Nitriles in organic synthesis: Synthesis of some new 2-heterocyclic benzothiazole derivatives

Article abstract of DOI:10.1080/10426500701323432

2-Cyanomethylbenzothiazole (1) was utilized for the synthesis of several new 2- heteryl benzothiazole derivatives as pyrazolyl, thienyl, pyridyl, benzoimidazolyl, and benzothiazolyl derivatives.

Full text of DOI:10.1080/10426500701323432

This article was downloaded by: [University of Sydney]
On: 12 April 2013, At: 05:29
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
Nitriles in Organic Synthesis:
Synthesis of Some New 2-
Heterocyclic Benzothiazole
Derivatives
A. A. Fadda a , M. E. A. Zaki b , K. Samir a & F. A.
Amer a
a Chemistry Department, Faculty of Science,
Mansoura University, Mansoura, Egypt
b Photochemistry Department, National Research
Center, Dokki, Cairo, Egypt
Version of record first published: 15 Jun 2007.
To cite this article: A. A. Fadda , M. E. A. Zaki , K. Samir & F. A. Amer (2007): Nitriles
in Organic Synthesis: Synthesis of Some New 2-Heterocyclic Benzothiazole Derivatives,
Phosphorus, Sulfur, and Silicon and the Related Elements, 182:8, 1845-1856
To link to this article: http://dx.doi.org/10.1080/10426500701323432
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-
and-conditions
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden.
The publisher does not give any warranty express or implied or make any
representation that the contents will be complete or accurate or up to

date. The accuracy of any instructions, formulae, and drug doses should be
independently verified with primary sources. The publisher shall not be liable
for any loss, actions, claims, proceedings, demand, or costs or damages
whatsoever or howsoever caused arising directly or indirectly in connection
with or arising out of the use of this material.

Phosphorus, Sulfur, and Silicon, 182:1845–1856, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701323432
Nitriles in Organic Synthesis: Synthesis of Some New
2-Heterocyclic Benzothiazole Derivatives
A. A. Fadda
Chemistry Department, Faculty of Science, Mansoura University,
Mansoura, Egypt
M. E. A. Zaki
Photochemistry Department, National Research Center, Dokki,
Cairo, Egypt
K. Samir
F. A. Amer
Chemistry Department, Faculty of Science, Mansoura University,
Mansoura, Egypt
2-Cyanomethylbenzothiazole (1) was utilized for the synthesis of several new 2-
heteryl benzothiazole derivatives as pyrazolyl, thienyl, pyridyl, benzoimidazolyl,
and benzothiazolyl derivatives.
Keywords 2-cyanomethylbenzothiazole; benzothiazole; pyrazole
INTRODUCTION
In recent times there have been broad developments in investiga-
tions of both the synthesis and the study of the chemical proper-
ties of benzothiazole derivatives which have valuable pharmacologi-
cal activity.1−3 Several lines of evidence support the hypothesis that
2-cyanomethylbenzothiazole (1) has resulted in much interest in their
synthesis and chemistry. For the past decade, we have been explor-
ing the synthetic potential, scope, and limitations of activated nitriles
in heterocyclic synthesis.4−6 Several new approaches for the synthesis
of five members, six members and their fused heterocyclic derivatives
have been developed during the work for this article.
Received October 5, 2006; accepted February 25, 2007.
Address correspondence to K. Samir, Chemistry Department, Faculty of Science,
Mansoura University, Mansoura, Egypt. E-mail: khaled samirm@yahoo.com
1845

1846
A. A. Fadda et al.
RESULTS AND DISCUSSION
As an extension of this work, and in continuation of our interest in the
synthesis of new heterocycles incorporating a benzothiazole nucleus,
we report here the behavior of 1 towards some reactive bifunctional
reagents as a facile and convenient route to some heterocyclic deriva-
tives containing a benzothiazole moiety.
As a part of this investigation, the reactions of α-, β-unsaturated
nitriles (4a,b)6 and certain activated bifunctional reagents have been
investigated. The resulting benzothiazole derivatives have latent func-
tional substituents, which have potential for further chemical transfor-
mations and new routes for the preparation of substituted benzothia-
zole derivatives in excellent yields with possible biological activity.
Thus, it has been found that 4a with hydrazine hydrate
in boiling ethanol afforded β-hydrazino-β-arylamino-α-2-benzothi-
azolylacrylonitrile (5)—not the expected aminopyrazole derivative (6)
(Scheme 1).
SCHEME 1

Heterocyclic Benzothiazole Derivatives
1847
SCHEME 2
Structure 5 was proved by IR spectrum which revealed stretching
frequencies of CN at 2216 cm,−1 NH2 at 3418, 3581 cm−1, and 3347 cm−1
due to NH. The 1H NMR (DMSO-d6) for compound 5 showed multiplet
bands at δ 6.9–8.1 ppm for (aromatic protons) and broad band at δ
9.0–9.5 ppm (-NHNH2 and NH). On the other hand, treatment of the
key intermediate 4a,b with hydrazine hydrate in DMF gave a single
product, which analyzed correctly for C16H13N5S (6a) and C17H15N5S
(6b) (Scheme 2).
The structure of 6 was inferred from its spectral data. Thus, the
infrared spectrum of (6b) showed bands at 3189, 3229, and 3419 cm−1
,
corresponding to the NH and NH2 groups, respectively, and was avoid
a band due to the cyano group. The 1H NMR spectra revealed a singlet
at δ 2.3, assigned for the methyl protons, a broad band located at δ 6.48,
assignable to amino group, and a multiplet at δ 7.0–8.2, assigned for
aromatic protons. The NH protons appeared at δ 10 and 12 ppm. The
formation of 6a,b is assumed to proceed via the replacement of the SCH3
group by the hydrazine moiety to give the intermediates 5a,b, which
then cyclized via the cyano group to afford the final isolable products
6a,b.
In fact, the structures of 6a,b were further confirmed by alternative
synthesis. Thus, it has been found that, treatment of 7a with hydrazine
hydrate in boiling ethanol for a long period of time produced the inter-
mediate 5a. Refluxing 5a in DMF led to the formation of a product
1
identical in all respects (m.p., mixed m.p., and IR, H NMR) with 6a
(Scheme 3).
Moreover, structure 6a was also proved by mass spectroscopy which
gave the fragments m/z 307, 291, 261, 199, 173, 108.
The aminopyrazole derivative 9 was prepared by reaction of 3 with
hydrazine hydrate in DMF/TEA to yield the intermediate 8, which

1848
A. A. Fadda et al.
SCHEME 3
undergoes intramolecular nucleophilic cycloaddition reaction affording
compound 9.
All attempts to prepare 3-amino-5-arylamino-4-[2-benzothiazolyl]-
pyrrazole (6a,b) by reaction of 3-amino-5-methylthio-4-[2-benzot
hazolyl]-pyrazole (9) with the appropriate aryl amines in DMF/TEA,
EtOH/TEA or fusion failed (Scheme 4).
The structure of (9) was established by IR spectrum, which showed no
absorption bands in the region 2150–2300 cm,−1 indicating the absence
of cyano group which involved in the cyclization process, in addition
to the presence of 3396, 3338 cm−1 (NH2), 3296 cm−1 (NH), 1623 cm−1
1
(C N), and 1601 cm−1 (C C). The H NMR (DMSO-d6) spectrum of
this product showed singlet signal at δ 2.7 (CH3S-), multiplet signal for
aromatic protons, and two amino protons at δ 7.3 - 8.1 and singlet signal
at δ 8.25 ppm due to (NH) proton. The mass spectrum of structure (9)
showed m/z 262 (M+, 100%), 261, 247, 246, 216, 215, 200, 199, 174, 108,
107.
All attempts to use the amino-imino tautomeric character in 6a,b
in the synthesis of pyrrazolo[2,3-a]pyrimidine derivatives by reaction
SCHEME 4

Heterocyclic Benzothiazole Derivatives
1849
SCHEME 5
with arylidene of activated nitriles (e.g., arylidene of malonitrile, ethyl
cyanoacetate, . . . etc), in boiling ethanol and in presence of a base such
as piperidine, triethylamine, sodium hydride or sodium ethoxide failed
(Scheme 5).
Moreover, it has been found the adduct (4a,b) behaved differ-
ently towards o-phenylenediamine and o-aminothiophenol when com-
pared with their reaction with ethylenediamine and ethanolamine5.
Thus, when (4a,b) reacted with o-phenylenediamine and/or o-
aminothiophenol in DMF and in presence of triethylamine afforded the
intermediate 11 (Scheme 6). The formation of 11 from 4 is assumed
to proceed via replacement of the SCH3 group with aromatic amines
which then cyclized by loss of the aryl amine fragment affording the
final product 12a,b (Scheme 7).
The formation of α-[benzothiazole-2-yl]α-[2-benzimidazolyl] acetoni-
trile (12a) and α,α-di[2-benzothiazolyl]acetonitrile (12b), instead of the
expected benzo[b]diazepine and benzo[b]oxazepine derivatives (13a,b)
were confirmed by their IR spectra which showed for compound (12a,b)
absorption band at 2188, 2198 cm−1 respectively, due to (C≡N) and this
SCHEME 6

1850
A. A. Fadda et al.
SCHEME 7
confirm that cyano group not involved in the cyclization, in addition
compound 12a showed band at 3153 cm−1 (NH) and no spectrum band
for NH2 group has been observed for both compounds 12a and 12b.
Mass spectrum of 12a showed m/e 290 (M+, 100%), 289, 288, 262, 199,
182, 173.
Melting points and spectral data of compounds 12a,b, were in agree-
ment with that prepared by Augstin and Dolling.7
Continuing our interest in thiophene chemistry, and with a view
directed towards preparing biological active heterocycles, we wish
to broaden the scope of the Gewald reaction utilizing heterocyclic
cyanomethylenes as candidates for facile synthetic route to hete-
rocyclic thiophenes. The work resulted in the formation of some

Heterocyclic Benzothiazole Derivatives
1851
new aminothiophene derivative. Thus, 3-amino-4-[2-benzothiazolyl]-
2-cyano-5-phenylaminothiophene (16) was prepared by action of
chloroacetonitrile on the sodium salt derivative 7a to form the interme-
diate β-cyanomethylthio-β-phenylamino-α-2-benzothiazolyl acryloni-
trile (15), which undergoes intramolecular cyclization by nucleophilic
addition of active methylene group to the cyano group affording the
amino-imino tautomeric form (16) (Scheme 8).
SCHEME 8
The structure of compound 16 was established by both elemental and
spectral data. The IR spectrum showed absorption bands at 3399, 3328
cm−1 (NH2 group), 3227 cm−1 (NH), 2180 cm−1 (CN), and 1637 cm−1
1
( C N). While its H NMR (DMSO-d6) spectrum revealed multiplet
band at δ 6.9–8.0 ppm due to nine aromatic protons and NH2, a singlet
signal at δ11.6 ppm (NH).The NH2 and NH protons disappeared on the
addition of few drops of deuterium oxide. The mass spectrum showed
m/z 348 (M+ base peak 100%), 347, 320, 315, 288, 245, 238, 180, 174,
135, 108, 77.
In attempts to prepare pyridobenzothiazole through the reaction
of 1 with β-diketone as reported with benzimidazolylacetonitrile by
Chuiguk and Volovenko8 and Kuz’menho et al.,9 the reaction of 1 with
acetyl acetone yielded the unexpected single product (as examined by
TLC) which analyzed correctly for C23H16N4S2 and its mass spectrum
exhibited a molecular ion peak at m/e 412 (M+ 20%). These data indi-
cated that one mole of acetylacetone was condensed with two moles of
1 to give 18. This product was formulated as 1,5-di[benzothiazol-2-yl]-
1,5-dicyano-2,4-dimethyl-1,4-pentadiene (18) (Scheme 9).
The structure of 18 was confirmed on the basis of elemental anal-
ysis, IR, and mass spectrum. Its IR spectrum showed the complete

1852
A. A. Fadda et al.
SCHEME 9
disappearance of CO band (or its enolic band) already present in the
parent compound (acetyl acetone), also the IR spectrum showed band
at 2192 cm−1 due to CN group. Moreover, its mass spectrum showed a
molecular ion peak at m/e 412 (M+ 20%), in addition to the following
fragments 397, 359, 213, 173 (100%) and 134.
Finally, we found that heating 2-cyanomethylbenzothiazole (1) in ab-
solute ethanol in presence of a few drops of TEA, a product of molecular
formula C27H18N6S3 was obtained 21. Structure 21 was proposed for
the reaction product based on IR spectrum which revealed the presence
of two amino stretching bands at 3450-3200 cm−1, three C N stretch-
ing bands at 1600-1630 cm−1. A mass spectrum measurement gave an
additional evidence for trimerization of 1 which showed M+ = 522. In
1
addition H NMR showed singlet signal at δ 5.1 for methylene group,
two broad bands at δ 6.1 and 6.7 (D2O exchangeable) due to two NH2
group and multiplet band at δ 7.0–8.2 for twelve aromatic protons. A
logical mechanism for this reaction is based on first dimerization of
1 in the basic medium to give compound 19, the latter in turn reacts
with a third molecule of 1 to give 20 which spontaneously cyclized di-
rectly to afford 2,4-diamino-3,5-di(benzothiazol-2-yl)-6-[(benzothiazol-
2-yl)-methyl]pyridine (21) (Scheme 10). Such pyridine formation finds
parallels with the reported literature.10
EXPERIMENTAL
Melting points were uncorrected. Elemental analysis was carried out in
the Microanalytical Unit Faculty of Science, Cairo University. IR spec-
tra were recorded on Pye Unicam SP-1000cm−1 spectrometer using KBr
1
wafer technique. H NMR spectra were determined on Varian Gemini
200 MHz NMR spectrometers using TMS as an internal standard with

Heterocyclic Benzothiazole Derivatives
1853
SCHEME 10
(δ = 0 ppm). Mass spectra were determined on GC-MS.QP-100 EX.
Schimadzye (Japan).
Synthesis of β-Hydrazino-β-phenylamino-α-2-
benzothiazolylacrylo-nitrile (5)
A mixture of 4a (1.615 g, 0.005 mol) and hydrazine hydrate (0.25 ml,
0.005 mol) in 20 ml of absolute ethanol was refluxed until the evolution
of methyl thiol was ceased (2–3 h). The reaction mixture was concen-
trated to its half volume and left to cool at room temperature and the
resulting material recrystallized from ethanol to give (5).
Compound 5. white crystals; yield (65%); m.p. 197◦C; IR(cm−1
)
(KBr) at 2216 (CN), 3418, 3581 (NH2), and 3347 (NH); 1H NMR:
(DMSO-d6): δ = 6.9–8.1 (m, 9H, aromatic protons) and 9.0–9.5 (m (br),
4H, NHNH2 and NH); MS: m/z 307 (M+); Anal. calcd. For C16H13N5S:
C, 62.52; H, 4.26; N, 22.78; S, 10.43. Found: C, 62.34; H, 4.30; N, 22.76;
S, 10.39.
Synthesis of 3-Amino-5-arylamino-4-[2-benzothiazolyl]-
pyrazole (6a,b)—
Method A
A mixture of 4a,b (0.01 mol) and hydrazine hydrate (0.01 mol) in
dimethylformamide was refluxed for 8 h. The reaction mixture was
cooled to room temperature and then poured onto ice cold water (100
ml). The precipitated solid material was filtered off, dried well, and

1854
A. A. Fadda et al.
recrystallized from dimethylformamide-ethanol mixture (1:3) to give
(6a,b).
Method B
A solution of 5 (0.01 mol) in dimethylformamide (20 ml) containing
a catalytic amount of triethylamine (4 drops) was refluxed for 4 h and
then cooled to room temperature. The reaction mixture was poured onto
ice cold water (100 ml). The precipitated solid material was filtered off,
dried well, and recrystallized from dimethylformamide-ethanol (1:3) to
give(6a,b)
Compound 6a. white crystals; yield (74%); m.p. > 300◦C; IR(cm−1
)
(KBr) 3217 (NH), 3325, 3465 (NH2); 1H NMR: (DMSO-d6): δ = 6.2
(broad, 2H, NH2), 7.0–7.9 (m, 10H, aromatic protons and NH), and
8.9(s, 1H, NH) ; MS 307(M+); Anal. calcd. For C16H13N5S: C, 62.52; H,
4.26; N, 22.78; S, 10.43. Found: C, 62.54; H, 4.19; N, 22.57; S, 10.42.
Compound 6b. Page crystals; yield (71%); m.p. > 300◦C; IR(cm−1
)
(KBr) 3189 (NH), 3229, 3419 (NH2); 1H NMR: (DMSO-d6): δ = 2.3 (s, 3H,
CH3), 6.48 (broad, 2H, NH2), 7.0–8.2 (m, 8H, aromatic protons), 10.0 (s,
1H, NH), 12.0( s, 1H, NH); MS 321(M+); Anal. calcd. For C17H15N5S: C,
63.53; H, 4.70; N, 21.79; S, 9.98. Found: C, 63.49; H, 4.69; N, 21.82; S,
9.97.
Synthesis of 3-Amino-5-methylthio-4-[2-benzothiazolyl]-
pyrazole (9)
To a solution of (3) (0.01 mol) in dimethylformamide (20 ml) was added
hydrazine hydrate (0.32 ml, 0.01 mol). The reaction mixture was re-
fluxed for six h, then left to cool at room temperature. The reaction
mixture was then poured onto ice cold water (100 ml). The precipi-
tated solid material was filtered off, dried well, and recrystallized from
ethanol to give (9).
Compound 9. Yellow crystals; yield (62%); m.p. 190◦C; IR(cm−1
)
1
(KBr) 1623 (C N), 3296 (NH), 3338, 3396 (NH2); H NMR: (DMSO-
d6): δ = 2.7(s, 3H, SCH3), 7.3-8.1 (m, 6H, aromatic protons and NH2),
and 8.25(s, 1H, NH); MS 262 (M+); Anal. calcd. For C11H10N4S2: C,
50.36; H, 3.84; N, 21.36; S, 24.44. Found: C, 50.31; H, 3.89; N, 21.40; S,
24.42.

Heterocyclic Benzothiazole Derivatives
1855
Synthesis of α-[benzothiazole-2-yl]- α-[2-benzimidazolyl]-
acetonitrile (12a) and di[benzothiazol-2-yl]acetonitrile (12b):
A mixture of (4a,b, 0.01 mol) and o-phenylenediamine or o-
aminothiophenol (0.01 mol) in dimethylformamide (25 ml) was refluxed
for 12 h. After cooling, the reaction mixture was poured onto ice cold
water (100 ml). The resulting, solid product was collected by filtration,
dried well, and recrystallized from dimethylformamide-ethanol (1:3) to
obtain (12a,b).
Compound 12a. Yellow crystals; yield (81%); m.p. > 300◦C; IR(cm−1
)
(KBr) 2188(CN), 3153(NH); 1H NMR: (DMSO-d6): δ = 4.38 (s, 1H, CH),
7.2–8.0 (m, 8H, aromatic protons), and 8.8(s, 1H, NH); MS 290 (M+);
Anal. calcd. For C16H10N4S: C, 66.19; H, 3.47; N, 19.30; S, 11.04. Found:
C, 66.12; H, 3.50; N, 19.26; S, 11.02.
Compound 12b. Yellow crystals; yield (65%); m.p. 290◦C; IR(cm−1
)
1
(KBr) 2198 (CN); H NMR: (DMSO-d6): δ = 4.40 (s, 1H, CH), 7.2–8.0
(m, 8H, aromatic protons); MS 307 (M+); Anal. calcd. For C16H9N3S2:
C, 62.52; H, 2.95; N, 13.67; S, 20.86. Found: C, 62.50; H, 3.01; N, 13.65;
S, 20.85.
Synthesis of 3-Amino-4-[2-benzothiazolyl]-2-cyano-5-
phenylamino-thiophene (16)
To a well stirred solution of (1, 0.01 mol) in absolute ethanol (25 ml) con-
taining sodium ethoxide (0.015 mol) was added phenyl isothiocyanate
(1.2 ml, 0.01 mol). The reaction mixture was stirred for one hour at room
temperature. To the above reaction mixture, chloroacetonitrile (0.01
mol) was added and refluxed for 8 h. The reaction mixture was poured
onto ice cold water (100 ml) and acidified by 2N HCl. The precipitated
product was collected by filtration, washed with water several times,
and recrystallized from dimethylformamide-ethanol to obtain (16).
Compound 16. Yellow crystals; yield (89%); m.p. 196◦C; IR(cm−1
)
(KBr) 2180 (CN), 3227 (NH) and 3399, 3328 (NH2); 1H NMR: (DMSO-
d6): 6.9–8.0 (m, 11H, aromatic protons and NH2) and 11.6(s, 1H, NH) ;
MS 348 (M+); Anal. calcd. For C18H12N4S2: C, 62.05; H, 3.47; N, 16.08;
S, 18.40. Found: C, 62.04; H, 3.45; N, 16.10; S, 18.38.
Synthesis of 1,5-Di[benzothiazol-2-yl]-1,5-dicyano-2,4-
dimethyl-1,3-pentadiene (18)
A mixture of (1, 0.01 mol) and acetyl acetone (0.01 mol) in dry dimethyl-
formamide (20 ml) containing a catalytic amount of triethylamine

1856
A. A. Fadda et al.
(3 drops) was heated under reflux for 6 h. The reaction mixture was
poured onto ice cold water. The separated solid product was filtered off
and recrystallized from dimethylformamide-ethanol (1:5) to give (18).
Compound 18. Yellow crystals; yield (68%); m.p. 192◦C; IR(cm−1
)
(KBr) 2192 (CN); 1H NMR: (DMSO-d6): δ = 1.8 (s, 6H, 2CH3), 2.7(s, 2H,
CH2), and 7.2–8.2 (m, 8H, aromatic protons); MS 412 (M+); Anal. calcd.
For C23H16N4S2: C, 66.96; H, 3.91; N, 13.58; S, 15.55. Found: C, 66.93;
H, 3.94; N, 13.60; S, 15.54.
Synthesis of 2,4-Diamino-3,5-di[benzothiazol-2-yl]-6-
[(benzothiazol-2-yl)methyl]pyridine (21)
2-Cyanomethylbenzothiazole (1) (0.01 mole) was dissolved in ethanol
(20 ml), a few drops of triethylamine (4 drops) were added; then the
solution was refluxed for 6 h. The solid product obtained after cooling
was collected by filtration and recrystallized from ethanol-DMF (1:3) to
give (21).
Compound 21. Yellow crystals; yield (68%); m.p. 226◦C;
IR(cm−1)(KBr) 3200–3450 (two NH2).; 1H NMR: (DMSO-d6): δ =
5.1 (s, 2H, CH2), 6.1(s, 2H, NH2), 6.7(s, 2H, NH2), and 7.0–8.2 (m, 12H,
aromatic protons); MS 522 (M+); Anal. calcd. For C27H18N6S3: C, 62.05;
H, 3.47; N, 16.08; S, 18.40. Found: C, 62.06; H, 3.46; N, 16.10; S, 18.41.
REFERENCES
[1] N. Pietrancosta, A. Moumen, R. Dono, P. Lingor, V. Planchamp, F. Lamballe, M.
Ba¨hr, J. Kraus, and F. Maina, J. Med. Chem., 49(12), 3645–3652, (2006).
[2] N. Bharatham, K. Bharatham, and K. Woo Lee, Bull. Korean Chem. Soc., 27(2),
266–272 (2006).
[3] R. Jeon, Y. Jung Kim, Y. Cheon, and J. Ryu, Arch. Pharm. Res., 29(5), 394–399
(2006).
[4] M. E. A. Zaki, A. A. Fadda, K. Samir, and F. A. Amer, Phosphorus, Sulfur, and Silicon
and the Related Elements., 181(8), 1815–1823 (2006).
[5] A. A. Fadda, F. A. Amer, M. E. A. Zaki, and Kh. Samir, Phosphorous, Sulfur and
Silicon, 155, 59–66 (1999).
[6] M. E. A. Zaki, A. A. Fadda, K. Samir, and F. A. Amer, Chemistry of Heterocylic
Compounds, 39(9), 1413–1419 (2003).
[7] M. Augustin and W. Dolling, J. Prakt Chemie, 324(1), 3–11 (1982).
[8] V. A. Chuiguk and Yu. M. Volovenko, Khim. Geterotsikl. Soedin, 4, 530–532 (1975).
[9] V. V. Kuz’menko, V. N. Komissarov, and A. M. Simonov, Khim. Geterotsikl. Soedin,
(11), 1497–502 (1981).
[10] T. Takeshima, M. Yokoyama, N. Fukada, and M. Akano, J. Org. Chem., 35, 7 (1970).