S(2)- or N(3)-substituted 2-mercapto-5-(4-pyridyl)-1,3,4-oxadiazoles

Article abstract of DOI:10.1023/A:1020646123616

New S₍₂₎- or N₍₃₎-substituted 2-mercapto-5-(4-pyridyl)-1,3,4-oxadiazoles have been obtained and characterized. The direction of substitution depends on the structure of the initial reactants and the reaction conditions. The synthesis of several thiosemicarbazides has been effected from isonicotinic acid hydrazide.

Full text of DOI:10.1023/A:1020646123616

Chemistry of Heterocyclic Compounds, Vol. 38, No. 7, 2002
S
(2)- OR N(3)-SUBSTITUTED
2
-MERCAPTO-5-(4-PYRIDYL)-
,3,4-OXADIAZOLES
1
A. Rutavicius and Z. Kuodis
New S(2)- or N(3)-substituted 2-mercapto-5-(4-pyridyl)-1,3,4-oxadiazoles have been obtained and
characterized. The direction of substitution depends on the structure of the initial reactants and the
reaction conditions. The synthesis of several thiosemicarbazides has been effected from isonicotinic
acid hydrazide.
Keywords: isonicotinic acid hydrazide, 2-mercapto-5-(4-pyridyl)-1,3,4-oxadiazole, 5-(4-pyridyl)-1,3,4-
thiadiazol-2-ylethylamine, thiosemicarbazides.
1
,3,4-Oxadiazoles containing a pyridine fragment possess antibacterial, antitubercular, and tranquilizing
properties [1].
2
-Mercapto-5-(4-pyridyl)-1,3,4-oxadiazole exists in thione and thiol forms and this determines its ability
to be substituted at sulfur or nitrogen. We [2] and others [3] previously carried out the alkylation of
1
,3,4-oxadiazoline-2-thiones with alkyl halides in alkaline medium and obtained S-alkyl derivatives in this way.
Oxadiazole 1 interacts with alkyl halides with the formation of thioethers 2a-d in the presence of alkali in
methanol or water under interphase catalysis conditions using triethylbenzylammonium chloride (TEBA) as
catalyst. As might have been expected, absorption bands for the C=S bond were absent from the IR spectra of
-1
compounds 2a-d. Two absorption bands were detected in the IR spectrum of thioether 2d at 3170 and 3340 cm
corresponding to the stretching vibration of the NH group, also an absorption band for the C=O bond
-1
(
1
amide I band) at 1685 cm , and a band corresponding to the NH deformation vibration (amide II band) at
2
-1
620 cm .
For the N-alkylation of compound 1 we chose several polar electrophilic reagents which interact with
the ambident NH–C=S system only at the more electronegative nitrogen atom possessing the highest electron
density [4]. On brief heating of compound 1 with formalin in a methanol–2-propanol mixture, the N-hydroxy-
methylated derivative 3 is formed, and under the conditions of the Mannich reaction oxadiazole 1 forms the
aminomethylated derivatives 4a,b with secondary amines. Heating compound 1 with 4-chlorophenyl isocyanate
gives benzamide 5, which was confirmed by the presence in the IR spectrum of an intense band for the
-
1
-1
stretching vibrations of the C=O bond at 1625 cm , a band for the C=S bond (1538 cm ), and bands for the
-1
-1
stretching vibrations of the NH group (3200 cm ). An absorption band for the C=S bond at 1534 cm was also
observed in the IR spectrum of compound 6, synthesized by the addition of oxadiazole 1 to 4-vinylpyridine in
acetic acid. Oxidation of compound 1 with potassium hexacyanoferriate leads to the disulfide 7.
_
_________________________________________________________________________________________
Institute of Chemistry, Vilnius LT-2600, Lithuania; e-mail: zenonas.kuodis@takas.lt. Translated from
Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 961-967, July, 2002. Original article submitted
February 26, 2001.
8
52
0009-3122/02/3807-0852$27.00©2002 Plenum Publishing Corporation

N
O
N
O
N
OH
N
N
N
SR
N
S
2
a–d
3
CH O
2
N
N
R'
N
CH O, HR'
N
2
N
O
O
SH
S
1
4
a,b
OCN
K [Fe(CN) ]
3
6
Cl
NaOH
Cl
O
N
N
N
N
N
N
N
H
N
O
S
O
S
2
5
N
7
N
N
N
O
S
N
O
6
N
N
2
a R = CH CH=CH ; b R = Bn;
c R = CH2C
CCH2S
2
2
;
d R =
CH CNH ; 4a R' = N
; b R' =
N
O
2
2
O
Isonicotinic acid hydrazide readily adds to the appropriate isothiocyanate and forms thiosemicarbazides
8
a-c (Table 1). Bands were observed in the IR spectrum of thiosemicarbazide 8b for the stretching vibrations of the
-1
C=O bond at 1674 and for C=S at 1519 and 1315 cm [5]. The formation of thiosemicarbazide 8b is indicated
1
by the appearance in its H NMR spectrum (Table 2) of the characteristic multiplicity of the proton signals of
CH
2
(4.76, d, J = 5 Hz), Ph (7.25, s), NHCH (8.55, br. t), and HN-NH (9.41, s and 10.62 ppm, s) groups.
2
S
HN NH
O
R
NH
HN NH2
O
RNCS
N
N
8
a–c
8
a R = Et; b R = Bn; c R = CH OCH C CH
2
2
8
53

On heating in anhydrous phosphoric acid thiosemicarbazide 8a is cyclized into thiadiazole 9, in the IR
spectrum of which bands for the stretching vibrations of C=S and C=O bonds were missing, but an absorption
-1
band at 2950 cm indicates the stretching vibration of the NH group.
N
S
Ac O
N
S
2
N
H PO
N
3
4
N
N
8a
CH(OC H )
2
5 3
N
N
H
O
9
10
On heating thiadiazole 9 in a mixture of acetic anhydride and ethyl orthoformate the NH group is
1
acetylated with the formation of amide 10, in the H NMR spectrum of which (Table 2) a triplet at 1.22 ppm
corresponds to the methyl group protons in the C
2
H fragment, and a singlet at 1.80 ppm to the protons of the
5
CH
at 1670 cm .
3
CO group. An intense band was observed in the IR spectrum for the stretching vibrations of the C=O bond
-1
The interaction of [5-(4-pyridyl)-1,3,4-oxadiazol-2-ylthio]acetic acid hydrazide (11) with ethyl
isothiocyanate leads to thiosemicarbazide 12.
We noted previously in [2] that the condensation of hydrazide 11 with acetylacetone leads to the
formation of a pyrazole system. However on heating hydrazide 11 with other dicarbonyl compounds, such as
ethyl acetoacetate, the linear hydrazone derivative 13 was obtained. An analogous ketohydrazone structure, also
existing in the tautomeric enehydrazine form, was also detected when obtaining benzoylhydrazones from esters
of 2,4-dioxobutanoic acid [6]. Analogously with the data of [6] an intense absorption band was observed at
-
1
1
1
720 cm in the IR spectrum of hydrazone 13, indicating the presence of an ester grouping, and a band at
-1 -1
675 cm for the C=O bond of the amide group. An absorption was observed at 2950 cm for the stretching
1
vibrations of the NH group. The presence in the H NMR spectrum of hydrazone 13 of characteristic signals for
the protons of the CH C= (3.43 ppm, s) and NH (10.64 ppm, s) groups point in its favor. However signals were
2
absent from the spectrum of hydrazone 13 for the proton of the =CH group, distinctive for the enehydrazone
form, which have been detected [6] at 4.60-4.80 ppm.
N
N
N
S
H
N
O
NCS
S
N
H
N
H
O
1
2
N
O
O
O
N
N
H
N
O
S
NH2
O
11
N
O
N
N
O
CH(OEt)3
H
N
S
N
O
1
3
O
N
O
N
N
H
N
S
N
O
O
14
8
54

TABLE 1. Characteristics of the Synthesized Compounds 2-10 and 12-14
Found, %
Com-
pound
Empirical
formula
——————
Calculated, %
mp, °С*
Yield, %
С
H
N
2
2
2
2
3
4
4
5
а
b
c
C
C
C
C
C
C
C
C
10
14
18
H
H
H
8
9
N
3
OS
54.85
4.21
4.14
19.32
19.17
57-58
108-109
149-150
215-216
243-245
110-112
142-143
>250
56
68
65
63
64
47
51
71
5
4.76
11
N
3
OS
62.57
4.30
15.77
6
2.44
4.12
15.60
12
N
6
O
2
S
S
S
2
52.88
2.99
2.96
20.67
20.57
5
2.94
d
9
H
N
4
3
O
2
2
45.81
3.50
23.82
4
5.77
3.41
23.72
8
H
7
H
H
H
N
O
45.73
3.49
3.37
20.25
20.09
4
5.94
56.59
6.50
a
13
12
14
16
N
4
4
OS
5.91
20.40
5
5.83
20.28
b
14
N
O
2
S
51.92
5.12
5.07
20.01
20.13
5
1.78
9
ClN
4
OS
53.29
2.92
17.84
5
3.08
2.86
17.69
(dec.)
6
7
8
8
8
C
C
C
C
C
14
14
H
12
N
4
OS
59.22
4.30
4.26
19.61
19.71
123-124
236-238
212-213
216-217
162-163
81
70
87
85
51
5
9.13
H
8
N
6
O
2
S
2
47.35
2.29
23.45
4
7.18
48.30
8.20
2.26
23.59
a
b
c
9
H
12
N
4
N
N
OS
5.44
5.40
24.80
24.98
4
14
11
H
14
4
4
OS
58.77
5.01
19.75
5
8.71
4.92
19.57
H
12
O
2
S
50.12
4.49
4.58
21.31
21.20
4
9.98
(dec.)
9
1
1
1
1
C
C
C
C
C
9
H
10
N
4
S
52.44
4.99
27.25
160-162
186-187
152-153
107-108
150-152
66
88
90
61
68
5
2.40
4.89
27.16
0
2
3
4
11
12
15
12
H
H
H
H
12
14
17
13
N
4
6
5
5
OS
53.37
4.99
4.87
22.45
22.57
5
3.21
42.66
2.59
N
N
N
O
O
O
2
4
3
S
S
S
2
4.22
24.99
4
4.17
24.84
49.73
4.81
4.72
19.41
19.27
4
9.58
46.81
4.33
22.90
4
6.90
4.26
22.79
_
*
______
Compounds 2a-c were recrystallized from a 2-propanol–octane mixture,
compound 14 from 2-propanol.
On heating hydrazide 11 with ethyl orthoformate the ethoxymethylenehydrazide 14 was formed, in the
1
H NMR spectrum of which the signal at 1.36 ppm belongs to the proton of the methyl group, that at 6.95 to the
proton of the =CH group, and the signal at 10.76 ppm to the NH group proton.
The antitubercular activity of some of the compounds synthesized in this work has been investigated.
Initial investigations showed that compounds 4a,b and 8a at concn. 6.25 mg/ml displayed antitubercular activity
in relation to Mycobacterium tuberculosis H37Rv (ATCC 27294) (98-99%). The investigation was carried out
under the program: the Tuberculosis Antimicrobial Acquisition and Coordinating Facility (TAACF) through a
research and development contract with the U.S. National Institute of Allergy and Infectious Diseases.
8
55

1
TABLE 2. Data of H NMR Spectra of Compounds 2-10, 12-14
Com-
pound
1Н NMR spectrum, δ, ppm, J (Hz)*
2
2
а
3.91 (2H, d, J = 7, SCH
2
); 5.04 (1H, d, J = 7, =CH
2
-cis); 5.21 (1H, d, J = 7, =CH -trans);
2
7
.71 (2H, d, J = 6, 3-, 5-H); 8.64 (2H, d, J = 6, 2-, 6-H)
b
4.51 (2H, s, SCH
2
); 7.27 (5H, m, Ph); 7.71 (2H, d, J = 6, 3-, 5-H);
8
.64 (2H, d, J = 6, 2-, 6-H)
2
2
3
4
c
4.13 (4H, s, SCH
4.01 (2H, s, SCH
5.22 (2H, s, NCH
2
); 7.69 (2H, d, J = 6, 3-, 5-H); 8.63 (2H, d, J = 6, 2-, 6-H)
); 7.68 (2H, d, J = 6, 3-, 5-H); 8.63 (2H, d, J = 6, 2-, 6-H)
d
2
2
); 7.57 (2H, d, J = 6, 3-, 5-H); 8.58 (2H, d, J = 6, 2-, 6-H)
а
1.36 (6H, m, CH
2
CH CH ); 2.62 (4H, m, CH NCH ); 4.84 (2H, s, NCH N);
2
2
2
2
2
7
.57 (2H, d, J = 6, 3-, 5-H); 8.56 (2H, d, J = 6, 2-, 6-H)
4
b
2.62 (4H, m, CH
2
OCH
2
); 3.42 (4H, m, CH
2
NCH
2
); 4.89 (2H, s, NCH N);
2
7
.60 (2H, d, J = 6, 3-, 5-H); 8.57 (2H, d, J = 6, 2-, 6-H)
5
6
7.38 (5H, m, Ph); 7.79 (2H, d, J = 6, 3-, 5-H); 8.79 (2H, d, J = 6, 2-, 6-H)
3.02 (2H, t, J = 7, CH ); 4.24 (2H, t, J = 7, NCH ); 7.07 (2H, d, J = 6, 3'-, 5'-H);
.58 (2H, d, J = 6, 3-, 5-H); 8.27 (2H, d, J = 6, 3'-, 5'-H); 8.62 (2H, d, J = 6, 2-, 6-H)
7.53 (2H, d, J = 6, 3-, 5-H); 8.53 (2H, d, J = 6, 2-, 6-H)
2
2
7
7
8
a
b
c
0.98 (3H, t, J = 7, CH
3
); 3.24 (2H, m, CH
); 8.53 (2H, d, J = 6, 2-, 6-H);
.11 and 10.33 (1H, s and 1H, s, C(О)NHNH)
); 7.25 (5H, m, Ph); 7.80 (2H, d, J = 6, 3-, 5-H);
.55 (1H, br. t, NH); 8.71 (2H, d, J = 6, 2-, 6-H);
.41 and 10.62 (1H, s and 1H, s, C(О)NHNH)
2
); 7.58 (2H, d, J = 6, 3-, 5-H);
7
9
.84 (1H, br. t, NHCH
2
8
8
4.76 (2H, d, J = 5, CH
2
8
9
2.16 (1H, t, J = 3, ≡CH); 4.19 (2H, d, J = 3, CH
C≡); 5.15 (2H, d, J = 6, CH
NH);
2
2
7
8
.60 (1H, s, NH); 7.93 (2H, d, J = 6, 3-, 5-H);
.80 (2H, d, J = 6, 2-, 6-H); 9.20 and 10.31 (1H, s and 1H, s, C(О)NHNH)
9
1
1
1.11 (3H, t, J = 7, CH
3
); 3.29 (2H, m, CH ); 7.49 (2H, d, J = 6, 3-, 5-H);
2
7
.49 (1H, s, NH); 8.42 (2H, d, J = 6, 2-, 6-H)
0
2
1.22 (3H, t, J = 7, CH
3
); 1.80 (3H, s, CH
3
CO); 4.11 (2H, q, J = 7, CH );
2
7
.60 (2H, d, J = 6, 3-, 5-H); 8.44 (2H, d, J = 6, 2-, 6-H)
0.98 (3H, t, J = 7, CH
3
); 3.17 (2H, m, CH
2
); 4.04 (2H, s, SCH );
2
7
9
.67 (2H, d, J = 6, 3-, 5-H); 8.55 (2H, d, J = 6, 2-, 6-H);
.04 (1H, s, NH); 9.51 (1H, s, NH); 9.95 (1H, s, NH)
); 1.97 (3H, s, CH C=); 3.43 (2H, s, CH
); 4.58 (2H, s, SCH ); 7.88 (2H, d, J = 6, 3-, 5-H);
.88 (2H, d, J = 6, 2-, 6-H); 10.64 (1H, s, NH)
); 4.15 (2H, m, CH CH
.95 (1H, s, =CH); 7.90 (2H, d, J = 6, 3-, 5-H); 8.83 (2H, d, J = 6, 2-, 6-H);
0.76 (1H, s, NH)
1
1
3
4
1.21 (3H, t, J = 7, CH
3
3
2
C=);
4
8
.17 (2H, m, CH
2
CH
3
2
1.28 (3H, t, J = 7, CH
3
2
3
); 4.55 (2H, s, SCH );
2
6
1
_
*
______
In DMSO-d
6
.
EXPERIMENTAL
1
The H NMR specra were measured on a Tesla BS 567 A (80 MHz) instrument, internal standard was
HMDS, solvent was DMSO-d . The IR spectra were recorded on a UR 10 spectrometer in KBr disks.
6
The initial 2-mercapto-5-(4-pyridyl)-1,3,4-oxadiazole (1) and [5-(4-pyridyl)-1,3,4-oxadiazol-2-ylthio]-
acetic acid (11) were obtained by the procedure of [2].
The physicochemical and spectral characteristics of the synthesized compounds are given in Tables 1
and 2.
2
-(2-Propenyl)thio-5-(4-pyridyl)-1,3,4-oxadiazole (2a). Oxadiazole 1 (2.69 g, 15 mmol) was added to
a solution of NaOH (0.6 g, 15 mmol) in water (30 ml), the solution obtained was filtered, TEBA (0.2 g) was
dissolved in it, and allyl bromide (1.82 g, 15 mmol) was added dropwise with stirring. The mixture was stirred
for 2 h at room temperature. The precipitated crystals of 2a were filtered off and washed with water.
8
56

2
-Benzylthio-5-(4-pyridyl)-1,3,4-oxadiazole (2b) was obtained from oxadiazole 1 (2.69 g, 15 mmol)
and benzyl chloride (1.90 g, 15 mmol) analogously to thioether 2a.
,4-Di-[5-(4-pyridyl)-1,3,4-oxadiazol-2-ylthio]-2-butyne (2c) was obtained from oxadiazole 1 (2.69 g,
5 mmol) and 1,4-dichloro-2-butyne (0.95 g, 7.5 mmol) analogously to thioether 2a.
1
1
[
5-(4-Pyridyl)-1,3,4-oxadiazol-2-ylthio]acetic Acid Amide (2d). Oxadiazole 1 (10.76 g, 60 mmol) and
chloroacetamide (5.6 g, 60 mmol) were added to a solution of KOH (3.8 g, 68 mmol) in methanol (200 ml). The
mixture was stirred for 3 h at 60°C, evaporated, and diluted with water. The crystals of amide 2d were filtered
off, and washed with water.
3
-Hydroxymethyl-5-(4-pyridyl)-1,3,4-oxadiazole-2(3H)-thione (3). Formalin (25%) (6 ml) was added
to the solution obtained by heating oxadiazole 1 (5.37 g, 30 mmol) in 2-propanol (100 ml), and methanol
10 ml) to 75°C. The mixture was stirred at room temperature for 3 h. The solution was partially evaporated, the
precipitated crystals of compound 3 were filtered off, and washed with 2-propanol.
(
3
-Piperidinomethyl-5-(4-pyridyl)-1,3,4-oxadiazole-2(3H)-thione (4a). Oxadiazole 1 (2.69 g,
1
4
5 mmol), methanol (30 ml), formalin (1.5 ml), and piperidine (1.28 g, 15 mmol) were mixed and heated to
0°C. The solution obtained was stored for 3 d, partially evaporated, the precipitated crystals of compound 4a
were filtered off, and washed with 2-propanol.
3
-Morpholinomethyl-5-(4-pyridyl)-1,3,4-oxadiazole-2(3H)-thione (4b) was obtained from a mixture
of oxadiazole 1 (2.69 g, 15 mmol), formalin (1.5 ml), and morpholine (1.3 g, 15 mmol) analogously to
compound 4a.
3
-(4-Chlorophenyl)carbamoyl-5-(4-pyridyl)-1,3,4-oxadiazole-2(3H)-thione (5).
A
mixture of
oxadiazole 1 (2.69 g, 15 mmol), 4-chlorophenyl isocyanate (2.3 g, 13.6 mmol), and dioxane (100 ml) was heated
for 2 h at 90°C, and filtered. The filtrate was partially evaporated, and diluted with water. The precipitated
crystals of amide 5 were filtered off, and washed with water.
3
-(2-Ethyl-4-pyridyl)-5-(4-pyridyl)-1,3,4-oxadiazole-2(3H)-thione (6). A mixture of oxadiazole 1
(
1
2.69 g, 15 mmol) , 4-vinylpyridine (1.57 g, 15 mmol), and glacial acetic acid (30 ml) was stirred for 2 h at
00°C, partially evaporated, and poured into cold 5% NaOH solution. The precipitated amorphous solid
compound 6 was filtered off and washed with water.
Di-[5-(4-pyridyl)-1,3,4-oxadiazol-2-yl] Disulfide (7). A solution of potassium hexacyanoferriate
(
(
3.3 g, 10 mmol) in water (20 ml) was added to the solution obtained from NaOH (0.4 g, 10 mmol), water
20 ml), and oxadiazole 1 (1.79 g, 10 mmol). After 1 h the solution was filtered, partially evaporated, and
diluted with acetone. The precipitated crystals of disulfide 7 were filtered off, and washed with acetone.
4
-Ethyl-1-(4-pyridylcarbonyl)thiosemicarbazide (8a). A mixture of isonicotinic acid hydrazide
(
5.44 g, 40 mmol), dioxane (200 ml), and ethyl isothiocyanate (3.48 g, 40 mmol) was heated for 2 h at 90°C, the
solution was partially evaporated, and diluted with diethyl ether. The crystals of thiosemicarbazide 8a were
filtered off, and washed with ether.
4
-Benzyl-1-(4-pyridylcarbonyl)thiosemicarbazide (8b). Benzyl isothiocyanate (13.26 g, 100 mmol)
was added to the solution obtained from isonicotinic acid hydrazide (13.7 g, 100 mmol) and dioxane (500 ml) at
0°C. The mixture was stirred for 30 min at 70°C. The precipitated crystals of thiosemicarbazide 8b were
filtered off, and washed with dioxane.
6
4
-(2-Oxa-4-pentynyl)-1-(4-pyridylcarbonyl)thiosemicarbazide (8c) was obtained from isonicotinic
acid hydrazide (6.85 g, 50 mmol), dioxane (200 ml), and 2-oxa-4-pentynyl isothiocyanate (6.35 g, 50 mmol) [7]
analogously to compound 8b.
2
-Ethylamino-5-(4-pyridyl)-1,3,4-thiadiazole (9). A solution of thiosemicarbazide 8a (1.8 g, 8 mmol)
in anhydrous phosphoric acid (30 ml) was heated at 110°C for 1 h 30 min. The mixture was cooled, diluted with
water (100 ml), and neutralized with aqueous ammonia. The precipitated thiadiazole 9 was filtered off, and
washed with water.
8
57

2
-(N-Acetylethylamino)-5-(4-pyridyl)-1,3,4-thiadiazole (10). Thiadiazole 9 (1.04 g, 5 mmol) was
dissolved in ethyl orthoformate (20 ml) and acetic anhydride (15 ml). The mixture was heated for 2 h at 100°C,
distilling off the volatile components through a fractionating column, and partially evaporated. The residue was
diluted with diethyl ether, the precipitated crystals of thiadiazole 10 were filtered off, and washed with ether.
4
-Ethyl-1-[5-(4-pyridyl)-1,3,4-oxadiazol-2-ylthioacetyl]thiosemicarbazide (12) was obtained from
hydrazide 11 (1.26 g, 5 mmol), ethyl isothiocyanate (0.44 g, 5 mmol), and dioxane (40 ml) analogously to
thiosemicarbazide 8a.
[
5-(4-Pyridyl)-1,3,4-oxadiazol-2-ylthio]acetic Acid 4-Ethoxy-4-oxo-2-butylidenehydrazide (13). A
solution of hydrazide 11 (1.26 g, 5 mmol) in ethyl acetoacetate (40 ml) was heated for 2 h at 105°C, distilling
off the volatile components through a fractionating column, and partially evaporated. The precipitated crystals
of hydrazide 13 were filtered off, and washed with ether.
[
5-(4-Pyridyl)-1,3,4-oxadiazol-2-ylthio]acetic Acid Ethoxymethylidenehydrazide (14). A solution of
hydrazide 11 (1.26 g, 5 mmol) in ethyl orthoformate (40 ml) was heated at 105-110°C for 2 h, distilling off the
volatile components through a fractionating column, and partially evaporated. The mixture was diluted with
hexane, the precipitated crystals of hydrazide 14 were filtered off, and washed with hexane.
REFERENCES
1
.
H. Liszkiewicz, T. Glowiak, M. W. Kowalska, M. Rutkowska, A. Szelag, J. Barczynska,
L. Kedzierska-Gozdzik, F. Blaszczyk, and W. Dziewiszek, Pol. J. Chem., 73, 321 (1999).
A. Rutavicius, S. Valyulene, and Z. Kuodis, Khim. Geterotsikl. Soedin., 966 (2000).
V. Yakubkene and P. Vainilavicius, Khim. Geterotsikl. Soedin., 1125 (1998).
S. I. Yakimovich, V. I. Nikolaev, and O. A. Afonina, Zh. Org. Khim., 15, 922 (1979).
A. Rutavicius and V. Mozolis, Trud. Akad. Nauk LitSSR, Ser. B, 5(126), 81 (1981).
2
3
4
5
.
.
.
.
8
58