Condensation of 1-substituted 5-aminopyrazoles with β-dicarbonyl compounds

Article abstract of DOI:10.1023/A:1026106707180

Condensation of N-substituted 5-aminopyrazoles with β-diketones occurs with the formation of pyrazolo[4,5-b]pyridines. Depending on the conditions, their reaction with β-keto acids can give either 6-oxo- or 4-oxopyrazolo[4,5-b]pyridines.

Full text of DOI:10.1023/A:1026106707180

Chemistry of Heterocyclic Compounds, Vol. 39, No. 7, 2003
CONDENSATION OF 1-SUBSTITUTED
5-AMINOPYRAZOLES WITH
β-DICARBONYL COMPOUNDS
N. L. Nam, I. I. Grandberg, and V. I. Sorokin
Condensation of N-substituted 5-aminopyrazoles with β-diketones occurs with the formation of
pyrazolo[4,5-b]pyridines. Depending on the conditions, their reaction with β-keto acids can give either
6-oxo- or 4-oxopyrazolo[4,5-b]pyridines.
Keywords: 5-aminopyrazoles, β-diketones, β-keto esters, pyrazolopyridines.
In the case of acetoacetic ester and 5-amino-3-methyl-1-phenylpyrazole (1a), an acetoacetic ester
condensation reaction with aminopyrazoles leads to type 3 amides or type 2 crotonates. Thermal condensation
or condensation in the presence of acids then gives the pyrazolopyridones 4a and 5a respectively [1, 2].
R2
R
+
H
R2C–CH(R)–CN
O
R1NHNH2
+
N
H2N
N
R1
1a–d
R
3
R3
R3
R2
R4
OH
R2
R4
MeCOOH
120 oC
N
N
N
120 oC
O
N
H
N
N
N
O
N
H
R1
R2
R1
4a–f
3
O
O
+
H
R2
20 oC,
R4 = H
OEt
N
220 oC
R3
N
H
N
H
N
R3
R1
R1
5a,d
2
1 a-d R = H, e, f R = Me, a, c, f R1 = Ph, b, d, e R1 = Me, a, b R2 = Me, c, d R2 = Ph,
e, f R2 = Et; 4 a, b, e, f R1 = Ph, c, d R1 = Me, a-f R2 = Me, a, c, e, f R3 = Me, b, d R3 = CF3,
a-d R4 = H, e R4 = Et, f R4 = Me; 5 a R1 = Ph, R2 = R3 = Me; d R1 = R2 = Me, R3 = CF3
__________________________________________________________________________________________
K. A. Timiryazev Agricultural Academy, Moscow 127550, Russia; e-mail: intelbioscan@mtu-net.ru.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 1080-1085, July, 2003. Original article
submitted September 5, 2000; revision submitted February 19, 2003.
0009-3122/03/3907-0937$25.00©2003 Plenum Publishing Corporation
937

We have broadened the scope of use of this condensation of 5-aminopyrazoles and have synthesized the
1-substituted 5-aminopyrazoles 1b-d from diacetonitrile and benzoylacetonitrile. By using β-imino-α-
methylbutyronitrile (dipropiononitrile) in this reaction we have also prepared two novel 5-aminopyrazoles
which are substituted in position 4 (1e,f). With the aim of producing pyrazolopyridines (which are of interest as
physiologically active compounds) we have also studied other β-diketones (methylacetylacetone and
hexafluoroacetylacetone) and β-keto esters (trifluoroacetoacetic, methyl- and ethylacetoacetic esters) in the
reaction. There also remains open the question of the condensation route in the case of the keto esters and hence
their type 4 and 5 structures. In addition, the position of the hydroxy and methyl groups in the pyridine nucleus
for the cyclization was not resolved by the authors in the work [3, 4]. Our observation [2] that only one of the
pyrazolopyridones forms a picrate is indirect proof of the positioning of the keto group in the pyridine nucleus
and the structure of the condensation product (amide or crotonate, see the Scheme). Since the most basic
nitrogen atom in the pyrazolopyridine structure is that in the pyridine ring it is logical to propose that compound
4a has more weakly basic properties than compound 5a (2-hydroxypyridine is a weaker base than
4-hydroxypyridine by a factor of 1500 [5]).
It is known [1, 2] that 4-oxopyrazolo[4,5-b]pyridine 5a, forming the picrate, is exclusively obtained
from the crotonate 2 when it is slowly introduced into refluxing dowtherm whereas all of the other variants of
the cyclization reaction give the product 6-oxopyrazolo[4,5-b]pyridine 4a. Hence the use of the cyclization
reaction conditions of heating both components at 150°C without solvent should give the 6-oxopyrazolo-
[4,5-b]pyridines. In all cases except one the actual compounds 4 were obtained (see Experimental) without
isomeric contamination and in high yields. Unfortunately, the 6-oxopyrazolopyridines obtained do not form
crystals suitable for X-ray analysis and hence an absolute structural proof could not be achieved.
CF3
CF3
Me
N
Me
N
CF3COCH2COOEt + 1b
N
N
O
N
H
HO
N
Me
Me
4d'
4d (85%)
OH
Me
N
5d (15%)
N
F3C
N
Me
5d'
However, in the case of the condensation of 5-amino-1,3-dimethylpyrazole (1b) with
1
trifluoroacetoacetic ester a mixture of pyrazolopyridines was obtained. The H NMR spectrum showed both
signals for the pyrazolopyridone 4d and the presence of doubled signals for a second component (15%) which
was initially assigned the lactim form 4d'.
However, a change in the solvent from DMSO-d6 to C6D6 when recording the spectrum did not change
the ratio of these peaks (85 and 15%) and it is clear that the compound is actually the second isomer 5d. A
detailed study of the spectrum of the mixture in C6D6 showed that the signal for the CH3 group in position three
of the pyrazole nucleus at 2.43 ppm in the main product (85%) is split to an indistinct quadruplet (J = 1.65 Hz)
and this can only be explained by a through space interaction with the CF3 group. This is additional proof that
the condensation occurs to form the 6-oxopyrazolopyridine 4d as the main product since such an a splitting is
not possible in the isomer 5d. It should also be noted that the methyl groups in the pyrazole nucleus undergo a
marked shift to higher field when the spectra are recorded in C6D6 (1-CH3 from 3.84 to 2.99 and 3-CH3 from
938

2.42 to 1.60 ppm). This is because the equilibrium between the hydroxy and keto forms 5d' and 5d is shifted in
non-polar solvent towards the hydroxy form 5d'. As the same time, the position of the signals for compound 4 is
little affected by solvent. This evidently supports the keto form and also indirectly confirms our deductions
regarding the direction of cyclization.
The condensation of all of the β-diketones (with the exclusion of dibenzoylmethane) with the
5-aminopyrazoles occurs without complication upon heating equimolar amounts of both components at
150-160°C with distillation of water. All of the pyrazolo[4,5-b]pyridines 6 are obtained in high yields (60-95%).
R3
R2
R4
R3COCH(R4)COR3
N
+
1b,c
N
R3
N
R1
6a–d
6 a-c R1 = Ph, d R1 = Me, a-d R2 = Me, a, b, d R3 = Me, c R3 = CF3, a, c, d R4 = H, b R4 = Me
EXPERIMENTAL
IR spectra were taken on a Perkin-Elmer instrument (for KBr tablets), UV spectra on a Specord M-40
1
spectrometer, and H NMR spectra on a Bruker AM-300 instrument (300 MHz) using DMSO-d6. TLC was
carried out on Silufol plates and revealed using iodine or an alcoholic solution of FeCl3 with subsequent heating.
5-Amino-3-methyl-1-phenylpyrazole (1a) was prepared according to method [6].
β-Imino-α-methylbutyronitrile. Finely divided sodium (27.6 g, 1.2 mol) was added to a three necked
two liter flask fitted with a reflux condenser and a Hershberg stirrer. A solution of dry propiononitrile (110 g,
2 mol) in absolute ether (150 ml) and absolute alcohol (1 ml) was added dropwise with vigorous stirring (care,
evolution of hydrogen, all of the work was carried out in a fume cupboard). After the end of the addition, the
reaction mixture was refluxed for 24 h with stirring and the precipitated sodium salt was filtered off and washed
with absolute ether (200 ml). The salt was then added carefully to water (150 ml). The separated oil was
extracted with ether and dried with sodium sulfate. Distillation then gave the β-imino-α-methylbutyronitrile
(36.7 g, 33.4%) with bp 109-111°C (4 mm Hg) which solidified to a crystalline mass with mp 46-47°C
(mp 46-48°C [7]). Rf 0.74 (methanol–acetone, 1:1). UV spectrum, λmax, nm (log ε): 256 (4.38). IR spectrum,
ν, cm-1: 1580, 2280, 3300. Found, %: C 65.3; H 9.1; N 25.3. C6H10N2. Calculated, %: C 65.5; H 9.1; N 25.5.
5-Amino-1,3-dimethylpyrazole (1b). A mixture of water (20 ml), isopropanol (30 ml),
methylhydrazine sulfate (47.5 g, 0.33 mol), and β-aminocrotonitrile (24.6 g, 0.3 mol) was heated on a steam
bath for 30 min. Concentrated hydrochloric acid (70 ml) was then added to the mixture which was refluxed
using the condenser for 8 h. The reaction mixture was then neutralized with sodium hydroxide solution (40%)
and evaporated to dryness on a rotary evaporator. The dry residue was extracted with isopropanol in a Soxhlet
apparatus for 18 h. Evaporation under vacuum gave the aminopyrazole (19.3 g, 58%) with bp 121-123°C
(9 mm Hg) which crystallized upon standing to a white crystalline mass with mp 52-54°C and Rf 0.57
(methanol–acetone, 1:1). UV spectrum, λmax, nm (log ε): 233 (3.81). IR spectrum, ν, cm-1: 1550, 1630, 3200.
Found, %: C 53.7; H 8.3; N 37.8. C5H9N3. Calculated, %: C 54.1; H 8.1; N 37.8.
5-Amino-1,3-diphenylpyrazole (1c). Initially phenylhydrazine (2.1 g, 0.02 mol) and then
benzoylacetonitrile (2.9 g, 0.02 mol) were introduced into a hot mixture of water (8 ml) and concentrated HCl
(2 ml) and the product was stirred for 15 min. A further 8 ml of concentrated HCl was added to the mixture
which was then heated to reflux for 2 h with use of a condenser. It was then neutralized using aqueous ammonia
939

until the appearance of an odour. The crystals obtained were dried and recrystallized from benzene–hexane (1:2)
to give the aminopyrazole (3.1 g, 66%) with mp 124°C (mp 124-125°C [7]) and Rf 0.67 (benzene–acetone, 5:1).
1
UV spectrum, λmax, nm (log ε): 261 (4.40); 281 (4.24). IR spectrum, ν, cm-1: 1490, 1560, 1620, 3200. H NMR
spectrum, δ, ppm: 5.85 (s, 4-H); 7.4-8.2 (m, Harom).
5-Amino-1-methyl-3-phenylpyrazole (1d). A mixture of benzoylacetonitrile (5.2 g, 0.0346 mol),
methylhydrazine sulfate (5.0 g, 0.036 mol), and water (10 ml) was refluxed for 30 min with use of a condenser.
Concentrated HCl (20 ml) was added and the product was heated for a further 3 h and neutralized with aqueous
ammonia to the appearance of an odour. The crystals produced were dried and recrystallized from a mixture of
benzene–hexane (1:2) to give the aminopyrazole (3.6 g, 60%) with mp 129°C and Rf 0.24 (benzene–acetone,
5:1). UV spectrum, λmax, nm (log ε): 251 (4.35); 333 (2.76). IR spectrum, ν, cm-1: 1510, 1560, 1620, 3200.
Found, %: C 68.9; H 6.4; N 24.6. C10H11N3. Calculated, %: C 69.4; H 6.4; N 24.3.
5-Amino-3-ethyl-4-methyl-1-phenylpyrazole (1f) was prepared similarly to compound 1c from
phenylhydrazine (0.03 mol) and β-imino-α-methylbutyronitrile (see above) (0.03 mol) in 81% yield.
Recrystallization from a mixture of benzene–hexane, 2:1 gave mp 62°C (mp 63°C [7]) and Rf 0.54 (benzene–
acetone, 5:1). UV spectrum, λmax, nm (log ε): 251 (4.22). IR spectrum, ν, cm-1: 1600, 1630, 3200, 3300.
5-Amino-3-ethyl-1,4-dimethylpyrazole (1e). A mixture of compound 1a (3.92 g, 0.04 mol), water
(15 ml), and methylhydrazine sulfate (6 g, 0.042 mol) was refluxed for 30 min. Concentrated HCl (25 ml) was
added and the product was refluxed for a further 3 h. The reaction product was basified with excess sodium
hydroxide. The crystals produced were recrystallized from a mixture of benzene–hexane (1:2) to give the
aminopyrazole 1e (4.1 g, 74%) with mp 111°C. UV spectrum, λmax, nm (log ε): 231 (3.77). IR spectrum, ν, cm-1:
1630, 3200, 3300. Found, %: C 59.8; H 9.4; N 30.1. C7H11N3. Calculated, %: C 60.4; H 9.4; N 30.2
b
A mixture of the pyrazole
3,4,6-Trimethyl-1-phenylpyrazolo[4,5- ]pyridine (6a).
1a
(1.75 g,
0.01 mol), acetylacetone (1.1 g, 0.011 mol), and acetic acid (6 ml) was heated for 8 h, water (5 ml) was added to
the hot solution, and the product was cooled. The precipitated crystals were filtered off, dried, and recrystallized
from a mixture of benzene–hexane (1:1) to give the pyrazolopyridine 6a (1.75 g, 74%) with mp 127-128°C
(mp 128°C [2]). 1H NMR spectrum, δ, ppm: 2.60 (3H, s, 3-CH3); 2.62 (3H, s, 4-CH3); 2.64 (3H, s, 6-CH3); 6.78
(s, 5-H); 7.23 (t, p-Harom); 7.48 (dd, m-Harom); 8.31 (d, o-Harom).
b
was prepared similarly to the
3,4,5,6-Tetramethyl-1-phenylpyrazolo[4,5- ]pyridine (6b)
pyrazolopyridine 6a from the pyrazole 1a (0.1 mol) and methylacetylacetone (0.011 mol) in 98% yield with mp
133-134°C and Rf 0.42 (benzene). UV spectrum, λmax, nm (log ε); 260(4.54); 317 (3.92). IR spectrum, ν, cm-1:
1
1595. H NMR spectrum, δ, ppm: 2.28 (3H, s, 3-CH3); 2.62 (3H, s, 5-CH3); 2.68 (3H, s, 6-CH3); 7.29 (t,
p-Harom); 7.44 (dd, m-Harom)l; 8.32 (d, o-Harom). Found, %: C 76.5; H 6.9; N 16.7. C16H17N3. Calculated, %:
C 76.5; H 6.8; N 16.7.
b
was prepared similarly to
3-Methyl-1-phenyl-4,6-di(trifluoromethyl)pyrazolo[4,5- ]pyridine (6c)
compound 6a from the aminopyrazole 1a (0.01 mol) and hexafluoroacetylacetone (0.011 mol) in 94% yield with
mp 78-79°C and Rf 0.91 (ethyl acetate–hexane, 1:2.5). Visualization in UV light. UV spectrum, λmax, nm (log ε):
221 (4.02); 255 (4.48); 282 (3.88). IR spectrum ν, cm-1: 1600. 1H NMR spectrum, δ, ppm: 2.75 (3H, s, 3-CH3);
7.85 (s, 5-H); 7.38 (t, p-Harom); 7.62 (dd, m-Harom); 8.31 (d, o-Harom). Found, %: C 51.8; H 2.7; N 12.1.
C14H9F6N3. Calculated, %: C 76.5; H 6.8; N 16.7.
b
was prepared similarly to
1,3,4,6-Tetramethylpyrazolo[4,5- ]pyridinium hydrochloride (6d)
compound 6a from the pyrazole 1b (0.02 mol) and acetylacetone (0.022 mol). After heating the reaction mixture
it was distilled in vacuo to give the non crystallizing pyrazolopyridine 6d (2.39 g) with bp 161-162°C
(9 mm Hg). The product was dissolved in absolute ether (30 ml) and the hydrochloride 6d was precipitated by
passage of a stream of dry HCl to give 2.6 g (61%) with mp 242-243°C (in a sealed capillary). UV spectrum,
1
λmax, nm (log ε): 222 (4.46); 268 (3.65); 275 (3.68); 305 (3.82). IR spectrum, ν, cm-1: 1610, 1630. H NMR
spectrum, δ, ppm: 4.15 (3H, s, 1-CH3); 2.59 (3H, s, 3-CH3); 2.69 (3H, s, 4-CH3); 2.69 (3H, s, 6-CH3); 6.82 (s,
5-H). Found, %: C 56.3; H 6.8; N 19.9. C10H13N3.HCl. Calculated, %: C 56.7; H 6.2; N 19.9.
940

b
was prepared similarly to the
3,4-Dimethyl-6-oxo-1-phenylpyrazolo[4,5- ]pyridine (4a)
pyrazolopyridine 6a from compound 1a (0.01 mol) and acetoacetic ester (0.011 mol). Recrystallization from
60% ethanol gave a 65% yield with mp 189-190°C (mp 188-190°C [2]) and Rf 0.10 (ethyl acetate–hexane,
1:2.5). The same compound could be prepared in 35% yield by mixing both components without solvent and by
heating for 3 h with distillation of water and alcohol [2]. UV spectrum, λmax, nm (log ε): 221 (4.10); 259 (4.57);
1
292 (4.16). IR spectrum, ν, cm-1: 1660 (s), 2960. H NMR spectrum, δ, ppm: 2.58 (3H, s, 3-CH3); 2.58 (3H, s,
4-CH3); 6.31 (s, 5-H); 7.20 (t, p-Harom); 7.48 (dd, m-Harom); 8.22 (d, o-Harom).
b
The crotonate from pyrazole
3,6-Dimethyl-4-oxo-1-phenylpyrazolo[4,5- ]pyridine (5a).
1a
and
acetoacetic ester was prepared as described in method [1]. The crotonate (2.85 g, 0.01 mol) was added
portionwise to gently refluxing diphenyl ether (30 ml) and it was heated under gentle reflux for 20 min. After
cooling and dilution with hexane (100 ml) the precipitate was separated, refluxed in hexane (20 ml) for 10 min,
again separated and then dried and recrystallized from 60% alcohol to give the pyrazolopyridine (2.05 g, 86%)
with mp 214°C. Picrate 184°C (methanol) (mp 184°C [2]). Rf 0.3 (ethyl acetate–hexane, 1:1). UV spectrum,
1
λmax, nm (log ε): 250 (4.60); 300 (3.80). IR spectrum, ν, cm-1: 1630 (med.), 2960. H NMR spectrum, δ, ppm:
2.62 (3H, s, 3-CH3); 3.11 (3H, s, 6-CH3); 6.42 (s, 5-H); 7.18 (t, p-Harom); 7.43 (dd, m-Harom); 8.32 (d, o-Harom);
11.08 (s, NH).
b
was prepared similarly to
3-Methyl-6-oxo-4-trifluoromethyl-1-phenylpyrazolo[4,5- ]pyridine (4b)
the 6-oxo derivative 4a in 93% yield by heating the 5-aminopyrazole 1a (0.01 mol) and trifluoroacetoacetic
ester (0.011 mol) in acetic acid and recrystallization from 70% alcohol to give mp 175-176°C and Rf 0.53 (ethyl
acetate–hexane, 1:5). UV spectrum, λmax, nm (log ε): 258 (4.55); 294 (4.12). IR spectrum, ν, cm-1: 1615, 1660
1
(s). H NMR spectrum, δ, ppm: 2.57 (3H, s, 3-CH3); 6.85 (s, 5-H); 11.9 (s, NH); 7.27 (t, p-Harom); 7.48 (dd,
m-Harom); 8.15 (d, o-Harom). Found, %: C 57.4; H 3.5; N 14.3. C13H11F3N3O. Calculated, %: C 57.4; H 3.4;
N 14.3.
b
was prepared similarly to compound
1,3,4-Trimethyl-6-oxopyrazolo[4,5- ]pyridine (4c)
4a
from the
pyrazole 1b and acetoacetic ester in 67% yield and was recrystallized from 70% alcohol to give mp 261-263°C
and Rf 0.65 (methanol). UV spectrum, λmax, nm (log ε): 233 (3.82); 300 (4.08); 320 (3.86). IR spectrum, ν, cm-1:
1605, 1640. 1H NMR spectrum, δ, ppm: 3.72 (3H, s, 1-CH3); 2.44 (3H, s, 3-CH3); 2.44 (s, 4-CH3); 6.06 (s, 5-H);
11.35 (s, NH). Found, %: C 59.5; H 6.2; N 23.2. C9H11N3O. Calculated, %: C 61.0; H 6.2; N 23.7.
b
was prepared similarly to the
1,3-Dimethyl-6-oxo-4-trifluoromethylpyrazolo[4,5- ]pyridine (4d)
derivative 4a in 57% yield and recrystallized from 50% alcohol to give mp 210-211°C and Rf 0.39 (ethyl
acetate–hexane, 1.5:2.5). UV spectrum, λmax, nm (log ε): 247 (3.34); 306 (4.11). IR spectrum, ν, cm-1: 1650.
1H NMR spectrum, δ, ppm: 3.87 (3H, s, 1-CH3); 2.39 (3H, s, 3-CH3); 6.72 (s, 5-H); 12.18 (br s, NH). The
1
1,3-dimethyl-4-oxo-6-trifluoromethylpyrazolo[4,5-b]pyridine 5d was also formed in 15% yield. H NMR
spectrum (DMSO-d6), δ, ppm: 3.84 (3H, s, 1-CH3); 2.42 (3H, s, 3-CH3); 6.43 (s, 5-H); 12.18 (br. s, NH).
1H NMR spectrum (C6D6), δ, ppm (J, Hz): 4d 3.63 (3H, s, 1-CH3); 2.43 (3H, J = 1.65, 3-CH3); 6.54 (s, 5-H);
13.2 (br. s, NH); 5d (15%) 2.99 (3H, s, 1-CH3); 1.60 (3H, s, 3-CH3); 6.65 (s, 5-H); 13.2 (br. s, NH). Found, %:
C 46.8; H 3.5; N 18.3. C9H8F3N3O. Calculated, %: C 46.8; H 3.5; N 18.2.
b
The pyrazole
5-Ethyl-3,4-dimethyl-6-oxo-1-phenylpyrazolo[4,5- ]pyridine (4e).
1a
(1.75 g,
0.01 mol), propionic acid (10 ml), ethylacetoacetic ester (1.9 g, 0.011 mol), and SOCl2 (1 drop) were added to a
flask for distillation. The mixture was heated for 12 h with distillation of water and alcohol such that the
temperature of the reaction mixture was not lower than 140°C with the addition of the propionic acid as
necessary. The reaction mixture was then diluted with water (5 ml) and left overnight in the fridge. The
precipitated crystals were filtered off and recrystallized from 80% alcohol to give the pyrazolopyridine 4e
(1.21 g, 45%) with mp 182-183°C, Rf 0.62 (benzene–acetone, 5:1). UV spectrum, λmax, nm (log ε): 260 (4.54);
1
297 (4.15). IR spectrum, ν, cm-1: 1625. H NMR spectrum, δ, ppm: 2.56 (3H, s, 3-CH3); 2.62 (3H, s, 4-CH3);
1.11 (1H, t, 5-CH3); 2.68 (2H, q, 5-CH2); 7.19 (t, p-Harom); 7.45 (dd, m-Harom); 8.21 (d, o-Harom). Found, %:
C 71.4; H 6.3; N 15.7. C16H17N3O. Calculated, %: C 71.9; H 6.4; N 15.7.
941

b
was prepared similarly to the pyridine
3,4,5-Trimethyl-6-oxo-1-phenylpyrazolo[4,5- ]pyridine (4f)
4a in 47% yield. Recrystallization from 60% alcohol gave mp 234-235°C and Rf 0.59 (benzene–acetone, 5:1).
UV spectrum, λmax, nm (log ε): 258 (4.27); 261 (4.28); 263 (4.28); 266 (4.27); 300 (4.00). IR spectrum, ν, cm-1:
1
1635. H NMR spectrum, δ, ppm: 2.19 (3H, s, 3-CH3); 2.54 (3H, s, 4-CH3); 2.62 (3H, s, 5-CH3); 7.20 (t,
p-Harom); 7.42 (dd, m-Harom); 8.24 (d, o-Harom); 10.95 (br. s, NH). Found, %: C 71.5; H 5.9; N 16.8. C15H15N3O.
Calculated, %: C 71.1; H 5.9; N 16.6.
REFERENCES
1.
2.
3.
4.
5.
S. V. Tabak, I. I. Grandberg, and A. N. Kost, Zh. Obshch. Khim., 34, 2756 (1964).
S. V. Tabak, I. I. Grandberg, and A. N. Kost, Khim. Geterotsikl. Soedin., 116 (1965).
S. Checchi, M. Ridi, and P. Papini, Gazz. Chim. Ital., 85, 1160 (1955).
M. Ridi, P. Papini, and S. Checchi, Gazz. Chim. Ital., 91, 973 (1961).
L. S. Efros (editor), Physical Methods in the Chemistry of Heterocyclic Compounds [in Russian],
Khimiya, Leningrad (1966), p. 92.
6.
7.
8.
N. L. Nam, I. I. Grandberg, and V. I. Sorokin, Izv. Timiryazev Agricultural Academy, 172 (2000).
I. I. Grandberg, Din-Vei-Py, and A. N. Kost, Zh. Obshch. Khim., 21, 2311 (1961).
I. I. Grandberg and N. F. Krokhina, Khim.-Farm. Zh., No. 1, 16 (1968).
942