Synthesis and antimicrobial activity of novel C-linked imidazole glycoconjugates

Article abstract of DOI:10.1016/j.bmcl.2007.11.118

Novel C-linked imidazole derivatives have been synthesized in good to excellent yields and characterized by analytical and spectral analysis. Four of the newly synthesized compounds exhibited moderate antibacterial activity.

Full text of DOI:10.1016/j.bmcl.2007.11.118

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1167–1171
Synthesis and antimicrobial activity of novel C-linked
imidazole glycoconjugates
Lingaiah Nagarapu,* A. Satyender, B. Rajashaker, K. Srinivas, P. Rupa rani,
K. Radhika and G. Subhashini
Organic Chemistry Division II, Indian Institute of Chemical Technology, FCL Ist Floor,
Taranaka, Hyderabad, Andhra Pradesh 500 007, India
Received 14 September 2007; revised 31 October 2007; accepted 30 November 2007
Available online 5 December 2007
Abstract—Novel C-linked imidazole derivatives have been synthesized in good to excellent yields and characterized by analytical
and spectral analysis. Four of the newly synthesized compounds exhibited moderate antibacterial activity.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The prevalence of imidazoles in natural products and
pharmacologically active compounds has instituted a di-
verse array of synthetic approaches to these heterocy-
cles.1 Sugar annulated imidazoles have received
particular interest due to the remarkable capacity of
naturally occurring nagstatin (Fig. 1) to inhibit glucos-
aminidases.2 In 1992 Aoyagi, Aoyama and their co-
workers published the structure of the natural product
nagstatin and showed that this imidazolo sugar is a very
potent inhibitor of some glucosaminidases, for example,
with a Ki value of 4 nM for the N-acetyl-b-D-glucosa-
minidase of bovine kidney enzyme.2
sive and essential role of carbohydrate molecules in the
complex machinery of various glycoconjugate biological
activities.6 Glycosylation of heterocyclic compounds
that are rich in biological activity is a field of increasing
interest, for example, synthesis of various guanidinogly-
cosides displays improved biological properties (anti-
inflammatory and anti-HIV activities) with respect to
the non-glycosylated guanidino derivatives.7
Recently Sharma et al.,8 reported an efficient synthesis
of tri substituted glycosyl imidazoles in the presence of
ZrCl4. Though few methods are available for tri substi-
tuted glycosyl imidazoles, there appear no reports for
the synthesis of C-2 glycosyl tetra substituted imida-
zoles. This fact has rekindled an increased interest in
us to obtain C-glycosyl tetra substituted imidazoles.
The discovery of nagstatin proved to be of interest for
the defined deciphering of glycosidase-catalyzed hydro-
lysis of polysaccharides by way of the so-called lateral
protonation mechanism.3–5
In continuation of our work for the synthesis of tetra-
substituted imidazoles,9 we now aim at the synthesis
and collection of monoglycosylated tetra substituted
The importance of sugar annulated imidazoles has gen-
erated an interest in us to explore a route leading to a
collection of hitherto scarcely investigated C-glycosyl
tetra substituted imidazoles. Aside from simple expecta-
tion that the sugar residues having hydroxyl groups in
the imidazole moiety should increase the water solubility
and bioavailability, other interesting biological proper-
ties may arise from glycosylation considering the exten-
HO
HO
N
COOH
N
HO
NHAc
NAGSTATIN (1)
Keywords: Bioactive; C-2 chiral polyhydroxy imidazoles; Potassium
dodecatungstocobaltate trihydrate.
*
Corresponding author. Tel.: +91 40 27160123; fax: +91 40
27193382; e-mail: nagarapu2@yahoo.co.in
Figure 1.
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.11.118

1168
L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1167–1171
Ph
Ph
Ph
O
O
O
O
O
N
Ph
OH
OH
O
a
O
+
R -NH
+
2
O
N
R
O
O
O
2
3
4
a, R = Benzyl
5
4-Me-Phenyl
b, R =
c, R = 4-F-Phenyl
d, R = 3-Me-Phenyl
b
Ph
N
Ph
HO
N
HO
R
OH
HO
OH
R = Benzyl
6
Scheme 1. Reagents and conditions: (a) K5C0W12O40Æ3H2O + NH4OAc (1.6 equiv), 140 ꢁC, 2 h; (b) DOWEX 50WX8 (H+), EtOH/H2O (1:1), 60 ꢁC,
3 h.
Ph
K CoW
O
.3H O
N
1
Ph
Ph
O
O
5
12 40
2
(1.0 mol%)
Ph
1
+
Sugar - CHO
+
R -NH
2
Sugar
N
o
140 C, 2 h
R
9
2
7
8
Scheme 2.
imidazole scaffolds via parallel synthesis employing suit-
able carbohydrate component as one of the reactants.
Thus, a new and novel method for the synthesis of tetra
substituted imidazole derivatives with glycosyl moiety at
C-2 position has been developed using appropriate su-
gar aldehydes, benzil, aromatic amines and ammonium
acetate by using potassium dodecatungstocobaltate tri-
hydrate (PDTC) (1.0 mol%) as a heterogeneous catalyst
under solvent-free conditions at 140 ꢁC (Scheme 1) in
60.0–68.4% yields. All the nine compounds were
screened for their antibacterial and antifungal activities.
substituted novel (C-2 chiral) imidazolyl sugars (9a–d)
using benzil, various sugar aldehydes, amines, ammo-
nium acetate and K5CoW12O40Æ3H2O (Scheme 2) (Table
1). The structures of compounds 5a–d, 6, and 9a–d were
deduced from 1H and 13C NMR, IR, mass spectral data
and elemental analysis.10 It was found that in the ab-
sence of PDTC there was no progress in the four-com-
ponent condensation reaction.
All the newly synthesized compounds 5a–d, 6, and 9a–d
were screened for antibacterial and antifungal activi-
ties.11 All the compounds 5a–d, 6, and 9a–d showed
activity against Gram-negative and Gram-positive bac-
teria. Compounds 5c, 5d, 9c, and 9d showed moderate
antibacterial activity against Pseudomonas aeruginosa.
Compound 9d showed more activity towards Gram-po-
sitive bacteria (i.e., Bacillus subtilis) (Table 2).
The four-component condensation of benzil (2), amine
(3a–d), mannose diacetonide (4) and ammonium acetate
in the presence of 1.0 mol% of K5CoW12O40Æ3H2O at
140 ꢁC for 2 h gave mannosyl imidazoles (5a–d). The
spectral data of compound 5a were in agreement with
the proposed structure. Corresponding C-2 chiral poly-
hydroxy imidazole (6) was obtained (71.52%) as colour-
less solid from 5a by cleavage of acetal with Dowexꢂ
(50WX8-H+) in EtOH: H2O (1:1) at 60 ꢁC (Scheme 1).
All the compounds were screened for antifungal activ-
ity against Saccharomyces cerevisiae, Aspergillus niger,
Rhizopus oryzae, and Candida albicans by agar cup dif-
fusion method12 using Amphotericin-B as standard.
However, none of the compounds showed antifungal
activity.
Under the same conditions, this approach can be re-
peated for the synthesis of hitherto unreported tetra-

L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1167–1171
1169
Table 1.
20
½aꢀD
Entry
R0-NH2 (7a–d)
Sugar
Time (h)
2
Yield (%)a
a
b
c
4-F-phenyl
ꢁ38.6
ꢁ42.8
ꢁ64.0
60
65
55
9a =
O
O
O
O
4-Methyl-phenyl
2
2
9b =
9c=
O
O
O
Benzyl
HO
O
O
O
O
9d =
d
4-Methyl-phenyl
2
ꢁ18.8
63
O
a Isolated yield.
Table 2. Antibacterial activities as MIC (150 lg/mL) for 5a–d, 6 and 9a–d
Compound
Gram positive organisms
Gram negative organisms
B. subtilis
S. aureus
S. epidermidis
E. coli
P. aeroginosa
K. pneumoniae
5a
5b
75
75
75
75
150
150
150
75
75
75
37.5
75
75
75
5c
5d
150
150
75
150
150
75
75
75
75
75
37.5
75
150
75
6
9a
75
75
75
75
75
37.5
75
37.5
150
37.5
75
75
75
9b
9c
150
150
37.5
6.25
1.526
150
150
37.5
1.562
6.25
75
150
75
150
75
2.35
7.81
75
75
9d
Streptomycin
Penicillin
1.562
3.125
3.125
12.5
3.125
6.25
York, 1989; p 241; (b) Grammitt, M. R.; Katrizky, A.
R.; Boulton, A. J.. In Advances in Heterocyclic
Chemistry; Academic: New York, 1970; Vol. 12, p
103.
In conclusion, C-2 chiral tetra substituted imidazoles
were synthesized, characterized and screened for anti-
bacterial and antifungal activities. Compounds 5c, 5d,
9c, and 9d showed moderate antibacterial activity.
Though the compounds showed no antifungal activity,
the promising results in antibacterial activity would give
scope for further work in this area.
2. (a) Aoyagi, T.; Suda, H.; Uotani, K.; Kojima, F.;
Aoyama, T.; Horiguchi, K.; Hamada, M.; Takeuchi, T.
J. Antibiot. 1992, 45, 1404; (b) Aoyama, T.; Naganawa, N.
H.; Suda, H.; Uotani, K.; Aoyagi, T.; Takeuchi, T.
J. Antibiot. 1992, 45, 1557.
3. Heightman, H.; Vasella, T. Angew. Chem. Int. Ed. 1999,
38, 750.
4. Tatsuta, K.; Miura, S.; Ohta, S.; Gunji, H. J. Antibiot.
1995, 48, 286.
5. Sinnott, M. L. Chem. Rev 1990, 1171.
6. (a)Essentials of Glycobiology; Varki, A., Cummings, R.,
Esko, J., Freeze, H., Hart, G., Marth, J., Eds.; Cold
Spring Harbor Laboratory Press: New York, 1993; (b)
Varki, A. Glycobiology 1993, 3, 97.
Acknowledgments
A.S. thanks CSIR-New Delhi, for financial assistance,
the Director and the Head, Organic-II, IICT, for their
support.
References and notes
7. Lin, P.; Lee, C. L.; Sim, M. M. J. Org. Chem. 2001, 66,
8243.
1. (a) Grammitt, M. R.; Katrizky, A. R.; Boulton, A. J..
In Advances in Heterocyclic Chemistry; Academic: New
8. Sharma, G. V. M.; Jyothi, Y.; Sree Lakshmi, P. Synth.
Commun. 2006, 36, 2991.

1170
L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1167–1171
9. (a) Nagarapu, L.; Satyender, A.; Srinivas, K. J. Mol.
Synthesis of imidazolo C-2 chiral pentahydroxy com-
pound (6). A stirred solution of 5a (1.8 g, 3.57 mmol) in
ethanol (40 mL) and water (40 mL), containing Dowexꢂ
(50WX8-H+) resin (1.0 g), was heated to 60 ꢁC for 3 h. The
reaction mixture was cooled to room temperature and
filtered; the resin was washed with ether (2 · 60 mL) and
extracted with MeOH-aq.NH3 (2 · 200 mL). The com-
bined extracts were concentrated to give 6 (1.08 g, 71.52
Catal. A: Chem. 2006, 266, 104; (b) Srinivas, K.; Srinivasu,
V. N. V.; Dhanraj, B. O.; Nagarapu, L. J. Mol. Catal. A:
Chem. 2006, 266, 109.
10. Experimental: Melting points were measured with Fie-
scher–Johns melting point apparatus and are not cor-
rected. 1H NMR spectra were recorded with an AVANCE
300 Bruker at 300 MHz and Gemini 200 MHz in CDCl3.
Chemical shifts relative to TMS as internal standard are
given as d values in ppm. 13C NMR spectra were recorded
20
%) as a colourless powder, m.p. 210–212 ꢁC, ½aꢀD ꢁ4.1 (c
1, MeOH), IR (KBr): t3447, 1643, 1383, 1206, 1140,
1
in CDCl3and in DMSO-d6 on
a
Varian (75 MHz)
1038ꢁ1, H NMR (DMSO-d6, 300 MHz): d 3.62 (m, 3H,
spectrometer. IR spectra were taken with a Perkin-Elmer
1725K FT-IR spectrophotometer. EI-MS mass spectra
were measured at 70 eV (EI).
50-Ha, 50-Hb & 30-H), 3.82 (d, 1H, 40-H, J = 6.7 Hz), 4.30–
4.40 (t · d, 2H, -CH2Ph, J = 4.5 Hz), 4.86 (t, 1H, 20-H,
J = 6.0 Hz), 5.31 (d, 1H, 10-H, J = 6.0 Hz), 4.07, 5.72 (2 br
s, 2 · OH), 5.08, 5.11, 5.24, (3s, 3 · OH), 6.89-7.64(m,
15H, Ar-H). 13C NMR (CDCl3, 75 MHz): d 22.50, 28.43,
29.89, 31.68, 39.44, 46.83, 63.55, 67.14, 70.34, 71.07, 71.71,
79.18, 126.05, 126.34, 127.23, 128.12, 128.36, 128.80,
128.96. ESI: m/z = 461 [M++H]. Anal. Calcd for
C27H28N2O5: C, 70.42; H, 6.13. Found: C, 70.46; H, 6.16.
General procedure for C-2 mannosyl tetra substituted
imidazoles (5a–d) and C-2 glycosyl tetra substituted
imidazoles (9a–d): To a mixture of benzil (2.0 mmol),
amine (2.0 mmol), mannose diacetonide (2.0 mmol) or
sugar aldehyde (2.0 mmol), ammonium acetate (3.2 mmol)
and K5CoW12O40.3H2O (64 mg, 1.0 mol%) were mixed
well and heated at 140 ꢁC for 2 h. The reaction mixture
was cooled to room temperature, added acetone (10 mL),
filtered (to remove catalyst), filtrate was concentrated
under reduced pressure and syrupy residue was purified by
column chromatography using EtOAc: hexane (1:9) as
eluent to obtain pure compounds.
20
Compound 9a. Syrup, Yield: 60%, ½aꢀD ꢁ38.6 (c 0.4,
CHCl3), IR (KBr): t 1653, 1602, 1508, 1445,1376ꢁ1,1H
NMR (CDCl3, 300 MHz): d 1.33, 1.45 (2 s, 2 · CH3), 4.20
(t, 1H, 10-H, J = 6.2 Hz), 4.73 (m, 2H, 20a-H, 20b-H), 7.00–
7.48 (m, 14H, Ar-H). 13C NMR (CDCl3, 75 MHz): d
25.99, 26.22, 67.63, 69.77,110.15, 115.64, 126.58, 127.20,
129.93, 130.73, 131.68, 134.23, 137.70, 144.46, 163.78. ESI:
m/z = 415 [M++H]. Anal. Calcd for C26H23N2O2: C,
75.34; H, 5.59. Found: C, 74.98; H, 6.62.
Compound 5a. Colourless powder, Yield: 61%, m.p. 124–
20
126 ꢁC, ½aꢀD ꢁ8.1 (c 1, CHCl3), IR (KBr): t 3331, 3053,
,
2982, 2930, 1890, 1515, 1382ꢁ1 1H NMR (CDCl3,
20
200 MHz): d 1.31, 1.41, 1.46 (3s, 12H, 4· –CH3), 3.67
(brs, 1H, –OH), 4.02 (m, 3H, 50-Ha, 50-Hb, 30-H), 4.98 (d,
1H, 20-H, J = 4.6 Hz), 5.07 (dd, 2H, –CH2Ph), 5.16 (dd,
1H, 40-H, J=2.3), 5.26 (d, 1H, 10-H, J = 3.9 Hz), 6.84–7.43
(m, 15H, Ar-H). 13C NMR (CDCl3, 75 MHz): d 25.61,
29.14, 47.53, 66.76, 70.09, 71.40, 77.67, 109.42, 119.43,
126.53, 128.23, 129.09, 131.21, 137.31, 144.03. ESI:
m/z = 541 [M++H]. Anal. Calcd for C33H36N2O5: C,
73.31; H, 6.71. Found: C, 73.38; H, 6.82.
Compound 9b. Syrup, Yield: 65%, ½aꢀD ꢁ42.8 (c 0.5,
CHCl3), IR (KBr): t 1666, 1603, 1512, 1446,1374ꢁ1 1H
,
NMR (CDCl3, 300 MHz): d 1.35, 1.50 (2 s, 2 · CH3), 2.35
(s, 3H, Ar-CH3), 4.18 (t, 1H, 10-H, J = 6.0 Hz), 4.73 (m,
2H, 20a-H, 20b-H), 7.05–7.24 (m, 14H, Ar-H). 13C NMR
(CDCl3, 75 MHz):
d
21.06, 26.08, 26.28, 67.85,
69.79,110.08, 120.29, 126.44, 127.29, 128.65, 129.43,
129.87, 130.51, 130.78, 133.08, 134.49, 137.69, 138.42,
144.43. ESI: m/z = 411 [M++H]. Anal. Calcd for
C27H26N2O2: C, 79.00; H, 6.38. Found: C, 79.18; H, 6.42.
Compound 9c. Colourless powder, Yield: 60.8%, m.p.
Compound 5b. Colourless powder, Yield: 68.4%, m.p.
20
162–164 ꢁC, ½aꢀD ꢁ72.2ꢁ (c 1, CHCl3), IR (KBr): t 3454,
20
2361, 1384ꢁ1
,
1H NMR (CDCl3, 200 MHz): d 1.30,1.38,
115–117 ꢁC, ½aꢀD ꢁ64.1 (c 0.6, CHCl3), IR (KBr): t 3479,
1.55 (3s, 12H, 4· –CH3), 2.37 (s, 3H, Ar-CH3), 3.55 (d,
1H, 40-H, J = 3.1 Hz), 3.95–4.09 (m, 3H, 50-Ha, 50-Hb, 30-
H), 4.72 (d, 1H,10-H, J = 7.8 Hz), 5.25 (dd, 1H, 20-H,
J = 3.1, 6.1 Hz), 7.06–7.50 (m, 14H, Ar-H). 13C NMR
(CDCl3, 75 MHz): d 21.06, 25.33, 26.76, 65.50, 70.14,
76.54, 109.16, 126.43, 127.28, 129.39, 130.73, 138.32,
143.78. ESI: m/z = 541 [M+ + H]. Anal. Calcd for
C33H36N2O5: C, 73.31; H, 6.71. Found: C, 73.29; H, 6.74.
Compound 5c. Colourless powder, Yield: 65.4%, m.p.
2924, 1601, 1446, 1380, 1211ꢁ1 1H NMR (CDCl3,
,
300 MHz): d 1.30 (2s, 6H, 2 · CH3), 4.52 (d, 1H, 40-H,
J = 1.7 Hz), 4.61 (d, 1H, 30-H, J = 4.2 Hz), 4.73 (d, 1H, 20-
H, J = 2.5 Hz), 5.0 (d, 1H, –CH2Ph, J = 2.5 Hz), 5.3 (t,
1H, –CH2Ph, J = 5.1 Hz.), 6.01 (d, 1H, 10-H, J = 3.4 Hz),
6.5 (s, 1H, –OH), 6.99–7.39 (m, 15H, Ar-H). 13C NMR
(CDCl3, 75 MHz): d 26.71, 29.62, 47.30, 72.44, 84.52,
105.88, 111.66, 126.75, 128.18, 128.59, 128.88, 129.14,
130.04, 130.13, 130.91, 131.01, 131.09, 133.50, 136.18,
136.27, 136.35, 136.44,136.47, 143.25. ESI: m/z = 469
[M++H]. Anal. Calcd for C29H28N2O4: C, 74.34; H,
6.02. Found: C, 73.98; H, 6.12.
120–122 ꢁC, IR (KBr): t 3447, 2925, 1880, 1522, 1372ꢁ1
,
1H NMR (CDCl3, 200 MHz): d 1.21, 1.26, 1.32 (3s, 12H,
4· –CH3), 3.5 (br s, 1H, -OH), 4.0 (m, 4H, 50-Ha, 50-Hb,
30-H, 20-H), 4.7 (d, 1H, 10-H, J = 8.3 Hz), 5.2 (dd, 1H, 40-
H, J = 2.8 Hz), 7.0–7.4 (m, 14H, Ar-H). ESI: m/z = 545
[M++H]. Anal. Calcd for C32H33N2O5F: C, 70.57; H, 6.10.
Found: C, 70.55; H, 6.06.
20
Compound 9d. Syrup, Yield: 60.8%, ½aꢀD ꢁ18.8 (c 0.45,
CHCl3), IR (KBr): t 3446, 2925, 1741, 1668, 1604, 1447,
1
1376, 1258, 1071ꢁ1, H NMR (CDCl3, 200 MHz): d 1.32,
1.43, 1.49 (3s, 12H, 4 · CH3), 2.06 (s, 3H, Ar-CH3), 3.93–
4.27 (m, 3H, 20-H, 40-H, 50-H), 4.57 (dd, 1H, 30-H,
J = 6.7 Hz), 5.46 (d, 1H, 10-H, J = 4.5 Hz), 6.5 (s, 1H, –
OH), 7.31–7.50 (m, 14H, Ar-H). 13C NMR (CDCl3,
75 MHz): d 20.85, 24.11, 24.46, 24.91, 25.92, 29.65,
63.44, 65.93, 70.67, 71.05, 96.28, 101.55, 108.72, 109.61,
120.26, 126.95, 128.26, 128.80, 129.54, 130.14, 131.66,
132.65, 164.11. Anal. Calcd for C33H28N2O5: C, 74.42; H,
5.29. Found: C, 74.38; H, 5.42.
Compound 5d. Colourless powder, Yield: 67%, m.p. 145–
147 ꢁC, IR (KBr): t 3444, 3043, 2924, 1888, 1511, 1385ꢁ1
,
1H NMR (CDCl3, 300 MHz): d1.22, 1.31, 1.38, 1.54 (4s,
12H, 4· –CH3), 2.3 (s, 3H, Ar-CH3), 3.5 (br s, 1H, –OH),
3.9 (m, 4H, 50-Ha, 50-Hb, 30-H, 20-H), 4.7 (d, 1H, 10-H,
J = 8.3 Hz), 5.1 (dd, 1H, 40-H, J = 3.0 Hz), 7.0–7.4 (m,
14H, Ar-H). 13C NMR (CDCl3, 75 MHz): d 21.10, 25.33,
26.76, 26.93, 29.64, 66.52, 70.11, 70.69, 78.57, 109.18,
110.14, 125.14, 126.48, 127.85, 128.02, 129.13, 130.70,
135.39, 138.73, 143.56. ESI: m/z = 541 [M+ + H]. Anal.
Calcd. for C33H36N2O5: C, 73.31; H, 6.71. Found: C,
73.29; H, 6.73.
11. Antimicrobial assay: All the newly synthesized com-
pounds 5a–d, 6, and 9a–d were screened in vitro for
antibacterial activity against Gram-positive Bacillus sub-
tilis, Staphyllococcus aureus, Staphyllococcus epidermidis,

L. Nagarapu et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1167–1171
1171
Gram-negative Escherichia coli, Pseudomonas aeruginosa
and Klebsiella pneumoniae and for anti-fungal activity
against Saccharomyces cerevisiae, Aspergillus niger, Rhi-
zopus oryzae and Candida albicans by agar cup diffusion
method.12 The compounds showing some growth inhibi-
tion zone in this method were further tested by the broth
dilution method13 to determine their MIC values, which
are summarized in Table 2. The synthesized compounds
and reference drugs were dissolved in DMSO-H2O (50%)
at a concentration of 150 lg/mL. The concentrations were
further made by twofold dilution with culture medium and
bacterial solution at the first tube.
12. E. Margery Linday, Practical Introduction to Microbiol-
ogy, E & F.N. Spon, UK (1962) 177.
13. NCCLS, Methods for Dilution Antimicrobial Susceptibil-
ity Tests for Bacteria, Which Grows Aerobically (fifth ed.),
NCCLS, Villanova, PA (2000) Approved Standard M
7-A5.