Synthesis of 2-phenyl-1H-imidazo[4,5-b]pyridine as type 2 diabetes inhibitors and molecular docking studies

Article abstract of DOI:10.1007/s00044-017-1806-0

A series of imidazo[4,5-b]pyridines (3–32) was synthesized and evaluated for their ability to inhibit Baker’s yeast α-glucosidase enzyme. The IC₅₀ values for all compounds were in the range of 13.5–93.7 μM with compound 15, a 2,4-dihydroxy-substituted analog, displayed the most potent activity potential. Structure–activity relationship strongly suggested the presence of hydroxyl group at aromatic side chain as the main contributing factor towards the inhibitory potential. Findings also suggested that compounds having hydroxyl groups at ortho and para positions are able to inhibit α-glucosidase enzyme efficiently. This experimental observation was further supported by docking studies carried out on human intestinal maltase-glucoamylase enzyme (PDB ID: 3TOP). The –NH– group of imidazo-pyridine of compound 15 formed H-bond with Asp1526, while both hydroxyls of catechol formed H-bond with Asp1279. Imidazo-pyridine ring was well stabilized by π–π stacking with Phe1560, and other hydrophobic interactions involving side chain of Pro1159, Tyr1167, Asp1157, Met1421, Trp1369, Pro1318, and Lys1460. The catechol ring also forms several hydrophobic interactions with Phe1560, Trp1523, Trp1418, His1584, Try1251, Ile1218 and Trp1355.

Full text of DOI:10.1007/s00044-017-1806-0

MEDICINAL
CHEMISTRY
RESEARCH
Med Chem Res
DOI 10.1007/s00044-017-1806-0
ORIGINAL RESEARCH
Synthesis of 2-phenyl-1H-imidazo[4,5-b]pyridine as type 2 diabetes
inhibitors and molecular docking studies
Muhammad Taha1,2 Nor Hadiani Ismail1,2 Syahrul Imran1,2 Izzatul Ainaa2
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Manikandan Selvaraj3,4 Mohd syukri baharudin1,2 Muhammad Ali5
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Khalid Mohammed Khan6 Nizam Uddin7
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Received: 29 August 2016 / Accepted: 13 February 2017
© Springer Science+Business Media New York 2017
Abstract A series of imidazo[4,5-b]pyridines (3–32) was
synthesized and evaluated for their ability to inhibit Baker’s
yeast α-glucosidase enzyme. The IC50 values for all com-
pounds were in the range of 13.5–93.7 µM with compound
15, a 2,4-dihydroxy-substituted analog, displayed the most
potent activity potential. Structure–activity relationship
strongly suggested the presence of hydroxyl group at aro-
matic side chain as the main contributing factor towards the
inhibitory potential. Findings also suggested that com-
pounds having hydroxyl groups at ortho and para positions
are able to inhibit α-glucosidase enzyme efficiently. This
experimental observation was further supported by docking
studies carried out on human intestinal maltase-
glucoamylase enzyme (PDB ID: 3TOP). The –NH– group
of imidazo-pyridine of compound 15 formed H-bond with
Asp1526, while both hydroxyls of catechol formed H-bond
with Asp1279. Imidazo-pyridine ring was well stabilized by
π–π stacking with Phe1560, and other hydrophobic inter-
actions involving side chain of Pro1159, Tyr1167,
Asp1157, Met1421, Trp1369, Pro1318, and Lys1460. The
catechol ring also forms several hydrophobic interactions
with Phe1560, Trp1523, Trp1418, His1584, Try1251,
Ile1218 and Trp1355.
Electronic supplementary material The online version of this article
(doi:10.1007/s00044-017-1806-0) contains supplementary material,
which is available to authorized users.
* Muhammad Taha
taha_hej@yahoo.com
muhamm9000@puncakalam.uitm.edu.my
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Keywords Imidazo[4,5-b]pyridine Type-2 Diabetes
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1
Pharmacokinetic prediction Molecular docking
Atta-ur-Rahman Institute for Natural Product Discovery,
Universiti Teknologi MARA (UiTM), Puncak Alam Campus,
Bandar Puncak Alam 42300 Selangor, Malaysia
2
Faculty of Applied Science Universiti Teknologi MARA (UiTM),
40450 Shah Alam, Selangor, Malaysia
3
Integrative Pharmacogenomics Institute (iPROMISE), Universiti
Teknologi MARA (UiTM), Puncak Alam Campus, Bandar
Puncak Alam 42300 Selangor Darul Ehsan, Malaysia
Introduction
Diabetes mellitus is a metabolic disorder that increases
blood glucose levels (hyperglycemia), resulting in various
medical complications like retinopathy, cardiovascular
problems, nephropathy and neuropathy (Chen et al. 2013).
The number of diabetes mellitus patients increased from
153 million to 347 million between the years 1980–2008
(Danaei et al. 2011), while the International Diabetes Fed-
eration estimated an increase up to 552 million by the year
2030 (Whiting et al. 2011). Type-II diabetes mellitus,
common in developed countries, is resulted from reduced
and impaired insulin secretion (Porte 1991; Taylor et al.
4
Faculty of Pharmacy, Universiti Teknologi MARA (UiTM),
Puncak Alam Campus, Bandar Puncak Alam 42300 Selangor
Darul Ehsan, Malaysia
5
Department of Chemistry, COMSATS Institute of Information
Technology, University Road, Abbottabad, Khyber Pakhtunkhwa
22060, Pakistan
6
H. E. J. Research Institute of Chemistry, International Center for
Chemical and Biological Sciences, University of Karachi, Karachi
75270, Pakistan
7
Batterje Medical College for Science & Technology, Jeddah
21442, Kingdom of Saudi Arabia

Med Chem Res
1994; Butler et al. 2003; Wu 2007; Tang et al. 2010). This
is usually controlled by suppressing absorption and diges-
tion of dietary carbohydrates, through inhibition of diges-
tive enzymes like α-glucosidase and α-amylase (Wang et al.
2010). Glucose produced through hydrolysis of carbohy-
drates by α-glucosidase is being absorbed into blood stream,
thus increases postprandial blood glucose level. Considered
as a crucial target for drug, the inhibition of α-glucosidase
can be an effective strategy in controlling type-II diabetes
mellitus (Puls et al. 1977; Shim et al. 2003; Samad et al.
1988; O’Dea and Turton 1985). Current regiment for
α-glucosidase inhibitors blocks glucose production by
occupying the active site and prevents hydrolysis of
α-(1–4)-linked D-glucose residues from the non-reducing
end (Charron et al. 1986). This reduces the rate of glucose
absorption and, therefore, decreases plasma glucose level.
α-Glucosidase inhibition has attracted plenty of attention
from pharmaceutical industry as a treatment method of
diseases like diabetics, viral infections, hepatitis and cancer
(Humphries et al. 1986; Park et al. 2008; Storr et al. 2008).
Although many α-glucosidase inhibitors have been repor-
ted, most of them are sugars or sugar-derived such as
acarbose (Lefebvre and Scheen 1994). In recent times,
number of α-glucosidase inhibitors were discovered and
studied, with several non-saccharide compounds including
substrate analogs (Luo et al. 2001; Kumar et al. 2011;
Wehmeier and Piepersberg 2004), which are taken orally
for the treatment of diabetes. Currently available α-gluco-
sidase inhibitors like miglitol (Scott and Spencer 2000),
voglibose (Matsuo et al. 1992), acarbose (Schmidt et al.
1977) and nojirimycin (Asano et al. 1994) are having some
unwanted side effects such as diarrhea, abdominal disten-
sion, meteorism, hepatoxicity and flatulence (Hsiao et al.
2006; Hollander 1992). Due to these limitations and asso-
ciated absorptivity issues, (Reuser and Wisselaar 1994;
Scheen 2003), new and potent α-glucosidase inhibitors are
highly desired. To overcome the undesired, plenty of efforts
have been put in to design and develop more effective non-
glycosidic-based α-glucosidase inhibitors (Adisakwattana
et al. 2004).
In the present study, a library of compounds (3–32)
consisting of imidazopyridine scaffold has been synthe-
sized. Imidazo[4,5-b]pyridine, which contains a fused imi-
dazole ring, is reported for various activities such as
antibacterial (Mallemula et al. 2013), anticancer (Temple
et al. 1987), antitubercular (Bukowski and Kaliszan 1991),
antiulcer (Loriga et al. 1992), analgesic (Clark et al. 1978)
and antioxidant (Lavanya et al. 2011). Compounds having
heterocyclic rings possess versatile biological properties
and easily interact with the nature (Narasimhan et al. 2011;
Imran et al. 2015a; Taha et al. 2015a; Taha et al. 2016a,
2016b). Imidazole has also been reported to show in vivo
antihyperglycemic activity for treatment of type-II diabetes
mellitus by stimulating insulin secretion and decreasing
insulin resistance of peripheral tissues via reduction of
hyperglycemia (Plant and Henquin 1990). Interestingly, it
has been found that imidazole moiety is additionally con-
sidered significant for antidiabetic activity (Brownson and
Hipkiss 2000). Encouraged by these previous findings, we
decided to identify new α-glucosidase inhibitors and reveal
new perspectives in the biological potentials of newly
synthesized imidazo[4,5-b]pyridine. α-Glucosidase inhibi-
tory potential of imidazo[4,5-b]pyridines and related com-
pounds with similar pharmacophores like benzothiazole (A)
(Taha et al. 2015b) Quinoline (B) (Taha et al. 2015c),
benzimidazole (C) (Taha et al. 2016), benzimidazole
thiourea (D) (Zawawi et al. 2017) have been widely
reported (Fig. 1).
Material and Methods
NMR experiments were performed on Ultra Shield Bruker
FT NMR 500 MHz, CHN analysis was performed on a
Fig. 1 Structures a–e similar
with target compounds recently
published

Med Chem Res
Carlo Erba Strumentazion-Mod-1106, Italy. Electron
impact mass spectra (EI-MS) were recorded on a Finnigan
MAT-311A, Germany. Thin layer chromatography was
performed on pre-coated silica gel aluminum plates (Kie-
selgel 60, 254, E. Merck, Germany). Chromatograms were
visualized using UV at 254 and 365 nm.
C, 51.34; H, 3.31; N, 13.82; found: C, 51.35; H, 3.30; N,
13.81
2-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)benzene-1,4-
diol (4)
1
Synthesis
Yield: 96.6%; Mp: 353 °C; H NMR (500 MHz, DMSO-
d6): δ 13.16 (s, 1H, NH), 9.17 (s, 1H), 8.46 (s, 1H), 7.52
(s,1H), 6.90 (s,1H). 13C-NMR (125 MHz, DMSO-d6): δ
153.6, 151.1, 150.4, 146.4, 140.3, 133.9, 127.5, 118.7,
117.4, 116.9, 115.2, 113.4; HREI-MS: m/z calcd for
C12H8BrN3O2 [M]+ 304.9800; found 304.9793; anal. calcd
for C12H8BrN3O2: C, 47.08; H, 2.63; N, 13.73; found: C,
47.02; H, 2.65; N, 13.72
2-Phenyl-1H-imidazo[4,5-b]pyridines (3–31)
Equimolar amount of 5-bromopyridine-2,3-diamine 1,
benzaldehydes 2 and sodium metabisulfite were dissolved
in 40 ml of Dimethylformamide and refluxed at 130 °C for
6 h. Then reaction mixture was poured into ice water to
allow product precipitation. The precipitate was filtered and
crystallized from ethanol to afford pure product.
6-Bromo-2-(4-chlorophenyl)-1H-imidazo[4,5-b]pyridine
(5)
4-(6-Bromo-1H-imidazo[4,5-b]pyridin-2yl) phenol (1)
1
1
Yield: 95.6%; Mp: 320 °C; H NMR (500 MHz, dimethyl
Yield: 84.0%; Mp: 340 °C; H NMR (500 MHz, DMSO-
sulfoxide (DMSO)-d6): δ 8.43 (d, 1H, J = 2.0 Hz), 8.26 (d,
1H, J = 2.0 Hz), 8.09 (d, 1H, J = 2.0 Hz), 8.08 (d, 1H, J =
2.0 Hz), 6.98 (s, 1H), 6.96 (s, 1H). 13C-NMR (125 MHz,
DMSO-d6): δ 161.7, 153.2, 144.5, 140.3, 133.5, 128.6,
126.7, 126.7, 121.9, 119.4, 115.23, 115.2; HREI-MS: m/z
calcd for C12H8BrN3O [M]+ 288.9851; found 288.9847;
anal. calcd for C12H8BrN3O: C, 49.68; H, 2.78; N, 14.48;
found: C, 49.66; H, 2.79; N, 14.47
d6): δ 834 (d, 1H, J = 2.0 Hz), 8.25 (d, 1H, J = 8.5 Hz),
8.18 (s, 1H), 6.72 (s, 1H). 13C-NMR (125 MHz, DMSO-d6):
δ 154.2, 144.2, 141.4, 134.3, 133.5, 132.7, 129.2, 127.1,
126.3, 114.4; HREI-MS: m/z calcd for C12H7BrClN3
[M]+ 306.9512; found 306.9519; anal. calcd for
C12H7BrClN3: C, 46.71; H, 2.29; N, 13.62; found: C, 46.72;
H, 2.31; N, 13.64
6-Bromo-2-(3,4-dimethoxyphenyl)-1H-imidazo[4,5-b]
pyridine (2)
6-Bromo-2-(pyridin-3-yl)-1H-imidazo[4,5-b]pyridine (6)
1
Yield: 56.0%; Mp: 340 °C; H NMR (500 MHz, DMSO-
1
Yield: 55.3%; Mp: 278 °C, H NMR (500 MHz, DMSO-
d6): δ 8.75 (s, 1H), 8.74 (s, 1H), 8.56 (d, 1H, J = 8.0 Hz),
8.34 (s, 1H), 7.47 (d, 1H, J = 2.0 Hz), 7.65 (t, 1H). 13C-
NMR (125 MHz, DMSO-d6): δ 154.2, 151.0, 148.8, 143.4,
139.4, 134.7, 132.7, 130.1, 129.0, 127.5, 118.9; HREI-MS:
m/z calcd for C11H7BrN4 [M]+ 273.9854; found 273.9847;
anal. calcd for C11H7BrN4: C, 48.02; H, 2.56; N, 20.37;
found: C, 48.01; H, 2.54; N, 20.38
d6): δ 8.35 (d, 1H, J = 2.0 Hz), 8.18 (s, 1H), 7.83 (s, 1H),
7.82 (s, 1H), 7.16 (d, 1H, J = 8.5 Hz), 3.88 (s, 3H, OCH3),
3.85 (s, 3H, OCH3). 13C-NMR (125 MHz, DMSO-d6): δ
155.8, 153.9, 150.4, 143.4, 139.4, 134.7, 128.6, 125.9,
121.9, 119.4, 114.2, 110.5, 57.7, 57.7; HREI-MS: m/z calcd
for C14H12BrN3O2 [M]+ 333.0113; found 333.0117; anal.
calcd for C14H12BrN3O2: C, 50.32; H, 3.62; N, 12.57;
found: C, 50.33; H, 3.64; N, 12.55
6-Bromo-2-p-tolyl-1H-imidazo[4,5-b]pyridine (7)
6-Bromo-2-(4-methoxyphenyl)-1H-imidazo [4,5-b]pyridine
(3)
1
Yield: 88.2%; Mp: 318 °C; H NMR (500 MHz, DMSO-
d6): δ 8.39 (d, 1H, J = 2.0 Hz), 8.23 (d, 1H, J = 2.0 Hz),
8.13 (s, 1H), 8.12 (s, 1H), 7.39 (d, 1H, J = 2.0 Hz), 2.40 (s,
3H, CH3). 13C-NMR (125 MHz, DMSO-d6): δ 151.2,
143.4, 142.4, 137.7, 132.7, 130.2, 130.2, 128.7, 127.2,
127.2, 126.9, 119.4, 20.1; HREI-MS: m/z calcd for
C13H10BrN3 [M]+ 287.0058; found 287.0064; anal. calcd
for C13H10BrN3: C, 54.19; H, 3.50; N, 14.58; found: C,
54.21; H, 3.51; N, 14.56
1
Yield: 73.3% ; Mp: 296 °C, H NMR (500 MHz, DMSO-
d6): δ 13.60 (s, 1H, NH ), 8.38 (s, 1H), 8.24 (s, 1H), 8.19 (d,
1H, J = 8.5 Hz), 7.15 (d, 1H, J = 8.5 Hz). 13C-NMR (125
MHz, DMSO-d6): δ 161.47, 152.34, 144.68, 140.23,
133.87, 127.56, 126.79, 126.79, 125.43, 118.34, 114.76,
114.76, 58.0; HREI-MS: m/z calcd for C13H10BrN3O [M]+
303.0007; found 303.0013; anal. calcd for C13H10BrN3O:

Med Chem Res
6-Bromo-2-(2-fluorophenyl)-1H-imidazo[4,5-b] pyridine
(8)
J = 2.0 Hz). 13C-NMR (125 MHz, DMSO-d6): δ 152.4,
148.6, 145.4, 142.4, 135.7, 128.6, 120.6, 117.1, 116.4,
113.5, 111.1; HREI-MS: m/z calcd for C12H8BrN3O2 [M]+
304.9800; found 304.9808; anal. calcd for C12H8BrN3O2:
C, 47.08; H, 2.63; N, 13.73; found: C, 47.06; H, 2.64; N,
13.71
1
Yield: 62.0%; Mp: 226 °C; H NMR (500 MHz, DMSO-
d6): δ 8.46 (d, 1H, J = 2.0 Hz), 8.25 (s, 1H), 8.22 (t, 1H),
7.65–7.60 (m, 3H), 7.48–7.41 (m, H). 13C-NMR (125 MHz,
DMSO-d6): δ 162.4 (d, J = 262.3 Hz), 147.0, 142.4, 139.4,
132.7, 130.3, 128.9, 127.4, 124.1, 118.9, 117.4, 114.5;
HREI-MS: m/z calcd for C12H7BrFN3 [M]+ 290.9807;
found 290.9801; anal. calcd for C12H7BrFN3: C, 49.34; H,
2.42; N, 14.39; found: C, 49.35; H, 2.43; N, 14.37
6-Bromo-2-o-tolyl-1H-imidazo[4,5-b]pyridine (13)
1
Yield: 84.1%; Mp: 226 °C; H NMR (500 MHz, DMSO-
d6): δ 8.44 (d, 1H, J = 2.0 Hz), 8.28 (d, 1H, J = 2.0 Hz),
7.80 (d, 1H, J = 7.5 Hz), 7.47–7.37(m, 3H), 2.61 (s, 3H,
CH3). 13C-NMR (125 MHz, DMSO-d6): δ 149.2, 143.4,
139.4, 133.6, 132.8, 131.7, 130.2, 128.5, 127.1, 126.6,
125.1, 115.4, 22.2; HREI-MS: m/z calcd for C13H10BrN3
[M]+ 287.0058; found 287.0058; anal. calcd for
C13H10BrN3: C, 54.19; H, 3.50; N, 14.58; found: C, 54.18;
H, 3.52; N, 14.57
3-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)phenol (9)
Yield: 67.0%; Mp: above 350 °C; 1H NMR (500 MHz,
DMSO-d6): δ 8.43 (s, 1H), 8.27 (s, 1H), 7.66 (d, 1H, J =
6.0 Hz), 7.38 (t.1H), 6.97 (d, 1H, J = 8.5 Hz). 13C-NMR
(125 MHz, DMSO-d6): δ 158.7, 152.4, 144.7, 140.3, 133.8,
132.4, 130.6, 128.6, 118.0, 117.4, 116.1, 113.4; HREI-MS:
m/z calcd for C12H8BrN3O [M]+ 288.9851; found
288.9856; anal. calcd for C12H8BrN3O: C, 49.68; H, 2.78;
N, 14.48; found: C, 49.76; H, 2.77; N, 14.47
4-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)benzene-1,3-
diol (14)
Yield: 80.5%; Mp: above 350 °C; 1H NMR (500 MHz,
DMSO-d6): δ 8.43 (d, 1H, J = 2.0 Hz), 8.26 (s,1H), 7.96 (d,
1H, J = 8.5 Hz), 6.49 (s, 1H), 6.47 (s, 1H), 6.44 (d, 1H, J =
2.0 Hz). 13C-NMR (125 MHz, DMSO-d6): δ 164.6, 159.7,
153.2, 146.4, 140.6, 133.3, 129.5, 127.2, 118.7, 109.3,
104.8, 105.1; HREI-MS: m/z calcd for C12H8BrN3O2 [M]+
304.9800; found 304.9792; anal. calcd for C12H8BrN3O2:
C, 47.08; H, 2.63; N, 13.73; found: C, 47.07; H, 2.64;
N, 13.75.
5-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)-2-
methoxyphenol (10)
1
Yield: 56.0%; Mp: 276 °C; H NMR (500 MHz, DMSO-
d6): δ 8.38 (d, 1H, J = 1.5 Hz), 8.20 (d, 1H, J = 2.0 Hz),
7.68 (s, 1H), 7.67 (d, 1H, J = 2.0 Hz), 7.13 (s, 1H), 7.11 (s,
1H), 3.87 (s, 3H, OCH3). 13C-NMR (125 MHz, DMSO-d6):
δ 151.5, 149.3, 148.2, 145.4, 142.4, 132.7, 128.6, 123.8,
118.0, 117.2, 115.3, 109.3, 58.7; HREI-MS: m/z calcd for
C13H10BrN3O2 [M]+ 318.9956; found 318.9948; anal.
calcdanal. calcd for C13H10BrN3O2: C, 48.77; H, 3.15; N,
13.13 found C, 48.78; H, 3.13; N, 13.12
2-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)-4-
methoxyphenol (15)
1
Yield: 83.6%; Mp: 326 °C; H NMR (500 MHz, DMSO-
6-Bromo-2-(pyridin-4-yl)-1H-imidazo[4,5-b]pyridine (11)
d6): δ 13.30 (s, 1H, NH), 8.49 (s, 1H), 8.43 (s,1H), 8.25
(s,1H), 7.73 (d, 1H, J = 3.0 Hz), 7.08 (d, 1H, J = 2.5 Hz),
7.06 (d, 1H, J = 2.5 Hz), 7.01(s, 1H), 3.81 (s, 3H,OCH3).
13C-NMR (125 MHz, DMSO-d6): δ 155.7, 152.1, 150.2,
143.4, 139.4, 132.7, 128.6, 117.4, 116.1, 114.9, 113.6,
112.0, 55.0; HREI-MS: m/z calcd for C13H10BrN3O2 [M]+
318.9956; found 318.9965; anal. calcd for C13H10BrN3O2:
C, 48.77; H, 3.15; N, 13.13; found C, 48.79; H, 3.16;
N, 13.12
1
Yield: 80.7%; Mp: 336 °C, H NMR (500 MHz, DMSO-
d6): δ 8.79 (s, 1H), 8.48 (d, 1H, J = 2.0 Hz), 8.36 (s, 1H),
8.14 (d, 1H, J = 6.0 Hz). 13C-NMR (125 MHz, DMSO-d6):
δ 153.2, 151.4, 151.4, 143.4, 139.4, 140.8, 133.7, 128.6,
123.5, 123.5, 115.4; HREI-MS: m/z calcd for C11H7BrN4
[M]+ 273.9854; found 273.9861; anal. calcd for
C11H7BrN4: C, 48.02; H, 2.56; N, 20.37; found: C, 48.03;
H, 2.54; N, 20.36
6-Bromo-2-(furan-2-yl)-1H-imidazo[4,5-b]pyridine (16)
4-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)benzene-1,2-
diol (12)
1
Yield: 99.6%; Mp: 328 °C; H NMR (500 MHz, DMSO-
d6): δ 8.41 (d, 1H, J = 2.0 Hz), 8.02 (d, 1H, J = 1.0 Hz),
7.36 (s, 1H), 7.35 (s, 1H), 6.79 (t.1H). 13C-NMR (125 MHz,
DMSO-d6): δ 148.5, 148.8, 144.8, 144.0, 140.3, 133.7,
127.6, 118.4, 117.1, 112.0; HREI-MS: m/z calcd for
1
Yield: 97.0%; Mp: 350 °C, H NMR (500 MHz, DMSO-
d6): δ 8.35 (d, 1H, J = 2.0 Hz), 8.16 (s, 1H), 7.90 (d, 1H,
J = 8.5 Hz), 7.65 (s, 1H), 7.55 (s, 1H), 7.53 (d, 1H,

Med Chem Res
C10H6BrN3O [M]+ 262.9694; found 262.9687; anal. calcd
for C10H6BrN3O: C, 45.48; H, 2.29; N, 15.91; found: C,
45.49; H, 2.28; N, 15.90
for C12H7BrFN3: C, 49.34; H, 2.42; N, 14.39; found: C,
49.34; H, 2.42; N, 14.39
6-Bromo-2-(2-nitrophenyl)-1H-imidazo[4,5-b]pyridine (21)
6-Bromo-2-(3-methoxyphenyl)-1H-imidazo[4,5-b]pyridine
(17)
1
Yield: 48.2%; Mp: 216 °C; H NMR (500 MHz, DMSO-
1
d6): δ 13.93 (s, 1H, NH), 8.12 (s, 1H), 8.10 (s, 1H), 8.01 (d,
1H, J = 1.0 Hz), 8.00 (d, 1H, J = 1.5 Hz), 7.93 (t, 1H), 7.85
(t, 1H). 13C-NMR (125 MHz, DMSO-d6): δ 150.7, 148.1,
144.2, 143.3, 135.2, 132.2, 130.3, 129.4, 128.0, 126.2,
124.3, 116.5; HREI-MS: m/z calcd for C12H7BrN4O2 [M]+
317.9752; found 317.9759; anal. calcd for C12H7BrN4O2:
C, 45.17; H, 2.21; N, 17.56; found: C, 45.18; H, 2.19; N,
17.54
Yield: 99.7%; Mp: 272 °C; H NMR (500 MHz, DMSO-
d6): δ 13.76 (s, 1H, NH), 8.43 (s, 1H), 8.33 (s,1H), 7.84 (d,
1H, J = 8.0 Hz), 7.81 (s, 1H), 7.50 (t, 1H), 7.15 (d, 1H, J =
2.0 Hz), 7.13 (d, 1H, J = 2.0 Hz), 3.88 (s, 3H,OCH3). 13C-
NMR (125 MHz, DMSO-d6): δ 162.4, 151.9, 144.8, 140.3,
133.7, 131.6, 129.0, 127.6, 119.3, 118.4, 114.6, 114.0,
58.3; HREI-MS: m/z calcd for C13H10BrN3O [M]+
303.0007; found 303.0014; anal. calcd for C13H10BrN3O:
C, 51.34; H, 3.31; N, 13.82; found: C, 51.35; H, 3.33;
N, 13.80
6-Bromo-2-(thiophen-2-yl)-1H-imidazo[4,5-b]pyridine (22)
3-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)benzene-1,2-
diol (18)
1
Yield: 89.0%; Mp: 310 °C; H NMR (500 MHz, DMSO-
d6): δ 13.80 (s, 1H, NH), 7.98 (d, 1H, J = 1.0 Hz), 7.97(d,
1H, J = 1.5 Hz), 7.85 (s, 1H), 7.84 (s, 1H), 7.29 (t,1H). 13C-
NMR (125 MHz, DMSO-d6): δ 148.1, 143.3, 140.4, 139.8,
130.4, 130.0, 127.5, 127.0, 126.3, 116.1; HREI-MS: m/z
calcd for C10H6BrN3S [M]+ 278.9466; found 278.9475;
anal. calcd for C10H6BrN3S: C, 42.87; H, 2.16; N, 15.00;
found: C, 42.89; H, 2.15; N, 14.98
1
Yield: 95.0%; Mp: 298 °C; H NMR (500 MHz, DMSO-
d6): δ 8.48 (s, 1H), 8.43 (s, 1H), 7.58 (d, 1H, J = 8.0 Hz),
6.97 (d, 1H, J = 7.5 Hz), 6.85 (t, 1H). 13C-NMR (125 MHz,
DMSO-d6): δ 153.5, 148.9, 146.6, 145.4, 142.3, 136.7,
124.6, 121.1, 119.5, 117.4, 115.1, 112.4 (s); HREI-MS: m/z
calcd for C12H8BrN3O2 [M]+ 304.9800; found 304.9809;
anal. calcd for C12H8BrN3O2: C, 47.08; H, 2.63; N, 13.73;
found: C, 47.07; H, 2.62; N, 13.75
2-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)phenol (23)
1
6-Bromo-2-m-tolyl-1H-imidazo[4,5-b]pyridine (19)
Yield: 85.0%; Mp: 318 °C; H NMR (500 MHz, DMSO-
d6): δ 12.71 (s, 1H, NH), 8.50 (d, 1H, J = 1.5 Hz), 8.15 (d,
1H, J = 8.5 Hz), 7.38 (t, 1H), 7.04 (s, 1H). 13C-NMR (125
MHz, DMSO-d6): δ 158.9, 153.0, 148.1, 143.3, 131.0,
130.3, 129.8, 126.3, 121.0, 118.4, 116.1, 111.7; HREI-MS:
m/z calcd for C12H8BrN3O [M]+ 288.9851; found
288.9859; anal. calcd for C12H8BrN3O: C, 49.68; H, 2.78;
N, 14.48; found: C, 49.69; H, 2.76; N, 14.46
1
Yield: 68.7%; Mp: 292 °C; H NMR (500 MHz, DMSO-
d6): δ 13.72 (s, 1H, NH), 8.40 (s, 1H), 8.08 (s, 1H), 8.03 (d,
1H, J = 7.5 Hz), 7.47 (t, 1H), 7.39 (d, 1H, J = 7.5 Hz), 2.43
(s, 3H,CH3). 13C-NMR (125 MHz, DMSO-d6): δ 152.9,
145.8, 141.4, 136.3, 135.7, 132.5, 130.1, 128.9, 127.6,
126.6, 124.0, 118.4, 21.0; HREI-MS: m/z calcd for
C13H10BrN3 [M]+ 287.0058; found 287.0058; anal. calcd
for C13H10BrN3: C, 54.19; H, 3.50; N, 14.58 found: C,
54.19; H, 3.50; N, 14.58
2-(6-Bromo-1H-imidazo[4,5-b]pyridin-2-yl)-5-
methoxyphenol (24)
6-Bromo-2-(3-fluorophenyl)-1H-imidazo[4,5-b]pyridine
(20)
1
Yield: 87.5%; Mp: 314 °C; H NMR (500 MHz, DMSO-
d6): δ 12.90 (s, 1H, NH), 8.42 (d, 1H, J = 2.0 Hz), 8.33 (s,
1H), 8.04 (s, 1H), 8.02 (s, 1H), 8.66 (d, 1H, J = 2.0 Hz),
8.65 (d, 1H, J = 2.5 Hz), 8.61 (d, 1H, J = 2.5 Hz). 13C-
NMR (125 MHz, DMSO-d6): δ 160.6, 160.3, 153.9, 148.1,
143.5, 130.3, 130.6, 126.2, 116.5, 108.0, 107.6, 102.1,
56.0; HREI-MS: m/z calcd for C13H10BrN3O2 [M]+
318.9956; found 318.9962; anal. calcd for C13H10BrN3O2:
C, 48.77; H, 3.15; N, 13.13; found: C, 48.78; H, 3.14;
N, 13.14
1
Yield: 89.0%; Mp: 325 °C; H NMR (500 MHz, DMSO-
d6): δ 13.89 (s, 1H, NH), 8.50 (s, 1H), 8.45(s, 1H), 8.39 (s,
1H), 8.23 (s, 1H), 8.11 (d, 1H, J = 7.5 Hz), 8.04 (d, 1H, J =
2.0 Hz), 8.02 (d, 1H, J = 2.0 Hz), 7.67–7.63 (m, 3H), 7.44
(t, 1H). 13C-NMR (125 MHz, DMSO-d6): δ 164.6 (d, J =
259.3 Hz), 152.9, 144.8, 141.4, 131.7, 130.5, 129.5, 127.6,
120.0, 119.4, 114.5, 114.3, 113.2; HREI-MS: m/z calcd for
C12H7BrFN3 [M]+ 290.9807; found 290.9807; anal. calcd

Med Chem Res
6-Bromo-2-(2-chlorophenyl)-1H-imidazo[4,5-b]pyridine (25)
7.47 (t, 1H). 13C-NMR (125 MHz, DMSO-d6): δ 165.8 (d,
J = 264.3 Hz), 148.7, 148.1, 143.3, 130.4, 129.8, 129.8,
126.3, 116.5, 116.5, 116.1, 115.5; HREI-MS: m/z calcd for
C12H7BrFN3 [M]+ 290.9807; found 290.9813; anal. calcd
for C12H7BrFN3: C, 49.34; H, 2.42; N, 14.39; found: C,
49.35; H, 2.43; N, 14.38
1
Yield: 61.0%; Mp: 239 °C; H NMR (500 MHz, DMSO-
d6): δ 13.64 (s, 1H, NH), 7.88 (s, 1H), 7.70 (s, 1H), 7.68 (d,
1H, J = 1.0 Hz), 7.62 (t, 1H), 7.55 (t, 1H). 13C-NMR (125
MHz, DMSO-d6): δ 148.1, 144.3, 143.3, 132.1, 131.7,
130.6, 130.0, 130.3, 128.8, 127.8, 126.2, 116.5; HREI-MS:
m/z calcd for C12H7BrClN3 [M]+ 306.9512; found
306.9521; anal. calcd for C12H7BrClN3: C, 46.71; H, 2.29;
N, 13.62; found: C, 46.73; H, 2.27; N, 13.63
α-Glucosidase inhibition assay
The α-glucosidase inhibition assay had been carried out
using Baker’s yeast α-glucosidase (EC 3.2.1.20) and p-
6-Bromo-2-(3-chlorophenyl)-1H-imidazo[4,5-b]pyridine
(26)
nitrophenyl-α-D-glucopyranoside. The samples (5 µg ml−1
)
were prepared by dissolving the compounds (3–32) in
DMSO. Test samples (10 µL), which had been prepared,
were reconstituted in 100 µL of phosphate buffer (100 mM)
at pH 6.8 in 96-well micro-plate and incubated with 50 µL
of Baker’s yeast α-glucosidase for 5 min before 50 µL of p-
nitrophenyl-α-D-glucopyranoside (5 mM) was added. After
incubating for 5 min, the absorbance was measured at 405
nm using SpectraMax plus384 (Molecular Devices Cor-
poration, Sunnyvale, CA, USA). Blank in which the sub-
strate was replaced with 50 µL of buffer were analyzed to
accurately determine the background absorbance. Positive
control samples (acarbose and voglibase) were prepared to
contain 10 µL DMSO instead of test samples.
1
Yield: 86.7%; Mp: 330 °C; H NMR (500 MHz, DMSO-
d6): δ 13.79 (s, 1H, NH), 8.46 (d, 1H, J = 2.0 Hz), 8.33 (s,
1H), 8.28 (s, 1H), 8.21–8.19 (m, 3H), 7.65–7.63 (m, 3H),
7.61 (s, 1H). 13C-NMR (125 MHz, DMSO-d6): δ 148.1,
144.3, 143.3, 132.1, 131.7, 130.6, 130.0, 130.3, 128.8,
127.8, 126.2, 116.5; HREI-MS: m/z calcd for C12H7BrClN3
[M]+ 306.9512; found 306.9508; anal. calcd for
C12H7BrClN3: C, 46.71; H, 2.29; N, 13.62; found: C, 46.72;
H, 2.30; N, 13.60
6-Bromo-2-(3-nitrophenyl)-1H-imidazo[4,5-b]pyridine (27)
1
Yield: 82.0%; Mp: 324 °C; H NMR (500 MHz, DMSO-
Pharmacokinetic predictions of the phenyl-imidazo-
pyridine derivatives
d6): δ 9.07 (d, 1H, J = 1.5 Hz), 8.66 (d, 1H, J = 7.5 Hz),
8.45 (d, 1H, J = 2.0 Hz), 8.38 (s, 1H), 8.37 (s, 1H), 8.32 (s,
1H), 7.89 (t, 1H). 13C-NMR (125 MHz, DMSO-d6): δ
150.7, 148.1, 144.1, 143.3, 135.4, 132.3, 130.3, 129.4,
128.0, 126.2, 124.3, 115.1; HREI-MS: m/z calcd for
C12H7BrN4O2 [M]+ 317.9752; found 317.9758; anal. calcd
for C12H7BrN4O2: C, 45.17; H, 2.21; N, 17.56; found: C,
45.18; H, 2.20; N, 17.54
In silico pharmacokinetic and absorption distribution meta-
bolism excretion properties were evaluated for the phenyl-
imidazo-pyridine derivatives using the QikProp program
(Duffy and Jorgensen 2000), implemented in Maestro
[Maestro, Version 9.2, Schrodinger, LLC, New York, NY,
USA, 2011]. Prior to QikProp, the derivatives were neu-
tralized using ligprep module of Maestro. Principally, four
descriptors were taken into account for assessment: Lipins-
ki’s rule of five is a rule of thumb to assess drug likeness
(Lipinski et al. 1997), the predicted aqueous solubility (QP
log S), the predicted octanol/water partition coefficient (QP
logP) and the predicted Caco-2 cell permeability (QPPcaco).
6-Bromo-2-(pyridin-2-yl)-1H-imidazo[4,5-b]pyridine (28)
1
Yield: 60.7%; Mp: 318 °C; H NMR (500 MHz, DMSO-
d6): δ 8.78 (t, 1H), 8.47 (d, 1H, J = 2.0 Hz), 8.38 (s, 1H),
8.36 (d, 1H, J = 1.5 Hz), 8.22 (s, 1H), 8.06–8.03 (m, 3H),
7.60–7.58 (m, 3H). 13C-NMR (125 MHz, DMSO-d6): δ
149.41, 148.11, 147.20, 143.37, 143.09, 141.77, 130.43,
126.61, 126.23, 124.38, 116.51; HREI-MS: m/z calcd for
C11H7BrN4 [M]+ 273.9854; found 273.9861; anal. calcd for
C11H7BrN4: C, 48.02; H, 2.56; N, 20.37; found: C, 48.03;
H, 2.57; N, 20.36
Molecular docking studies
Glide docking (Schrödinger 2015), a program for complete
solution for ligand–receptor docking in small molecule drug
discovery suite, was employed to identify the binding
modes of the 30 phenyl-imidazo-pyridine derivatives (3–32)
accountable for the activity. The α-glucosidase protein
structure (pdb id: 3top) was used in the study and acarbose.
The reference molecular binding site was considered as
center with 12 Å radius for grid box generation. The stan-
dard precision (SP) mode was selected as preferred mode
6-Bromo-2-(4-fluorophenyl)-1H-imidazo[4,5-b]pyridine
(29)
1
Yield: 92.7%; Mp: 336 °C; H NMR (500 MHz, DMSO-
d6): δ 13.70 (s, 1H, NH), 8.43 (s, 1H), 8.30–8.27 (m, 3H),

Med Chem Res
Scheme 1 Synthesis of 2-
phenyl-1H-imidazo[4,5-b]
pyridines (3–32)
during the Glide docking process and Glide gscore was
taken into account for analysis. While, GOLD (Genetic
Optimization for Ligand Docking) version 5.1 (Verdonk
et al. 2003) was considered as docking validation program.
Gold scoring fitness function was chosen. The genetic
algorithm was set with search efficiency of 100%. The
acarbose inhibition site on α-glucosidase protein structure
of hydroxy substitutions at different position of aromatic
side chain. Based on structure activity relationship, it was
observed that presence of hydroxyl group on the aromatic
side chain significantly enhanced the inhibitory potential of
certain derivatives against α-glucosidase enzyme. These
findings suggest that presence of hydroxyl groups at a
particular position is pertinent for compounds to be active
against a-glucosidase. In the current series, observation on
IC50 values in Table 1 shows that compounds having
hydroxyl substituent at ortho and para positions are able to
efficiently inhibit α-glucosidase enzyme.
The importance of hydroxyl at ortho and para positions
can be evidently observed from the activity trend. Com-
parison between compounds 15 and 12 revealed that
transformation of para-hydroxy substituent to methoxy
causes a significant decrease in the inhibition potential.
Comparison between the activities of these compounds also
showed that meta-substituted hydroxyl group is not con-
tributing significantly towards that activity. On the other
hand, comparison between IC50 values of compounds 17
and 27 is suggestive of that hydroxyl group at para position
is not as significant as hydroxyl group at ortho position is.
The results for these compounds showed that replacing
hydroxyl at para position with a methoxy group does not
affect the activity significantly. The significant contribution
of hydroxyl group at ortho position can also be observed
through the results displayed by compounds 6 and 21. It
was observed for these two derivatives, besides the ortho-
substituted hydroxyl group, additional hydroxyl group at
other position does not show much affect on the IC50
values. This also confirms and strengthens the earlier
hypothesis that hydroxyl group at meta position is less
likely to play any significant role in the inhibitory potential
of α-glucosidase enzyme. However, observation on activity
of compound 18 suggests that hydroxyl at position C-5 for
compound 6 is quite important as any attempt to replace this
hydroxyl group reduces the activity significantly as being
observed for compound 18.
̊
(pdb id: 3TOP) with a cavity of 12 A radius defined as
active site. The results divergent by less than 1.5 Å in ligand
all atom root-mean-square deviations were clustered col-
lectively. Pymol (Pymol 2010), and Maestro visualization
were used to analyze the best clusters and top rank scored
binding mode.
Results and discussion
Chemistry
The synthetic route towards derivatives 3–32 is shown in
Scheme 1. The starting material, 5-bromopyridine-2,3-dia-
mine 1, was treated with various substituted aromatic
aldehyde metabisulfite adducts to form compounds 3–32.
The structures of derivatives 3–32 were confirmed using
spectroscopic techniques such as NMR, MS and were fur-
ther confirmed using CHN analysis.
α-Glucosidase inhibition
In our ongoing efforts of identifying new α-glucosidase
inhibitors (Imran et al. 2015b, 2015c; Taha et al. 2015c),
derivatives of imidazo[4,5-b]pyridine (3–32) were eval-
uated for their ability to inhibit Baker’s yeast α-glucosidase.
The IC50 values for all compounds were in the range of
13.5–93.7 µM (Table 1). The in vitro activities of the deri-
vatives were compared with the IC50 value of acarbose
(IC50 = 38.25 0.12 µM) and voglibase (IC50 = 23.40
0.10 µM); the standard inhibitors. Compound 15, a 2,4-
dihydroxy-substituted derivative displayed the most potent
activity with an IC50 value of 13.5 0.15 µM. Besides
compound 15, other compounds that were found to be
active also include compounds 6 (IC50 = 19.9 0.2 µM), 7
(IC50 = 29.4 0.3 µM), 17 (IC50 = 23.7 0.3 µM), 21
(IC50 = 19.4 0.2 µM), 26 (IC50 = 21.2 0.2 µM), 27
(IC50 = 23.5 0.2 µM) and 31 (IC50 = 33.3 0.3 µM).
General observation suggests that the inhibitory potential
of these derivatives is highly dependent upon the presence
The phenyl-imidazo-pyridine derivatives and their four
pharmacokinetic descriptor properties were analyzed by
QikProp program, which consists of predicted Lipinski’s
rule-of-five, the predicted octanol/water partition coefficient
(QP logP), predicted aqueous solubility (QP log S) and the
predicted Caco-2 cell permeability (QPPcaco) and their
results were listed in Table 1. Based on the results displayed
in Table 1, all of the pharmacokinetic parameters of phenyl-
imidazo-pyridine derivatives were within the acceptable

Med Chem Res
Table 1 Synthesis of imidazo[4,5-b]pyridines 3–32
Table 1 continued
No.
R
No.
R
No.
R
No.
R
3
18
15
30
4
5
19
20
16
17
31
32
6
7
21
22
range for drug-likeness. These compounds possess that
desired molecular properties in drug’s pharmacokinetics in
the human bodies that are crucial for being a perfect drug
candidate.
Molecular docking studies were performed in order to
better understand the inhibition mechanism of active com-
pounds and to further establish the interaction of hydroxyl
at different position towards the active site of α-glucosidase
enzyme. The compounds were docked into the binding
pocket of α -glucosidase enzyme to further confirm the
inhibition potential and also the docked Glide gscore was
reported for newly synthesized 3–32 compounds (Table 1).
8
9
23
24
10
11
25
26
Docking study
Glide docking tool was used for the protein–ligand inter-
action study. The phenyl-imidazo-pyridine derivatives
along with the acarbose were docked using Glide (Glide
gscore is shown in Table 1) targeting the human intestinal
α-glucosidase C-terminal domain. Flexibility of the ligand
was taken into consideration to predict the correct con-
formation and to obtain minimum energy structures. The
top ranked pose of each molecule was chosen on the basis
of docking score (S) for further analysis. The results were
analyzed considering the best binding mode in glide
docking program and each compound’s binding mode
agreement with the GOLD docking. The best binding mode
of each compound were considered for interaction pattern
(H-bonding, hydrophobic and π–π interactions). Our
analysis showed that the active molecules of the
12
27
13
14
28
29

Med Chem Res
Table 2 In vitro α-glucosidase
inhibition activity of derivatives
3–32 and their predicted
d
Compound Lipinski’s rule of QPlog P
o/w
QPlog Sc QP Pcaco
Glide
IC50 (μM SEMa)
fivea
[nm s−1
]
gscoree
b
pharmacokinetic properties
3
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2.316
3.335
3.176
1.685
3.559
2.161
3.375
3.307
2.317
2.47
−3.723
−4.567
−4.258
−3.397
−4.752
−3.505
−4.585
−4.25
784.35
−5.404
−5.061
−5.064
−6.282
−5.504
−4.777
−5.159
−5.244
−5.216
−5.218
−4.454
−4.834
−6.655
−5.142
−6.058
−5.464
−5.753
−5.135
−6.563
−5.278
−5.152
−5.319
−5.325
−6.055
−5.824
−5.455
−5.133
−5.286
−5.532
−5.517
−7.388
−8.511
39.60 0.39
N. A.
4
2584.794
2586.619
346.775
2588.832
1395.823
2588.557
2830.205
786.896
849.941
309.016
1395.594
283.907
2655.38
179.01
5
N. A.
6
19.90 0.19
29.40 0.28
N. A.
7
8
9
N. A.
10
43.40 0.49
93.70 0.96
N. A.
11
−3.723
−4.001
−4.197
−3.506
−3.434
−4.466
−4.176
−3.945
−3.241
−4.231
−3.375
−4.586
−4.376
−4.141
−3.958
−3.665
−3.896
3.501
12
13
2.287
2.161
1.627
3.357
1.785
2.505
2.347
3.168
1.746
3.375
3.332
2.355
2.961
2.423
2.536
3.501
3.558
2.297
2.441
3.333
−7.274
−2.839
N. A.
14
N. A.
15
13.50 0.15
N. A.
16
17
23.70 0.25
46.60 0.45
63.40 0.65
N. A.
18
986.481
2181.849
2589.198
420.069
2589.771
2589.144
418.67
19
20
21
19.40 0.20
N. A.
22
23
N. A.
24
59.60 0.60
79.40 0.80
21.20 0.21
23.50 0.23
39.40 0.36
N. A.
25
2412.615
1140.025
1142.428
2572.037
2589.319
310.445
1668.021
2588.969
0.044
26
27
28
29
−4.75
30
−4.219
−3.916
−4.376
0.716
N. A.
31
33.30 0.33
36.30 0.37
38.25 0.12
23.40 0.10
32
Acarbose
voglibase
a
0.308
8.911
Predicted rule of five: range of recommended values is maximum number 4
b
c
d
e
f
Predicted octanol/water partition coefficient (QP log Po/w): range of recommended values = −2.0− + 6.5
Predicted aqueous solubility (QP log S): values less than −6 or greater than −1 are undesirable
Predicted apparent Caco-2 cell permeability (QP Pcaco): value < 25 is poor
Predicted Glide gscore from Glide docking using SP mode
N.A no activity
phenyl-imidazo-pyridine derivatives displayed substantial
binding within the acarbose inhibition site i.e. active site of
α-glucosidase (Table 2).
There was clear demarcation among the active and inac-
tive derivatives in terms of the docked Glide gscore. The
presence of the hydroxyl group at ortho, meta and para
position on the phenyl ring of the phenyl-imidazo-pyridine
derivatives shows activity. The four active most compounds
are compound 14 (13.5 0.2 μM/gscore: −6.655),
compound 21 (19.4 0.2 μM/gscore: −6.563), compound 6
(19.9 0.2 μM/gscore:−6.282), and compound 26 (21.2
0.2 μM/gscore: −6.055). In the case of inactive compounds,
presence of hydrophobic groups on the phenyl ring, more
specifically presence of methoxy group, will result in the loss
of activity profile (compounds 4, 5, 9, 12, 17, 21 and 23). On
the other hand the presence of halogen such as Cl or F at the
ortho/para position of the phenyl-imidazo-pyridine deriva-
tives displayed inhibitory activity (compounds 7 and 10).

Med Chem Res
Fig. 2 Illustration of predicted binding mode of bromo phenyl-
imidazo-pyridine derivatives in the binding site of α-glucosidase. a.
Compound 15 (blue color) b. Compound 21 (pink color),
c. Compound 6 (brown color), d. Compound 27 (light blue color).
Key residues and compounds are shown stick and the H-bond inter-
actions are represented in yellow dashed lines
Binding mode of the four most active compounds have
been discussed much in detail. The most active among the
series is compound 15. The imidazo-pyridine NH formed
H-bond with Asp1526, while the both the hydroxyl group
of the catechol formed H-bond with Asp1279. Phe1560
formed π–π stacking with the imidazo-pyridine ring.
Additionally the compounds are also stabilized by hydro-
phobic interactions of the imidazo-pyridine ring with the
side chain of Pro1159, Tyr1167, Asp1157, Met1421,
Trp1369, Pro1318, and Lys1460. Likewise the catechol ring
forms hydrophobic interaction with Phe1560, Trp1523,
Trp1418, His1584, Try1251, Ile1218 and Trp1355
(Fig. 1a).
The second most active molecule in the series was
compound 21 that is shown in Fig. 2b. Asp1526 residue
formed H-bond with both imidazo-pyridine NH and with
hydroxyl group of catechol. Hydrophobic interaction is
formed between bromoimidazo pyridine ring and side
chains of Asp1157, Pro1159, Lys1460, Trp1369, Phe1560
and Trp1355. While the benzene ring of catechol moiety
formed hydrophobic interactions with side chains of
Met1421, Arg1510, Tyr1167, Trp1523, Trp1418, Ile1315,
Tyr1251, Ile1280 and Trp1355.
The binding mode of the compound 6 as in Fig. 2c
showed that one of the hydroxyl group of the α‐hydro-
quinone forms H-bond with NH of the imidazole ring of

Med Chem Res
binding mode of the phenyl-imidazo-pyridine potent deri-
vatives shows close binding mode orientation similarity to
that of acarbose and voglibase. Hence this series of deri-
vatives could be a good future drug candidate equivalent to
that of well-known anti-diabetic drug such as acarbose and
voglibase.
Consequently, the binding mode analysis of active
compounds of the phenyl-imidazo-pyridine derivatives in
the active site of α-glucosidase is facilitated by stable
hydrogen bond network with the key residues in the active
site. Addition, the hydrophobic interactions also sig-
nificantly contribute for the stability of the interaction.
Fig. 3 The comparison of the binding mode of acarbose (forest green
color stick) and voglibase (hot pink color stick) along with the four
most potent compound’s binding mode represented with same color
code as in Fig. 1
Conclusions
In conclusion, new compounds possessing imidazo[4,5-b]
pyridine have been synthesized and identified as potent α-
glucosidase inhibitor. The inhibitory results are well sup-
ported by structure-activity relationship, pharmacokinetic
prediction and docking studies. Structure–activity relation-
ship revealed that inhibitory potentials for derivatives 3–32
were dependent on presence of hydroxyl substituents at
different positions of aromatic side chain, especially at
ortho-positions and para-positions. Molecular docking
analysis clearly demonstrated that binding mode of active
compounds of the phenyl-imidazo-pyridine derivatives in
the active site of α-glucosidase is facilitated by stable
hydrogen bond network with the key residues in the active
site. Additionally, the hydrophobic interactions also sig-
nificantly contributed for the stability of the interactions.
Pharmacokinetic parameters predicted for imidazo[4,5-b]
pyridine derivatives were within the acceptable range for
drug-likeness and it suggests that these compounds can
potentially be the plausible drug candidates.
His1584. Additionally, the benzene ring of catechol moiety
forms hydrophobic interactions with side chains of
Arg1510, Phe1559, Trp1523, Trp1418, Arg1582, Asp1526,
Asp1279, Tyr1251, Ile1280 and Trp1355. While the
imidazo-pyridine ring with the side chain of Met1421,
Phe1560, Tyr1167, Lys1460, Asp1157, Pro1159 and
Trp1369.
Figure 2d shows the interaction of the compound 26
revealing its binding mode. Asp1526 being the key residue
forms H-bond with phenol OH and with imidazo-pyridine
NH. While the bromo group forms hydrophobic interaction
with Pro1159 and Lys1460, whereas bromoimidazo pyr-
idine ring forms hydrophobic interaction with Trp1369,
Phe1560 and Asp1157. Likewise the phenol ring forms
hydrophobic interaction with Met1421, Tyr1167, Trp1418,
Tyr1251, Ile1280, Trp1523 and Trp1355.
Analyzing both α-glucosidase inhibition data and the
molecular interaction study clearly suggests that the
hydroxyls attached to the phenyl ring of the phenyl-
imidazo-pyridine derivatives are responsible for the activity,
while the presence of hydrophilic group over the phenyl
ring was accountable for the loss of activity. Alternatively
presence of halogen at the ortho/para position was also
showing activity and the presence of methoxy group over
the ring decreases the activity index considerably.
Acknowledgements Dr. Muhammad Taha would like to acknowl-
edge the Ministry of Higher Education for financial support under the
Fundamental Research Grant Scheme (FRGS) with sponsorship
reference numbers FRGS/1/2016/STG01/UiTM/02/2 and Universiti
Teknologi MARA for the financial support under LESTARI grant 600-
RMI/DANA /5/3/ lestari (54/2013). The author (Dr. M.A.) is also
grateful to HEC Pakistan for providing a research grant vide 20-1933/
NRPU/R&D/HEC/12/5017.
Comparison of the binding mode of the acarbose and
voglibase (anti-diabetic drug) between the four most potent
compound at the active site of α-glucosidase showed that
the phenyl ring of phenyl-imidazo-pyridine derivatives are
exactly positioned in the same orientation to that of the
cyclohexene and cyclohexane position of acarbose and
voglibase, respectively. Likewise, potent derivatives have
their imidazopyridine rings of the phenyl-imidazo-pyridine
positioned exactly in similar orientation with one of the
cyclic hexane as shown in Fig. 3. Therefore the predicted
Compliance with ethical standards
Conflict of interest The authors declare that they have no com-
peting interest.
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