1-Chloro-2,6-dinitro-4-perfluoroalkylthiobenzenes in the synthesis of heterocycles

Article abstract of DOI:10.1023/A:1022138711847

1-Chloro-2,6-dinitro-4-perfluoroalkylthiobenzenes was obtained in first time by perfluoroalkylation of bis(4-chloro-3,5-dinitrophenyl)disulfide in the presence of xenon bisperfluoroalkylcarboxylates. At the interaction of these compounds with potassium et

Full text of DOI:10.1023/A:1022138711847

Chemistry of Heterocyclic Compounds, Vol. 38, No. 11, 2002
1-CHLORO-2,6-DINITRO-
4-PERFLUOROALKYLTHIOBENZENES
IN THE SYNTHESIS OF HETEROCYCLES
S. M. Sipyagin1, V. S. Enshov1, S. A. Kashtanov1, and J. S. Thrasher2
1-Chloro-2,6-dinitro-4-perfluoroalkylthiobenzenes was obtained in first time by perfluoroalkylation of
bis(4-chloro-3,5-dinitrophenyl)disulfide in the presence of xenon bisperfluoroalkylcarboxylates. At the
interaction of these compounds with potassium ethylxanthogenate only substitution of chlorine atom
occurred. The reaction with sodium N,N-dimethyldithiocarbamate leads to the nucleophilic substitution
of nitro group with formation of 1,3-benzodithiol-2-one, but on action of ethyl thioglycolate the
intramolecular condensation occurs with formation of benzothiazoles N-oxides.
Keywords:
1-ethylthio-2,6-dinitro-4-trifluoromethylthiobenzene,
2-ethoxycarbonyl-7-nitro-5-
perfluoroalkylthiobenzothiazole
N-oxide,
7-nitro-6-trifluoromethylthio-1,3-benzodithiol-2-one,
1-chloro-2,6-dinitro-4-perfluoroalkylthiobenzenes, heterocyclization, perfluoroalkylation of aromatic
disulfides.
The replacement of the hydrogen atom in aromatic and heterocyclic systems by perfluoroalkyl groups
may significantly affect the physical and biological properties of such molecules [1]. However, the introduction
of "superlipophilic" perfluoroalkylthio groups SRF (RF = CF3, C2F5, etc.), which have the highest lipophilicity
indices [2], may substantially enhance the biological effect of the resultant compounds [3].
In light of our current scientific interests, we are developing a synthesis of new synthones containing
both superlipophilic substituents and functional groups, which are capable of reacting with various nucleophilic
agents. For example, this might involve aromatic and heterocyclic compounds activated by other electron-
withdrawing substituents [4]. Such an approach, in our view, might lead to new compounds with potential
biological activity.
In the present work, we describe heterocyclization involving two new 1-chloro-3,5-dinitro-4-
perfluoroalkylthiobenzenes synthesized by the perfluoroalkylation of bis(4-chloro-3,5-dinitrophenyl) disulfide
(1). This is the first report of the preparation of 1 by the reduction of the corresponding sulfonyl chloride by an
HBr/PhOH mixture in acetic acid according to Lukashevich [5]. We have recently developed a method for the
perfluoroalkylation of heterocyclic thiols and disulfides based on the thermolytic reactions of xenon(II)
bis(perfluoroalkylcarboxylates) [6]. In the case of disulfide 1, this method was extended to aromatic
compounds. In general, the perfluoroalkylation of 1 may be initiated by electron transfer from the disulfide
RSSR to the xenon bisperfluoroalkylcarboxylate molecule to give a radical–cation and radical–anion [7]. Then,
__________________________________________________________________________________________
1
Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka,
2
Moscow Region, Russia; e-mail: sip@icp.ac.ru. Department of Chemistry, The University of Alabama,
Tuscaloosa, AL 35487-0336, USA; e-mail: fluorine@bama.ua.edu. Translated from Khimiya
Geterotsiklicheskikh Soedinenii, No. 11, pp. 1559-1565. November, 2002. Original article submitted April 12,
2000.
0009-3122/02/3811-1375$27.00©2002 Plenum Publishing Corporation
1375

the radical–anion decomposes to give a perfluoroalkyl radical, perfluoroalkanoate, carbon dioxide, and xenon,
·
while the radical–cation may decompose to give RS· and RS+. The recombination of RS· and RF leads to the
expected products 2a or 2b.
2 RFCOOH + XeF2
O2N
(RFCOO)2Xe + 2 HF
NO2
Cl
NO2
NO2
Cl
(RFCOO)2Xe
– 2CO2, – Xe
R1
Cl
S
S
O2N
NO2
2a,b
2 a R1 = SCF3, b R1 = SC2F5
1
.
.
–
+
.
RSSR + (RFCOO)2Xe
RSSR + (RFCOO)2Xe
.
–
–
(RFCOO)2Xe
RFCOO + RFCOO + Xe
.
.
RFCOO
RF
+
+
CO2
RS
.
.
+
+
RSSR
RS
.
.
RSRF
RS
+
RF
R = 1-chloro-2,6-dinitrophenyl;
RF = CnF2n+1 (when n = 1–2)
The greatest yield of 2a (70%) was achieved when the disulfide–perfluoroalkylcarboxylic acid–xenon
difluoride ratio was 1:5:4. When smaller amounts of the perfluorocarboxylic acid and XeF2 were used, some of
disulfide 1 remains unreacted in the reaction mixture. A significant effect of the solvents (trifluoroacetic acid,
perfluoropropionic acid, methylene chloride, and acetonitrile) on the yields of 2a and 2b was found. The best
results were obtained when the perfluoroalkylation was carried out in methylene chloride, in which disulfide 1
has high solubility. Products 2a and 2b are light brown or yellow crystalline compounds.
The 19F NMR spectrum of 2a shows a singlet for the CF3S group at -41.62 ppm. The quartet at
-90.88 ppm and triplet at -82.74 ppm confirm the presence of a C2F5S fragment in 2b. The 13C NMR spectrum of
1
2a has five signals. The quartet with JC–F = 310 Hz is assigned to the CF3 group. The triplet at 126.6 ppm
(3JC–SCF = 2.7 Hz) is assigned to the carbon atom directly attached to the SCF3 fragment, while the triplet at
134.2 p3pm (4JC–SCF = 1.1 Hz) is ascribed to C(4) and C(5). The singlet at 149.8 ppm belongs to the carbon atom
3
bound to the chlorine atom.
The chlorine atom and one of the nitro groups in 1-chloro-2,6-dinitro-4-trifluoromtehylbenzene are good
leaving groups in reactions with nucleophilic agents, while two-step intramolecular substitution reactions may
occur in the presence of bifunctional sulfur-containing reagents such as potassium ethyl xanthate or sodium
N,N-dimethyldithiocarbamate to give condensed heterocycles [8, 9].
Thus, the formation of thianthrene 4 through pathway A would be expected in the reaction of 2a with
potassium ethyl xanthate (Scheme 1). However, in fact, pathway B is realized, involving initial attack of the
chlorine atom in synthone 2a by the ethyl xanthate anion to give intermediate 3, which then converts to
ethylthio derivative 5 through loss of a COS molecule.
An attempt to synthesize a 1,3-benzdithiol-2-one derivative in the reaction of dinitrochlorobenzene 2a
with sodium N,N-dimethyldithiocarbamate proved more successful. We are the first to report the preparation of
a 1,3-benzodithiol-2-one containing a CF3S group 9 in 40% yield and thereby expand the scope of this general
method for the synthesis of this important class of organic compounds, which are precursors to
tetrathiafulvenes. The formation of 1,3-benzodithiol-2-one 9 may be seen as the result of initial replacement of
1376

Scheme 1
NO2
–
+
S
K S C(=S)OEt
2a
F3CS
S
OEt
NO2
K S C(=S)OEt
3
B
NO2
SEt
NO2
–
+
A
F3CS
– EtSC(=S)OEt
– COS
5
NO2
NO2
S
S
SCF3
–
F3CS
S
F3CS
NO2
NO2
4
the chlorine atom in synthone 2a by the N,N-dimethyldithiocarbamate fragment, subsequent loss of one of the
nitro groups at a nitrite ion from σ-complex
7 as the result of intramolecular attack of the negatively charged
sulfur atom of mesomeric form 6b, and, finally, hydrolysis of iminium intermediate 8 in the presence of water
molecules found in the initial sample of sodium N,N-dimethyldithiocarbamate:
CF3
S
CF3
S
–
+
.
Na S (=S)NMe2 2H2O
2a
O2N
NO2
O2N
NO2
S
–
S
S
S
+
NMe2
6b
NMe2
6a
NO2
F3CS
F3CS
H2O
F3CS
– ON=O
S
S
S
S
S
+
+
NMe2
NMe2
O
–
–
S
NO2
NO2
NO2
9
7
8
On the other hand, an radical–ion mechanism may be proposed for the intramolecular cyclization of
intermediate 6a by analogy with our previous findings [10]. The presence of the 1,3-dithiol-2-one fragment in 9
is indicated by the singlet for the carbonyl carbon atom in the 13C NMR spectrum at 189.0 ppm, the IR bands at
1704 and 1666 cm-1 characteristic for the C=O bond, and the major mass spectral decomposition pathway with
loss of CO by the molecular ion [11].
1377

Another possibility for the use of 2a and 2b for the synthesis of heterocyclic compounds may be seen in
the reaction with ethyl thioglycolate [12], leading to derivatives of benzothiazole N-oxide 11. In this case, the
chlorine atom is replaced by the ethyl thioglycolate fragment (intermediates 10a and 10b) and subsequent
intramolecular condensation with a nitro group leading to heterocyclization. The conversion of ethyl ester 11a
into hydroxamic acid 12 provides additional information on the reactivity of these heterocyclic compounds:
NO2
HSCH2COOEt
1
2a,b
R
S
CH2COOEt
Et3N
10a,b
– H2O
NO2
_
–
O
O
R1
+
+
F3CS
N
N
NH2OH
(11a)
CONHOH
COOEt
S
S
NO2
NO2
11a,b
12
2, 10, 11 a R1 = SCF3, b R1 = SC2F5
The 13C NMR spectrum of the benzothiazole fragment in 11a shows seven signals. The two strongest
signals at 132.4 and 132.7 ppm belong to C(4) and C(6) bound to protons. The signals for C(5) and C(7) attached to
the NO2 and SCF3 groups in the benzene ring are seen at 142.4 and 123.4 ppm, respectively, and are rather
similar to the values for the analogous atoms in 9. The thiazole ring carbon atoms are seen as singlets at 126.1,
137.4, and 147.0 ppm and are ascribed to C(2), C(8), and C(9), respectively. The ethoxycarbonyl fragment is seen
as a carbonyl carbon at 156.5 ppm, while a strong IR band is observed for the carbonyl group at 1740 cm-1. The
N–O fragment is indicated in the finding of strong [M - O]+ ion peaks in the mass spectra, typical for the
decomposition of the molecular ions of heterocyclic N-oxides [13].
EXPERIMENTAL
Column chromatography was carried out on Merck silica gel 60, 230-400 mesh. Thin-layer
chromatography was carried out on Merck silica gel 60 F254. The 1H, 13C, and 19F NMR spectra were taken on a
Bruker AM-360 spectrometer at 360 MHz or AM-500 spectrometer at 500 MHz using TMS or CFCl3 as the
internal standard. The IR spectra were taken on Biorad FTS-40 FT-IR and Specord M-80 spectrometers. The
GC/MS analysis was carried out on a Hewlett–Packard 5890 GC/MS unit at 70 eV using a 30-m capillary
column packed with HP1 oil. The high-resolution mass spectra were taken on a VG Autospec mass
spectrometer. The measurement precision was ±0.002 dalton.
Bis(4-chloro-3,5-dinitrophenyl) Disulfide (1). This starting aromatic disulfide was obtained in a three-
step synthesis. The potassium salt of 4-chloro-3,5-dinitrobenzenesulfonic acid is obtained in the first step from
chlorobenzene. In the second step, this salt is converted to the corresponding sulfonyl chloride according to
Ullmann [15]. In the third step, sulfonyl chloride (20 g, 66.4 mol) is dissolved with stirring in glacial acetic acid
(340 ml) saturated with 35 g of gaseous HBr. Then, phenol (6.9 g) is added to the reaction mixture and carefully
heated to 55-60°C. An exothermal reaction ensued. The reaction mixture was stirred for 26 h at 60°C and a light
yellow precipitate formed. After cooling, the precipitate was filtered off, washed with acetic acid and ethanol,
and dried to give 14 g (90%) 1. The precipitate was recrystallized from glacial acetic acid to give 13.3 g of
yellow crystals; mp 188-189°C (acetic acid). IR spectrum (vaseline oil), ν, cm-1: 3063 (C–H), 1536, 1351, 1278,
1378

1129, 1054, 914, 883, 722. 1H NMR spectrum (CDCl3), δ, ppm: 8.09 (s). Mass spectrum, m/z (I, %): 466 ([M]+,
73), 234 ([M/2 + 1]+, 100), 233 ([M/2]+, 23), 218 (8), 188 (31). Found: m/z 465.883 [M]+. C12H4Cl2N4O8S2.
Calculated: M 465.885.
Perfluoroalkylation of Bis(4-chloro-3,5-dinitrophenyl) Disulfide (1) (General Method). A sample
of disulfide 1 (4.0 g, 8.57 mmol) was added with stirring to a mixture of XeF2 and perfluoroalkylcarboxylic acid
in CH2Cl2 (15-20 ml) at -30°C (the disulfide–XeF2–perfluoroalkylcarboxylic acid mole ratio was 1:4:5). Stirring
was continued and the mixture was gradually let warm to room temperature. The end of the reaction was
defined as the cessation of gas evolution. The reaction mixture was washed with saturated aqueous NaHCO3.
The organic layer was separated and dried over Na2SO4. After removal of the solvent in vacuum, the residue
was subjected to chromatography on a silica gel column using 1:2 hexane–benzene as the eluent.
1-Chloro-2,6-dinitro-4-trifluoromethylthiobenzene (2a) was obtained in 70% yield as light yellow crystals;
1
mp 35-37°C. H NMR spectrum (CDCl3), δ, ppm: 8.27 (s). 19F NMR spectrum, δ, ppm: -41.62 (s). 13C NMR
spectrum, δ, ppm, J (Hz): 123.6, 126.6 (q, 3JC–SCF = 2.7); 128.3 (q, JC–F = 31.0.1); 134.2 (q, 4JC–SCF = 1.1); 149.8.
Mass spectrum, m/z (I, %): 302 ([M]+, 100), 2833 ([M - F]+, 5), 256 ([M - NO2]+, 1), 233 ([M -3CF3]+, 2), 210
([M - 2NO2 - CF3]+, 16), 69 [CF3]+, 9). Found: m/z 301.937 [M]+. C7H2ClF3N2O4S. Calculated: M 301.938.
1-Chloro-2,6-dinitro-4-pentafluoroethylthiobenzene (2b) was obtained in 66% yield as yellow
crystals; mp 65-67°C. 1H NMR spectrum (CDCl3), δ, ppm, J (Hz): 8.27 (s). 19F NMR spectrum, δ, ppm, J (Hz):
-90.88 (2F, q, JF–F = 3.3); -82.74 (3F, t, JF–F = 3.3). Mass spectrum, m/z (I, %): 352 ([M]+, 100), 316 ([M - HCl]+,
1), 283 ([M - CF3]+, 6), 237 ([M - CF3 - NO2]+, 2), 233 ([M - C2F5]+, 4), 191 (2), 187 (1), 106 (20). Found:
m/z 351.934 [M]+. C8H2ClF5N2O4S. Calculated: M 351.934.
Reaction of 1-Chloro-2,6-dinitro-4-trifluoromethylthiobenzene (2a) with Potassium Ethyl
Xanthate. A sample of 2a (2.48 mmol) was added in a single portion to a stirred solution of potassium ethyl
xanthate (0.44 g, 2.75 mol) in DMF (3 ml). The reaction mixture was heated to 80°C and maintained at this
temperature with stirring for 10 h and then at room temperature for an additional 5 h. The solvent was removed
in vacuum. The residue was washed with 10% hydrochloric acid and extracted with chloroform. The organic
layer was removed and dried over Na2SO4. The solvent was evaporated. The residue was subjected to
chromatography on a silica gel column using 1:2 benzene–hexane as the eluent).
1-Ethylthio-2,6-dinitro-4-trifluoromethylthiobenzene (7b) was obtained in 28% yield as orange
crystals; mp 50-52°C. 1H NMR spectrum, δ, ppm, J (Hz): 1.26 (3H, t, J = 7.5, CH3); 2.94 (2H, q, J = 7.5, CH2);
19
8.12 (2H, s, Ar). F NMR spectrum: -41.63 (s). Mass spectrum, m/z (I, %): 328 ([M]+, 54), 300 ([M - C2H4]+,
32), 283 ([M - C2H4 - OH]+, 67), 270 ([M - C2H4 - NO]+, 73), 223 ([M - C2H4 - HNO2]+, 26), 206 ([M - C2H4 -
2HNO2]+, 100), 176 (82). Found: m/z 327.978 [M]+. C9H7F3N2O4S2. Calculated: M 327.980.
Reaction of 1-Chloro-2,6-dinitro-4-trifluoromethylthiobenzene (2a) with Sodium
N,N-Dimethyldithiocarbamate. A solution of sodium N,N-dimethyldithiocarbamate (0.5 g) in DMSO (6 ml)
was added dropwise to a solution of 2a (1 g, 2.8 mmol) in DMSO (6 ml) stirred at room temperature. Stirring
was continued for 24 h. Then, water (150 ml) was added and the mixture was extracted with three 75-ml
chloroform portions. The combined extracts were washed thrice with water and dried over Na2SO4. The solvent
was evaporated and the residue was subjected to chromatography on a silica gel column with 1:1 benzene–
hexane as the eluent.
4-Nitro-6-trifluoromethylthio-1,3-benzodithiol-2-one (9) was obtained in the above reaction as
yellow crystals; mp 93-94°C. Yield of 9 0.33 g (40%). IR spectrum (CHCl3), ν, cm-1: 1704, 1666 (C=O), 1542,
1
1339 (N–O), 1173, 1146, 1105 (CF3). H NMR spectrum (CDCl3), δ, ppm, J (Hz): 8.11 (d, J = 1.25, Ar); 8.62
13
(d, J = 1.25, Ar). 19F NMR spectrum, δ, ppm: -42.07 (s, SCF3). C NMR spectrum, δ, ppm, J (Hz): 124.8 (q,
JC
= 2.4, C(5)); 129.4 (q, JC–F = 308, CF3); 130.9, 135.4 (C(4), C(6)); 134.2, 138.2 (C(1), C(2)); 143.6 (br. s,
(5)–SCF3
C(3)); 189.0 (s, C=O). Mass spectrum, m/z (I, %): 368 ([M]+, 100), 294 ([M - F]+, 2), 285 ([M - CO]+, 57), 239
([M - CO - NO2]+, 4), 216 (3), 209 (3), 170 (11). Found: m/z 312.915 [M]+. C8H2F3NO3S3. Calculated:
M 312.914.
1379

Reactions of 1-Chloro-2,6-dinitro-4-perfluoroalkylthiobenzenes (2a,b) with Ethyl Thioglycolate
(General Method). A sample of triethylamine (1.2 ml, 8.4 mmol) was added with stirring to a mixture of
2,6-dinitro-4-perfluoroalkylchlorobenzene 2a or 2b (7.6 mmol) and ethyl thioglycolate (0.9 g, 7.6 mmol) in
ethanol with ice cooling. The reaction solution was stirred at room temperature for 2 h. The precipitate formed
was filtered off, washed with water and then ethanol, and dried.
2-Ethoxycarbonyl-7-nitro-5-trifluoromethylthiobenzothiazole N-Oxide (11a) was obtained in 75%
yield in the above reaction as orange crystals; mp 134-136°C. IR spectrum (vaseline oil), ν, cm-1: 1740 (C=O).
1H NMR spectrum (CDCl3), δ, ppm, J (Hz): 1.46 (3H, t, J = 7.2, CH3); 4.53 (2H, q, J = 7.2, CH2); 8.84 (2H, s,
19
13
Ar). F NMR spectrum, δ, ppm: -42.03 (s). C NMR spectrum, δ, ppm, J (Hz): 14.1 (Me), 63.3 (CH2); 125.7
(C(5)); 126.1 (C(2)); 128.6 (q, JC–F = 310.0, CF3); 132.4, 132.7 (C(4), C(6)); 137.4 (C(8)); 142.4 (C(7)); 147.0 (C(9));
159.5 (C=O). Mass spectrum, m/z (I, %): 368 ([M]+, 6), 352 ([M - O]+, 39), 333 ([M - O - F]+, 14), 322
([M - NO2]+, 307 ([M - O - EtO]+, 39), 280 ([M - O - EtO - HCN]+, 100), 250 (20), 234 (27), 222 (13). Found:
m/z 367.976 [M]+. C11H7F3N2O5S2. Calculated: M 367.975.
2-Ethoxycarbonyl-7-nitro-5-pentafluoroethylthiobenzothiazole N-Oxide (11b) was obtained in
66.7% yield in the above reaction as orange crystals; mp 145-147°C. IR spectrum (vaseline oil), ν, cm-1: 1740
(C=O). 1H NMR spectrum (CDCl3), δ, ppm, J (Hz): 1.48 (3H, t, J = 7.2, CH3); 4.56 (2H, q, J = 7.2, CH2); 8.85
(1H, s, Ar); 8.88 (1H, d, J = 1.7, Ar). 19F NMR spectrum, δ, ppm: -91.15 (2F, s, CF2); -82.62 (3F, s, CF3). Mass
spectrum, m/z (I, %): 418 ([M]+), 20), 402 ([M - O]+, 55), 386 (15), 372 ([M - NO2]+, 17), 357 ([M - O - EtO]+,
48), 346 ([M - EtO - HCN]+, 28), 330 ([M - O -EtO - HCN]+, 100), 311 (14), 284 (25). Found: m/z 417.973
[M]+. C12H7F5N2O5S2. Calculated: M 417.972.
7-Nitro-5-trifluoromethylthiobenzothiazolyl-2-hydroxamic Acid N-Oxide (12). A mixture of
hydroxylamine hydrochloride (0.11 g, 1.63 mol) and sodium hydroxide (0.13 g) in methanol was added with
stirring to a solution of 9b (0.5 g, 1.36 mmol) in methanol. Stirring was continued for about 5 h. The solvent
was evaporated and the residue was dissolved in water. The solution was acidified by adding hydrochloric acid.
The precipitate formed was filtered off, washed with water and then methanol, and dried to give 10 as orange
crystals in 51% yield; mp 229-231°C. IR spectrum (vaseline oil), ν, cm-1: 3450, 3150 (OH, NH), 1650 (C=O).
1H NMR spectrum (DMSO-d6), δ, ppm: 8.54 (1H, s, Ar); 8.53 (1H, s, Ar); 11.83 (1H, br. s, NH). 19F NMR
spectrum, δ, ppm: -42.45. Mass spectrum, m/z (I, %): 355 ([M]+, 2), 339 ([M - O]+, 3), 323 (12), 305 (14),
295 (15), 280 (53), 275 (27), 250 (35), 234 (29), 206 (16). Found: m/z 354.955 [M]+. C9H4F3N3O5S2. Calculated:
M 354.954.
REFERENCES
1.
R. E. Banks (editor), Organofluorine Chemicals and their Industrial Applications, Horwood Ltd,
Chichester (1979), p. 125.
2.
3.
T. Fujita, J, Iwasa, and C. Hansch, J. Am. Chem. Soc., 86, 5175 (1964).
L. M. Yagupol'skii, Aromatic and Heterocyclic Compounds with Fluorine-Containing Substituents
[in Russian], Naukova Dumka, Kiev (1988), p. 275.
4.
A. M. Sipyagin, V. S. Enshov, C. P. Bateman, B. D. Mullen, and J. S. Thrasher, Abstracts of the Twelfth
European Symposium on Fluorine Chemistry, Berlin, Germany (1998), PII-96.
V. O. Lukashevich, Dokl. Akad. Nauk SSSR, 103, 627 (1955).
5.
6.
V. S. Enshov, S. A. Kashtanov, I. V. Efremov, I. A. Pomytkin, A. M. Sipyagin, and N. N. Aleinikov,
Khim. Geterotsikl. Soedin., 1703 (1995).
7.
8.
9.
M. Yoshida, T. Yoshida, M. Kobayashi, and N. Kamigata, J. Chem. Soc. Perkin Trans. 1, 909 (1989).
K. Rasheed and J. D. Warkentin, J. Org. Chem., 44, 267 (1979).
J. J. D'Amico, C. C. Tung, W. E. Dahl, D. J. Dahm, J. Org. Chem., 42, 2896 (1977).
1380

10.
11.
A. M. Sipyagin and I. V. Efremov, Khim. Geterotsikl. Soedin., 1088 (1996).
A. M. Sipyagin, S. V. Pal'tsun, A. V. Piyuk, N. N. Sveshnikov, V. I. Kozlovskii, and Z. G. Aliev, Zh.
Org. Khim., 29, 1872 (1993).
12.
13.
K. Wagner, H. Heitzer, and L. Oehlmann, Chem. Ber., 106, 640 (1973).
N. S. Vul'fson, V. G. Zaikin, and A. I. Mikaya, Mass Spectrometry of Organic Compounds [in Russian],
Khimiya, Moscow (1986), p. 139.
14.
15.
Organic Syntheses, Coll. Vol. 4 (1963), p. 364.
F. Ullmann, G. Engi, N. Wosnessensky, E. Kuhn, and E. Herre, Liebigs Ann. Chem, 366, 103 (1909).
1381