Synthesis and some transformations of 2-(2′-thienyl)benzothiazole

Article abstract of DOI:10.1134/S1070427210090181

Condensation of ortho-aminothiophenol with 2-thenoyl chloride in 1-methyl-2-pyrrolidone was used to synthesize 2-(2′-thienyl)benzothiazole. Its reactions of electrophilic (nitration, bromination, oxymethylation, formylation, acylation) and radical (nitration, arylation) substitution were studied.

Full text of DOI:10.1134/S1070427210090181

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2010, Vol. 83, No. 9, pp. 1602–1606. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © A.A. Aleksandrov, M.M. El’chaninov, 2010, published in Zhurnal Prikladnoi Khimii, 2010, Vol. 83, No. 9, pp. 1504–1508.
ORGANIC SYNTHESIS AND INDUSTRIAL
ORGANIC CHEMISTRY
Synthesis and Some Transformations
of 2-(2'-Thienyl)benzothiazole
A. A. Aleksandrov and M. M. El’chaninov
South-Russian State Technical University (Novocherkassk Polytechnic Institute),
Novocherkassk, Rostov-on-Don oblast, Russia
Received November 10, 2009
Abstract—Condensation of ortho-aminothiophenol with 2-thenoyl chloride in 1-methyl-2-pyrrolidone was
used to synthesize 2-(2'-thienyl)benzothiazole. Its reactions of electrophilic (nitration, bromination, oxy-
methylation, formylation, acylation) and radical (nitration, arylation) substitution were studied.
DOI: 10.1134/S1070427210090181
The mutual influence of heterocyclic rings in
bihetaryls is an interesting and as yet poorly studied
problem. The most scarce are data on the reactivity of
compounds in which benzothiazole and π-excessive
thiophene rings are linked to each other. The reactivity
of compounds of this kind is at times rather unex-
pected. It has been shown that 2-(2-thienyl)oxazole is
formylated at the oxazole ring, although the π-
excessiveness of the thiophene ring is higher.
Previously, compound I has been synthesized in
41% yield from the Zn-salt of 2-aminothiophenol by
its boiling with 2-thenoyl chloride [2], followed by
treatment of 2-thienylbenzothiazoline with FeCl3 in an
ethanol solution [3]. Synthesis of compound I by
condensation of ortho-aminothiophenol with 2-formyl-
thiophene under heating to 96°C was described in [4].
The best result was obtained in interaction of ortho-
aminophenol with 2-thenoyl chloride in 1-methyl-2-
pyrrolidone (yield 86%). The thus obtained 2-(2'-
thienyl)benzothiazole (I) was introduced into reactions
with electrophilic reagents: mixture of nitric and poly-
phosphoric (PPA) acids, bromine in dichloroethane,
formalin in the presence of hydrochloric acid, urotro-
pin in PPA, and carboxylic acids in PPA. Also, reac-
tions of radical substitution were performed: nitration
in dilute nitric acid and Meerwein arylation:
The goal of our study was to develop or choose a
convenient synthesis method and examine the relative
reactivity of 2-(2'-thienyl)benzothiazole (I) and com-
pare it with that of the benzimidazole analog (II):
N
S
N
S
N
S
CH3
I
II
N
S
S
O
R'COOH/(CH2)6N4
PPA
S
S
HNO3 (diluted)
R
Br2/Dichloroethane
R'
Ia, Ic
I
Ie−Ii
p-O2NC6H4N2Cl
HNO3
CH2O
HCl
PPA
NO2
S
S
N
S
S
NO2
CH2OH
O2N
Ib
Id
Ij
R = NO2 (Ia), Br (Ic); R' = H (Ie), Me (If), CH2COMe (Ig), Ph (Ih), p-FC6H4 (Ii).
1602

SYNTHESIS AND SOME TRANSFORMATIONS OF 2-(2'-THIENYL)BENZOTHIAZOLE
1603
Comparison of the chemical shifts of protons of the
2-thienyl group in H NMR spectra of compounds I
exceeding 17%. The return of the starting compound I
together with unidentified impurities was 69%.
1
and II shows that, in these compounds, protons H-4'
are the most strongly shielded (7.15 and 7.25 ppm),
H-3' protons are less shielded (both 7.51 ppm), and
H-5' protons are shielded to an even lesser extent (7.66
and 7.52 ppm). The values of the last chemical shifts
indicate that the deshielding (electron-acceptor) effect
of the 2-benzothiazolyl group exceeds that of the 2-
benzimidazolyl group, and, therefore, a decrease in the
relative reactivity of the thiophene ring in compound I
would be expected.
Oxidation of aldehyde Ie with potassium bichro-
mate in 20% sulfuric acid gives, similarly to the formyl
derivative of compound II, a stable carboxylic acid.
This is due to the lower basicity of compound I as
compared with benzimidazole II, which presumably
diminishes the probability of formation of the easily
decarboxylated betaine:
O
H
O
S
K2Cr2O7
H2SO4
S
OH
The results obtained confirm our assumption. For
example, the nitration of 2-(2'-thienyl)benzothiazole
occurs in boiling in dilute nitric acid (d = 1.32 g cm–3)
to give 2-(5'-nitro-2'-thienyl)benzothiazole (Ia) in 37%
yield, with 50% of the starting compound I returned.
In nitration with fuming nitric acid (d = 1.51 g cm–3) in
PPA, compound Ia is also formed at 20°C, and 5',6-
dinitro derivative Ib, at 95°C. It should be noted for
comparison that 2-(2'-thienyl)benzimidazole (II) is
nitrated with a mixture of fuming nitric acid and acetic
anhydride at 0°C to give a mononitro derivative [5]. In
addition, compound Ia was produced by counter
synthesis via ipso-substitution of bromine with a nitro
group in 2-(5'-bromo-2'-thienyl)benzothiazole (Ic).
Bromination of compound I with bromine in di-
chloroethane also results in substitution at the position
of the 5'-thiophene ring. In contrast to the case of
benzimidazole II, no formation of 4',5'-dibromo
derivative or substitution in the benzothiazole moiety
occurs even upon prolonged boiling with a threefold
excess of bromine.
Ie
Ik
O−
O
H
N+
S
I
×
−CO2
S
Various kinds of acetylation of compound I, tested
in the study, failed to produce positive results. Taking
into account the successful experience of acetylation of
furylbenzimidazole [7], we used the method suggested
by Gardner for acetylation of phenols and phenyl
ethers [8], in which compound I was treated with
acetic acid or its anhydride in PPA at 110–120°C.
Similarly to compound II, 2-(2'-thienyl)benzothiazole
(I) is rather difficultly acylated into the position of the
5'-thiophene ring to give ketone If in a 28% yield. The
reaction is accompanied by formation of a by-product
whose amount increases with temperature. Pres-
umably, this by-product is diketone Ig formed via
acylation of the –COMe group in monoketone If.
Column chromatography was used to isolate 2-(5'-
acetacetyl-2'-thienyl)benzothiazole (Ig) in 32% yield.
The IR spectrum of compound Ig contains two bands
of stretching vibrations of the C=O group at 1630 and
1680 cm–1. The benzoylation of compound I with
benzoic and para-fluorobenzoic acids was also per-
formed using the Gardner method, but at a higher
temperature (170°C). In this case, 5'-benzoyl deriva-
tives are formed in moderate yields.
Because, in contrast to compound II, benzothiazole
I does not enter into the chloromethylation by para-
form and conc. HCl, we hydroxymethylated compound
I by boiling in a 40% solution of formaldehyde in the
presence of catalytic amounts of hydrochloric acid (d =
1.19 g cm–3). After boiling for 12 h, carbinol Id was
isolated from the reaction mixture in ~16% yield; the
return of the starting compound I was 77%.
Taking into account the negative result obtained in
formylation of compound I with the Vielsmeier
reagent in [6], we used in this study a method that had
been used previously for formylation of benzimi-
dazoles: specifically by treatment of benzothiazole I
with urotropin in PPA. Compared with benzimidazole
II, which forms a 5' formyl-substituted derivative in
91% yield [7], the corresponding 2-(5'-formyl-2'-
thienyl)benzothiazole (Ie) is obtained in a yield not
Benzothiazole I was arylated by Meerwein by
treatment with para-nitrophenyldiazonium chloride in
an aqueous–acetone solution with CuCl2 as a catalyst.
The yield of 2-(5'-para-nitrophenyl-2'-thienyl)benzo-
thiazole (Ij) was 42%.
As demonstrated by the experimental data, the
thiophene ring in compound I is, in contrast to
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 9 2010

1604
ALEKSANDROV, EL’CHANINOV
Table 1. Yields, melting points, and elemental analysis data for 2-(2'-thienyl)benzothiazoles Ia–Ik
Found, %
Calculated, %
H
Comp.
no.
Yield,
%
mp,°С
(EtOH)
Formula
C
H
N
6.73
10.55
14.02
–
C
N
6.45
10.68
13.76
–
I
86
43
27
54
16
17
28
32
47
55
42
63
98–99
61.17
49.98
–
2.97
2.66
–
C11H7NS2
60.80
50.37
–
3.25
2.31
–
Ia
Ib
Ic
Id
Ie
If
205–206
202–203
87–88
C11H6N2O2S2
C11H5N3O4S2
C11H6BrNS2
C12H9NOS2
C12H7NOS2
C13H9NOS2
C15H11NO2S2
C18H11NOS2
C18H10FNOS2
C17H10N2O2S2
C12H7NO2S2
44.82
57.93
59.03
60.49
60.12
66.88
64.07
59.97
55.46
1.88
3.55
3.17
3.77
3.42
3.71
3.15
3.22
2.47
44.60
58.27
58.75
60.21
59.78
67.26
63.70
60.34
55.16
2.04
3.67
2.88
3.50
3.68
3.45
2.97
2.98
2.70
118–119
140–141
135–136
190–191
134–135
172–173
167–168
243–244
5.83
5.66
–
5.66
5.71
–
Ig
Ih
Ii
4.27
–
4.59
–
3.88
7.85
–
4.13
8.28
–
Ij
Ika
a Compound Ik was recrystallized from water.
substance II, less reactive. For example, we failed to
obtain a sulfo derivative despite numerous attempts to
sulfate compound I under various conditions (mixture
of PPA and H2SO4, concentrated H2SO4, sodium
benzene sulfonate in PPA). We could not obtain,
either, a product of chloromethylation under the action
of paraform and conc. HCl on the starting benzo-
thiazole (I). This is presumably due to protonation of
the benzothiazole moiety in compound I by the strong
acids, enhancement of its electron-acceptor properties,
and the resulting deactivation of the thiophene ring.
the resulting compounds, verified y TLC on plates
with Al2O3 of II degree of activity by Brokman
(development with an iodine vapor) in CH2Cl2 and on
Silufol UV 254 plates in CH2Cl2. The elemental
analysis was made with a Perkin-Elmer 2400 analyzer.
The melting points were determined by the capillary
method on a PTP instrument. The yield, melting
points, and spectral characteristics of the compounds
obtained are listed in Tables 1 and 2.
2-(2'-Thienyl)benzothiazole (I). To a solution of
1.25 g (10 mmol) of ortho-aminothiophenol in 10 ml
of 1-methylpyrrolidone was added 1.47 g (10 mmol) of
thiophene-2-carbonyl chloride. The mixture was boiled
for 2 h and a preliminarily cooled reaction mass was
poured into 50 ml of cold water. The precipitate
formed was separated, thoroughly washed with cold
water, and crystallized from ethanol.
It is a common knowledge that, together with
benzimidazoles and benzoxazoles, some derivatives of
2-hetarylbenzothiazoles are known as antiparasitic
agents. For example, 2-(5'-nitro-2'-thienyl)benzothia-
zole (Ia) synthesized by condensation of ortho-amino-
phenol with 5-nitro-2-thiophenaldehyde in the pres-
ence of Pb(OAc)4 exhibits a high antichromonal and
antihelminth activity [9].
2-(5'-Nitro-2'-thienyl)benzothiazole (Ia). a. A solu-
tion of 1.09 g (5 mmol) of compound I in 25 ml of
nitric acid (d = 1.32 g cm–3) was boiled for 3 h. The
reaction mass was cooled and poured into 150 ml of
cold water. The precipitate formed was filtered off,
washed two to three times with a small amount of cold
water and dried in air.
EXPERIMENTAL
IR spectra were recorded with a Specord 75-IR
1
spectrometer in Vaseline oil and chloroform, and H
NMR spectra, with a Varian Unity 300 instrument
(300 MHz, CDCl3, TMS as internal standard). The
reaction course was monitored, and the individuality of
b. A 1.48-g portion (5 mmol) of compound Ic was
dissolved in 15 ml of acetic acid and 1.04 g (15 mmol)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2010

SYNTHESIS AND SOME TRANSFORMATIONS OF 2-(2'-THIENYL)BENZOTHIAZOLE
1605
Table 2. Spectral characteristics of 2-(2'-thienyl)benzothiazoles Ia–Ik
Comp. no. IR spectrum, ν, cm–1
1Н NMR spectrum, δ, ppm (J, Hz; CDCl3)
7.15 d.d (1H, J5,4=5.1, J4,3=4.0, H-4'); 7.38 t (1H, J = 7.7, H-6arom); 7.48 t (1H, J = 7.7,
I
–
H-5arom); 7.51 d (1H, J = 4.2, Н-3'); 7.66 d (1H, J = 5.5, H-5'); 7.86 d (1H, J = 7.9, Н-7arom);
8.04 d (1H, J = 7.9, H-4arom
)
7.44 t (1H, J = 7.1, H-6arom); 7.53 t (1H, J = 8.0, Н-5arom); 7.51 d (1H, J = 4.4, Н-3'); 7.89 d
1370 (sym.), 1530,
1550 (assym.) (NO2)
Ia
Ib
Ic
Id
(1H, J = 7.0, H-7arom); 7.91 d (1H, J = 4.4, H-4'); 8.06 d (1H, J = 8.2, H-4arom
7.55 d (1H, J = 4.2, H-3'); 7.95 d (1H, J = 4.2, H-4'); 8.22 d (1H, J = 8.0, H-4arom); 8.46 d
)
1370 (sym.), 1530,
1550 (assym.) (NO2)
(1H, J = 8.0, H-5arom); 8.94 s (1Н, Н-7arom
7.10 d (1H, J = 4.0, H-4'); 7.38 t (1H, J = 7.7., Н-6arom); 7.39 d (1H, J = 4.0, H-3'); 7.48 t
(1H, J = 7.7, H-5arom); 7.85 d (1H, J = 7.6, Н-7arom); 8.01 d (1H, J = 7.9, Н-4arom
)
–
)
2.02 (1Н, br.s, ОН); 4.89 (2Н, s, СН2); 7.03 d (1H, J = 3.7, Н-4'); 7.37 t (1H, J = 7.2,
Н-6arom); 7.47 t (1H, J = 7.5, Н-5arom); 7.54 d (1H, J = 3.7, Н-3'); 7.85 d (1H, J = 8.0,
1130 (OH)
Н-7arom); 8.02 d (1H, J = 8.1, Р-4arom
)
7.44 t (1H, J = 7.5, Н-6arom); 7.52 t (1H, J = 7.7, Н-5arom); 7.67 d (1H, J = 3.9, Н-3'); 7.72 d
(1H, J = 3.9, Н-4'); 7.90 d (1H, J = 7.8, Н-7arom); 8.07 d (1H, J = 7.8, Р-4arom); 9.97 s (1Н,
СНО)
1680 (C=O)
1660 (C=O)
Ie
If
2.61 s (3Н, СН3); 7.42 t (1H, J = 7.5, Н-6arom); 7.52 t (1H, J = 7.7, Н-5arom); 7.65 d (1H, J =
4.0, Н-3'); 7.70 d (1H, J = 4.0, H-4'); 7.89 d (1H, J = 7.8, Н-7arom); 8.07 d (1H, J = 8.0,
Н-4arom
)
2.37 s (3Н, СН ); 6.17 d (1H, J = 1.4, СН ); 6.65 d (1H, J = 2.2, СН ); 7.42 t (1H, J = 7.5,
1680 (C=O), 1630
(C=O)
3
2
2
Ig
Ih
Ii
Н-6arom); 7.52 t (1H, J = 7.7, Н-5arom); 7.55 d (1H, J = 4.0, H-3'); 7.63 d (1H, J = 4.0, H-4');
7.89 d (1H, J = 7.8, Н-7arom); 8.06 d (1H, J = 8.1, Н-4arom
)
7.43 t (1H, J = 7.2, Н-6arom); 7.53 t (3Н, J = 7.5, H-3,4,5); 7.63 t (1H, J = 7.2, Н-5arom); 7.67
d (1H, J = 4.0, H-3'); 7.70 d (1H, J = 4.0, H-4'); 7.90 d (1H, J = 8.2, Н-7arom); 7.91 d (2Н,
1640 (C=O)
1670 (C=O)
J = 7.2, H-2,6); 8.09 d (1H, J = 8.5, Н-4arom
)
7.20 d (1H, J = 8.6, H-3); 7.23 d (1H, J = 8.6, H-5); 7.44 t (1H, J = 7.4, Н-6arom); 7.53 t (1H,
J = 8.2, Н-5arom); 7.67 d (1H, J = 4.0, H-3'); 7.70 d (1H, J = 4.0, H-4'); 7.91 d (1H, J = 7.7,
Н-7arom); 7.95 d (1H, J = 5.4, H-2); 7.98 d (1H, J = 5.4, H-6); 8.09 d (1H, J = 8.3,
Н-4arom
)
7.12 d (1H, J = 3.8, H-4'); 7.38 (1H, t, J = 7.7, Н-6arom); 7.48 t (1H, J = 7.7, H-5arom); 7.52 d
(1H, J = 3.8, H-3'); 7.80 d (2H, J = 8.0, H-2,6); 7.85 d (1H, J = 8.0, Н-7arom); 8.07 d (1H,
J = 7.7, Н-4arom); 8.30 d (2Н, J = 8.0, H-3,5)
–
Ij
7.40–7.60 m (2H, Н-5,6arom); 7.72 d (1H, J = 4.0, H-4'); 7.82 d (1H, J = 4.0, H-3'); 8.00–
Ika
1700 (C=O), 3490
(OH)
8.15 m (2H, Н-4,7arom); 13.22 br.s (1Н, СООН)
a 1H NMR spectrum of compound Ik was recorded in DMSO-d6.
of sodium nitrite was introduced in portions. The
mixture was boiled for 2 h, cooled, poured into 50 ml
of water, and the target compound was isolated
similarly to run a. A mixing test of the compounds
synthesized by methods a and b did not give any
depression of the melting point.
2-(5'-Bromo-2'-thienyl)benzothiazole (Ic). To a
solution of 1.09 g (5 mmol) of compound I in 25 ml of
dichloroethane was added 0.8 ml (15 mmol) of
bromine and the mixture was boiled for 8 h. Dichloro-
ethane was evaporated and the residue was dissolved
in methylene chloride and chromatographed on a
2.5×15-cm column packed with Al2O3 (methylene
chloride as eluent).
6-Nitro-2-(5'-nitro-2'-thienyl)benzothiazole (Ib).
To a solution of 1.09 g (5 mmol) of benzothiazole I in
20 g of PPA was added 0.63 ml (15 mmol) of nitric
acid (d = 1.51 g cm–3, and the mixture was heated to
90–95°C for 3 h under permanent agitation. Then, the
mixture was poured into 100 ml of water and
neutralized with 25% aqueous ammonia, and the
precipitate of compound Ib was separated.
2-(5-Hydroxymethyl-2-thienyl)benzothiazole (Id).
A mixture of 1.09 g (5 mmol) of benzothiazole I,
35 ml of formalin, and 3–4 drops of hydrochloric acid
was boiled for 12 h and then was poured into 100 ml of
cold water and neutralized with 25% aqueous
ammonia until its odor appeared in the mixture. The
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 83 No. 9 2010

1606
ALEKSANDROV, EL’CHANINOV
precipitate was filtered off, thoroughly washed with
cold water and chromatographed on a 2.5×5-cm
column packed with Al2O3 (chloroform as eluent).
2.18 g (10 mmol) of 2-(2'-thienyl)benzothiazole in
10 ml of acetone and a solution of 0.2 g of CuCl2 in
1 ml of water were added. The reaction mass was
agitated for 24 h at 20°C. The resulting precipitate of
compound Ij was filtered off and washed with 50%
aqueous ammonia.
2-(5'-Formyl-2'-thienyl)benzothiazole (Ie). A mix-
ture of 1.09 g (5 mmol) of benzothiazole I, 20 g of
PPA, and 5.6 g (40 mmol) of urotropin was heated to
100–120°C for 10 h under vigorous agitation. Then,
the reaction mass was cooled, diluted with 50 ml of
water, and neutralized with aqueous ammonia. The
reaction product was dissolved in methylene chloride
and chromatographed on a 2.5×15-cm column packed
with Al2O3 (methylene chloride as eluent).
2-(5'-Carboxy-2'-thienyl)benzothiazole (Ik). A
mixture of 0.61 g of compound Ie and 1.47 g (5 mmol)
of K2Cr2O7 in 10 ml of a 20% solution of sulfuric acid
was heated on a boiling water bath for 2 h. Then, the
reaction mass was cooled, with crystals of acid Ik
precipitated. The crystals were separated and
recrystallized from water.
2-(5'-Acetyl-2'-thienyl)benzothiazole (If). To a
solution of 1.09 g (5 mmol) of compound I in 20 g of
PPA was added 0.95 ml (10 mmol) of acetic anhydride
and the resulting mixture was heated to 110–120°C for
16 h under permanent agitation. Then, the reaction
mass was poured into 100 ml of water and neutralized
with a solution of ammonia. The precipitate formed
was filtered off, dissolved in 50 ml of chloroform, and
chromatographed on a 2.5×20-cm column packed with
Al2O3 (chloroform as eluent).
CONCLUSIONS
(1) A method for synthesis of 2-(2-thienyl)benzo-
thiazole in 1-methyl-2-pyrrolidone was found. The
method provides a 86% yield of the desired product.
(2) A decrease in the reactivity of the thiophene
ring in 2-(2-thienyl)benzothiazole, relative to that of 1-
methyl-2-(thienyl)benzimidazole, was observed because
of the higher electron-acceptor properties of the 2-benzo-
thiazolyl group.
2-(5'-Acetoacetyl-2'-thienyl)benzothiazole (Ig).
After compound If was eluted from the column, the
elution was continued to give by-product Ig as yellow
crystals recrystallized from ethanol.
REFERENCES
1. Belen’kii, L.I., Cheskis, M.A., Zvolinskii, V.P., and
Obukhov, A.E., Khim. Geterotsikl. Soedin., 1986, no. 6,
pp. 826–831.
2. Bogert, M.T. and Stull, A., J. Am. Chem. Soc., 1926,
vol. 48, no. 2, pp. 248–251.
3. Emerson, W.S. and Patrick, T.M., J. Org. Chem., 1949,
2-(5'-Benzoyl-2'-thienyl)benzothiazole (Ih). A mix-
ture of 1.09 g (5 mmol) of compound was mixed with
20 g of PPA and 2.44 g (20 mmol) of benzoic acid was
agitated at 170–180°C for 8 h. The reaction product
was isolated similarly to the acetyl derivative (If).
vol. 14, no. 3, pp. 790–796.
4. Capitan-Vallvey, L.F. and Espinosa, P., Polyhedron,
1983, vol. 2, no. 11, pp. 1147–1153.
5. El’chaninov, M.M., Oleinikova, L.Ya., and Simonov, A.M.,
Khim. Geterotsikl. Soedin., 1979, no. 8, pp. 1047–1049.
6. Kornilov, M.Yu., Ruban, E.M., Fedchuk, V.N., et al.,
2-[5'-(para-Fluorobenzoyl)-2'-thienyl]benzothia-
zole (Ii). To a solution of 1.09 g (5 mmol) of com-
pound I in 20 g of PPA was added 2.8 g (20 mmol) of
para-fluorobenzoic acid and the mixture was heated to
170–180°C for 6 h. The reaction product was isolated
similarly to substance If.
Zh. Org. Khim., 1973, vol. 9, no. 12, pp. 2577–2582.
2-[5'-(para-Nitrophenyl)-2'-thienyl]benzothiazole
(Ij). para-Nitroaniline chloride produced from 1.38 g
(10 mmol) of para-nitroaniline, 20 ml of water, and
8 ml of conc. HCl was diazotated with a solution of
0.69 g (10 mmol) of NaNO3 in 3 ml of water. The
reaction mass was agitated for 30 min at 5°C and then
7. El’chaninov, M.M., Simonov, A.M., and Oleinikova, L.Ya.,
Khim. Geterotsikl. Soedin., 1983, no. 10, pp. 1311–1313.
8. Gardner, P.D., J. Am. Chem. Soc., 1951, vol. 76,
pp. 4550–4554.
9. Dunn, G.L., Actor, P., and Di Pasquo, V.J., J. Med.
Chem., 1966, vol. 9, no. 5, pp. 751–753.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 82 No. 9 2010