Synthesis of novel imidazo[l,2-a]pyridines and evaluation of their antifungal activities

Article abstract of DOI:10.3906/kim-1307-14

New 2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)-N-substituted hydrazinecarbothioamides (4a-j), N'-(3-substitu-ted-4-oxo-1,3-thiazolidin-2- ylidene)imidazo[1,2-a]pyridine-2-carbohydrazides (5a-f ), and N-(nonsubstituted/4-substitu-ted phenyl)-5-(imidazo[1,2-a]p

Full text of DOI:10.3906/kim-1307-14

Turk J Chem
Turkish Journal of Chemistry
(2014) 38: 581 – 591
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http://journals.tubitak.gov.tr/chem/
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c
⃝ TUBITAK
doi:10.3906/kim-1307-14
Research Article
Synthesis of novel imidazo[1,2-a]pyridines and evaluation of their antifungal
activities
Fu¨sun GOKTAS¸1,∗, Nesrin CESUR1, Dilek S¸ATANA2, Meltem UZUN2
¨
1
˙
Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Istanbul University, Istanbul, Turkey
˙
2
˙
˙
˙
Department of Medical Microbiology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey
Received: 18.07.2013
•
Accepted: 27.12.2013
•
Published Online: 11.06.2014
•
Printed: 10.07.2014
Abstract:New 2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -substituted hydrazinecarbothioamides (4a–j), N ’-(3-substitu-
ted-4-oxo-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazides (5a–f), and N -(nonsubstituted/4-substitu-
ted phenyl)-5-(imidazo[1,2-a]pyridine-2-yl)-1,3,4-oxadiazole-2-amines (6a–d) were synthesized from imidazo[1,2-a]pyri-
dine-2-carbohydrazide (3) and evaluated for antifungal activity against Microsporum gypseum NCPF 580, M. canis,
Trichophyton tonsurans NCPF 245, T. rubrum, Candida albicans ATCC 10231, and C. parapsilosis ATCC 22019 using
amphotericin B as the standard. The chemical structures of the compounds were confirmed by elemental analysis, IR,
1 H NMR, 13 C NMR, HMBC (13 C, 1 H), and mass spectra. Most of the tested compounds showed moderate antifungal
activity. Hydrazinecarbothioamide derivatives 4h and 4f exhibited the highest activity against M. canis (MIC: 2 µg
mL−1 and 4 µg mL−1 , respectively).
Key words: Imidazo[1,2-a]pyridine, hydrazinecarbothioamide, 4-oxo-1,3-thiazolidine, 1,3,4-oxadiazole, antifungal ac-
tivity
1. Introduction
Heterocycles are important molecular building blocks that are involved in the structural composition of crucial
chemicals for humans, including pharmaceuticals, natural resources, veterinary and agricultural products,
analytical reagents, and dyes. Imidazo[1,2-a]pyridine, a fused bicyclic 5-6-heterocycle with 1 ring junction
nitrogen atom and 1 extra nitrogen atom in the 5-membered ring, is of interest because of the occurrence of
its derivatives in biologically active compounds and the pharmacology of the system has also been extensively
investigated.1
Human fungal infections have increased in the last 2 decades due to the increasing number of immuno-
compromised patients or those undergoing anticancer chemotherapy or transplantation.2 On the other hand,
the current antifungal therapy suffers from toxicity, nonoptimal pharmacokinetics, and some serious adverse
drug interactions. New chemotherapeutic agents with higher efficiency, a broader spectrum, and lower toxicity
are urgently needed for the treatment of fungal infections.3
In previous papers, we reported the synthesis and biological activity of a series of imidazo[1,2-a]pyridines
as antibacterials and antifungals.4−6 The present work is an extension of our ongoing efforts toward the develop-
ment and identification of new antifungal imidazo[1,2-a]pyridine derivatives bearing hydrazinecarbothioamide
(4a–j), 1,3-thiazolidine (5a–f), or 1,3,4-oxadiazole (6a–d) moieties.
∗Correspondence: fusung@istanbul.edu.tr
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2. Results and discussion
2.1. Chemistry
Ethyl imidazo[1,2-a]pyridine-2-carboxylate hydrobromide (2) was obtained from 2-aminopyridine and ethyl
bromopyruvate by a 2-step procedure.7 Heating 2 with hydrazine in ethanol gave imidazo[1,2-a]pyridine-2-
carbohydrazide (3),8 and 3 was reacted with alkyl or aryl isothiocyanates to achieve 2-(imidazo[1,2-a]pyridin-
2-ylcarbonyl)-N - substituted hydrazinecarbothioamide (4a–j) (Scheme 1).
COOC2H5
N+
N
. Br-
BrCH2COCOOC2H5
+
O
NH2
NH2
1
N
3
H2NNH2
RNCS
N
.HBr
COOC2H5
CONHNH2
N
N
2
O
O
N
BrCH2COOC2H5
N
N
NH NH
S
N
NH
NHR
N
S
4
R
5
N
R= CH ( ), C H ( ), C H ( ), CH C H ( ),
4a 4b 4c 4d
3
2
5
3
7
2
6
5
O
CH CH C H ( ),C H ( ),4-CH C H (
4e 4f
)
4g
2
2
6
5
6
5
3
6
4
R= CH ( ), C H ( ), C H ( ),
5a 5b 5c
4-ClC H ( ), 4-BrC H ( ), 4-FC H ( )
4h 4i 4j
3
2
5
3
7
6
4
6
4
6
4
CH C H ( ), CH CH C H ( ),
5d 5e
2
6
5
2
2
6
5
I2 / NaOH
4-CH C H ( )
5f
3
6
4
NHR
O
N
N
N
N
6
R= C H ( ), 4-ClCH ( ), 4-BrC H ( ), 4-FC H (
6a 6b 6c
)
6d
6
5
6
4
6
4
6
4
Scheme 1. Synthesis of compounds 4–6.
The structures of compounds 4a–j were assigned by elemental analyses and spectral data; 4a–j were
confirmed by their IR spectra, which displayed absorption peaks at 3395–3106 cm−1 for N–H, 1682–1670
cm−1 for C=O, and 1156–1144 cm−1 corresponding to C=S stretching vibrations. 1 H NMR spectra showed
N2 –H, N1 –H, and N–H resonances in the 10.50–10.12 ppm, 9.87–9.19 ppm, and 9.77–7.89 ppm regions,
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GOKTAS¸ et al./Turk J Chem
respectively.6,9−11 The C5 –, C3 –, C8 –, C7 –, and C6 –H resonances of the imidazo[1,2-a]pyridine system
appeared in the 8.66–8.52 ppm, 8.53–8.39 ppm, 7.66–7.53 ppm, 7.43–7.29 ppm, and 7.07–6.91 ppm regions,
respectively. HMBC (13 C–1 H) experiments were performed to establish the interfragment relationship and
assign the proton and carbon signals of the prototype compounds 4a, g, and j. The HMBC spectra of 4a, g,
and j exhibited resonances arising from C=S at 182.96–181.50 ppm and C=O at 162.43–160.93 ppm (Scheme
2)12 and C8a , C2 , C5 , C7 , C8 , C3 , and C6 resonances of the imidazo[1,2-a]pyridine residue appeared in the
144.58–144.61, 138.71–138.75, 128.33–128.35, 127.17–127.21, 118.04–118.07, 116.22–116.25, and 114.01–114.03
ppm regions, respectively.13 The mass spectra of 4a–j also confirmed their molecular weights.
128.33
116.22
31.66
162.43
O
C
S
N
114.01
127.17
NH NH C NH CH
3
N
138.75
182.96
144.58
118.04
Scheme 2. 13 C NMR data of compound 4a.
N ’-(3-alkyl/aryl-4-oxo-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazide derivatives (5a–
f) were prepared by reacting 2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)-N - substituted hydrazinecarbothioamides
(4a–j) with ethyl bromoacetate in the presence of sodium acetate. A new C=O band (1720–1697 cm−1) in the
IR spectra of 5a–f was particularly diagnostic for 4-oxo-1,3-thiazolidine formation.4−6,10,14,15 Further support
was obtained from the 1 H NMR spectra of 5a–f, which showed signals due to the CH2 protons at the 5 position
of 4-oxo-1,3-thiazolidine ring at about 4.22–4.06 ppm.5,6,15,16
After cyclization, the absence of resonances assigned to the N1 –H and N–H protons of the hydrazinecar-
bothioamides (4a–f) provided evidence of 4-oxo-1,3-thiazolidine formation. HMBC (13 C–1 H) experiments of
5a, b, and d, chosen as prototypes, made it possible to differentiate the carbon atoms of 4-oxo-1,3-thiazolidine
C=O and C2 and also of amide C=O (Scheme 3). Abundant ions [M + H]+ in the APCI+ or ESI+ mass
spectra of 5a–f confirmed their molecular weights.
O
C
115.81
139.13
159.15
29.92
128.36
NH
N
CH
3
N
172.09
O
N
N
113.97
144.63
160.65
S
127.25 118.00
33.49
Scheme 3. 13 C NMR data of compound 5a.
On the other hand, 2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -substituted hydrazinecarbothioamides (4f–
j) were oxidatively cyclized to 1,3,4-oxadiazole derivatives (6a–d), using iodine and potassium iodide in ethanolic
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sodium hydroxide by the elimination of H2 S. The IR spectra of 1,3,4-oxadiazole derivatives showed N–H and
C=N bands at about 3244–3107 cm−1 and 1669–1480 cm−1 , respectively. The absence of a C=O band in the
IR spectra of 6a–d supported the 1,3,4-oxadiazole structure. In the 1 H NMR spectra, the disappearance of
CONH and CSNH signals (4f–j) and the appearance of a new signal at about 10.91–10.64 ppm confirmed the
formation of an oxadiazole ring.16,17 HMBC (13 C–1 H) experiments of representative 6d also confirmed the
structure of the oxadiazole ring (Scheme 4).17 The APCI+ or ESI+ mass spectra of 6a–d showed abundant
ions [M + H]+ with different intensities.
119.42 (d,J= 8.00 Hz)
128.13
135.83
113.88
130.61
160.40
NH
116.37 (d,J= 23.01 Hz)
158.06 (d,J= 237.77 Hz)
N
O
114.13
127.23
N
N
F
N
116.37 (d,J= 23.01 Hz)
119.42 (d,J= 8.00 Hz)
155.26
145.70
117.84
Scheme 4. 13 C NMR data of compound 6d.
The purity of the synthesized compounds was established by elemental analysis.
2.2. Antifungal activity
The antifungal activities of compounds 4a–c, f–j, 5a, b, f, and 6a–d were investigated against Microsporum
gypseum NCPF 580, M. canis, Trichophyton tonsurans NCPF 245, T. rubrum, Candida albicans ATCC 10231,
and C. parapsilosis ATCC 22019 by microdilution method. Antifungal activity data are given in the Table. All
tested compounds exhibited varying degrees of antifungal activity; the highest activities were demonstrated by
compounds 4h and 4f against M. canis at 2 µg mL−1 and 4 µg mL−1 , respectively.
Table. Antifungal activity data of compounds 4–6 (MIC µg mL−1).
T. tonsurans
NCPF245
M. gypseum C. albicans
C. parapsilosis
Compound
T. rubrum M. canis
NCPF 580
ATCC 10231 ATCC 22019
4a
4b
4c
4f
4g
4h
4i
32
32
32
16
8
32
32
16
8
16
8
32
32
16
4
16
2
32
32
32
16
16
16
16
16
32
32
32
32
32
32
32
0.5
64
64
32
32
64
32
64
64
64
64
32
32
32
32
32
0.5
64
64
64
32
64
32
64
64
64
64
32
32
32
64
64
0.5
16
8
8
8
4j
16
32
32
16
32
32
32
32
16
32
32
32
32
16
16
16
2
16
32
32
16
32
32
32
32
0.5
5a
5b
5f
6a
6b
6c
6d
Amphotericin B 0.25
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3. Experimental
3.1. Chemistry
Melting points were determined with a Buchi 530 apparatus in open capillary tubes and are uncorrected.
IR spectra were recorded on KBr disks, using a PerkinElmer Model 1600 FT-IR spectrophotometer. 1 H
UNIT Y
NMR spectra were obtained in DMSO-d6 , with Varian
INOVA 400 (500 MHz), or Bruker (200 MHz)
spectrophotometers using TMS as the internal standard. 13 C NMR spectra were recorded at 150 and 75 MHz
using the instruments mentioned above. EI and APCI mass spectra were determined with a Finnigan LCQ
mass spectrometer. Elemental analyses were performed on a Thermo Finnigan Flash EA1112. Chemicals were
purchased from Merck (Darmstadt, Germany), Fluka, and Sigma-Aldrich Chemical Co.
2-Amino-1-(3-ethoxy-2,3-dioxopropyl)pyridinium bromide (1)7
To a suspension of 2-aminopyridine (0.09 mol) in dimetoxyethane (50 mL) was added ethylbromopyruvate (0.1
mol) and the reaction mixture was stirred for 2 h at room temperature. The precipitate was filtered, washed
with H2 O, and used without further purification.
Ethyl imidazo[1,2-a]pyridine-2-carboxylate hydrobromide (2)7
Compound 1 (0.04 mol) in ethanol 96% (100 mL) was refluxed for 2 h. Ethanol was evaporated to 1/5 volume
under reduced pressure, and then ether was added to give a solid residue. The crude product was filtered and
used without further purification.
Imidazo[1,2-a]pyridine-2-carbohydrazide (3)8
A mixture of 0.03 mol 2 and 0.3 mol of hydrazine was heated for 2 h. After cooling, the precipitate was filtered,
washed with cold water, and crystallized from ethanol 96%. Yield: 93%, mp 195–197 ◦ C. IR (cm−1): 3429,
3317 (N–H), 1654 (C=O); 1 H NMR δ (ppm): 4.55 (2H, broad s, NH2); 6.96 (1H, t, J = 6.7 Hz, C6 –H); 7.32
(1H, t, J = 6.8 Hz, C7 –H); 7.58 (1H, d, J = 9.1 Hz, C8 –H); 8.36 (1H, s, C3 –H); 8.58 (1H, d, J = 6.8 Hz,
C5 –H); 9.48 (1H, s, CONH).
General procedure for the synthesis of 2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -substituted
hydrazinecarbothioamide (4a–j)
First 0.075 mol of 3, 0.075 mol of appropriate alkyl/aryl isothiocyanate, and 40 mL of absolute ethanol were
refluxed 30 min. The solid formed was filtered and recrystallized from ethanol (96%).
2-(Imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -methylhydrazinecarbothioamide (4a). Yield: 60%,
mp 265–266 ◦ C. IR (cm−1): 3374, 3106 (N–H), 1670 (C=O), 1155 (C=S); 1 H NMR δ (ppm): 2.85 (3H, d, J
= 4.4 Hz, –CH3); 6.99 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.35 (1H, dd, J7,8 = 9.1 Hz, J7,6 = 6.8 Hz, C7 –H);
7.60 (1H, d, J = 9.2 Hz, C8 –H); 7.89 (1H, broad s, N–H); 8.46 (1H, s, C3 –H); 8.59 (1H, d, J5,6 = 6.8 Hz,
C5 –H); 9.30 (1H, s, N1 –H); 10.19 (1H, s, N2 –H); 13 C NMR (HMBC) δ (ppm): 182.96 (C=S); 162.43 (C=O);
144.58 (imidazo[1,2-a]pyridine C8a); 138.75 (imidazo[1,2-a]pyridine C2); 128.33 (imidazo[1,2-a]pyridine C5);
127.17 (imidazo[1,2-a]pyridine C7); 118.04 (imidazo[1,2-a]pyridine C8); 116.22 (imidazo[1,2-a]pyridine C3);
114.01 (imidazo[1,2-a] pyridine C6); 31.66 (imidazo[1,2-a] pyridine CH3); MS [APCI+] (m/z): 250 ([M +
H]+ , 8), 79 (100). Anal. calcd. for C10 H11 N5 OS: C: 48.18; H: 4.45; N: 28.09. Found: C: 47.82; H: 4.17; N:
27.65.
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N -Ethyl-2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)hydrazinecarbothioamide (4b). Yield: 94%,
mp 251–252 ◦ C. IR (cm−1): 3262, 3140 (N–H), 1670 (C=O), 1149 (C=S); 1 H NMR δ (ppm): 1.05 (3H, t, J
= 7.1 Hz, CH3); 3.42–3.48 (2H, m, –CH2 –); 7.00 (1H, t, J6,7 = J6,5 = 6.7 Hz, C6 –H); 7.36 (1H, dd, J7,8
=
9.2 Hz, J7,6 = 6.7 Hz, C7 –H); 7.61 (1H, d, J8,7 = 9.2 Hz, C8 –H); 7.91 (1H, s, N–H); 8.47 (1H, s, C3 –H); 8.60
(1H, d, J5,6 = 6.7 Hz, C5 –H); 9.24 (1H, s, N1 –H); 10.16 (1H, s, N2 –H). MS EI (m/z): 265 ([M + 2], 3), 264
(MH+ , 10), 263 (M+ , 36), 176 (100). Anal. calcd. for C11 H13 N5 OS: C: 50.17; H: 4.98; N: 26.60. Found: C:
50.52; H: 4.81; N: 26.74.
2-(Imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -propylhydrazinecarbothioamide (4c). Yield: 59%,
mp 192–194 ◦ C. IR (cm−1): 3265, 3140 (N–H), 1676 (C=O), 1144 (C=S); 1 H NMR δ (ppm): 0.73 (3H, t, J
= 7.4 Hz, CH3); 1.41 (2H, m, –CH2 CH2 CH3); 3.25 (m, –CH2 CH2 CH3 and H2 O); 6.91 (1H, t, J6,7 = J6,5
= 6.8 Hz, C6 –H); 7.27 (1H, dd, J7,8 = 9.1 Hz, J7,6 = 6.8 Hz, C7 –H); 7.53 (1H, d, J8,7 = 9.1 Hz, C8 –H); 7.84
(1H, broad s, N–H); 8.39 (1H, s, C3 –H); 8.52 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.19 (1H, s, N1 –H); 10.12 (1H,
s, N2 –H). MS [APCI+] (m/z): 278 ([M + H]+ , 30), 177 (100). Anal. calcd. for C12 H15 N5 OS: C: 51.97; H:
5.45; N: 25.25. Found:C: 51.44; H: 4.86; N: 24.99.
N -Benzyl-2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)hydrazinecarbothioamide (4d). Yield: 83%,
mp 240–242 ◦ C. IR (cm−1): 3374, 3152 (N–H), 1673 (C=O), 1145 (C=S); 1 H NMR δ (ppm): 4.72 (2H, d, J
= 6.0 Hz, –CH2 –); 7.00 (1H, t, J6,5 = J6,7 = 6.8 Hz, C6 –H); 7.21–7.38 (6H, m, C7 –H, phenyl); 7.61 (1H, d,
J8,7 = 9.2 Hz, C8 –H); 8.50 (2H, s, C3 –H, N–H); 8.60 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.46 (1H, s, N1 –H); 10.35
(1H, s, N2 –H). MS [ESI–] (m/z): 324 ([M–H]− , 67), 290 (100). Anal. calcd. for C16 H15 N5 OS: C: 59.06; H:
4.65; N: 21.52. Found C: 58.61; H: 4.72; N: 20.86.
2-(Imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -(2-phenylethyl)hydrazinecarbothioamide (4e).
Yield: 79%, mp 195–197 ◦ C. IR (cm−1): 3395, 3135 (N–H), 1682 (C=O), 1148 (C=S); 1 H NMR δ (ppm):
2.81 (2H, t, J = 7.8 Hz, –CH2 C6 H5); 3.63 (2H, t, J = 7.8 Hz, –N–CH2 –); 7.01 (1H, t, J6,7 = J6,5 = 6.8
Hz, C6 –H); 7.17–7.29 (5H, m, phenyl); 7.38 (1H, t, J7,8 = 9.1 Hz, C7 –H); 7.62 (1H, d, J7,8 = 9.1 Hz, C8 –H);
8.01 (1H, broad s, N–H); 8.50 (1H, s, C3 –H); 8.62 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.39 (1H, s, N1 –H); 10.23
(1H, s, N2 –H). MS [APCI+] (m/z): 340 ([M + H]+ , 90), 219 (100). Anal. calcd. for C17 H17 N5 OS: C: 60.16;
H: 5.05; N: 20.63. Found: C: 59.85; H: 4.70; N: 20.50.
2-(Imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -phenylhydrazinecarbothioamide (4f). Yield: 72%,
mp 188–190 ◦ C. IR (cm−1): 3260, 3107 (N–H), 1669 (C=O), 1156 (C=S).1 H NMR δ (ppm): 7.01 (1H, t,
J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.14 (1H, dd, J = 7.4, 9.0 Hz, phenyl 4–H); 7.30–7.40 (3H, m, C7 –H, phenyl 2,
6–H); 7.51 (2H, dd, J = 6.8, 7.5 Hz, phenyl 3, 5–H), 7.64 (1H, d, J8,7 = 9.1 Hz, C8 –H); 8.51 (1H, s, C3 –H);
8.62 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.73 (1H, broad s, N–H); 9.87 (1H, s, N1 –H); 10.41 (1H, s, N2 –H). MS
[ESI–] (m/z): 310([M–H]− , 23), 276(100). Anal. calcd. for C15 H13 N5 OS. 0.5 H2 O: C: 56.23; H: 4.40; N:
21.86. Found C: 56.91; H: 4.20; N: 21.69.
2-(Imidazo[1,2-a]pyridin-2-ylcarbonyl)-N -(4-methylphenyl)hydrazinecarbothioamide (4g).
Yield: 59%, mp 198–200 ◦ C. IR (cm−1): 3320, 3141 (N–H), 1671 (C=O), 1146 (C=S). 1 H NMR δ (ppm): 2.27
(3H, s, –CH3), 7.00 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.12 (2H, d, J = 8.1 Hz, phenyl 3, 5–H); 7.34–7.39
(3H, m, C7 –H, phenyl 2, 6–H); 7.64 (1H, d, J8,7 = 9.2 Hz, C8 –H); 8.50 (1H, s, C3 –H); 8.62 (1H, d, J5,6
=
6.8 Hz, C5 –H); 9.43 (2H, broad s, N1 –H, N–H); 10.50 (1H, s, N2 –H).13 C NMR (HMBC) δ (ppm): 181.50
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GOKTAS¸ et al./Turk J Chem
(C=S); 162.00 (C=O); 144.61 (imidazo[1,2-a]pyridine C8a); 138.72 (imidazo[1,2-a]piridin C2); 137.40 (phenyl
C1); 134.57 (phenyl C4); 129.17 (phenyl C3 , C5); 128.35 (imidazo[1,2-a]pyridine C5); 127.19 (imidazo[1,2-
a]pyridine C7); 125.68 (phenyl C2 , C6); 118.07 (imidazo[1,2-a]pyridine C8); 116.22 (imidazo[1,2-a]pyridine
C3); 114.02 (imidazo[1,2-a]pyridine C6); 21.21 (imidazo[1,2-a] pyridine CH3). MS [APCI+] (m/z): 326 ([M
+ H]+ , 14), 145 (100). Anal. calcd. for C16 H15 N5 OS: C: 59.06; H: 4.65; N: 21.52. Found C: 58.27; H: 4.55;
N: 21.70.
N -(4-Chlorophenyl)-2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)hydrazinecarbothioamide (4h).
Yield: 75%, mp 214–216 ◦ C. IR (cm−1): 3260, 3142 (N–H), 1671 (C=O), 1149 (C=S). 1 H NMR δ (ppm):
6.92 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.26–7.30 (3H, m, C7 –H, phenyl 3,5-H); 7.43 (2H, s, phenyl 2,6-H);
7.55 (1H, d, J7,8 = 9.1 Hz, C8 –H); 8.41 (1H, s, C3 –H); 8.52 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.68 (1H, broad s,
N–H); 9.70 (1H, s, N1 –H); 10.34 (1H, s, N2 –H). MS [ESI–] (m/z): 344.0 ([M–H]− , 67); 310 (100). Anal. calcd.
for C15 H12 ClN5 OS: C: 52.10; H: 3.50; N: 20.25. Found C: 51.62; H: 3.52; N: 20.24.
N -(4-Bromophenyl)-2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)hydrazinecarbothioamide (4i).
Yield: 75%, mp 209–210 ◦ C. IR (cm−1): 3260, 3147 (N–H), 1668 (C=O), 1154 (C=S); 1 H NMR δ (ppm): 7.07
(1H, dd, J6,7 = 9.5 Hz, J6,5 = 6.9 Hz, C6 –H); 7.43 (1H, dd, J7,8 = 8.5 Hz, J7,6 = 7.3 Hz, C7 –H); 7.46–7.54
(4H, m, phenyl); 7.66 (1H, d, J8,7 = 8.5 Hz, C8 –H); 8.53 (1H, s, C3 –H); 8.66 (1H, d, J5,6 = 6.9 Hz, C5 –H);
9.77 (1H, broad s, N–H); 9.86 (1H, s, N1 –H); 10.47 (1H, s, N2 –H). MS [ESI–] (m/z): 390([M–H]− , 24), 356
(100). Anal. calcd. for C15 H12 BrN5 OS. H2 O: C: 44.12; H: 3.45; N: 17.15. Found: C: 43.74; H: 2.83; N: 17.46.
N -(4-Fluorophenyl)-2-(imidazo[1,2-a]pyridin-2-ylcarbonyl)hydrazinecarbothioamide (4j).
Yield: 67%, mp 200–203 ◦ C. IR (cm−1): 3330, 3147 (N–H), 1682 (C=O), 1184 (C=S). 1 H NMR δ (ppm):
7.00 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.10–7.19 (2H, m, phenyl 3,5-H); 7.33–7.45 (3H, m, C7 –H, phenyl
2,6-H); 7.64 (1H, d, J8,7 = 9.2 Hz, C8 –H); 8.51 (1H, s, C3 –H); 8.62 (1H, d, J5,6 = 6.8 Hz, C5 –H); 9.74 (1H, s,
N–H); 9.80 (1H, s, N1 –H); 10.39 (1H, s, N2 –H). 13 C NMR (HMBC) δ (ppm): 181.87 (C=S); 160.93 (C=O);
157.97 (d, J = 242.03 Hz, phenyl C4); 144.61 (imidazo[1,2-a]pyridine C8a); 138.71 (imidazo[1,2-a]pyridine
C2); 136.31 (d, J = 2.4 Hz, phenyl C1); 128.35 (imidazo[1,2-a]pyridine C5); 128.09 (phenyl C2 , C6); 127.21
(imidazo[1,2-a]pyridine C7); 118.05 (imidazo[1,2-a]pyridine C8); 116.25 (imidazo[1,2-a]pyridine C3); 115.29
(d, J = 23 Hz, phenyl C3 , C5); 114.03 (imidazo[1,2-a]pyridine C6). MS [APCI–] (m/z): 328 ([M–H]− , 21),
294 (100). Anal. calcd. for C15 H12 FN5 OS. 0.5 H2 O: C: 53.24; H: 3.87; N: 20.70. Found C: 53.91; H: 3.51; N:
20.73.
General procedure for the synthesis of N ’-(3-substituted-4-oxo-1,3-thiazolidin-2-ylidene)imidazo
[1,2-a]pyridine-2-carbohydrazide (5a–f)
First 0.0035 mol of appropriate hydrazinecarbothioamide (4a–e, 4g) and 0.0055 mol of ethyl bromoacetate were
refluxed in absolute ethanol (30 mL) in the presence of anhydrous CH3 COONa (0.04 mol) for 3 h. The reaction
mixture was cooled and the solid thus obtained was filtered, washed with water, and purified by crystallization
from an ethanol–water mixture.
N ’-(3-Methyl-4-oxo-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazide (5a).
Yield: 86%, mp 281–283 ◦ C. IR (cm−1): 3298, 3145 (N–H), 1697 (thia. C=O), 1670 (C=O). 1 H NMR δ (ppm):
3.13 (3H, s, –CH3); 4.08 (2H, s, thia. CH2); 7.00 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.37 (1H, dd, J7,8 = 9.2
Hz; J7,6 = 6.8 Hz; C7 –H) 7.64 (1H, d, J8,7 = 9.2 Hz, C8 –H); 8.46 (1H, s, C3 –H); 8.60 (1H, d, J5,6 = 6.8 Hz,
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C5 –H); 10.42 (1H, s, CONH). 13 C NMR δ (ppm): 172.09 (thia. C=O); 160.65 (thia. C2); 159.15 (CONH);
144.63 (imidazo[1,2-a]pyridine C8a); 139.13 (imidazo[1,2-a]pyridine C2); 128.36 (imidazo[1,2-a]pyridine C5);
127.25 (imidazo[1,2-a]pyridine C7); 118.00 (imidazo[1,2-a]pyridine C8); 115.81 (imidazo[1,2-a]pyridine C3);
113.97 (imidazo[1,2-a]pyridine C6); 33.49 (thia. C2); 29.92 (CH3). MS [APCI+] (m/z): 290 ([M + H]+ , 100).
Anal. calcd. for C12 H11 N5 O2 S: C: 49.82; H: 3.83; N: 24.21. Found C: 49.38; H: 3.94; N: 24.55.
N ’(-3-Ethyl-4-oxo-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazide (5b).
Yield: 92%, mp 223–224 ◦ C. IR (cm−1): 3296, 3148 (N–H); 1707 (thia. C=O), 1680 (C=O). 1 H NMR δ
(ppm): 1.58 (3H, t, J = 6.3 Hz, –CH2 CH3); 3.75 (2H, q, J = 7.0 Hz, –CH2 CH3); 4.08 (2H, s, thia. CH2);
7.00 (1H, t, J6,7 = J6,5 = 6.7 Hz, C6 –H); 7.37 (1H, t, J7,8 = 9.2 Hz; J7,6 = 6.7 Hz; C7 –H) 7.64 (1H, d, J8,7
= 9.2 Hz, C8 –H); 8.46 (1H, s, C3 –H); 8.59 (1H, d, J5,6 = 6.7 Hz, C5 –H); 10.58 (1H, s, CONH).13 C NMR
(HMBC) δ (ppm): 171.86 (thia. C=O); 160.07 (thia. C2); 159.20 (CONH); 144.63 (imidazo[1,2-a]pyridine
C8a); 139.17 (imidazo[1,2-a]pyridine C2); 128.35 (imidazo[1,2-a]pyridine C5); 127.22 (imidazo[1,2-a]pyridine
C7); 118.00 (imidazo[1,2-a]pyridine C8); 115.77 (imidazo[1,2-a]pyridine C3); 113.95 (imidazo[1,2-a]pyridine
C6); 38.29 (–CH2 –CH3); 33.42 (thia. C5); 12.81 (–CH2 –CH3). MS [APCI +] (m/z): 304 ([M + H]+ ,100).
Anal. calcd. for C13 H13 N5 O2 S: C: 51.47; H: 4.32; N: 23.09. Found C: 51.79; H: 4.19; N: 22.75.
N ’(-4-Oxo-3-propyl-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazide (5c).
Yield: 78%, mp 124–126 ◦ C. IR (cm−1): 3295, 3137 (N–H); 1708 (thia. C=O); 1673 (C=O). 1 H NMR δ (ppm)
0.86 (3H, t, J = 7.5 Hz, –CH3); 1.61–1.67 (2H, m, –CH2 CH2 CH3); 3.65 (2H, t, J = 7.3 Hz, –CH2 CH2 CH3);
4.08 (2H, s, thia. CH2); 6.98 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.36 (1H, dd, J7,8 = 9.2 Hz; J7,6 = 6.8 Hz;
C7 –H) 7.62 (1H, d, J8,7 = 9.2 Hz, C8 –H); 8.45 (1H, s, C3 –H); 8.59 (1H, d, J5,6 = 6.8 Hz, C5 –H); 10.45 (1H,
s, CONH). MS [ESI +] (m/z): 318 ([M + H]+ , 100). Anal. calcd. for C14 H15 N5 O2 S. H2 O: C: 50.13; H: 5.11;
N: 20.88. Found C: 50.67; H: 4.93; N: 20.72.
N ’-(3-Benzyl-4-oxo-1,3-thiazolidin-2-ylidene)imidazo[1,2-a]pyridine-2-carbohydrazide (5d).
Yield: 79%, mp 242–244 ◦ C. IR (cm−1): 3298, 3142 (N–H), 1700 (thia. C=O); 1685 (C=O). 1 H NMR δ
(ppm) 4.16 (2H, s, thia. CH2); 4.91 (2H, s, N–CH2); 7.01 (1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.27–7.39
(4H, m, phenyl 3,4,5-H, C7 –H); 7.44 (2H, d, J = 7.0 Hz, phenyl 2,6-H); 7.64 (1H, d, J = 9.2 Hz, C8 –H); 8.45
(1H, s, C3 –H); 8.60 (1H, d, J5,6 = 6.8 Hz, C5 –H); 10.49 (1H, s, CONH).13 C NMR (HMBC) δ (ppm) 172.14
(thia. C=O); 160.01 (thia. C2); 159.22 (CONH); 144.64 (imidazo[1,2-a]pyridine C8a); 139.13 (imidazo[1,2-
a]pyridine C2); 136.58 (phenyl C1); 129.06 (phenyl C3 /C5); 128.65 (phenyl C2 /C6); 128.36 (imidazo[1,2-
a]pyridine C5); 128.22 (phenyl C4); 127.22 (imidazo[1,2-a]pyridine C7); 118.02 (imidazo[1,2-a]pyridine C8);
115.79 (imidazo[1,2-a]pyridine C3); 113.96 (imidazo[1,2-a]pyridine C6); 46.27 (N-CH2); 33.38 (thia. C5). MS
[APCI +] (m/z): 366 ([M + H]+ , 100). Anal. calcd. for C18 H15 N5 O2 S. 0.5 C2 H5 OH: C: 58.75; H: 4.67; N:
18.03. Found C: 58.86; H: 5.14; N: 18.36.
N ’-[4-Oxo-3-(2-phenylethyl)-1,3-thiazolidin-2-ylidene]imidazo[1,2-a]pyridine-2-carbohydra-
zide (5e). Yield: 84%, mp 214–216 ◦ C. IR (cm−1): 3278, 3160 (N–H); 1720 (thia. C=O); 1693 (C=O), 1 H
NMR δ (ppm) 3.04 (2H, t, J = 7.8 Hz, CH2 –C6 H5); 3.98 (2H, t, J = 7.0 Hz, N–CH2); 4.06 (2H, s, thia.
CH2); 7.07 (1H, t, J6,5 = J6,7 = 6.5 Hz, C6 –H); 7.27–7.45 (5H, m, phenyl, C7 –H); 7.70 (1H, d, J8,7 = 9.3
Hz, C8 –H); 8.52 (1H, s, C3 –H); 8.67 (1H, d, J5,6 = 6.5 Hz, C5 –H); 10.58 (1H, s, CONH). MS [ESI +] (m/z):
380 ([M + H]+ , 35), 362 (100). Anal. calcd. for C19 H17 N5 O2 S. H2 O: C: 57.41; H: 4.81; N: 17.62. Found C:
57.79; H: 4.75; N: 17.63.
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N ’-[3-(4-Methylphenyl)-4-oxo-1,3-thiazolidin-2-ylidene]imidazo[1,2-a]pyridine-2-carbohyd-
razide (5f). Yield: 66%, mp 254–255 ◦ C. IR (cm−1): 3319, 3135 (N–H); 1712 (thia. C=O); 1693 (C=O).
1 H NMR δ (ppm): 2.37 (3H, s, –CH3); 4.22 (2H, s, thia. CH2); 7.00 (1H, t, J6,7 = J6,5 = 6.9 Hz, C6 –H);
7.22–7.50 (5H, m, phenyl and C7 –H); 7.64 (1H, d, J8,7 = 9.1 Hz, C8 –H); 8.44 (1H, s, C3 –H); 8.58 (1H, d,
J5,6 = 6.9 Hz, C5 –H); 10.38 (1H, s, CONH). MS [ESI +] (m/z): 366 ([M + H]+ , 100). Anal. calcd. for
C18 H15 N5 O2 S: C: 59.16; H: 4.14; N: 19.17. Found C: 59.03; H: 4.00; N: 19.17.
General procedure for the synthesis of N -(nonsubstituted/4-substituted phenyl)-5-(imidazo[1,2-
a]pyridine-2-yl)-1,3,4-oxadiazole-2-amine (6a–d).
The appropriate hydrazinecarbothioamides (4f–j) (0.0035 mol) were suspended in ethanol 96% (30 mL), and
aqueous sodium hydroxide (5 mL, 4 N) and iodine in potassium iodide solution (aqueous 5%) were added with
shaking at room temperature until the color of iodine persisted. The solid separated was filtered, and purified
by crystallization from ethanol 96%.
5-(Imidazo[1,2-a]pyridin-2-yl)-N -phenyl-1,3,4-oxadiazol-2-amine (6a). Yield: 86%, mp 236–
238 ◦ C. IR (cm−1): 3229, 3186 (N–H); 1662, 1546, 1480 (C=C, C=N). 1 H NMR δ (ppm) 6.93 (2H, t, J =
7.1 Hz, phenyl 4–H, C6 –H); 7.28 (3H, t, J = 7.2 Hz, phenyl 2, 6–H, C7 –H); 7.55 (3H, t, J = 8.3 Hz, phenyl
3,5-H, C8 –H); 8.48 (1H, s, C3 –H); 8.54 (1H, d, J5,6 = 6.8 Hz, C5 –H); 10.64 (1H, s, NH). MS [APCI +] (m/z):
278 ([M + H]+ , 16); 161 (100). Anal. calcd. for C15 H11 N5 S. 2 H2 O: C: 57.50; H: 4.83; N: 22.35. Found C:
57.81; H: 4.78; N: 22.33.
N -(4-Chlorophenyl)-5-(imidazo[1,2-a]pyridin-2-yl)-1,3,4-oxadiazol-2-amine (6b). Yield: 92%,
mp 308–310 ◦ C. IR (cm−1): 3220, 3107 (N–H); 1645, 1589, 1494 (C=C, C=N). 1 H NMR δ (ppm) 7.03 (1H,
t, J6,7 = J6,5 = 6.9 Hz, C6 –H); 7.38 (1H, dd, J7,6 = 6.9 Hz, J7,8 = 8.3 Hz, C7 –H); 7.44 (2H, d, J = 8.8 Hz,
phenyl 2,6-H); 7.65 (3H, d, J = 8.9 Hz, phenyl 3,5-H, C8 –H); 8.58 (1H, s, C3 –H); 8.63 (1H, d, J5,6 = 6.9 Hz,
C5 –H); 10.91 (1H, s, NH). MS [ESI +] (m/z): 312 ([M + H]+ , 36); 161 (100). Anal. calcd. for C15 H10 ClN5 O.
0.5 H2 O: C: 56.16; H: 3.45; N: 21.38. Found C: 56.79; H: 3.63; N: 21.73.
N -(4-Bromophenyl)-5-(imidazo[1,2-a]pyridin-2-yl)-1,3,4-oxadiazol-2-amine (6c).
Yield: 96%, mp 284–286 ◦ C. IR (cm−1): 3230, 3110 (N–H); 1669, 1580, 1484 (C=C, C=N). 1 H NMR
δ (ppm): 7.04 (1H, dd, J6,7 = 7.8 Hz, J6,5 = 6.8 Hz, C6 –H); 7.38 (1H, dd, J7,8 = 9.1 Hz, J7,6 = 7.8 Hz,
C7 –H); 7.54–7.62 (4H, m, phenyl); 7.66 (1H, d, J8,7 = 9.1 Hz, C8 –H); 8.58 (1H, s, C3 –H); 8.63 (1H, d, J5,6
=
6.8 Hz, C5 –H); 10.91 (1H, s, NH). MS [APCI +] (m/z): 358 ([M + 2 + H]+ , 100); 356 ([M + H]+ , 94). Anal.
calcd. for C15 H10 BrN5 O 0.5 H2 O: C: 49.33; H: 3.04; N: 19.17. Found C: 49.01; H: 3.77; N: 18.88.
N -(4-Fluorophenyl)-5-(imidazo[1,2-a]pyridin-2-yl)-1,3,4-oxadiazol-2-amine (6d). Yield: 94%,
mp 226–228 ◦ C. IR (cm−1): 3244, 3170 (N–H); 1628, 1593, 1457 (C=C, C=N). 1 H NMR δ (ppm): 7.02
(1H, t, J6,7 = J6,5 = 6.8 Hz, C6 –H); 7.21 (2H, t, J = 8.78, 9.28 Hz, phenyl 2,6-H); 7.36, 7.38 (1H, dd, J7,8
= 9.3 Hz, J7,6 = 6.8 Hz, C7 –H); 7.61–7.64 (3H, m, phenyl 3,5-H, C8 –H); 8.55 (1H, s, C3 –H); 8.62 (1H, d,
J5,6 = 6.8 Hz, C5 –H); 10.71 (1H, s, NH). 13 C NMR (HMBC) δ (ppm): 160.40 (oxadia. C2); 158.06 (d,
J = 237.77 Hz, phenyl C4); 155.26 (oxadia. C5); 145.70 (imidazo[1,2-a]pyridine, C8a); 135.83 (phenyl C1);
130.61 (imidazo[1,2-a]pyridine, C2); 128.13 (imidazo[1,2-a] pyridine, C5); 127.23 (imidazo[1,2-a]pyridine, C7);
119.42 (d, J = 8.43 Hz, phenyl C2 /C6); 117.84 (imidazo[1,2-a]pyridine, C8); 116.37 (d, J = 23.01 Hz, phenyl
C3 /C5); 114.13 (imidazo[1,2-a]pyridine, C6); 113.88 (imidazo[1,2-a]pyridine, C3). MS [ESI +] (m/z): 296
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([M + H]+ , 40); 161 (100). Anal. calcd. for C15 H10 FN5 O: C: 61.02; H: 3.41; N: 23.72. Found C: 60.60; H:
3.17; N: 23.66.
3.2. Antifungal activity
Microdilution was conducted according to a standard protocol by the National Committee for Clinical Labora-
tory Standards (NCCLS).18,19 RPMI 1640 broth with L-glutamine without sodium bicarbonate was used and
buffered with 3-(N-morfolino) propanesulfonic acid (MOPS). The medium was adjusted to pH 7.0 at 25 ◦ C. Am-
photericin B was provided by Sigma as the standard. All compounds were dissolved in 100% dimethylsulfoxide
according to NCCLS methods.18,19 The final concentrations were 64 to 0.03 µg/mL for all compounds.
Preparation of inoculum suspensions was based mainly on the NCCLS guidelines and described previo-
usly.19,20 The isolates were subcultured onto potato dextrose agar (PDA) plates at 28 ◦ C, over 7–14 days.
The fungal colonies were covered with 1 mL of sterile 0.85% saline, and suspensions were made by gently
probing the surface with the tip of a Pasteur pipette. The resulting mixture of conidia and hyphal fragments
was withdrawn and transferred to a sterile tube. Heavy particles were allowed to settle for 15–20 min at
room temperature; the upper suspension was mixed with a vortex for 15 s. The turbidity of supernatants
was measured spectrophotometrically at 530 nm, and transmission was adjusted to 65% to 75%. These stock
suspensions were diluted 1:50 in RPMI medium to obtain the final inoculum sizes, which ranged from 0.4 ×
104 to 5 × 104 CFU/mL.
Preparation of inoculum suspensions of C. albicans and C. parapsilosis was based mainly on the NCCLS
guidelines18 and described previously.21 Yeasts were grown on Sabouraud dextrose agar for 24 h at 35 ◦ C and
from the 24- to 48-h-old culture was suspended in 5 mL of sterile 0.85% saline.
The turbidity of mixed suspension was measured at 530 nm to obtain 75% to 77% transmission and
adjusted to 1 × 106 to 5 × 106 CFU/mL by spectrophotometric method. These stock suspensions were
diluted 1:50 in RPMI medium, and further diluted 1:20 with medium to obtain the 2-fold test inoculum (1 ×
103 to 5 × 103 CFU/mL). The (2-fold) inoculum was diluted 1:1 when wells were inoculated and the desired
final inoculum size was achieved (0.5 × 103 to 2.5 × 103 CFU/mL).
Microdilution plates (96 U-shaped) were prepared and frozen at –70 ◦ C until needed. Rows 2 to 12
contained the series of compound dilutions in 100-µL volumes and the first row contained 100 µL of compound-
free medium, which served as the growth control. Each well was inoculated on the day of the test with 100
µL of the corresponding inoculum. This step brought the compound dilutions and inoculum size to the final
test concentrations given above. The microplates of dermatophytes were incubated at 28 ◦ C for 7 days and
the microplates of yeasts were incubated at 35 ◦ C for 24 and 48 h. The microplates were read visually with
the aid of an inverted reading mirror after 7 days for dermatophytes and after 24 and 48 h for yeasts. For all
compounds, the MIC was defined as the lowest concentration showing 100% inhibition of growth.
Acknowledgment
˙
This work was supported by Istanbul University Scientific Research Projects (Project Number: T-662/05042005).
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