A novel one-pot synthesis of 2-alkylthio-1,3,4-oxadiazoles in water

Article abstract of DOI:10.1007/BF03249086

A facile and one-pot protocol for the synthesis of 2-alkylthio-1,3,4- oxadiazoles is reported. This green method relies on the reaction of acid hydrazides with CS² and an alkyl halide. The reaction is carried out under mild and environmentally

Full text of DOI:10.1007/BF03249086

J. Iran. Chem. Soc., Vol. 8, No. 2, June 2011, pp. 525-530.
JOURNAL OF THE
Iranian
Chemical Society
A Novel One-Pot Synthesis of 2-Alkylthio-1,3,4-oxadiazoles in Water
F. Aryanasab, H. Maleki and M.R. Saidi*
Department of Chemistry, Sharif University of Technology, Tehran, Iran
(Received 4 August 2010, Accepted 15 October 2010)
A facile and one-pot protocol for the synthesis of 2-alkylthio-1,3,4-oxadiazoles is reported. This green method relies on the
reaction of acid hydrazides with CS2 and an alkyl halide. The reaction is carried out under mild and environmentally friendly
procedure in water with high to excellent yields. Thirteen different valuable alkylthio-1,3,4-oxadiazoles are synthesized from
cheap and easily available CS2 with this method. This is the first report for the synthesis of 1,3,4-oxadiazoles in water.
Keywords: Alkyl-2-thio-1,3,4-oxadiazoles, Hydrazids, Carbon disulfide, Alkyl halides, Water
INTRODUCTION
Several methods have been reported for the synthesis of
1,3,4-oxadiazoles in the literature [23-31]. Most of these
methods are multi-step and generally involve cyclization of
acid hydrazides with a variety of anhydrous reagents such as
thionyl chloride [32,33], phosphorous oxychloride [34] and
sulfuric acid [35], usually under harsh reaction conditions.
Recently, some efficient techniques have been reported for the
synthesis of 1,3,4-oxadiazoles, especially from readily
available carboxylic acids and acid hydrazides [36-38].
However, these protocols made use of expensive catalysts, and
required long reaction time for the completion of the reactions
[39,40].
The development of efficient and environmentally friendly
chemical processes for the preparation of new biologically
active molecules poses a major challenge in organic synthesis
[41,42]. Organic reactions in water without using harmful
organic solvents have attracted a great deal of interest in both
academic research and industry. In addition to the
environmental concerns, there are beneficial effects of
aqueous medium on the rate and selectivity of important
organic transformations [44-52] which must be taken into
account.
1,3,4-Oxadiazoles are a class of heterocycles which have
attracted significant interest in medicinal chemistry. They have
demonstrated a broad spectrum of biological properties in both
pharmaceutical and agrochemical fields such as antibacterial,
anti-inflammatory, antimitotic, antiarrhythmic and insecticidal
activities [1-8].
The widespread use of 1,3,4-oxadiazoles as a scaffold in
medicinal chemistry renders this moiety as an important
bioactive class of heterocycles. These molecules are also
utilized as pharmacophores due to their favorable metabolic
profile and ability to engage in hydrogen bonding [9-13].
In particular, marketed antihypertensive agents such as
tiodazosin [14] and nesapidil [15], as well as antibiotics such
as furamizole [16] contain the oxadiazole nucleus. They are
also useful as HIV integrase inhibitors and the angiogenesis
inhibitors [16-17]. Moreover, 1,3,4-oxadiazole derivatives are
among the most widely employed electron-transporting and
hole-blocking materials in the development of organic light-
emitting diodes (OLEDs), which are used in energy efficient,
full-color and fiat-panel displays [18-22].
Sequential transformations, multi-components, and one-pot
*Corresponding author. E-mail: saidi@sharif.edu
reactions
have
always been more effective and

Aryanasab et al.
environmentally acceptable, thus greener as compared to
2-(Butylthio)-5-phenyl-1,3,4-oxadiazole (2b) [13]. 1H
multi-steps reactions [53-56]. They offer significant
advantages over the conventional linear step syntheses, by
reducing time, saving money, energy and raw-materials. At
the same time, diversity can be achieved for building up
libraries by simply changing each component [57,58].
NMR (500 MHz, CDCl3) δ:, 8.03 (m, 2H), 7.50 (m, 3H), 3.30

(t, 2H), 1.80 (m, 2H), 1.50 (m, 2H), 1.00 (t, 3H). 13C NMR
(125 MHz, CDCl3) δ, 165.9, 164.7, 131.8, 129.3, 127.1, 124.3,
32.7, 31.7, 22.2, 14.0.
2-(Benzylthio)-5-phenyl-1,3,4-oxadiazole (2c) [43].
1H NMR (500 MHz, CDCl3): δ, 8.02 (m, 2H), 7.51 (m, 4H),
7.34 (m, 4H), 4.56 (s, 2H). 13C NMR (125 MHz, CDCl3): δ,
166.1, 164.1, 136.0, 131.9, 129.4, 129.2, 128.8, 128.5, 127.1,
124.2, 37.2. MS [m/z (%)] 268 (M+, 47.0), 132 (81.6), 107
(55.0), 91 (100).
EXPERIMENTAL
General
NMR spectra were recorded on a 500 MHz NMR
spectrometer using CDCl3 as the solvent. Chemical shifts
have been expressed in (ppm). Water and other solvents
were distilled before use. Silica gel was used for column
chromatography, diluted with petroleum ether and ethyl
acetate. Alkyl halides, CS2, and other chemicals were
purchased from Merck, and used without further
purification.
2-Methylmercapto-5-(2-hydroxyphenyl)-1,3,4-oxa-
1
diazole (2d) [27]. H NMR (500 MHz, CDCl3): δ, 9.86 (s,
1H), 7.68 (m, 1H), 7.43 (m, 1H), 7.10 (m, 1H), 6.96 (m, 1H),
2.81 (s, 3H). 13C NMR (125 MHz, CDCl3): δ, 165.6, 164.6,
157.7, 133.8, 126.7, 120.1, 117.9, 108.3, 14.6.
2-Butylmercapto-5-(2-hydroxyphenyl)-1,3,4-oxadiazole
1
(2e) [65-67]. H NMR (500 MHz, CDCl3): δ, 9.90 (s, 1H),
7.67 (dd, 1H), 7.40 (dt, 1H), 7.10 (dd, 1H), 6.96 (dt, 1H), 3.35
(t, 2H), 1.80 (m, 2H), 1.50 (m, 2H), 1.00 (t, 3H). 13C NMR
(125 MHz, CDCl3): δ, 165.4, 164.1, 157.7, 133.8, 126.7,
120.1, 117.9, 108.4, 32.7, 31.7, 22.2, 13.92. MS [m/z (%)] 250
(M+, 42.7), 203 (25.5), 161 (27.0), 121 (100).
General Procedure for the One-Pot Preparation of 5-
Aryl-2-thio-1,3,4-oxadiazoles in Water
Acid hydrazide (1 mmol), CS2 (3 mmol) and triethylamine
(1.5 mmol) were combined in water (5 ml) in a 25 ml round-
bottom flask. The mixture was stirred at room temperature for
1 h. Then alkyl halide (1.2 mmol) was added and the mixture
was stirred for additional 6 h. Then the mixture was heated in
a 90 °C oil bath to bring the mixture to reflux for 8 h, and then
was cooled to room temperature. EtOAc (10 ml) and 1 N HCl
(10 ml) were added, and the mixture was vigorously stirred for
5 min. The aqueous layer was removed and extracted with
EtOAc (10 ml) and the combined organic layers were dried
over anhydrous Na2SO4. The solvent was removed under
reduced pressure to give the desired product. The crude
product was purified by flash column chromatography and in
some cases they were crystallized from ethanol to provide the
pure product. All the compounds were characterized on the
basis of their spectroscopic data.
2-Allylmercapto-5-(2-hydroxyphenyl)-1,3,4-oxadiazole
1
(2f) [13]. H NMR (500 MHz, CDCl3): δ, 9.87 (s, 1H), 7.67
(m, 1H), 7.42 (m, 1H), 7.09 (m, 1H), 6.97 (m, 1H), 5.98 (m,
1H), 5.43 (dd, 1H), 5.27 (dd, 1H), 3.95 (d, 2H). 13C NMR (125
MHz, CDCl3): δ, 165.6, 163.2, 157.7, 133.9, 131.8, 126.7,
121.3, 120.8, 118.0, 108.3, 35.6. MS [m/z (%)] 234 (M+,
90.9), 161 (9.1), 121 (63.6), 119 (100), 82 (45.5).
2-Benzylmercapto-5-(2-hydroxyphenyl)-1,3,4-oxa-
1
diazole (2g) [13]. H NMR (500 MHz, CDCl3): δ, 9.89 (s,
1H), 7.68 (dd, 1H), 7.33-7.48 (m, 6H), 7.12 (m, 1H), 6.98 (m,
1H), 4.56 (s, 2H). 13C NMR (125 MHz; CDCl3): δ, 165.6,
163.44, 155.8, 135.7, 133.9, 129.5, 129.3, 128.6, 126.7, 120.1,
118.0 108.3, 37.3. MS [m/z (%)] 284 (M+, 30), 121 (43.3), 91
(100).
2-(Methylthio)-5-(p-tolyl)-1,3,4-oxadiazole (2h) [13].
Spectroscopic Data
1H NMR (500 MHz, CDCl3)
δ:, 7.90 (d, 2H), 7.20 (d, 2H),

2-(Ethylthio)-5-phenyl-1,3,4-oxadiazole
1H NMR (500 MHz, CDCl3):
δ, 7.95 (m, 2H), 7.45 (m, 3H),
3.3 (q, 2H), 1.5 (t, 3H). 13C NMR (125 MHz, CDCl3) δ
165.9,
(2a)
[27].
3.85 (s, 3H), 2.40 (s, 3H). 13C NMR (125 MHz, CDCl3): δ,
167.1, 143.6, 130.0, 129.3, 127.9, 52.1, 22.0.


2-(Ethylthio)-5-(p-tolyl)-1,3,4-oxadiazole (2i) [13]. 1H
NMR (500 MHz, CDCl3): δ, 7.90 (d, 2H), 7.20 (d, 2H), 3.33
164.5, 131.8, 129.3, 127.0, 124.2, 27.3, 15.2.
526

A Novel One-Pot Synthesis of 2-Alkylthio-1,3,4 Oxadiazoles in Water
(q, 2H), 2.42 (s, 3H), 1.52 (t, 3H). 13C NMR (125 MHz;
CDCl3): δ, 166.0, 164.0, 142.1, 130.0, 127.0, 121.5, 27.3,
22.0, 15.2.
O
N
N
Et3N, H2O
NH2
+ CS2
+ EtI
Ar
N
H
Ar
SR
R = ethyl
rt, then reflux
O
2a
1a
2-(Butylthio)-5-(p-tolyl)-1,3,4-oxadiazole
(2j)
[13].
Scheme 1. Reaction of benzohydrazide with ethyl iodide and
1H NMR (500 MHz, CDCl3)

: ,δ7.86 (d, 2H), 7.60 (d, 2H),
CS2
3.2 (t, 2H), 2.4 (s, 3H), 1.8 (m, 2H), 1.5 (m, 2H), 0.90 (t, 3H).
13C NMR (125 MHz, CDCl3): δ, 166.0, 164.3, 142.2, 130.1,
130.0, 127.0, 126.7, 121.5, 32.6, 31.7, 22.5, 22.1, 14.0. MS
[m/z (%)] 248 (M+), 201, 159, 132, 91 (100).
O
O
Et3N, H2O
H
N
S
NH2
+ CS2
+ EtI
Ar
N
H
Ar
N
H
rt, then reflux
2-(Methylthio)-5-(4-pyridyl)-1,3,4-oxadiazole
(2k).
S
1a
3a
1H NMR (500 MHz, CDCl3)
δ:, 8.80 (d, 2H), 7.80 (d, 2H),

2.82 (s, 3H). 13C NMR (125 MHz. CDCl3); δ, 166.8, 164.3,
H
N
-H2S
N
N
N
151.2, 131.1, 120.3, 15.0.
SH
Ar
S
2-(Methylthio)-5-(4-pyridyl)-1,3,4-oxadiazole
(2l).
Ar
S
O
O
1H NMR (500 MHz, CDCl3)
δ:, 8.80 (d, 2H), 7.80 (d, 2H),

4a
2a
3.26 (q, 2H), 1.29 (t, 3H). 13C NMR (125 MHz; CDCl3): δ,
Scheme 2. Proposed mechanism for the synthesis of 1,3,4-
166.81, 164.26, 151.18, 131.11, 120.33, 46.57, 15.02.
oxadiazoles
2-(Buthylthio)-5-(4-pyridyl)-1,3,4-oxadiazole
(2m).
1H NMR (500 MHz, CDCl3): δ, 8.79 (d, 2H), 7.85 (d, 2H),
3.30 (t, 2H), 1.8 (m, 2H), 1.5 (m, 2H), 0.90 (t, 3H). 13C NMR
(125 MHz, CDCl3): δ, 166.5, 164.0, 150.9, 131.3, 120.4, 32.7,
31.6, 22.5, 14.1. MS [m/z (%)] 235 (M+, 19.0), 188 (100), 106
(75.0), 78 (100).
CS2 and ethyl iodide in the presence of an equimolecular
amount of triethylamine at room temperature, and then at
reflux temperature, gave 1,3,4-oxadiazoles 2a in 98% yield
(Scheme 1).
We studied the scope and limitations of this protocol with
respect to the acid hydrazides and alkyl halides. This
procedure was quite general and a wide range of acid
hydrazides and alkyl halides were used which afforded
excellent yields (Table 1). This two-step, one-pot procedure
provides an access to a variety of 2-thio-1,3,4-oxadiazoles in
an efficient manner.
In addition to benzohydrazide (Table 1, entries 1-3), other
heterocyclic hydrazides such as salicylic hydrazide (entries 4-
7), 4-methylbenzohydrazide (entries 8-10) and 4-pyridyl
hydrazide (entries 11-13) reacted efficiently to afford the
corresponding 2-thio-1,3,4-oxadiazoles in good yields.
We have proposed a mechanism for the reaction in
question on the basis of the formation of acyldithiocarbazate
3a, which then undergoes cyclization to furnish target
molecule 2a (Scheme 2). Similarly, this type of
transformations have been reported in the literature using
thiosemicarbazides for the synthesis of 2-amino-1,3,4-
oxadiazoles [28]. This implies that the reaction might be a
combination of two processes. First, the reaction of acid
RESULTS AND DISCUTION
In our previous works, we reported a green, one-pot and
simple method for the synthesis of dithiocarbamates from
amines, CS2 and different nucleophile acceptors such as alkyl
halides, activated olefins and epoxides, under solvent-free and
under aqueous conditions [59-63].
In continuation of our research to find new, green and
efficient methods for the synthesis of novel dithiocarbamate
derivatives, herein we describe a novel one-pot synthesis of 5-
aryl-2-thio-1,3,4-oxadiazoles in water. 5-Aryl-2-thio-1,3,4-
oxadiazoles exert anti-tubercular effect against mycobacterium
tuberculosis. Tuberculosis (TB) is one of the major causes of
death and suffering worldwide among the infectious diseases
[64]. To the best of our knowledge, this is the first report
regarding the synthesis of this class of heterocycles in water.
In our initial investigation, we used acid hydrazides instead
of amines and carbon disulfide with various alkyl halides. The
treatment of a suspension of benzohydrazide 1a in water with
527

Aryanasab et al.
Table 1. Synthesis of 1,3,4-Oxadiazoles in Water
Entry Acid Hydrazide Alkyl Halide
Product
Yield%
98
N
N
CH3CH2I
1
Ph
Ph
S
S
O
O
2a
N
N
N
O
n-BuBr
90
2
3
NH2
N
H
2b
N
PhCH2Br
90
Ph
Ph
S
O
2c
N
N
CH3I
95
4
S
O
N
2d
OH
N
O
n-BuBr
92
86
5
6
S
NH2
O
N
2e
H
OH
N
OH
N
Br
S
O
N
2f
OH
N
Ph
7
8
85
95
PhCH2Br
S
O
2g
OH
N
N
CH3I
S
O
O
2h
NH2
N
N
N
H
9
CH3CH2I
97
94
S
O
2i
N
N
N
10
n-BuBr
S
S
O
O
2j
N
CH3I
O
11
97
NH2
2k
N
N
H
N
N
N
N
CH3CH2I
n-BuBr
12
13
95
90
S
S
O
O
2l
N
N
N
2
528

A Novel One-Pot Synthesis of 2-Alkylthio-1,3,4 Oxadiazoles in Water
hydrazide with CS2 and an alkyl halide yields intermediate 3a,
3289.
followed by a cyclization of 3a to produce 4a. By the H2S
elimination, 2-thio-1,3,4 oxadiazole 2a is formed.
[16] M. Ogata, H. Atobe, H. Kushida, K.J. Yamamoto,
Antibiot 24 (1971) 443.
[17] A.A. El-Emam, O.A. Al-Deeb, M. Al-Omar, J.
Lehmann, Bioorg. Medicinal Chem. 12 (2004) 5107.
[18] M. Guan, Z.Q. Bian, Y.F. Zhou, F.Y. Li, Z.J. Li, C.H.
Huang, Chem. Commun. (2003) 2708.
[19] Q. Pei, Y. Yang, Chem. Mater. 7 (1995) 1568.
[20] C. Wang, G.Y. Jung, Y. Hua, C. Pearson, M.R. Bryce,
M.C. Petty, A.S. Batsanov, A.E. Goeta, J.A.K. Howard,
Chem. Mater. 13 (2001) 1167.
ACKNOWLEDGEMENTS
Financial support of this work by Sharif University of
Technology is gratefully acknowledged.
REFERENCES
[1]
[2]
B.S. Holla, R. Gonsalves, S. Shenoy, Eur. J. Med.
Chem. 35 (2000) 267.
P. Brown, D.J. Best, N.J.P. Broom, R. Cassels, P.J.
O’Hanlon, T.J. Mitchell, N.F. Osborne, J.M. Wilson, J.
Med. Chem. 40 (1997) 2563.
[21] J. Wang, R. Wang, J. Yang, Z. Zheng, M.D. Carducci,
T. Cayou, N. Peyghambarian, G.E. Jabbour, J. Am.
Chem. Soc. 123 (2001) 6179.
[22] Y. Li, J. Liu, H. Zhang, X. Yang, Z. Liu, Medicinal
Chem. Lett. 16 (2006) 2278.
[3]
[4]
E. Palaska, G. Sahin, P. Kelicen, N.T. Durlu, G.
Altintok, Il Farmaco 57 (2002) 101.
K.M. Lokanatha Rai, N. Linganna, Il Farmaco 55
(2000) 389.
[23] I.R. Baxendale, S.V. Ley, M. Martinelli, Tetrahedron
61 (2005) 5323.
[24] B.J. Brown, I.R. Clemens, J.K. Neesom, Synlett (2000)
131.
[5]
[6]
G.W. Adelstein, J. Med. Chem. 16 (1973) 309.
X. Zheng, Z. Li, Y. Wang, W. Chen, Q. Huang, C. Liu,
G.J. Song, Fluorine Chem. 123 (2003) 163.
C.H. Lee, H.I. Cho, K.J. Lee, Bull. Korean Chem. Soc.
22 (2001) 1153.
V. Polshettiwar, R.S. Varma, Tetrahedron Lett. 49
(2008) 879.
D. Leung, W. Du, C. Hardouin, H. Cheng, I. Hwang,
B.F. Cravatt, D.L. Boger, Bioorg. Med. Chem. Lett. 15
(2005) 1423.
[25] F.T. Coppo, K.A. Evans, T.L. Graybill, G. Burton,
Tetrahedron Lett. 45 (2004) 3257.
[26] C.T. Brain, J.M. Paul, Y. Loong, P.J. Oakley,
Tetrahedron Lett. 40 (1999) 3275.
[27] C.T. Brain, S.A. Brunton, Synlett (2001) 382.
[28] V.K. Tandon, R.B. Chhor, Synth. Commun. 31 (2001)
1727.
[29] S.H. Mashraqui, S.G. Ghadigaonkar, R.S. Kenny,
Synth. Commun. 33 (2003) 2541.
[7]
[8]
[9]
[30] F. Bentiss, M. Lagrenee, D. Barbry, Synth. Commun.
31 (2001) 935.
[10] C. Chen, C.H. Senanayake, T.J. Bill, R.D. Larsen, T.R.
Verhoeven, P.J. Reider, J. Org. Chem. 59 (1994) 3738.
[11] H. Chen, Z. Li, Y. Han, J. Agric. Food Chem. 48 (2000)
5312;.
[12] X. Zou, L. Lai, G. Jin, Z. Zhang, J. Agric. Food Chem.
50 (2002) 3757.
[13] F. Macaev, G. Rusu, S. Pogrebnoi, A. Gudima, E.
Stingaci, L. Vlad, N. Shvets, F. Kandemirli, A.
Dimogloa, R. Reynoldsd, Bioorg. Medicinal Chem. 13
(2005) 484.
[14] S. Vardan, S. Mookherjee, R. Eich, Clin. Pharm. Ther.
34 (1983) 290.
[31] M. Dabiri, P. Salehi, M. Baghbanzadeh, M.
Bahramnejad, Tetrahedron Lett. 47 (2006) 6983.
[32] V.L. Pachhamia, A.R. Parekh, J. Ind. Chem. Soc. 66
(1989) 250. (ca. 1989 Chem. Abstr. 112:7434).
[33] M. Al-Talib, H. Tastoush, N. Odeh, Synth. Commun.
20 (1990) 1811.
[34] A.B. Theocharis, N.E. Alexandrou, J. Heterocycl.
Chem. 27 (1990) 1685.
[35] F.W. Short, L.N. Long, J. Heterocycl. Chem. 6 (1969)
707.
[36] Y. Wang, D.R. Sauer, S.W. Djuric, Tetrahedron Lett.
47 (2006) 105.
[15] R. Schlecker, P.C. Thieme, Tetrahedron 44 (1988)
529

Aryanasab et al.
[37] A. Souldozi, A. Ramazani, Tetrahedron Lett. 48 (2007)
Int. Ed. 39 (2000) 3168.
1549.
[54] N.K. Terret, M. Gardener, D.W. Gordon, R.J.
Kobylecki, J. Steele, Tetrahedron 51 (1995) 8135.
[55] L.A. Thomson, J.A. Ellman, Chem. Rev. 96 (1996) 555.
[56] J.A. Ellman, Acc. Chem. Res. 29 (1996) 132.
[57] A. Domling, E.I. Herdtweck, I. Ugi, Acta Chem. Scand.
52 (1998) 107.
[38] H.A. Rajapakse, H. Zhu, M.B. Young, B.T. Mott,
Tetrahedron Lett. 47 (2006) 4827.
[39] S.J. Dolman, F. Gosselin, P.D. O’Shea, I.W. Davies, J.
Org. Chem. 71 (2006) 9548.
[40] E.V. Zarudnitskii, I.I. Pervak, A.S. Merkulov, A.A.
Yurchenko, A.A. Tolmachev, Tetrahedron 64 (2008)
10431.
[58] S.S. Palimkar, R.J. Lahoti, K.V. Srinivasan, Green
Chem. 9 (2007) 146.
[41] R.T. Baker, W. Tumas, Science 284 (1999) 1477.
[42] P.T. Anastas, J.C. Warner, Green Chemistry: Theory
and Practice, Oxford University Press, Oxford, 1998.
[43] A.A. Atalla, A.M.K. El-Dean, A.M. Gaber, S.M.
Radwan, Phosphorus, Sulfur and Silicon and Related
Elements 88 (1994) 233.
[44] P.A. Grieco, Organic Synthesis in Water, Blackie
Academic & Professional, London, 1998.
[45] C.J. Li, T.H. Chan, Organic Reactions in Aqueous
Media, Wiley, New York, 1997.
[59] N. Azizi, F. Aryanasab, M.R. Saidi, Org. Lett. 8 (2006)
5275.
[60] N. Azizi, F. Aryanasab, L. Torkiyan, A. Ziyaei, M.R.
Saidi, J. Org. Chem. 71 (2006) 3634.
[61] A. Ziyaei, M.R. Saidi, Can. J. Chem. 84 (2006) 1515.
[62] N. Azizi, F. Ebrahimi, E. Akbari, F. Aryanasab, M.R.
Saidi, Synlett (2007) 2797.
[63] N. Azizi, B. Pourhasan, F. Aryanasab, M.R. Saidi,
Synlett (2007) 1239.
[64] F. Macaev, G. Rusu, S. Pogrebnoi, A. Gudima, E.
Stingaci, L. Vlad, N. Shvets, F. Kandemirli, A.
Dimoglo, R. Reynolds, Bioorg. Med. Chem. 13 (2005)
4842.
[46] C.J. Li, Chem. Rev. 105 (2005) 3095.
[47] S. Kobayashi, K. Manabe, Acc. Chem. Res. 35 (2002)
209.
[48] U.K. Lindstrom, Chem. Rev. 102 (2002) 2751.
[49] A. Lubineau, J. Auge, Y. Queneau, Synthesis (1994)
741.
[50] C.J. Li, Chem. Rev. 93 (1993) 2023.
[51] S. Shimizu, N. Shimada, Y. Sasaki, Green Chem. 8
(2006) 608.
[65] I.R. Baxendale, S.V. Ley, M. Martinelli, Tetrahedron
61 (2005) 5323.
[66] J.Y. Hwang, H.S. Choi, D.H. Lee, Y.D. Gong, J. Comb.
Chem. 7 (2005) 816.
[67] E.L.P. Chekler, H.M. Elokdah, J. Butera, Tetrahedron
Lett. 49 (2008) 6709.
[52] X. Deng, N.S. Mani, Green Chem. 8 (2006) 835.
[53] For a Review See: A. Domling, I. Ugi, Angew. Chem.
[68] L.M. Alam, G.I. Koldobskii, Rus. J. Org. Chem. 33
(1997) 1149.
530