Synthesis of azolo[a]pyridines from 5-(bromomethyl)hept-4-en-3-one and 5-bromopent-3-en-2-one derivatives

Article abstract of DOI:10.1007/s10593-012-0931-4

Alkyl derivatives of 1H-imidazo[1,2-a]pyridin-4-ium, 5H-pyrido[1,2-a] benzimidazol-10-ium, 1H-[1,2,4]-triazolo[4,3-a]pyridin-4-ium, and 3-methylthiazolo[3,2-a]pyridin-4-ium bromides were obtained in two stages from (4Z)-5-(bromomethyl)-2,2,6,6-tetramethylhept-4-en-3-one, 5-bromo-4-methylpent-3- en-2-one, or (3E)-5-bromopent-3-en-2-one by alkylation of 1-alkyl-1H-imidazoles, 1-alkyl-1H-benz-imidazoles, 1-methyl-1H-1,2,4-triazole, and 4-methylthiazole and subsequent cyclization of the quaternary azolium salts in the presence of bases.

Full text of DOI:10.1007/s10593-012-0931-4

Chemistry of Heterocyclic Compounds, Vol. 47, No. 11, February, 2012 (Russian Original Vol. 47, No. 11, November, 2011)
SYNTHESIS OF AZOLO[a]PYRIDINES FROM
5-(BROMOMETHYL)HEPT-4-EN-3-ONE AND
5-BROMOPENT-3-EN-2-ONE DERIVATIVES
L. M. Potikha1*, A. R. Turelik1, and V. A. Kovtunenko1
Alkyl derivatives of 1H-imidazo[1,2-a]pyridin-4-ium, 5H-pyrido[1,2-a]benzimidazol-10-ium, 1H-[1,2,4]-
triazolo[4,3-a]pyridin-4-ium, and 3-methylthiazolo[3,2-a]pyridin-4-ium bromides were obtained in two
stages from (4Z)-5-(bromomethyl)-2,2,6,6-tetramethylhept-4-en-3-one, 5-bromo-4-methylpent-3-en-
2-one, or (3E)-5-bromopent-3-en-2-one by alkylation of 1-alkyl-1H-imidazoles, 1-alkyl-1H-benz-
imidazoles, 1-methyl-1H-1,2,4-triazole, and 4-methylthiazole and subsequent cyclization of the
quaternary azolium salts in the presence of bases.
Keywords: 5-(bromomethyl)hept-4-en-3-one, 5-bromopent-3-en-2-one, imidazo[1,2-a]pyridine, pyrido-
[1,2-a]benzimidazole, thiazolo[3,2-a]pyridine, [1,2,4]triazolo[4,3-a]pyridine.
γ-Halo-substituted unsaturated ketones are highly reactive compounds and are the synthetic equivalents
of 1,4-dielectrophilic synthons. On account of their relatively poor availability they were used predominantly for
the production of pyrroles in reactions with primary amines [1-5]. We recently found convenient methods for the
synthesis of the azolo[a]pyridine system derivatives on the basis of (2Z)-4-bromo-1,3-diphenyl-2-buten-1-one
(γ-bromodypnone) and its derivatives [6-8]. While continuing investigations in this direction we turned to γ-halo
ketones of the aliphatic series.
The structural features of (4Z)-5-(bromomethyl)-2,2,6,6-tetramethylhept-4-en-3-one (1) make it possible
to regard it as an analog of γ-bromodypnone; there are nonenolizing groups at the carbonyl group and at the
C=C double bond, and their volume leads to configurational stability for the molecule in the form of the
(Z)-isomer [9]. Properties of this compound have hardly been studied at all, and only the formation of a
quaternary triphenylphosphonium salt [9] and intramolecular cyclization to 2,4-di-tert-butylfuran [10] are
known.
We have established that compound 1, like γ-bromodypnone, forms quaternary azolium salts 2 with high
yields when a mixture of the bromo ketone 1 and azole is kept in benzene at room temperature for several days
(Scheme 1), as described in [7, 8] (Table 1). Difficulties in the isolation of the alkylation products 2b,e in pure
form only arose in the case of reactions with 1-benzyl-1H-imidazole and 1-methyl-1H-1,2,4-triazole. When the
reaction is conducted under standard conditions (benzene, 25°, 2-3 days) the reaction mixture still contains the
initial azole as impurity (15-20%), which subsequently initiates cyclization of the salts 2b,e during
_______
*To whom correspondence should be addressed, e-mail: potikha_l@mail.ru.
1Taras Shevchenko Kyiv National University, 64 Volodymyrska St., Kyiv 01033, Ukraine.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1725-1734, November, 2011.
Original article submitted December 1, 2010; revision submitted October 5, 2011.
1436
0009-3122/12/4711-1436©2012 Springer Science+Business Media, Inc.

attempts of their purification by recrystallization. Increase of the reaction time or carrying out the reaction with
heating also leads to appearance of cyclization products in the reaction mixture.
Scheme 1
Bu-t
_
N
X
Br
+
N
t-Bu
Br
COBu-t
COBu-t
C6H6
X
2a–f
1
EtOH, K2CO3
Bu-t
_
_
Br
_
_
Me
Br
Br
Br
+
Bu-t
+
Bu-t
Bu-t
+
+
N
N
N
N
N
N
N
R
S
N
Bu-t
Me
R
t-Bu
t-Bu
3a,b
t-Bu
4a,b
6
5
N
N
X
; 2a, 3a R = Me; 2b, 3b R = Bn;
2a,b
=
N
R
Me
N
N
X
N
X
N
N
X
N
S
N
; 2f
; 2c, 4a R = Me; 2d, 4b R = Bn; 2e
=
=
2c,d
=
N
N
Me
R
When the salts 2a-f are heated in ethanol in the presence of K2CO3 intramolecular cyclization occurs
with formation of 1-alkyl-6,8-di-tert-butyl-1H-imidazo[1,2-a]pyridin-4-ium 3a,b, 5-alkyl-2,4-di-tert-butyl-
5H-pyrido[1,2-a]benzimidazol-10-ium 4a,b, 6,8-di-tert-butyl-1-methyl-1H-[1,2,4]triazolo[4,3-a]pyridin-4-ium
(5), and 6,8-di-tert-butyl-3-methylthiazolo[3,2-a]pyridin-4-ium (6) bromides respectively. This method was the
best one for production of the quaternary salts 3-6. The use of other bases (Et3N, morpholine, MeONa) as
initiators of cyclization either reduces the yield of the desired products 5 and 6 or leads to the formation of
mixtures consisting of the starting salt 2, the desired products 3 and 4, and hydroxy derivatives of
azolo[a]pyridine. The structure of compounds 2a-f and their cyclization products 3-6 was established by the data
from the IR and 1H NMR spectra, in which a series of analogies with the spectra of the quaternary azolium salts
of γ-bromodypnone and the corresponding diarylazolo[a]pyridines bromides are observed [7, 8] (Table 2).
Earlier in the case of quaternary azolium salts of γ-bromodypnone it was shown that the structure of
undehydrated products from their cyclization is determined by the reaction mechanism, i.e., by the structure of
the ylide generated during the action of the bases [7, 8]. One of the deciding factors here was the nature of the
base; the ylide 7 is formed preferentially in the presence of a relatively weak base (Et3N) (Scheme 2), while the
ylides 8 and 9 are formed in the presence of stronger bases (morpholine, MeONa).
The same relationships are observed for the quaternary imidazolium and benzimidazolium salts of the
bromoketone 1. Thus, pure 2,4-di-tert-butyl-4-hydroxy-5-methyl-4,5-dihydro-1H-pyrido[1,2-a]benzimidazol-
10-ium bromide (10) is formed when the salt 2c is heated with Et3N in acetone, while a mixture of compounds
10+11 (1:2), the main component of which 2,4-di-tert-butyl-4-hydroxy-5-methyl-4,5-dihydro-3H-pyrido[1,2-a]-
benzimidazol-10-ium bromide is the product from cyclization of an ylide of type 9, is formed in the presence of
1
MeONa (MeOH, 0-5°C). The isomers 10 and 11 are easily identified by the data from the H NMR spectra.
1437

Scheme 2
R
R
R
_
_
Br
Br
+
N
+
N
+
N
COR
COR
H
COR
X
X
X
20
2
8
R
R
+
N
+
N
COR
COR
X
X
9
7
Thus, in the spectrum of compound 10 the signals of the methylene group 1-CH2 are observed in the form of an
AB spin system (5.30 and 4.98 ppm) with 2J = 18.0 Hz, and the signal of H-3 is at 5.83 ppm. For compound 11,
the signal of the 3-CH2 group is in the region of 3.07 ppm in the form of a narrow multiplet and H-1 is at
7.35 ppm. Like its 2,4-diaryl-substituted analogs [7] 4-hydroxy-3H-pyridobenzimidazolium bromide 11 is
unstable and loses a molecule of water when heated or during chromatographic separation of the mixture. When
heated in ethanol in the presence of K2CO3 the hydroxy derivatives 10 and 11 are easily transformed into the
corresponding pyridobenzimidazolium bromide 4a.
Scheme 3
Bu-t
Bu-t
_
_
Br
Br
+
+
N
N
Et3N
MeONa
MeOH
2a,c
Bu-t
10
+
Bu-t
N
Me2CO
N
HO
HO
Me
10
Me
11
Ac2O
_
Bu-t
Br
Bu-t
+
+
N
N
_
Br
Bu-t
N
N
HO
Bu-t
Me
HO
12
Me
A
According to previously obtained data [7], 1-alkyl-8-hydroxy-7,8-dihydro-1H-imidazo[1,2-a]pyridin-
4-ium salts are distinguished by even lower stability to heating in the presence of bases. We tried a new method
for cyclization of azolium salts by heating them in Ac2O, but it only proved moderately suitable for imidazole
derivatives. Under these conditions the salt 2a undergoes cyclization to 6,8-di-tert-butyl-8-hydroxy-1-methyl-
7,8-dihydro-1H-imidazo[1,2-a]pyridin-4-ium bromide (12). It was not possible to isolate compound 12 in the
1
pure form, and it was only characterized by the difference in H NMR spectrum. Thus, the spectrum contained
signals for the methine proton H-5 in the aromatic protons resonance region (6.70 ppm) and for the methylene
group in the form of an AB system in the upfield region with δ < 3.0 ppm (2J = 15.0 Hz).
1438

TABLE 1. Physicochemical Characteristics of the Synthesized Compounds
Found, %
——————
Com-
pound
Empirical
formula
Calculated, %
Mp*, °С
Yield, %
C
H
Br
N
С16H27BrN2O
55.90
55.98
8.05
7.93
23.22
23.28
8.18
8.16
132-134
86
2а
С20H29BrN2O
С26H33BrN2O
С16H26BrNOS
С16H25BrN2
С22H29BrN2
С20H27BrN2
С26H31BrN2
С15H24BrN3
С16H24BrNS
С20H29BrN2O
С10H13BrN2
С16H17BrN2
С14H15BrN2
С9H12BrN3
60.95
61.07
7.40
7.43
20.35
20.31
7.10
7.12
188-190
210-212
192-194
250-252
210-212
315-317
294-296
178-180
202-204
248-250
216-218
204-206
310-312
262-264
218-220
262-264
77
73
74
90*2
85
87
84
48
86
89*2
63
57
51
48
55
41
2с
66.48
66.52
7.18
7.09
17.00
17.02
6.00
5.97
2d
53.28
53.33
7.21
7.27
22.15
22.17
3.91
3.89
2f
58.90
59.08
7.81
7.75
24.55
24.56
8.60
8.61
3a
65.80
65.83
7.20
7.28
19.95
19.91
7.00
6.98
3b
63.94
64.00
7.21
7.25
21.31
21.29
7.48
7.46
4a
69.02
69.17
6.95
6.92
17.73
17.70
6.23
6.21
4b
55.15
55.22
7.38
7.41
24.52
24.49
12.90
12.88
5
56.10
56.13
7.08
7.07
23.36
23.34
4.10
4.09
6
60.95
61.07
7.40
7.43
20.33
20.31
7.14
7.12
10
49.75
49.81
5.40
5.43
33.10
33.14
11.65
11.62
14a
60.61
60.58
5.46
5.40
25.15
25.19
8.82
8.83
14b
57.70
57.75
5.21
5.19
27.45
27.44
9.60
9.62
15
44.60
44.65
4.97
5.00
33.02
33.00
17.33
17.36
16
С9H11BrN2
47.65
47.60
5.00
4.88
35.17
35.18
12.35
12.34
18
С8H10BrN3
42.10
42.13
4.39
4.42
35.08
35.03
18.40
18.42
19
_______
*Solvents: MeCN (compounds 2a,c,d,f), MeOH (compounds 3a,b, 4a,b),
MeNO2 (compounds 5, 6), EtOH (compounds 10, 14a,b, 15, 16, 18, 19).
*2Yield by method A.
The mechanism of cyclization under these conditions probably includes a stage of the dienol A
formation, and it is this that determines the structure of the reaction product as the 7,8-dihydro derivative.
Heating the quaternary benzimidazolium, triazolium, and thiazolium salts 2c,e,f in Ac2O does not lead to
cyclization. Like the salts 10 and 11, compound 12 is easily transformed into the corresponding imidazo-
pyridinium bromide 3a when heated (EtOH, K2CO3).
The possibility of formation of diazolium ylides with structures 8 and 9 by the action of strong bases
(Scheme 2) obviously removes restrictions of the γ-halo-substituted unsaturated ketones configuration used for
synthesis of azolo[a]pyridines. 5-Bromo-4-methylpent-3-en-2-one (13) (Scheme 4) represents an equilibrium
1439

1440

1441

mixture of the (E)- and (Z)-isomers (~56:44) [11]. From the ketone 13, according to the scheme developed for
the case of the bromo ketone 1, high yields (48-63%) of 1-alkyl-6,8-dimethyl-1H-imidazo[1,2-a]pyridin-4-ium
(14a,b), 2,4,5-trimethyl-5H-pyrido[1,2-a]benzimidazol-10-ium (15), and 1,6,8-trimethyl-1H-[1,2,4]triazolo[4,3-a]
pyridin-4-ium (16) bromides were obtained. The transformation of (3E)-5-bromopent-3-en-2-one (17) into
1,8-dimethyl-1H-imidazo[1,2-a]pyridin-4-ium (18) and 1,8-dimethyl-1H-[1,2,4]triazolo[4,3-a]pyridin-4-ium
(19) bromides was realized in the same way with high yields (41% and 55% respectively). The intermediate
products from alkylation of the azoles by the ketones 13 and 17 were not isolated in the pure form since in
addition to the quaternary salts, the reaction mixtures also contained the products from their further cyclization.
1
The conclusion about the qualitative composition of the reaction mixtures was based on data from their H
NMR spectra and the pure cyclization products (Table 2). In our opinion the relatively high yields of
compounds 14 and 15 and the fact that the products 18 and 19 are formed indicate that the mechanism of
cyclization in these cases includes a stage with the formation of ylides of type 8 from salts with structure 20
((E)-isomers) (Scheme 2).
It should be noted that the 1,8-dimethyl-1H-imidazo[1,2-a]pyridin-4-ium salt in the form of the iodide
was described earlier [12, 13], but its method of synthesis involved alkylation of the corresponding 8-methyl-
imidazo[1,2-a]pyridine. Thus, we have found a new method for the production of derivatives of the
azolo[a]pyridine system alkyl-substituted in the pyridine part of the molecule by annelation of the four-carbon
fragment to the azole.
Scheme 4
Br
N+
Br
N+
Me
COMe
Me
Me
Br
1, 2
1, 2
N
N
N
13
1, 2
Me
R
Me
Me
16
14
Me
Me
Br
N+
N
Me
15
Br
Br
N+
N+
COMe
1, 2
1, 2
N
N
N
Br
Me
Me
Me
Me
17
19
18
14a R = Me, b R = Bn
N
1:
, C6H6, r.t.; 2: K2CO3, EtOH, 
N
R
EXPERIMENTAL
1
The IR spectra were recorded using KBr pellets on a Perkin Elmer Spectrum BX instrument. The H
NMR spectra were recorded on a Bruker Avance DRX-500 instrument (500 MHz) with TMS as internal
standard. The purity of the obtained compounds was monitored by HPLC mass spectrometry on an Agilent 1100
1442

instrument with an Agilent LC/MSD SL selective detector (sample in a CF3CO2H matrix, EI ionization).
Elemental analysis for CHNOS was conducted on a Vario Macro cube elemental analyzer, and elemental
analysis for Br was done by the Schöniger flask method.
(4Z)-5-(Bromomethyl)-2,2,6,6-tetramethylhept-4-en-3-one (1) was obtained by the known method [10].
5-Bromo-4-methylpent-3-en-2-one (13) (content 80%), obtained by the bromination of mesityl oxide [11], was
used for the synthesis of products 14-16. (3E)-5-Bromopent-3-en-2-one (17) (content 86%), obtained by
acylation of 3-bromo-1-propene [14], was used for the synthesis of products 18 and 19.
3-[(2Z)-2-tert-Butyl-5,5-dimethyl-4-oxohex-2-en-1-yl]-1-methyl-1H-imidazol-3-ium (2a), 1-Alkyl-
3-[(2Z)-2-tert-butyl-5,5-dimethyl-4-oxohex-2-en-1-yl]-1H-benzimidazol-3-ium (2c,d), and 3-[(2Z)-2-tert-
Butyl-5,5-dimethyl-4-oxohex-2-en-1-yl]-4-methyl-1,3-thiazol-3-ium (2f) Bromides (General Method).
Diazole or benzimidazole (3.83 mmol) was added to a solution of bromoheptenone 1 (1.0 g, 3.83 mmol) in
benzene (30 ml) . The mixture was kept at room temperature for 1-2 days. The precipitate was filtered off,
washed with acetone, and recrystallized from MeCN.
6,8-Di-tert-butyl-1-methyl-1H-imidazo[1,2-a]pyridin-4-ium Bromide (3a). A. K2CO3 (0.62 g,
4.5 mmol) was added to a solution of the salt 2a (1.03 g, 3.00 mmol) in ethanol (40 ml). The mixture was
boiled for 30 min. The precipitate was filtered off, the solvent was evaporated, and hexane (30 ml) was added.
The colorless precipitate was filtered off and washed with diethyl ether.
B. The salt 2a (1.03 g, 3.00 mmol) was dissolved in Ac2O (30 ml) and heated at 50°C for 30 min. The
solvent was evaporated under vacuum, and an oily residue was obtained. It contained 78% of 6,8-di-tert-butyl-
8-hydroxy-1-methyl-7,8-dihydro-1H-imidazo[1,2-a]pyridin-4-ium bromide (12), 20% of the starting salt 2a,
and < 10% of imidazo[1,2-a]pyridinium bromide 3a. The mixture was dissolved in ethanol (40 ml), and the
reaction was then continued as in method A. Yield 0.79 g (81%).
1-Benzyl-6,8-di-tert-butyl-1H-imidazo[1,2-a]pyridin-4-ium Bromide (3b). 1-Benzyl-1H-imidazole (0.8
g, 3.83 mmol) was added to a solution of bromoheptenone 1 (1.0 g, 3.83 mmol) in benzene (30 ml). The mixture
was kept at room temperature for two days. The precipitate was filtered off and washed with acetone. A mixture
containing 82% of 1-benzyl-3-[(2Z)-2-tert-butyl-5,5-dimethyl-4-oxohex-2-en-1-yl]-1H-imidazol-3-ium bromide
2b was obtained. The reaction was then continued according to method A for the synthesis of product 3a.
5-Alkyl-2,4-di-tert-butyl-5H-pyrido[1,2-a]benzimidazol-10-ium (4a,b) and 6,8-Di-tert-butyl-
3-methylthiazolo[3,2-a]pyridin-4-ium (6) Bromides (General Method). The compounds were obtained by
method A for the synthesis of compound 3a. The heating time was 45 min.
6,8-Di-tert-butyl-1-methyl-1H-[1,2,4]triazolo[4,3-a]pyridin-4-ium Bromide (5). 1-Methyl-1H-1,2,4-
triazole (0.32 g, 3.83 mmol) was added to a solution of bromoheptanone 1 (1.0 g, 3.83 mmol) in benzene (30
ml). The mixture was kept at room temperature for four days. The solvent was evaporated, and a mixture
containing 80% of 4-[(2Z)-2-tert-butyl-5,5-dimethyl-4-oxohex-2-en-1-yl]-1-methyl-1H-1,2,4-triazol-4-ium
bromide (2e) was obtained. The reaction was then continued according to method A for the synthesis of
compound 3a.
2,4-Di-tert-butyl-4-hydroxy-5-methyl-4,5-dihydro-1H-pyrido[1,2-a]benzimidazol-10-ium
Bromide
(10). A. A mixture of the benzimidazolium salt 2c (1 g, 2.54 mmol) and Et3N (4 ml) in acetone (25 ml) was
heated for 45 min. After cooling, the precipitate was filtered off and washed with acetone.
B. Sodium (0.25 g, 11.0 mmol) was dissolved in MeOH (15 ml), and salt 2c (0.45 g, 1.15 mmol) was
added with stirring to the MeONa solution cooled to 0-5°C. The mixture was stirred for a further 1.5 h while the
temperature was maintained at 5-10°C. The solvent was evaporated, water (20 ml) was added, and the product
was extracted with chloroform. The combined extracts were dried over Na2SO4 (anh.), the solvent was
evaporated, and a 2:1 mixture of 2,4-di-tert-butyl-4-hydroxy-5-methyl-4,5-dihydro-3H-pyrido[1,2-a]-
benzimidazol-10-ium bromide (11) and 2,4-di-tert-butyl-4-hydroxy-5-methyl-4,5-dihydro-1H-pyrido-
[1,2-a]benzimidazol-10-ium bromide (10) was obtained in the oily residue.
1443

1-Alkyl-6,8-dimethyl-1H-imidazo[1,2-a]pyridin-4-ium Bromides 14a,b, 2,4,5-Trimethyl-5H-pyrido-
[1,2-a]benzimidazol-10-ium Bromide (15), and 1,6,8-Trimethyl-1H-[1,2,4]triazolo[4,3-a]pyridin-4-ium
Bromide (16) (General Method). 1-Alkyl-1H-imidazole, 1-methyl-1H-benzimidazole, or 1-methyl-1H-1,2,4-tri-
azole (4.0 mmol) respectively was added to a solution of 80% 5-bromo-4-methylpent-3-en-2-one (13) (0.89 g,
4.0 mmol) in benzene (40 ml). The mixture was kept at room temperature for 1-2 days. The solvent was
evaporated, the oily residue was dissolved in ethanol (30 ml), K2CO3 (0.62 g, 4.5 mmol) was added, and the
mixture was boiled for 30 min. The precipitate was filtered off, the solvent was evaporated, and 30 ml of Et2O
was added. The colorless precipitate was filtered off and washed with Et2O.
1,8-Dimethyl-1H-imidazo[1,2-a]pyridin-4-ium Bromide (18) and 1,8-dimethyl-1H-[1,2,4]triazolo-
[4,3-a]pyridin-4-ium bromide (19) were obtained by the method used for the synthesis of compounds 14-16
from 86% (3E)-5-bromopent-3-en-2-one (17) (0.76 g, 4.00 mmol).
REFERENCES
1.
2.
3.
F. Aydogan and A. S. Demir, Tetrahedron, 61, 3019 (2005).
A. S. Demir, A. C. Igdir, and N. B. Gunay, Tetrahedron: Asymmetry, 16, 3170 (2005).
R. A. Nadzhafova, G. G. Ibragimov, R. A. Gadzhily, S. K. Zeynalova, and Kh. S. Khalilov, Azerb.
Khim. Zh., № 3, 32 (2001).
4.
R. A. Gadzhili, V. M. Fedoseev, N. A. Netkacheva, Ch. N. Akhmedov, and M. Sh. Sultanova, Khim.
Geterotsikl. Soedin., 998 (1989). [Chem. Heterocycl. Comp., 25, 837 (1989)].
M. N. Alberti, G. C. Vougioukalakis, and M. Orfanopoulos, J. Org. Chem., 74, 7274 (2009).
V. Kovtunenko, L. Potikha, and A. Turov, Synth. Commun., 34, 3609 (2004).
L. M. Potikha, A. R. Turelik, V. A. Kovtunenko, and A. V. Turov, Khim. Geterotsikl. Soedin., 95
(2010). [Chem. Heterocycl. Comp., 46, 82 (2010)].
5.
6.
7.
8.
L. M. Potikha, A. R. Turelik, V. A. Kovtunenko, and A. V. Turov, Khim. Geterotsikl. Soedin., 275
(2010). [Chem. Heterocycl. Comp., 46, 223 (2010)].
9.
10.
11.
12.
13.
14.
M. J. Miller, M. H. Lyttle, and A. Streitwieser, J. Org. Chem., 46, 1977 (1981).
E. E. van Tamelen, and T. H. Whitesides, J. Am. Chem. Soc., 90, 3894 (1968).
D. V. С. Awang and A. Vincent, J. Org. Chem., 37, 2625 (1972).
W. W. Paudler and L. S. Helmick, J. Org. Chem., 33, 1087 (1968).
G. Maury and C. Pigiere, Tetrahedron, 37, 91 (1981).
I. I. Ibragimov, E. I. Mamedov, A. T. Ismailov, A. G. Aliev, Sh. Z. Mekhtieva, V. G. Dzhafarov, and
V. I. Belyaeva, Zh. Org. Khim., 26, 1648 (1990).
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