7-Aryl-4a-hydroxy-4a,6,7,7a-tetrahydro-isoxazolo[4,5-b]quinuclidines as NO donors

Article abstract of DOI:10.1023/A:1011616924260

7-Aryl-4a-hydroxy-4a,6,7,7a-tetrahydroisoxazolo[4,5-b]quinuclidines have been prepared from 2-arylmethylene-3-quinuclidones and hydroxylamine and they are able to release NO upon mild oxidation with K₃[Fe(CN₆)] in basic medium.

Full text of DOI:10.1023/A:1011616924260

Chemistry of Heterocyclic Compounds, Vol. 37, No. 5, 2001
7-ARYL-4a-HYDROXY-
4a,6,7,7a-TETRAHYDRO-
b
ISOXAZOLO[4,5- ]QUINUCLIDINES
AS NO DONORS
L. N. Koikov1, N. V. Alekseeva2, N. B. Grigor'ev1, V. I. Levina1, K. F. Turchin1,
T. Ya. Filipenko1, and V. G. Granik2
7-Aryl-4a-hydroxy-4a,6,7,7a-tetrahydroisoxazolo[4,5-b]quinuclidines have been prepared from
2-arylmethylene-3-quinuclidones and hydroxylamine and they are able to release NO upon mild
oxidation with K3[Fe(CN6)] in basic medium.
Keywords: arylmethylene quinuclidine, hydroxylamine, NO donors, quinuclidone oxime,
hydrogenation.
Certain 2-arylmethylene- and 2-arylmethyl-3-quinuclidone oximes are able to generate NO upon mild
oxidation and to stimulate soluble guanylate cyclase i.e. the enzyme catalyzing the synthesis of cyclic
5'-guanosine monophosphate which is finally responsible for a whole series of biological effects [1-4]. In this
work we have carried out an investigation of the reaction of Z-2-arylmethylene-3-quinuclidones with
hydroxylamine with the aim of preparing novel, potential NO donors.
Hydroxylamine and its derivatives can take part in reaction with , -unsaturated compounds to form the
α β
corresponding oximes or via addition of hydroxylamine at the electron deficient -carbon atom and subsequent
β
cyclization to isoxazoles [5, 6] or a compound of the 4a-hydroxytetrahydroisoxazole series for a derivative of
2-methylene-3-quinuclidone [7].
A study of the reaction mixtures obtained by treating the 3-quinuclidone derivatives 1a-d with
hydroxylamine hydrochloride has shown that, along with the corresponding oximes 2-4 there are formed in the
reaction the hydrogenated isoxazolo[4,5-b]quinuclidines 5a-d which could be separated and identified by
spectroscopic methods (Tables 1-3).
Changing hydroxylamine hydrochloride for the base in the reaction with compound 1b lowers the yield
of the isomeric oximes 2/3b from 39 to 23% and the bicyclic derivative 6b becomes the main reaction product
(yield 51%). It is likely that protonation activates the carbonyl group and accelerates the oximation and that
-addition predominates in its absence, (in agreement with reported data [6]). In fact, in the case of the reaction
β
of the 2-pyridylmethylene ketone (1e) with hydroxylamine the dominating process is addition at the -position
β
of the , -unsaturated system to give the cyclic tautomer of the hydroxyamino derivative 4a-hydroxy-7-(2-
α β
pyridyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5-b]quinuclidine (5e) as the principal product.
__________________________________________________________________________________________
1
Center for the Chemistry of Medicinal Substances, All Russia Science Research Chemico-
2
Pharmaceutical Institute, Moscow 119815. Russian Federation State Science Center, Science Research
Institute for Intermediate Products and Dyes, Moscow 103787. Translated from Khimiya Geterotsiklicheskikh
Soedinenii, No. 5, pp. 673-680, May, 2001. Original article submitted May 20, 1999.
620
0009-3122/01/3705-0620$25.00©2001 Plenum Publishing Corporation

TABLE 1. Characteristics of Compounds Synthesized
Found, %
——————
mp, °C,
solvent
Yield,
%
Compound
Empirical formula
Rf*
Calculated, %
С
H
N
C13H14N2O
73.0
72.9
6.9
6.6
13.1
13.1
112-114
(dec.),
0.66
42
1e
i-PrOH
2/3c HCl
5b·2HCl
5c·HCl
C15H19N2O2·HCl
61.4
61.1
6.6
6.5
9.3
9.5
196-198, 0.51
16
4
acetone
C14H18N2O3·2HCl·2H2O
C15H20N2O3·HCl·H2O
C14H18N2O3·2HCl·H2O
C13H17N3O2·HCl
44.6
7.0
7.5
148-150, 0.33
water
45.0
7.0
7.5
55.1
54.6
6.6
7.0
8.4
8.5
181-183, 0.22
17
8
i-PrOH
5d·2HCl
5e·HCl
47.5
5.9
7.5
176-178, 0.14
47.3
6.2
7.9
i-PrOH
54.6
55.0
6.7
6.4
14.8
14.8
192
0.23
54
(dec.),
MeOH
2
C15H20N2O3·H2O
61.0
61.2
7.4
7.5
9.5
9.5
167-168,
MeOH
*
51
15
6
7·NH2OH·2HCl C13H15N3O·2HCl·NH2OH 46.7
6.4
16.7
175-185
0.54
46.6
6.0
16.7
(dec.),
i-PrOH
C13H15N3O·0.5H2O
65.8
65.5
6.7
6.7
17.0
17.0
145-147, 0.54
87
7
(dec.),
CHCl3
_______
* CHCl3–MeOH, 50:3.
*2 Hydrolyzed to 5b upon chromatography.
OCH3
4HO
O
O
O
O
3
2
CH3OH
5
4a
7a
9
NH
8
6NH
+
NH2
N
N
N
N
7
Ar
Ar
Ar
ArCHO
NaOH
6
5b–e
7
OH
N
OH
N
4
N
O
.
H2NOH HCl
5
6
OH
3
2
8
+
+
7
Ar
9
N
N
N
N
Ar
Ar
Ar
3a–d
4a,b
2a–d
1a–e
1–7 a Ar = Ph, b Ar = 2-HOC6H4, c Ar = 2-MeOC6H4, d Ar = 4-HOC6H4, e Ar = 2-Py
Oximes 2-4 were not separated from the reaction mass. It is impossible to rule out an additional factor
promoting the predominant - addition in the reaction of ketone 1e with NH2OH·HCl which is the formation of
β
a stable cation (B) to which the hydroxylamine base also adds. The actual basicity of the quinuclidine nitrogen
in (Z)-2-arylmethylene-3-quinuclidones is low (according to L. S. Khabarova for the (Z)-2-(3'-chlorophenyl)-
methylene-3-quinuclidone in 50% ethanol the pKa is 4.54 0.05).
±
621

TABLE 2. Chemical Shifts in the 1H NMR Spectra of the Sythesized Compounds, δ, ppm
5,8-CH2 (3,9-СH2)*2 6,7-CH2 (2,8-СH2)*2
Compound
Solvent
4-H*
9-H (7-Н)*2
3'-H (m)
4'-H (m)
5'-H (m)
6'-H (m)
7a-Н
(m)
(m)
CDCl3
CDCl3
CD3OD
DMСО-d6
3.88
2.62
3.95
3.88
2.63
2.00-2.30
2.01
2.04, 2.19
1.90-2.10
1.90-2.20
3.28, 3.57
2.90-3.15
3.57, 3.74
3.46, 3.64
3.30-3.70
7.19 (s)
7.14 (s)
7.25 (s)
7.28 (s)
5.13 (d)
6.88
8.56
6.99
6.85
7.0
7.33
7.66
7.32
7.16
7.35
6.84
7.16
6.96
6.75
7.0
7.35
8.60
7.29
7.94
7.35
—
—
—
—
4.26 (d)
1b
1e
3b·HCl
4b·HCl
5b·2HCl
DMСО-d6–D2O,
1:1
5c·HCl*3
5d*4
CD3OD
2.21
2.05
2.40
2.05
2.57
2.74
1.50-2.00
1.30-1.75
1.65-2.05
1.30-1.80
1.85
2.65-3.25
2.45-3.00
3.10-3.55
2.50-3.15
2.77, 2.99
3.32, 3.66
4.72 (d)
4.11 (q)
4.82
4.50 (d)
—
7.02
7.34
—
7.85 (t)
7.10
7.69 (t)
8.18 (t)
6.99
7.46
3.41 (d)
2.96 (d)
3.83 (d)
3.02 (d)
—
DMСО-d6
DMСО-d6
DMСО-d6
CDCl3
6.72 (d)
7.70 (d)
6.78
8.81 (d)
8.30 (d)
6.72 (d)
7.40 (q)
6.75
7.26 (q)
7.72 (q)
7.20 (d)
8.59 (d)
7.20
8.57 (d)
8.81 (d)
5e·2HCl*5
6*6
7*7
7·2HCl·NH2OH*8 DMСО-d6
1.94, 2.14
—
—
_______
* The 4-H signal is a quintet; the remaining signals are multiplets unless indicated otherwise in the brackets.
*2 Numbering in brackets relates to series 5 and to compound 6.
*3 OMe: 3.89 (s).
*4 6-H: 5.92 (d), J6--H, 7-H = 13.2, J7-H, 7a-H = 6.2.
*5 6-H: 6.95 (br. s), OH: 7.01 (br. s), +NH: 11.4, J6-H, 7-H = 12.5, J7-H, 7a-H = 5.2.
*6 OMe: 3.15 (s).
*7 NH2+: 6.70 (br. s) and 9.33 (br. s).
*8 NH3 : 10.47, NH+: 12.3, NH, OH: 8.98, 9.61, and 10.16 (all br. s).

TABLE 3. Chemical Shifts in the 13C NMR Spectra of the Synthesized Compounds, δ, ppm
2-C
3-C
5,8-C
6, 7-C
9-C
Compound
Solvent
4-C
1'-C
2'-C
3'-C
4'-C
5'-C
6'-C
(7a-C)*
(4a-C)*
(3,9-C)*
(2,8-C)*
(7-C)*
(CD3)2CO–D2O, 1:1 129.9
153.0
152.7
22.8
24.7
20.6
22.3
49.7
51.1
117.6
122.5
117.4
120.4
152.7
157.3
115.6*2
116.4*2
132.0*3
132.0*3
120.8*2
119.9*2
132.6*3
132.4*3
3b·HCl
4b·HCl
DMSO-d6–CD3OD,
133.2
1:1
5c·HCl*4
DMSO-d6–CD3OD,
1:2
62.6
107.0
32.7
23.1; 23.7 41.3; 49.7
78.4
124.8
159.5
112.3
130.9
122.1
130.8
5d·2HCl
DMSO-d6
CDCl3
DMSO-d6
64.9
118.5
110.2
105.3
205.3
195.0
31.2
40.5
38.3
22.0; 22.3 40.3; 48.5
79.0
127.7
—
—
129.0
115.4
127.4
124.0
157.2
136.0
139.3
115.4
124.3
126.8
129.0
148.3
149.8
26.5
21.2
49.4
51.2
147.4*3
148.7*3
150.6*3
149.4*3
7
7·2HCl
·NH2OH
_______
* Numbering in brackets relates to series 5
*2,*3 Possible reversed signal assignments are marked with the same characters.
*4 OCH3: 56.2.

O
–
.
.
O
O
.
.
.
.
H
N
H
+
–2H+
H
.
.
NH OH
H
+
NH2OH HCl
2
N
N
H
OH
–
.
.
.
.
.
N
H
.
.
H
+
+
.
–
.
.
.
.
Cl
.
N
Cl
N
N
B
OH
N
OH
O
OH
N
NH
.
.
N
H
H
.
H
N
H
.
+
.
+
.
–
N
Cl
–
Cl
5
It is indeed likely that the electron acceptor properties of the pyridine ring, increased by protonation and
the presence of the stabilization due to the formation of hydrogen bonds, promotes the reductive fission of the
N–O bond and that this leads to the formation of the enamino ketone 7 (initially separated as the NH2OH·HCl
complex) during reduction of 5e·nHCl or its open form with excess hydroxylamine.
The cyclic structure of compounds 5a-e is confirmed by the absence of absorption due to C=O or C=N
groups in their IR spectra and the presence of several bands in the region 3100-3400 cm-1 for the absorption of
1
the OH and NH groups. The H NMR spectra show signals for the 7-H (4.1-4.8 ppm) and 7a-H protons
(3.0-4.0 ppm), the vicinal situation of which follows from the presence of a spin-spin coupling J7,7a ~ 5.2 Hz.
The placement of the nitrogen at position 6 rather than 5 of the pentacyclic ring follows from the presence of a
spin-spin coupling J7,6 ~ 12.5 Hz for the 7-H and 6-H protons in the spectrum of 5e·HCl (Table 2). The ketal
nature of atom C(4a) was confirmed by its greater shielding (13C 105-107 ppm, Table 3) when compared with
δ
the carbonyl carbon (~ 200 ppm for ketones 1). Analogous differences are seen in the 13C NMR chemical shifts
for 4a-hydroxy-4aH-chromeno[3,2-b]quinuclidines [2].
The enamino ketone structure of compound 7 follows from the 1H and 13C NMR spectra which show the
presence in the molecule of a 2'-substituted pyridine ring and a quinuclidine system with an exocyclic double
1
bond at atom C(2) and keto group at C(3). In addition, the H NMR spectrum of compound 7 shows two broad
singlets, corresponding to protons exchanging for deuterium in CD3OD and assigned to the NH2 group at the
exocyclic double bond. The presence in the 13C NMR spectrum of a signal at 205 ppm shows the presence of a
keto group in compound 7. The 1H (in CD3OD) and 13C (in CD3OD and DMSO-d6) NMR spectra of compound
7·NH2OH·2HCl show the same signals as in the spectra of 7, however the chemical shifts of the nuclei which
correspond to these identical signals differ in themselves. The origins of these differences can be judged by the
1
low field region of the H NMR spectrum of 7·NH2OH·2HCl where (in addition to the broad signals due to the
protons of the NH2 group) there are observed three broad singlets at 10.16, 10.47 and 12.3 ppm with intensities
of 1, 3, and 1 proton units. The lowest field of these signals can be assigned to the quinuclidine ring proton and
+
the two remaining to the NH3 and OH of the hydroxylamine hydrochloride. Hence compound 7 presents itself
as a 1:1 complex of the hydrochloride of 7 with hydroxylamine hydrochloride.
As in previous work, testing of the ability of the novel 7-aryl-4a-hydroxy-4a,6,7,7a-
tetrahydroisoxazolo[4,5-b]quinuclidines 5b-e to behave as NO donors was carried out via their oxidation with
K3[Fe(CN)6] in NaOH (0.4 N) at 80°C and subsequent detection of the nitroprusside anion formed using a
differential pulsed method [2]. The yield of NO (%) was calculated as a ratio of peaks for the second wave of
sodium nitroprusside for a solution of the investigated oxime (htest) and a standard solution of the same
concentration (href):
624

h
test/href·100%
3-6
5b·2HCl
5c·HCl
3-10
5d·2HCl
1.5-1.7
17-30
100
5e·HCl
Sodium nitroprusside
(c = 4·10-4 M, 4% EtOH–H2O)
As is evident from the data given, all of the compounds 5b-e can generate NO upon oxidation. In
contrast to previously studied oximes [1, 2], in this case the degree of formation of the nitric oxide is not a
consequence of the effect of the o-hydroxy group since both the isomeric hydroxy- (5b,d) and the 2-methoxy
derivative (5c) form NO to an insignificant degree. The most powerful NO donor is the pyridine derivative 5e
and this exceeds in activity the o-hydroxy derivative 4b from the previous series.
The probable mechanism for formation of the NO can be described by the following scheme:
OH
O
O
O
+
H
N
H
N
–H
NH
Ar
–
OH
O
N
N
N
N
N
Ar
Ar
–
–
O
O
O
Ox
–
+ NO
N
O
N
OH
OH
N
+
Ar
Ar
Ar
H
O
OH
O
O
N
N
Ar
Ar
Hence, in this study, we have investigated the methods of synthesis and certain reactions of 7-aryl-4a-
hydroxy-4a,6,7,7a-tetrahydroisoxazolo[4,5-b]quinuclidines and shown that these cyclic tautomers of
-hydroxyamino ketones can generate NO through mild oxidation.
γ
EXPERIMENTAL
1
NMR spectra were taken on a Varian Unity Plus 400 instrument at 400 MHz for the H nucleus and
100.6 MHz for the 13C nucleus and mass spectra on a Finnigan MAT SSQ 710. The melting points were
determined in a sealed capillary. TLC was carried out on Silufol UV-254 plates and revealed using UV light and
the Dragendorff reagent. The parameters for the compounds obtained are given in Table 1.
Z
The 3-quinuclidone hydrochloride (0.124 mol) and
( )-2-(2'-Pyridyl)methylene-3-quinuclidone (1e).
NaOH (0.124 mol) were refluxed in ethanol (96%, 150 ml) for 30 min and a further amount of NaOH
(0.124 mol) and pyridine-2-aldehyde (0.124 mol) were then added. The product was refluxed for 10 h (until
disappearance of the quinuclidone by TLC) and the ethanol distilled off. The residue was dissolved in water
(100 ml), extracted with chloroform (3 × 150 ml), the extracts dried with K2CO3, and the chloroform distilled
off. The residue was extracted with refluxing petroleum ether (60-70°C boiling range, 3 × 70 ml). After
evaporation of the petroleum ether from the combined extracts, crystallization from 2-propanol (150 ml) gave
the bright yellow ketone 1e.
625

b
4a-Methoxy-7-(2'-hydroxyphenyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5- ]quinuclidine (6b), 4a-Hydroxy-
b
7-(2'-hydroxyphenyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5- ]quinuclidine Dihydrochloride (5b·2HCl), and
the Mixture of Bases (2/3b). A solution of NaOH (100 mmol) in water (9.4 ml) was added to 1b (67 mmol) and
NH2OH·HCl (100 mmol) in MeOH (300 ml) and the suspension was allowed to stand at room temperature
overnight. The NaCl precipitate was separated and the filtrate was evaporated to a reduced volume (100 ml) and
diluted with water (200 ml). The precipitate (13.4 g) was refluxed with chloroform (100 ml) to remove the
mixture of bases 2/3b (3.0 g). The residue after extraction was dissolved in refluxing MeOH (250 ml) and the
solution then evaporated to 100 ml to give 6b as light yellow crystals (9.4 g). When the methanol mother liquors
of 6b were evaporated to lower volume (50 ml) the product 4b (0.7 g, 4.2%) was obtained. Attempted
recrystallization of 6b from refluxing water caused a rapid precipitation of yellow crystalline starting ketone 1b.
Solution of 6b (1.6 g) in aqueous methanol (2:1, 30 ml) and acidification with an excess of concentrated
hydrochloric acid to pH 3 caused the precipitation of the dihydrochloride 5b (50%).
b
4a-Hydroxy-7-(2'-methoxyphenyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5- ]quinuclidine Hydrochloride
(5c·HCl) and 2-(2'-Methoxyphenyl)methylene-3-quinuclidone Oxime Hydrochlorides (2/3c·HCl). A. A
solution of 1c [2] (31 mmol) and NH2OH·HCl (34 mmol) in MeOH (170 ml) was allowed to stand overnight at
room temperature, the solvent was evaporated, and the product was decomposed using a saturated K2CO3
solution, extracted with chloroform (3 × 50 ml), the extracts dried with K2CO3, and the chloroform was distilled
off. The residue was refluxed with acetone (150 ml) and the insoluble precipitate which contained 5c was
recrystallized from MeOH to give 5c (1.6 g). This was dissolved in MeOH (10 ml) and converted to the
hydrochloride by the addition of the calculated amount of a solution of HCl in anhydrous ether. Evaporation to
1/3 volume and addition of acetone (20 ml) gave 5c·HCl (1.6 g) which was separated. The acetone solution after
removal of 5c was evaporated and the residue was crystallized from methanol to give 2/3c (1.5 g, 17%). It was
dissolved in acetone and acidified with the calculated amount of ethereal HCl to give 2/3c·HCl (1.5 g) (the
isomeric ratio was 37: 63).
B. A solution of 1c (2.5 mmol), NH2OH·HCl (2.7 mmol) and NaOH (1.35 mmol) in 96% EtOH (50 ml)
was left at room temperature for one day. According to 1H NMR data the reaction product contained 2/3/5c·HCl
in the ratio 73:8:22.
C. A solution of 1c (2.5 mmol), NH2OH (2.7 mmol), and AcOH (1.25 mmol) in 96% EtOH (50 ml) was
1
left at room temperature for one day. According to H NMR data the reaction product contained 2/3/5c·HCl in
the ratio 75:13:13.
b
4a-Hydroxy-7-(4'-hydroxyphenyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5- ]quinuclidine Dihydrochloride
(5d·2HCl) and 2-(4'-Hydroxyphenyl)methylene-3-quinuclidone Oxime Hydrochlorides (2/3d·HCl). A.
NH2OH·HCl (20.5 mmol) was added to a solution of the sodium salt 1d [2] (18.6 mmol) in MeOH (200 ml) and
the suspension obtained was left overnight at room temperature. The MeOH was evaporated off, the residue was
dissolved in water (100 ml), and the precipitate was separated to give the mixture 2/3d (2.2 g). This was
suspended in water (20 ml) and converted to the hydrochloride by the addition of the calculated amount of 0.5 N
HCl to give 2/3d·HCl (2.2 g, 44%) in the ratio 26: 74 and identical to that obtained previously [2]. After 2 days,
a precipitate (0.4 g), containing 5d mixed with 2d, was obtained from the aqueous extract. Extraction of this
mixture with refluxing chloroform gave 5d (0.3 g) which was suspended in MeOH and converted to the
hydrochloride by the addition of an excess of a solution of HCl in anhydrous ether.
B. A suspension of 1d (10 mmol) and NH2OH·HCl (12 mmol) in MeOH (200 ml) was left overnight at
1
room temperature. According to H NMR, the clear solution obtained contained 2d (57%), 5d (20%), and side
products (approximately 23%).
b
4a-Hydroxy-7-(2'-pyridyl)-4a,6,7,7a-tetrahydroisoxazolo[4,5- ]quinuclidine Hydrochloride (5e·HCl),
Z
the Dihydrochloride Complex of 2-Amino-( )-2-(2'-pyridyl)methylene-3-quinuclidone with NH2OH
Z
A
solution of
(7e·NH2OH·2HCl), and 2-Amino-( )-2-(2'-pyridyl)methylene-3-quinuclidone (7e).
Z-2-(2'-pyridyl)methylene-3-quinuclidone (1e, 14 mmol) and NH2OH·HCl (16 mmol) in MeOH (60 ml) was left
at room temperature overnight (the color of the reaction mixture changed from yellow to crimson) and the light
626

pink precipitate of 5e·HCl separated. The mother liquor was evaporated, the residue was dissolved in
2-propanol (50 ml), and NH2OH·HCl (3 mmol) was added. After 10 days, the dihydrochloride of 7e with
NH2OH was filtered off. The 7e base was prepared by treatment of the complex with K2CO3 solution and
extraction with chloroform. After drying with K2CO3 and distillation of the chloroform, the residue was
transferred to a silica gel column and eluted with a mixture (100 ml) of MeOH and CHCl3 (1:10).
This work was supported by grant 96-04-48325 from the Russian fund for Basic Research.
REFERENCES
1.
L. N. Koikov, N. V. Alekseeva, N. B. Grigor'ev, V. I. Levina, K. F. Turchin, T. Ya. Filipenko,
M. D. Mashkovskii, M. E. Kaminka, V. B. Nikitin, G. N. Engalycheva, M. A. Kalinkina, I. S. Severina,
I. K. Ryaposova, and V. G. Granik, Khim. -Farm. Zh., No. 5, 26 (1997).
L. N. Koikov, N. V. Alekseeva (Alexeeva), N. B. Grigor'ev (Grigoryev), V. I. Levina, K. F. Turchin,
T. Ya. Filipenko, I. S. Severina, I. K. Ryaposova, and V. G. Granik, Mendeleev Commun., No. 3, 94
(1996).
V. G. Granik, S. Yu. Ryabova, and N. B. Grigor'ev, Usp. Khim., 66, 792 (1997).
T. Ya. Filipenko, O. I. Gorbyleva, K. F. Turchin, O. S. Anisimova, E. M. Peresleni, E. E. Mikhlina,
Yu. N. Sheinker, and L. N. Yakhontov, Khim. Geterotsikl. Soedin., 666 (1981).
A. P. Bezhan, K. N. Zelenin, L. A. Sviridova, I. A. Motorina, A. Yu. Ershov, G. A. Golubeva, and
Yu. G. Bundel', Khim. Geterotsikl. Soedin., 823 (1989).
J. Suwinski and C. Ctroszkiewicz, Roczn. Chem., 46, 1077 (1972).
V. A. Bondarenko, K. F. Turchin, E. E. Mikhlina, and L. N. Yakhontov, Khim. Geterotsikl. Soedin., 948
(1981).
2.
3.
4.
5.
6.
7.
627