Reactions of 3-aroylacrylates with α-aminoazoles

Article abstract of DOI:10.1007/s10593-011-0864-3

Dihydro derivatives of pyrazolo[3,4-b]pyridine-, pyrazolo[1,5-a]pyrimidine- , and [1,2,4]triazolo-[1,5-a]pyrimidinecarboxylates have been prepared by cyclocondensation of β-aroylacrylates with 5-aminopyrazoles and 3-amino-1,2,4-triazole. Heating dihydro[1,2,4]triazolo[1,5-a]pyrimidine-7- carboxylates with hydrazine hydrate led to recyclization of the pyrimidine ring to form 6-arylpyridazin-3(2H)-ones.

Full text of DOI:10.1007/s10593-011-0864-3

Chemistry of Heterocyclic Compounds, Vol. 47, No. 8, November, 2011 (Russian Original Vol. 47, No. 8, August, 2011)
REACTIONS OF 3-AROYLACRYLATES
WITH -AMINOAZOLES*
N. N. Kolos1**, L. U. Kovalenko1, and V. A. Borovskoy2
Dihydro derivatives of pyrazolo[3,4-b]pyridine-, pyrazolo[1,5-a]pyrimidine-, and [1,2,4]triazolo-
[1,5-a]pyrimidinecarboxylates have been prepared by cyclocondensation of -aroylacrylates with
5-aminopyrazoles and 3-amino-1,2,4-triazole. Heating dihydro[1,2,4]triazolo[1,5-a]pyrimidine-
7-carboxylates with hydrazine hydrate led to recyclization of the pyrimidine ring to form
6-arylpyridazin-3(2H)-ones.
Keywords: 3-amino-1,2,4-triazole, 5-aminopyrazole, 6-arylpyridazin-3(2H)-ones, dihydropyrazolo[3,4-b]-
pyridines, dihydropyrazolo[1,5-a]pyrimidines, dihydro[1,2,4]triazolo[1,5-a]pyrimidines, 3-aroylacrylic acids
esters, cyclocondensation.
3-Aroylacrylic acids and their functionalized derivatives are convenient bielectrophilic reagents for the
synthesis of six-membered heterocycles [1-5]. The formal [4+2], [3+3] or [2+4] cycloaddition schemes correspond
to interaction of these compounds with 1,4-, 1,3- or 1,2-binucleophiles respectively. Reactions of 3-aroylacrylic
acids with -aminoazoles remain the least studied, few publications being concerned with this problem [6, 7].
However, a convenient method for the synthesis of the azolopyridine or azolopyrimidine systems is the reaction of
chalcone type enone systems with -aminoazoles [8]. The synthetic availability of 3-aroylacrylic acids [9, 10], the
high reactivity of an activated ethylene bond [11-13], and the possibility of functionalization of the carboxyl group
point to the evident advantages of these bielectrophiles over the chalcones.
In our work we have studied the cyclocondensation of the 3-aroylacrylates 1a-d with the
5-aminopyrazoles 2a,b and the 3-aroylacrylates 1d-g with 3-amino-1,2,4-triazole (3).
There are literature reports of the formation of both pyrazolo[1,5-a]pyrimidines [14-17] and
pyrazolo[3,4-b]pyridines [18-20] in the reaction of 5-aminopyrazoles with compounds having enone structural
fragments. The use of 3-aroylacrylic acid derivatives in similar reactions permits the synthesis of functionalized
dihydroazoloazine systems [6].
Treatment of the esters 1a-d with 5-aminopyrazole 2a in ethanol gives high yields of the pyrazolo-
[3,4-b]pyridines 4a-d as yellow crystals. The formation of the pyrazolopyridine bicycle is associated above all
with the nucleophilic centers of the 5-aminopyrazole molecule [18, 19]. The synthesis of pyrazolopyridines 4a-d
involves the formation of an α-adduct at the pyrazole 2a C-4 atom.
_______
*Dedicated to the blessed memory of Academician M. O. Lozinskii.
**To whom correspondence should be addressed, e-mail: kolos_n@mail.ru.
1V. N. Karazin Kharkiv National University, 4 Svoboda Sq., Kharkiv 61077, Ukraine.
2 Institute for Single Crystals, 60 Lenin Ave., Kharkiv 61178, Ukraine; e-mail: borovskoy@gmail.com.
__________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1198-1204, August, 2011.
Original article submitted April 9, 2011.
983
0009-3122/11/4708-0983©2011 Springer Science+Business Media, Inc.

1
The H NMR spectra confirm the structure of the products obtained. The proton signals for the
dihydropyridine ring appear as a typical ABX system, the aromatic protons are in the region 7.10-8.00 ppm as
multiplets, and the methyl groups appear as three-proton singlets. The absence of a low-field NH proton signal
1
in the H NMR spectra points to the existence of the products 4a-d in the imine and not enamine tautomeric
form in DMSO-d6 solutions.
CO2R
CO2R O
NH2
Me
N
Me
R1
Me
R1
R1
EtOH
CO2R
N
NH2
N
+
N
N

N
N
O
Ph
Ph
2a
Ph
4a–d
1a–d
1, 4 a–c R = Me, a R1 = H, b R1 = 4-MeO, c R1 = 4-Br; 1, 4 d R = Et, R1 = 4-Br
The IR spectra of pyrazolopyridines 4a-d show strong absorption at 1720 cm-1 (ester C=O) and the
absence of a band at 1640-1670 cm-1 for a C=C–NH band which confirms the imine structure in the crystals
[18].
The reaction of the 5-aminopyrazole 2b and esters 1b,d,e occurs under similar experimental conditions
to give the pyrazolo[1,5-a]pyrimidines 5b,d,e in somewhat lower yields than for the pyrazolo[3,4-b]pyridines
4a-d.
In the amine molecule 2b, the most nucleophilic center proves to be the N-1 atom of the pyrazole ring
and this participates in addition to the activated C=C bond of the esters 1.
Formation of the pyrazolo[1,5-a]pyrimidine ring is chiefly confirmed from the presence in the 1H NMR
spectra of the obtained products 5b,d,e of a singlet signal for proton H-3 and also by the double doublets for the
protons of the methylene and methine groups, as for the derivatives 4a-d. According to the IR data, compounds
5b,d,e also exist in the imine tautomeric form.
Me
R1
EtOH
CO2R
+

N
O
NH2
N
H
2b
1b,d,e
Me
Me
N
N
N
NH2
CO2R
N
O
N
R1
R1
RO2C
5b,d,e
1, 5 b R = Ме, R1 = 4-MeO; d R = Et, R1 = 4-Br; e R = Et, R1 = H
984

Cyclocondensation of esters 1d-g was also carried out with the 3-aminotriazole 3. It has been reported
that formation of [1,2,4]triazolo[1,5-a]pyrimidine systems based on ,-unsaturated ketones and amine 3 needs
quite rigorous conditions, in fact refluxing the reagents in DMF or butanol [22]. Since the reaction of esters 1d-g
and amine 3 when refluxing in DMF is accompanied by tarring we carried out this reaction by refluxing in
ethanol and this resulted in the preparation of the triazolo[1,5-a]pyrimidines 6d-g in good yields.
CO2Et
N
N
N
R1
N
EtOH

CO2Et +
NH2
R1
N
H
N
N
H
O
1d–g
3
6d–g
N2H4·H2O
H2NHNCO
R1
N
N
R1
O
+
3
N
N
H
N
N
H
7e,f
8e,f
1, 6 d R1 = H; g R1 = 3,4-di-Cl; 1, 6, 8 e R1 = 4-Br; f R1 = 4-Cl
The 1H NMR spectra of the products 6d-g show proton signals for the 4,7-dihyropyrimidine fragment as
an AB system with the triazole ring proton as a singlet together with a singlet of NH proton which disappears
upon addition of D2O and this confirms the formation of the triazolo[1,5-a]pyrimidine ring. The halogen
substituents in the aryl radical prove to have an effect on the chemical shift of the H-6 proton (compare
compounds 6d and 6g) whereas the position of the H-7 signal is virtually unchanged throughout the series 6d-g.
Compound 6g shows a low-field shift in the NH proton signal with the introduction of electron-acceptor
substituents and this is indirect evidence supporting the given structure and, consequently, the route of formation
of the pyrimidine ring in molecules 6d-g.
The IR spectra of the triazolopyrimidines 6d-g show strong absorption bands at 1730-1740 and 1655
cm-1 which confirm the presence of ester and endocyclic vinylene groups respectively. The region 3200 cm-1
shows a medium intensity broad band for the secondary amino group. This data confirms that the
triazolopyrimidines 6d-g containing an acceptor triazole ring exist as the enamine tautomeric form in DMSO-d6
solutions and in the solid phase and this agree well with literature data for the tautomerism of [1,2,4]triazolo-
[1,5-a]pyrimidine systems [8, 14].
Modification of the ester function was carried out by reaction of compounds 6e,f with hydrazine
hydrate. However, the products of this reaction proved to be the 6-arylpyridazin-3(2H)-ones 8e,f reported in the
literature [23] rather than the expected hydrazides 7e,f. This points to recyclization of the dihydropyrimidine
ring during the reaction. Compounds 8e,f are also formed readily by treatment 3-aroylacrylic acids with
hydrazine [23].
Hence, the reaction of -aroylacrylates with -aminoazoles occurs through a stage of formation of an
-adduct involving the most nucleophilic center of the -aminoazole and permits the preparation of annelated
azoloazine systems containing a carboxylate group.
985

EXPERIMENTAL
1
IR spectra were recorded on an IR-75 instrument as KBr tablets. H NMR spectra were taken on a
Varian Mercury VX-200 instrument (200 MHz) as DMSO-d6 solution with TMS as internal standard. Elemental
analysis was carried out on a LECO CHNS-900 instrument. Melting points were determined on a Kofler hot
stage. The course of the reaction and purity of the compounds obtained was monitored using TLC on Silufol
UV-254 plates in the system toluene–ethyl acetate (2:1).
3-Aroylacrylates were prepared by known methods [24, 25]. Commercial 5-aminopyrazoles 2a,b and
3-aminotriazole 3 were used from the Merck company.
Alkyl 6-Aryl-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazolo[3,4-b]pyridine-4-carboxylates (4) and
Alkyl 5-Aryl-2-(4-methylphenyl)-6,7-dihydropyrazolo[1,5-a]pyrimidine-7-carboxylates (5) (General
Method). A mixture of the corresponding ester 1 (10 mmol) and the corresponding 5-aminopyrazole 2
(12 mmol) in ethanol (10 ml) was refluxed for 3 h. The solution was cooled and the precipitate was filtered off
and recrystallized from ethanol.
Methyl 3-Methyl-1,6-diphenyl-4,5-dihydro-1H-pyrazolo[3,4-b]pyridine-4-carboxylate (4a). Yield
70%; mp 125-126ºC. IR spectrum, , cm-1: 1610 (C=N), 1725 (C=O), 2905, 2960 (CH2). 1H NMR spectrum, ,
ppm (J, Hz): 2.25 (3H, s, 3-CH3); 3.00 (1H, dd, 2J =17.7, 3J = 9.5) and 3.63 (1H, dd, 2J = 17.7, 3J = 2.7, 5-CH2);
3
3
3.58 (3H, s, OCH3); 4.10 (1H, dd, J = 9.5, J = 2.7, H-4); 7.29 (1H, t, J = 7.3, H 1-Ph); 7.46-7.53 (5H, m,
H 6-Ph); 7.89 (2H, d, J = 7.6, H 1-Ph); 8.04 (2H, d, J = 7.6, H 1-Ph). Found, %: C 72.99; H 5.58; N 12.15.
C21H19N3O2. Calculated, %: C 73.03; H 5.54; N 12.17.
Methyl 6-(4-Methoxyphenyl)-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazolo[3,4-b]pyridine-4-carb-
oxylate (4b). Yield 69%; mp 129-130ºC. IR spectrum, , cm-1: 1610 (C=N), 1730 (C=O), 2900, 2950 (CH2).
1H NMR spectrum, , ppm (J, Hz): 2.24 (3H, s, 3-CH3); 2.91 (1H, dd, J = 17.4, J = 9.0) and 3.56 (1H, dd,
2
3
3
3
3
2J = 17.4, J = 3.0, 5-CH2); 3.62 (3H, s, CO2CH3); 3.82 (3H, s, C6H4OCH3); 4.05 (1H, dd, J = 9.0, J = 3.0,
H-4); 7.04 (2H, d, J = 9.0, o-H Ar); 7.28 (1H, t, J = 8.0, p-H Ph); 7.44-7.48 (2H, m, m-H Ph); 7.90 (2H, d,
J = 8.0, o-H Ph); 8.01 (2H, d, J = 9.0, m-H Ar). Found, %: C 70.34; H 5.67; N 11.21. C22H21N3O3. Calculated,
%: C 70.38; H 5.64; N 11.19.
Methyl 6-(4-Bromophenyl)-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazolo[3,4-b]pyridine-4-carb-
oxylate (4c). Yield 71%; mp 133-134ºC. IR spectrum, , cm-1: 1620 (C=N), 1720 (C=O), 2905, 2950 (CH2).
1H NMR spectrum, , ppm (J, Hz): 2.25 (3H, s, 3-CH3); 3.0 (1H, dd, J = 17.5, J = 9.1) and 3.57 (1H, dd,
2
3
3
3
3
2J = 17.5, J = 2.9, 5-CH2); 3.60 (3H, s, OCH3); 4.10 (1H, dd, J = 9.1, J = 2.9, H-4); 7.30 (1H, t, J = 7.7,
p-H Ph); 7.50-7.54 (2H, m, m-H Ph); 7.71 (2H, d, J = 7.7, o-H Ar); 7.86 (2H, d, J = 7.9, m-H Ar); 7.97 (2H, d,
J = 7.9, o-H Ph). Found, %: C 59.40; H 4.24; N 9.94. C21H18BrN3O2. Calculated, %: C 59.45; H 4.28; N 9.90.
Ethyl
6-(4-Bromophenyl)-3-methyl-1-phenyl-4,5-dihydro-1H-pyrazolo[3,4-b]pyridine-4-carb-
oxylate (4d). Yield 67%; mp 81-82ºC. IR spectrum, , cm-1: 1620 (C=N), 1730 (C=O), 2900, 2950 (CH2).
1H NMR spectrum, , ppm (J, Hz): 1.12 (3H, t, J = 7.0, OCH2CH3); 2.26 (3H, s, 3-CH3); 2.97 (1H, dd, 2J = 17.9,
3J = 9.5) and 3.58 (1H, dd, 2J = 17.9, 3J = 2.6, 5-CH2); 4.02 (2H, q, J = 7.0, OCH2CH3); 4.07 (1H, dd, 3J = 9.5,
3J = 2.6, H-4); 7.31 (1H, t, J = 7.7, p-H Ph); 7.50-7.54 (2H, m, m-H Ph); 7.71 (2H, d, J = 8.4, o-H Ar); 7.97 (2H,
d, J = 8.4, m-H Ar); 7.86 (2H, d, J = 7.7, o-H Ph). Found, %: C 60.34; H 4.54; N 9.49. C22H20BrN3O2.
Calculated, %: C 60.28; H 4.60; N 9.59.
Methyl
5-(4-Methoxyphenyl)-2-(4-methylphenyl)-6,7-dihydropyrazolo[1,5-a]pyrimidine-7-carb-
oxylate (5b). Yield 59%; mp 174-176ºC. IR spectrum, , cm-1: 1610 (C=N), 1740 (C=O), 2900, 2950 (CH2).
1H NMR spectrum, , ppm (J, Hz): 2.31 (3H, s, C6H4CH3); 3.51 (1H, dd, J = 17.4, 3J = 9.0) and 3.75 (1H, dd,
2
3
3
3
2J = 17.4, J = 4.4, 6-CH2); 3.62 (3H, s, CO2CH3); 3.82 (3H, s, C6H4OCH3); 5.41 (1H, dd, J = 9.0, J = 4.4,
H-7); 6.81 (1H, s, H-3); 7.04 (2H, d, J = 8.8, o-H Ar); 7.21 (2H, d, J = 8.0, o-H Tol); 7.71 (2H, d, J = 8.0, m-H
Tol); 8.01 (2H, d, J = 8.8, m-H Ar). Found, %: C 70.30; H 5.60; N 11.23. C22H21N3O2. Calculated, %: C 70.38;
H 5.64; N 11.19.
986

Ethyl
5-(4-Bromophenyl)-2-(4-methylphenyl)-6,7-dihydropyrazolo[1,5-a]pyrimidine-7-carboxy-
late (5d). Yield 55%; mp 159-160ºC. IR spectrum, , cm-1: 1610 (C=N), 1735 (C=O), 2900, 2970 (CH2).
1H NMR spectrum, , ppm (J, Hz): 1.10 (3H, t, J = 7.0, CH2CH3); 2.31 (3H, s, C6H4CH3); 3.57 (1H, dd,
3
2
3
2J = 18.3, J = 8.4) and 3.68 (1H, dd, J = 18.3, J = 2.6, 6-CH2); 4.07 (2H, q, J =7.0, CH2CH3); 5.41 (1H, dd,
3
3J = 8.4, J = 2.6, H-7); 6.91 (1H, s, H-3); 7.22 (2H, d, J = 8.1, o-H Tol); 7.72 (2H, d, J = 8.1, m-H Tol); 7.79
(2H, d, J = 8.8, o-H Ar); 7.98 (2H, d, J = 8.8, m-H Ar). Found, %: C 60.26; H 4.64; N 9.63. C22H20BrN3O2.
Calculated, %: C 60.28; H 4.60; N 9.59.
Ethyl 2-(4-Methylphenyl)-5-phenyl-6,7-dihydropyrazolo[1,5-a]pyrimidine-7-carboxylate (5e).
1
Yield 57%; mp 155-156ºC. IR spectrum, , cm-1: 1610 (C=N), 1735 (C=O), 2910, 2970 (CH2). H NMR
2
spectrum, , ppm (J, Hz): 1.10 (3H, t, J = 7.0, CH2CH3); 2.30 (3H, s, C6H4CH3); 3.55 (1H, dd, J =18.3,
2
3
3
3J = 8.4) and 3.63 (1H, dd, J = 18.3, J = 2.6, 6-CH2); 4.06 (2H, q, J = 7.0, CH2CH3); 5.40 (1H, dd, J = 8.4,
3J = 2.6, H-7); 6.88 (1H, s, H-3); 7.20 (2H, d, J = 8.1, o-H Tol); 7.63-7.75 (3H, m, H Ph); 7.72 (2H, d, J = 8.1,
m-H Tol); 7.90 (2H, d, J = 8.8, o-H Ph). Found, %: C 73.59; H 5.83; N 11.74. C22H21N3O2. Calculated, %:
C 73.52; H 5.89; N 11.69.
Ethyl 5-Aryl-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylates (6) (General Method). A
mixture of ester 1 (10 mmol) and amine 3 (12 mmol) in ethanol (10 ml) was refluxed for 4 h. The solution was
cooled and the precipitate was filtered off, washed with hot water and then ethanol, and dried in air.
Ethyl 5-Phenyl-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylate (6d). Yield 68%; mp
182-184ºC. IR spectrum, , cm-1: 1655 (C=C), 1740 (C=O), 3200 (NH). 1H NMR spectrum, , ppm (J, Hz): 1.19
(3H, t, J = 7.0, CH2CH3); 4.15 (2H, q, J = 7.0, CH2CH3); 5.15 (1H, d, J = 4.3, H-6); 5.79 (1H, d, J = 4.3, H-7);
7.40-7.60 (5H, m, H Ph); 7.74 (1H, s, H-2); 10.17 (1H, br. s, NH). Found, %: C 62.17; H 5.19; N 20.70.
C14H14N4O2. Calculated, %: C 62.21; H 5.22; N 20.73.
Ethyl 5-(4-Bromophenyl)-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylate (6e). Yield
1
70%; mp 160-162ºC. IR spectrum, , cm-1: 1655 (C=C), 1745 (C=O), 3200 (NH). H NMR spectrum, , ppm
(J, Hz): 1.19 (3H, t, J = 7.0, CH2CH3); 4.12 (2H, q, J = 7.0, CH2CH3); 5.19 (1H, d, J = 4.0, H-6); 5.78 (1H, d,
J = 4.0, H-7); 7.53 (2H, d, J = 8.5, m-H Ar); 7.62 (2H, d, J = 8.5, o-H Ar); 7.73 (1H, s, H-2); 10.19 (1H, br. s,
NH). Found, %: C 48.20; H 3.68; N 16.10. C14H13BrN4O2. Calculated, %: C 48.16; H 3.75; N 16.05.
Ethyl 5-(4-Chlorophenyl)-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylate (6f). Yield
1
75%; mp 172-174ºC. IR spectrum, , cm-1: 1655 (C=C), 1740 (C=O), 3210 (NH). H NMR spectrum, , ppm
(J, Hz): 1.18 (3H, t, J = 7.0, CH2CH3); 4.15 (2H, q, J = 7.0, CH2CH3); 5.20 (1H, d, J = 4.3, H-6); 5.78 (1H, d,
J = 4.3, H-7); 7.47 (2H, d, J = 8.5, m-H Ar); 7.61 (2H, d, J = 8.5, o-H Ar); 7.73 (1H, s, H-2); 10.19 (1H, br. s,
NH). Found, %: C 55.20; H 4.25; N 18.42. C14H13ClN4O2. Calculated, %: C 55.18; H 4.30; N 18.39.
Ethyl 5-(3,4-Dichlorophenyl)-4,7-dihydro-[1,2,4]triazolo[1,5-a]pyrimidine-7-carboxylate (6g).
1
Yield 74%; mp 124-126ºC. IR spectrum, , cm-1: 1655 (C=C), 1738 (C=O), 3200 (NH). H NMR spectrum, ,
ppm (J, Hz): 1.19 (3H, t, J = 7.0, CH2CH3); 4.15 (2H, q, J = 7.0, CH2CH3); 5.33 (1H, d, J = 4.2, H-6); 5.79 (1H,
d, J = 4.2, H-7); 7.56-7.96 (3H, m, H Ar); 7.75 (1H, s, H-2); 10.28 (1H, br. s, NH). Found, %: C 49.50; H 3.60;
N 16.48. C14H12Cl2N4O2. Calculated, %: C 49.58; H 3.57; N 16.52.
6-(4-Bromophenyl)pyridazin-3(2H)-one (8e). A solution of compound 6e (0.35 g, 1 mmol) in
hydrazine hydrate (95%, 2.5 ml, 50 mmol) was heated on a water bath for 5 h. The precipitate formed was
filtered off, washed with water, and crystallized from ethanol to give the product 8e (0.15 g, 60%) with mp
1
228-229ºC (mp 228-229ºC [23]). IR spectrum, , 1665 (C=O), 3256 (NH). H NMR spectrum, , ppm (J, Hz):
6.96 (1H, d, J = 9.9, H-5); 7.59 (2H, d, J = 8.8, o-H Ar); 7.88 (2H, d, J = 8.8, m-H Ar); 8.07 (1H, d, J = 9.9,
H-4); 13.21 (1H, br. s, NH).
6-(4-Chlorophenyl)pyridazin-3(2H)-one (8f) was prepared similarly to compound 8e. Yield 57%; mp
1
220-221ºC (mp 220-221ºC [23]). IR spectrum, , 1660 (C=O), 3260 (NH). H NMR spectrum, , ppm (J, Hz):
6.98 (1H, d, J = 9.9, H-5); 7.52 (2H, d, J = 8.8, o-H Ar); 7.86 (2H, d, J = 8.8, m-H Ar); 8.02 (1H, d, J = 9.9,
H-4); 13.21 (1H, br. s, NH).
987

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