DPTA-catalyzed one-pot regioselective synthesis of polysubstituted pyridines and 1,4-dihydropyridines

Article abstract of DOI:10.1016/j.tet.2012.03.104

With diphenylammonium triflate (DPAT) as a catalyst, the highly substituted pyridines and dihydropyridines were prepared under solvent-free conditions from aldehydes, ketones, and amines via a one-pot multi-component reaction. The advantages of this protocol include excellent yields, environmentally benign source of nitrogen, mild reaction conditions, and simple manipulation. Different source of nitrogen like urea, thiourea, inorganic ammonium salts, and organic amines were studied. In addition, a novel way was developed for the conversion of primary aliphatic amines into alcohols.

Full text of DOI:10.1016/j.tet.2012.03.104

Tetrahedron 68 (2012) 4138e4144
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
DPTA-catalyzed one-pot regioselective synthesis of polysubstituted pyridines
and 1,4-dihydropyridines
*
*
Jianjun Li , Ping He, Chuanming Yu
Key Laboratory for Green Pharmaceutical Technologies and Related Equipment of Ministry of Education, College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
With diphenylammonium triflate (DPAT) as a catalyst, the highly substituted pyridines and dihy-
dropyridines were prepared under solvent-free conditions from aldehydes, ketones, and amines via
a one-pot multi-component reaction. The advantages of this protocol include excellent yields, envi-
ronmentally benign source of nitrogen, mild reaction conditions, and simple manipulation. Different
source of nitrogen like urea, thiourea, inorganic ammonium salts, and organic amines were studied. In
addition, a novel way was developed for the conversion of primary aliphatic amines into alcohols.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 17 February 2012
Accepted 27 March 2012
Available online 2 April 2012
Keywords:
Diphenylammonium triflate
1,4-Dihydropyridines
Polysubstituted pyridine
Aliphatic amines
Multicomponent coupling
1. Introduction
reactions and occurrence of side reactions, laborious or complex
work-up and purification, and the use of metal catalyst. Green
The pyridines, especially the polysubstituted pyridines, and
their intermediate 1,4-dihydropyridines (1,4-DHPs) are important
class of biologically active heterocycles.1 Most of these pyridines
have potential pharmaceutical activities and could be used as an-
timalarial, vasodilator, anesthetic, anticovulsant, antiepileptic, an-
titumor, and agrochemicals, such as fungicide, pesticide, and
herbicide.2 1,4-DHPs have also been successfully utilized for the
generation of libraries for diverse medicinal application.3 There-
fore, the synthesis of highly substituted pyridines and 1,4-DHPs has
always attracted the attention of many synthetic chemists. Poly-
substituted pyridines are usually synthesized from acetophenones,
benzaldehydes, and nitrogen source (such as ammonium acetate,
synthesis10 of pyridines and 1,4-DHPs is still in great demand and
also challenging.
2. Results and discussion
A green chemical process always involves considerations, such
as atom economy, using cost-effective catalysts without any addi-
tive, employing nontoxic solvent or with a solvent-free system and
procedural simplicity.11 Therefore, to overcome the problems as-
sociated with the synthesis of pyridines, it is highly desirable to
develop a powerful method to meet the requirement of green
chemistry in both academia and industry. Stable and easily handled
solid ammonium triflate catalysts, such as diphenylammonium
triflate (DPAT), as a novel type of organocatalyst have been applied
in a variety of reactions and displayed great catalytic activity and
efficiency.12 In connection with our continuing studies on the de-
velopment of one-pot multi-component reactions catalyzed by
ammonium triflate to synthesize biologically important heterocy-
cles,13 we present herein a straightforward convergent one-pot
synthesis of polysubstituted pyridines and 1,4-dihydropyridines
in ethanol or under solvent-free conditions promoted by DPAT
(Fig. 1). Literature survey revealed that, there is no report on the
synthesis of polysubstituted pyridines from inorganic ammonium
salt or aliphatic amine as an environmentally benign source of
nitrogen for this conversion.
urea, thiourea, guanidine hydrochloride) with L
-proline,4 I2,5 Prey-
ssler type heteropolyacid,6 HClO4eSiO2,7 etc. as the catalysts. In
addition, the oxidation of 1,4-DHPs provides an easy access to
pyridine derivatives.8 1,4-DHPs are traditionally synthesized by
condensation of an aldehyde, b-ketoester, and ammonia (Hantzsch
reaction). In recent years, a number of improved protocols have
been reported, which include the use of microwave, ionic liquids,
TMSI, I2, Yb(OTf)3, CAN, and Sc(OTf)3.9 To our knowledge, despite
the potential utility of the methods published so far, they exhibited
varying degree of successes as well as limitations, such as multistep
* Corresponding authors. Tel./fax: þ86 571 88871087; e-mail addresses:
lijianjun@zjut.edu.cn (J. Li), ycm@zjut.edu.cn (C. Yu).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.03.104

J. Li et al. / Tetrahedron 68 (2012) 4138e4144
4139
Table 2
Solvent-free synthesis of polysubstituted pyridines catalyzed by DPATa
Productb
R
Ar
Time (h)
Yieldc (%)
Fig. 1. Diphenylammonium triflate (DPAT).
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-CH3C6H4
4-CH3C6H4
4-ClC6H4
Furan-2-yl
Furan-2-yl
C6H5
4.0
6.5
6.5
6.0
7.0
4.5
5.0
6.0
5.0
5.0
5.0
6.5
7.0
12.0
12.0
96
54
62
83
51
94
83
74
93
70
92
56
55
Trace
Trace
3-HOC6H4
4-HOC6H4
4-CH3OC6H4
4-FC6H4
4-ClC6H4
2,4-Cl2C6H3
Thiophen-2-yl
C6H5
4-HOC6H4
C6H5
Thiophen-2-yl
C6H5
Initial studies showed that better results could be obtained in
solvent-free condition at 120 ꢀC compared with those in the sol-
vents (CH3CN, EtOH, DMF, CH2Cl2, PhMe). Then we investigated the
condensation reaction of benzaldehyde, acetophenone and differ-
ent source of nitrogen at 120 ꢀC in the presence of DPAT as catalyst,
and the results are listed in Table 1. It was found that when DPAT
was used, the reaction proceeded very smoothly and gave product
4a in moderate to high yields. Most excitingly, ammonium bi-
carbonate and sal volatile are the most effective nitrogen sources
for the conversion (Table 1, entries 5 and 6). The reaction in the
presence of other ammonium sources required longer time with
a considerable amount of 1a and 2a remained unreacted. With the
success, focus was then shifted to optimization. The above model
reaction was carried out under different DPAT percentages (1 mol %,
2 mol %, 5 mol %, 10 mol %, Table 1). It was observed that 2 mol %
loading of the DPAT provided the best yield (Table 1, entry 7).
4-NO2C6H4
H
C6H5
a
Reaction conditions: aldehyde (1.0 mmol), ketone (2.0 mmol), NH4HCO3
(1.2 mmol), and DPAT (0.02 mmol) at 120 ꢀC under solvent-free conditions.
b
All products, except product 4j, are known compounds and were identified by
NMR, MS, and IR.
c
Isolated yield base on 1.
were reacted with acetophenone and NH4HCO3 under the same
conditions, however, relatively lower yields were afforded, which
could be attributed to the formation of less reactive imines.14 The
aldehyde was then extended to heterocyclic and aliphatic alde-
hydes. Thiophene-2-carbaldehyde afforded the corresponding
products (4h, 4l) in lower yields (74%, 56%, respectively).
Table 1
Optimization of the condition for the one-pot synthesis of polysubstituted pyridinea
On the basis of the above results, we further studied the reaction
of 3,4-dihydronaphthalen-1(2H)-one 5, NH4HCO3 and aldehydes
under similar conditions. It was found that the corresponding
products (6aec, 6e) could also be obtained in good yields (Table 3).
Formaldehyde afforded the corresponding product (6d) in 43%
yield. Compared with aromatic aldehydes, heterocyclic or aliphatic
aldehydes afforded lower yields of the corresponding products
(Table 2, 4h, 4l, 4m and Table 3, 6d).
Entry
Nitrogen source
Temperature
(ꢀC)
Catalyst
(mol %)
Time
(h)
Yieldb
(%)
1
2
Urea
120
120
10
10
8.5
8.0
42
50
Guanidine
hydrochloride
CH3COONH4
Thiourea
NH4HCO3
(NH4)2CO3
NH4HCO3
NH4HCO3
NH4HCO3
NH4HCO3
NH4HCO3
NH4HCO3
3
4
5
6
7
8
9
10
11
12
120
120
120
120
120
120
120
80
10
10
10
10
2
5
1
10
10
10
5.5
8.0
4.0
4.5
4.0
4.0
4.0
8.0
8.0
4.0
78
49
97
90
96
95
75
54
75
93
Table 3
Three-component
reaction of
aldehydes and
NH4HCO3
with
3,4-
dihydronaphthalen-1(2H)-one catalyzed by DPAT under the optimal conditionsa
100
140
a
Reaction condition: benzaldehyde (1.0 mmol), acetophenone (2.0 mmol), and
nitrogen source (1.2 mmol), DPAT(Ph2NH2OTf) (0.02 mmol) as catalyst, solvent-free
condition.
b
Isolated yield based on 1a.
Product
R
Time (h)
Yieldb (%)
6a
6b
6c
6d
6e
C6H5
4.5
6.5
6.0
8.0
7.0
91
65
80
43
75
Under the optimal reaction conditions, a range of aldehydes
were treated with various ketones in order to examine the scope of
the reaction, and several representative examples are summarized
in Table 2. The resulting products were washed with ethanol and
characterized by NMR and MS. It was found that most of the re-
actions proceeded smoothly to afford the corresponding poly-
substituted pyridines in moderate to excellent yields (51e96%). All
the aromatic aldehydes carrying either electron-donating or
electron-withdrawing substitutes reacted efficiently to give im-
proved yields compared with those of the classical synthetic routes.
The aromatic aldehydes with such substituents as OH, F, and NO2
3,4-(CH3O)2C6H3
4-HOC6H4
H
Thiophen-2-yl
a
Reaction conditions: aldehyde (1.0 mmol), 3,4-dihydronaphthalen-1(2H)-one
(2.0 mmol), NH4HCO3 (1.2 mmol), and DPAT (0.02 mmol) at 120 ꢀC under solvent-
free conditions.
b
Isolated yield based on 1.
Recently, Kumar15 reported a method for synthesis of N-
substituted-1,4-DHPs by multistep reaction of aldehyde, aniline,

4140
J. Li et al. / Tetrahedron 68 (2012) 4138e4144
and acetoacetateester. In order to examine the scope of this re-
action, aliphatic amine was employed as nitrogen source. Initially,
isopropylamine (1.2 mmol) was employed instead of NH4HCO3 to
react with benzaldehyde (1 mmol) and acetophenone (2 mmol)
(Scheme 1). Surprisingly, no expected N-substituted 1,4-DHPs were
detected in any case.15 Instead, the pure 2,4,6-triphenylpyridine
was obtained in high yield by filtration and washing with etha-
nol. Thus, we set out to explore the mechanism of this trans-
formation. The reaction afforded an unexpected by-product, which
was separated by distillation and identified as isopropyl alcohol.
Scheme 1. Synthesis of polysubstituted pyridines using isopropylamine as nitrogen
source.
Scheme 2. Proposed mechanism for the reaction of aldehydes, ketones, and aliphatic
amine.
To confirm the findings, a series of reactions were carried out to
generalize this transformation and the results are shown in Table 4.
2,4,6-Triarylpyridines and alcohol were got in high yields except
when tert-butylamine was used. The reason may be the steric
hindrance. When aromatic amine was used instead of aliphatic
amine, no desired product was detected. Therefore, the protocol
also provided a new way to convert aliphatic amine into the cor-
responding alcohol.
It is well-known that carbonyl compounds (2 and 8) involving in
the different pKa value in the active hydrogen show different re-
activity. Therefore, various ketones were selected in a one-pot re-
action and used their reactivity for construction various pyridines.
The mixture of 4-hydroxybenzaldehyde, acetophenone, 4-
methylacetophenone, and NH4HCO3 were treated under the opti-
mized conditions. However, this reaction showed low regiose-
lectivity and leads to mixture of products, which were elucidated
by 1H NMR and MS. Similar results were obtained by other
substituted acetophenone. Recently, Mukhopadhyay groups4
employed indan-1,3-dione and 2-acetylthiophene to develop an
efficient regiospecific synthesis of highly substituted pyridines.
Considering the distinct difference in the pKa value between ketone
2 and 1,3-diketone 8, we then employed cyclohexane-1,3-dione to
react with benzaldehyde, acetophenone, and NH4HCO3 in ethanol
promoted by DPAT under reflux conditions. In this case, only one
product was isolated in good yield. Unfortunately, we could not get
the expected polysubstituted pyridines in any case. Instead, the
pure 9-phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione 9a
(Table 5, entry 1) was obtained by filtration and washing with
ethanol. The results demonstrate that cyclohexane-1,3-dione have
higher reactivity than acetophenone due to its small pKa.
Due to the vast medicinal utility of 2,4-diaryl-1,4-
dihydropyridines 9 and 10, various methods to prepare these
compounds have been reported.17 However, many of these
methods involved multistep synthesis and were non-selective.
Therefore, it is still necessary to develop an efficient procedure
for selective syntheses of this type of compounds in one-pot re-
action using different pKa value of ketone in the active hydrogen for
control of their reaction sequence. First, benzaldehyde (1.2 mmol),
cyclohexane-1,3-dione (1.0 mmol), acetophenone (1.0 mmol), and
ammonium bicarbonate (1.2 mmol) were dissolved in ethanol and
stirred at room temperature for 6 h. The mixture was then refluxed
for 5 h in the presence DPAT, which led to 9e in good yield (83%) and
selectivity.
Table 4
Synthesis of polysubstituted pyridines using various aliphatic amines as nitrogen
sourcea
Entry R5
Ar
Time Pyridine
(h)
Alcohol
Product Yieldb (%) Product Yieldc (%)
1
2
3
4
5
6
7
8
Isopropyl C6H5
5.0
4.5
4.0
4.5
4a
4a
4a
4k
4i
4a
4a
4a
92
89
95
94
93
97
Trace
Trace
7a
7b
7c
7c
7c
7d
7e
7f
90
85
91
93
90
94
Trace
Trace
Propyl
n-Butyl
n-Butyl
n-Butyl
Benzyl
t-Butyl
Phenyl
C6H5
C6H5
4-ClC6H4
4-CH3C6H4 5.0
C6H5
C6H5
C6H5
5.0
8.0
8.0
a
Reaction condition: aldehyde (1 equiv), ketone (2 equiv), aliphatic amine
(1.2 equiv), and DPAT (0.02 equiv) at 120 ꢀC under solvent-free conditions.
b
Isolated yield of pyridine base on 1a.
Yield of alcohol detected by GC base on 1a.
c
A probable mechanism for the DPAT-catalyzed synthesis of
highly substituted pyridines has been proposed (Scheme 2). Thus,
isopropylamine reacted with aromatic ketone to form a complex
(A),16 containing nucleophilic anion (RNHꢁ). This intermediate A
further underwent in situ Michael addition to 1,3-diaryl-2-propen-
1-one (B) generated from condensation of the aldehyde and ketone
to yield intermediate C. Intermediate C is then cyclized and sub-
sequently dehydrogenated to afford the aromatized products and
alcohol.
Encouraged by the remarkable results obtained with the above
reaction conditions, and in order to explore the generality and
scope of this new protocol, we used various aldehydes, ketones, and
cyclic 1,3-diketones to react with NH4HCO3 under the above

J. Li et al. / Tetrahedron 68 (2012) 4138e4144
4141
Table 5
The condensation reaction of aldehyde, ketone, NH4HCO3, and 1,3-dicarbonyl compound catalyzed by DPATa
Entry
1
R
R1
R2
8
Time (h)
Productsc
Yieldd (%)
b
C6H5
d
d
8a
3
94
96
9a
b
2
3
C6H5
d
d
8b
8a
3
9b
C6H5
CH3
COOEt
11
93
79
9c
4
2-ClC6H4
CH3
COOEt
8a
11
9d
5
6
C6H5
C6H5
H
H
8a
8a
11
11
83
85
9e
C6H5
4-CH3C6H4
9f
7
8
4-CH3OC6H4
C6H5
H
8b
11
78
10a
C6H5
4-CH3C6H4
H
H
8b
8b
11
11
73
67
10b
10c
9
C6H5
3-ClC6H4
a
Reaction condition: aldehyde (1.2 mmol), 1,3-dicarbonyl compound (1.0 mmol), ketone (1.0 mmol), and ammonium bicarbonate were dissolved in ethanol and stirred at rt
for 6 h then heat to reflux for 5 h in the presence DPAT.
b
Reaction condition: aldehyde (1 mmol), 1,3-dicabonyl compound (2.0 mmol), and ammonia bicarbonate were dissolved in ethanol at reflux temperature by DPAT.
Major products and the yield of other one less than 5%.
Isolated yield base on 8.
c
d

4142
J. Li et al. / Tetrahedron 68 (2012) 4138e4144
conditions. Interestingly, the reaction of 1,3-dimethyluracil, benz-
aldehyde, acetylbenzene, and NH4HCO3 under the above optimized
conditions was found to produce polysubstituted pyridine (10) in
good yields. Various methods for the oxidation of 1,4-DHPs have
been reported.8 Based on the above results and literature, it could
be inferred that products 10 are the oxidation products of 1,4-DHPs
by atmospheric oxygen as a stoichiometric oxidant. However, pilot
study showed that the reaction was equally successful and afforded
10 in comparable yields when performed in an inert atmosphere.
Thus, the exact progress of this reaction is still unknown.
In summary, we have developed a novel, one-pot and solvent-
free conditions for the regioselective synthesis of polysubstituted
pyridines and 1,4-dihydropyridines using DPAT as the catalyst. The
advantages of this method include environmentally benign source
of nitrogen, simple work-up procedure, easy manipulation, good
atom economy, and employment of cost-effective catalyst. Fur-
thermore, water and carbon dioxide were the only by-products,
which added to its attractiveness. In addition, a new way to de-
compose primary aliphatic amines to alcohol was reported. Further
studies on the effect of substituents on the cyclohexane-1,3-dione
and the confirmation of the mechanism proposed for the oxida-
tive process are in progress.
778, 695 cmꢁ1; 1H NMR (400 MHz, DMSO-d6)
d 9.84 (s, 1H), 8.30 (d,
J¼6.8 Hz, 4H), 8.10 (s, 2H), 7.90 (d, J¼8.4 Hz, 2H), 7.55e7.44 (m, 6H),
6.92 (d, J¼8.4 Hz, 2H); 13C NMR (100 MHz, DMSO-d6)
d 158.5, 156.0,
149.1, 138.7, 128.9, 128.5, 128.4, 127.9, 126.7, 115.6, 115.4; MS (ESI):
m/z: 324 [Mþ1]þ.
3.2.4. 4-(40-Methoxyphenyl)-2,6-diphenylpyridine (4d). Yield 83%;
mp 102.7e103.8 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 8.31 (d,
J¼7.2 Hz, 4H), 8.14 (s, 2H), 8.02 (d, J¼8.8 Hz, 2H), 7.54 (t,
J¼7.2 Hz, 4H), 7.48e7.45 (m, 2H), 7.10 (d, J¼8.8 Hz, 2H), 3.85 (s,
3H); 13C NMR (100 MHz, DMSO-d6)
d 160.1, 156.1, 148.8, 138.7,
129.6, 128.9, 128.5, 128.4, 126.7, 115.7, 114.3, 55.3; MS (ESI): m/z:
338 [Mþ1]þ.
3.2.5. 4-(40-Fluorophenyl)-2,6-diphenylpyridine (4e). Yield 51%; mp
203.4e204.6 ꢀC; IR (KBr): nmax¼3184, 3074, 1692, 1606, 1412, 1240,
855, 778 cmꢁ1; 1H NMR (400 MHz, DMSO-d6)
d
8.00e7.95 (m, 8H),
166.3, 165.6,
7.30e7.25 (m, 8H); 13C NMR (100 MHz, DMSO-d6)
d
163.1, 131.8, 131.8, 131.7, 131.6, 128.3, 115.4, 115.2, 115.1, 114.9; MS
(ESI): m/z: 326 [Mþ1]þ.
3.2.6. 4-(40-Chlorophenyl)-2,6-diphenylpyridine (4f). Yield 94%; mp
125.9e127.5 ꢀC; IR (KBr): nmax¼3063, 1600, 1545, 1491, 1384, 1091,
3. Experimental section
825, 774, 692 cmꢁ1; 1H NMR (400 MHz, DMSO-d6)
d
7.95e7.91 (m,
166.9,
8H), 7.21e7.17 (m, 8H); 13C NMR (100 MHz, DMSO-d6)
d
3.1. General considerations
164.8, 162.4, 156.3, 147.9, 138.4, 131.4, 131.3,128.5, 126.7,116.2, 114.6,
114.4; MS (ESI): m/z: 342 [Mþ1]þ.
Analytical grade solvents and commercially available reagents
were used without further purification. Melting points were de-
3.2.7. 4-(20,40-Dichlorophenyl)-2,6-diphenylpyridine
(4g). Yield
8.16 (d,
termined on a Buchi B-540 capillary melting point apparatus and
83%; mp 134.5e136.4 ꢀC; 1H NMR (400 MHz, CDCl3)
d
€
uncorrected. IR spectra was recorded on an AVATAR-370, samples
were prepared as KBr plates. 1H NMR and 13C NMR spectra were
recorded at VARAIN-400 using DMSO-d6 or CDCl3 as the solvent
with tetramethylsilane (TMS) as an internal standard. Chemical
J¼7.2 Hz, 4H), 7.71 (s, 2H), 7.51e7.37 (m, 8H), 7.23 (s, 1H); 13C NMR
(100 MHz, CDCl3)
d 156.8, 147.3, 139.0, 136.9, 134.8, 132.9, 131.5,
129.9, 129.1, 128.6, 127.4, 127.0, 119.1; MS (ESI): m/z: 376 [Mþ1]þ.
shifts are given in
d
relative to TMS, the coupling constants J are
3.2.8. 4-(Thiophen-2-yl)-2,6-diphenylpyridine (4h). Yield 74%; mp
162.3e163.6 ꢀC; IR (KBr): nmax¼3036, 1594, 1550, 1494, 1407, 865,
given in hertz. Mass spectra were measured with Thermo Finnigan
LCQ-Advantage. High resolution mass spectral (HRMS) analyze
were measured on an Agilent 6210 TOF LC/MS using ESI or EI
(electrospray ionization) techniques.
772, 686 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.28 (d, J¼6.8 Hz,
4H), 8.11 (s, 2H), 8.09 (d, J¼3.2 Hz, 1H), 7.79 (d, J¼4.8 Hz, 1H),
7.54 (t, J¼6.8 Hz, 4H), 7.50e7.46 (t, J¼7.2 Hz, 2H), 7.28e7.26 (m,
1H); 13C NMR (100 MHz, DMSO-d6)
d 156.4, 142.8, 140.5, 138.2,
3.2. General procedure for the synthesis of polysubstituted
pyridines (4aen, 6aee)
129.2, 128.7, 128.6, 128.2, 127.0, 126.7, 114.5; MS (ESI): m/z: 314
[Mþ1]þ.
DPAT (0.006 g, 0.02 mmol) was added to a mixture of aldehyde
(1.0 mmol), ketone (2.0 mmol), and NH4HCO3 (1.2 mmol) at room
temperature. The mixture was stirred at 120 ꢀC and the progress
monitored by TLC. The resulting mixture was washed with ethanol
and dried to provide the product.
3.2.9. 4-Phenyl-2,6-dip-tolylpyridine
(4i). Yield
93%;
mp
157.4e157.9 ꢀC; IR (KBr): nmax¼3346, 2987, 1660, 1511, 1226, 1130,
841, 761 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.21 (d, J¼8.0 Hz,
4H), 8.10 (s, 2H), 8.00 (d, J¼6.8 Hz, 2H), 7.57e7.49 (m, 3H), 7.34 (d,
J¼8.0 Hz, 4H), 2.39 (s, 6H); 13C NMR (100 MHz, DMSO-d6)
d 156.1,
149.1, 138.5, 137.6, 135.8, 129.1, 129.0, 128.8, 127.1, 126.6, 115.7, 20.9;
3.2.1. 2,4,6-Triphenylpyridine (4a). Yield 96%; mp 134.6e135.3 C;
MS (ESI): m/z: 336 [Mþ1]þ.
ꢀ
IR (KBr): nmax¼3057, 1662, 1594, 1549, 1494, 1398, 757, 693 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.32 (d, J¼6.8 Hz, 4H), 8.18 (s, 2H),
3.2.10. 4-(40-Hydroxyphenyl)-2,6-dip-tolylpyridine (4j). Yield 70%;
mp 161.8e163.0 ꢀC; IR (KBr) 3433, 2928, 1596, 1544, 1492, 1444,
8.03 (d, J¼6.8 Hz, 2H), 7.58e7.46 (m, 9H). 13C NMR (100 MHz,
DMSO-d6)
d
156.3, 149.4, 138.6, 137.6, 129.1, 129.0, 128.9, 128.6,
1193, 773, 702 cmꢁ1 1H NMR (400 MHz, DMSO-d6)
; d 9.83 (s,
127.2, 126.8, 116.5; MS (ESI): m/z: 308 [Mþ1]þ.
1H), 8.19 (d, J¼8.4 Hz, 4H), 8.03 (s, 2H), 7.88 (d, J¼8.4 Hz, 2H),
7.32 (d, J¼8.0 Hz, 4H), 6.92 (d, J¼8.8 Hz, 2H), 2.39 (s, 6H); 13C
3.2.2. 4-(30-Hydroxyphenyl)-2,6-diphenylpyridine (4b). Yield 54%;
mp 214.6e216.2 ꢀC; IR (KBr): nmax¼3437, 3033, 1596, 1543, 1493,
NMR (100 MHz, DMSO-d6) d 158.4, 155.9, 148.9, 138.3, 136.0,
129.0, 128.3, 128.1, 126.5, 115.6, 114.8, 20.9; MS (ESI): m/z: 352
[Mþ1]þ; HRMS-ESI: calcd for C25H22NO: 352.1701; found:
352.2062.
1200, 778, 698 cmꢁ1 1H NMR (400 MHz, DMSO-d6)
; d 9.64 (s, 1H),
8.31e8.28 (m, 4H), 8.09 (s, 2H), 7.53 (t, J¼7.6 Hz, 4H), 7.49e7.40 (m,
5H), 6.92e6.89 (m, 1H). 13C NMR (100 MHz, DMSO-d6)
d
157.7, 156.1,
149.5, 138.9, 138.5, 129.9, 129.0, 128.5, 126.7, 117.8, 116.4, 116.0,
3.2.11. 2,6-Bis(4-chlorophenyl)-4-phenylpyridine (4k). Yield 92%;
mp 124.8e126.4 ꢀC; IR (KBr): nmax¼3090, 1601, 1544, 1492, 1091,
113.9; MS (ESI): m/z: 324 [Mþ1]þ.
833, 762 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.37 (d, J¼8.8 Hz,
3.2.3. 4-(40-Hydroxyphenyl)-2,6-diphenylpyridine (4c). Yield 62%;
mp 197.0e198.3 ꢀC; IR (KBr): nmax¼3033,1603,1518,1282,1211, 831,
4H), 8.24 (s, 2H), 8.05 (d, J¼6.8 Hz, 2H), 7.60 (d, J¼8.8 Hz, 4H),
7.58e7.51 (m, 3H); 13C NMR (100 MHz, DMSO-d6)
d
155.0,

J. Li et al. / Tetrahedron 68 (2012) 4138e4144
4143
149.6, 137.1, 133.9, 129.0, 128.6, 128.5, 128.4, 127.2, 127.1, 116.7; MS
366 [Mþ1]þ; HRMS-ESI: calcd for C25H20NS: 366.1316; found:
(ESI): m/z: 376 [Mþ1]þ.
366.1307.
3.2.12. 2,6-Di(furan-2-yl)-4-(thiophen-2-yl) pyridine (4l). Yield
56%; mp 161.7e162.4 ꢀC; IR (KBr): nmax¼3096, 1617, 1566, 1495,
3.3. General procedure for synthesis of 1,4-dihydropyridine 9
and pyridine 10
1394, 1000, 829, 743, 695 cmꢁ1 1H NMR (400 MHz, DMSO-d6)
;
d
7.97 (d, J¼4.0 Hz, 1H), 7.89 (dd, J¼1.6, 0.8 Hz, 2H), 7.80 (s, 2H), 7.78
(dd, J¼5.2, 1.2 Hz, 1H), 7.27e7.24 (m, 3H), 6.70 (dd, J¼3.6, 2.0 Hz,
2H); 13C NMR (100 MHz, DMSO-d6)
152.3, 149.0, 144.1, 142.2,
To a mixture of aldehyde (1.0 mmol), cyclohexane-1,3-dione
(2.0 mmol), and NH4HCO3 (1.2 mmol) was added DPAT (0.006 g,
0.02 mmol) in ethanol (3 mL). The reaction was stirred at reflux for
3 h and the progress was monitored by TLC. The resulting mixture
was washed with ethanol and dried to provide the product (9a
and b).
d
139.8, 128.9, 128.2, 126.9, 112.2, 111.8, 109.9; MS (ESI): m/z: 294
[Mþ1]þ.
3.2.13. 2,6-Di(furan-2-yl)-4-phenylpyridine (4m). Yield 55%; mp
127.9e128.1 ꢀC; IR (KBr): nmax¼3096, 1619, 1583, 1560, 1495, 1004,
3.3.1. 9-Phenyl-3,4,6,7,9,10-hexahydroacridine-1,8(2H,5H)-dione
758, 740 cmꢁ1; 1H NMR (400 MHz, DMSO-d6)
d
7.91e7.87 (m, 6H),
(9a). Yield 94%; mp>250 ꢀC; IR (KBr): nmax¼3171, 3048, 2944,1636,
7.57e7.51 (m, 3H), 7.28 (d, J¼3.2 Hz, 2H), 6.70 (dd, J¼3.2, 1.6 Hz,
1602, 1483, 1364, 1234, 1180, 1136, 960, 706 cmꢁ1 1H NMR
;
(400 MHz, DMSO-d6) d 9.42 (s, 1H), 7.17e7.09 (m, 4H), 7.02 (m, 1H),
2H); 13C NMR (100 MHz, DMSO-d6)
d
152.6, 149.0, 148.9, 144.2,
136.9, 129.4, 129.1, 126.8, 114.0, 112.2, 109.7; MS (ESI): m/z: 288
[Mþ1]þ.
4.89 (s, 1H), 2.57e2.52 (m, 4H), 2.20 (m, 4H), 1.90 (m, 2H), 1.84e1.69
(m, 2H); 13C NMR (100 MHz, DMSO-d6)
d 193.7, 150.4, 146.7, 127.0,
126.8, 124.6, 112.1, 36.5, 31.8, 26.1, 20.5.
3.2.14. 7-Phenyl-5,6,8,9-tetrahydrodibenzo [c,h]acridine (6a). Yield
91%; mp 162.3e164.2 ꢀC; IR (KBr): nmax¼2953, 1603, 1548, 1498,
3.3.2. 3,3,6,6-Tetramethyl-9-phenyl-3,4,6,7-tetrahydroacridine-1,8
1390, 757, 737, 702 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.41 (d,
(2H,5H)-dione (9b). Yield 96%; mp>250 ꢀC; 1H NMR (400 MHz,
J¼7.6 Hz, 2H), 7.54e7.50 (m, 2H), 7.47e7.45 (m, 1H), 7.41e7.37 (m,
DMSO-d6)
d
9.28 (s, 1H), 7.15e7.12 (m, 4H), 7.01 (dd, J¼8.4, 4.4 Hz,
2H), 7.34e7.24 (m, 6H), 2.80 (t, J¼7.2 Hz, 4H), 2.60e2.56 (m, 4H);
1H), 4.79 (s,1H), 2.44 (d, J¼16.8 Hz, 2H), 2.32 (d, J¼16.8 Hz, 2H), 2.16
(d, J¼16.0 Hz, 2H), 1.98 (d, J¼16.0 Hz, 2H), 1.00 (s, 6H), 0.85 (s, 6H).
To a mixture of aldehyde (1.2 mmol), ketone (1.0 mmol), cy-
clohexane-1,3-dione or 1,3-dimethyluracil (1.0 mmol), and
NH4HCO3 (1.2 mmol) was added DPAT (0.006 g, 0.04 mmol) in
ethanol (3 mL). The reaction was stirred at room temperature for
6 h, then refluxed for 5 h and the progress was monitored by TLC.
The resulting mixture was washed with ethanol and dried to pro-
vide the product (9cef, 10aec).
13C NMR (100 MHz, DMSO-d6)
d 148.9, 147.2, 137.4, 136.7, 134.3,
128.6, 128.4, 128.2, 127.7, 127.2, 126.9, 126.4, 124.6, 27.2, 25.3; MS
(ESI): m/z: 360 [Mþ1]þ.
3.2.15. 7-(3,4-Dimethoxyphenyl)-5,6,8,9-tetrahydrodibenzo[c,h] ac-
ridine (6b). Yield 65%; mp 183.8e184.9 ꢀC; IR (KBr): nmax¼2925,
1605, 1512, 1391, 1256, 856, 751, 733 cmꢁ1 1H NMR (400 MHz,
;
DMSO-d6)
d
8.40 (d, J¼7.6 Hz, 2H), 7.38 (t, J¼7.6 Hz, 2H), 7.32 (t,
J¼7.6 Hz, 2H), 7.26 (d, J¼7.2 Hz, 2H), 7.08 (d, J¼8.0 Hz, 1H), 6.87 (s,
1H), 6.80 (d, J¼8.0 Hz,1H), 3.82 (s, 3H), 3.75 (s, 3H), 2.81 (t, J¼7.2 Hz,
3.3.3. Ethyl 2-methyl-5-oxo-4-phenyl-1,4,5,6,7,8-hexahydroquino-
4H), 2.66 (t, J¼7.2 Hz, 4H); 13C NMR (100 MHz, DMSO-d6)
d
148.9,
line-3-carboxylate (9c). Yield 93%; mp 241.4e243.2 ꢀC; 1H NMR
148.5, 147.9, 147.2, 137.4, 134.5, 128.9, 128.8, 128.5, 127.4, 126.6,
124.4, 120.6, 112.2, 111.6, 55.6, 55.5, 27.3, 25.4; MS (ESI): m/z: 420
[Mþ1]þ.
(400 MHz, DMSO-d6) d 9.11 (s, 1H), 7.18e7.11 (m, 4H), 7.07e7.03 (m,
1H), 4.88 (s, 1H), 3.97 (q, J¼7.2 Hz, 2H), 2.53e2.42 (m, 2H), 2.28 (s,
3H), 2.25e2.12 (m, 2H), 1.94e1.80 (m, 1H), 1.74 (m, 1H), 1.12 (t,
J¼7.2 Hz, 3H); 13C NMR (100 MHz, DMSO-d6)
d 194.2, 166.5, 151.1,
3.2.16. 7-(4-Hydroxyphenyl)-5,6,8,9-tetrahydrodibenzo[c,h] acridine
147.5, 144.6, 127.5, 127.1, 125.4, 110.9, 103.4, 58.9, 36.7, 35.6, 26.1,
20.8, 18.2, 14.2.
(6c). Yield 80%; mp 235.5e236.7 ꢀC; IR (KBr) 3419, 2930, 1612,
1514, 1390, 1200, 844, 757 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d 8.56
(d, J¼7.6 Hz, 2H), 7.40 (t, J¼7.6 Hz, 2H), 7.30 (t, J¼7.2 Hz, 2H), 7.19 ( d,
J¼6.4 Hz, 2H) 7.06 (d, J¼6.4 Hz, 2H), 6.94 (d, J¼8.4 Hz, 2H), 4.94 (s,
1H), 2.83 (t, J¼6.8 Hz, 4H), 2.67 (t, J¼7.6 Hz, 4H); 13C NMR (100 MHz,
3.3.4. Ethyl 4-(2-chlorophenyl)-2-methyl-5-oxo-1,4,5,6,7,8-hexahyd-
roquinoline-3-carboxylate (9d). Yield 79%; mp 236.6e237.8 ꢀC; 1H
NMR (400 MHz, DMSO-d6) d 9.13 (s, 1H), 7.25 (m, 1H), 7.22e7.12 (m,
CDCl3)
d
154.8, 149.8, 147.0, 137.7, 135.2, 130.1, 129.9, 129.1, 128.5,
2H), 7.05 (m, 1H), 5.18 (s, 1H), 3.92 (m, 2H), 2.46 (dd, J¼6.8, 5.2 Hz,
2H), 2.27e2.20 (m, 3H), 2.20e2.04 (m, 2H), 1.87 (m, 1H), 1.78e1.63
(m, 1H), 1.07 (t, J¼7.2 Hz, 3H).
127.3, 126.9, 125.2, 115.4, 77.3, 77.0, 76.7, 28.3, 26.0; MS (ESI): m/z:
376 [Mþ1]þ; HRMS-ESI: calcd for C27H22NO: 376.1701; found:
376.1693.
3.3.5. 2,4-Diphenyl-4,6,7,8-tetrahydroquinolin-5(1H)-one (9e). Yield
3.2.17. 5,6,8,9-Tetrahydrodibenzo[c,h]acridine (6d). Yield 43%; mp
160.1e160.7 ꢀC; IR (KBr): nmax¼2928, 1604, 1551, 1417, 744 cmꢁ1; 1H
83%; mp 206.3e208.6 ꢀC; 1H NMR (400 MHz, DMSO-d6)
d 8.67 (s,
1H), 7.49e7.43 (m, 2H), 7.40e7.30 (m, 3H), 7.26e7.18 (m, 4H),
7.12e7.06 (m, 1H), 5.21 (dd, J¼5.6, 1.8 Hz, 1H), 4.58 (d, J¼5.6 Hz, 1H),
2.72e2.52 (m, 2H), 2.31e2.09 (m, 2H), 2.02e1.72 (m, 2H).
NMR (400 MHz, CDCl3)
d
8.50 (d, J¼3.6 Hz, 2H), 7.37 (t, J¼7.2 Hz,
2H), 7.31e7.20 (m, 5H), 2.95 (s, 8H); 13C NMR (100 MHz, CDCl3)
d
150.6, 138.1, 137.0, 135.3, 130.7, 128.8, 127.8, 127.3, 125.3, 28.7, 28.4;
MS (ESI): m/z: 284 [Mþ1]þ.
3.3.6. 4-Phenyl-2-p-tolyl-4,6,7,8-tetrahydroquinolin-5(1H)-one
(9f). Yield 85%; mp 203.5e205.4 ꢀC; IR (KBr): nmax¼3279, 3059,
3.2.18. 7-(Thiophen-2-yl)-5,6,8,9-tetrahydrodibenzo[c,h]
acridine
2950, 1582, 1485, 1387, 1186, 823, 702 cmꢁ1 1H NMR (400 MHz,
;
(6e). Yield 75%; mp 171.8e172.5 ꢀC; IR (KBr) 2938, 1604, 1547,
DMSO-d6)
d
8.61 (s, 1H), 7.35 (d, J¼8.0 Hz, 2H), 7.24e7.15 (m,
1391, 756, 744, 702 cmꢁ1
;
1H NMR (400 MHz, DMSO-d6)
d
8.40
6H), 7.09 (m, 1H), 5.17 (d, J¼5.6 Hz, 1H), 4.57 (d, J¼5.6 Hz, 1H),
2.69e2.52 (m, 2H), 2.30 (s, 3H), 2.26e2.12 (m, 2H), 1.97e1.74
(m, 2H).
(d, J¼8.0 Hz, 2H), 7.77 (d, J¼4.8 Hz, 1H), 7.39 (t, J¼7.2 Hz, 2H),
7.34 (t, J¼7.2 Hz, 2H), 7.28e7.23 (m, 3H), 7.11 (d, J¼3.2 Hz, 1H),
2.84 (t, J¼7.6 Hz, 4H), 2.71 (t, J¼7.6 Hz, 4H); 13C NMR
(100 MHz, DMSO-d6)
d
149.0, 140.2, 137.3, 136.0, 134.1, 130.0,
3.3.7. 4-(4-Methoxyphenyl)-7,7-dimethyl-2-phenyl-7,8-dihydroqui-
nolin-5(6H)-one (10a). Yield 78%; mp 143.4e144.9 ꢀC; IR (KBr):
128.7, 127.8, 127.4, 127.1, 126.6, 124.4, 27.1, 25.3; MS (ESI): m/z:

4144
J. Li et al. / Tetrahedron 68 (2012) 4138e4144
nmax¼2958, 1691, 1532, 1240, 1183, 1029, 825, 787, 700 cmꢁ1
;
1H
References and notes
NMR (400 MHz, DMSO-d6)
d 8.17 (m, 2H), 7.65 (s, 1H),
1. (a) Constable, E. C.; Housecroft, C. E.; Neuburger, M.; Phillips, D.; Raithby, P. R.;
Schofield, E.; Sparr, E.; Tocher, D. A.; Zehnder, M.; Zimmermann, Y. J. Chem. Soc.,
Dalton Trans. 2000, 2219e2228; (b) Cave, G. W. V.; Hardie, M. J.; Roberts, B. A.;
Raston, C. L. Eur. J. Org. Chem. 2001, 3227e3231; (c) Jetti, R. K. R.; Nagia, A.; Xue,
F.; Mak, T. C. W. Chem. Commun. 2001, 919e920; (d) Clyde-Watson, Z.; Bampos,
N.; Sanders, J. K. M. New J. Chem. 1998, 1135e1138.
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. Eur. J. Med. Chem. 2004, 39, 433e447; (b) Enyedy, I. J.;
Sakamuri, S.; Zaman, W. A.; Johnson, K. M.; Wang, S. Bioorg. Med. Chem. Lett.
2003, 13, 513e517; (c) Pillai, A. D.; Rathod, P. D.; Franklin, P. X.; Patel, M.;
Nivsarkar, M.; Vasu, K. K.; Padh, H.; Sudarsanam, V. Biochem. Biophys. Res.
Commun. 2003, 301, 183e186; (d) Lowe, G.; Droz, A. S.; Vilaivan, T.; Weaver, G.
W.; Park, J. J.; Pratt, J. M.; Tweedale, L.; Kelland, L. R. J. Med. Chem. 1999, 42,
3167e3174; (e) Karki, R.; Thapa, P.; Kang, M. J.; Jeong, T. C.; Nam, J. M.; Kim, H.
L.; Na, Y.; Cho, W. J.; Kwon, Y.; Lee, E. S. Bioorg. Med. Chem. 2010, 18, 3066e3077.
3. (a) Triggle, D. J. Cell. Mol. Neurobiol. 2003, 23, 293e303; (b) DeSimone, R. W.;
Currie, K. S.; Mitchell, S. A.; Darrow, J. W.; Pippin, D. A. Comb. Chem. High
Throughput Screening 2004, 7, 473e493.
7.54e7.45 (m, 3H), 7.29 (d, J¼8.8 Hz, 2H), 6.95 (d, J¼8.8 Hz, 2H),
3.80 (s, 3H), 3.13 (s, 2H), 2.57 (s, 2H), 1.08 (s, 6H); 13C NMR
(100 MHz, DMSO-d6)
d 197.2, 171.5, 162.9, 157.5, 151.0, 137.3,
131.8, 129.9, 129.6, 128.6, 127.1, 120.9, 113.7, 113.2, 55.1, 53.2,
47.1, 32.4, 27.9.
3.3.8. 7,7-Dimethyl-4-phenyl-2-p-tolyl-7,8-dihydroquinolin-5(6H)-
one (10b). Yield 73%; mp 152.4e153.4 ꢀC; IR (KBr): nmax¼2949,
1683, 1582, 1533, 1277, 826, 760, 695 cmꢁ1 1H NMR (400 MHz,
;
DMSO-d6)
3.13 (s, 2H), 2.55 (s, 2H), 2.37 (s, 3H), 1.08 (s, 6H); 13C NMR
(100 MHz, DMSO-d6) 196.8, 162.7, 157.6, 151.0, 139.8, 139.7, 134.3,
d
8.09 (d, J¼8.0 Hz, 2H), 7.63 (s, 1H), 7.45e7.23 (m, 7H),
d
129.2, 127.9, 127.5, 127.3, 127.0, 123.4, 120.4, 53.0, 46.9, 32.3, 27.8,
20.9; MS (ESI): m/z: 342 [Mþ1]þ; HRMS-ESI: calcd for C24H24NO:
342.1858; found: 342.1856.
4. Mukhopadhyay, C.; Tapaswi, P. K.; Butcher, R. J. Tetrahedron Lett. 2010, 51,
1797e1802.
5. Ren, Y. M.; Cai, C. Monatsh. Chem. 2009, 140, 49e52.
6. Heravi, M. M.; Bakhtiari, K.; Daroogheha, Z.; Bamoharram, F. F. Catal. Commun.
2007, 8, 1991e1994.
3.3.9. 2-(3-Chlorophenyl)-7,7-dimethyl-4-phenyl-7,8-dihy-
droquinolin-5(6H)-one (10c). Yield 67%; mp 129.8e130.3 ꢀC; IR
(KBr): nmax¼2960, 1689, 1578, 1533, 1369, 1276, 1239, 803, 762,
7. Nagarapu, L.; Aneesa; Peddiraju, R.; Apuri, S. Catal. Commun. 2007, 8,
1973e1976.
8. (a) Liao, X. L.; Lin, W. B.; Lu, J.; Wang, C. Tetrahedron Lett. 2010, 51, 3859e3861;
(b) Gorjizadeh, M.; Abdollahi-Alibeik, M. Chin. Chem. Lett. 2011, 22, 61e64; (c)
Zhang, W.; Chen, Z. Y. Chin. Chem. Lett. 2007, 18, 1443e1446.
701 cmꢁ1 1H NMR (400 MHz, DMSO-d6)
; d 8.28 (m, 1H), 8.17 (d,
J¼6.8 Hz, 1H), 7.76 (s, 1H), 7.54 (m, 2H), 7.39e7.32 (m, 5H), 3.15 (s,
2H), 2.57 (s, 2H), 1.08 (s, 6H);13C NMR (100 MHz, DMSO-d6)
d
196.8,
9. (a) Yamada, S.; Misono, T.; Ichikawa, M.; Morita, C. Tetrahedron 2001, 57,
8939e8949; (b) Yamada, S.; Morita, C. J. Am. Chem. Soc. 2002, 124, 8184e8185;
(c) Ishar, M. P. S.; Kumar, K.; Kaur, S.; Kumar, S.; Girdhar, N. K.; Sachar, S.;
Marwaha, A.; Kapoor, A. Org. Lett. 2001, 3, 2133e2136.
162.8, 155.8, 151.2, 139.5, 139.1, 133.6, 130.4, 129.6, 128.0, 127.5,
127.4, 126.7, 125.6, 124.1, 121.2, 109.1, 53.0, 46.9, 32.3, 27.8; MS (ESI):
m/z: 362 [Mþ1]þ; HRMS-ESI: calcd for C23H21ClNO: 362.1312;
found: 362.1317.
ꢀ
10. (a) Anastas, P. T.; Warner, J. C. Green Chem. 1998, ; (b) Horvath, I. T.; Anastas, P. T.
Chem. Rev. 2007, 107, 2169e2173; (c) Sheldon, R. A. Chem. Commun. 2008,
3352e3365; (d) Li, C. J.; Trost, B. M. Proc. Natl. Acad. Sci. 2008, 105,
13197e13202.
11. Zhao, Y. N.; Li, J.; Li, C. J.; Yin, K.; Ye, D. Y.; Jia, X. S. Green Chem. 2010, 12,
1370e1372.
Acknowledgements
12. Iida, A.; Osada, J.; Nagase, R.; Misaki, T.; Tanabe, Y. Org. Lett. 2007, 9, 1859e1862.
13. (a) Li, J. J.; Sun, J.; Su, W. K. Lett. Org. Chem. 2010, 7, 314e318; (b) Li, J. J.; Lu, L. M.;
Su, W. K. Tetrahedron Lett. 2010, 51, 2434e2437; (c) Li, J. J.; Tang, W. Y.; Lu, L. M.
Tetrahedron Lett. 2008, 49, 7117e7120.
14. Khan, A. T.; Khan, M. M.; Bannuru, K. K. R. Tetrahedron 2010, 66, 7762e7772.
15. Kumar, B. R. P.; Masih, P.; Lukose, C. P.; Abraham, M.; Priya, D.; Xavier, R. M.; Saji,
K.; Adhikary, L. J. Heterocycl. Chem. 2009, 46, 336e339.
16. Shen, Z. L.; Jiang, X. Z. J. Mol. Catal. A: Chem. 2004, 213, 193e198.
17. (a) Wang, X. S.; Shi, D. Q.; Tu, S. J. Synth. Commun. 2002, 32, 3449e3454; (b)
Chen, W. Y.; Lu, J.; Jin, J. R. Lett. Org. Chem. 2006, 3, 317e319; (c) Mekheimer, R.
A.; Hameed, A. A.; Sadek, K. U. Green Chem. 2008, 10, 592e593; (d) Undale, K. A.;
Shaikh, T. S.; Gaikwad, D. S.; Pore, D. M. C. R. Chimie 2011, 14, 511e515; (e)
Kumar, A.; Sharma, S.; Tripathi, V. D.; Maurya, R. A.; Srivastava, S. P.; Bhatia, G.;
Tamrakar, A. K.; Srivastava, A. K. Bioorg. Med. Chem. 2010, 18, 4138e4148.
We are grateful to the National Natural Science Foundation of
China (No. 20806073). We also thank professor Xiaoxia Wang of
Zhejiang Normal University for helpful discussions.
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2012.03.104. These data in-
clude MOL files and InChIKeys of the most important compounds
described in this article.