Synthesis of 5H-thiazolo[3,2-a]pyrimidines

Article abstract of DOI:10.1007/s10593-013-1172-x

A convenient method has been developed for preparing 5H-thiazolo[3,2-a] pyrimidines by the reaction of tetrahydropyrimidine-2-thiones with halocarboxylic acid esters or with 3-bromopentane-2,4-dione.

Full text of DOI:10.1007/s10593-013-1172-x

Chemistry of Heterocyclic Compounds, Vol. 48, No. 10, January, 2013 (Russian Original Vol. 48, No. 10, October, 2012)
SYNTHESIS OF 5H-THIAZOLO[3,2-a]PYRIMIDINES
A. K. Shiryaev1*, N. S. Baranovskaya1, and M. S. Eremin1
A convenient method has been developed for preparing 5H-thiazolo[3,2-a]pyrimidines by the reaction
of tetrahydropyrimidine-2-thiones with halocarboxylic acid esters or with 3-bromopentane-2,4-dione.
Keywords: 3-bromopentane-2,4-dione, halocarboxylic acid esters, tetrahydropyrimidines, thiazolo-
pyrimidines, cyclization.
The search for convenient methods of preparing dihydropyrimidines [1, 2] has been motivated by their
ability to regulate the action of calcium channels [3]. Significantly less attention has been devoted to studying
the chemical properties of dihydropyrimidines, but work has recently appeared relating to the synthesis of
condensed dihydropyrimidine derivatives [3] in connection to their broad spectrum of biological activity.
5H-Thiazolo[3,2-a]pyrimidines are glutamate receptor antagonists [4] and acetylcholinesterase inhibitors [5],
and they show anti-inflammatory, antiparkinsonian, and antiherpes activity [6].
In the synthesis of thiazolo[3,2-a]pyrimidines from 1,2,3,4-tetrahydropyrimidine-2-thiones, cyclization
can occur at either the N-1 or N-3 atoms of the pyrimidine ring [7]. In the majority of cases, cyclization happens
at the N-3 atom to give 5H-thiazolo[3,2-a]pyrimidines, e.g. when treating tetrahydropyrimidine-2-thiones with
-haloketones [4, 5], 1,2-dibromoethane [7, 8], -halocarboxylic acids [7, 8] or their acyl halides [9], methyl
chloroacetate [10], or bromomalonodinitrile [11]. However, methyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-
tetrahydropyrimidine-5-carboxylate cyclizes with iodoacetamide at the N-1 atom, to give a 7H-thiazolo[3,2-
a]pyrimidine [12].
With the aim of studying the selectivity of this cyclization, we have treated different pyrimidine-
2-thiones 1a-g with -halocarboxylates and with 3-bromopentane-2,4-dione. We have found that the reaction of
pyrimidine-2-thiones 1a-g with chloroacetate does not require the use of triethylamine (as previously reported
[10]) and the 5H-thiazolo[3,2-a]pyrimidines 2a-g are formed simply by heating the reagents without solvent.
R1
R1
O
O
O
O
120°C, 20 min
+
Cl
R
NH
R
N
OEt
S
·HCl
Me
N
H
1a–g
S
Me
N
2a–g
a–e R = OEt, f, g R = Me; a, f R1 = H, b R1 = 2-OH, c R1 = 2-Cl, d R1 = 2-NO2, e, g R1 = 4-OMe
_______
*To whom correspondence should be addressed, e-mail: andrey_shiryaev@yahoo.com.
1Samara State Technical University, 244 Molodogvardeiskaya St., Samara 443100, Russia.
________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1662-1667, October, 2012.
Original article submitted September 23, 2011.
1550
0009-3122/13/4810-1550©2013 Springer Science+Business Media New York

The thiazolopyrimidines 2a-g were isolated as hydrochlorides with high melting points. They are
characterized by a high frequency IR absorption band in the range of 1774-1754 cm-1, which is assigned to the
carbonyl group stretching vibration of the thiazolone ring. The methylene groups in this bicyclic structure are
diastereotopic, and their 1H NMR signals appear as doublets with a rather large spin-spin coupling, or in the form of
multiplets. The selectivity of the cyclization at the tetrahydropyrimidine ring N-3 atom is confirmed by the
13
1
appearance of C NMR signals for the carbon atoms C-5 (53.9-55.7 ppm) and C-7 (145.0-148.6 ppm) and a H
NMR signal at 5.61-6.20 ppm for the proton on the C-5 atom within a narrow range for all of the synthesized
thiazolopyrimidines 2a-g with the exception of the nitro derivative 2d, which shows a C-5 signal (50.0 ppm) and its
bonded proton (6.74 ppm). The signals of the indicated atoms should differ markedly for the two possible 5H- and
7H-thiazolo[3,2-a]pyrimidine isomers. For each of compounds 2a-g these signals do not deviate far from the mean
values and correspond to literature data for ethyl 7-methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]-
pyrimidine-6-carboxylate in CDCl3: 53.9 (C-5), 150.8 (C-7), and 6.05 ppm (H-5) [6]. This data provides arguments
against assigning the thiazolo[3,2-a]pyrimidines 2b-d (having an ortho substituent in the benzene ring at C-5 atom)
to 7H-isomers, in spite of the fact that an ortho substituent in the benzene ring at the C-4 atom of a
dihydropyrimidine ring can direct an electrophilic attack to the N-1 atom [3].
To examine a possible cyclization at the N-1 atom, we have carried out reactions of N-3-substituted
tetrahydropyrimidine-2-thiones with ethyl chloroacetate. For the reaction of ethyl 3-acetyl-6-methyl-4-phenyl-
2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (prepared by method [7]) the reaction does not stop at the
sulfur atom alkylation stage, but proceeds further to form the 5H-thiazolopyrimidine 2a. During the reaction,
deacetylation of the N-3 atom occurs leading to the more stable 5H-isomer 2a and pointing to a thermodynamic
control. Attempts to alkylate the ethyl 3-(2-cyanoethyl)-6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate (prepared by method [13]) with ethyl chloroacetate proved unsuccessful. The reaction
did not occur, probably due to steric hindrance from the cyanoethyl group. Computational analysis of model
compounds by the B3LYP/6-31G* method (GAMESS program [14], correspondence of the optimized geometry
to energy minimum was checked by calculation of vibrational frequencies) confirmed that the more stable
product is that arising from cyclization at the N-3 atom: 6-acetyl-5,7-dimethyl-5H-[1,3]thiazolo[3,2-a]-
pyrimidin-3(2H)-one (2h) is by 27.7 kJ/mol more stable than 6-acetyl-5,7-dimethyl-7H-[1,3]thiazolo[3,2-a]-
pyrimidin-3(2H)-one (2i), very likely as a result of pyrimidine ring double bond conjugation in the structure of
2h.
Me
O
Me
Me
O
O
O
Me
N
N
S
S
Me
Me
N
N
2h
2i
The reaction of the pyrimidinethione 1a with bromomalonate, 3-bromoacetoacetic ester, and
3-bromopentane-2,4-dione also occurs upon heating. The reaction with the bromomalonate forms a mixture of
diastereomeric diethyl 7-methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-2,6-dicarb-
oxylates (3). The reaction with the 3-bromopentane-2,4-dione gives ethyl 3,7-dimethyl-5-phenyl-5H-
[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate (4), which is formed through loss of the acetyl group in a process
that is likely catalyzed by the hydrogen bromide liberated in the course of the reaction. When treating the
pyrimidinethione 1a with 3-bromoacetoacetate, a mixture of difficult to separate products was obtained,
apparently because of the primary alkylation product cyclization both at the acetyl and at the ester group and
also possibly as a result of loss of the acetyl group.
1551

O
Ph
N
O
Ph
N
Br
Br
O
Me
O
CO2Et
EtO2C
Ac
Ac
EtO
Me
N
EtO
Me
N
1a
90°C, 20 min
90°C, 20 min
S
S
·HBr
OEt
·HBr
3
4
Hence we have discovered a convenient and simple method for preparing 5-aryl-5H-thiazolo[3,2-a]-
pyrimidines by heating 5-aryl-1,2,3,4-tetrahydropyrimidine-2-thiones with 2-halocarboxylates and have shown
that a single isomer is formed in the reaction, most likely due to the thermodynamic factor.
EXPERIMENTAL
1
IR spectra were recorded on a Shimadzu FTIR8500S instrument for KBr pellets. H and 13C NMR
spectra were recorded on a JEOL JNM ECX-400 instrument (400 and 100 MHz, respectively) using DMSO-d6
with TMS as internal standard. Elemental analysis was performed on a EuroVector EA-3000 instrument.
Melting points were determined on a PTP-2 apparatus.
Ethyl 5-Aryl-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate Hydro-
chlorides 2a-e and 6-Acetyl-5-aryl-7-methyl-5H-[1,3]thiazolo[3,2-a]pyrimidin-3(2H)-one Hydrochlorides
2f,g (General Method). A mixture of ethyl chloroacetate (3.0 ml, 35 mmol) and the corresponding pyrimidine-
2-thione 1a-g [15] (3.6 mmol) was heated for 20 min at 120ºC. The precipitate formed was filtered off and
washed with EtOAc.
Ethyl
7-Methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate
Hydrochloride (2a). Yield 0.85 g (67%). Yellow crystals; mp 220-222ºC (decomp.). IR spectrum, , cm-1:
1762 (N–C=O), 1712 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 1.06 (3Н, t, 3J = 7.3, CH2CH3); 2.35 (3Н, s,
2
2
7-СН3); 3.91-4.08 (2Н, m, CH2CH3); 4.14 (1Н, d, J = 18.1) and 4.19 (1Н, d, J = 18.1, 2-СН2); 5.86 (1H, s,
5-CH); 7.20-7.35 (5H, m, Н Ph); 8.01 (1Н, br. s, N+H). 13C NMR spectrum, , ppm: 14.4 (СH2CH3); 21.2
(7-СН3); 33.9 (2-СН2); 55.5 (5-CH); 60.8 (CH2CH3); 108.0 (C-6); 128.2 (С Ph); 129.1 (С Ph); 129.2 (С Ph);
140.4 (С Ph); 148.6 (C-7); 164.8 (C-8a); 164.9 (COOEt); 171.8 (C-3). Found, %: С 54.23; Н 4.59; N 7.62.
C16H17ClN2O3S. Calculated, %: С 54.47; Н 4.86; N 7.94.
Ethyl 5-(2-Hydroxyphenyl)-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxy-
late Hydrochloride (2b). Yield 0.48 g (36%). Yellow crystals; mp 220-224ºC (decomp.). IR spectrum, , cm-1:
1761 (N–C=O), 1705 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 1.00 (3Н, t, 3J = 6.9, CH2CH3); 1.92 (3Н, s,
2
2
7-СН3); 3.95-4.05 (2Н, m, СH2CH3); 4.19 (1Н, J = 17.5) and 4.24 (1Н, d, J = 17.5, 2-СН2); 5.61 (1H, s,
5-CH); 6.50 (1H, br. s, OH); 6.88-7.00 (2H, m, H Ar); 7.20-7.30 (2H, m, H Ar); 10.30 (1Н, br. s, N+H).
13C NMR spectrum, , ppm: 14.3 (СH2CH3); 21.0 (7-СН3); 34.1 (2-СН2); 55.5 (5-CH); 60.8 (CH2CH3); 108.0
(C-6); 128.1 (C Ar); 128.2 (C Ar); 129.1 (C Ar); 129.2 (C Ar); 129.3 (C Ar); 140.4 (C Ar); 148.5 (C-7); 157.5
(C-8a); 164.9 (COOEt); 171.4 (C-3). Found, %: С 52.32; Н 4.96; N 7.28. C16H17ClN2O4S. Calculated, %:
С 52.10; Н 4.65; N 7.59.
Ethyl 5-(2-Chlorophenyl)-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxy-
late Hydrochloride (2c). Yield 0.85 g (61%). Yellow crystals; mp 220-223ºC (decomp.). IR spectrum, , cm-1:
1774 (N–C=O), 1689 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 1.02 (3Н, t, 3J = 6.9, CH2CH3); 2.31 (3Н, s,
2
2
7-СН3); 3.90-4.00 (2Н, m, СH2CH3); 4.10 (1Н, d, J = 17.9) and 4.16 (1Н, d, J = 17.9, 2-СН2); 6.20 (1H, s,
5-CH); 7.22-7.43 (4H, m, H Ar); 7.78 (1Н, br. s, N+H). 13C NMR spectrum, , ppm: 14.4 (СН2CH3); 21.2
(7-СН3); 33.4 (2-СН2); 53.9 (5-CH); 60.8 (CH2CH3); 107.0 (C-6); 128.2 (C Ar); 130.2 (C Ar); 130.7 (C Ar);
131.8 (C Ar); 133.2 (C Ar); 137.7 (C Ar); 148.6 (C-7); 164.8 (COOEt, C-8a); 171.2 (C-3). Found, %: С 49.40;
Н 4.49; N 7.05. C16H16Cl2N2O3S. Calculated, %: С 49.62; Н 4.16; N 7.23.
1552

Ethyl
7-Methyl-5-(2-nitrophenyl)-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carb-
oxylate Hydrochloride (2d). Yield 0.69 g (48%). Yellow crystals; mp 240-243ºC (decomp.). IR spectrum, , cm-1:
1774 (N–C=O), 1712 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 0.92 (3Н, t, 3J = 6.3, CH2CH3); 2.31 (3Н, s,
7-СН3); 3.77-3.95 (2Н, m, СH2CH3); 4.02 (2Н, s, 2-СН2); 6.74 (1H, s, 5-CH); 7.40-7.60 (2H, m, H Ar); 7.66-
7.76 (1H, m, H Ar); 7.90 (1H, d, J = 6.8, H Ar); 7.95 (1Н, br. s, N+H). 13C NMR spectrum, , ppm: 14.2
3
(СН2CH3); 22.9 (7-СН3); 32.8 (2-СН2); 50.0 (5-CH); 60.6 (CH2CH3); 106.6 (C-6); 125.0 (C Ar); 130.4 (2C Ar);
133.2 (C Ar); 134.9 (C Ar); 148.6 (C-7); 152.5 (C Ar); 162.1 (C-8a); 165.1 (COOEt); 171.8 (C-3). Found, %:
С 48.69; Н 4.38; N 10.83. C16H16ClN3O5S. Calculated, %: С 48.31; Н 4.05; N 10.56.
Ethyl 5-(4-Methoxyphenyl)-7-methyl-3-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxy-
late Hydrochloride (2e). Yield 0.90 g (65%). Yellow crystals; mp 214-216ºC (decomp.). IR spectrum, , cm-1:
1760 (N–C=O), 1710 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 1.06 (3Н, t, 3J = 7.3, CH2CH3); 2.35 (3Н, s,
2
2
7-СН3); 3.69 (3Н, s, ОСН3); 3.95-4.05 (2Н, m, СH2CH3); 4.15 (1Н, d, J = 18.2) and 4.21 (1Н, d, J = 18.2,
3
3
2-СН2); 5.84 (1H, s, 5-CH); 6.87 (2H, d, J = 7.8, H Ar); 7.19 (2H, d, J = 7.8, H Ar); 8.4 (1Н, br. s, N+H).
13C NMR spectrum, , ppm: 14.4 (СН2CH3); 20.7 (7-СН3); 34.2 (2-СН2); 55.1 (OCH3); 55.7 (5-CH); 60.9
(CH2CH3); 108.2 (C-6); 114.4 (C Ar); 114.5 (C Ar); 129.7 (C Ar); 132.3 (C Ar); 147.5 (C-7); 159.9 (C-8a);
164.8 (COOEt); 171.4 (C-3). Found, %: С 53.01; Н 5.22; N 7.10. C17H19ClN2O4S. Calculated, %: С 53.33;
Н 5.00; N 7.32.
6-Acetyl-7-methyl-5-phenyl-5H-[1,3]thiazolo[3,2-a]pyrimidine-3(2H)-one Hydrochloride (2f).
Yield 0.60 g (52%). Yellow crystals; mp 226-229ºC (decomp.). IR spectrum, , cm-1: 1759 (N–C=O), 1655
(CH3CO). 1H NMR spectrum, , ppm (J, Hz): 2.20 (3Н, s, СН3CO); 2.38 (3Н, s, 7-СН3); 4.18 (1Н, d, 2J = 18.2)
2
and 4.24 (1Н, d, J = 18.2, 2-СН2); 6.03 (1H, s, 5-CН); 7.24-7.35 (5H, m, Н Ph); 8.77 (1Н, br. s, N+H).
13C NMR spectrum, , ppm: 21.2 (7-СН3); 31.3 (CH3CO); 34.5 (2-СН2); 55.4 (5-CH); 117.1 (C-6); 128.5
(С Ph); 129.3 (С Ph); 129.4 (С Ph); 139.6 (С Ph); 145.4 (С-7); 165.8 (C-8а); 171.3 (C-3); 196.6 (MeCO).
Found, %: С 55.49; Н 4.85; N 8.60. C15H15ClN2O2S. Calculated, %: С 55.81; Н 4.68; N 8.68.
6-Acetyl-5-(4-methoxyphenyl)-7-methyl-5H-[1,3]thiazolo[3,2-a]pyrimidin-3(2H)-one Hydrochloride
(2g). Yield 0.66 g (42%). Yellow crystals; mp 223-225ºC (decomp.). IR spectrum, , cm-1: 1754 (N–C=O), 1654
(CH3CO). 1H NMR spectrum, , ppm (J, Hz): 2.19 (3Н, s, СН3CO); 2.39 (3Н, s, 7-СН3); 3.70 (3Н, s, ОСН3); 4.19
(2Н, s, 2-СН2); 6.00 (1H, s, 5-CН); 6.88 (2H, d, 3J = 7.8, H Ar); 7.22 (2H, d, 3J = 7.8, H Ar); 8.55 (1Н, br. s, N+H).
13C NMR spectrum, , ppm: 21.2 (7-СН3); 31.1 (CH3CO); 34.4 (2-СН2); 54.9 (OCH3); 55.7 (5-CH); 114.5 (C Ph);
114.6 (C Ph); 117.0 (C-6); 129.9 (C Ph); 131.6 (C Ph); 145.0 (C-7); 160.0 (C-8a); 171.4 (C-3); 196.7 (MeCO).
Found, %: С 54.64; Н 4.77; N 7.62. C16H17ClN2O3S. Calculated, %: С 54.47; Н 4.86; N 7.94.
5-Phenyl-5H-[1,3]thiazolo[3,2-a]pyrimidine hydrobromides 3 and 4 were prepared similarly to the
thiazolopyrimidines 2a-g by heating the ethyl 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-
5-carboxylate (1a) (1.0 g, 3.6 mmol) at 90ºC for 20 min with bromomalonic ester or 3-bromopentane-2,4-dione
(35.0 mmol), respectively.
Diethyl
7-Methyl-3-oxo-5-phenyl-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyrimidine-2,6-dicarboxy-
late Hydrobromide (3). Yield 0.91 g (59%). Yellow crystals; mp 150-153ºC (decomp.). IR spectrum, , cm-1:
1747 (N–C=O), 1720 (CO2Et). 1H NMR spectrum, , ppm (J, Hz): 1.06 (6Н, t, 3J = 7.1, 2CH2CH3); 2.35 (3Н, s,
7-СН3); 3.95-4.05 (4Н, m, 2СH2CH3); 5.58 (1H, s, 5-CH); 5.75 (1H, br. s, 2-СН); 7.20-7.40 (5H, m, Н Ph);
8.50 (1H, br. s, N+H). 13C NMR spectrum, , ppm: 14.2 (СH2CH3); 14.4 (СH2CH3); 17.7 (7-СН3); 52.9 (2-СН);
55.9 (5-CH); 60.9 (CH2CH3); 63.4 (CH2CH3); 104.9 (C-6); 127.5 (C Ph); 129.1 (C Ph); 129.3 (C Ph); 141.2
(C Ph); 144.6 (С-7); 156.7 (C-8а); 164.5 (C=O); 164.8 (C=O); 165.3 (C=O). Found, %: С 48.81; Н 4.44;
N 5.69. C19H21BrN2O5S. Calculated, %: С 48.62; Н 4.51; N 5.97.
Ethyl 3,7-dimethyl-5-phenyl-5H-[1,3]thiazolo[3,2-a]pyrimidine-6-carboxylate Hydrochloride (4).
1
Yield 0.66 g (52%). Yellow crystals; mp 248-251ºC (decomp.). IR spectrum, , cm-1: 1677 (CO2Et). H NMR
3
spectrum, , ppm (J, Hz): 1.15 (3Н, t, J = 7.1, CH2CH3); 2.17 (3Н, s, 3-СН3); 2.39 (3Н, s, 7-СН3); 4.00-4.12
(2Н, m, СH2CH3); 6.40 (1H, s, 5-CН); 7.17 (1H, s, Н-2); 7.25-7.40 (5H, m, Н Ph); 12.71 (1Н, br. s, N+H).
13C NMR spectrum, , ppm: 13.4 (CH3); 14.5 (СН3); 18.2 (СН3); 59.0 (5-CH); 61.1 (CH2CH3); 103.2 (C-2);
1553

109.8 (C-6); 127.8 (С Ph); 129.7 (С Ph); 129.8 (С Ph); 138.1 (С Ph); 140.6 (C-3(7)); 142.6 (C-7(3)); 160.9
(C-8а); 164.5 (C=O). Found, %: С 51.88; Н 4.63; N 6.92. C17H19BrN2O2S. Calculated, %: С 51.65; Н 4.84;
N 7.09.
This work was carried out with the financial support of the Ministry of Education and Science of the
Russian Federation Analytical Target Program "Development of the Higher School Scientific Potential"
(Procedure 1) with the use of the scientific equipment of the Collective Use Center of the Samara State
Technical University "Investigation of the Physicochemical Properties of Substances and Materials".
REFERENCES
1.
2.
3.
4.
S. V. Vdovina and V. A. Mamedov, Usp. Khim., 77, 1091 (2008).
J.-P. Wan and Y. Liu, Synthesis, 3943 (2010).
K. Singh, D. Arora, K. Singh, and S. Singh, Mini-Rev. Med. Chem., 9, 95 (2009).
J. Wichmann, G. Adam, S. Kolczewski, V. Mutel, and T. Woltering, Bioorg. Med. Chem. Lett., 9, 1573
(1999).
5.
6.
H. Zhi, L. Chen, L. Zhang, S. Liu, D. C. C. Wan, H. Lin, and C. Hu, ARKIVOC, xiii, 266 (2008).
S. J. Kashyap, P. K. Sharma, V. K. Garg, R. Dudhe, and N. Kumar, J. Adv. Sci. Res., 2, No. 3, 18
(2011).
7.
8.
C. O. Kappe and P. Roschger, J. Heterocycl. Chem., 26, 55 (1989).
A. Mobinikhaledi, N. Foroughifar, and F. Goodarzi, Phosphorus, Sulfur Silicon Relat. Elem., 178, 2539
(2003).
9.
A. Mobinikhaledi, M. Zendehdel, M. H. Nasab, and M. A. B. Fard, Heterocycl. Commun., 15, 451
(2009).
10.
11.
I. V. Kulakov, Zh. Org. Khim., 45, 1270 (2009).
N. Foroughifar, A. Mobinikhaledi, H. F. Jirandehi, and S. Memar, Phosphorus, Sulfur Silicon Relat.
Elem., 178, 1269 (2003).
12.
E. L. Khanina, R. M. Zolotoyabko, D. Kh. Muceniece, and G. Ya. Dubur, Khim. Geterotsikl. Soedin.,
1076 (1989). [Chem. Heterocycl. Compd., 25, 898 (1989).
13.
14.
X. Wang, Z. Quan, and Z. Zhang, Tetrahedron, 63, 8227 (2007).
M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. J. Jensen, S. Koseki, N.
Matsunaga, K. A. Nguyen, S. Su, T. L. Windus, M. Dupuis, and J. A. Montgomery, J. Comput. Chem.,
14, 1347 (1993).
15.
K. V. N. S. Srinivas and B. Das, Synthesis, 2091 (2004).
1554