Synthesis of fused pyrido[a]imidazoles

Article abstract of DOI:10.1080/00397910801997868

A new method based on the reaction of 4-bromo-1,3-diphenyl-2-buten-1-one with imidazoles and benzimidazoles was proposed to obtain imidazo[1,2-a] pyridinium and pyrido[2,1-b]benzimidazolium salts. Copyright Taylor & Francis Group, LLC.

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Synthesis of Fused Pyrido[ a
]imidazoles
Lyudmyla Potikha a , Volodymyr Kovtunenko a , Arsen
Turelyk a , Aleksandr Turov a & Andrei Tolmachev a
a Department of Chemistry, Taras Shevchenko
University of Kyiv, Kyiv, Ukraine
Available online: 12 Jun 2008
To cite this article: Lyudmyla Potikha, Volodymyr Kovtunenko, Arsen Turelyk,
Aleksandr Turov & Andrei Tolmachev (2008): Synthesis of Fused Pyrido[ a ]imidazoles,
Synthetic Communications: An International Journal for Rapid Communication of
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Synthetic Communications1, 38: 2061–2070, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801997868
Synthesis of Fused Pyrido[a]imidazoles
Lyudmyla Potikha, Volodymyr Kovtunenko, Arsen Turelyk,
Aleksandr Turov, and Andrei Tolmachev
Department of Chemistry, Taras Shevchenko University of Kyiv,
Kyiv, Ukraine
Abstract: A new method based on the reaction of 4-bromo-1,3-diphenyl-2-
buten-1-one with imidazoles and benzimidazoles was proposed to obtain imi-
dazo[1,2-a]pyridinium and pyrido[2,1-b]benzimidazolium salts.
Keywords: Annelation; Benzimidazolium ylide; c-Bromodypnone; Imidazo[1,2-
a]pyridine; Imidazolium ylide; Pyrido[2,1-b]benzimidazole
Application of heterocyclic ylides in the synthesis of fused systems with a
bridgehead nitrogen atom is one of the most general strategies for con-
structing complex structures. The strongly polarized triad of atoms,
dꢀ C0-Nþ -Cdꢀ, gives rise to an ambident reactivity of ylide species toward
electrophiles. The reaction can involve both cyclic and acyclic anionic
moieties of the ylide structure, the latter pathway being mostly inherent
in 1,3-dipolar cycloadditions to activated unsaturated compounds.[1]
For instance, it underlies the indolizine nucleus formation that results
from treating 1-(4-oxo-2,4-diphenyl-2-butenyl)pyridinium salts with
bases.[2] The dipnone residue has a twofold effect in this reaction: it acts
both as an active methylene site facilitating pyridinium ylide formation
and as a dipolarophile. The reported evidence suggests[3,4] that conver-
sions of this kind are also possible for 1-substituted imidazoles and
benzimidazoles. 4-Bromo-1,3-diphenyl-2-buten-1-one (c-bromodipnone)
(1), if reacted with heterocyclic bases, readily furnishes quaternary
salts already at room temperature.[2] Accordingly, we started from
Received in the U.K. October 1, 2007.
Address correspondence to Lyudmyla Potikha, Department of Chemistry,
Taras Shevchenko University, Volodymyrska Str. 64, 01033 Kyiv, Ukraine.
E-mail: potikha_l@mail.ru
2061

2062
L. Potikha et al.
1-R-imidazoles and 1-R-benzimidazoles to obtain, under similar
conditions, 3-alkyl-1-(4-oxo-2,4-diphenyl-2-butenyl)-3H-benzimidazol-
1-ium bromides (2a,b) and 1-R-3-(4-oxo-2,4-diphenyl-2-butenyl)-1H-imi-
dazol-3-ium bromides (3a–c) in high yields (74–92%, Scheme 1).
The salts prepared can be cyclized with bases of various strengths.
We applied the reagents most commonly used in such syntheses, namely
Et3N, pyridine, morpholine, and K2CO3. Heating salts 2a,b and 3a–c
with excess Et3N in acetone for 2 h (a short time compared to that neces-
sary for the cyclization of the analogous pyridinium salts) resulted in two
corresponding types of products. On the basis of elemental analysis, it
was clear that both of them contained a halogen atom, whereas the sup-
posed structures of their heterocyclic nuclei, pyrrolo[1,2-a]benzimidazole
and pyrrolo[1,2-a]imidazole, were at variance with the measured IR and
1
1H NMR spectra. The H NMR spectrum recorded in DMSO-d6 for
the product of conversion of 3-methylbenzimidazolium bromide 2a dis-
played a signal of the methylene group appearing as an AB spin
system (2J ¼ 16.0 Hz) at 5.81 ppm and
a one-proton singlet at
6.45 ppm. A one-proton singlet observed at 7.67 ppm (in the absorption
region of aromatic protons) proved sensitive to the water content
Scheme 1. Pyrido[a]imidazolium salts preparation.

Synthesis of Fused Pyrido[a]imidazoles
2063
in the solvent (DMSO-d6=H2O) and completely exchanged with D2O.
The products resulting from the conversion of compounds 2a,b and
3a–c in the presence of pyridine, morpholine, or K2CO3 all demonstrated
the same featureintheir1HNMRspectra:theone-protonsingletpeaksinthe
downfield region (8.05–10.18ppm), which exhibited no exchange with D2O.
The previously discussed spectral data, along with the mass spectro-
metric results for the reaction products, suggest the formation of
pyrido[2,1-b]benzimidazolium bromide from salts 2a,b and imidazo
[1,2-a]pyridinium bromide from salts 3a–c. A highly plausible mechanism
for this conversion implies that initially bases induce the formation of
benzimidazolium and imidazolium ylides, with the negatively charged
atom C(2) of the heterocycle. As the second stage, the ylides cyclize via
the addition of the dipnone residue to the carbonyl group. Salt 2a reacts
in this way in the presence of Et3N to afford 9-hydroxy-10-methyl-7,9-
diphenyl-6H,9H,10H-pyrido[2,1-b]benzimidazol-5-ium bromide (4). The
conversion of salts 2a,b and 3a–c in the presence of pyridine, morpholine,
or K2CO3 involves dehydration, thus leading to aromatic products,
10-alkyl-7,9-diphenyl-10H-pyrido[2,1-b]benzimidazol-5-ium
bromides
(5a,b) and 1-R-6,8-diphenyl-1H-imidazo[1,2-a]pyridin-4-ium bromides
(6a–c). The nature of the bases used exerts no significant effect on the
yields of salts 5a,b and 6a–c; however, morpholine appears to be the
most appropriate reagent (in addition to high yields, it affords the best
product purity).
For unequivocal structural determination of the products obtained,
the 13C NMR spectra of compounds 4, 5b, and 6a were measured, and
two-dimensional (2D) homonuclear (COSY) and heteronuclear (HMBC
and HMQC) correlation spectroscopy were employed. The 13C NMR
spectra were edited by the DEPT technique. Moreover, the 2D NOESY
experiments were performed to determine the spatial proximity of certain
protons. As a result, the spectra of the three structures under study
revealed the correlations of the pyridinic protons at positions 8 and
6 (5 and 7 for 6a). Formation of the fused (benz)imidazopyridine nucleus
is corroborated by the spatial proximity of the pyridinic proton 5-H (for
6a) or 8-H (for 5b) to the imidazolic proton 3-H or the benzimidazolic
proton 4-H, respectively. Besides, the presence of a hydroxyl group in
compound 4 is evidenced by the 2D NOESY spectral data: the negative
NOE values are detected for the signals of water protons and the presum-
ably hydroxylic proton (at 7.76 ppm), thus pointing to the exchange
process. The data on heteronuclear correlations for compounds 4, 5b,
and 6a (see Table 1) permitted us to completely assign 1H and
13C NMR signals and hence to draw a solid conclusion about the struc-
tures concerned. The signal assignments and the structurally significant
HMBC correlations (indicated by arrows) are demonstrated in Fig. 1.

2064

Synthesis of Fused Pyrido[a]imidazoles
2065
Figure 1. Signal assignments (1H NMR, d; 13C NMR, d) and the structurally
significant HMBC correlations for substrances 5b (A), 4 (B), and 6a (C).
EXPERIMENTAL
General
1H and 13C NMR spectra were measured on a Varian-400 instrument at 400
and 100MHz, respectively, using tetramethylsilane as the internal standard.
Chemical shifts are reported in ppm. IR spectra were recorded in KBr disks
on a Pye Unicam SP3-300 spectrometer. Mass spectra used Agilent 100 series
(chemical ionization (CI), acetonytril, 0.05% formic acid). Melting points
were determined on a Boetius apparatus and are uncorrected. 4-Bromo-
1,3-diphenyl-2-buten-1-one was prepared by the known procedure.[5]
3-Methyl-1-(4-oxo-2,4-diphenyl-2-butenyl)-3H-benzimidazol-1-ium
Bromide (2a)
To a solution of c-bromodypnone 1 (10.7 g, 35.5 mmol) in benzene
(50 mL), 1-methyl-1H-benzimidazole (4.69 g, 35.5 mmol) was added.

2066
L. Potikha et al.
The mixture was left alone at room temperature for 2 days, and the solid
precipitated was filtered off, washed thoroughly with benzene and acet-
one, and crystallized from nitromethane to yield 2a (14.15 g, 92%) as
ꢁ
þ
=
=
white crystals. Mp 182–184 C. IR (KBr): n1670 (C O), 1620 (C N ),
1220, 770 cmꢀ1
;
1H NMR (DMSO-d6): d 10.17 (s, 1H, 2-H), 8.15
3
(d, 2H, J ¼ 8.0 Hz, 200-H, 600-H), 8.09 (m, 1H, 7-H), 7.95 (m, 1H, 4-H),
7.80 (m, 2H, 20-H, 60-H), 7.70–7.55 (m, 6H, 5-H, 6-H, 300-H–500-
H, ¼ CHCO-), 7.36 (m, 3H, 30-H–50-H), 6.21 (s, 2H, CH2), 4.10 (s, 3H,
CH3). Anal. calcd. for C24H21BrN2O: C, 66.52; H, 4.88; Br 18.44; N,
6.46. Found: C, 66.48; H, 4.85; Br, 18.45; N, 6.48.
3-Ethyl-1-(4-oxo-2,4-diphenyl-2-butenyl)-3H-benzimidazol-1-ium
Bromide (2b)
Compound 2b was prepared by the procedure described for 2a, using
5.19 g (35.5 mmol) of 1-ethyl-1H-benzimidazole. Yield 12.07 g (76%);
ꢁ
1
mp: 180–182 C (CH3NO2); H NMR (DMSO-d6): d 10.27 (s, 1H, 2-H),
3
8.14 (d, 2H, J ¼ 8.0 Hz, 200-H, 600-H), 8.04 (m, 1H, 7-H), 7.98 (m, 1H,
4-H), 7.73 (m, 2H, 20-H, 600-H), 7.70–7.57 (m, 6H, 5-H, 6-H, 300-H–50-H,
CHCO-), 7.33 (m, 3H, 3 -H–5 -H), 6.25 (s, 2H, C(1)H2), 4.53 (q, 2H,
0
=
3J ¼ 6.5 Hz, C(3)H2), 1.45 (t, 3H, 3J ¼ 6.5 Hz, CH3). Anal. calcd. for
C25H23BrN2O: C, 67.12; H, 5.18; Br, 17.86; N, 6.26. Found: C, 67.09;
H, 5.14; Br, 17.88; N, 6.25.
1-Methyl-3-(4-oxo-2,4-diphenyl-2-butenyl)-1H-imidazol-3-ium
Bromide (3a)
Compound 3a was prepared by the procedure described for 2a, using
2.91 g (35.5 mmol) of 1-methyl-1H-imidazole. Yield 12.92 g (95%); mp:
ꢁ
1
219–220 C (CH3NO2); H NMR (DMSO-d6): d 10.38 (s, 1H, 2-H), 8.13
3
(d, 2H, J ¼ 8.0 Hz, 200-H, 600-H), 7.78 (m, 2H, 200-H, 60-H), 7.72–7.67
3
00
00
00
=
(m, 4H, 4-H, 5-H, CHCO-, 4 -H), 7.57 (t, 2H, J ¼ 7.5 Hz, 3 -H, 5 -
H), 7.45 (m, 3H, 30-H–50-H), 5.83 (s, 2H, CH2), 3.88 (s, 3H, CH3). Anal.
calcd. for C20H19BrN2O: C, 62.67; H, 5.00; Br, 20.85; N, 7.31. Found: C,
62.62; H, 4.93; Br, 20.87; N, 7.35.
1-Ethyl-3-(4-oxo-2,4-diphenyl-2-butenyl)-1H-imidazol-3-ium Bromide (3b)
Compound 3b was prepared by the procedure described for 2a, using
3.41 g (35.5 mmol) of 1-ethyl-1H-imidazole. Yield 12.27 g (87%); mp:
ꢁ
þ
=
=
200–202 C (CH3NO2); IR (KBr): n 1670 (C O), 1630 (C N ), 1470,

Synthesis of Fused Pyrido[a]imidazoles
2067
1230, 1160, 790, 710 cmꢀ1; 1H NMR (DMSO-d6): d 9.56 (s, 1H, 2-H), 8.11
(d, 2H, 3J ¼ 7.5 Hz, 20-H, 60-H), 7.78 (m, 3H, 4-H, 20-H, 60-H), 7.72 (s, 1H,
3
0
=
5-H), 7.67 (t, 1H, J ¼ 7.5 Hz, 4 -H), 7.62 (s, 1H, CHCO-), 7.56 (t, 2H,
3J ¼ 7.5 Hz, 30-H, 50-H), 7.42 (m, 3H, 30-H–50-H), 5.87 (s, 2H, C(3)H2),
4.22 (q, 2H, J ¼ 7.0 Hz, C(1)H2), 1.39 (t, 3H, J ¼ 7.0 Hz, CH3). Anal.
calcd. for C21H21BrN2O: C, 63.48; H, 5.33; Br, 20.11; N, 7.05. Found:
C, 63.42; H, 5.30; Br, 20.13; N, 7.09.
3
3
3-(4-Oxo-2,4-diphenyl-2-butenyl)-1-(2-pyridinyl)-1H-imidazol-3-ium
Bromide (3c)
Compound 3c was prepared by the procedure described for 2a, using
5.15 g (35.5 mꢁmol) of 2 (1H-imidazol-1-yl)pyridine. Yield 11.72 g (74%);
1
mp: 222–224 C (CH3NO2); H NMR (DMSO-d6): d 10.46 (s, 1H, 2-
3
H), 8.60 [d, 1H, J ¼ 4.8 Hz, 6-H(Py)], 8.56 (s, 1H, 5-H), 8.15 [m, 4H,
4-H(Py), 5-H(Py), 20-H, 60-H], 8.00 (s, 1H, 4-H), 7.86 (d, 2H, 3J ¼ 7.0 Hz,
0
0
0
0
=
Hz, 2 -H, 6 -H), 7.69 (m, 2H, 4 -H, CHCO-), 7.59 [m, 3H, 3-H(Py), 3 -
H, 50-H], 7.44 (m, 3H, 30-H–50-H), 6.04 (s, 2H, CH2). Anal. calcd. for
C24H20BrN3O: C, 64.58; H, 4.52; Br, 17.90; N, 9.41. Found: C, 64.53;
H, 4.48; Br, 17.91; N, 9.45.
9-Hydroxy-10-methyl-7,9-diphenyl-6H,9H,10H-pyrido[2,1-
b]benzimidazol-5-ium Bromide (4)
The mixture of 3-methyl-benzimidazolium bromide 2a (4.33 g,
10.0 mmol) and Et3N (15 mL) in acetone (50 mL) was heated under reflux
for 2 h, and the solid precipitated after cooling to room temperature was
filtered off, washed thoroughly with acetone, and crystallized from ace-
tonitrile to yield 4 (3.55 g, 82%) as white crystals. Mp 287–289ꢁC. IR
1
(KBr): n3400 (OH), 1550, 1450, 1060, 760 cmꢀ1; H NMR (DMSO-d6):
3
3
d 8.32 (d, 1H, J ¼ 8.0 Hz, 4-H), 8.03 (d, 1H, J ¼ 8.0 Hz, 1-H), 7.80–
3
7.74 (m, 5H, 2-H, 3-H, 20-H, 60-H, OH), 7.54 (d, 2H, J ¼ 7.5 Hz, 20-H,
60-H), 7.49–7.39 (m, 6H, 30-H–50-H, 30-H–50-H), 6.45 (s, 1H, 8-H), 5.81
(dd, 2H, J ¼ 16.0 Hz, CH2), 3.77 (s, 3H, CH3). 13C NMR (DMSO-d6):
2
d149.28 (C-9a), 140.9 (C-10), 135.56 (C-10), 132.53 (C-10a), 130.72 (C-
4a), 129.88, 129.73 (2C), 129.6 (2C), 129.4, 129.3 (C-7), 128.09 (C-2),
127.45 (C-3), 126.48 (2C), 126.2 (2C), 125.84 (C-8), 115.0 (C-4), 114.0
(C-1), 69.51 (C-9), 46.11 (C-6), 33.06 (CH3). MS: m=z 337.2 ([M-Br-
OH]þ , 100%), 338.2 (40%), 353.0 ([M-Br]þ, 60%). Anal. calcd. for
C24H21BrN2O: C, 66.52; H, 4.88; Br, 18.44; N, 6.46; Found: C, 66.46;
H, 4.86; Br, 18.47; N, 6.47.

2068
L. Potikha et al.
10-Methyl-7,9-diphenyl-10H-pyrido[2,1-b]benzimidazol-5-ium
Bromide (5a)
The mixture of 3-methyl-benzimidazolium bromide 2a (4.33 g,
10.0 mmol) and morpholine (15 mL) in acetone (50 mL) was heated under
reflux for 2 h, and the solid precipitated after cooling to room tempera-
ture was filtered off, washed thoroughly with acetone, and crystallized
from acetonitrile to yield 5a (3.28 g, 79%) as white crystals. Mp 292–
ꢁ
þ
1
294 C. IR (KBr): n1620 (C N ), 1530, 1490, 770, 700 cmꢀ1; H NMR
=
3
(DMSO-d6): d 10.13 (s, 1H, 6-H), 9.05 (d, 1H, J ¼ 8.0 Hz, 4-H), 8.41
3
3
(s, 1H, 8-H), 8.13 (d, 1H, J ¼ 8.0 Hz, 1-H), 8.04 (d, 2H, J ¼ 8.0 Hz,
20-H, 60-H), 7.91 (t, 1H, 3J ¼ 8.0 Hz, 2-H), 7.79 (t, 1H, 3J ¼ 8.0 Hz,
3-H), 7.73 (m, 2H, 20-H, 60-H), 7.64 (m, 3H, 30-H–50-H), 7.58 (t, 2H,
3J ¼ 8.0 Hz, 30-H, 50-H), 7.51 (t, 1H, J ¼ 8.0 Hz, 40-H), 3.65 (s, 3H,
3
CH3). MS: m=z 337.2 ([M-Br]þ , 100%), 339.2 (40%). Anal. calcd. for
C24H19BrN2: C, 69.41; H, 4.61; Br, 19.24; N, 6.72. Found: C, 69.37; H,
4.56; Br, 19.26; N, 6.75.
10-Ethyl-7,9-diphenyl-10H-pyrido[2,1-b]benzimidazol-5-ium Bromide (5b)
Compound 5b was prepared by the procedure described for 4, using
4.47 g (10.0 mmol) of 3-ethyl-benzimidazolium bromide 2b. Yield,
ꢁ
þ
=
3.09 g (72%); mp: 310–311 C (CH3CN); IR (KBr): n 1620 (C N ),
1
1500, 1480, 780, 700 cmꢀ1; H NMR (DMSO-d6): d 10.07 (s, 1H, 6-H),
9.01 (d, 1H, 3J ¼ 8.0 Hz, 4-H), 8.46 (s, 1H, 8-H), 8.15 (d, 1H, 3J ¼ 8.0 Hz,
1-H), 8.07 (d, 2H, 3J ¼ 8.0 Hz, 20-H, 60-H), 7.91 (t, 1H, 3J ¼ 8.0 Hz, 2-H),
3
7.80 (t, 1H, J ¼ 8.0 Hz, 3-H), 7.75 (m, 2H, 20-H, 60-H), 7.66 (m, 3H, 30-
H–50-H), 7.57 (t, 2H, 3J ¼ 8.0 Hz, 30-H, 50-H), 7.50 (t, 1H, 3J ¼ 8.0 Hz, 40-
H), 4.11 (q, 2H, J ¼ 6.8 Hz, CH2), 1.06 (t, 3H, J ¼ 6.8 Hz, CH3). 13C
NMR (DMSO-d6): d 140.0 (C-9a), 139.5 (C-8), 134.42 (C-10, C-10),
133.39 (C-10a), 130.52 (C-40), 130.48 (C-2), 130.33 (C-20, C-60), 129.96
(C-30, C-50), 129.83 (C-40), 129.57 (C-7), 129.45 (C-30, C-50), 128.15 (C-
20, C-60), 127.46 (C-4a), 127.06 (C-9), 126.53 (C-6), 125.67 (C-3), 115.6
(C-4), 113.4 (C-1), 41.42 (CH2), 14.51 (CH3). MS: m=z 351.2 ([M-Br]þ ,
100%), 321.0 (20%). Anal. calcd. for C25H21BrN2: C, 69.94; H, 4.93;
Br, 18.61; N, 6.52. Found: C, 69.89; H, 4.90; Br, 18.60; N, 6.54.
3
3
1-Methyl-6,8-diphenyl-1H-imidazo[1,2-a]pyridin-4-ium Bromide (6a)
Compound 6a was prepared by the procedure described for 2a, using
3.83 g (10.0 mmol) ofꢁ 1-methyl-imidazolium bromide 3a. Yieþld, 3.25 g
=
(89%); mp: 352–354 C (i-PrOH); IR (KBr): n 1610 (C N ), 1500,

Synthesis of Fused Pyrido[a]imidazoles
2069
760, 700 cmꢀ1; 1H NMR (DMSO-d6): d 9.54 (s, 1H, 5-H), 8.57 (s, 1H, 3-
H), 8.29 (s, 1H, 2-H), 8.08 (s, 1H, 7-H), 7.87 (d, 2H, 3J ¼ 7.5 Hz, 20-H, 60-
3
H), 7.65 (m, 2H, 20-H, 60-H), 7.59 (m, 3H, 30-H–50-H), 7.54 (t, 2H, J ¼
3
7.5 Hz, 30-H, 50-H), 7.48 (t, 1H, J ¼ 7.5 Hz, 40-H), 3.45 (s, 3H, CH3). 13C
NMR (DMSO-d6): d 137.12 (C-4a), 134.78 (C-10), 134.36 (C-7), 134.04 (C-
10), 130.79 (C-20, C-60), 130.18 (C-40), 130.07 (C-30, C-50), 129.84 (C-6),
129.77 (C-40), 129.25 (C-2), 129.1 (C-30, C-50), 127.79 (C-20, C-60), 127.58
(C-8), 126.62 (C-5), 115.84 (C-3), 38.10 (CH3). MS: m=z 285.2 ([M-Br]þ ,
100%), 286.2 (30%). Anal. calcd. for C20H17BrN2: C, 65.76; H, 4.69; Br,
21.88; N, 7.67. Found: C, 65.72; H, 4.64; Br, 21.90; N, 7.69.
1-Ethyl-6,8-diphenyl-1H-imidazo[1,2-a]pyridin-4-ium Bromide (6b)
Compound 6b was prepared by the procedure described for 2a, using
3.97 g (10.0 mmol) of 1-ethyl-imidazolium bromide 3b. Yield, 3.3 g
ꢁ
1
(87%); mp: 262–264 C (i-PrOH); H NMR (DMSO-d6): d 9.73 (s, 1H,
5-H), 8.75 (s, 1H, 3-H), 8.47 (s, 1H, 2-H), 8.05 (s, 1H, 7-H), 7.85 (d,
3
2H, J ¼ 7.5 Hz, 20-H, 60-H), 7.65 (m, 2H, 20-H, 60-H), 7.60 (m, 3H, 30-
3
3
H–50-H), 7.50 (t, 2H, J ¼ 7.5 Hz, 30-H, 50-H), 7.48 (t, 1H, J ¼ 7.5 Hz,
3
3
40-H), 3.97 (q, 2H, J ¼ 6.0 Hz, CH2), 1.11 (t, 3H, J ¼ 6.0 Hz, CH3).
MS: m=z 299.3 ([M-Br]þ , 100%), 300.3 (30%). Anal. calcd. for
C21H19BrN2: C, 66.50; H, 5.05; Br, 21.07; N, 7.39. Found: C, 66.45; H,
5.00; Br, 21.10; N, 7.41.
6,8-Diphenyl-1-(2-pyridinyl)-1H-imidazo[1,2-a]pyridin-4-ium Bromide (6c)
Compound 6c was prepared by the procedure described for 2b, using
4.46 g (10.0 mmol) of 1-(2-pyridinyl)-imidazolium bromide 3c. Yield
ꢁ
1
2.78 g (65%); mp: 295–296 C (acetone); H NMR (DMSO-d6): d 9.83
(s, 1H, 5-H), 8.91 (s, 1H, 3-H), 8.58 (s, 1H, 2-H), 8.29 (s, 1H, 7-H),
3
3
8.13 [d, 1H, J ¼ 3.5 Hz, 6-H (Py)], 7.82 (d, 2H, J ¼ 7.5 Hz, 20-H, 60-
H), 7.67 [m, 1H, 4-H (Py)], 7.57 (m, 2H, 20-H, 60-H), 7.51 [m, 1H, 5-H
3
3
(Py)], 7.41 [d, 1H, J ¼ 8.0 Hz, 3-H (Py)], 7.30 (t, 1H, J ¼ 7.5 Hz, 40-
H), 7.20–7.12 (m, 5H, 30-H–50-H, 30-H, 50-H). Anal. calcd. for
C24H18BrN3: C, 67.30; H, 4.24; Br, 18.66; N, 9.81. Found: C, 67.28; H,
4.20; Br, 18.66; N, 9.82.
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