Solvent-free preparation of 2,4,6-triaryl pyridines using silver(I) nitrate adsorbed on silica gel nanoparticles (AgNO3-Nano SiO2) as an efficient catalyst

Article abstract of DOI:10.2174/157017811799304214

Three-component condensation of substituted acetophenones, aromatic aldehydes, and ammonium acetate catalyzed by silver(I) nitrate adsorbed on silica gel nanoparticles (AgNO₃-nano SiO₂) has been accomplished for the synthesis of a series of 2,4,6-triaryl pyridines in good to excellent yields under solvent-free conditions.

Full text of DOI:10.2174/157017811799304214

Letters in Organic Chemistry, 2011, 8, 717-721
717
Solvent-Free Preparation of 2,4,6-triaryl Pyridines Using Silver(I) Nitrate
Adsorbed on Silica Gel Nanoparticles (AgNO3-Nano SiO2) as an Efficient
Catalyst
Mohammad R.M. Shafiee* and Raheleh Moloudi
Faculty of Sciences, Islamic Azad University –Najafabad Branch, Najafabad, Esfahan, P.O. Box 517, Iran
Received March 29, 2011: Revised July 18, 2011: Accepted August 08, 2011
Abstract: Three-component condensation of substituted acetophenones, aromatic aldehydes, and ammonium acetate
catalyzed by silver(I) nitrate adsorbed on silica gel nanoparticles (AgNO3-nano SiO2) has been accomplished for the
synthesis of a series of 2,4,6-triaryl pyridines in good to excellent yields under solvent-free conditions.
Keywords: AgNO3-nano SiO2, 2,4,6-triaryl pyridine, krohnke pyridine, solvent-free.
INTRODUCTION
Various kinds of silver salts such as silver nitrate, silver
chloride, silver sulfadiazine, and metallic silver nanoparticles
have proved to be most effective as it has good antibacterial
efficacy against bacteria, viruses and other eukaryotic micro-
organisms [8]. In addition, they have shown good catalytic
properties in organic transformations [9, 10]. In this area
silver nitrate has ever been used as an antibiotic to treat some
infectious diseases and burn wounds [8]. To the best of our
knowledge, catalytic silver-based processes for the
preparation of 2,4,6-triaryl pyridines are unknown. Given the
different forms of Ag(I) compounds, we decided to explore
the possibility of using AgNO3 as a catalyst. AgNO3 is stable
under ambient conditions.
Nowadays, development of heterocyclic containing
nitrogen atom and especially Pyridine rings has received
great demand due to their pharmacological and biological
activities [1]. Moreover, these structures show different
activities such as anesthetic, anti-malarial, anti-oxidant, anti-
convulsant, anti-bacterial, anti-HIV, anti-cancer and anti-
parasitic properties [2]. In this area 2,4,6-Triarylpyridines
(commonly named as Krohnke pyridines) have been found to
be highly prominent building blocks in supra-molecular
chemistry and are important because of their biological
activities [2]. Therefore, preparation of Krohnke pyridine
skeletons has attracted considerable attention from the past
and in recent years.
The use of heterogeneous solid supported Lewis acid
catalysts instead of their unsupported salts have some
advantages and they are desirable to achieve effective and
catalyst handling, product purification and to decrease waste
production. Some examples of them have been performed as
inexpensive catalysts that can be easily separated, reused and
is not contaminated by the products [11]. In this Letter,
AgNO3-nano SiO2 was employed as useful and novel
catalyst for the synthesis and easy purification of 2,4,6-
triaryl pyridines from the cyclo-condensation reaction of
acetophenones, benzaldehydes and NH4OAc in high yields
(Scheme 1).
In traditional form 2,4,6-triaryl pyridines (Krohnke
pyridines) have been synthesized via three more common
procedures: through the reaction of N-phenacylpyridinium
salts with ꢀ,ꢁ-unsaturated ketones in the presence of
NH4OAc (Krohnke method) [3]. This method has been
modified via replacement of N-phenacylpyridinium salts
with N-phenacylisoquinolinium salts [4] and ꢂ-
pyrrolidinoacetophenones [5]. Another method is based on
the reaction of ꢀ,ꢁ-unsaturated ketones and NH4OAc (or
urea/thiourea) in the presence of a catalytic amount of
Brønsted acid catalysts such as acetic acid [6]. One of the
limitations of these methods is that the pyridinium salts and
the unsaturated ketones have to be synthesized first, so these
methods are relatively expensive. Newly method is cyclo-
condensation reaction of acetophenones, benzaldehydes and
NH4OAc using conventional heating and refluxing
approaches in the presence of Brønsted and Lewis acid
catalysts [7] such as H14[NaP5W30O110] [7a], SiO2-HClO4
[7b], [HO3S(CH2)4MIM][HSO4] [7c], I2 [7d] and wet 2,4,6-
trichloro-1,3,5-triazine (TCT) [7g].
RESULTS AND DISCUSSION
Initially, the cyclo-condensation reaction of 4-
methoxyacetophenone, 4-chlorobenzaldehyde and NH4OAc
was chosen as a model and the influence of AgNO3-nano
SiO2 as catalyst was examined to find the optimal conditions
for the reaction (Table 1, Scheme 2). As revealed in Table 1,
in the absence of catalyst no product was obtained (Table 1,
Entry 1). Our investigations show that the use of catalyst is
unavoidable for this transformation.
Among the tested solvents such as n-hexane, ethanol,
*Address correspondence to this author at the Faculty of Sciences, Islamic
Azad University –Najafabad Branch, Najafabad, Esfahan, P.O. Box 517,
Iran; Tel: +98-3312291004; Fax: +98-3312291016;
water,
dichloromethane
and
solvent-free
system,
condensation of 4-methoxyacetophenone (2 mmol), 4-
chlorobenzaldehyde (1 mmol) and ammonium acetate (1.3
E-mail: mohammadreza.mohammadshafiee@gmail.com
1875-6255/11 $58.00+.00
© 2011 Bentham Science Publishers

718 Letters in Organic Chemistry, 2011, Vol. 8, No. 10
Shafiee and Moloudi
Ar'
N
AgNO3 -nanoSiO2
heat, solvent-free
Ar'CHO + ArCOCH3
+ NH4OAc
Ar
Ar
Scheme 1. Preparation of 2,4,6-triaryl pyridines using AgNO3-nano SiO2.
Cl
CH3
O
CHO
AgNO3-nanoSiO2
+ NH4OAc
+
heat; Solvent-free
N
OMe
Scheme 2. Synthesis of 2,6-bis(4-methoxyphenyl)-4-(4-chlorophenyl)pyridine.
Cl
MeO
OMe
mmol) is more facile and proceeded to give highest yield,
under solvent free conditions (Table 1, Entries 2-6).
chlorobenzaldehyde (1 mmol), 4-methoxyacetophenone (2
mmol) and ammonium acetate (1.3 mmol) in the presence of
AgNO3 has been established (Scheme 3).
Next, the effect of temperature and amount of catalyst on
the rate of reaction was investigated (Table 1, Entries 7-11).
Finally, we achieved an optimized condition using AgNO3-
nano SiO2 (0.05 g, 0.03 mmol of AgNO3) as the catalyst and
100 °C. The results are summarized in Table 1.
As revealed from our investigations, in the presence of
AgNO3 product was obtained in high yield but in longer
reaction time than that AgNO3-nano SiO2.
To explain the formation of 2,4,6-triaryl pyridines via the
one-pot multi-component reaction, we have proposed a
plausible reaction mechanism, which is illustrated in Scheme
4. Firstly, the interaction of aldehyde with empty ꢀ orbital of
Lewis acid was occurred to form a cation intermediate (A).
Similarly, aromatic ketone can be activated and equilibrated
with an enolic form (B). In continue the formation of ꢁ,ꢂ-
unsaturated ketones could be prepared from the reaction of A
and B. The second step is the addition of ammonia (resulted
from NH4OAc) to ꢁ,ꢂ-unsaturated ketone to form the
intermediate (C). An enolic form of aromatic ketone was
reacted with intermediate (C) to produce (D) as a new
intermediate. The cyclization of (D) was occurred and
We then investigated the reaction scope of this catalytic
system and its tolerance of functional groups in the case of
other acethophenone and benzaldehyde derivatives. As
shown in Table 2, the optimized reaction conditions were
applicable to various substrates. Whether using various
acethophenones or various benzaldehydes, AgNO3-nano
SiO2 efficiently promoted the reaction with good to excellent
yields. The effect of the nature of substituents on the
aromatic ring showed no noticeable effect on this translation
as they were obtained in high yields with short reaction time.
To shows the utility of AgNO3-nano SiO2 than that
unsupported state of AgNO3, a model reaction of 4-
Table 1. Optimization of the Reaction Conditions in the Preparation of 2,6-bis(4-methoxyphenyl)-4-(4-chlorophenyl)pyridine
Entry
Catalyst (g)
T (°C)
Solvent
Time (min)
Yield (%)a
1
-
100
Reflux
Reflux
Reflux
Reflux
100
-
200
300
180
60
-
2
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.1
H2O
-
3
CH2Cl2
80
50
75
93
90
70
-
4
n-Hexane
5
Ethanol
240
45
6
-
-
-
-
-
-
7
120
40
8
80
120
480
40
9
r.t.
10
100
85
84
11
0.03
100
80
aIsolated Yield.

Solvent-Free Preparation of 2,4,6-triaryl Pyridines
Letters in Organic Chemistry, 2011, Vol. 8, No. 10 719
Table 2. Preparation of 2,4,6-triaryl Pyridine Derivatives Using AgNO3-nano SiO2
Product
Acethophenone
Aldehyde
Time (min)
Yield (%)a
m.p. (°C) [lit. m.p.]ref
1a
2a
4-Methoxyacethophenone
4-Methoxyacethophenone
4-Methoxyacethophenone
4-Methoxyacethophenone
4-Hydroxyacethophenone
4-Chloroacethophenone
4-Chloroacethophenone
4-Chloroacethophenone
4-Bromoacethophenone
4-Bromoacethophenone
4-Bromoacethophenone
4-Bromoacethophenone
Benzaldehyde
4-Methylbenzaldehyde
4-Chlorobenzaldehyde
4-Nitrobenzaldehyde
Benzaldehyde
70
60
80
84
93
95
83
90
94
83
87
82
93
88
102-104 [100-103]6
157-159 [155-157]6
112-114 [115-116]6
146-148 [143-144]6
199-201 [197]7a
3a
45
4a
25
5a
120
45
6a
Benzaldehyde
125-127 [125-127]6
202-204 [201-203]7b
260-262 [264-265]7d
103-105 [103-105]7c
166-168 [-]
7a
4-Methylbenzaldehyde
4-Chlorobenzaldehyde
Benzaldehyde
30
8a
135
70
9a
10a
11a
12a
4-Chlorobenzaldehyde
4-Fluorobenzaldehyde
4-Methoxybenzaldehyde
60
80
146-148 [-]
30
149-150 [-]
aIsolated Yield; All known products have been reported previously in the literature and were characterized by comparison of IR and NMR spectra with authentic samples [5-7].
Cl
CH3
O
O
H
AgNO3 (0.03 mmol)
+ NH4OAc
+
100 oC
N
solvent-free
H3CO
OCH3
OCH3
Cl
Time: 60 min
Yield: 87%
Scheme 3. Synthesis of 2,6-bis(4-methoxyphenyl)-4-(4-chlorophenyl)pyridine in the presence of AgNO3 as catalyst.
intermediate (E) was established. This intermediate was
equilibrated with (F). Finally, the oxidation of intermediates
E and F was occurred to form the appropriate product.
spectra were measured in DMSO-d6 relative to TMS (0.00
ppm). IR spectra were recorded on a PerkineElmer 781
spectrophotometer. Melting points were determined in open
capillaries with a BUCHI 510 melting point apparatus. TLC
was performed on silica gel polygram SIL G/UV 254 plates.
SEM was taken by a Philips (XL30 model) photographs to
examine the shape and size of nano silica.
CONCLUSION
In conclusion, we have presented a highly efficient and
powerful method for the preparation of 2,4,6-triaryl
pyridines catalyzed by AgNO3-nano SiO2. The reactions
were carried out under solvent-free conditions with short
reaction times and produce the corresponding products in
good yields. The present methodology offers several
advantages such as good yields, simple procedure, shorter
reaction times and milder conditions and the products were
purified without resorting to chromatography.
Preparation of Silica Nanoparticles
Silica nanoparticles were prepared by the hydrolysis of
tetraethyl orthosilicate (TEOS) with aqueous ethanol
catalyzed by oxalic acid. Typically, 50 mL of TEOS and 10
mL oxalic acid (35 w%) was added to a stirring mixture of
50 mL of aqueous ethanol (50%) respectively. The mixture
was stirred at room temperature until a gel was formed and
was subsequently dried by stirring overnight at 60 °C and
24h at 120 °C, respectively to remove the excess solvents, as
well as some of the water, from the pores. In continue, the
dry mixture was grind in a mortar and passed through
standard sieves. Finally, the xerogel was heated from room
temperature to 1400 °C at a rate of 4 °C/min in an argon
flow. The temperature was kept at 1400 °C for 1h to remove
EXPERIMENTAL
All reagents were purchased from Merck and Aldrich and
used without further purification. All yields refer to isolated
products after purification. The NMR spectra were recorded
on a Bruker Avance DPX 300 and 400 MHz instrument. The

720 Letters in Organic Chemistry, 2011, Vol. 8, No. 10
Shafiee and Moloudi
organic contaminate. Xerogel particles 29-43 nm in size was
used in the next preparation procedure.
Ag
O
O
SiO2-AgNO3
SiO2-AgNO3
A
Ph
Ar
H
Ph
H
OH
O
B
Ar
A + B
O
NH4OAc
Ar
Ph
O
NH
OH
NH2
Ar
+
Ar
Ph
Ar
Ar
Ph
D
C
Ph
Ph
Cyclization
D
Ar
N
Ar
Ar
N
Ar
E
H
Ph
F
[ox]
E or F
Ar
N
Ar
Scheme 4. Proposed mechanism for the formation of 2,4,6-triaryl
pyridines.
Fig. (1). SEM micrographs of silica gel nanoparticles (nano SiO2).
Morphological Investigations by SEM
dissolved in CH2Cl2. The catalyst was removed by simple
filtration. Solvent was evaporated and the solid product was
purified by recrystallization procedure in ethanol. More
purification can be occurred by recrystallization in hexane.
Selected data:
The dispersal state of nano silica was characterized by
SEM. Fig. (1) shows the SEM micrographs of silica gel
nanoparticles (nano SiO2). Based on the SEM observation,
the particles contain irregular shapes with wide size
distribution.
2,6-bis(4-bromophenyl)-4-(4-chlorophenyl)pyridine (10a)
1H-NMR (300 MHz, DMSO-d6): ꢀ = 7.53 (d, J = 8.5 Hz,
2H), 7.79 (d, J = 8.7 Hz, 4H), 7.94 (d, J = 8.5 Hz, 4H), 7.98
(s, 2H), 8.11 (d, J = 8.6 Hz, 2H) ppm; 13C NMR (75 MHz,
DMSO-d6): ꢀ= 123.3, 128.3, 129.8, 131.5, 131.6, 132.7,
134.4, 136.1, 137.2, 143.9, 189.1 ppm; IR (KBr, cm-1): 1655,
1593, 1487, 1402, 1328, 1215, 1096, 1032, 1006, 982, 814;
[Found: C, 55.33; H, 2.85; N, 2.84 C23H14Br2ClN; Requires
C, 55.29; H, 2.82; N, 2.80%].
Silver(I) Nitrate Adsorbed on Silica Gel (AgNO3-nano
SiO2)
The actual role for the preparation of AgNO3-nano SiO2
is relatively similar to the procedure for the preparation of
SiO2-FeCl3 that was reported by Tal and co-workers [12].
Typically, in a 250 ml flask, silver(I) nitrate (AgNO3 (1 g)
was dissolved in acetone (50 mL) and silica gel
nanoparticles (10 g) was added to this mixture and was
vigorously stirred under rotary evaporation until water was
completely evaporated. This powder was kept in oven at 100
°C for 1h to give the active catalyst.
2,6-bis(4-bromophenyl)-4-(4-fluorophenyl)pyridine (11a)
1H-NMR (400 MHz, DMSO-d6): ꢀ = 7.30 (t, J = 8.8 Hz,
2H), 7.72-7.82 (m, 5H), 7.88-8.02 (m, 5H), 8.10 (d, J = 8.4
Hz, 2H) ppm; 13C NMR (100 MHz, DMSO-d6): ꢀ= 116.3,
116.5, 122.0, 127.8, 131.0, 131.7, 131.72, 131.8, 131.9,
132.3, 136.9, 143.7, 162.7, 165.2, 188.6 ppm; IR (KBr, cm-
1): 1658, 1592, 1505, 1411, 1330, 1210, 1159, 1107, 1070,
997, 819; [Found: C, 57.21; H, 2.97; N, 2.94 C23H14Br2FN;
Requires C, 57.17; H, 2.92; N, 2.90%].
Typical Procedure
To
a
mixture of benzaldehyde (1 mmol), 4-
methoxyacethophenone (2 mmol) and ammonium acetate
(1.3 mmol) was added to AgNO3-nano SiO2 (0.05 g, 0.03
o
mmol of AgNO3) and mixture was heated at 100 C in an oil
2,6-bis(4-bromophenyl)-4-(4-methoxyphenyl)pyridine (12a)
bath for the appropriate time (Table 2). The progress of the
1H-NMR (400 MHz, DMSO-d6): ꢀ = 3.82 (s, 3H), 7.02
(d, J = 8.4 Hz, 2H), 7.71-7.87 (m, 8H), 8.08 (d, J = 8.4 Hz,
reaction was monitored by TLC. After completion of the
reaction, mass was cooled to 25 C and the mixture was
o

Solvent-Free Preparation of 2,4,6-triaryl Pyridines
Letters in Organic Chemistry, 2011, Vol. 8, No. 10 721
4H) ppm; 13C NMR (100 MHz, DMSO-d6): ꢀ= 55.9, 114.9,
119.6, 127.5, 127.7, 130.9, 131.4, 132.2, 137.3, 145.0, 162.0,
188.6 ppm; IR (KBr, cm-1): 3002, 2931, 1655, 1591, 1509,
1459, 1330, 1303, 1255, 1214, 1171, 1032, 1006, 981, 818;
[Found: C, 58.27; H, 3.49; N, 2.86; C24H17Br2NO; Requires
C, 58.21; H, 3.46; N, 2.83%].
synthesis of 2,4,6-triarylpyridines. Tetrahedron Lett., 2006, 47(33),
5957-5960.
[7]
(a) Heravi, M.M.; Bakhtiari, K.; Daroogheha, Z.; Bamoharram,
F.F. An efficient synthesis of 2,4,6-triarylpyridines catalyzed by
heteropolyacid under solvent-free conditions. Catal. Commun.,
2007, 8(12), 1991-1994; (b) Nagarapu, L.; Peddiraju, A.R.; Apuri,
S. HClO4–SiO2 as a novel and recyclable catalyst for the synthesis
of 2,4,6-triarylpyridines under solvent-free conditions. Catal.
Commun., 2007, 8(12), 1973-1976; (c) Davoodnia, A.; Bakavoli,
M.; Moloudi, R.; Tavakoli-Hoseini, N.; Khashi, M. Highly
efficient, one-pot, solvent-free synthesis of 2,4,6-triarylpyridines
using a Brønsted-acidic ionic liquid as reusable catalyst. Monatsh.
Chem., 2010, 141(8), 867-870; (d) Ren, Y.M.; Cai, C. Three-
components condensation catalyzed by molecular iodine for the
synthesis of 2,4,6-triarylpyridines and 5-unsubstituted-3,4-
ACKNOWLEDGEMENTS
We are thankful to the Islamic Azad University –
Najafabad Branch Research Council for the partial support
of this research.
dihydropyrimidin-2(1
H )-ones under solvent-free conditions,
Monatsh. Chem., 2009, 140(1), 49-52; (e) Mukhopadhyay, C.;
Tapaswi, P.K.; Butcher, R.J. l-Proline-catalyzed one-pot
expeditious synthesis of highly substituted pyridines at room
temperature. Tetrahedron Lett., 2010, 51(13), 1797-1802; (f) He,
Q.; Wang, J.; Feng, R. PEG-400 Catalyzed Reaction of the
Synthesis of 2,4,6-Triarylpyridines and Crystal Structure of 2,4,6-
Tris(4ꢁ-chlorophenyl)pyridine. Lett. Org. Chem., 2010, 7(2), 172-
177; (g) Maleki, B.; Azarifar, D.; Veisi, H.; Hojati, S.F.;
Salehabadi, H.; Nejat Yami, R. Wet 2,4,6-trichloro-1,3,5-triazine
(TCT) as an efficient catalyst for the synthesis of 2,4,6-
triarylpyridines under solvent-free conditions. Chin. Chem. Lett.,
2010, 21(11), 1346-1349.
(a) Rai, M.; Yadav, A.; Gade, A. Silver nanoparticles as a new
generation of antimicrobials. Biotechnol. Adv., 2009, 27(1), 76-83;
(b) Choi, O.; Deng, K.K.; Kim, N.J.; Ross Jr., L.; Surampalli, Y.Z.;
Hu, R. The inhibitory effects of silver nanoparticles, silver ions,
and silver chloride colloids on microbial growth. Water Res., 2008,
42(12), 3066-3074; (c) Bugla-Ploskonska, G.; Leszkiewicz, A.;
Borak, B.; Jasiorski, M.; Drulis-Kawa, Z.; Baszczuk, A.;
Maruszewski, K.; Doroszkiewicz, W. Bactericidal properties of
silica particles with silver islands located on the surface. Int. J.
Antimicrob. Agents, 2007, 29(6), 746-748.
REFERENCES
[1]
Eicher T.; Hauptmann, S. The Chemistry of Heterocycles:
Structure, Reactions, Syntheses, and Applications, 2nd ed. Wiley-
VCH: Weinheim, 2003.
[2]
(a) Kim, B.Y.; Ahn, J.B.; Lee, H.W.; Kang, S.K.; Lee, J.H.; Shin,
J.S.; Ahn, S.K.; Hong, C.I.; Yoon, S.S. Synthesis and biological
activity of novel substituted pyridines and purines containing 2,4-
thiazolidinedione. Eur. J. Med. Chem., 2004, 39(5), 433-447; (b)
Enyedy, I.J.; Sakamuri, S.; Zaman, W.A.; Johnson, K.M.; Wang, S.
Pharmacophore-Based discovery of substituted pyridines as novel
dopamine transporter inhibitors. Bioorg. Med. Chem. Lett., 2003,
13(3), 513-517; (c) Pillai, A.D.; Rathod, P.D.; Franklin, P.X.; Patel,
M.; Nivsarkar, M.; Vasu, K.K.; Padh, H.; Sudarsanam, V. Novel
drug designing approach for dual inhibitors as anti-inflammatory
agents: Implication of pyridine template. Biochem. Biophys. Res.
Commun., 2003, 301(1), 183-186; (d) Vaz, A.D.; Coon, M.J.;
Peegel, H.; Meenon, K.M. Substituted pyridines: nonsteroidal
inhibitors of human placental aromatase cytochrome P-450. Drug
Metab. Dispos., 1992, 20(1), 108-112; (e) Selvaraj, M.; Lee, T.G.
Highly selective synthesis of 2,6-bis(4-methylphenyl)pyridine over
novel mesoporous solid acid catalysts. Microporous Mesoporous
Mater., 2005, 85(1-2), 52-58.
[8]
[9]
(a) Pauvert, M.; Collet, S.; Guingant, A. Silver nitrate-promoted
ring enlargement of 1-tribromomethyl-1,2-dihydro- and 1-
tribromomethyl-1,2,3,4-tetrahydro-isoquinoline
derivatives:
[3]
(a) Krohnke, F.; Zecher, W.; Curtze, J.; Drechsler, D.; Pfleghar, K.;
Schnalke, K.E.; Weis, W. Syntheses Using the Michael Adddition
of Phridinium Salts. Angew. Chem., Int. Ed. Engl., 1962, 1(12),
626-632; (b) Krohhnke, F. The Specific Synthesis of Pyridines and
Oligopyridines. Synthesis, 1976, (1), 1-24; (c) Newkome, G.R.;
Hager, D.C.; Kiefer, G.E. Chemistry of heterocyclic compounds.
Part 119. Synthesis of halogenated terpyridines and incorporation
of the terpyridine nucleus into a polyetheral macrocycle. J. Org.
Chem., 1985, 51(6), 850-853.
application to the synthesis of the anti-anginal zatebradine.
Tetrahedron Lett., 2003, 44(22), 4203-4206; (b) Schadel, U.;
Gruner, M.; Habicher, W.D. The silver nitrate oxidation of
2,2,4,6,7-pentamethylcoumaran-5-ol. Tetrahedron, 2002, 58(25),
5081-5086; (c) Orsini, A.; Viterisi, A.; Bodlenner, A.; Weibel, J.-
M.; Pale, P. A chemoselective deprotection of trimethylsilyl
acetylenes catalyzed by silver salts. Tetrahedron Lett., 2005,
46(13), 2259-2262.
(a) Hayes, S.J.; Knight, D.W.; Menzies, M.D.; O’Halloran, M.;
Tan, W.-F. An efficient furan synthesis using heterogeneous
catalysis. Tetrahedron Lett., 2007, 48(43), 7709-7712; (b)
Sharland, C.M.; Singkhonrat, J.; NajeebUllah, M.; Hayes, S.J.;
Knight, D.W.; Dunford, D.G. An efficient and general synthesis of
pyrroles. Tetrahedron Lett., 2011, 52(18), 2320-2323.
Ertl, G.; Knözinger, H.; Weitkamp, J. Preparation of solid catalysts,
Wiley-VCH: Weinheim, 1999.
Tal, D.M.; Einan, E.; Mazur, Y. Reactions in dry media: reactions
of cholesterol and cholestanes on silica bound ferric chloride:
Cholestane-diacholestene rearrangement. Tetrahedron, 1981,
37(24), 4327-4330.
[10]
[4]
(a) Tewari, R.S., Misra, N.K.; Dubey, A.K. Studies on
cycloimmonium ylids. Synthesis of some new 2,4,6-triaryl-
substituted pyridines via pyridinium, picolinium and
isoquinolinium ylids. J. Heterocyclic Chem., 1980, 17(5), 953-955;
(b) Tewari, R.S.; Dubey, A.K. Studies on cycloimmonium ylides.
Synthesis of some 2,4,6-triaryl-substituted pyridines via
isoquinolinium ylides. J. Chem. Eng. Data, 1980, 25(1), 91-92.
Borthakur, M.; Dutta, M.; Gogoi, S.; Boruah, R.C. Microwave-
Promoted and Lewis Acid Catalysed Synthesis of 2,4,6-
Triarylpyridines Using Urea as Benign Source of Ammonia.
Synlett, 2008, (20), 3125-3128.
[11]
[12]
[5]
[6]
Adib, M.; Tahermansouri, H.; Aali Koloogani, S.; Mohammadi, B.;
Bijanzadeh, H.R. Kröhnke pyridines: an efficient solvent-free