Catalyst-free one-pot synthesis of 2,4,6-triaryl-1,4-dihydropyridines in ionic liquid and their catalyzed activity on two simple Diels-Alder reactions

Article abstract of DOI:10.1080/00397910701818446

A series of 2,4,6-triaryl-1,4-dihydropyridines bearing a hydroxyl group was synthesized greenly by the cascade aldol/Michael/addition reaction of aromatic aldehyde, acetophenone, and NH4OAc (1:2:excessive) in ionic liquid [BmIm][BF4] without any catalyst. The results of their catalyzed activity on two simple Diels-Alder reactions indicated that this kind of compound containing a poly aryl ring can be used as an effective catalyst on the Diels-Alder reaction. This method, because of its environmental friendliness, simplicity, mild conditions, effectiveness, and lower costs, is suitable for the synthesis of arrays of compounds. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701818446

This article was downloaded by: [University Of Pittsburgh]
On: 24 June 2013, At: 01:47
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office:
Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International
Journal for Rapid Communication of
Synthetic Organic Chemistry
Publication details, including instructions for authors and subscription
information:
http://www.tandfonline.com/loi/lsyc20
Catalyst‐Free One‐Pot Synthesis of
2,4,6‐Triaryl‐1,4‐dihydropyridines in Ionic
Liquid and Their Catalyzed Activity on Two
Simple Diels–Alder Reactions
Hui Wu a b , Yu Wan a b , L. L. Lu c , Yang Shen a , L. Ye a & F. R. Zhang a
a School of Chemistry and Chemical Engineering, Xuzhou Normal University,
Xuzhou, Jiangsu, China
b Key Laboratory of Biotechnology on Medical Plants of Jiangsu Province,
Xuzhou, Jiangsu, China
c Department of Chemical and Textile Engineering, Jiangyin Polytechnic
College, Jiangyin, Jiangsu, China
Published online: 15 Feb 2008.
To cite this article: Hui Wu , Yu Wan , L. L. Lu , Yang Shen , L. Ye & F. R. Zhang (2008): Catalyst‐Free One‐Pot
Synthesis of 2,4,6‐Triaryl‐1,4‐dihydropyridines in Ionic Liquid and Their Catalyzed Activity on Two Simple
Diels–Alder Reactions, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:5, 666-673
To link to this article: http://dx.doi.org/10.1080/00397910701818446
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial
or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or
distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the
contents will be complete or accurate or up to date. The accuracy of any instructions, formulae,
and drug doses should be independently verified with primary sources. The publisher shall not
be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or
howsoever caused arising directly or indirectly in connection with or arising out of the use of this
material.

Synthetic Communicationsw, 38: 666–673, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701818446
Catalyst-Free One-Pot Synthesis
of 2,4,6-Triaryl-1,4-dihydropyridines in
Ionic Liquid and Their Catalyzed Activity
on Two Simple Diels–Alder Reactions
Hui Wu,1,2 Yu Wan,1,2 L. L. Lu,3 Yang Shen,1 L. Ye,1
and F. R. Zhang1
1School of Chemistry and Chemical Engineering, Xuzhou Normal
University, Xuzhou, Jiangsu, China
2Key Laboratory of Biotechnology on Medical Plants of Jiangsu
Province, Xuzhou, Jiangsu, China
3Department of Chemical and Textile Engineering, Jiangyin Polytechnic
College, Jiangyin, Jiangsu, China
Abstract: A series of 2,4,6-triaryl-1,4-dihydropyridines bearing a hydroxyl group
was synthesized greenly by the cascade aldol/Michael/addition reaction of
aromatic aldehyde, acetophenone, and NH4OAc (1:2:excessive) in ionic liquid
[BmIm][BF4] without any catalyst. The results of their catalyzed activity on two
simple Diels–Alder reactions indicated that this kind of compound containing a
poly aryl ring can be used as an effective catalyst on the Diels–Alder reaction.
This method, because of its environmental friendliness, simplicity, mild conditions,
effectiveness, and lower costs, is suitable for the synthesis of arrays of
compounds.
Keywords: catalyzed activity, Diels–Alder reaction, synthesis, 2,4,6-triaryl-1,4-
dihydropyridines
Received December 19, 2006
Address correspondence to Hui Wu, School of Chemistry and Chemical Engineer-
ing, Xuzhou Normal University, Xuzhou, Jiangsu 221116, China. E-mail: wuhui72@
yahoo.com.cn
666

2,4,6-Triaryl-1,4-dihydropyridines
INTRODUCTION
667
1,4-Dihydropyridines are important Ca2þ-channel regulators and are mainly
used to treat cardiovascular disease, such as hypertension and congestive car-
diopathy.[1–4] 2,4,6-Triaryl-1,4-dihydropyridines are useful intermediates in
[5]
the synthesis of Krohnke-type pyridines (2,4,6-triarylpyridines). So, the
¨
development of new methods for the synthesis of this kind of compound is
an ever-expanding area in organic chemistry and medicinal chemistry.
In general, they were synthesized by the Hantzsch reaction.[6–10] Eynde
and coworkers developed a synthesis with higher yields than the one from
pyridine, aldehydes, and SOCl2 in CH2Cl2.[11] Evdokimov and
coworkers[12] obtained target molecules through the reaction of aldehyde,
malononitrile, and sulfur alcohol in ethanol. Breitenbucher and
coworkers[13] developed a resin-support route to such compounds, but
violent conditions (1408C, xylene, trifluoroacectic acid (TFA)) were
necessary. These methods always used volatile and very toxic organic
solvents and displayed only moderate to low final yields; at least four steps
were necessary. To adopt the principles of green chemistry, Alajarin
et al.[14] reported a microwave-assisted synthesis using ethanol as solvent in
a few minutes, but it had poor recyclability and was difficult to industrialize.
Most of products were symmetric, and there were few reports of 2,4,6-triaryl-
1,4-dihydropyridine in these methods for asymmetric 1,4-dihydropyridine.
Probably because of the deactivation and steric effect of the hydroxyl group
on the carbonyl group, this kind of reaction, including 20-hydroxyacetophe-
none, is difficult and has never been reported. As one hydroxyl group is
added on the benzene ring, the yield will reduce 30–40%.[15] We have to
conquer this difficulty because the hydroxyl group is a general group in the
structure of most natural products and often displays biological activity.[16]
Green chemistry is the utilization of a set of principles that reduces or
eliminates the use or products. Ionic liquid, which has unique properties of
nonvolatility, nonflammability, recyclability, and ability to dissolve a wide
range of materials, is useful for generating hazardous substances in the
design, manufacture, and application of chemicals and has been the subject
of considerable interest as environmentally benign reaction media in
organic synthesis. Recently, many reports about the applications of ionic
liquids have been published.[17–21]
Herein, we observe a new mild and simple protocol for the synthesis of
2,4,6-triaryl-1,4-dihydropyridines through the reaction of aromatic
aldehydes with acetophenone and NH4OAc in one pot (Scheme 1). Their
catalytic functions on two simple Diels–Alder reactions were also studied.
At the onset of the research, an effort was made to establish the best
condition for the model reaction of 4-cyanobenzaldehyde, 20-hydroxyaceto-
phenone, and ammonium acetate. Representative results are summarized in
Table 1. [BmIm][BF4] was selected as the most effective solvent to
generated the desired product due to its biggest pKa value of HBF4.

668
H. Wu et al.
Scheme 1. Synthesis of 2,4,6,-triaryl-1,4-dihydropyridines 3.
The scope of the reaction was investigated, and the results are summar-
ized in Table 2. It indicates that the steric effect of substituted group on the
aryl group of the aldehyde strongly influenced the yields. When aromatic
aldehydes has substitutes in the o-position reacted with 20-hydroxyacetophe-
none, the reaction time were a little longer and the yields were much lower
compared with m- or p-aromatic aldehydes because of steric hindrance.
30-Nitroacetophenone and 40-nitroacetophenone gave high yields (56.7–
65.6%), and the reaction times were shorter than that of 20-hydroxyacetophe-
none. The electronic nature of the substitutes had almost no influence on the
yields. 40-Chlorobenzaldehyde and 40-bromobenzaldehyde reacted with
30-nitroacetophenone to give higher yields than others.
Why did the Mannich reaction not take place in this experiment? We
speculated the reason was that the ionic liquid weakened the nucleophilicity
of NH3 released from NH4OAc. That proves that ionic liquid has good selec-
tivity and is superior as a reaction medium.
By far, there were no reports about the Diels–Alder reaction catalyzed
by 2,4,6-triaryl-1,4-dihydropyridines. Because 2,4,6-triarylpyranium (TPþ) had
been used as a photoinduced electron-transfer reagent,[22] we used our products
to catalyze two simple Diels–Alder reactions, and the results are shown in Table 3.
Table 3 indicated that all of products could increase the yields and shorten
the reaction time of both Diels–Alder reactions. Theoretically, as the catalyst
of a concerted reaction, their optimal catalytic activity should come from this
kind of structure in which both electron-withdrawing group (EWG) and
Table 1. Effect of ionic liquid on the yield of 3a
IL
pKaa
Isolated yield (%)
[BmImb][Br]
[BmIm][BF4]
[BmIm][ClO4]
[bpyc][Br]
29
0.5
44.0
63.5
10.1
27.6
38.6
9.1
211
29
0.5
[bpy][BF4]
[bpy][ClO4]
211
aThe pKa value of the parent acid of the anions.
b[BmIm] ¼ 1-n-butyl-3-methylimidazolium.
c[bpy]¼ 1-n-butylpyridinium.

2,4,6-Triaryl-1,4-dihydropyridines
669
Table 2. Synthesis of 2,4,6-triaryl-1,4-dihydropyridines
Compound
Ar1
Ar2
Mp(8C)
.300
286.5–287.1
253.7–254.8
235.9–236.7
Yields (%) Times (h)
3a
3b
3c
3d
3e
3f
4-CNC6H4
4-ClC6H4
4-BrC6H4
2-OHC6H4
2-OHC6H4
2-OHC6H4
63.5
63.3
68.5
54.4
66.7
78.2
75.6
24
20
23
24
24
23
24
2-OCH3C6H4 2-OHC6H4
2-ClC6H4
4-ClC6H4
4-BrC6H4
4-NO2C6H4 186.5–187.6
3-NO2C6H4 189.5–190.2
3-NO2C6H4 187.3–188.6
3g
electron-donating group (EDG) were contained. This had been confirmed by
correlative experiments, especially in reaction 1: compound 3a–d gave
higher yields than others. If the electronic nature of two substitutents were
uniform, the EWG one (3e–g) showed better catalytic activity than EDG
(3d). From the comparability in structure between 2,4,6-triaryl-1,4-dihydro-
pyridines and 2,4,6-triarylpyranium, we supposed that this reaction might
also be rationalized as a photo-electron-transform (PET)-catalyzed a synchro-
nous cation radical Diels–Alder reaction. The catalytic activity of compound
3 may be contribute to its long conjugated system and nonplanar framework,
both of which are propitious to electron transfer from diene to dienophile.[23]
Further experiments must be carried out to explain it.
EXPERIMENTAL
Melting points were measured with a Fisher-Johns melting-point apparatus
without correction. IR spectra were recorded on a Nicolet Nexus 670
Table 3. Catalyzed function of 3 on two simple Diels–Alder reactions
Catalyst
Time (min)
Yield (%)
Catalyst
Time (min)
Yield (%)
—
3a
3b
3c
3d
3e
3f
60
60
55
53
54
50
52
56
44 (44)[24]
68.6
69.4
—
3a
3b
3c
3d
3e
3f
156
154
153
149
149
152
150
150
75.1
88.7
85.5
85.2
80.2
83.3
81.6
82.1
70.1
50.8
52.3
58.6
3g
59.3
3g

670
H. Wu et al.
spectrometer in KBr. The proton nuclear magnetic resonance (1H NMR)
spectra were measured on a Bruker AM-400 spectrometer with Me4Si
(TMS) as the internal reference and DMSO-d6 as solvent. Elemental
analyses were performed by a Carlo Erba 1106 elemental analysis meter
and a Foss-Heraeus Vario EL instrument. Mass spectra were determined on
a Finnigan 8230 mass spectrometer. Ionic liquids were synthesized
according to the literature.[18,19]
General Procedure
A dry 50-mL flask was charged with aromatic aldehydes (1) (1 mmol),
2-hydroxylacetophenone (2) (2 mmol), NH4OAc (5 mmol), and ionic liquid
(2 mL). The mixture was stirred at 808C for 20–24 h and then was poured
into water (40 mL). The precipitate was washed with water two or three
times and purified by column chromatography to give 2,4,6-triaryl-1,4-dihy-
dropyridines (3). Diels–Alder reactions were run according to the literature
method.[24]
Data
3a: 4-(4-Cyanophenyl)-2,6-bi(2-hydroxyphenyl)-1,4-dihydropyridine
IR (KBr, n, cm21): 3350, 3101, 2230, 1597, 1561, 1525, 1416, 1224, 847, 750,
1
636; H NMR (400 MHz, DMSO-d6) (d, ppm): 8.81 (2H, s, pyridine-H),
8.60–8.65 (4H, m, Ar-H), 8.36 (1H, d, J ¼ 8.0 Hz, Ar-H), 8.10 (4H, d,
J ¼8.4 Hz, Ar-H), 7.85 (1H, s, NH), 7.47 (1H, t, J ¼ 8.4 Hz, Ar-H), 7.05–
7.10 (2H, m, Ar-H), 6.25 (1H, s, NH), 4.41–4.44 (1H, m, CH). EI-MS (m/
z): 366.1 [M]þ. C24H18N2O2 found (%): C, 78.58; H, 4.85; N, 7.60; calcd.
(%): C, 78.67; H, 4.95; N, 7.65.
3b: 4-(4-Chlorophenyl)-2,6-bi(2-hydroxyphenyl)-1,4-dihydropyridine
IR (KBr, n, cm21): 3390, 3028, 2680, 1606, 1541, 1494, 1486, 1411, 1295,
1
1213, 1093, 982, 831, 756; H NMR (400 MHz, DMSO-d6) (d, ppm): 8.13
(2H, d, J ¼ 7.6 Hz, Ar-H), 7.87 (2H, d, J ¼ 8.0 Hz, Ar-H), 7.80 (2H, s,
3
4
pyridine-H), 7.41 (2H, dd, J ¼ 8.4 Hz, J ¼ 1.6 Hz, Ar-H), 7.33 (1H, t,
J ¼ 8.0 Hz, Ar-H), 7.23 (1H, d, J ¼ 8.0 Hz, Ar-H), 7.19 (1H, d, J ¼ 8.0
Hz, Ar-H), 7.14 (1H, d, J ¼ 8.0 Hz, Ar-H), 7.08 (1H, d, J ¼ 8.4 Hz, Ar-H),
6.98 (1H, t, J ¼ 7.2 Hz, Ar-H), 6.90 (1H, d, J ¼ 8.4 Hz, Ar-H), 6.32 (1H, s,
NH), 4.42–4.45 (1H, m, CH). EI-MS (m/z): 375.1 [M]þ. C23H18ClNO2
found (%): C, 73.58; H, 4.85; N, 3.70; calcd. (%): C, 73.50; H, 4.83; N, 3.73.

2,4,6-Triaryl-1,4-dihydropyridines
671
3c: 4-(4-Bromophenyl)-2,6-bi(2-hydroxyphenyl)-1,4-dihydropyridine
IR (KBr, n, cm21): 3350, 3301, 3179, 3020, 1600, 1540, 1490, 1407, 1212,
982, 829, 756; 1H NMR (400 MHz, DMSO-d6) (d, ppm): 8.04 (2H, s,
3
4
pyridine-H), 7.94 (1H, dd, J ¼ 7.6 Hz, J ¼ 1.2 Hz, Ar-H), 7.54 (2H, d,
J ¼ 8.4 Hz, Ar-H), 7.49 (2H, d, J ¼ 8.4 Hz, Ar-H), 7.45 (1H, d, J ¼ 8.0
Hz, Ar-H), 7.35 (2H, d, J ¼ 8.4 Hz, Ar-H), 7.14 (2H, d, J ¼ 8.4 Hz, Ar-H),
7.02 (2H, t, J ¼ 8.0 Hz, Ar-H), 6.95 (1H, d, J ¼ 8.4 Hz, Ar-H), 6.52 (1H, s,
NH), 4.40–4.43 (1H, m, CH). EI-MS (m/z): 419.1 [M]þ; C23H18BrNO2
found (%): C, 65.58; H, 4.35; N, 3.40; calcd. (%): C, 65.73; H, 4.32; N, 3.33.
3d: 4-(2-Methoxylphenyl)-2,6-bi(2-hydroxyphenyl)-1,4-
dihydropyridine
IR (KBr, n, cm21): 3360, 3347, 2570, 1600, 1532, 1426, 1308, 1072, 967, 852,
773; 1H NMR (400 MHz, DMSO-d6) (d, ppm): 11.09 (1H, s, OH), 7.96 (2H, s,
pyridine-H), 7.88 (1H, d, J ¼ 8.0 Hz, Ar-H), 7.65 (1H, d, J ¼ 8.0 Hz, Ar-H),
7.12–7.16 (1H, m, Ar-H), 6.84–6.93 (2H, m, Ar-H), 6.71 (2H, t, J ¼ 8.0 Hz,
Ar-H), 6.02 (1H, s, NH), 3.99–4.02 (1H, m, CH), 3.48 (3H, s, OCH3). EI-MS
(m/z): 371.2 [M]þ C24H21NO3 found (%): C, 77.57; H, 5.80; N, 3.70; calcd.
(%): C, 77.61; H, 5.70; N, 3.77.
3e: 4-(2-Chlorophenyl)-2,6-bi(4-nitrophenyl)-1,4-dihydropyridine
IR (KBr, n, cm21): 3348, 3306, 3106, 2807, 1642, 1598, 1513, 1350, 1103,
863, 756, 693; 1H NMR (400 MHz, DMSO-d6) (d, ppm): 8.34 (2H, s,
pyridine-H), 8.29 (2H, d, J ¼ 8.8 Hz, Ar-H), 8.16 (1H, d, J ¼ 8.4 Hz, Ar-
3
4
H), 8.10 (2H, d, J ¼ 9.2 Hz, Ar-H), 7.63 (1H, dd, J ¼ 7.6 Hz, J ¼ 1.6 Hz,
3
4
Ar-H), 7.53 (2H, dd, J ¼ 8.0 Hz, J ¼ 2.0 Hz, Ar-H), 7.44–7.47 (2H, m,
3
4
Ar-H), 7.36 (2H, dd, J ¼ 8.8 Hz, J ¼ 1.6 Hz, Ar-H), 6.23 (1H, s, NH),
4.39–4.42 (1H, m, CH). EI-MS (m/z): 433.2 [M]þ. C23H16ClN3O4 found
(%): C, 63.57; H, 3.80; N, 9.70; calcd. (%): C, 63.67; H, 3.72; N, 9.69.
3f: 4-(4-Chlorophenyl)-2,6-bi(3-nitrophenyl)-1,4-dihydropyridine
IR (KBr, n, cm21): 3448, 3092, 2740, 2373, 1602, 1519, 1355, 883, 729, 674;
1H NMR (400 MHz, DMSO-d6) (d, ppm): 9.03 (2H, s, pyridine-H), 8.64 (1H,
d, J ¼ 8.0 Hz, Ar-H), 8.49 (2H, dd, 3J ¼ 8.0 Hz, 4J ¼ 1.6 Hz, Ar-H), 8.38 (2H,
d, J ¼ 8.0 Hz, Ar-H), 7.76 (2H, t, J ¼ 8.0 Hz, Ar-H), 7.65 (1H, d, J ¼ 8.4 Hz,
Ar-H), 6.24 (1H, s, NH), 4.52–6.57 (1H, m, CH). EI-MS (m/z): 433.2 [M]þ.
C23H16ClN3O4 found (%): C, 63.52; H, 3.70; N, 9.73; calcd. (%): C, 63.67; H,
3.72; N, 9.69.

672
H. Wu et al.
3g: 4-(4-Bromophenyl)-2,6-bi(3-nitrophenyl)-1,4-dihydropyridine
1
IR (KBr, n, cm21): 3435, 3048, 2623, 2284, 1600, 1525, 926, 874, 693; H
NMR (400 MHz, DMSO-d6) (d, ppm): 9.03 (2H, s, pyridine-H), 8.61 (1H,
d, J ¼ 8.0 Hz, Ar-H), 8.48 (2H, d, J ¼ 8.0 Hz, Ar-H), 8.38 (2H, d, J ¼ 7.6
Hz, Ar-H), 7.75 (2H, t, J ¼ 8.0 Hz, Ar-H), 7.69 (1H, d, J ¼ 8.0 Hz, Ar-H),
3
4
7.62 (2H, dd, J ¼ 8.4 Hz, J ¼ 1.6 Hz, Ar-H), 6.25 (1H, s, NH), 4.54–6.57
(1H, m, CH). EI-MS (m/z): 477.2 [M]þ. C23H16BrN3O4 found (%): C,
57.66; H, 3.30; N, 8.75; calcd. (%): C, 57.76; H, 3.37; N, 8.79.
CONCLUSION
Overall, we have demonstrated a versatile new approach to synthesize a range
of 1,4-dihydropyridines bearing aryl groups on the 2, 4, and 6 positions. The
significance of our finding also relates to reducing the use of organic solvents
as potentially toxic and hazardous materials, as well as its simplicity, mild
conditions, and inherent lower costs. Moreover, the findings give further
credence to the synthesis of activated natural products and analogous
natural products. This kind of compound is first used as an effective catalyst
on simple Diels–Alder reactions.
ACKNOWLEDGMENT
Financial support from the Natural Sciences Foundation in Jiangsu Province
(No. BK2007028), Surpassing Project in Jiangsu Province (No. QL200607),
and the Postdoctoral Foundation of Xuzhou Normal University (No.
2003009) is gratefully acknowledged.
REFERENCES
1. Liang, J. C.; Yeh, J. L.; Wang, C. S.; Liou, S. F.; Tsai, C. H.; Chen, I. J. Bioorg.
Med. Chem. 2002, 10 (3), 719–730.
2. Tasaka, S.; Ohmori, H.; Gomi, N.; Iion, M.; Machida, T.; Kiue, A.; Naito, S.;
Kuwano, M. Bioorg. Med. Chem. Lett. 2001, 11 (2), 275–277.
3. Mikhail, F. G.; Dinesh, V. P.; Bruce, P. E.; Supriya, J.; Jesse, D. C.; Eric, M. G.
Bioorg. Med. Chem. Lett. 1998, 6 (7), 883–889.
4. Tanabe, H.; Tasaka, S.; Ohmori, H.; Gomi, N.; Sasaki, Y.; Machida, T.; Iion, M.;
Kiue, A.; Naito, S.; Kuwano, M. Bioorg. Med. Chem. 1998, 6 (11), 2219–2227.
5. Kiselyov, A. S. Tetrahedron Lett. 1995, 36 (51), 9297–9300.
6. Eisner, U.; Elvidge, J. A.; Linstead, R. P. J. Chem. Soc. 1953, 1372–1379.
7. Kamal, A.; Qureshi, A. Tetrahedron 1963, 19, 513–520.
8. Wiley, R.; Ridgway, J. S. J. Org. Chem. 1961, 26 (2), 595–597.
9. Biellmann, J. F.; Callot, H. J.; Goeldner, M. P. Chem. Commun. 1969, 141–142.

2,4,6-Triaryl-1,4-dihydropyridines
673
10. (a) Dondoni, A.; Massi, A.; Minghini, E. Synlett 2002, 2002 (1), 89–92;
(b) Gevariya, H.; Desai, B.; Vora, V.; Shah, A. Heterocyclic Commun. 2001,
7 (5), 481–484; (c) Sobolev, A.; Franssen, M. C. R.; Vigante, B. Tetrahedron:
Asymmetry 2001, 12 (23), 3251–3256; (d) Yamada, S.; Misono, T.;
Ichikawa, M.; Morita, C. Tetrahedron 2001, 57 (43), 8939–8949; (e) Obika, S.;
Nishiyama, T.; Tatematsu, S. Tetrahedron 1997, 53 (9), 3073–3082;
´
(f) Caballero, E.; Puebla, P.; Sanchez, M. Tetrahedron: Asymmetry 1996, 7 (7),
1985–1994.
11. Eynde, J. J. V.; Dorazio, P.; Mayence, A. Tetrahedron 1992, 48 (7), 1263–1268.
12. Evdokimov, N. M.; Magedov, I. V.; Kireev, A. S. Organic Letters 2006, 8 (5),
899–902.
13. Breitenbucher, J. G.; Figliozzi, G. Tetrahedron Lett. 2000, 41 (22), 4311–4315.
14. Alajarin, R.; Jordan, P.; Vaquero, J. J.; Alvarez-Builla, J. Synthesis 1995, 4,
389–391.
15. Chen, W. M.; Guo, H. X. Hecheng Huaxue 1999, 7, 422–426.
16. Kai, T.; Nicole, D.; Jacques, P. R. Org. Lett. 2004, 6, 4595–4597.
17. Vincenzo, C.; Angelo, N.; Li, L. G. Tetrahedron Lett. 2000, 41, 8973–8976.
18. Lee, C. W. Tetrahedron Lett. 1999, 40, 2461–2464.
19. Hagiwara, H.; Sayuri, T.; Okabe, T.; et al. Green Chem. 2002, 4 (5), 461–463.
20. Deng, Y. Q.; Shi, F.; Peng, J. J.; et al. Mol. Catal. A: Chem. 2001, 165 (1), 33–36.
21. Wu, H.; Lu, L. L.; Shen, Y.; Wan, Y.; Yu, K. B. Synth. Commun. 2006, 36,
1193–1200.
22. Zhang, W.; Jia, X. D.; Yang, L.; Liu, Z. L. Tetrahedron Lett. 2002, 43, 9433–9436.
23. Singh, S.; Sharma, V. K.; Gill, S.; Sahoto, R. I. K. J.Chem. Soc., Perkin Trans. 1
1985, 437.
24. Zeng, S. Q.; Zeng, H. P. Experiment of Organic Chemistry, 3rd Edn.; Publishing
Company of Advance Education: Beijing, 2000, Chapter 3, pp. 140–142.