Synthesis, antioxidant and antimicrobial properties of novel pyridyl-carbonyl thiazoles as dendrodoine analogs

Article abstract of DOI:10.3906/kim-2008-8

Marine compound dendrodoine was first obtained from tunicate species (Dendrodo grossularia). It has a five-membered ring, namely, it is a heterocycle thiadiazole, which is found rarely in natural sources. Following its biological activities, novel analogs have been investigated recently. Synthesis of the analogs for this study is realized with uncommon thiazole closure, including methylene-carbonyl condensation. Structures are elucidated by NMR (¹H, ¹³C) and HRMS spectrums. As an alkaloid derivative, antioxidant properties were evaluated with DPPH and FRAP assays and antimicrobial effect with microdilution method. Among the series, 3bc-3cf showed higher antioxidant activity than those having 3 or 4-pyridyl substituents. There is lesser activity for 2-pyridyl activity for 2-pyridyl containing group, which may be a result of intramolecular interactions. No activity was observed against gram-negative bacteria at 250 μg/mL. 3ae and 3ce showed activity at 64 μg/mL against S. aureus and 3ae showed activity at 16 μg/mL against S. epidermidis gram-positive bacteria. Chloramphenicol showed activity against all microorganisms at 8-16 μg/mL. Sixteen original dendrodoine analogs have been defined by close/higher activity compared to dendrodoine analogs and Trolox.

Full text of DOI:10.3906/kim-2008-8

Turkish Journal of Chemistry
Turk J Chem
2020) 44: 1733-1741
TÜBİTAK
doi:10.3906/kim-2008-8
(
©
http://journals.tubitak.gov.tr/chem/
Research Article
Synthesis, antioxidant and antimicrobial properties of novel pyridyl-carbonyl thiazoles
as dendrodoine analogs
1
,
2
3
1
Zafer ŞAHİN *, Sevde Nur BİLTEKİN , Leyla YURTTAŞ , Şeref DEMİRAYAK 
1
Department of Pharmaceutical Chemistry, School of Pharmacy, İstanbul Medipol University, İstanbul, Turkey
2
Department of Pharmaceutical Microbiology, School of Pharmacy, İstanbul Medipol University, İstanbul, Turkey
3
Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Anadolu University, Eskişehir, Turkey
Received: 06.08.2020
Accepted/Published Online: 06.11.2020
Final Version: 16.12.2020
Abstract: Marine compound dendrodoine was first obtained from tunicate species (Dendrodo grossularia). It has a five-membered ring,
namely, it is a heterocycle thiadiazole, which is found rarely in natural sources. Following its biological activities, novel analogs have been
investigated recently. Synthesis of the analogs for this study is realized with uncommon thiazole closure, including methylene-carbonyl
1
13
condensation. Structures are elucidated by NMR ( H, C) and HRMS spectrums. As an alkaloid derivative, antioxidant properties were
evaluated with DPPH and FRAP assays and antimicrobial effect with microdilution method. Among the series, 3bc-3cf showed higher
antioxidant activity than those having 3 or 4-pyridyl substituents. ere is lesser activity for 2-pyridyl activity for 2-pyridyl containing
group, which may be a result of intramolecular interactions. No activity was observed against gram-negative bacteria at 250 μg/mL. 3ae
and 3ce showed activity at 64 μg/mL against S. aureus and 3ae showed activity at 16 μg/mL against S. epidermidis gram-positive bacteria.
Chloramphenicol showed activity against all microorganisms at 8–16 μg/mL. Sixteen original dendrodoine analogs have been defined
by close/higher activity compared to dendrodoine analogs and Trolox.
Key words: Antꢀoxꢀdant, antꢀmꢀcrobꢀal, thꢀazole, dendrodoꢀne
1
. Introduction
Marine tunicates are animals living in different deepness levels of oceans and are usually attached to docks. Dendrodoine
is an alkaloid that was isolated from Dendroda grossular indigenous in North Britain in 1980[1]. Dendrodoine synthesis
was accomplished 4 years af er the first isolation. It has a thiadiazole ring which is rarely found in natural compounds, and
it is known to be a part of functional synthetic compounds [2]. Beyond its unique thiadiazole ring, it is also a bioisoster
of aminothiazoles. Aminothiazoles have a diverse biological spectrum including anticancer, antimicrobial, antioxidant,
antidiabetic, anticonvulsant, antiinflammatory, antihypertensive, and protective activities. ere are approved drugs,
such as meloxicam, cefdinir, famotidine, bearing this moiety [3–5].
Atoms with lone-pair electrons can be very dangerous if they are converted to reactive oxygen species. ese reactive
molecules can interact with cellular mechanisms. In biological mechanisms, O reduces to H O via electron transfer
2
2
pathways such as NADP/NADPH. And this process produces free-radicals (FRs) or reactive oxygen species (ROSs).
6]. FRs or ROSs have homeostatic functions at normal conditions and they are normally produced in small amounts;
[
however, overexpression of them may propagate harmful chain reactions using DNA, RNA, proteins, and lipids as
substrate. Finally, these reactions may be the cause of various pathogenic processes such as tumor growth and oxidative
stress [7–9].
An analog synthesis is a functional approach in medicinal chemistry. In this respect, natural compounds have been
extensively researched. ere is only a few studies about dendrodoine analogs (Figure 1), and these studies are usually
based on antioxidant activity [1,10-14].
In this study, we have synthesized trisubstituted aminothiazole compounds bearing pyridoyl and phenyl groups and
have evaluated their antioxidant and antimicrobial properties. DPPH free-radical scavenging and FRAP iron reducing
properties were measured for antioxidant capacity. Besides, antimicrobial properties were evaluated against 8 gram-
positive and gram-negative bacteria.
*
Correspondence: zsahin@medipol.edu.tr
1733
is work is licensed under a Creative Commons Attribution 4.0 International License.

ŞAHİN et al. / Turk J Chem
Figure 1. Structures of the dendrodoine analogs.
2
. Materials and methods
2
.1. Synthesis

e reactants necessary for the synthesis process were purchased from Sigma-Aldrich Chemical Corp. Melting point of title
1
molecules were accomplished by a Stuart melting point apparatus and experiments were performed in duplicate. H-NMR
1
3
and C-NMR spectrums were recorded in Bruker 300 MHz UltraShield NMR and Bruker 75 MHz UltraShield NMR,
respectively. DMSO-d was used as solvent and TMS was used as standard. High resolution mass spectrums were recorded
6
in Shimadzu 8040 LC/MS/MS ITTOF system with the electron spray method (ESI). IC values of the title molecules were
5
0
calculated using GraphPad Prism sof ware version 7.02.
2
.1.1. Synthesis of benzoyl thioureas (2a-f)
Ammonium thiocyanate (500 mmol) was dissolved in acetone. Af er it becomes a clear solution, 500 mmol benzoyl
chloride was added to it quite slowly. e mixture became white blurry af er 30 min. An equivalent mole of corresponding
amine (500 mmol) was added. e mixture was then boiled for 6 h. e finalization of the process was decided with thin-
layer chromatography using ethyl acetate 1:1 petroleum ether solvent system. Precipitation was filtered off, and mixed with
water. e obtained solid material was recrystallized using ethyl alcohol [15].
2
.1.2. Synthesis of 2-bromoacetylpyridines (1a-c)
Acetyl pyridine (250 mmol) was dissolved in chloroform. Af er 15 min stirring, 250 mmol bromine (diluted in chloroform)
was added quite slowly to the mixture. It was checked with thin-layer chromatography that reaction ended, and then solid
material was gathered on the bottom. Because of the basic nitrogen, each compound was formed as HBr salt. Compounds
were used as HBr salt in the next step [16].
2
.1.3. Synthesis of target compounds (3aa-af, 3ba-bf, 3cc-cf)
e target molecules were obtained using a distinct ring closure method including methylene-carbonyl condensation
Figure 2). During reactions, bromoacetylpyridines (1a-c) (20 mmol) and (2a-f) (20 mmol) were mixed and heated in
ethyl alcohol. When it was certain that the reaction ended, the mixture was lef for cooling (Table 1). When it reached 15–
0 °C, it was poured into cold water (50 mL), and then it was added to water and neutralization was made using NaHCO .

(
2

3
e final compounds were dissolved and recrystallized using ethyl alcohol [17-21].
(
2-(Dimethylamino)-4-phenylthiazol-5-yl)(pyridin-2-yl)methanone (3aa)
1
Yield: 82%. m.p.: 160–161 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 3.19 (6H, s, N(CH ) ), 7.25–7.32 (3H, m, Ar-
6
3 2
H), 7.43–7.53 (3H, m, Ar-H), 7.83 (1H, d, J: 7.76 Hz, Ar-H), 7.91 (1H, dt, J: 8.58, 1.70 Hz, Ar-H), 8.52 (1H, d, J: 4.92 Hz,
1
3
Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 40.5(N-CH ), 116.3, 123.2, 126.8, 127.8, 128.8, 129.9, 136.9, 137.9, 148.2,
6
3
+
1
3
54.9 (thiazole C ), 163.8 (pyridine C ), 173.9 (thiazole C ), 182.7 (C=O). HRMS (m/z): [M+H] calcd for C H N OS:
10.1005; found: 310.1009.
4
2
2
17 15
3
(
2-(Diethylamino)-4-phenylthiazol-5-yl)(pyridin-2-yl)methanone (3ab)
1
Yield: 75%. m.p.: 112–114 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.22 (6H, t, J: 7.03 Hz, (CH ) ), 3.57 (4H, q, J:
6
3 2
7
.06 Hz, N(CH ) ), 7.23–7.30 (3H, m, Ar-H), 7.42–7.49 (3H, m, Ar-H), 7.80–7.91 (2H, m, Ar-H), 8.48 (1H, d, J: 4.76 Hz,
2 2
13
Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 12.8 (CH ), 48.8 (N-CH ), 115.9, 123.3, 126.7, 127.3, 127.8, 128.8, 129.6,
6
3
2
1
29.9, 130.3, 130.8, 136.9, 137.8, 148.2, 155.1 (thiazole C ), 163.7 (pyridine C ), 172.3 (thiazole C ), 182.9 (C=O). HRMS
4 2 2
+
(
m/z): [M+H] calcd for C H N OS: 338.1314; found: 338.1322.
19 19 3
(
4-Phenyl-2-(pyrrolidin-1-yl)thiazol-5-yl)(pyridin-2-yl)methanone (3ac)
1
Yield: 70%. m.p.: 180-182 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 2.01 (4H, brs, (CH ) ), 3.51 (4H, brs, N(CH ) ),
6
2
2
2 2
1
3
7.27–7.32 (3H, m, Ar-H), 7.42–7.53 (3H, m, Ar-H), 7.81–7.91 (2H, m, Ar-H), 8.53 (1H, d, J: 4.66 Hz, Ar-H). C-NMR
1734

ŞAHİN et al. / Turk J Chem
Figure 2. Synthesis of the target compounds.
(
(
75 MHz, DMSO-d , ppm) δ: 25.6 (CH ), 49.9 (N-CH ), 115.7, 123.2, 126.8, 127.8, 128.3, 128.7, 129.9, 137.1, 148.2, 155.0
6 2 2
+
thiazole C ), 164.1 (pyridine C ), 170.5 (thiazole C ), 182.5 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 336.1152;
4
2
2
19 17
3
found: 336.1165.
(
4-Phenyl-2-(piperidin-1-yl)thiazol-5-yl)(pyridin-2-yl)methanone (3ad)
1
Yield: 68%. m.p.: 124.5 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.63 (6H, brs, (CH ) ), 3.60 (4H, brs, N(CH ) ),
6
2
3
2 2
7
.24–7.31 (3H, m, Ar-H), 7.44 (2H, d, J: 7.65 Hz, Ar-H), 7.47–7.52 (1H, m, Ar-H), 7.83 (1H, d, J: 7.58 Hz, Ar-H), 7.90 (1H,
13
dt, J: 7.55, 1.67 Hz, Ar-H), 8.51 (1H, d, J: 4.81 Hz, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 23.9 (CH ), 25.3 (CH ),
6
2
2
4
9.2 (N-CH ), 115.9, 123.3, 126.8, 127.8, 128.8, 129.9, 136.9, 137.9, 148.2, 154.9 (thiazole C ), 163.7 (pyridine C ), 173.6
2 4 2
+
(
thiazole C ), 182.8 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 350.1322; found: 350.1328.
2
2
0
1
9
3
(
2-(Azepan-1-yl)-4-phenylthiazol-5-yl)(pyridin-2-yl)methanone (3ae)
1
Yield: 85%. m.p.: 127–129 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.55 (4H, brs, (CH ) ), 1.79 (4H, brs, (CH ) ),
6
2
2
2 2
3
7
1
.66 (4H, brs, N(CH ) ), 7.22-7.33 (3H, m, Ar-H), 7.42–7.50 (3H, m, Ar-H), 7.82 (1H, d, J: 7.88 Hz, Ar-H), 7.89 (1H, dt, J:
2 2
13
.50, 1.73 Hz, Ar-H), 8.49 (1H, d, J: 4.81 Hz, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 27.5 (CH ), 115.8, 123.2, 126.7,
6
2
27.8, 128.8, 129.9, 136.9, 137.8, 148.2, 155.1 (thiazole C ), 163.7 (pyridine C ), 173.1 (thiazole C ), 182.7 (C=O). HRMS
4
2
2
+
(m/z): [M+H] calcd for C H N OS: 364.1480; found: 364.1478.
21 21 3
(
2-Morpholino-4-phenylthiazol-5-yl)(pyridin-2-yl)methanone (3af)
1
Yield: 90%. m.p.: 149–151 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 3.60 (4H, t, J: 4.88 Hz, N(CH ) ), 3.74 (4H, t, J:
6
2 2
4
7
.36 Hz, O(CH ) ), 7.25–7.33 (3H, m, Ar-H), 7.44 (2H, dd, J: 7.49, 1.92 Hz, Ar-H), 7.50–7.55 (1H, m, Ar), 7.86 (1H, td, J:
2 2
13
.67, 1.22 Hz, Ar-H), 7.93 (1H, td, J: 7.84, 1.74 Hz, Ar-H), 8.53 (1H, d, J: 4.53 Hz, Ar-H). C-NMR (75 MHz, DMSO-d ,
6
ppm) δ: 48.1 (N-CH ), 65.8 (CH -O), 116.2, 123.3, 127.01, 127.8, 128.9, 129.9, 136.7, 138.0, 148.2, 154.6 (thiazole C ), 163.2
2
2
4
+
(pyridine C ), 174.1 (thiazole C ), 182.9 (C=O). HRMS (m/z): [M+H] calcd for C H N O S: 352.1106; found: 352.1114.
2 2 19 17 3 2
(
2-(Dimethylamino)-4-phenylthiazol-5-yl)(pyridin-3-yl)methanone (3ba)
1
Yield: 68%. m.p. : 174–176 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 3.19 (6H, s, N(CH ) ), 7.05–7.17 (4H, m, Ar-
6
3 2
H), 7.23–7.26 (2H, m, Ar-H), 7.83 (1H, td, J: 7.82, 2.02 Hz, Ar-H), 8.38–8.42 (1H, m, Ar-H), 8.52 (1H, d, J: 4.92 Hz, Ar-H).
1
3
C-NMR (75 MHz, DMSO-d , ppm) δ: 40.3 (N-CH ), 122.4, 123.2, 128.0, 128.3, 129.1, 129.2, 130.4, 134.6, 136.2, 149.4,
6
3
+
1
3
51.5 (thiazole C ), 161.0 (pyridine C ), 171.9 (thiazole C ), 185.9 (C=O). HRMS (m/z): [M+H] calcd for C H N OS:
4
2
2
17 15
3
10.0998; found: 310.1009.
(
2-(Diethylamino)-4-phenylthiazol-5-yl)(pyridin-3-yl)methanone (3bb)
1
Yield: 62%. m.p.: 143–144 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.23 (6H, t, J:6.90 Hz, (CH ) ), 3.58 (4H, q, J:
6
3 2
7
1
1
1
.05 Hz, N(CH ) ), 7.05–7.11 (3H, m, Ar-H), 7.14–7.20 (1H, m, Ar-H), 7.23–7.26 (2H, m, Ar-H), 7.65 (1H, td, J: 8.06,
2 2
13
.95 Hz, Ar-H), 8.38–8.42 (2H, m, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 12.7 (CH ), 46.0 (N-CH ), 121.7, 123.1,
6
3
2
28.0, 128.3, 129.0, 129.2, 130.4, 134.7, 135.0, 136.1, 149.4, 151.5 (thiazole C ), 161.1 (pyridine C ), 170.4 (thiazole C ),
4
2
2
+
85.8 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 338.1323; found: 338.1322.
1
9
19
3
(
4-Phenyl-2-(pyrrolidin-1-yl)thiazol-5-yl)(pyridin-3-yl)methanone (3bc)
1
Yield: 83%. m.p.: 176–177 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 2.02 (4H, brs, (CH ) ), 3.51 (4H, brs, N(CH ) ),
6
2
2
2 2
1
3
7
.05–7.25 (6H, m, Ar-H), 7.64 (1H, td, J: 7.91, 2.09 Hz, Ar-H), 8.37–8.41 (2H, m, Ar-H). C-NMR (75 MHz, DMSO-d ,
6
1735

ŞAHİN et al. / Turk J Chem
Table 1. R groups of the synthesized compounds.
N
R1
S
O
N
R2
N
3
3
3
aa-3af
ba-3bf
cc-3cf
Isolated
yield (%)
Isolated
yield (%)
R1
R2
R1
R2
3
3
3
3
3
3
3
3
aa
ab
ac
ad
ae
af
2-pyridyle
2-pyridyle
2-pyridyle
2-pyridyle
2-pyridyle
2-pyridyle
3-pyridyle
3-pyridyle
Dimethylamine
Diethylamine
Pyrrolidine
Piperidine
Hexamethylamine
Morpholine
82
75
70
68
85
90
68
62
3bc
3-pyridyle
3-pyridyle
3-pyridyle
3-pyridyle
4-pyridyle
4-pyridyle
4-pyridyle
4-pyridyle
Pyrrolidine
Piperidine
Hexamethylamine
Morpholine
Pyrrolidine
Piperidine
Hexamethylamine
Morpholine
83
66
70
84
76
70
78
69
3bd
3be
3bf
3cc
3cd
3ce
3cf
ba
bb
Dimethylamine
Diethylamine
ppm) δ: 25.7 (CH ), 50.2 (N-CH ), 122.1, 123.1, 128.0, 128.3, 129.0, 129.2, 130.4, 134.7, 135.0, 136.1, 149.3, 151.5 (thiazole
2
2
+
C ), 161.3 (pyridine C ), 168.3 (thiazole C ), 185.85 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 336.1151; found:
4
2
2
19 17
3
3
36.1165.
(
4-Phenyl-2-(piperidin-1-yl)thiazol-5-yl)(pyridin-3-yl)methanone (3bd)
1
Yield: 66%. m.p.: 143–145 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.64 (6H, brs, (CH ) ), 3.62 (4H, brs, N(CH ) ),
6
2
3
2 2
1
3
7
.04–7.20 (4H, m, Ar-H), 7.22–7.25 (2H, m, Ar-H), 7.66 (1H, td, J: 7.98, 2.07 Hz, Ar-H), 8.38–8.42 (2H, m, Ar-H). C-NMR
(
75 MHz, DMSO-d , ppm) δ: 23.8 (CH ), 25.3 (CH ), 49.4 (N-CH ), 121.8, 123.2, 128.0, 129.2, 130.3, 134.6, 136.2, 149.4,
6 2 2 2
+
1
3
51.6 (thiazole C ), 161.0 (pyridine C ), 171.5 (thiazole C ), 186.0 (C=O). HRMS (m/z): [M+H] calcd for C H N OS:
4
2
2
20 19
3
50.1322; found: 350.1320.
(
2-(Azepan-1-yl)-4-phenylthiazol-5-yl)(pyridin-3-yl)methanone (3be)
1
Yield: 70%. m.p.: 130–132 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.56 (4H, brs, (CH ) ), 1.80 (4H, brs, (CH ) ),
6
2
2
2 2
3
8
1
.66 (4H, brs, N(CH ) ), 7.05–7.17 (4H, m, Ar-H), 7.23-7.25 (2H, m, Ar-H), 7.65 (1H, td, J: 7.84 Hz, 1.78 Hz, Ar-H),
2 2
13
.37–8.42 (2H, m, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 27.5 (CH ), 121.7, 123.1, 128.0, 129.2, 130.4, 134.7,
6
2
+
35.0, 136.1, 149.4, 151.5 (thiazole C ), 161.1 (pyridine C ), 171.2 (thiazole C ), 185.8 (C=O). HRMS (m/z): [M+H] calcd
4
2
2
for C H N OS: 364.1480; found: 364.1478.
2
1
21
3
(
2-Morpholino-4-phenylthiazol-5-yl)(pyridin-3-yl)methanone (3bf)
1
Yield: 84%. m.p.: 146–149 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 3.61 (4H, t, J: 4.37 Hz, N(CH ) ), 3.74 (4H, t, J:
6
2 2
4
.37 Hz, O(CH ) ), 7.06–7.18 (4H, m, Ar-H), 7.23–7.26 (2H, m, Ar-H), 7.68 (1H, td, J: 7.98 Hz, 2.06 Hz, Ar-H), 8.39–8.44
2 2
13
(
2H, m, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ: 48.2 (N-CH ), 65.8 (CH -O), 122.4, 123.2, 128.1, 129.3, 130.4,
6
2
2
+
1
34.4, 134.8, 136.3, 149.5, 151.7 (thiazole C ), 160.4 (pyridine C ), 172.0 (thiazole C ), 186.3 (C=O). HRMS (m/z): [M+H]
4
2
2
calcd for C H N O S: 352.1113; found: 352.1114.
1
9
17
3
2
(
4-Phenyl-2-(pyrrolidin-1-yl)thiazol-5-yl)(pyridin-4-yl)methanone (3cc)
1
Yield: 76%. m.p.: 169–171 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 2.03 (4H, brs, (CH ) ), 3.52 (4H, brs, N(CH ) ),
6
2
2
2 2
1
3
7
2
.04–7.09 (2H, m, Ar-H), 7.15-7.24 (5H, m, Ar-H), 8.29 (2H, d, J: 4.47 Hz, Ar-H). C-NMR (75 MHz, DMSO-d , ppm) δ:
6
5.7 (CH ), 50.2 (N-CH ), 122.5, 127.9, 129.4, 130.2, 135.0, 146.1, 149.6 (thiazole C ), 162.0 (pyridine C ), 168.6 (thiazole
2
2
4
2,6
+
C ), 186.1 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 336.1155; found: 336.1165.
2
19 17
3
(
4-Phenyl-2-(piperidin-1-yl)thiazol-5-yl)(pyridin-4-yl)methanone (3cd)
1
Yield: 70%. m.p.: 135–137 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.63 (6H, brs, (CH ) ), 3.61 (4H, brs, N(CH ) ),
6
2
3
2 2
1
3
7
.04–7.09 (2H, m, Ar-H), 7.16–7.24 (5H, m, Ar-H), 8.30 (2H, d, J: 4.14 Hz, Ar-H). C-NMR (75 MHz, DMSO-d , ppm)
6
δ: 23.7 (CH ), 25.3 (CH ), 49.4 (N-CH ), 121.4, 122.5, 127.9, 129.4, 130.2, 134.9, 146.0, 149.7 (thiazole C ), 161.7 (pyridine
2
2
2
4
+
C ), 171.7 (thiazole C ), 186.2 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 350.1322; found: 350.1322.
2
,6
2
20 19
3
1736

ŞAHİN et al. / Turk J Chem
(
2-(Azepan-1-yl)-4-phenylthiazol-5-yl)(pyridin-4-yl)methanone (3ce)
1
Yield: 78%. m.p.: 144–146 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 1.56 (4H, brs, (CH ) ), 1.79 (4H, brs, (CH ) ),
6
2
2
2 2
1
3
3
.60 (4H, brs, N(CH ) ), 7.04–7.09 (2H, m, Ar-H), 7.16–7.24 (5H, m, Ar-H), 8.30 (2H, d, J: 4.32 Hz, Ar-H). C-NMR (75
2 2
MHz, DMSO-d , ppm) δ: 27.5 (CH ), 53.2 (N-CH ), 121.3, 122.5, 127.9, 129.34, 130.2, 135.0, 146.2, 149.6 (thiazole C ),
6
2
2
4
+
1
3
61.8 (pyridine C ), 171.4 (thiazole C ), 186.0 (C=O). HRMS (m/z): [M+H] calcd for C H N OS: 364.1475; found:
2,6 2 21 21 3
64.1478.
(
2-Morpholino-4-phenylthiazol-5-yl)(pyridin-4-yl)methanone (3cf)
1
Yield: 69%. m.p.: 171–172 °C. H-NMR (300 MHz, DMSO-d , ppm) δ: 3.62 (4H, t, J: 4.23 Hz, N(CH ) ), 3.74 (4H, t, J:
6
2 2
1
3
4
.2 Hz, O(CH ) ), 7.05–7.10 (2H, m, Ar-H), 7.17–7.25 (5H, m, Ar-H), 8.32 (2H, d, J: 4.05 Hz, Ar-H). C-NMR (75 MHz,
2 2
DMSO-d , ppm) δ: 48.3 (N-CH ), 65.8 (CH -O), 122.0, 122.5, 127.9, 129.5, 130.2, 134.8, 145.9, 149.6 (thiazole C ), 161.3
6
2
2
4
+
(
pyridine C ), 172.2 (thiazole C ), 186.5 (C=O). HRMS (m/z): [M+H] calcd for C H N O S: 352.1106; found: 352.1114.
2,6 2 19 17 3 2
2
2
2
.2. Antioxidant activity
.2.1. DPPH radical scavenging assay
,2-diphenyl-1-picrylhydrazyl (DDPH) sweep test was used for one type of antioxidant activity as reported by Lu et al. [22].
DDPH has a strong absorbance at 517 nm. DPPH starts to react by existence of a hydrogen donor (free-radical scavenger/
antioxidant). e color change depends on the number of electrons captured and is determined with spectrophotometric
measurement. Compounds and standard gallic acid were prepared in the concentration from 1.25 µM to 25 µM. ese,
in six different concentrations (1.25 µM, 2.5 µM, 5 µM, 10 µM, 12.5 µM, 25 µM), were added to 190 µL of simultaneously
made solution of DPPH (0.1 mM in methanol). e reaction blend was kept in darkness for 40 min, and then the optical
density was measured against the blank at 517 nm. e capability of the compounds of scavenging the DPPH radical was
calculated as percent inhibition (%).
Here Ac is the absorbance of the control and At is the absorbance of the compounds; DPPH radical scavenging activity
(
%) = [(Ac – At)/Ac] × 100. Af er the concentration calculations, IC values of each compound were calculated using
50
GraphPad Prism. e results were calculated by comparison with the standard antioxidants, gallic acid and ascorbic acid.
e assay was carried out in triplicate [22].
.2.2. Ferric reducing antioxidant power (FRAP assay)

2
FRAP test was performed with the method of Benzie and Strain’s (1996) with some modifications [23,24]. To prepare
FRAP freshly just before use, 300 mM acetate buffer (pH 3.6), 10 mM TPTZ solution (10 mM TPTZ in 40 mM HCl), and
2
0 mM FeCl 6H O in a 10:1:1 ratio were mixed. Ten-microliter compounds of different concentrations (0.312 μM, 0.625
3. 2
μM, 1.25 μM, 2.5 μM, 3.75 μM, 5 μM) dissolved in DMSO below 1% were mixed with 230 uL of FRAP reagent and 10 uL
dH O in a 96-well microtiter plate. e plate reading at 593 nm was performed af er 8 min using an ELISA plate reader.
2
Trolox was the standard antioxidant molecule. e assay was carried out in triplicate.
2
.3. Antimicrobial activity
Antimicrobial activity test was applied to various gram-positive and gram-negative bacteria strains including E. coli
ATCC 8739), S. aureus (ATCC 29213), B. spizizenii (ATCC 6633), K. pneumoniae (Clinical isolate), P. aeruginosa (ATCC
027), E. faecalis (ATCC 29212), S. epidermidis (ATCC 12228), VRE (Clinical isolate). ese organisms were inoculated to
(
9
the midlog phase in Muller Hinton Broth (MHB) at 37 °C. Broth microdilution procedure is a more user-friendly method
that enables the testing of multiple antimicrobial agents. e method was carried out by the relevant 2018 CLSI standard.
Compounds were dissolved in DMSO below 1% concentration and were added to 96-well plates. ese compounds were
diluted serially to make ten various concentrations ranging between 0.5 μM and 256 μM. Bacterial inoculum suspensions
5
were primed at 1 × 10 cfu/mL concentrations. Plates were incubated at 37 °C for 24 h. Positive or negative control was
set to wells with and without bacteria, respectively. Chloramphenicol and sulfamethoxazole were used as standards.
Minimum inhibitory concentration (MIC) was determined by visual inspection af er the change in turbidity. Experiments
were performed in triplicate [25,26].
3
. Results
3
.1. Chemistry
Synthesis of compounds was realized in good yield between 65% and 90%. Structures of compounds were enlightened
by spectral data. In summary, methyl groups of compounds 3aa and 3ba (N(CH ) ) were observed at 3.19 ppm with
3
2
6
H integration. Methyl groups of compound 3ab and 3bb were observed at 1.23 ppm with 6H integration, and J values
were 7.03 and 6.90, respectively. Methylene group peaks on the same compounds were observed at 3.58 ppm with 4H
integration as a quartet. Aromatic hydrogen count was consistent with the total number on the molecules, and the most
1737

ŞAHİN et al. / Turk J Chem
shif ed hydrogen is the one on the neighborhood of pyridine nitrogen. ese hydrogens were observed between 8.29
1
3
and 8.53 ppm. In C NMR, carbonyl peaks were specifically observed approximately at 180 ppm. Following that, 2nd
position of thiazole carbon was observed around 170 ppm. e 2nd position of pyridine and 4th position of thiazole were
the following peaks for all compounds in descending order. Methyl or ethyl groups in the neighborhood of amine were
recorded at 40 and 52 ppm, respectively. Other aliphatic carbons were recorded at 12 and 27 ppm, which represents methyl
and ethyl groups on the amine, but not directly bound to nitrogen, respectively. HRMS results satisfied the molecular
weights with high accuracy.
3
.2. Antioxidant activity
Antioxidant activity was performed by two different common methods namely DPPH radical scavenging and ferric
reducing ability (FRAP). DPPH activity was measured as IC values with mM unit whereas FRAP results are presented as
5
0
Trolox equivalent antioxidant capacity (TEAC). e higher TEAC result means the higher antioxidant capacity.
In DPPH scavenging activity, 2-pyridyl derivatives (3aa-af) showed up to 65 mM IC value, except 3ad. Remarkably, 3
5
0
and 4-pyridyl derivatives showed less than 38 mM IC , except for 3ba and 3bb. In terms of scavenging activity, standard
ascorbic acid exhibited 3 mM. e most active compounds in the series were 3cf and 3bd with 28.10 and 26.02 mM,
respectively (Table 2).
5
0
In FRAP reducing ability, experiments were performed at 5 μM concentrations. To examine the results, compounds
showed a narrow range between 90.34 and 137.43 as TEAC. ese results are approximately the quarter of gallic acid
results on the same assay. In summary, compounds did not show higher or equal activity to standards (Table 2); however,
they still have antioxidant capacity as novel chemicals.
3
.3. Antimicrobial activity
Final compounds were examined for antibacterial properties on 8 microbial strains (Table 3). e results were presented
as MIC (μg/mL) values. According to the results, there is not a considerable activity for gram-negative bacteria E. coli, P.
aeruginosa, and K. pneumoniae. Compound 3ae (128 μg/mL) was the only compound that showed more than 256 μg/mL
activity against these strains. As for gram-positive bacteria, 3ae, 3af, 3cc, 3cd, 3ce, and 3cf exhibited ≥128 μg/mL activity
against different strains. Specifically, 3ae and 3ce showed 64 μg/mL activity against S. aureus. 3ae also exhibited 16 μg/mL
activity against S. epidermidis. Chloramphenicol was used as a wide-spectrum standard and sulfamethoxazole was used
as sulfonamide standard. Chloramphenicol showed 8–16 μg/mL activity against all bacteria as expected. Considerably, its
MIC value is not too far from the results of 3ae. Sulfamethoxazole was not effective against gram-positive bacteria except
for S. aureus (64 μg/mL). However, it showed high effect against E. coli (8 μg/mL) and P. aeruginosa (32 μg/mL) gram-
negative bacteria.
4
. Discussion
Synthesis of sixteen compounds was accomplished. In thiazole synthesis, ring closure is accomplished by methylene-
carbonyl condensation. is process runs when there is carbonyl attached to thioamide moiety, unlike in the Hantzsch
method. In this way, it is possible to synthesize trisubstituted thiazoles. In this method, sulfur basically attacks alkyl halide
(
CH -Br) at the first stage and then bromine leaves the structure, prompting nucleophilic substitution. In the final step,
2
the methylene, which stands between the carbonyl and the sulfur, forms a condensation and water withdrawal with the
carbonyl arising from the thioamide moiety (Figure 2) [17–21].
Oceans are rich in many living organisms, including truncates and they have many different biologically active
compounds. Antioxidant activity is one of the most important of those activities and compounds IC values are reported
5
0
to be between 26.02 and 150.74 mM in DPPH test. 3 and 4-pyridyl derivatives showed higher activity compared to
2
-pyridyles. Compounds 3ba and 3bb showed less activity compared to the rest of 3 and 4-pyridyl derivatives (3g-p).

is can be interpreted as that the cyclic amine substituents is preferable, not alkyl amines (3ba, 3bb). Standard ascorbic
acid was measured as 3.21 mM and this activity seems 9 times of our most active compound. However, in another study,
the IC value of ascorbic acid was reported as 38.78 mM [27]. Trolox equivalent antioxidant capacity for FRAP assay was
5
0
measured between 90.34 and 137.43, where gallic acid TEAC value was 476.8 and caffeic acid was 272 [28]. Synthesized
compounds have antioxidant capacity approximately half of the standard caffeic acid and ¼ of gallic acid. According to
antioxidant activity results, compounds have considerable antioxidant capacity but not equal to or higher than standard
strong antioxidants such as ascorbic acid, gallic acid, and caffeic acid. DPPH results were measured between 1.25 µM
and 25 µM concentrations. Interestingly, dendrodoine analog (Figure 1) was found 16.66 times less active compared to
Trolox (measured at 12.28 µM [12]) whereas 3cf (IC : 26.02 µM) had close or higher activity to Trolox (Reported in the
5
0
literature: 15–52 µM) [29,30]. In this respect, our compounds seem more active than dendrodoine analogs with respect
to antioxidant results and have a close activity to Trolox compared to the literature results. Examining the antimicrobial
1738

ŞAHİN et al. / Turk J Chem
Table 2. IC values (μM) for DPPH scavenging ability of the compounds. Antioxidant activity of the compounds
5
0
(
5 mM) as measured with FRAP assay. Data represented as mean ± SE from three individual experiments. (C:
Compound, aa: ascorbic acid, ga: gallic acid)
IC (mM)
TEAC
IC (mM)
TEAC
50
50
C
C
DPPH
FRAP
DPPH
FRAP
3
3
3
3
3
3
3
3
aa
87.95 ± 2.94
150.74 ± 3.99
74.37 ± 3.11
34.10 ± 0.65
100.18 ± 3.05
82.47 ± 1.55
66.13 ± 2.36
95.91 ± 2.24
3.21 ± 0.01
103.66 ± 2.12
91.43 ± 0.65
93.12 ± 1.42
102.22 ± 1.47
90.34 ± 2.87
101.56 ± 0.67
137.43 ± 0.99
123.53 ± 0.38
3bc
3bd
3be
3bf
3cc
3cd
3ce
3cf
ga
26.13 ± 0.08
28.10 ± 0.12
37.93 ± 2.06
31.10 ± 0.90
37.76 ± 3.04
31.13 ± 2.40
32.84 ± 2.08
26.02 ± 1.55
0.8 ± 0.04
123.89 ± 0.45
121.17 ± 5.21
120.67 ± 2.90
121.65 ± 0.88
124.99 ± 1.62
107.23 ± 2.89
122.12 ± 0.98
128.88 ± 2.47
ab
ac
ad
ae
af
ba
bb
aa
Table 3. Screening for MIC of the compounds using the microdilution method. (e capital S represents
strains with its number: S1: E. coli, S2: P. aeruginosa, S3: K. pneumoniae, S4: B. spizizenii, S5: S. aureus, S6: E.
faecalis, S7: S. epidermidis, S8: VRE. Ch: Chloramphenicol, Sm: Sulfamethoxazole)
Microorganisms and minimal inhibitory concentration (μg/mL)
Gram-negative bacteria
Gram-positive bacteria
C. no
S1
S2
S3
S4
S5
S6
S7
S8
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
aa
ab
ac
ad
ae
af
ba
bb
bc
bd
be
bf
cc
>256
>256
>256
>256
>128
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>8
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>16
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>8
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>16
>256
>256
>256
>256
>64
>128
>256
>256
>256
>256
>256
>256
>256
>128
>64
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>128
>8
>256
>256
>256
>256
>16
>128
>256
>256
>256
>256
>256
>256
>128
>128
>128
>128
>8
>256
>256
>256
>256
>128
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>256
>16
cd
ce
cf
>256
>16
>64
Ch*
Sm*
>4
>32
>256
>256
>256
>256
>256
results, compounds do not have significant antimicrobial activity except for 3ae. As a result, 16 dendrodoine analogs have
been defined successfully with their antioxidant and antimicrobial properties.
Conflict of interest
e authors declare that there is no conflict of interest.
1739

ŞAHİN et al. / Turk J Chem
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