New derivatives of 1H-imidazo[4,5-b]pyridine-2(3H)-thione. Synthesis of thiosemicarbazides and their cyclic analogues

Article abstract of DOI:10.1080/10426500701648077

A series of 2-(arylthiosemicarbazide)-imidazo[4,5-b]pyridines (6-9) were prepared by reaction of 2-(carbazoylalkylthio)-imidazo[4,5-b]pyridines (4 and 5) with selected arylisothiocyanates. The thiosemicarbazides 6 and 7 were cyclized with 2-bromoacetophenone to give the derivatives of imidazo[4,5-b]pyridine 10 and 11. Compounds (2-11) were characterized by elemental analysis as well as IR and s1H NMR spectroscopy. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500701648077

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New Derivatives of 1H-
imidazo[4,5-b]pyridine-2(3H)-
thione. Synthesis of
Thiosemicarbazides and Their
Cyclic Analogues
Hanna Liszkiewicz a
a Wroclaw Medical University, Department of Drugs
Technology, Wroclaw Medical University, Poland
Published online: 04 Jun 2008.
To cite this article: Hanna Liszkiewicz (2008) New Derivatives of 1H-imidazo[4,5-
b]pyridine-2(3H)- thione. Synthesis of Thiosemicarbazides and Their Cyclic Analogues,
Phosphorus, Sulfur, and Silicon and the Related Elements, 183:6, 1402-1409
To link to this article: http://dx.doi.org/10.1080/10426500701648077
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Phosphorus, Sulfur, and Silicon, 183:1402–1409, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701648077
New Derivatives of 1H-imidazo[4,5-b]pyridine-2(3H)-
thione. Synthesis of Thiosemicarbazides and Their Cyclic
Analogues
Hanna Liszkiewicz
Wroclaw Medical University, Department of Drugs Technology, Wroclaw
Medical University, Poland
A series of 2-(arylthiosemicarbazide)-imidazo[4,5-b]pyridines (6–9) were prepared
by reaction of 2-(carbazoylalkylthio)-imidazo[4,5-b]pyridines (4 and 5) with se-
lected arylisothiocyanates. The thiosemicarbazides 6 and 7 were cyclized with 2-
bromoacetophenone to give the derivatives of imidazo[4,5-b]pyridine 10 and 11.
s1
Compounds (2–11) were characterized by elemental analysis as well as IR and
H
NMR spectroscopy.
Keywords 1H,3H-2-Thioxoimidazo[4,5-b]pyridine derivatives; hydrazide; structures;
synthesis; thiazole; thiosemicarbazide
INTRODUCTION
The thiazole ring is a pharmacophoric group; condensed or substi-
tuted with different heterocyclic systems it makes part of many drugs1
with various pharmacological activities, e.g., Penicillin, Thiamine, Lev-
amisole, Amiphenazole, Mebendazole, and Bleomycin. According to lit-
erature data derivatives of various heterocyclic systems as quinazoli-
nones, benzimidazoles, indoles, etc. containing in their structure the
thiazole moiety also show pharmacological and biological activities
like anti-inflammatory,2 anti-fungi,3,4 and anti-microbial as well as
anti-viral5 activity. They are also active antibacterial (Mycobacterium
tuberculosis),6 CN depressant,7 and anticonvulsive8 agents.
Thiazoles are classically obtained using the Hantzsch reaction,9
i.e. the condensation of thiourea with an amide acetal to form a
thiocarbamoylamidine, followed by base promoted cyclization with
a phenacyl halide. Cyclocondensation of thiosemicarbazides10 or
Received 16 June 2007; accepted 16 August 2007.
Address correspondence to Hanna Liszkiewicz, Wroclaw Medical University, Depart-
ment of Drugs Technology, Wroclaw Medical University, Nankiera 1 Sq. 50-140 Wroclaw,
Poland. E-mail: hanna@ktl.am.wroc.pl
1402

1H-imidazo[4,5-b]pyridine-2(3H)-thione Derivatives
1403
isothiosemicarbazones4,8 with a phenacyl halide, chloroacetic acid or
oxalyl chloride is also possible.
In recent years, the synthesis and biological studies of thiosemi-
carbazides, thiosemicarbazones and thiazolidine derivatives were
described.4,8,10,13,14 The goal of the present study was to synthe-
size thiosemicarbazide derivatives of 1H-imidazo[4,5-b]pyridine-2(3)-
thione and of their cyclic analogs in order to use them for biological
research, e.g., concerning a possible in vitro antiproliferative activity
(Scheme 1). The 1H-imidazo[4,5-b]pyridine-2(3H)-thione is a universal
substrate for the synthesis of substituted and condensed compounds
containing the 1,3-diamino and the 2-thioxo-thiole group. Synthesis,
structure, and in vitro antiproliferative activity of these derivatives
were described in our previous paper.11
RESULTS AND DISCUSSION
The easily prepared sodium salt of 1H-imidazo[4,5-b]pyridine-2(3)-
thione (1a) was used in the alkylation with ethyl bromoacetate or ethyl
2-chloropropionate in ethanol to obtain 2-ethoxycarbonylmethylthio-
3H-imidazo[4,5-b]pyridine (2) or 2-(1-ethoxycarbonyl-1-ethylthio)-3H-
imidazo[4,5-b]pyridine (3). The esters 2 and 3 were reacted with 80%
hydrazine hydrate in ethanol to give 2-carbazoylmethylthioimidazo
[4,5-b]pyridine (4) and 2-(carbazoyl-1-ethylthio)-3H-imidazo[4,5-b]
pyridine (5). The hydrazides 4 and 5 were treated with selected sub-
stituted arylisothiocyanates to obtain the thiosemicarbazide deriva-
tives 6–9. The thiosemicarbazides 6 and 7 were cyclized with 2-
bromoacetophenone to give the corresponding derivatives 10 and 11.
The structures of compounds 2–11 were confirmed by elemental anal-
yses as well as by IR and 1H NMR spectroscopy. The IR spectra of
the esters 2 and 3 exihibit characteristic C O bands in the region of
1750–1730 cm−1. The methylene S CH2 protons of compound 2 show
1
in the H NMR spectrum a singlet at 4.25 ppm. For compound 3, a
quartet at 4.67 ppm is observed for the S CH proton. The IR spectra of
the thiosemicarbazide derivatives 6–9 show characteristic NH bands in
the region of 3320–s3290 cm−1. The C O bands of these compounds are
observed at 1690–1665 cm−1. The IR spectra of compounds 10 and 11 ex-
hibited NH and C O bands at 3420-3400 cm−1 and at 1680–1670 cm−1
respectively, which are attributed to the CO NH N group. The H
,
1
NMR spectra display a single CONH resonance at 8.85–8.50 ppm. The
1
absence of the H NMR signals for the thiosemicarbazide moiety and
the appearance of signals for CH at 6.95–6.81 ppm and for CH2 at
4.90–4.55 ppm confirm the presence of the thiazole ring in compounds
10 and 11. The new compounds prepared are intended for biological

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H. Liszkiewicz
SCHEME 1

1H-imidazo[4,5-b]pyridine-2(3H)-thione Derivatives
1405
research and can be suitable as starting materials for further synthe-
ses, as well.
EXPERIMENTAL
Melting points (uncorrected) were measured with a Boethius melting
point apparatus. Analyses of the new compounds were performed on a
Perkin Elmer 2400 analyzer and satisfactory results within 0.4% of
the calculated values were obtained. IR spectra (in KBr) were recorded
with an IR 75 spectrophotometer. 1H NMR spectra were obtained with
a Bruker ARX 300 MHz instrument at room temperature using DMSO-
d6or CDCl3 as solvent. Chemical shifts are referred to the residual sol-
vent signal at δ = 2.50 ppm. The course of the reactions and the purity
of the products were checked by TLC (Kieselgel G, Merck) using diethyl
ether : ethanol = 5:1 as eluent.
1H-Imidazo[4,5-b]pyridine-2(3H)-thione (1)12
Yield: 1.15 g (77%); m.p. 318–320◦C. IR (KBr), ν (cm−1): 3050 (CH); 1625,
1520 (C N), 1430, 1250, 1180 (C S); 920, 840, 780 (Ar); 705 (C S), 1H
NMR (DMSO-d6): δ = 13.10 (s, 1H, NH); 12.70 (s, 1H, NH); 8.08 (dd,
J = 4.8 Hz, J = 1.2 Hz, 1H, H-5); 7.48 (dd, J = 7.8 Hz, J = 1.2 Hz,
1H, H-7); 7.13 (dd, J = 7.8 Hz, J = 4.8 Hz, 1H, H-6). Anal. Calcd. for
C6H5N3S (151.19): C, 47.67; H, 3.33; N, 27.79%; Found: C, 47.57; H,
3.30; N, 27.94%.
General Procedure for Compounds 2 and 3
A mixture of 0.01 mol of 1H-imidazo[4,5-b]pyridine-2(3H)-thione (2)
and 0.01 mol of NaOH in 50 mL of absolute ethanol was refluxed
for 0.5 h. After cooling 0.01 mol of ethyl bromoacetate or ethyl 2-
chloropropionate was added. The mixture was refluxed for 5 h. The
precipitate formed was filtered off, washed with water, dried, and re-
crystallized from ethanol.
2-Ethoxycarbonylmethylthio-3H-imidazo[4,5-b]pyridine (2)
Yield: 1.77 g (75%); white solid, m.p. 149–150◦C. IR (KBr), ν(cm−1):
3400 (CH); 3150 (CH); 1730 (COOC2H5); 1630, 1570 (CN); 1400 (S CH);
960, 875, 780 (Ar). 1H NMR (DMSO-d6): δ = 13.24 (s, 1H, NH); 8.21 (dd,
J = 7.9 Hz, J = 5.2 Hz, 1H, H-5); 7.84 (d, J = 7.9 Hz, 1H, H-7), 7.18
(dd, J = 7.9 Hz, J = 5.2 Hz, 1H, H-6); 4.25 (s, 2H, CH2); 4.09 (q, J = 7.1
Hz, 2H, CH2CH3); 1.18 (t, J = 7.1 Hz, 3H, CH2CH3). Anal. Calcd. for

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H. Liszkiewicz
C10H11N3O2S(237.06); C, 50.50; H, 4.64; N, 17.55%; Found: C, 50.62; H,
4.68; N, 17.72%.
2-(1-Ethoxycarbonyl-1-ethylthio)-3H-imidazo[4,5-b]pyridine
(3)
Yield: 2,10 g (84%); white solid, m.p. 119–120◦C. IR (KBr) ν (cm−1):
3060, 2990 (CH); 1750 (COOC2H5); 1520 (NH); 1460 (C N); 1400, 1315,
1280 (C S); 1170 (COOC2H5). 1H NMR (CDCl3): δ = 12.77 (s, 1H, NH);
8.44 (d, J = 5.2 Hz, 1H, H-5), 8.12 (d, J = 7.9 Hz, 1H, H-7), 7.29 (dd,
J = 7.9 Hz, J = 5.2 Hz, 1H, H-6); 4.67 (q, J = 7.2 Hz, 1H, S-CH); 4.17
(q, J = 7.1 Hz, 2H, CH2 CH3); 1.67 (d, J = 7.2 Hz, 3H, CH3 CH); 1.20
(t, J = 7.1 Hz, 3H, CH2 CH3). Anal. Calcd. for C11H13N3O2S (251.30);
C, 52.57; H, 5.22; N, 16.73%; Found: C, 53.15; H, 5.31; N, 17.07%.
General Procedure for Preparation of Compounds 4 and 5
A mixture of compound 2 or 3 0.01 mol and 80% hydrazine hydrate
(0.02 mol) in 30 mL of ethanol was refluxed for 20 h. The solvent was
evaporated and the resulting residue was crystallized from ethanol.
2-Carbazoylmethylthioimidazo[4,5-b]pyridine (4)
Yield: 1.98 g (88%); m.p. white solid, m.p. 189–190◦C. IR (KBr) ν
(cm−1): 3350, 3290 (NH2); 1670 (CONH); 1630, 1590 (NH); 970, 880,
1
760 (Ar); 700 (C S). H NMR (DMSO-d6): δ = 12.83 (s, 1H, NH); 9.42
(br, 1H, CONH); 8.20 (dd, J = 4.8 Hz, J = 1.4 Hz, 1H, H-5); 7.82 (dd,
J = 7.9 Hz, J = 1.4 Hz, 1H, H-7); 7.15 (dd, J = 7.9 Hz, J = 4.8 Hz,
1H, H-6); 4.33 (br, 2H, NH2); 4.03 (s, 2H, S CH2). Anal. Calcd. for:
C8H9N5OS(233.25); C, 43.04; H, 4.06; N, 31.37%; Found: C, 43.34; H,
3.68; N, 30.95%.
2-(Carbazoyl-1-ethylthio)-3H-imidazo[4,5-b]pyridine (5)
Yield: 1.3 g (55%); m.p. white solid, m.p. 176–178◦C. IR (KBr) ν(cm−1):
3440, 3300 (NH2); 3080, 3040, 2940 (CH); 1650 (CONH); 1550, 1500
(NH); 1460 (C N); 1425, 1370, 1275 (C-S); 1120 (NH). 1H NMR (DMSO-
d6): δ = 13.00 (s, 1H, NH); 9.51 (br, 1H, CONH); 8.21 (d, J = 4.8 Hz,
1H, H-5); 7.84 (d, J = 7.9 Hz, 1H, H-7); 7.16 (dd, J = 7.9 Hz, J = 4.8
Hz, 1H, H-6); 4.64 (q, J = 6.9 Hz, 1H, S CH); 4.37 (br, 2H, NH2); 1.53
(d, J = 6.9 Hz, 3H, CH CH3). Anal. Calcd. for C9H11N5SO (237.28); C,
45.56; H, 4.67; N, 29.52%; Found: C, 46.03; H, 4.70; N, 30.01%.
General Procedure for Thiosemicarbazides 6 and 9
A solution of 0.01 mol of hydrazide 4 or 5 in ethanol (50 mL) and 0.01 mol
of the appropriate isothiocyanate was refluxed for 6–8 h. The solution

1H-imidazo[4,5-b]pyridine-2(3H)-thione Derivatives
1407
was concentrated at reduced pressure to 20 mL, the solid formed was
filtered off and crystallized from n-butanol.
2-[4-(Phenylthiosemicarbazide)-carbonylmethylthio]-
3H-imidazo[4,5-b]pyridine (6)
Yield: 2.7 g (75%); white solid, m.p. 191–192◦C. IR (KBr) ν(cm−1):
3230 (NH); 1665 (CONH); 1570 (NH); 1300, 1245, 1130 (C S). 1H NMR
(DMSO-d6): δ = 13.28 (s, 1H, NH); 10.47 (br, 1H, CONH); 9.76 (br, 1H,
CONHNH); 9.62 (br, 1H, NH Ph); 8.15-7.01 (m, 7H, Ar H); 4.18 (s,
2H, CH2). Anal. Calcd. for C15H14N601S2 (358.45); C, 50.26; H, 3.94; N,
33.45%; Found: C, 50.42; H, 3.85; N, 33.21%.
2-[4-(p-Chlorophenylthiosemicarbazide)-carbonylmethylthio]-
3H-imidazo[4,5-b]-pyridine (7)
Yield: 3.3 g (84%); white solid, m.p. 199–200◦C. IR (KBr) ν (cm−1);
3225 (NH); 1420, 1260, 1210 (C S); 790, 760 (Ar). 1H NMR (DMSO-d6):
δ = 13.30 (s, 1H, NH); 10.51 (br, 1H, CONH); 9.89 (br, 1H, CONHNH);
9.69 (s, 1H, NH C6H4Cl); 8.17 (d, J = 4.9 Hz, 1H, H-5); 7.59 (d, J = 7.8
Hz, 1H, H-7); 7.35 (m, 4H, Ar); 7.16 (dd, J = 7.8 Hz, J = 4.9 Hz, 1H,
H-6); 4.19 (s, 2H, CH2). Anal. Calcd. for: C15H13N6Cl1OS (392.88); C,
45.86; H, 3.34; N, 21.39%; Found: C, 45.53; H, 3.16; N, 21.10%.
2-[4-(p-Bromophenylthiosemicarbazide)-carbonylmethylthio]-
3H-imidazo[4,5-b]-pyridine (8)
Yield: 3.8 g (86%); white solid, m.p. 204–205◦C. IR (KBr) ν(cm−1):
3260 (NH); 1690 (CONH); 1540 (NH); 1260, 1215, 1140 (C S); 790, 760
(Ar). 1H NMR (DMSO-d6): δ = 13.28 (s, 1H, NH); 10.49 (br, 1H, CONH);
9.95 (br, 1H, CONHNH); 9.67 (s, 1H, NH C6H4Br); 8.17 (d, J = 4.9
Hz, 1H, H-5); 7.61 (d, J = 7.9 Hz, 1H, H-7); 7.41 (m, 4H, Ar); 7.11 (dd,
J = 7.9 Hz, J = 4.9 Hz, 1H, H-6); 4.18 (s, 2H, CH2). Anal. Calcd. for:
C15H13N6Br1OS (437.33); C, 41.20; H, 3.00; N, 19.22%; Found: C, 41.10;
H, 2.80; N, 18.80%.
2-[4-(p-Anisylthiosemicarbazide)-carbonylmethylthio]-
3H-imidazo[4,5-b]pyridine (9)
Yield: 3.21 g (83%); white solid, m.p. 194–195◦C. IR (KBr) ν(cm−1):
3320, 3290 (NH); 2975, 2880 (CH); 1680 (CONH); 1600, 1540 (NH);
1425, 1390, 1260, 1230, 1180 (C S); 840, 800, 760 (Ar). 1H NMR
(DMSO-d6): δ = 13.28 (s, 1H, NH); 10.45 (br, 1H, CONH); 9.68 (br, 1H,
CONHNH); 9.52 (s, 1H, NH C6H4OCH3); 8.16 (d, J = 4,8 Hz, 1H, H-5);
7.58 (d, J = 7.9 Hz, 1H, H-7); 7.70 (m, 5H, Ar H, H-6); 4.17 (s, 2H, CH2);

1408
H. Liszkiewicz
3.74 (s, 3H, OCH3). Anal. Calcd. for: C16H16N6OS (388.46); C, 49.47; H,
4.15; N, 21.63%; Found: C, 49.00; H, 3.96; N, 21.31%.
General Procedure for Compounds 10 and 11
A mixture of 0.01 mol of the thiosemicarbazide 10 or 11, 0.01 mol of
2-bromoacetophenone and 0.04 mol of freshly dehydrated sodium ac-
etate in 80 mL of absolute ethanol was refluxed for 4 h. The solvent
from the solution thus obtained was evaporated under reduced pres-
sure, the residue was diluted with H2O (100 mL) and left overnight.
The precipitate obtained was filtered, washed with cold water, dried,
and crystallized from n-butanol.
2-[3-(p-Chlorophenyl-4-phenyl-3H-thiazol-2-ylidene)-3-
carbazoylmethylthio]-3H-imidazo[4,5-b]pyridine (10)
Yield: 2.39 g (47%), m.p. white solid, m.p. 192–194◦C. IR (KBr)
ν(cm−1): 3060, 2900 (CH); 1670 (CONH); 1600, 1495 (NH); 1395, 1280,
1
1200, 1160 (C S); 960, 830, 750 (Ar). H NMR (CDCl3): δ = 11.87 (s,
1H, NH); 8.85 (br, 1H, CONH); 8.25 (d, J = 5.2 Hz, 1H, H-5); 8.10 (d,
J = 7.8 Hz, 1H, H-7); 7.91 (m, 2H, Ar H); 7.46 (m, 7H, Ar H); 7.16 (dd,
J = 7.8 Hz, J = 5.2 Hz, 1H, H-6); 6.95 (s, 1H, CH-thiazoline); 4.55 (s,
2H, S CH2). Anal. Calcd. for: C23H17N6Cl1O1S2 (492.06): C, 56.09; H,
3.48; N, 17.07%; Found: C, 56.01; H, 3.54; N, 17.54%.
2-[3-(p-Bromophenyl-4-phenyl-3H-thiazol-2-ylidene)-3-
carbazoylmethylthio]-3H-imidazo[4,5-b]pyridine (11)
Yield: 2.14 g (39%); m.p. white solid, m.p. 198–200◦C. IR (KBr)
ν(cm−1): 3060, 2930 (CH); 1680 (CONH); 1600, 1495 (NH); 1330, 1265,
1200, 1170 (C-S); 960, 840, 750 (Ar). 1H NMR (CDCl3): δ = 11.95 (s, 1H,
NH); 8.50 (s, 1H, CONH); 8.28 (d, J = 4.9 Hz, 1H, H-5); 7.97 (m, 2H,
Ar H); 7.80 (d, J =7.6 Hz, 1H, H-7); 7.37 (m, 7H, Ar H); 7.16 (dd, J =
7.6 Hz, J = 4.9 Hz, 1H, H-6); 6.81 (s, 1H, CH-thiazoline); 4.90 (s, 2H,
S CH2). Anal. Calcd. for C23H17N6BrOS(537.45); C, 51.40; H, 3.19; N,
15.64%; Found: C, 50.35; H, 3.05; N, 15.36%.
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