Molybdenum- and tungsten-based coordination polymers as catalysts for an efficient and rapid synthesis of hexahydro-5-oxoquinoline-3-carboxylates and 1,4-Dihydropyridine-3,5-dicarboxylates

Article abstract of DOI:10.1002/hlca.201000354

Hexahydro-5-oxoquinoline-3-carboxylates and 1,4-dihydropyridine-3,5- dicarboxylates were synthesized efficiently and rapidly (2 min) in the presence of molybdenum- and tungsten-based coordination polymers [M(Bu ₃Sn)₂O₄)]_n (M=Mo or W) as catalysts (Schemes 1 and 2; Tables 2 and 3). The products were formed at room temperature in excellent yields (90-98%). The catalysts worked under heterogeneous conditions and were recyclable. The earlier reports for the application of these polymers to conduct organic synthesis are limited. The present method explores a new and useful application of these catalysts. Copyright

Full text of DOI:10.1002/hlca.201000354

Helvetica Chimica Acta – Vol. 94 (2011)
885
Molybdenum- and Tungsten-Based Coordination Polymers as Catalysts for an
Efficient and Rapid Synthesis of Hexahydro-5-oxoquinoline-3-carboxylates
and 1,4-Dihydropyridine-3,5-dicarboxylates1)
by Biswanath Das*, Malampati Srilatha, Boyapati Veeranjaneyulu, and Boddu Shashi Kanth
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad-500007, India
(phone: þ 91-40-7193434; fax: þ 91-40-7160512; e-mail: biswanathdas@yahoo.com)
Hexahydro-5-oxoquinoline-3-carboxylates and 1,4-dihydropyridine-3,5-dicarboxylates were synthe-
sized efficiently and rapidly (2 min) in the presence of molybdenum- and tungsten-based coordination
polymers [M(Bu3Sn)2O4)]n (M ¼ Mo or W) as catalysts (Schemes 1 and 2; Tables 2 and 3). The products
were formed at room temperature in excellent yields (90 – 98%). The catalysts worked under
heterogeneous conditions and were recyclable. The earlier reports for the application of these polymers
to conduct organic synthesis are limited. The present method explores a new and useful application of
these catalysts.
Introduction. – The Hantzsch reaction for the synthesis of 1,4-dihydropyridine-3,5-
dicarboxylates [1 – 3] involves a one-pot condensation of an aldehyde with a b-keto
ester and NH3 in AcOH or in alcohol under reflux for several hours [4]. This reaction
has subsequently been carried out by employing different catalysts [5], ionic liquids [6],
and microwaves [7]. However, the harsh reaction conditions, high temperatures, long
reaction times, and variable yields are the drawbacks associated with many of the
procedures. Here, we report an efficient and rapid synthesis of hexahydro-5-oxo-
quinoline-3-carboxylates and 1,4-dihydropyridine-3,5-dicarboxylates under mild reac-
tion conditions.
Results and Discussion. – In continuation of our work [8] on the development of
useful synthetic methodologies, we observed that hexahydroquinoline derivatives D
can conveniently be synthesized by a multicomponent reaction of an aldehyde A, ethyl
acetoacetate (C), a cyclohexane-1,3-dione B, and AcONH4 in the presence of a
molybdenum- or tungsten-based coordination polymer [M(Bu3Sn)2O4)]n (M ¼ Mo or
W) [9] at room temperature (Scheme 1).
Initially, the reaction of benzaldehyde, dimedone (¼ 5,5-dimethylcyclohexane-1,3-
dione), ethyl acetoacetate (¼ethyl 3-oxobutanoate), and AcONH4 was conducted at
room temperature in the presence of various catalysts including Brønsted and Lewis
acids (Table 1). Considering the reaction times and yields, molybdenum- and tungsten-
based coordination polymers [M(Bu3Sn)2O4)]n (M ¼ Mo or W) were found to be most
effective, and their efficiency was almost equal. Within only 2 min, the product D1 was
formed in excellent yield (97 – 98%) in the presence of either of the two catalysts. On
1
)
Part 206 in the series, ꢂStudies on novel synthetic methodologiesꢃ.
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Helvetica Chimica Acta – Vol. 94 (2011)
Scheme 1. Multicomponent Reaction of Aldehydes A, Ethyl Acetoacetate (C), Cyclohexane-1,3-diones B,
and AcONH4 in the Presence of Molybdenum- or Tungsten-Based Coordination Polymer
a
[M(Bu3Sn)2O4)]n (M ¼ Mo or W) at Room Temperature. See Table 2.
the other hand, Na2MoO4 and Na2WO4 could not catalyze the reaction efficiently
(reaction time 15 min, yield 38 – 41%). Bu3SnCl was also less efficient (reaction time
15 min, yield 35%).
Table 1. Activity of Different Catalysts in the Synthesis of Ethyl 1,4,5,6,7,8-Hexahydro-2,7,7-trimethyl-5-
oxo-4-phenylquinoline-3-carboxylate (D1) by Reaction of Benzaldehyde, Dimedone, Ethyl Acetoacetate,
a
and AcONH4
)
Catalyst
Reaction time
Yield [%]b)
TsOH
2 h
2 h
82
86
80
87
73
41
38
35
97
98
HY-Zeolite
Amberlyst-15
PEG-SO3H
K-10 Clay
Na2WO4
Na2MoO4
Bu3SnCl
[W(Bu3Sn)2O4]n
[Mo(Bu3Sn)2O4]n
20 min
20 min
20 min
15 min
15 min
15 min
2 min
2 min
a) Reaction conditions: benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol),
AcONH4 (1 mmol), and catalyst (0.5 mmol) in MeCN (2 ml). b) Yields of isolated pure product after
column chromatography.
As both the catalysts, the molybdenum- and tungsten-based coordination polymer
[M(Bu3Sn)2O4)]n (M ¼ Mo or W), were almost equally effective for the synthesis of the
hydroquinolinecarboxylate D1, the former catalyst was used to prepare a series of the
compounds D from various aldehydes A and cyclohexane-1,3-diones B (Scheme 1,
Table 2). The conversions proceeded rapidly within 2 min, and the products D were
formed in excellent yields (90 – 98%). The time economic achievement is remarkable
in this procedure. Aromatic, heteroaromatic, and aliphatic aldehydes underwent the
conversion smoothly. The aromatic aldehydes contained both electron-donating and
electron-withdrawing groups. A sterically hindered aldehyde such as naphthalene-2-

Helvetica Chimica Acta – Vol. 94 (2011)
887
Table 2. [Mo(Bu3Sn)2O4]n-Catalyzed Synthesis of Ethyl Hexahydro-5-oxoquinoline-3-carboxylates D by
a
Reaction of Aldehydes A, Cyclohexane-1,3-diones B, Ethyl Acetoacetate (C), and AcONH4
)
Entry
R1
R
Product
Yield [%]b)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
Me
H
H
Me
Me
Me
Me
D1
D2
D3
D4
D5
D6
D7
D8
98
94
95
95
92
93
95
94
93
96
96
93
91
97
98
97
90
98
96
91
90
92
92
4-MeꢀC6H4
4-MeOꢀC6H4
4-OHꢀC6H4
4-FꢀC6H4
4-BrꢀC6H4
2,4-Cl2C6H3
cinnamyl
furan-2-yl
D9
4-OH,3-MeOC6H3
4-CNꢀC6H4
2-ClꢀC6H4
4-NO2ꢀC6H4
naphthalen-2-yl
3-Cl,4-FC6H3
naphthalen-2-yl
3-NO2ꢀC6H4
3,4,5-(MeO)3C6H2
4-CNꢀC6H4
1H-indol-3-yl
Pr
D10
D11
D12
D13
D14
D15
D16
D17
D18
D19
D20
D21
D22
D23
i-Bu
hexyl
a) See Scheme 1; for reaction conditions, see Table 1. b) Yields of isolated pure compounds after column
chromatography.
carboxaldehyde (Table 2, Entry 14) and an acid-sensitive aldehyde such as cinnamal-
dehyde (¼(2E)-3-phenylprop-2-enal; Table 2, Entry 8) afforded the desired hydro-
quinolinecarboxylates also in high yields.
The present method was also applied to the synthesis of Hantzsch 1,4-dihydropyr-
idine derivatives 3 involving the reaction of aldehydes 1, ethyl acetoactate (2), and
AcONH4 (Scheme 2). In this case also, both the molybdenum- and tungsten-based
coordination polymers [M(Bu3Sn)2O4)]n (M ¼ Mo or W) catalyzed the reaction
efficiently with almost equal activity. The former catalyst was thus used to prepare a
series of 1,4-dihydropyridine-3,5-dicarboxylates 3 (Table 3). The conversion required
only 2 min to afford the products 3 in excellent yields. The 1,4-dihydropyridine-3,5-
dicarboxylates were prepared from aromatic, heteroaromatic, and aliphatic aldehydes.
Again, aromatic aldehydes containing both electron-donating and electron-withdraw-
ing groups underwent the conversion smoothly. The structures of the products were
1
established from their spectral data (IR, H- and 13C-NMR, and MS).
The catalysts [M(Bu3Sn)2O4)]n (M ¼ Mo or W) could easily be prepared by
following the reported method [9]. They worked under heterogeneous conditions and
could easily be separated from the reaction mixture. They were recycled three times
with almost equal activity (Table 4). Recently, heterogeneous catalysts have gained

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Helvetica Chimica Acta – Vol. 94 (2011)
Scheme 2. Multicomponent Reaction of Aldehydes 1, Ethyl Acetoacetate (2), and AcONH4 in the
Presence of a Molybdenum- or Tungsten-Based Coordination Polymer [M(Bu3Sn)2O4)]n (M ¼ Mo or W)
at Room Temperature. See Table 3.
Table 3. [Mo(Bu3Sn)2O4]n-Catalyzed Formation of 1,4-Dihydropyridine-3,5-dicarboxylates 3 by the
a
Reaction of Aldehydes 1, Ethyl Acetoacetate (2), and AcONH4
)
R
Product
Yield [%]b)
Ph
3a
3b
3c
3d
3e
3f
3g
97
96
94
95
97
94
93
4-NO2ꢀC6H4
3-NO2ꢀC6H4
4-MeꢀC6H4
4-(EtOOC)furan-2-ylc)
i-Pr
Pr
a) Reaction conditions as in Table 1, but ethyl acetoacetate (2 mmol). b) Yields of isolated pure
compounds after column chromatography. c) R derived from ethyl 5-formylfuran-3-carboxylate (1e).
much importance due to eco-economic benefits. In the present cases, they were
conveniently applied to prepare hexahydroquinoline and 1,4-dihydropyridine deriva-
tives. Earlier applications of these catalysts for organic synthesis are limited [9][10].
Table 4. Catalyst Recycling and Variation of Yield of D1a)
No. of cycles
Yield [%]b)
1
2
3
98, 97
96, 95
96, 94
a) Reaction conditions: benzaldehyde (1 mmol), dimedone (1 mmol), ethyl acetoacetate (1 mmol),
AcONH4 (1 mmol), and catalyst (0.5 mmol) in MeCN (2 ml) were used. b) Yields of isolated pure
compound after column chromatography, obtained with molybdenum- or tungsten-based polymer
catalyst (first and second value, resp.).
In conclusion, we described the utilization of molybdenum- and tungsten-based
coordination polymers [M(Bu3Sn)2O4)]n (M ¼ Mo or W) for a facile and convenient
synthesis of hexahydroquinoline and 1,4-dihydropyridine derivatives at room temper-
ature. The simple experimental procedure, the mild reaction conditions, the rapid
conversion (2 min), the excellent yields (90 – 98%), and the wide applicability and
utilization of recyclable heterogeneous catalysts are the notable advantages of the
present method.

Helvetica Chimica Acta – Vol. 94 (2011)
889
The authors thank CSIR and UGC, New Delhi, for financial assistance. They are also thankful to the
NMR, MS, and IR Divisions of the IICT for spectra recording.
Experimental Part
General. The catalysts, molybdenum- and tungsten-based coordination polymers [M(Bu3Sn)2O4)]n
(M ¼ Mo or W), were prepared according to [9]. CC ¼ Column chromatography. IR Spectra:
PerkinꢀElmer RX FT-IR spectrometer; in cmꢀ1. NMR Spectra: Varian Gemini 200 MHz (1H) and
500 MHz (13C) spectrometer; d in ppm, J in Hz. ESI-MS: VG-Autospec micromass spectrometer; in m/z.
Ethyl 1,4,5,6,7,8-Hexahydro-5-oxoquinoline-3-carboxylates D: General Procedure. To a stirred soln.
of aldehyde A (1 mmol), dimedone or cyclohexane-1,3-dione B (1 mmol), ethyl acetoacetate (C;
1 mmol), and AcONH4 (1 mmol) in MeCN (2 ml), catalyst (0.5 mmol) was added at r.t., and the mixture
was stirred well. After completion of the reaction (TLC monitoring), the mixture was filtered, and the
residue washed with AcOEt (3 ꢁ 5 ml) and dried to recover the catalyst. The filtrate was diluted with
AcOEt (10 ml), washed with dist. H2O (10 ml) and then with brine (3 ꢁ 5 ml). The org. layer was dried
(Na2SO4) and concentrated and the crude product subjected to CC (silica gel, AcOEt/hexane): pure D.
The catalyst was recycled three times to get the product with almost equal yield.
Diethyl 1,4-Dihydropyridine-3,5-dicarboxylates 3: General Procedure. As described for D, with
aldehyde 1 (1 mmol), ethyl acetoacetate (2; 2 mmol), AcONH4 (1 mmol) in MeCN (2 ml), and catalyst
(0.5 mmol): pure 3.
Here, also the catalyst was recycled three times to generate the product with almost equal yield.
Data of New Products (cf. Tables 2 and 3). Ethyl 4-(4-Cyanophenyl)-1,4,5,6,7,8-hexahydro-2,7,7-
trimethyl-5-oxoquinoline-3-carboxylate (D11). White solid. M.p. 199 – 2028. IR: 3274, 1714, 1605, 1492,
1214. 1H-NMR (200 MHz, CDCl3): 8.60 (br. s, 1 H); 7.51 (d, J ¼ 8.0, 2 H); 7.39 (d, J ¼ 8.0, 2 H); 4.99 (s,
1 H); 4.02 (q, J ¼ 7.0, 2 H); 2.37 (s, 3 H); 2.33 – 2.28 (m, 2 H); 2.20 – 2.01 (m, 2 H); 1.15 (t, J ¼ 7.0, 3 H);
1.08 (s, 3 H); 0. 85 (s, 3 H). 13C-NMR (50 MHz, CDCl3): 194.2; 166.3; 152.6; 149.1; 145.2; 130.8; 128.0;
118.1; 110.1; 109.7; 102.9; 59.0; 50.2; 40.0; 36.9; 31.5; 29.2; 26.6; 18.0; 14.2. ESI-MS: 365 ([M þ H]þ).
Anal. calc. for C22H24N2O3: C 72.52, H 6.59, N 7.69; found: C 72.45, H 6.55, N 7.66.
Ethyl
1,4,5,6,7,8-Hexahydro-2,7,7-trimethyl-4-(naphthalen-2-yl)-5-oxoquinoline-3-carboxylate
(D14). Light yellow solid. M.p. 201 – 2048. IR: 3199, 1678, 1604, 1492, 1381. 1H-NMR (CDCl3,
200 MHz): 9.18 (br. s, 1 H); 7.83 – 7.71 (m, 3 H); 7.61 – 7.59 (m, 1 H); 7.49 – 7.37 (m, 3 H); 5.02 (s, 1 H);
3.99 (q, J ¼ 7.0, 2 H); 2.52 ꢀ 2.41 (m, 2 H); 2.32 (s, 3 H); 2.21 – 1.90 (m, 2 H); 1.12 (t, J ¼ 7.0, 3 H); 1.01 (s,
3 H); 0.82 (s, 3 H). 13C-NMR (CDCl3, 50 MHz): 194.5; 167.1; 149.9; 145.0; 144.9; 132.9; 131.5; 127.7;
127.2; 127.1; 126.5; 125.9; 125.2; 125.1; 110.0; 103.5; 59.2; 50.1; 39.9; 36.2; 32.2; 29.1; 26.2; 18.5; 14.1. ESI-
MS: 390 ([M þ H]þ). Anal. calc. for C25H27NO3: C 77.12, H 6.94, N 12.33; found: C 77.05, H 6.90, N 12.28.
Ethyl 4-(3-Chloro-4-fluorophenyl)-1,4,5,6,7,8-hexahydro-2,7,7-trimethyl-5-oxoquinoline-3-carboxy-
late (D15). White solid. M.p. 162 – 1668. IR: 3283, 1710, 1605, 1491, 1383, 1222. 1H-NMR (CDCl3,
200 MHz): 8.72 (br. s, 1 H); 7.22 (d, J ¼ 8.0, 1 H); 7.13 – 7.09 (m, 1 H); 6.95 (t, J ¼ 8.0, 1 H); 4.85 (s, 1 H);
4.08 – 3.94 (m, 2 H); 2.40 – 2.21 (m, 5 H); 2.19 – 2.00 (m, 2 H); 1.19 (t, J ¼ 7.0, 3 H); 1.02 (s, 3 H); 0.90 (s,
3 H). 13C-NMR (CDCl3, 50 MHz): 193.8; 166.2; 155.6 (d, J ¼ 28.0); 149.1; 145.2; 145.0; 129.7; 127.2; 119.1
(d, J ¼ 10.0); 115.2 (d, J ¼ 10.0); 110.0; 103.2; 59.0; 50.2; 39.6; 35.1; 31.8; 29.3; 26.6; 18.0; 14.0. ESI-MS:
394, 392 ([M þ H]þ). Anal. calc. for C21H23ClFNO3: C 64.45, H 5.88, N 3.58; found: C 64.37, H 5.85, N
3.55.
Ethyl 1,4,5,6,7,8-Hexahydro-2-methyl-4-(naphthalen-2-yl)-5-oxoquinoline-3-carboxylate (D16).
1
Light yellow solid. M.p. 220 – 2228. IR: 3286, 1694, 1610, 1481, 1226. H-NMR (200 MHz, CDCl3): 8.72
(br. s, 1 H); 7.81 – 7.53 (m, 4 H); 7.42 – 7.29 (m, 3 H); 5.12 (s, 1 H); 4.01 (q, J ¼ 7.0, 2 H); 2.57 – 2.42 (m,
2 H); 2.34 (s, 3 H); 2.28 – 2.17 (m, 2 H); 1.98 – 1.72 (m, 2 H); 1.15 (t, J ¼ 7.0, 3 H). 13C-NMR (50 MHz,
CDCl3): 194.2; 166.1; 150.8; 150.7; 145.1; 145.0; 132.5; 131.8; 126.6; 126.1; 125.1; 125.0; 124.8; 111.5; 104.7;
58.4; 30.4; 30.2; 25.8; 20.2; 18.5; 14.3. ESI-MS: 362 ([M þ H]þ). Anal. calc. for C23H23NO3: C 76.45, H
6.37, N 3.87; found: C 76.41, H 6.34, N 3.85.
Ethyl 1,4,5,6,7,8-Hexahydro-2-methyl-5-oxo-4-(3,4,5-trimethoxyphenyl)quinoline-3-carboxylate
1
(D18). Light brown solid. M.p. 157 – 1608. IR: 3275, 1695, 1605, 1488, 1379, 1226. H-NMR (200 MHz,

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Helvetica Chimica Acta – Vol. 94 (2011)
CDCl3): 8.61 (br. s, 1 H); 6.44 (s, 2 H); 4.92 (s, 1 H); 4.03 (q, J ¼ 7.0, 2 H); 3.72 (s, 6 H); 3.70 (s, 3 H);
2.58 – 2.39 (m, 2 H); 2.32 (s, 3 H); 2.28 – 2.25 (m, 2 H); 2.02 – 1.83 (m, 2 H); 1.20 (t, J ¼ 7.0, 3 H).
13C-NMR (50 MHz, CDCl3): 195.0; 167.5; 152.5; 150.6; 145.1; 144.4; 136.8; 111.0; 105.2; 104.9; 60.4; 59.2;
55.1; 36.2; 35.7; 26.0; 21.3; 19.7; 14.1. ESI-MS: 402 ([M þ H]þ). Anal. calc. for C22H27NO6: C 65.83, H
6.73, N 3.49; found: C 65.75, H 6.70, N 3.46.
Ethyl 4-(4-Cyanophenyl)-1,4,5,6,7,8-hexahydro-2-methyl-5-oxoquinoline-3-carboxylate (D19). Light
yellow solid. M.p. 245 – 2488. IR: 3289, 2230, 1703, 1608, 1482, 1380, 1286. 1H-NMR (200 MHz, CDCl3):
8.82 (br. s, 1 H); 7.48 (d, J ¼ 8.0, 2 H); 7.37 (d, J ¼ 8.0, 2 H); 4.99 (s, 1 H); 3.98 (q, J ¼ 7.0, 2 H); 2.58 – 2.39
(m, 2 H); 2.31 (s, 3 H); 2.25 – 2.12 (m, 2 H); 1.99 – 1.76 (m, 2 H); 1.12 (t, J ¼ 7.0, 3 H). 13C-NMR (50 MHz,
CDCl3): 194.4; 165.1; 153.0; 151.1; 145.4; 131.0; 128.5; 118.2; 110.7; 108.8; 102.6; 59.1; 37.9; 26.3; 21.0;
18.4; 14.2. ESI-MS: 337 ([M þ H]þ). Anal. calc. for C20H20N2O3: C 71.42, H 5.95, N 8.33; found: C 71.35,
H 5.93, N 8.29.
Ethyl 1,4,5,6,7,8-Hexahydro-4-(1H-indol-3-yl)-2,7,7-trimethyl-5-oxoquinoline-3-carboxylate (D20).
Brown semi-solid. IR: 3226, 1695, 1584, 1484, 1382, 1213. 1H-NMR (200 MHz, CDCl3): 9.64 (br. s,
1 H); 8.34 (br. s, 1 H); 7.58 (d, J ¼ 8.0, 1 H); 7.21 (d, J ¼ 8.0, 1 H); 6.98 – 6.82 (m, 3 H); 5.20 (s, 1 H); 4.02 –
3.90 (m, 2 H); 2.30 – 2.19 (m, 5 H); 2.04 – 1.97 (m, 2 H); 1.12 (t, J ¼ 7.0, 1 H); 1.02 (s, 3 H); 0.82 (s, 3 H).
13C-NMR (50 MHz, CDCl3): 196.6; 166.1; 164.2; 163.5; 135.4; 127.2; 124.2; 123.8; 123.4; 120.2; 119.6;
118.2; 111.0; 101.4; 61.3; 51.2; 45.6; 39.0; 31.5; 27.8; 24.2; 14.1. ESI-MS: 401 ([M þ Na]þ). Anal. calc. for
C23H26N2O3: C 73.01, H 6.87, N 7.40; found: C 72.95, H 6.84, N 7.36.
Diethyl 4-[4-(Ethoxycarbonyl)furan-2-yl]-1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (3e).
Brown semi-solid. IR: 3333, 1730, 1700, 1666, 1498, 1381, 1281. 1H-NMR (200 MHz, CDCl3): 8.45 (br. s,
1 H); 7.19 (br. s, 1 H); 6.52 (br. s, 1 H); 5.73 (s, 1 H); 4.22 (q, J ¼ 7.0, 2 H); 4.04 – 3.92 (m, 4 H); 2.23 (s,
6 H); 1.34 (t, J ¼ 7.0, 3 H); 1.12 (t, J ¼ 7.0, 6 H). 13C-NMR (50 MHz, CDCl3): 166.5; 163.0; 146.6; 145.1;
139.6; 111.2; 110.3; 103.2; 59.2; 59.0; 32.1; 18.4; 14.8. ESI-MS: 392 ([M þ H]þ). Anal. calc. for C20H25NO7:
C 61.38, H 6.39, N 3.58; found: C 61.31, H 6.35, N 3.55.
REFERENCES
[1] M. Kawase, A. Shah, H. Gaveruja, N. Motohashi, H. Sakagami, A. Varga, J. Molnar, Bioorg. Med.
Chem. 2002, 10, 1051; R. Shan, C. Velazquez, E. E. Knaus, J. Med. Chem. 2004, 47, 254; Y. Sawada,
H. Kayakiri, Y. Abe, T. Mizutani, N. Inamura, M. Asano, C. Hatori, I. Aramori, T. Oku, H. Tanaka, J.
Med. Chem. 2004, 47, 2853.
[2] T. Godfraind, R. Miller, M. Wibo, Pharmacol. Rev. 1986, 38, 321; R. Mannhold, B. Jablonka, W.
Viogdt, K. Schoenafinger, K. Schravan, Eur. J. Med. Chem. 1992, 27, 229.
[3] V. Clusa, Drugs Future 1995, 20, 135; R. Boer, V. Gekeler, Drugs Future 1995, 20, 499.
[4] A. Hantzsch, Justus Liebigs Ann. Chem. 1882, 215, 1.
[5] G. Sabitha, G. S. K. K. Reddy, C. S. Reddy, J. S. Yadav, Tetrahedron Lett. 2003, 44, 4129; L. M. Wang,
J. Sheng, L. Zhang, J.-W. Han, Z.-Y. Fan, H. Tian, C.-T. Qian, Tetrahedron 2005, 61, 1539; S. Ko,
M. N. V. Sastry, C. Lin, C.-F. Yao, Tetrahedron Lett. 2005, 46, 5771; G. V. M. Sharma, K. L. Reddy,
P. S. Lakshmi, P. R. Krishna, Synthesis 2006, 55; A. Kumar, R. A. Maurya, Tetrahedron 2007, 63,
1946; C. S. Reddy, M. Raghu, Indian J. Chem., Sect. B 2008, 47, 1578; A. Kumar, R. A. Mourya,
Synlett 2008, 883.
[6] S.-J. Ji, Z.-Q. Jiang, J. Lu, T.-P. Loh, Synlett 2004, 831; R. Sridhar, P. T. Perumal, Tetrahedron 2005, 61,
2465.
[7] S.-J. Tu, S.-F. Zhou, X. Ding, P.-J. Cai, H. Wang, J.-C. Feng, Chin. J. Org. Chem. 2001, 21, 313; S.-J. Tu,
C.-X. Yu, X.-H. Lrie, C.-S. Yao, F. Lrie, Y. Gao, Chin. J. Struct. Chem. 2002, 21, 99.
[8] B. Das, G. Satyalakshmi, K. Suneel, K. Damodar, J. Org. Chem. 2009, 74, 8400; B. Das, P.
Balasubramanyam, B. Veearnjaneyulu, G. C. Reddy, J. Org. Chem. 2009, 74, 9505; B. Das, J. P. N.
Kumar, A. S. Kumar, K. Damodar, Synthesis 2010, 914; B. Das, C. R. Reddy, D. N. Kumar, M.
Krishnaiah, R. Narendar, Synlett 2010, 391.
[9] A. Bordoloi, S. B. Halligudi, Adv. Synth. Catal. 2007, 349, 2085.

Helvetica Chimica Acta – Vol. 94 (2011)
891
[10] M. Abrantes, A. Valente, M. Pillinger, I. S. Gonealves, J. Rocha, C. C. Ramao, J. Catal. 2002, 209,
237; M. Abrantes, A. Valente, M. Pillinger, I. S. Gonealves, J. Rocha, C. C. Ramao, Chem. – Eur. J.
2003, 9, 2685; A. Bordoloi, F. Lefebvre, S. B. Halligudi, J. Mol. Catal. A: Chem. 2007, 270, 177.
Received September 15, 2010