The synthesis of highly fluorescent heterocyclic compounds: Pyrido[2',1':2,3]imidazo[4,5-b]quinoline-12-yl cyanides

Article abstract of DOI:10.1016/j.dyepig.2010.01.013

The syntheses and fluorescence properties of novel derivatives of pyrido[2',1':2,3]imidazo[4,5-. b]quinoline are described. The key intermediates (4-substituted)(3-hydroxyimino-2,3-dihydroimidazo[1,2-. a]pyridin-2-yliden)methyl cyanides were synthesized by reacting the imidazo[1,2-. a]pyridine with arylacetonitriles via nucleophilic substitution of hydrogen. The compounds were deep violet in colour. Acylation of hydroxyl group led to heterocyclization and gave highly fluorescent pyrido[2',1':2,3]imidazo[4,5-. b]quinoline.

Full text of DOI:10.1016/j.dyepig.2010.01.013

Dyes and Pigments 86 (2010) 266e270
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Dyes and Pigments
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The synthesis of highly fluorescent heterocyclic compounds: Pyrido[20,10:2,3]
imidazo[4,5-b]quinoline-12-yl cyanides
*
Mohammad Rahimizadeh , Mehdi Pordel, Mehdi Bakavoli, Hossein Eshghi
Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran
a r t i c l e i n f o
a b s t r a c t
The syntheses and fluorescence properties of novel derivatives of pyrido[20,10:2,3]imidazo[4,5-b]quino-
line are described. The key intermediates (4-substituted)(3-hydroxyimino-2,3-dihydroimidazo[1,2-a]
pyridin-2-yliden)methyl cyanides were synthesized by reacting the imidazo[1,2-a]pyridine with aryla-
cetonitriles via nucleophilic substitution of hydrogen. The compounds were deep violet in colour.
Acylation of hydroxyl group led to heterocyclization and gave highly fluorescent pyrido[20,10:2,3]imidazo
[4,5-b]quinoline.
Article history:
Received 27 September 2009
Received in revised form
11 January 2010
Accepted 13 January 2010
Available online 1 February 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Quinolines
Imidazo[1,2-a]pyridine
Dye
Absorption
Emission
Fluorescence
1. Introduction
peripheral benzodiazepine receptor (PBR) [21]. Based on these
aspects, it was decided to synthesize a heterocyclic system that
Fluorescent heterocyclic compounds are of interest in many
disciplines such as emitters for electroluminescence devices [1],
molecular probes for biochemical research [2], in traditional textile
and polymer fields [3], fluorescent whitening agents [4] and photo
conducting materials [5]. Quinolines are well-known fluorescent
compounds of high quantum yield and have attracted much
attention owing to their potential high technology applications
[6e9] including fluorescence probes in chemosensors [10,11].
Quinolines are, in general, comparatively easy to prepare, and
numerous derivatives have been described for potential use as
biologically active materials [12,13]. Several classical synthetic
methods can be used to prepare quinolines, including the Skraup
synthesis that involves heating aniline with glycerol and nitro-
benzene in the presence of sulfuric acid [14], the Combes quinoline
synthesis [15], Friedländer synthesis [16] and other methods
[17e19]. Imidazo[1,2-a]pyridine derivatives are also well-known
fluorescent compounds, as exemplified by imidazo[1,2-a]pyr-
idinium salts, which are used to prepare styryl dyes [20] and imi-
dazopyridine-7-nitrofurazan conjugates which have been utilized
as fluorescent probes for the localization and function of the
contained both imidazo[1,2-a]pyridine and quinoline rings and to
study their fluorescence properties. Continuing the authors’ study
of the nucleophilic substitution reactions of hydrogen [22e25], it
was decided to use these reactions in the synthesis of novel
derivatives of (3-hydroxyimino-2,3-dihydroimidazo[1,2-a]pyridin-
2-yliden)-2-phenylacetonitrile 3aee as key intermediates. In this
paper, a new method for the synthesis of several novel, highly
fluorescent derivatives of pyrido[20,10:2,3]imidazo[4,5-b]quinolines
4aee, is described.
2. Experimental
2.1. Equipment and materials
Melting points were recorded on an Electrothermal type 9100
melting point apparatus. The IR spectra were obtained on a 4300
Shimadzu spectrometer and only noteworthy absorptions are
listed. The 1H NMR (500 MHz) and the 13C NMR (125 MHz) spectra
were recorded on a Bruker Avance DRX-500 Fourier transformer
spectrometer. Chemical shifts are reported in ppm downfield from
TMS as internal standard; coupling constance J are given in Hertz.
The mass spectra were scanned on a Varian Mat CH-7 at 70 eV.
Elemental analyses were performed on a Thermo Finnigan Flash EA
microanalyzer. Absorption spectra were recorded on an Agilent
* Corresponding author. Tel.: þ98 511 879 7022; fax: þ98 511 879 6416.
E-mail address: rahimizh@yahoo.com (M. Rahimizadeh).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.01.013

M. Rahimizadeh et al. / Dyes and Pigments 86 (2010) 266e270
267
R
R
N
N
N
N
N
N
..
KOH, MeOH
4 h, reflux
HO
N
N
3a: R=H (92%)
N
HO
N
CN
OH
R
R
3b: R=Cl (90%)
3c: R=Br (93%)
3d: R=OMe (82%)
3e: R=Me (87%)
NO2
N
CH2CN
C
N
N
1
2a-e
3a-e
D
E
Fig. 1. Synthesis of new dyes 3aee.
Fig. 3. A resonance from hydroxyl to cyanide group.
2.2.2. (4-Chlorophenyl)(3-hydroxyimino-2,3-dihydroimidazo[1,2-
a]pyridin-2-yliden)methyl cyanide (3b)
8453 spectrophotometer. Fluorescence spectra were recorded
using Shimadzu RF-1501 spectrofluorophotometer. UVevis and
fluorescence scans were recorded from 350 to 700 nm. The fluo-
rescence quantum yields (VF) of compounds 4aee were deter-
mined via comparison methods, using fluorescein as a standard
sample in 0.1 M NaOH and MeOH solution [26]. All measurements
were carried out at room temperature. Compound 1 was obtained
according to the published methods [27]. Other reagents were
commercially available.
Compound 3b was obtained as dark red powder, yield (2.66 g,
90%), mp 217e219 ꢀC; 1H NMR (DMSO-d6)
d
6.58 (dd, J ¼ 8.6 Hz,
J0 ¼ 6.8 Hz, 1H), 7.08 (d, J ¼ 8.9 Hz, 1H), 7.44 (d, J ¼ 8.7 Hz, 2H), 7.62
(dd, J ¼ 8.9 Hz, J0 ¼ 8.6 Hz, 1H), 8.16 (d, J ¼ 8.7 Hz, 2H), 9.01 (d,
J ¼ 6.8 Hz, 1H), 13.51 (br s, 1H); IR (KBr): 3455 cmꢁ1 (OH),
2195 cmꢁ1 (CN). MS (m/z) 296 (Mþ), 298 (Mþ þ 2). Anal. Calcd for
C15H9ClN4O (296.7): C, 60.72; H, 3.06; N, 18.88. Found: C, 60.41; H,
2.91; N, 18.63.
2.2.3. (4-Bromophenyl)(3-hydroxyimino-2,3-dihydroimidazo[1,2-a]
pyridin-2-yliden)methyl cyanide (3c)
2.2. General procedure for the synthesis of 3aee from 1 to 2aee
Compound 3c was obtained as dark red powder, yield (3.17 g,
Compounds 1 (10 mmol) and 2aee (12 mmol) were added with
stirring to a solution of KOH (20 g, 357 mmol) in methanol (80 mL).
The mixture was refluxed with stirring for 4 h, and then poured into
water. After neutralization with dilute HCl solution, the precipitate
was collected by filtration, washed with water, followed by EtOH
and acetone, and then air dried to give pure 3aee.
93%), mp 219e220 ꢀC; 1H NMR (DMSO-d6)
d
6.60 (dd, J ¼ 8.6 Hz,
J0 ¼ 6.9 Hz, 1H), 7.08 (d, J ¼ 9.1 Hz, 1H), 7.55e7.70 (m, 3H), 8.10 (d,
J ¼ 8.7 Hz, 2H), 9.01 (d, J ¼ 6.9 Hz, 1H), 13.51 (br s, 1H); IR (KBr):
3455 cmꢁ1 (OH), 2195 cmꢁ1 (CN). MS (m/z) 341 (Mþ), 343 (Mþ þ 2).
Anal. Calcd for C15H9BrN4O (341.1): C, 52.81; H, 2.66; N, 16.42.
Found: C, 52.53; H, 2.51; N, 16.69.
2.2.1. 2-(3-Hydroxyimino-2,3-dihydroimidazo[1,2-a]pyridin-2-
yliden)-2-phenylacetonitrile (3a)
2.2.4. (3-Hydroxyimino-2,3-dihydroimidazo[1,2-a]pyridin-2-
yliden)(4-methoxyphenyl)methyl cyanide (3d)
Compound 3a was obtained as dark red powder, yield (2.41 g,
Compound 3d was obtained as dark red powder, yield (2.39 g,
92%), mp 195e197 ꢀC; 1H NMR (DMSO-d6)
d
6.58 (dd, J ¼ 8.6 Hz,
82%), mp 175e177 ꢀC; 1H NMR (DMSO-d6)
d 3.76 (s, 3H), 6.50 (dd,
J0 ¼ 7.0 Hz, 1H), 7.03 (d, J ¼ 9.2 Hz, 1H), 7.13e7.72 (m, 4H), 8.13 (d,
J ¼ 8.4 Hz, 2H), 9.03 (d, J ¼ 7.0 Hz, 1H), 13.35 (br s, 1H); IR (KBr):
3450 cmꢁ1 (OH), 2196 cmꢁ1 (CN). MS (m/z) 262 (Mþ). Anal. Calcd
for C15H10N4O (262.3): C, 68.69; H, 3.84; N, 21.36. Found: C, 68.35;
H, 3.70; N, 21.14.
J ¼ 8.6 Hz, J0 ¼ 6.8 Hz,1H), 7.02 (d, J ¼ 8.8 Hz, 2H), 7.12 (d, J ¼ 9.2 Hz,
1H), 7.52 (dd, J ¼ 9.2 Hz, J0 ¼ 8.6 Hz, 1H), 8.07 (d, J ¼ 8.8 Hz, 2H),
8.95 (d, J ¼ 6.8 Hz, 1H), 13.27 (br s, 1H); IR (KBr): 3450 cmꢁ1 (OH),
2197 cmꢁ1 (CN). MS (m/z) 292 (Mþ). Anal. Calcd for C16H12N4O2
(292.3): C, 65.75; H, 4.14; N, 19.17. Found: C, 65.40; H, 3.99; N, 18.86.
2.2.5. (3-Hydroxyimino-2,3-dihydroimidazo[1,2-a]pyridin-2-
yliden)(4-methylphenyl)methyl cyanide (3e)
Compound 3e was obtained as dark red powder, yield (2.40 g,
R
N
N
87%), mp 173e175 ꢀC; 1H NMR (DMSO-d6)
d 2.31 (s, 3H), 6.54 (dd,
O
KOH
J ¼ 8.6 Hz, J0 ¼ 6.9 Hz,1H), 7.06 (d, J ¼ 9.2 Hz,1H), 7.23 (d, J ¼ 8.2 Hz,
2H), 7.58 (dd, J ¼ 9.2 Hz, J0 ¼ 8.6 Hz, 1H), 8.03 (d, J ¼ 8.2 Hz, 2H),
8.97 (d, J ¼ 6.9 Hz, 1H), 13.34 (br s, 1H); IR (KBr): 3450 cmꢁ1 (OH),
2197 cmꢁ1 (CN). MS (m/z) 276 (Mþ). Anal. Calcd for C16H12N4O
N
O
N
N
CN
MeOH
O2N
CH2CN
R
2a-e
1
A
0.4
3a
3b
0.3
0.2
0.1
0
3c
3d
3e
N
N
N
N
H
N
N
H
HO
O
N
O
CN
N
HO N
C N
CN
R
R
R
350
400
450
500
550
600
650
700
Wavelength (nm)
3a-e
C
B
Fig. 4. Visible absorption spectrum of compounds 3aee in dilute (3 ꢂ 10ꢁ5 M) ethanol
Fig. 2. Proposed reaction mechanism for the synthesis of prepared dyes 3aee.
solution.

268
M. Rahimizadeh et al. / Dyes and Pigments 86 (2010) 266e270
Table 1
Spectroscopic properties of dyes 3aee in EtOH solvent.
N
N
N
N
N
N
CH3COCl
Dye
3a
3b
3c
3d
3e
HO N
AcO N
C N
C N
AcO N
C N
lmax (nm)a
3
525
10.35
527
9.85
528
9.35
530
7.65
527
15.60
H
b
ꢂ 10ꢁ3 (Mꢁ1 cmꢁ1
)
R
a
R
R
Wavelengths of maximum absorbance (lmax).
Extinction coefficient.
b
3a-e
F
G
(276.3): C, 69.55; H, 4.38; N, 20.28. Found: C, 69.21; H, 4.23; N,
20.19.
N
N
N
N
N
AcO N
C N
C N
2.3. General procedure for the synthesis of 4aee from 3aee
R
R
Acetyl chloride (0.55 g, 7 mmol) was added with stirring to
a solution of 3aee (5 mmol) in dry pyridine (50 mL). After the
addition was completed, the mixture was refluxed for 4 h. The
pyridine was evaporated under reduced pressure. The resulting
solid residue was boiled in EtOH for 5 min. The precipitated solid
product was collected by filtration and washed well with water and
acetone to give pure compounds 4aee.
4a-e
H
Fig. 6. Proposed reaction mechanism for the synthesis of prepared compounds 4aee.
2.3.4. 3-Methoxypyrido[20,10:2,3]imidazo[4,5-b]quinolin-12-yl
cyanide (4d)
2.3.1. Pyrido[20,10:2,3]imidazo[4,5-b]quinolin-12-yl cyanide (4a)
Compound 4a was obtained as shiny yellow needles (pyridine),
Compound 4d was obtained as shiny orange needles (pyridine),
yield (1.31 g, 96%), mp 305e307 ꢀC; 1H NMR (CDCl3)
d 4.04 (s, 3H),
yield (1.09 g, 90%), mp 280e282 ꢀC; 1H NMR (CDCl3)
d
6.92e7.12
(m, 1H), 7.62e7.92 (m, 4H), 8.18e8.55 (m, 2H), 8.97 (d, J ¼ 6.8 Hz,
1H) ppm; 13C NMR (CDCl3):
102.53, 107.34, 109.35, 118.25, 120.59,
6.93e7.12 (m, 1H), 7.47 (dd, J ¼ 9.4 Hz, J0 ¼ 2.4 Hz, 1H), 7.60 (d,
J ¼ 2.4 Hz, 1H), 7.66e7.8 (m, 2H), 8.31 (d, J ¼ 9.4 Hz, 1H), 8.91 (d,
d
J ¼ 6.9 Hz, 1H); 13C NMR (CDCl3):
d 56.54, 103.81, 109.69, 110.74,
122.34, 129.47, 135.43, 136.75, 137.50, 145.33, 146.34, 147.90, 148.51,
149.12 ppm; IR (KBr): 2198 cmꢁ1 (CN). MS (m/z) 244 (Mþ). Anal.
Calcd for C15H8N4 (244.3): C, 73.76; H, 3.30; N, 22.94. Found: C,
73.41; H, 3.20; N, 22.71.
113.51, 118.91, 123.12, 133.02, 133.90, 137.69, 139.00, 143.57, 146.65,
147.50, 149.14, 160.30 ppm; IR (KBr): 2198 cmꢁ1 (CN). MS (m/z) 274
(Mþ). Anal. Calcd for C16H10N4O (274.3): C, 70.07; H, 3.67; N, 20.43.
Found: C, 69.71; H, 3.50; N, 20.19.
2.3.2. 3-Chloropyrido[20,10:2,3]imidazo[4,5-b]quinolin-12-yl
cyanide (4b)
2.3.5. 3-Methylpyrido[20,10:2,3]imidazo[4,5-b]quinolin-12-yl
cyanide (4e)
Compound 4b was obtained as shiny yellow needles (pyridine),
Compound 4e was obtained as shiny yellow needles (pyridine),
yield (1.28 g, 92%), mp 325e327 ꢀC; 1H NMR (CDCl3)
d 6.98e7.22
yield (1.16 g, 90%), mp 300e301 ꢀC; 1H NMR (CDCl3)
d 2.64 (s, 3H),
(m, 1H), 7.72e7.93 (m, 3H), 8.28 (d, J ¼ 8.8 Hz, 1H), 8.53 (d,
6.88e7.07 (m, 1H), 7.48e7.73 (m, 3H), 8.06 (d, J ¼ 1.9 Hz, 1H), 8.28
J ¼ 1.9 Hz, 1H), 8.96 (d, J ¼ 6.9 Hz, 1H) ppm; 13C NMR (CDCl3):
(d, J ¼ 8.6 Hz, 1H), 8.91 (d, J ¼ 6.4 Hz, 1H); 13C NMR (CDCl3):
d 22.23,
d
102.31, 107.21, 109.41, 118.40, 121.39, 122.21, 131.18, 133.01, 135.27,
102.91, 109.50, 110.11, 117.34, 127.45, 131.54, 133.26, 133.35, 136.54,
137.19, 142.56, 143.50, 146.13, 147.12, 149.07 ppm; IR (KBr):
2197 cmꢁ1 (CN). MS (m/z) 258 (Mþ). Anal. Calcd for C16H10N4
(258.3): C, 74.41; H, 3.90; N, 21.69. Found: C, 74.23; H, 3.79; N, 21.43.
136.83, 137.91, 146.02, 147.21, 148.09, 149.20 ppm; IR (KBr):
2199 cmꢁ1 (CN). MS (m/z) 278 (Mþ), 280 (Mþ þ 2). Anal. Calcd for
C15H7ClN4 (278.7): C, 64.64; H, 2.53; N, 20.10. Found: C, 64.29; H,
2.41; N, 19.88.
2.3.3. 3-Bromopyrido[20,10:2,3]imidazo[4,5-b]quinolin-12-yl
cyanide (4c)
3. Results and discussion
Compound 4c was obtained as shiny yellow needles (pyridine),
The new (4-substituted)(3-hydroxyimino-2,3-dihydroimidazo
[1,2-a]pyridin-2-yliden)methyl cyanides 3aee were synthesized
via the nucleophilic substitution of hydrogen of 3-nitro-imiadazo
[1,2-a]pyridine 1 with arylacetonitriles 2aee in basic MeOH solu-
tion [28] in excellent yields (Fig. 1). When R is an electron with-
drawing group such as NO2 group, the yield of the reaction is very
yield (1.50 g, 93%), mp 331e332ꢀC; 1H NMR (CDCl3)
d 7.02e7.19 (m,
1H), 7.65e7.89 (m, 3H), 8.30 (d, J ¼ 8.8 Hz, 1H), 8.51 (d, J ¼ 1.9 Hz,
1H), 8.95 (d, J ¼ 6.9 Hz, 1H) ppm; 13C NMR (CDCl3):
d 103.50, 109.11,
109.63, 118.31, 119.81, 121.70, 133.91, 133.99, 136.14, 137.61, 139.32,
146.09, 147.00, 148.56, 149.35 ppm; IR (KBr): 2199 cmꢁ1 (CN). MS
(m/z) 323 (Mþ), 325 (Mþ þ 2). Anal. Calcd for C15H7BrN4 (323.1): C,
55.75; H, 2.18; N, 17.34. Found: C, 55.43; H, 2.03; N, 17.16.
low, since the corresponding conjugated base is
a
weak
R
N
C
CN
N
C
N
C
N
N
acetyl chloride
Pyridine, 4 h, reflux
N
N
N
N
N
R
4a: R=H (90%)
4b: R=Cl (92%)
4c: R=Br (93%)
4d: R=OMe (96%)
4e: R=Me (90%)
N
..
N
N
CN
OH
N
N
R
N
N
R
N
R
3a-e
4a-e
J
K
I
Fig. 5. Synthesis of new fluorescent compounds 4aee.
Fig. 7. The resonance structures of compounds 4aee.

M. Rahimizadeh et al. / Dyes and Pigments 86 (2010) 266e270
269
Table 2
0.3
0.2
0.1
0
Photophysical data for absorption (abs) and fluorescence (flu) of 4aee.
4a
4b
4c
4d
4e
Dye
4a
4b
4c
4d
4e
lmax (nm)
443
13.50
480
446
6.00
485
0.90
447
19.30
485
448
26.50
495
445
27.40
485
3
ꢂ 10ꢁ4 (Mꢁ1 cmꢁ1
)
l
flu (nm)
a
VF
0.57
0.87
0.61
0.70
a
Quantum yield.
of 4d revealed the absence of the signal at
to one exchangeable proton (OH group) and two doublet signals at
7.02 ppm and 8.07 ppm attributed to four aromatic protons of p-
d 13.27 ppm assignable
d
d
380
400
420
440
460
480
500
methoxyphenyl ring of compound 3d and presence of the doublet
Wavelength (nm)
of doublet signal at
doublet signal at
signal at
d
7.47 ppm (J ¼ 9.4 Hz and J0 ¼ 2.4 Hz), the
Fig. 8. Visible absorption spectrum of compounds 4aee in dilute (1 ꢂ10ꢁ6 M) chlo-
d
7.60 ppm with meta J (2.4 Hz) and the doublet
roform solution.
d
8.31 ppm with ortho J (9.4 Hz) assignable to three
aromatic protons of methoxyphenyl ring. Moreover, the FT-IR
spectrum of 4d in KBr did not show the absorption band at
3450 cmꢁ1 corresponding to OH group. Furthermore, the mass
spectrum of this product shows the molecular ion m/z 274 (Mþ),
confirming its presumed structure. Analytical data are also in
accordance with the proposed structure for compound 4d. These
compounds were highly fluorescent. When a heteronitrogen is
singly bonded to carbon atoms in a heterocycle, as in pyrrole rings
(e.g. indole, carbazole), the transitions involving the non-bonding
nucleophile. A proposed mechanism to explain the formation of
compounds 3aee is shown in Fig. 2.
In spectral data of compounds 3aee, the presence of one
exchangeable peak at ranging 13.39e13.51 ppm in the 1H NMR
spectrum, a weak absorption band at ranging 2195e2198 cmꢁ1 and
a broad absorption band at ranging 3450e3455 cmꢁ1 assignable to
CN and OH group in the IR spectrum together with microanalytical
data strongly support the uncyclized structure of compounds 3aee.
As depicted in Fig. 3, in these compounds, there is a strong
resonance from OH (electron-donor) to CN (electron-acceptor)
group which can be witnessed in their absorption spectra in visible
(violet) area. Formation of two aromatic rings (in form B) is the
driving force for this fully extended conjugation.
electrons have properties similar to those of
pep* transitions. In
fact, the non-bonding orbital is perpendicular to the plane of the
ring, which allows it to overlap the p orbitals on the adjacent carbon
atoms. The relatively high fluorescence quantum yield of the
synthetic compounds 4aee can be explained in the same way
(Fig. 7).
The fluorescence absorption and emission spectra of
compounds 4aee were recorded at the concentration of 10ꢁ6 M in
chloroform as the solvent. The fluorescence excitation (lex) wave-
length at 271 nm (lex/nm) was used for all compounds 4aee. Figs. 8
and 9 show the visible absorption and emission spectra of
Fig. 4 shows the visible absorption spectrum of compounds
3aee in dilute (3 ꢂ 10ꢁ5 M) ethanol solution. Characteristics of
absorption spectra for 3aee in ethanol are presented in Table 1.
Values of extinction coefficient (
the plot of absorbance vs concentration, and have a precision of the
order of 5%. Absorbance intensity and extinction coefficient ( ) in
3) were calculated as the slope of
3
compounds 4aee. The lmax, lflu and fluorescence quantum yield
(VF) data are presented in Table 2.
compound 3e, with a methyl group attached to the para position of
the benzene ring, were the biggest values.
Acylation of compounds 3aee with acetyl chloride gave the new
pyrido[20,10:2,3]imidazo[4,5-b]quinolines 4aee, in excellent yields,
via the intramolecular electrophilic aromatic substitution (Fig. 5).
The possible mechanism for this transformation is shown in Fig. 6.
Other reagent such as SOCl2 and POCl3 act in same way but the yield
of reaction is not as good as acetyl chloride.
4. Conclusion
In conclusion, we have described a new and efficient synthetic
method for the synthesis of new tetracyclic ring systems containing
a quinoline nucleus which have strong fluorescence emission. The
precursors of these compounds, (3-hydroxyimino-2,3-dihy-
droimidazo[1,2-a]pyridin-2-yliden)-2-phenylacetonitrile deriva-
tives 3aee were prepared via nucleophilic substitution of hydrogen
on 3-nitro-imidazo[1,2-a]pyridine.
The structural assignments of compounds 4aee were based on
the analytical and spectral data. For example, the 1H NMR spectrum
1000
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800
4b
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