. (found) %: C 74.26 (74.31), H
6.84 (6.77), N 13.68 (13.64).
4,6-bis-(4-Chlorophenyl)-1,4-dihydropyrimidine-2(1H)-
one-3-yl-N,N-dimethyl methanamine (3e): Colour: pale red
crystalline solid, m.w. 375, m.p. > 250 ºC, Rf: 0.36; IR (KBr,
ν
max, cm-1): 747, 753, 3452, 1668, 1326, 3056, 2942, 1609,
945, 792; 1H NMR (300 MHz, CDCl3) δ: 2.4 (s, 6H, 2CH3),
2.9 (s, 2H, CH2), 9.2 (s, 1H, NH), 7.0-7.7 (m, 8H, Ar.H), 4.8
(d, 1H, CH), 8.1 (d, 1H, C=CH, J = 4.5 Hz); ESI-MS: Found
[M+2] 379; Elemental analyses for calcd. (found) %: C 60.63
(60.60), H 5.05 (5.09), N 11.17 (11.13).
RESULTS AND DISCUSSION
4-(Phenyl)-6-(4-hydroxyphenyl)-1,4-dihydropyrim-
idine-2(1H)-one-3-yl-N,N-dimethylmethanamine (3f): Colour:
pale yellowish amorphous solid, m.w. 323, m.p. 196-198 ºC,
Rf: 0.34; IR (KBr, νmax, cm-1): 3462, 3421, 1715, 1358, 3124,
2885, 1614, 916, 765; 1H NMR (300 MHz, CDCl3) δ: 2.9 (s,6H,
2CH3), 3.6 (s, 2H, CH2), 9.1 (s, 1H, NH), 5.6 (s, 1H, OH),
7.2-7.8 (m, 9H, Ar-H), 4.4 (d, 1H, CH), 6.3 (d, 1H, C=CH, J =
5.2 Hz); ESI-MS: Found [M+] 323; Elemental analyses for
calcd. (found) %: C 70.58 (70.51), H 6.50 (6.42), N 13.00
(12.57).
4-(4-Hydroxyphenyl)-6-(phenyl)-1,4-dihydropyrimidine-
2(1H)-one-3-yl-N,N-dimethylmethanamine (3g): Colour:
yellowish crystalline solid, m.w. 323, m.p. 237-239 ºC, Rf: 0.57;
IR (KBr, νmax, cm-1): 3420, 3232, 1682, 1202, 3174, 2933, 1588,
939, 790; 1H NMR (300 MHz, CDCl3) δ: 2.0 (s, 6H, 2CH3),
4.1 (s, 2H, CH2), 8.4 (s, 1H, NH), 4.8 (s,1H, OH), 7.1-8.0 (m,
9H, Ar-H), 4.8 (d, 1H, CH), 5.6 (d, 1H, C=CH, J = 7.1 Hz);
ESI-MS: Found [M+] 323; Elemental analyses for calcd.
(found) %: C 70.58 (70.49), H 6.50 (6.53), N 13.00 (12.52).
4,6-bis-(4-Hydroxyphenyl)-1,4-dihydropyrimidine-
2(1H)-one-3-yl-N,N-dimethylmethanamine (3h): Colour:
The synthesis of a series of O- and N-Mannich bases of
4,6-diaryl-3,4-dihydropyrimidinones is carried out as outlined
in Scheme-I. Initially 3,4-DHPMs (1a-m) were obtained by
modified Biginelli reaction by a multi-component system comp-
rising substituted benzaldehydes, substituted acetophenones
and urea in the presence of catalytic amount of p-toluene sulphonic
acid (PTSA) in ethanol [20] under microwave irradiation. Unlike
other lewis acid catalysts, PTSA does not require any anhydrous
conditions [21]. Simple distilled ethanol without further drying
was employed as solvent.
The titled compounds, O- and N-Mannich bases of DHPMs
(2a-j and 3a-k, respectively) were synthesized by taking equi-
molar quantity of DHPMs obtained in step 1 (1a-m), 37 %
formaldehyde, dimethylamine and either with or without K2CO3
under microwave irradiation [14]. All the compounds were
obtained in good yield (60-94 %). They were further purified
by recrystallization and column chromatography. The physical
data of the synthesized compounds is shown in Table-1 in
which only compound 2a was reported by Venkateswarlu et
al. [15] and all others are new molecules. To study the effect

Vol. 31, No. 8 (2019)
SAR Drug Likeness and Evaluation of Antioxidant Activity of Some Mannich Bases of Dihydropyrimidinones 1771
TABLE-1
TABLE-2
PHYSICAL DATA OF SYNTHESIZED
ANTIOXIDANT ACTIVITY (IC50, µM*) OF
COMPOUNDS (2a-j AND 3a-k)
MANNICH BASES (2a-j AND 3a-k)
Yield (%)
Reported Obtained*
Reaction
time (s)
DPPH
scavenging
activity
H2O2
scavenging
activity
NO
scavenging
activity
Iron
chelation
activity
Compd.
R
R1
Compd.
4-OCH3 3-NO2
4-Cl
-H
4-Cl
2-Cl
-H
4-OH
4-OH
-H
4-OH
4-OCH3
4-Cl
-H
-H
4-Cl
-H
135
225
260
340
200
160
220
310
380
280
200
280
170
200
290
400
320
340
380
190
220
64a
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
85
60
74
81
72
65
79
68
82
76
81
75
68
80
64
77
86
94
88
62
74
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
3a
3b
3c
3d
3e
3f
3g
3h
3i
0.27 0.043
0.33 0.023
0.36 0.081
0.44 0.029
0.40 0.062
0.19 0.084
0.16 0.045
0.13 0.074
0.29 0.059
0.23 0.042
0.24 0.056
0.35 0.069
0.37 0.091
0.26 0.051
0.45 0.072
0.15 0.036
0.16 0.024
0.12 0.038
0.29 0.073
0.19 0.045
0.51 0.064
0.24 0.087
0.31 0.052
0.32 0.042
0.42 0.081
0.39 0.066
0.17 0.023
0.20 0.036
0.15 0.021
0.35 0.032
0.27 0.056
0.27 0.087
0.36 0.052
0.34 0.042
0.33 0.081
0.39 0.066
0.22 0.023
0.19 0.036
0.16 0.021
0.44 0.032
0.30 0.056
0.37 0.046
0.16 0.091
0.29 0.065
0.28 0.045
0.34 0.076
0.37 0.023
0.20 0.096
0.18 0.054
0.17 0.032
0.26 0.046
0.25 0.038
0.26 0.091
0.37 0.065
0.38 0.045
0.39 0.076
0.45 0.023
0.29 0.096
0.24 0.054
0.19 0.032
0.35 0.046
0.33 0.038
0.41 0.027
0.15 0.044
0.38 0.053
0.36 0.078
0.21 0.049
0.42 0.092
0.27 0.084
0.24 0.073
0.23 0.082
0.20 0.068
0.18 0.026
0.27 0.044
0.35 0.053
0.41 0.078
0.51 0.049
0.47 0.092
0.25 0.084
0.22 0.073
0.17 0.042
0.20 0.068
0.16 0.026
0.29 0.058
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
4-Cl
3-NO2
3-Cl
4-OH
H
4-OH
3-NO2
3-NO2
4-Cl
H
4-Cl
H
4-Cl
4-OH
H
2j
3a
3b
3c
3d
3e
3f
3g
3h
3i
4-OH
4-OH
-H
4-OH
4-Cl
4-OH
3-NO2
3-NO2
3-NO2
3j
3j
3k
3k
aa[[RReeff.. 1155]]
*IC50 values were expressed as Mean SEM (standard error mean).
of substitution on antioxidant activity, a series of Mannich bases
were prepared by substituting the phenyl rings present at 4th
and 6th positions of DHPMs with various electron donating
and electron withdrawing groups. Regioselectivity was achieved
by employing weak base like K2CO3.
pared to compounds 3h and 2h due to the presence of single
hydroxyl groups. However, these compounds exhibited better
antioxidant activity compared to unsubstituted ones (3d) and
the compounds substituted with electron withdrawing groups
such as nitro and chloro groups (2i, 2d, 3e, 3k). The compounds
2j and 3j substituted with one hydroxyl group at p-position
on one phenyl ring and with nitro group at m-position in other
phenyl ring exhibited lower activity compared to compounds
with single hydroxyl group such as compounds 2g, 2f, 3g and
3f. The negative mesomeric effect of m-nitro group could be
the reason for this. Compounds 2a and 3a exhibited moderate
to higher activity compared to compounds 2i and 3i, respec-
tively. This may be due to predominant stabilization by electron
donating methoxy group counteracting the electron withdra-
wing effect of chloro and nitro groups on the other ring. The
compounds 2b, 2c, 3b and 3c are less active compared to
compounds 2i and 3i. These four compounds have one chloro
substitution on either of the rings destabilizing the radical
formed by virtue of its electronegativity. The same effect is
responsible for the least scavenging effects of compounds 2d,
2e, 3e and 3k. The compound 2e exhibits better activity than
compound 2d in O-Mannich bases (similarly compound 3e
exhibits more activity than compound 3k in N-Mannich bases).
The compounds 2e and 3e contains one chlorine group on
both phenyl rings. Among these, compound 2e exhibits better
activity than compound 3e. In compound 3e, two chlorine
groups are present in p-positions of two phenyl rings, which
would strongly destabilizes the rings, whereas in compound
2e, the two chlorine groups occupy m- and o- positions of the
two phenyl rings. However, variation in o-, m- and p-positions
has shown very little effect on scavenging. The least scavenging
ability of compounds 2d and 3k is attributed to the presence
Antioxidant activity
DPPH scavenging activity: Till date, antioxidant activity
of substituted DHPMs are unexplored. In the present study,
efforts have been laid down to evaluate their radical scavenging
potency. As presented in Table-2, O- and N-Mannich bases of
DHPMs (2a-j and 3a-k, respectively) with IC50 values in the
range of 0.13-0.19 µM showed higher DPPH radical scavenging
activities compared to standard ascorbic acid. This can be
attributed to the H+ donating capacity of hydroxyl aryl moieties
of DHPMs.
Structural activity relationship (SAR) studies revealed that
the phenolic group is essential for radical scavenging activity, as
the compounds with phenolic hydroxyl groups 3h, 2h, 3f, 3g,
2g and 2f exhibited profound activity. This could be attributed
to the stability of phenoxide ion formed in the due course. It is
well established that phenolic hydroxyl group is responsible
for antioxidant activity [22]. In compounds 3h and 2h, two
hydroxyl groups are present on phenyl rings (one on each) at
p-position, therefore the radical ion generated from the abstrac-
tion of either H atom (from either of hydroxyl groups) by
DPPH would be stabilized by other hydroxyl group and vice-
versa. Moreover, an additional stability can be attributed by
the formation of phenoxide ion. Wherein, the phenoxide ion
is in conjugation with dihydropyrimidine ring enhancing its
stability. Thus, compounds 3h and 2h with two hydroxyl
groups at p-position on phenyl rings exhibited highest activity.
The compounds 2g, 2f, 3g and 3f exhibited less activity com-

1772 Mamidala et al.
Asian J. Chem.
of strong electron withdrawing groups chlorine and nitro on
one phenyl ring each. In both compounds, chlorine is present
at p-position and nitro in m-position. Both the groups strongly
destabilize the rings and are responsible for least activity.
Alternatively, compound 3d with unsubstituted phenyl rings
exhibited better activity than the compounds substituted with
electron withdrawing groups (like compounds 3e, 3b, 3k, etc.)
but showed lesser activity compared to compounds with electron
donating groups (such as compounds 3h, 3g and 3a) for obvious
reasons mentioned above.
Among DPHM series, the above observations clearly
depicted that phenyl rings substituted with electron donating
groups is a prerequisite for the scavenging of free radicals along
with the DHPM pharmacophore, but alone it is insufficient
for radical scavenging activity. Comparatively N-Mannich
bases exhibited slightly better activity than O-Mannich bases.
N-Mannich bases might add additional stability to the ring
systems as two methyl groups present on 'N' are strongly electron
donating in nature. Furthermore, one more hydroxyl group
(as ketone oxygen at 2nd position of DHPMs can exist in enol
form) is added to the DHPM ring system in N-Mannich bases.
This hydroxyl moiety may also participate in radical scavenging
as well as ring stabilization, whereas the same is not possible
in O-Mannich bases.
H2O2 scavenging activity:Among the synthesized comp-
ounds, 2h and 3h exhibited highest activity in this method
also for the same reasons mentioned above. The compounds
substituted with electron donating groups such as hydroxyl,
methoxy (2f, 2g, 3f, 3g, 2a and 3a) showed moderate to good
activity. The compounds with electron withdrawing groups
such as chloro and nitro at various positions (2b, 2c, 3b, 3c,
2i, 2e, 3i, 3e and 3k) exhibited poor activity. This may be due
to its inability to donate a hydrogen atom to quench the peroxide
radical as discussed above.
nature of methoxy and hydroxyl groups (at p-position on other
phenyl ring) by stabilizing the ring system. SAR studies reveal
that all molecules containing nitro group such as 2i, 2j, 2d, 3i,
3k exhibited better activity compared to others. The comp-
ounds 2h and 3h were also found to exhibit good activity due
to the presence of hydroxyl group at p-position on phenyl
ring. Compounds containing neither nitro group nor hydroxyl
group exhibited poor activity due to lack of iron chelating
atoms. The presence of chlorine in most of these compounds
may decrease the iron chelation activity by virtue of its electron
negativity.
Drug likeness: The physico-chemical properties of all the
synthesized compounds 2a-j and 3a-k have been calculated
in silico and presented in Table-3. Total polar surface area
(TPSA) [25] is directly correlated with the transport of mole-
cules across membranes. All the compounds showed TPSA
values within the range of limit (< 140 Å). The clogP value is
a legitimate measure to determine their hydrophilicity.
Compounds with low clogP (< 5) values possess good perme-
ability, there by enhanced bioavailability. All the O- and N-
Mannich bases (2a-j and 3a-k) exhibited acceptable clogP
value in the range of 0.53-3.46. Interestingly compound 3h,
with excellent antioxidant activity was found to contain least
clogP value. Other molecular descriptors such as molecular
weight, hydrogen bond donor /acceptors and the number of
rotatable bonds (ROTB), were found to be in acceptable range
and follow the Lipinski's rule of five. The number of ROTB
were also found in between 4-6 (≥ 6) [26]. In a nutshell, in
silico evaluation of the physico-chemical parameters portrays
the drug likeness of synthesized compounds.
TABLE-3
PHYSICO-CHEMICAL PROPERTIES OF SYNTHESIZED
O-MANNICH (2a-j) AND N-MANNICH BASES (3a-k)
NO scavenging activity: Targeting the NO production is
a promising strategy in search of novel potent drugs against
inflammatory related degenerative diseases [23]. In terms of
NO inhibitory activity, Mannich bases of DHPMs substituted
with hydroxyl groups displayed excellent activity (3h, 3g, 2h
and 2g). In this case, it seems to be that presence of hydroxyl
group in p-position is a prerequisite. Further, the presence of
methoxy group at p-position of phenyl ring (2a and 3a) also
displayed better activity. This may be due to the strong electron
donating nature of the methoxy groups. But in the absence of
electron donating groups, the effect of electron withdrawing
groups is predominant as seen in compounds 2b, 2c, 2d, 2e,
3c, 3k, 3e. Optimum activity of compounds 2i and 3j is attri-
buted to the presence of hydroxyl group on phenyl ring at
p-position, inspite of having electron withdrawing groups.
Iron chelation activity:Accumulation of unmetabolized
excess of free elemental iron is toxic and builds up various
ROS via Fenton reaction [24]. Therefore, the synthesized comp-
ounds were also evaluated for their iron chelation efficiency.
Among all the compounds, 2a and 3j exhibited excellent metal
chelation capacity. This may be due to the involvement of oxygens
(of nitro group at m-position on phenyl ring) by forming coor-
dinate covalent bonds with metallic iron. Further the metal
chelation activity may be enhanced with the electron donating
Physico-chemical properties
Compd.
MW
TPSA
36.86
36.86
36.86
80
HBA
4
HBD
1
ROTB cLogP
2a
2b
2c
2d
2e
2f
2g
2h
2i
382.16
341.13
341.13
386.11
375.09
323.16
323.16
339.16
352.15
5
5
5
6
5
5
5
5
6
6
4
4
4
4
4
4
4
4
5
5
5
2.59
3.31
3.31
2.89
3.46
2.65
2.65
1.94
2.95
2.23
1.19
1.91
1.91
1.96
1.85
1.25
1.25
0.53
1.54
0.83
1.49
4
4
4
4
5
5
6
4
1
1
1
1
2
2
3
1
36.86
57.09
57.09
77.32
80
2j
368.15 100.23
5
2
3a
3b
3c
3d
3e
3f
3g
3h
3i
371.52
341.13
341.13
307.17
375.09
323.16
323.16
339.16
352.15
368.15
386.11
35.58
35.58
35.58
35.58
35.58
55.81
55.81
76.04
78.72
98.95
78.72
4
4
4
4
4
5
5
6
1
1
1
1
1
2
2
3
4
5
4
1
2
1
3j
3k
MW = Molecular weight; TPSA = Total polar surface area; HBA =
Hydrogen bond acceptor; HBD = Hydrogen bond donor; ROTB =
Number of rotatable bonds.

Vol. 31, No. 8 (2019)
SAR Drug Likeness and Evaluation of Antioxidant Activity of Some Mannich Bases of Dihydropyrimidinones 1773
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Compounds 3h, 3g, 2h and 2g exhibited better radical
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Structure activity relationship studies revealed the effect of
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improve their antioxidant properties. This adds a top up antioxidant
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ACKNOWLEDGEMENTS
The authors sincerely thank Indian Institute of Chemical
Technology, Hyderabad, India for providing the spectral data.
One of the authors (RM) is grateful to the management of St.
Peter's Institute of Pharmaceutical Sciences, Warangal, India
for their constant encouragement and providing all the necessary
facilities for completion of the work.
CONFLICT OF INTEREST
18. B. Ramesh and C.M. Bhalgat, Eur. J. Med. Chem., 46, 1882 (2011);
https://doi.org/10.1016/j.ejmech.2011.02.052.
19. E.V. Buravlev, O.G. Shevchenko, A.A. Anisimov and K.Y. Suponitsky,
Eur. J. Med. Chem., 152, 10 (2018);
The authors declare that there is no conflict of interests
regarding the publication of this article.
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