Novel four-component approach for the synthesis of polyfunctionalized 1,4-dihydropyridines in aqueous media

Article abstract of DOI:10.1002/hlca.201000228

The four-component reaction of dimethyl acetylenedicarboxylate (=dimethyl but-2-ynedioate; DMAD), aromatic aldehydes, and malononitrile (= propanedinitrile) leads to polyfunctionalized 1,4-dihydropyridine derivatives. The reaction proceeds at room temperature and in the presence of a catalytic amount (20%) of (NH₄)₂HPO₄ as a base in aqueous media.

Full text of DOI:10.1002/hlca.201000228

382
Helvetica Chimica Acta – Vol. 94 (2011)
Novel Four-Component Approach for the Synthesis of Polyfunctionalized
1,4-Dihydropyridines in Aqueous Media
by Melika Hadjebia), Mehri S. Hashtroudib), Hamid Reza Bijanzadeh*c), and Saeed Balalaie*a)
a) Peptide Chemistry Research Center, K. N. Toosi University of Technology, P.O. Box 15875-4416
Tehran, Iran (e-mail: Balalaie@kntu.ac.ir)
b) Marine Chemistry Laboratory, Iranian National Center for Oceanography (INCO),
P.O. Box 14155-4781 Tehran, Iran
c) Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175 Tehran, Iran
(e-mail: HR_bijan@yahoo.com)
Dedicated to Prof. Hooshang Pirelahi on the occasion of his 70th birthday
The four-component reaction of dimethyl acetylenedicarboxylate (¼dimethyl but-2-ynedioate;
DMAD), aromatic aldehydes, and malononitrile (¼ propanedinitrile) leads to polyfunctionalized 1,4-
dihydropyridine derivatives. The reaction proceeds at room temperature and in the presence of a
catalytic amount (20%) of (NH4)2HPO4 as a base in aqueous media.
Introduction. – Construction of complex molecules by using a one-pot multi-
component reaction (MCR) is a significant pathway in organic synthesis. This type of
reaction becomes increasingly important in organic, bioorganic, and medicinal
chemistry for the synthesis of polyfunctionalized molecules. MCRs have been applied
to generate highly diverse combinatorial libraries with high bond-forming efficiency
(BFE). Thus, there is a strong demand for the development of new MCRs that allow to
prepare an assembly of polyfunctionalized heterocycles [1].
The core structure of N-substituted dihydropyridines (DHPs) is an important
heterocyclic framework that can be found in numerous biologically active compounds.
For example, some of these compounds are Ca2þ-channel blocker and show
antihypertensive effects [2]. A series of cage-dimeric N-substituted 1,4-dihydropyr-
idines was evaluated as inhibitors of membrane efflux pump in multidrug-resistant
cancer [2][3][4]. Thus, the synthesis of 1,4-dihydropyridines is an important and useful
task in organic chemistry. So far, many protocols for the synthesis of DHPs have been
reported [5]. A well-known approach is the Hantzsch reaction [4][6]. Cyclocondensa-
tion of aldehydes, propiolates (¼ prop-2-ynoates), and AcO(NH4) or primary amines
in refluxing AcOH or under microwave irradiation leads to 4-substituted N-alkyl-1,4-
dihydropyridines [7].
As a part of our current studies on the development of new routes in heterocyclic
synthesis via novel one-pot MCRs [8], we report here a novel four-component reaction
for the synthesis of polyfunctionalized 1,4-dihydropyridines in aqueous media at room
temperature in good yields (Scheme 1). The reaction was performed in the presence of
catalytic amounts of (NH4)2HPO4 (20%). The obtained products contain one CꢀN,
ꢀ 2011 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 94 (2011)
383
Scheme 1
Table. Synthesis of Polyfunctionalized 1,4-Dihydropyridines via a Four-Component One-Pot Reaction at
Room Temperature (see Scheme 1)
Ar
Product
Time [h]
Yield [%]a)
Ph
5a
5b
5c
5d
5e
5f
5g
20
16
14
12
24
10
10
79
75
85
95
80
98
87
4-BrꢁC6H4
4-ClꢁC6H4
4-NꢀCꢁC6H4
3-OHꢁC6H4
3-NO2ꢁC6H4
4-NO2ꢁC6H4
a) Yield of isolated product.
one NH2 group, and two ester groups, which could be readily used for further
conversions. The results are summarized in the Table.
Results and Discussion. – The addition of nucleophiles across CꢀC bonds has
evolved to become a versatile tool for the synthesis of compounds with different
functional groups [9]. Typically, the addition of amines to acetylenedicarboxylate
(¼ but-2-ynedioate) derivatives has been extensively studied [10], and the DMADꢁ
primary-amine adducts are used as efficient starting materials for the synthesis of
heterocyclic skeletons (DMAD ¼ dimethyl but-2-ynedioate) [11].
In our experiments, the addition of benzylamine (1) to DMAD (2) under neat
conditions was studied, which led to a mixture of (Z)/(E) isomers. In solution, the (E)
isomer converts slowly to the (Z) isomer (see [12]) (Scheme 2). Reaction of
benzylamine with DMAD was completed in less than 3 h under neat conditions. The
assignments of the (E) and (Z) configurations of the DMADꢁbenzylamine adduct are
1
based on the chemical shifts of the olefinic and NH H-atoms in the H-NMR spectra.
Scheme 2

384
Helvetica Chimica Acta – Vol. 94 (2011)
The olefinic H-atom of the (E) and (Z) isomer appeared at d(H) 4.70 and 5.10,
respectively [12], and the NH H-atom of the (E) and (Z) isomer resonated at d(H)
5.20 and 8.40, respectively. The olefinic H-atom of the (Z) isomer appears at lower field
as a result of anisotropic deshielding by the ester C¼O groups. There is an efficient
intramolecular H-bonding in the structure of (Z) isomer. It was shown that the ease of
isomerization is related to the polarity of the solvent [13].
After formation of the DMADꢁbenzylamine adduct, a mixture containing the
aromatic aldehyde 3, malononitrile (¼ propanedinitrile; 4), and 20% (NH4)2HPO4
(DAHP) in H2O/EtOH 2 :1 was added to the adduct. The results for different aromatic
aldehydes are summarized in the Table. The four-component reaction of benzylamine,
DMAD, 4-chlorobenzaldehyde, and malononitrile was selected as a model, and the
effect of different solvents and bases were studied. The yields of this model reaction
(product 5c) in MeCN, CH2Cl2, MeOH, and H2O were 30, 25, 60, and 85%,
respectively. Carrying out the model reaction in the presence of 10% 1-methyl-1H-
imidazole or 10% DAHP as the base led to lower yields (24 and 14%, resp.).
Meanwhile, different primary amines were used instead of benzylamine under the
optimum conditions, for example, butylamine and aniline, but the best yield was
obtained with benzylamine. The amount of DAHP as a base was also optimized, and
the best yield for the synthesis of 5a – 5g was obtained with 20% DAHP.
The structure of compounds 5a – 5g were deduced from their spectroscopic and
mass spectrometric data, which displayed in each case the molecular-ion peak at the
1
appropriate m/z values. The H-NMR spectra of 5d in CDCl3 showed two s at d(H)
3.30 and 3.79 for two MeO groups. A distinguished signal for all of these compounds
appeared at d(H) 4.15 – 4.75 as a s which is due to HꢁC(4). The 13C-NMR spectra of
5a – 5g exhibited a characteristic signal at d(H) 37.9 – 38.6 for C(4). Furthermore, an
unusual signal at d(H) 60 – 66 for C(5) bearing the CꢀN group could be explained by
1
to the resonance structures displayed in the Figure. The H- and 13C-NMR spectra of
5a – 5g were similar to those of 5d, except for the signals of the aryl moieties, which
showed characteristic resonances in the appropriate regions of the spectra [10].
Figure. Resonance structures of the CN-substituted 1,4-Dihydropyridines 5a – 5g
To explain the mechanism of this multicomponent reaction, we propose a plausible
reaction pathway which is shown in Scheme 3. Knoevenagel condensation of an
aldehyde with malononitrile in the presence of DAHP as mild-base catalyst leads to the
formation of 2-arylidenemalononitrile A. Reaction of benzylamine with dimethyl
acetylenedicarboxylate gives the enamine B. Then, Michael addition of B to A yields
adduct C, which is transformed to intermediate D via intramolecular addition of the N-
atom as a nucleophile to the CꢀN bond. The tautomerization of the imino group to the

Helvetica Chimica Acta – Vol. 94 (2011)
385
Scheme 3. Possible Mechanism for the Synthesis of Polyfunctionalized 1,4-Dihydropyridines 5a – 5g
NH2 group then forms the desired products 5a – 5g. Thus, the reaction could proceed via
a domino Knoevenagel/Michael addition/cyclization reaction sequence.
For the investigation of the reaction mechanism, the 2-arylidenemalononitriles A
were formed from the reaction of benzaldehydes and malononitrile, and then these
were added to the benzylamineꢁDMAD adduct. The products were again 5a – 5g, but
obtained in lower yields (30 – 73%) compared to our one-pot method, and also longer
reaction times were required (12 – 36 h).

386
Helvetica Chimica Acta – Vol. 94 (2011)
Conclusions. – We demonstrated a convenient synthesis of polyfunctionalized 1,4-
dihydropyridine derivatives via a one-pot four-component reaction of benzylamine,
DMAD, aromatic aldehydes, and malononitrile in the presence of a catalytic amount
(20%) of DAHP at room temperature in aqueous media. The advantage of the present
procedure is that the reaction is performed under neutral conditions by simple mixing
of the starting materials, in good to high yields, with short reaction times, with 100%
atom-economy, with high bond-forming efficiency (BFE), and finally in aqueous media
under one-pot conditions.
S. B. would like to thank the Iran National Science Foundation (INSF) for the financial support. We
thank Prof. Rolf Gleiter for his invaluable comments. Partial support of this work by the Research Council
of K. N. Toosi University of Technology is appreciatively acknowledged.
Experimental Part
General. Commercially available materials were used without further purification. M.p.: Electro-
thermal-9100 apparatus; uncorrected. IR Spectra: ABB-FT-IR (FTLA 2000) spectrometer; ˜n in cmꢁ1. 1H-
and 13C-NMR Spectra: Bruker-DRX-300-Avance spectrometer; at 300 (1H) and 75 MHz (13C); CDCl3 as
solvent; d in ppm rel. to Me4Si as internal standard, J in Hz. GC/MS: Hewlett-Packard-5973 (EI, 70 eV)
instrument; in m/z (rel. %). Elemental analyses: Heraeus-CHN-O rapid analyzer for C, H, and N.
Polyfunctionalized 1,4-Dihydropyridines: General Procedure. A mixture of DMAD (2 mmol,
0.288 g) and benzylamine (2.4 mmol, 0.26 ml) was stirred vigorously at r.t. for 3 h (magnetic stirrer).
Then, a mixture of benzaldehyde derivative (2 mmol), malononitrile (2 mmol, 0.133 g), and
(NH4)2HPO4 (20%, 26.4 mg) in H2O/EtOH 2 :1 (10 ml) was added and stirred at r.t. for 10 – 24 h
(TLC monitoring). The solvent was evaporated and the precipitate purified by recrystallization from
petroleum ether/CH2Cl2.
Dimethyl 6-Amino-1-benzyl-5-cyano-1,4-dihydro-4-phenylpyridine-2,3-dicarboxylate (5a): Yield
318 mg (79%). M.p. 89 – 938 (dec.). IR (CHCl3): 3491, 3372, 3019, 2187, 1739, 1707, 1650, 1573.
1H-NMR: 3.63 (s, MeO); 3.75 (s, MeO); 4.08 (br. s, NH2); 4.60 (s, HꢁC(4)); 4.69 (B of AB, J ¼ 18, 1 H,
PhCH2); 4.79 (A of AB, J ¼ 18, 1 H, PhCH2); 7.19 – 7.42 (m, 10 arom. H). 13C-NMR: 38.4; 50.8; 52.2; 53.1;
66.7; 107.1; 120.3; 126.5; 126.9; 127.1; 128.5; 128.7; 129.3; 135.2; 142.1; 144.0; 151.0; 164.7; 165.8. EI-MS
(70 eV): 403 (Mþ), 326 (14), 344 (48), 91 (100), 106 (49). Anal. calc. for C23H21N3O4 (403.44): C 68.47, H
5.24, N 10.41; found: C 68.29, H 5.15, N 10.30.
Dimethyl 6-Amino-1-benzyl-4-(4-bromophenyl)-5-cyano-1,4-dihydropyridine-2,3-dicarboxylate
(5b). Yield 361 mg (75%). M.p. 146 – 1488 (dec.). IR (CHCl3): 3432, 3345, 2945, 2188, 1744, 1697,
1576. 1H-NMR: 3.64 (s, MeO); 3.77 (s, MeO); 4.15 (br. s, NH2); 4.56 (s, HꢁC(4)); 4.67 (B of AB, J ¼ 17.5,
1 H, PhCH2); 4.79 (A of AB, J ¼ 17.5, 1 H, PhCH2); 7.05 (d, J ¼ 8.4, 2 arom. H); 7.28 (d, J ¼ 8.4, 2 arom.
H); 7.30 – 7.43 (m, 5 arom. H). 13C-NMR: 38.0; 50.8; 53.5; 54.1; 66.2; 106.7;120.0; 121.1; 126.6; 128.7;
129.2; 131.7; 134.9; 142.2; 143.0; 151.1; 164.5; 165.5. EI-MS (70 eV): 483 (31, [M þ 2]þ), 481 (20, Mþ), 326
(72), 91 (100). Anal. calc. for C23H20BrN3O4 (482.33): C 57.27, H 4.18, N 8.71; found: C 57.13, H 4.10, N
8.71.
Dimethyl 6-Amino-1-benzyl-4-(4-chlorophenyl)-5-cyano-1,4-dihydropyridine-2,3-dicarboxylate (5c).
Yield 371 mg (85%). M.p. 146 – 1488 (dec.). IR (KBr): 3357, 3530, 2959, 2186, 1726, 1646, 1578. 1H-NMR:
3.64 (s, MeO); 3.77 (s, MeO); 4.18 (br. s, NH2); 4.57 (s, HꢁC(4)); 4.66 (B of AB, J ¼ 18, 1 H, PhCH2); 4.80
(A of AB, J ¼ 18, 1 H, PhCH2); 7.10 (d, J ¼ 8.4, 2 arom. H); 7.18 – 7.29 (m, 5 arom. H); 7.28 (d, J ¼ 8.4, 2
arom. H). 13C-NMR: 37.9; 50.8; 52.1; 53.1; 66.2; 106.9; 120.1; 126.7; 126.9; 128.4; 128.7; 128.8; 129.3;
132.9; 135.0; 142.2; 142.5; 151.1; 164.5; 165.5. EI-MS (70 eV): 439 (10, [M þ 2]þ), 437 (28, Mþ), 378 (66),
346 (14), 326 (30), 91 (100), 43 (42). Anal. calc. for C23H20ClN3O4 (437.88): C 63.09, H 4.60, N 9.60;
found: C 62.90, H 4.50, N 9.47.
Dimethyl 6-Amino-1-benzyl-5-cyano-4-(4-cyanophenyl)-1,4-dihydropyridine-2,3-dicarboxylate (5d).
Yield 406 mg (95%). M.p. 129 – 1318 (dec.). IR (KBr): 3490, 3349, 2227, 2187, 1726, 1652, 1579. 1H-NMR:
3.63 (s, MeO); 3.71 (s, MeO); 4.27 (br. s, NH2); 4.63 (s, HꢁC(4)); 4.65 (B of AB, J ¼ 18, 1 H, PhCH2); 4.80

Helvetica Chimica Acta – Vol. 94 (2011)
387
(A of AB, J ¼ 18, 1 H, PhCH2); 7.25 (d, J ¼ 9.0, 2 arom. H); 7.28 – 7.39 (m, 5 arom. H); 7.54 (d, J ¼ 9.0, 2
arom. H). 13C-NMR: 38.6; 50.9; 52.3; 53.3; 53.4; 65.3; 106.2; 110.9; 118.7; 119.8; 126.7; 127.7; 128.8; 129.3;
132.6; 134.7; 142.6; 148.9; 151.4; 164.3; 165.2. EI-MS (70 eV): 428 (32, Mþ), 369 (61), 326 (47), 337 (25),
106 (43), 91 (100). Anal. calc. for C24H20N4O4 (428.45): C 67.28, H 4.70, N 13.07; found: C 67.09, H 4.60, N
12.95.
Dimethyl 6-Amino-1-benzyl-5-cyano-1,4-dihydro-4-(3-hydroxyphenyl)pyridine-2,3-dicarboxylate
(5e). Yield 327 mg (78%). M.p. 144 – 1468 (dec.). IR (KBr): 3430, 3353, 2179, 1726, 1688, 1651, 1586.
1H-NMR: 3.68 (s, MeO); 3.83 (s, MeO); 4.10 (br. s, NH2); 4.54 (s, HꢁC(4)); 4.65 (B of AB, J ¼ 17.5, 1 H,
PhCH2); 4.80 (A of AB, J ¼ 17.5, 1 H, PhCH2); 6.51 (s, 1 arom. H); 6.70 (dd, J ¼8.0, 1 arom. H); 6.80 (d,
J ¼ 8.1, 1 arom. H); 7.16 (t, J ¼ 8.1, 1 arom. H); 7.30 – 7.40 (m, 5 arom. H). 13C-NMR: 38.0; 49.4; 51.9; 53.0;
60.8; 105.7; 113.6; 117.0; 121.3; 127.6; 128.2; 128.8; 129.2; 136.0; 142.6; 145.9; 151.6; 157.2; 164.1; 165.2. EI-
MS (70 eV): 419 (57, Mþ), 326 (60), 360 (80), 91 (100), 106 (12). Anal. calc. for C23H21N3O5 (419.44): C
65.86, H 5.04, N 10.01; found: C 65.62, H 4.95, N 9.90.
Dimethyl 6-Amino-1-benzyl-5-cyano-1,4-dihydro-4-(3-nitrophenyl)pyridine-2,3-dicarboxylate (5f).
Yield 439 mg (98%). M.p. 175 – 1778 (dec.). IR (KBr): 3414, 3360, 2941, 2180, 1740, 1709, 1652, 1531,
1421, 1343. 1H-NMR: 3.66 (s, MeO); 3.77 (s, MeO); 4.21 (br. s, NH2); 4.72 (s, HꢁC(4)); 4.74 (B of AB,
J ¼ 17.5, 1 H, PhCH2); 4.80 (A of AB, J ¼ 17.5, 1 H, PhCH2); 7.30 – 7.70 (m, 7 arom. H); 8.09 – 8.12 (m, 2
arom. H). 13C-NMR: 38.4; 51.0; 52.4; 53.4; 64.9; 105.9; 119.8; 121.9; 122.1; 122.3; 126.5; 128.7; 129.4;
129.6; 133.4; 134.7; 142.7; 146.3; 148.5; 151.5; 164.3; 165.3. EI-MS (70 eV): 448 (23, Mþ), 326 (91), 389
(73), 91 (100). Anal. calc. for C23H20N4O6 (448.43): C 61.60, H 4.49, N 12.49; found: C 61.45, H 4.38, N
12.40.
Dimethyl 6-Amino-1-benzyl-5-cyano-1,4-dihydro-4-(4-nitrophenyl)pyridine-2,3-dicarboxylate (5g).
Yield 389 mg (87%). M.p. 140 – 1428 (dec.). IR(KBr): 3428, 3344, 2953, 2187, 1744, 1688, 1654, 1626,
1574, 1425, 1347. 1H-NMR: 3.64 (s, MeO); 3.80 (s, MeO); 4.24 (br. s, NH2); 4.69 (AB (q), J ¼ 17.5,
PhCH2); 4.70 (s, HꢁC(4)); 7.26 – 7.40 (m, 7 arom. H); 8.13 (d, J ¼ 8.7, 2 arom. H). 13C-NMR: 38.5; 51.0;
52.3; 53.3; 65.3; 106.0; 124.1; 126.7; 127.8; 128.9; 129.4; 134.7; 142.8; 147.0; 150.9; 151.4; 164.2; 165.2. EI-
MS (70 eV): 402 (Mþ), 326 (67), 312 (38), 91 (100), 122 (10), 69 (57). Anal. calc. for C23H20N4O6
(448.43): C 61.60, H 4.49, N 12.49; found: C 61.47, H 4.37, N 12.39.
REFERENCES
[1] A. Dçmling, I. Ugi, Angew. Chem., Int. Ed. 2000, 39, 3168; A. Dçmling, Chem. Rev. 2006, 106, 17; A.
Dçmling, in ꢂMulticomponent Reactionsꢃ, Ed. Wiley – VCH, Weinheim, 2005, pp. 76 – 94; L. F.
Tietze, Chem. Rev. 1996, 96, 115.
[2] D. J. Triggle, D. A. Langs, R. A. Janis, Med. Res. Rev. 1989, 9, 123; T. Takenaka, O. Inagaki, M. Terai,
M. Asano, K. Kubo, J. Pharmacol. Exp. Ther. 1988, 244, 699; S. Visentin, B. Rolando, A. Di Stilo, R.
Fruttero, M. Novara, E. Carbone, C. Roussel, N. Vanthuyne, A. Gasco, J. Med. Chem. 2004, 47,
2688.
´
[3] M. Richter, J. Molnar, A. Hilgeroth, J. Med. Chem. 2006, 49, 2838; A. Hilgeroth, U. Baumeister,
F. W. Heinemann, Eur. J. Org. Chem. 2000, 245.
[4] S. Goldmann, J. Stoltefuss, Angew. Chem., Int. Ed. Engl. 1991, 30, 1559; D. M. Stout, A. I. Meyers,
Chem. Rev. 1982, 82, 223.
[5] A. Sausins, G. Duburs, Heterocycles 1988, 27, 269.
[6] A. Hantzsch, Chem. Ber. 1881, 14, 1637; U. Eisner, J. Kuthan, Chem. Rev. 1972, 72, 1.
[7] T. Chennat, U. Eisner, J. Chem. Soc., Perkin Trans. 1 1975, 926; S. Balalaie, E. Kowsari, Monatsh.
Chem. 2001, 132, 1551.
[8] M. J. Khoshkholgh, M. Lotfi, S. Balalaie, F. Rominger, Tetrahedron 2009, 65, 4228; M. J.
Khoshkholgh, S. Balalaie, H. R. Bijanzadeh, F. Rominger, J. H. Gross, Tetrahedron Lett. 2008, 49,
6965; M. J. Khoshkholgh, S. Balalaie, H. R. Bijanzadeh, J. H. Gross, Arkivoc 2009, ix, 114; M. J.
Khoshkholgh, S. Balalaie, S. H. R. Bijanzadeh, J. H. Gross, Synlett 2009, 55; M. Bararjanian, S.
Balalaie, F. Rominger, S. Barouti, Helv. Chim. Acta 2010, 93, 777; S. Balalaie, S. Abdolmohammadi,
B. Soleimanifard, Helv. Chim. Acta 2009, 92, 932; M. Bararjanian, S. Balalaie, F. Rominger, B.

388
Helvetica Chimica Acta – Vol. 94 (2011)
Movassagh, H. R. Bijanzadeh, J. Org. Chem. 2010, 75, 2806; M. Bararjanian, S. Balalaie, B.
Movassagh, H. R. Bijanzadeh, Tetrahedron Lett. 2010, 51, 3277.
[9] L. J. Goossen, M. Arndt, M. Blanchot, F. Rudolphi, F. Menges, G. Nieder-Schattenburg, Adv. Synth.
Catal. 2008, 350, 2701, and refs. cit. therein.
[10] J. I. Dickstein, S. I. Miller, in ꢂThe Chemistry of Functional Groupsꢃ, ꢂThe Chemistry of Carbon-
Carbon Triple Bondsꢃ, Part 2, Ed. S. Patai, Wiley, Chichester, 1978, Chapt. 19, pp. 813; R. Huisgen,
K. Herbig, A. Siegel, H. Huber, Chem. Ber. 1966, 99, 2526; R. Naitoh, Y. Nakamura, E. Katano, Y.
Nakamura, E. Okada, M. Asaoka, Heterocycles 2004, 63, 1009; J. P. Bouillon, B. Tinant, J. M.
Nuzillard, C. Portella, Synthesis 2004, 711; J. E. Dolfini, J. Org. Chem. 1965, 30, 1298; A. K. Ogawa,
C. Willoughby, R. Bergeron, K. P. Ellsworth, W. M. Geissler, R. W. Myers, J. Yao, G. Harris, K. T.
Chapmann, Bioorg. Med. Chem. Lett. 2003, 13, 3405; A. Ziayaei-Halimehjani, M. R. Saidi,
Tetrahedron Lett. 2008, 49, 1244.
[11] I. Yavari, S. Souri, Synlett 2007, 2969; I. Yavari, S. Souri, Synlett 2008, 1208.
[12] I. Yavari, A. Hossein-Nia, Magn. Reson. Chem. 1994, 30, 413; I. Yavari, A. Shaabani, H. Soleimani,
F. Nourmohammadian, H. R. Bijanzadeh, Magn. Reson. Chem. 1996, 34, 1003; K. Herbig, R.
Huisgen, H. Huber, Chem. Ber. 1966, 99, 2546
[13] Q. Zhu, H. Jiang, J. Li, M. Zhang, X. Wang, C. Qi, Tetrahedron 2009, 65, 4604.
Received June 7, 2010