Organotellurium chemistry: Remarkably facile preparation of benzo-1,3-tellurazoles

Article abstract of DOI:10.1002/jhet.1007

Benzo-1,3-tellurazoles carrying alkyl or aryl substituents in position 2 were prepared in a facile two-step sequence, starting with readily available 2-haloanilines. This approach relies on the preparation of bis(2-aminophenyl) ditellurides by nucleophilic halide displacement from 2-haloanilines with sodium telluride in N-methylpyrrolidone. Subsequent reductive cyclization with carboxylic acid halides or carboxylic anhydrides furnished benzo-1,3- tellurazoles in good yields.

Full text of DOI:10.1002/jhet.1007

120
Organotellurium Chemistry: Remarkably Facile Preparation of Benzo-
1,3-tellurazoles
Vol 50
a
b
c
*
Nathan C. McMullen, Frank R. Fronczek, and Thomas Junk
aDepartment of Chemistry, University of Louisiana at Monroe, Monroe, Louisiana 71209
bDepartment of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803
cDepartment of Chemistry, University of Louisiana at Lafayette, Lafayette, Louisiana 70504
*
E-mail: txj9137@louisiana.edu
Received October 27, 2010
DOI 10.1002/jhet.1007
View this article online at wileyonlinelibrary.com.
Benzo-1,3-tellurazoles carrying alkyl or aryl substituents in position 2 were prepared in a facile two-step
sequence, starting with readily available 2-haloanilines. This approach relies on the preparation of bis
(2-aminophenyl) ditellurides by nucleophilic halide displacement from 2-haloanilines with sodium telluride in
N-methylpyrrolidone. Subsequent reductive cyclization with carboxylic acid halides or carboxylic anhydrides
furnished benzo-1,3-tellurazoles in good yields.
J. Heterocyclic Chem., 50, 120 (2013).
INTRODUCTION
aromatic substrates [8]. The fact that electron-rich haloani-
lines suffer nucleophilic attack is remarkable and attests to
the high nucleophilicity of the Te2À anion. Indeed, this reac-
tion proceeds well only within a narrow range of conditions
and cannot be adapted to selenium analogues. Both the
reported reaction temperature and time constitute a compro-
mise between the fairly vigorous conditions needed for
halogen displacement and product decomposition. Other
solvents, such as DMSO, HMPA, DMF, and sulfolane, did
not produce satisfactory results. An increase of the reaction
temperature or extension of the reaction time much beyond
4.5 h resulted in drastically diminished yields for the
corresponding bis(2-aminophenyl) ditellurides 1–3, presum-
ably due to carbon–tellurium bond fission. The reaction
provided the highest yield, when a threefold molar excess of
sodium hydride was used. Initially, aryl tellurolates were gener-
ated, which underwent rapid air oxidation to the corresponding
ditellurides. Remarkably, 4-chloro-2-iodoaniline produced the
corresponding ditelluride 3 in rather modest yield, whereas
2-bromo-4-chloraniline and 2-iodo-4-fluoroaniline did not
generate ditellurides in synthetically useful quantities.
Strategies for the conversion of bis(2-aminophenyl)
ditellurides to benzo-1,3-tellurazoles 4–11 have been
reviewed recently [9]; methods reported here are optimized
for simplicity. The use of acid halides was found to be well
suited for the preparation of 2-arylbenzotellurazoles,
whereas acid anhydrides were best suited for the synthesis
of 2-alkylbenzotellurazoles such as 10. The synthesis of
2-arylbenzotellurazoles is facilitated by the fact that 2-
arylbenzotellurazolium hydrochlorides are poorly soluble
in the media employed for their synthesis, allowing most
by-products to be removed by filtration.
Benzo-1,3-tellurazoles constitute highly light- and air-
stable compounds, which have found interest in applications
as diverse as the preparation of telluracarbocyanine dyes
[1], potential anthelmintics [2], and organic polymers [3].
Several methods for their preparation have been described,
nearly all of which rely on displacing a suitable leaving group
by tellurium in the presence of a nitrogen-containing substit-
uent in ortho position (Scheme 1). These include treatment
of 2-nitrophenylboronic acid with tellurium tetrachloride
[4], the reaction of 2-(N-trimethylsilylamino)phenylmagne-
sium bromides with elemental tellurium [5], reaction of
2-lithionitrobenzene with benzyltellurocyanate [6], and reac-
tion of 2-acetamidophenylmercury chlorides with tellurium
tetrachloride [7]. All of these methods have disadvantages,
such as relatively inaccessible starting materials, highly toxic
intermediates, or tedious procedures. Consequently, benzo-
1,3-tellurazoles have remained relatively unexplored.
We now report the facile preparation of benzo-1,3-tellurazoles
in two steps, starting with readily available 2-haloanilines.
These were found to undergo nucleophilic displacement of
halogen by tellurium when treated with sodium telluride in
NMP, to yield bis(2-aminophenyl) ditellurides. Cyclization to
the target compounds was achieved in one additional step, by
treatment of the resulting bis(2-aminophenyl) ditellurides 1–3
with benzoyl chlorides or aliphatic acid anhydrides and subse-
quent reductive cyclization in a one-pot procedure (Scheme 2).
RESULTS AND DISCUSSION
The displacement of halogen from halobenzenes by Te2À
was reported before, but only for relatively electron-poor
© 2013 HeteroCorporation

January 2013
Organotellurium Chemistry: Remarkably Facile Preparation of Benzo-1,3-tellurazoles
121
Scheme 1
Scheme 2
Scheme 3
The behavior of bis(2-chloroacetamidophenyl) ditelluride
was investigated in further detail. It was observed that solu-
tions of bis(2-chloroacetamidophenyl) ditelluride are unsta-
ble and precipitate a mixture of elemental tellurium and a
colorless solid upon standing, both in the presence and
absence of light. A well-formed crystal, when isolated and
submitted to X-ray crystallography, identified this solid as
1,1-dichloro-2H-1,4-benzotellurazin-3(4H)-one 14. It is
plausible that this product results from disproportionation
via an enolate intermediate (Scheme 3, Fig. 1).
Because of its low yield, however, this reaction is poorly
suited for the preparation of 2H-1,4-benzotellurazin-3(4H)-
one 13. The synthesis of this compound was reported previ-
ously [10], but no experimental details were provided. Conse-
quently, they are reported in the following section. At
d 1.5 ppm, the methylene carbon of this compound is remark-
ably shielded. Product yields are summarized in Table 1.
EXPERIMENTAL
4-Chloro-2-iodoaniline was prepared according to the literature
[11]. NMP and THF were dried over molecular sieves. All other
chemicals were purchased in reagent-grade purity and used as
received. Tellurium was used as 200 mesh powder. Chromatographic
purifications were carried out using Acros Organics (Geel, Belgium)
activated aluminum oxide, neutral, 50–200 micron. Melting points
were recorded using a Mel-Temp EM-6 (Bibby Scientific US,
Burlington, NJ) apparatus. nmr spectra were recorded on a JEOL
(JEOL USA, Peabody, MA) Eclipse 300-MHz spectrometer. MS
was performed on a Finnigan (Thermo Finnigan LLC, San Jose,
CA) Mat GCQ mass spectrometer equipped with a direct probe
inlet. Only the most abundant masses are reported for isotope clusters
containing tellurium. ir spectra were recorded on a PerkinElmer
(Perkin Elmer Inc., Waltham, MA) Spectrum 100 FTIR spectrome-
ter. Elemental analyses were performed on an Exeter (Exeter Analyt-
ical Inc., North Chelmsford, MA) CE-440 Elemental Analyzer. Due
the possibility for the generation of trace amounts of highly toxic
hydrogen telluride at various stages of the following syntheses,
“all operations should be carried out under a well-vented hood.”
Bis(2-aminophenyl) ditelluride (1) and bis(2-amino-4-
methylphenyl) ditelluride (2). A 100-mL three-necked flask
fitted with mechanical stirring and a nitrogen purge line was
Figure 1. ORTEP plot of 1,1-dichloro-2H-1,4-benzotellurazin-3(4H)-one. [Color
figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Journal of Heterocyclic Chemistry
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N. C. McMullen, F. R. Fronczek, and T. Junk
Vol 50
charged with 1.0g (7.84 mmol) of tellurium powder, 0.94 g,
(23.51mmol) of 60% sodium hydride suspension in mineral oil
(the oil was not removed prior to use) and 10 mL of anhydrous
NMP. The flask was purged with nitrogen and immersed in a
Wood’s metal bath kept at 185Æ 2. The mixture was stirred
vigorously for 10 min, during which time the tellurium dissolved.
Following the dropwise addition of 1.35 g (7.84 mmol) of
2-bromoaniline and 1.46 g (7.84 mmol) of 2-bromo-4-
methylaniline, respectively, over approximately 1 min, the
mixture was heated and stirred for an additional 4.5 h. It was
then allowed to cool to RT, diluted by adding 80 mL of water,
and buffered by the addition of 1.3 g (24.3 mmol) of
ammonium chloride. A brisk stream of air was forced through
this solution for 2 h to oxidize the sodium aryl tellurolate to the
ditelluride, as well as to convert any unreacted sodium telluride
to elemental tellurium. A color change to reddish was noticed
during this time, and a mixture of tellurium and the target
compound precipitated. Subsequently, this mixture was
collected by vacuum filtration, washed with copious amounts
of water, and finally with 5 mL of petroleum ether to remove
any mineral oil introduced with sodium hydride. The remaining
solid was extracted with hot acetone in 20-mL portions until no
further red ditelluride dissolved; the combined extracts were
filtered and concentrated on a rotary evaporator. The crude
product was purified by crystallization from ethanol.
a reaction temperature of 175ꢀC, had to be further purified
by chromatography (dichloromethane : alumina) before being
crystallized from ethyl acetate : methyl-tert-butyl ether (1: 3). Yield,
0.42 g, 21%; mp 125–127ꢀC; ir (potassium bromide): 3320, 3411
1
(NH); H nmr (CDCl3): d 4.12 (NH), 6.63–7.61 (ArH); 13C nmr
(CDCl3): d 95.1, 114.3, 122.6, 131.6, 142.0, 149.1; m/z (30 eV EI):
125.9 (C5H5NCl, 100%), 253.9 (C6H5ClNTe, 70%), 380.8
(M+ À Te, 6%). Anal. Calcd for C12H10Cl2N2Te2: C, 28.35; H,
1.98; N, 5.51. Found: C, 28.31; H, 1.80; N, 5.84.
General procedure for preparation of 2-arylbenzotellurazoles.
A 25-mL round-bottom flask equipped with magnetic stirring and
reflux condenser was charged with the corresponding ditelluride
(1, 2, or 3, 0.7 mmol), dissolved in 2 mL of THF. The mixture was
stirred for 1 min until all ditelluride had gone into solution,
1.5 mmol of the corresponding benzoyl chloride added, and stirring
continued for 5 min. This was followed by an addition of 2 mL of
95% ethanol, 330 mg (2.5 mmol) of 50% aq hypophosphorous
acid, and 1.5 mL of 36% hydrochloric acid. The mixture was
subsequently heated to reflux for 15 min. A gradual color change
from orange to greenish was observed. Subsequently, the mixture
was chilled in a freezer for 1 h, and precipitated solids were
collected by filtration. These consisted mainly of the tellurazolium
hydrochlorides in addition to trace quantities of elemental
tellurium. The filter cake was placed in a beaker; 10 mL of 5% aq
ammonia solution and 10 mL of DCM were added. The beaker
was covered, set aside for several hours and occasionally stirred to
complete phase partitioning. The organic phase was collected, and
traces of extremely fine elemental tellurium were removed by
centrifugation (filtration proved ineffective). After solvent removal,
products were recrystallized from petroleum ether. All were
isolated as pale yellow needles.
2-Phenylbenzotellurazole (4, from 1 and benzoyl chloride). Yield
267 mg, 62%, mp 100–101ꢀC; 1H nmr (CDCl3): d 7.15–8.22
(ArH); 13C nmr (CDCl3): d 125.1, 126.6, 127.2, 128.6, 129.2,
131.1, 131.7, 134.4, 141.1, 162.2, 173.4; m/z (30 eV EI): 179.2
(M+ À Te, 41%), 206.0 (C6H4Te, 55%), 308.8 (M+, 100%). Anal.
Calcd for C13H9NTe: C, 50.89; H, 2.96; N, 4.57. Found: C,
50.83; H, 2.76; N, 4.58.
2-(4-Chlorophenyl)benzotellurazole (5, from 1 and 4-
chlorobenzoyl chloride). Yield 280 mg, 60%, mp 124–125ꢀC; 1H
nmr (CDCl3): d 7.15–8.22 (ArH); 13C nmr (CDCl3): d 125.3,
126.7, 127.3, 129.4, 129.7, 131.7, 134.7, 137.0, 139.7, 162.0,
Bis(2-aminophenyl) ditelluride (1). Yield 1.05 g, 61%, mp
1
104–105ꢀC; ir (potassium bromide): 3302, 3398 (NH); H nmr
(CDCl3): d 4.17 (NH), 6.81–7.70 (ArH); 13C nmr (CDCl3):
d 95.0, 113.7, 119.1, 131.7, 143.3, 150.7; m/z (30 eV EI): 92.0
(C6H6N, 100%), 184.1 (C12H12N2, 83%), 221.9 (C6H6NTe,
35%), 346.8 (C6H5NTe2, 14%). Anal. Calcd for C12H12N2Te2:
C, 32.80; H, 2.75; N, 6.38. Found: C, 32.72; H, 3.01; N, 6.18.
Bis(2-amino-4-methylphenyl) ditelluride (2). Yield 1.15 g,
63%, mp 116–117ꢀC; ir (potassium bromide): 3158, 3357 (NH);
1H nmr (CDCl3): d 2.18 (CH3), 4.06 (NH), 6.66–7.48 (ArH); 13
C
nmr (CDCl3): d 20.1, 95.3, 113.7, 128.4, 132.4, 143.3, 148.2; m/z
(30 eV EI): 106.1 (C7H8N, 100%), 212.2 (C14H16N2, 56%), 235.9
(C7H8NTe, 27%), 360.8 (C7H7NTe, 28%). Anal. Calcd for
C14H16N2Te2: C, 35.97; H,3.45; N,5.99. Found: C, 35.67; H,
3.31; N, 5.94.
Bis(4-chloro-2-aminophenyl) ditelluride (3). The crude product,
prepared as discussed previously from 4-chloro-2-iodoaniline at
Table 1
Yields of compounds prepared.
Compound
Formula
12H12N2Te2
C14H16N2Te2
C12H10Cl2N2Te2
C13H9NTe
C13H8ClNTe
C14H11NOTe
C14H11NTe
C14H10ClNTe
C13H8ClNTe
C10H11NTe
Yield (%)
Bis(2-aminophenyl) ditelluride (1)
C
61
63
21
62
60
64
56
74
69
74
81
70a
61
Bis(2-amino-4-methylphenyl) ditelluride (2)
Bis(4-chloro-2-aminopenyl) ditelluride (3)
2-Phenylbenzotellurazole (4)
2-(4-Chlorophenyl)benzotellurazole (5)
2-(4-Methoxyphenyl)benzotellurazole (6)
6-Methyl-2-phenylbenzotellurazole (7)
6-Methyl-2-(4-chlorophenyl)benzotellurazole (8)
6-Chloro-2-phenylbenzotellurazole (9)
2-Propylbenzotellurazole (10)
6-Chloro-2-methylbenzotellurazole (11)
1-Butyl-3,4-dihydro-3-oxo-2H-1,4-benzotellurazolium bromide (12)
2H-1,4-Benzotellurazin-3(4H)-one (13)
C8H6ClNTe
C12H16BrNOTe
C8H7NOTe
aApproximately, compound 12 was not isolated in pure form.
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Organotellurium Chemistry: Remarkably Facile Preparation of Benzo-1,3-tellurazoles
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171.5; m/z (30 eV EI): 206.02 (C6H4Te, 54%), 213.06 (M+ À Te,
32%), 342.80 (M+, 100%). Anal. Calcd for C13H8ClNTe: C,
45.75; H, 2.36; N, 4.10. Found: C, 45.84; H, 2.66; N, 4.18.
2-(4-Methoxyphenyl)benzotellurazole (6, from 1 and 4-
methoxybenzoyl chloride). Yield 305 mg, 64%, mp 112–113ꢀC;
1H nmr (CDCl3): d 3.87 (CH3O), 6.93–8.16 (ArH); 13C nmr
(CDCl3): d 55.6, 114.4, 124.8, 126.1, 127.1, 130.2, 131.6, 133.9,
134.2, 162.1, 162.2, 172.5; m/z (30 eV EI): 206.1 (C6H4Te, 36%),
281.0 (M+ À Te, 49%), 339.0 (M+, 100%). Anal. Calcd for
C14H11NOTe: C, 49.92; H, 3.29; N, 4.16. Found: C, 49.98; H,
3.61; N, 4.30.
6-Methyl-2-phenylbenzotellurazole (7, from 2 and benzoyl
chloride). Yield 251 mg, 56%, mp 84–85ꢀC; 1H nmr (CDCl3):
d 2.46 (CH3), 7.25–8.08 (ArH); 13C nmr (CDCl3): d 21.3, 126.0,
128.5, 128.6, 129.2, 130.9, 131.7, 134.4, 135.3, 141.2, 160.2,
171.8; m/z (30 eV EI): 293.2 (M+ À Te, 34%), 220.1 (C7H6Te,
22%), 322.9 (M+, 100%). Anal. Calcd for C14H11NTe: C, 52.41;
H, 3.46; N, 4.37. Found: C, 52.39; H, 3.39; N, 4.42.
170.9; m/z (30 eV EI): 151.2 (M+ À Te, 26%), 240.1 (C6H4ClTe,
32%), 280.9 (M+, 100%). Anal. Calcd for C8H6ClNTe: C, 34.42;
H, 2.17; N,5.02. Found: C, 34.40; H, 2.39; N, 5.00.
Reaction of bis(2-aminophenyl) ditelluride (1) with
chloroacetyl chloride. A 25-mL round-bottom flask equipped
with magnetic stirring and a reflux condenser was charged with
300 mg (0.68 mmol) of 1 and 5 mL of acetonitrile. The mixture
was stirred and briefly heated until all ditelluride had gone into
solution. Pyridine (119mg, 1.50mmol) was added, followed by
dropwise addition of 154mg (1.36 mmol) of chloroacetyl chloride.
The flask was capped and set aside for 5 days. After this time,
approximately 90 mg of solids had precipitated, which consisted
of a white crystalline solid mixed with elemental tellurium and
contained several well-formed crystals suitable for X-ray
crystallography. The white compound was identified as 1,
1-dichloro-2H-1,4-benzotellurazin-3(4H)-one, an ORTEP plot of
which is shown in Figure 1. In light of the low yield of this
procedure, no attempt was made to purify the product.
1-Butyl-3,4-dihydro-3-oxo-2H-1,4-benzotellurazolium bromide
(12) and 2H-1,4-benzotellurazin-3(4H)-one (13). A 50-mL
round-bottom flask equipped with magnetic stirring, reflux
condenser, and a nitrogen purge line was charged with 500 mg
(1.14 mmol) of 1 and 8 mL of methanol. The mixture was heated
to reflux, purged with nitrogen, and sodium borohydride added
until the color of the ditelluride had faded (approximately 110 mg
consumed). A solution of 327 mg (2.39 mmol, 5% excess) of 1-
bromobutane in 1-mL methanol was added, and refluxing
resumed for an additional 5 min. Subsequently, the flask was
allowed to cool to RT and 379mg (2.73mmol, 20% excess) of
bromoacetic acid added before capping and setting aside for
3 days. After this time, the crude telluronium salt 12 had
precipitated as colorless crystals. The flask was chilled in a freezer
for 1 h and the product collected by filtration, yield, 630 mg, 70%.
It was dried and used without purification.
6-Methyl-2-(4-chlorophenyl)benzotellurazole (8, from 2 and
4-chlorobenzoyl chloride). Yield 368 mg, 74%, mp 128–129ꢀC; 1H
nmr (CDCl3): d 2.47 (CH3), 7.25–8.07 (ArH); 13C nmr (CDCl3): d
21.4, 126.1, 128.7, 129.3, 129.5, 131.7, 134.7, 135.6, 136.8, 139.7,
160.1, 169.9; m/z (30 eV EI): 89.07 (84%), 227.09 (M+ À Te, 34%),
356.84 (M+, 100%). Anal. Calcd for C14H10ClNTe: C, 47.33; H,
2.84; N, 3.94. Found: C 47.09; H, 2.59; N, 3.93.
6-Chloro-2-phenylbenzotellurazole (9, from 3 and 4-benzoyl
chloride). Yield 329 mg, 69%, mp 130–131ꢀC; 1H nmr
(CDCl3): d 7.42–8.08 (ArH); 13C nmr (CDCl3): d 126.9, 127.8,
128.6, 129.3, 130.8, 131.0, 131.4, 135.5, 140.7, 160.7, 173.7;
m/z (30 eV EI): 213.2 (M+ À Te, 28%), 240.1 (C6H3ClTe,
34%), 342.9 (M+, 100%). Anal. Calcd for C13H8ClNTe: C,
45.75; H, 2.36; N,4.10. Found: C, 46.12; H, 2.59; N, 4.14.
General procedure for preparation of 2-alkylbenzotellurazoles.
A 25-mL round-bottom flask equipped with magnetic stirring and a
reflux condenser was charged with the corresponding ditelluride
(1 or 3, 0.7 mmol) and 5 mL of acetonitrile. The mixture was stirred
and briefly heated until all ditelluride had gone into solution.
Stirring was continued while the temperature was allowed to return
to below 50ꢀC, followed by addition of 1.5 mmol of the
corresponding acid anhydride. The mixture was subsequently heated
to reflux for approximately 2 min, followed by addition of 330 mg
(2.5 mmol) of 50% aq hypophosphorous acid. Heating at reflux was
resumed for 15 min, during which time a color change from orange
to pale gray-green was noticed. The mixture was subsequently
allowed to cool to RT and poured into 10 mL of a 5% aq ammonia
solution. The products separated as off-white crystals. They were
collected and dissolved in petroleum ether : methyl-tert-butyl ether
(3:1), and traces of polar impurities were removed with a
10 mm  1.5 cm alumina-packed flash column. The tellurazoles
were subsequently recrystallized from 95% ethanol.
The crude telluronium salt (12, 300 mg, 0.75 mmol) was placed
in an open-ended 15mm  15cm reaction tube equipped with
magnetic stirring, and 2 mL of DMF was added. The mixture was
heated to gentle boiling for 15 min, allowing 1-bromopropane to
escape. The tube was then allowed to cool to RT and 10mL of water
added. The product precipitated as crystalline solid and was
collected by filtration. It was dissolved in THF, traces of polar impu-
rities were removed with a 10mm 1.5 cm alumina-packed flash
column, and the product was crystallized from 95% ethanol. Yellow
needles, yield 120 mg, 61 %, mp 206–208ꢀC; ir (potassium
1
bromide): 3165, 3263 (NH), 1643 (CO); H nmr (CDCl3): d 3.38
(2H), 6.89–7.54 (ArH), 8.52 (NH); 13C nmr ([1H6]-DMSO): d 1.5,
103.7, 119.1, 123.6, 128.4, 136.5, 142.8, 169.1; m/z (30 eV EI):
132.07 (C8H6NO, 100%), 220.9 (C6H5NTe, 14%), 262.85 (M+,
54%). Anal. Calcd for C8H7NOTe: C,36.85; H, 2.72; N,5.37.
Found: C, 37.04; H, 3.11; N, 5.34.
2-Propylbenzotellurazole (10, from
1 and butanoic
Acknowledgments. This work was funded by the College of
Arts and Sciences at the University of Louisiana at Monroe.
We would like to thank Prof. Jason A. Carr for his assistance
with elemental analyses.
anhydride). Colorless plates, yield 276 mg, 74%, mp 71–72ꢀC;
1H nmr (CDCl3): d 1.04 (CH3), 1.84 (–CH2–), 3.02 (–CH2–),
7.11–8.10 (ArH); 13C nmr (CDCl3): d 13.7, 24.7, 45.4, 124.8,
125.7, 126.8, 131.8, 134.2, 160.6, 178.80; m/z (30 eV EI): 275.0
(M+, 100%). Anal. Calcd for C10H11NTe: C, 44.03; H, 4.06; N,
5.13. Found: C, 43.86; H, 4.44; N, 5.15.
REFERENCES AND NOTES
6-Chloro-2-methylbenzotellurazole (11, from 3 and acetic
anhydride). Colorless needles, yield 269 mg, 81%, mp
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1
126–127ꢀC; H nmr (CDCl3): d 2.84 (CH3), 7.37–7.94 (ArH); 13C
nmr (CDCl3): d 30.8, 125.9, 127.4, 130.4, 131.1, 136.6, 159.2,
Journal of Heterocyclic Chemistry
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N. C. McMullen, F. R. Fronczek, and T. Junk
Vol 50
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet