Recyclization of condensed carbethoxypyrimidines accompanied by substitution of a carbon atom into the heterocycle

Article abstract of DOI:10.1007/s10593-005-0176-6

Condensation in ethanol of ethyl ethoxymethyleneacetoacetate with systems containing an amidine fragment (substituted 3-aminopyrazoles and 3-amino-1,2,4-triazole) gave 6-carbethoxy-7-methylpyrazolo[1,5-a]pyrimidines. Addition of base to solutions of the o

Full text of DOI:10.1007/s10593-005-0176-6

Chemistry of Heterocyclic Compounds, Vol. 41, No. 4, 2005
RECYCLIZATION OF CONDENSED
CARBETHOXYPYRIMIDINES ACCOMPANIED
BY SUBSTITUTION OF A CARBON
ATOM INTO THE HETEROCYCLE
G. G. Danagulyan1, A. D. Mkrtchyan1, and G. A. Panosyan2
Condensation in ethanol of ethyl ethoxymethyleneacetoacetate with systems containing an amidine
fragment (substituted 3-aminopyrazoles and 3-amino-1,2,4-triazole) gave 6-carbethoxy-7-
methylpyrazolo[1,5-a]pyrimidines. Addition of base to solutions of the obtained bicyclic carbethoxy
derivatives in the course of several minutes caused rearrangement to 6-acetyl-7-hydroxypyrazolo-
[1,5-a]pyrimidine and 6-acetyl-7-hydroxy-1,2,4-triazolo[1,5-a]pyrimidine respectively.
A
more
prolonged refluxing in 15% aqueous alcohol solution of base caused 6-carbethoxy-7-methyl-2-
phenylpyrazolo[1,5-a]pyrimidine and 6-acetyl-7-hydroxy-2-phenylpyrazolo[1,5-a]pyrimidine to
recyclize to 7-methylpyrazolo[1,5-a]pyrimidine.
Keywords: pyrazolo[1,5-a]pyrimidine, 1,2,4-triazolo[1,5-a]pyrimidine, rearrangement, recyclization.
Among recyclizations of pyrimidines the most studied are Dimroth rearrangements [1] accompanied by
exchange of a ring heteroatom with an exocyclic nitrogen atom (N-N recyclization conditions). We have studied
a schematic relative of the Dimroth rearrangement in detail for conversion of pyrimidines to pyridine derivatives
(the Kost–Sagitullin rearrangement [2-6]). Under these conditions a nitrogen atom of the pyrimidine ring is
substituted by an exocyclic carbon atom occurring at the 2 position (N-C recyclization conditions). This
communication relates to our study of a further recyclization conversion of pyrimidines which, according to the
evidence, involves the substitution of an atom in the heterocycle and is a C-C recyclization. A similar
rearrangement has previously been noted in a series of 2-substituted 5-carbethoxy-4-methylpyrimidines. Upon
heating in sodium ethylate solution they are converted to the corresponding 2-substituted 5-acetyl-4-
hydroxypyrimidines [7]. We have previously shown [8] that 2-substituted 4-amino-5-carbethoxypyrimidines are
converted to the corresponding 5-carbamoyl-5-hydroxypyrimidines by treatment with base. It is also clear that
the transformation noted above for the 5-carbethoxy-4-methylpyrimidines is, in fact, only achieved when
treating the reaction mixture with water, i.e. under the action of hydroxide ion formed in solution.
In a continuation of this work we have studied the possibility of carrying out similar rearrangements for
condensed bicyclic pyrimidines, in particular for 6-carbethoxy-7-methylpyrazolo[1,5-a]- and 6-carbethoxy-7-
methyl-1,2,4-triazolo[1,5-a]pyrimidine derivatives. Model compounds for study were synthesized by
condensation of ethyl ethoxymethyleneacetoacetate in ethanol with systems containing amidine fragments, viz.
3-aminopyrazoles and 3-amino-1,2,4-triazole.
__________________________________________________________________________________________
1
Institute of Organic Chemistry, National Academy of Sciences of the Republic of Armenia, Yerevan
2
375094; e-mail: gdanag@email.com. Molecular Structure Research Center, National Academy of Sciences of
the Republic of Armenia, Yerevan 375014. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4,
pp. 569-576, April, 2005. Original article submitted October 7, 2004.
0009-3122/05/4104-0485©2005 Springer Science+Business Media, Inc.
485

R
CHOEt
X
R
X
MeCOCCO2Et
N
N
N
N
NH2
N
H
Me
CO2Et
1–4
1 R = OH, X = CH; 2 R = Ph, X = CH; 3 R = CH2CN, X = CCN; 4 R = H, X = N
The structures of all of the compounds including the presence in the pyrimidine ring of carbethoxy and
methyl groups (i.e. the occurrence of a condensation via the acetyl and ethoxymethylene fragments of the
reagent) were confirmed by spectroscopic methods and the position of the methyl group in the condensed
1
compounds was established on the basis of the H NMR spectra of their iodomethylates with the use of the
NOESY method. In particular, the spectrum of compound 5 (the iodomethylate of compound 2) shows a
response between the protons of the N-methyl group and the CH of the pyrazole ring which points to alkylation
1
of the N(4) atom. At the same time the H NMR spectrum shows a clear cross peak between the signal of the
same N-methyl group and the pyrimidine ring proton thus unambiguously proving their close positioning in the
molecule. The absence in the NOESY spectrum of any kind of interaction between the protons of the two methyl
groups also points to the positioning of the methyl group in compound 5, as for the other synthesized
compounds, at position 7 of the pyrazolo[1,5-a]pyrimidine ring, i.e. removed from the quaternary nitrogen atom.
Ph
Ph
H
+
N
Me
N
Me
_
MeI
MeI
N
N
–
N
N
+
I
2
I
H
Me
Me
CO2Et
CO2Et
5
The rearrangement of the pyrazolo[1,5-a]pyrimidines 1 and 2 and the triazolo[1,5-a]pyrimidine 4 to give
the corresponding 6-acetyl-7-hydroxy derivatives 6-8 occurs over several minutes, even at room temperature.
The rearrangement product of compound 3 could not be separated, evidently because concurrent process of
transformation of the nitrile groups occurs under these conditions to form a mixture of substances.
R
X
N
KOH
1, 2, 4
N
N
HO
6–8
COMe
6 R = OH, X = CH; 7 R = Ph, X = CH; 8 R = H, X = N
The 1H NMR spectra of the recyclization products show the absence of ester proton signals characteristic
of the starting materials and the appearance of a broadened signal for the hydroxyl group. The 13C NMR spectra
of the final transformation products differ from those of the starting materials and correspond to those expected.
The structures of the rearrangement products were also confirmed by mass spectrometry.
The rearrangement probably occurs by the following scheme:
486

R
R
N
X
X
_
N
O
OH
N
N
_
N
N
_
1, 2, 4
HO
Me
Me
CO2Et
CO2Et
R
N
R
N
X
X
N
_
N
N
N
6–8
EtO
O
O
COMe
COMe
Evidently, in the recyclization process under the action of hydroxide ion there occurs a fission of the
pyrimidine ring at the N–C(7) bond, subsequent rotation around the C(5)–C(6) single bond, and repeated
recyclization. As a result, the ester group C atom is included in the newly formed pyrimidine ring while the C(7)
atom which was situated in the pyrimidine ring appears outside the heterocycle.
It should be stressed that, in the example of the transformation of the condensed systems 1, 2, and 4, we
have not excluded the likelihood of other possible types of recyclization occurring as mentioned at the beginning
of our paper. Hence in the example of 6-carbethoxy-7-methylpyrazolo[1,5-a]pyrimidines a possible
transformation route might be a conversion to a pyrazolo[3,4-b]pyridine derivative by a Kost–Sagitullin type
rearrangement [9] via fission of the N–C(7) bond of the pyrimidine ring and subsequent cyclization by attack at
the C(4) atom of the azole. However, in the conditions indicated above we were unable to separate the expected
reaction products. Prolonged heating in a 15% aqueous alcohol solution of base, i.e. in the conditions reported
before for the conversion of pyrazolo[1,5-a]pyrimidines via a Kost–Sagitullin rearrangement [9], the
pyrazolo[1,5-a]pyrimidines 2 and 7 were unexpectedly for us transformed to 7-methyl-2-phenylpyrazolo[1,5-a]-
pyrimidine (9). The formation of the latter can be explained by the occurrence of a series of consecutive
reactions which include opening of the pyrimidine ring, cyclization, and decarboxylation. The consecutive chain
of recyclizactions is also evidenced by the chromatographic formation and disappearance of compound 7 in the
process of transformation of compound 2 to compound 9.
Ph
15% KOH
EtOH/H2O
15% KOH
EtOH/H2O
N
7
2
N
N
Me
9
MeI
Ph
N
Me
N
N
+
_
I
Me
10
487

The structure of the pyrazolopyrimidine 9 and its iodomethylate 10 were proved by the NOESY method
1
and also using mass spectrometry and H NMR and 13C NMR spectroscopy. In the NOESY NMR spectrum of
the iodomethylate 10, as in the spectrum of compound 5, a response was noted between the N-methyl group
protons and the pyrazole H-3 and pyrimidine H-5 ring protons. Recyclization of compound 7 to compound 9
occurs by the following scheme (recalling that the conversion of the carbethoxy derivative 2 to compound 9
takes place via the stage of formation of the acetyl derivative 7):
Ph
Ph
Ph
N
N
N
7
N
N
N
_
N
N
_
N
_
HO
O
HO
HO
Me
O
H
COMe
COMe
H
COOH
7a
Ph
Ph
_
N
N
– OH
N
N
N
N
9
Me
– CO2
Me
O
_
H
COOH
COOH
The formation of compound 9 rather than the realization of a Kost–Sagitullin rearrangement is explained
by the insufficient nucleophilicity of atom C(4) in the pyrazole ring of intermediate 7a when compared with atom
N(1).
TABLE 1. Parameters for the Synthesized Compounds
Found, %
——————
Calculated, %
Com-
pound
Empirical
formula
Yield, %
(method)
mp, °С
Rf*
С
H
N
1
2
3
4
5
6
7
8
9
C10H11N3O3
C16H15N3O2
C13H11N5O2
C9H10N4O2
C16H15N3O2·MeI
C8H7N3O3
54.45
54.30
68.52
68.31
57.78
57.99
52.35
52.42
48.51
48.24
49.36
49.75
66.32
66.40
47.05
47.19
5.12
5.01
5.41
5.37
4.02
4.12
4.71
4.89
4.25
4.29
3.58
3.65
4.26
4.38
3.48
3.39
18.86
19.00
14.90
14.94
26.18
26.00
26.97
27.17
10.12
9.93
21.63
21.75
16.43
16.59
31.67
31.45
180-182
146-147
126-128
99-100
0.66
0.72
0.71
0.78
—
84
99
77
90
93
58
67
60
250-251
240-241
262-263
260-262
94-95
0.72
0.63
0.62
0.65
C14H11N3O2
C7H6N4O2
C13H11N3
74.52
74.62
5.39
5.30
20.29
20.08
62 (А),
57 (B)
10
C13H11N3·MeI
47.74
47.88
3.89
4.02
11.70
11.97
280-281
—
80
_______
* Systems: toluene–acetone, 1:1 (compounds 1, 4); benzene–acetone, 3:1
(compounds 2, 3, 9); ethanol (compounds 6-8)
488

TABLE 2. 1H NMR and 13C NMR Spectra of the Condensed Pyrimidines 1-10
Com-
pound
1Н NMR spectrum, δ, ppm (J, Hz)
13C NMR spectrum, δ, ppm
1
1.43 (3H, t, J = 7.1, CH2CH3); 3.14 (3H, s,
CH3-7); 4.43 (2H, q, J = 7.1, OCH2);
6.16 (1H, s, H-3); 8.95 (1H, s, H-5);
9.65-10.4 (1H, br. s OH)
14.43 (CH2CH3); 15.29 (CH3-7);
61.69 (CH2); 83.39 (C(3)); 109.97 (C(6));
149.85 (C(7)); 150.34 (Cipso); 151.23 (C(5));
164.82 (CONH); 167.94 (C=O)
2
3
1.43 (3H, t, J = 7.1, CH2CH3); 3.29 (3H, s,
CH3-7); 4.43 (2H, q, J = 7.1, OCH2);
7.05 (1H, s, H-3); 7.43 (3H, m, Н-3',4',5');
8.03 (2H, m, Н-2',6'); 8.97 (1H, s, H-5)
14.39 (CH2CH3); 15.17 (CH3-7);
61.58 (CH2); 94.97 (C(3)); 110.5 (C(6));
126.85 (C(3') and C(5'));
128.92 (C(2') and C(6'));
129.57 (C(4')); 132.5 (C(1')); 149.8 (C(7));
149.95 (Cipso); 151.5 (C(2)); 157.9 (C(5));
164.9 (C=O)
1.44 (3H, t, J = 7.1, CH2CH3); 3.25 (3H, s,
14.37 (CH2CH3); 15.42 (CH3-7);
CH3-7); 4.17 (2H, s, CH2CN); 4.51 (2H, q, 17.96 (CH2-2); 62.65 (OCH2); 94.0 (C(3));
J = 7.1, OCH2); 9.20 (1H, s, H-5)
111.06 (C(6)); 114.03 (CH2CN);
114.40 (CN); 151.16 (C(7)); 151.52 (Cipso);
153.11 (C(2)); 154.17 (C(5)); 163.49 (C=O)
4
5
1.43 (3H, t, J = 7.1, CH2CH3); 3.27 (3H, s,
CH3-7); 4.45 (2H, q J = 7.1, ОCH2);
8.48 (1H, s, H-2); 9.32 (1H, s, H-5)
14.37 (CH2CH3); 15.81 (CH3-7);
62.31 (CH2); 113.48 (C(6)); 152.93 (C(7));
155.56 (Cipso); 155.75 (C(2)); 157.23 (C(5));
163.85 (C=O)
1.48 (3H, t, J = 7.1, CH2CH3); 3.21 (3H, s,
CH3-7); 4.46 (3H, s, N–CH3); 4.51 (2H, q,
J = 7.1, OCH2); 7.58 (3H, m, C6H5);
8.03 (1H, s, H-3); 8.21 (2H, m, C6H5);
9.74 (1H, s, H-5)
14.43 (CH2CH3); 17.71 (CH3-7);
43.81 (N–CH3); 62.08 (CH2); 90.41 (C(6));
107.18 (C(3)); 126.81 (C(3') and C(5'));
128.84 (C(2') and C(6')); 130.31 (C(1'));
130.43 (C(4')); 141.29 (Cipso); 148.40 (C(5));
156.31 (C(7)); 157.70 (C(2)); 164.95 (CO)
6
7
2.8 (3H, s, COCH3); 5.81 (1H, s, Н-3);
6.1-6.9 (1H, br. s OH);
8.59 (1H, s, H-5)
3.22 (3H, s, CH3); 7.03 (1H, s, H-3);
7.41 (3H, m, Н-3',4',5'); 7.91 (2H, m,
Н-2',6'); 8.85 (1H, s, H-5);
14.39 (CH3); 93.87 (C(3)); 110.67 (C(6));
126.11 (C(3') and C(5')); 128.1 (C(2') and C(6'));
128.58 (C(4')); 132.06 (C(1')); 144.26 (C(7));
149.63 (Cipso); 149.92 (C(2)); 156.41 (C(5));
165.55 (C=O)
13.18 (1H, br. s OH)
8
9
2.62 (3H, s, CH3); 4.5-5.6 (1H, br. s OH);
8.11 (1H, s, H-2); 8.51 (1H, s, H-5)
30.15 (CH3); 110.49 (C(6)); 145.78 (C(5));
149.95 (C(7)); 151.28 (C(2)); 154.4 (Cipso);
192.89 (C=O)
2.85 (3H, d, J = 0.8, СН3-7); 6.68 (1H, dq,
J1 = 0.8, J2 = 4.5, Н-6); 7.00 (1H, s,
17.40 (CH3); 93.83 (C(3)); 107.56 (C(6));
126.79 (C(3') and C(5'));
H-3); 7.43 (3H, m, Н-3',4',5'); 8.05 (2H, m, 128.93 (C(2') and C(6'));
Н-2',6'); 8.36 (1H, d, J = 4.5, Н-5)
129.07 (C(4')); 133.24 (C(1')); 146.24 (C(5));
148.72 (C(7)); 150.19 (Cipso); 155.92 (C(2)
)
10
3.11 (3H, s, CH3-7); 4.39 (3H, s,
N–CH3); 7.50–7.60 (3H, m, C6H5);
7.60 (1H, d, J = 6.2, H-6); 7.88 (1H, s,
Н-3); 8.16 (2H, dd, J1 = 7.9, J2 = 1.8,
C6H5); 9.21 (1H, d, J = 6.2, H-5)
17.67 (CH3-7); 43.76 (N–CH3); 90.95 (C(6));
107.18 (C(3)); 126.79 (C(3') and C(5'));
128.78 (C(2') and C(6')); 130.03 (ipso-C6H5);
130.39 (C(4')); 141.33 (Cipso); 148.37 (C(5));
156.41 (C(7)); 157.72 (C(2)
)
EXPERIMENTAL
1H NMR spectra were obtained in the Molecular Structure Research Center in the Armenian Republic
National Academy of Sciences (US CRDF RESC 17-5 program) on a Varian Mercury 300 instrument (300 and
75 MHz respectively) using CDCl3 (compounds 1-4, 9) or DMSO-d6 (5-8 and 10) with TMS internal standard
and a sample temperature of 303 K. Mass spectra were recorded on an MK-1321 spectrometer with direct
introduction of the sample into the ion source and an ionization energy of 70 eV. TLC was performed on Silufol
UV-254 plates and revealed using iodine vapor or the Ehrlich reagent.
489

The physicochemical and spectroscopic parameters are given in Tables 1 and 2.
6-Carbethoxy-2-hydroxy-7-methylpyrazolo[1,5-a]pyrimidine (1). A mixture of 3-aminopyrazol-5-
one [10] (1 g, 10 mmol), ethyl ethoxymethyleneacetoacetate (1.9 g, 10 mmol) and glacial acetic acid (15 ml) was
refluxed for 5-6 h. The major part of the acetic acid was removed by distillation in vacuo. The crystals formed
were filtered off, washed with acetone, and recrystallized from 50% ethanol to give compound 1 (1.85 g) as light
brown crystals.
6-Carbethoxy-7-methyl-2-phenylpyrazolo[1,5-a]pyrimidine (2).
A
mixture of 3-amino-5-
phenylpyrazole [11] (1.44 g, 9 mmol), ethyl ethoxymethyleneacetoacetate (1.7 g, 9 mmol), and absolute ethanol
(10 ml) was stirred for 10 min at about 20°C. After a completely solid mass had formed it was filtered off and
washed with acetone to give compound 2 (2.5 g).
6-Carbethoxy-3-cyano-2-cyanomethyl-7-methylpyrazolo[1,5-a]pyrimidine (3).
A
mixture of
3-amino-4-cyano-5-cyanomethylpyrazole [12] (1 g, 7 mmol), ethyl ethoxymethyleneacetoacetate (1.3 g,
7 mmol), and absolute ethanol (10 m) was refluxed for 4-5 h. The crystals formed were filtered off, washed with
hexane, and recrystallized from ethanol to give compound 3 (1.4 g) as orange crystals.
6-Carbethoxy-7-methyl-1,2,4-triazolo[1,5-a]pyrimidine (4). A mixture of 3-amino-1,2,4-triazole
(0.85 g, 10 mol), ethyl ethoxymethyleneacetoacetate (1.9 g, 10 mmol), and absolute ethanol (15 ml) was
refluxed for 1 h. The product was cooled and the crystals formed were recrystallized from ethanol to give
compound 4 (1.85 g).
6-Acetyl-2,7-dihydroxypyrazolo[1,5-a]pyrimidine (6). An alcoholic solution of KOH prepared from
KOH (0.56 g, 10 mmol) in absolute ethanol (10 ml) was added to a hot solution of compound 1 (1.1 g, 5 mmol)
in absolute ethanol (10 ml). The crystals formed were filtered off, dissolved in a minimum amount of water, and
acidified using dilute HCl solution to pH 6. The crystals were filtered off, washed with acetone, and
recrystallized from ethanol to give compound 6 (0.56 g).
6-Acetyl-7-hydroxy-2-phenylpyrazolo[1,5-a]pyrimidine (7). An alcoholic solution of KOH prepared
from KOH (0.23 g, 4 mmol) in absolute ethanol (10 ml) was added to a solution of compound 2 (0.57 g, 2 mmol)
in alcohol (15 ml). The instantly formed crystals were filtered off, dissolved in a minimum amount of water, and
acidified using dilute HCl solution to pH 6. The crystals formed were filtered off and recrystallized from ethanol
to give compound 7 (0.34 g). Mass spectrum, m/z (Irel, %): 253 [M]+ (100), 252 (12), 236 (14), 209 (14), 208
(11), 144 (12), 142(14), 127 (7), 77 (19), 67 (9), 28 (43).
6-Acetyl-7-hydroxy-1,2,4-triazolo[1,5-a]pyrimidine (8). Similarly to the above, a solution of KOH
prepared from KOH (0.28 g, 5 mmol) in absolute ethanol (5 ml) was poured into a hot alcoholic solution of the
triazolopyrimidine 4 (0.35 g, 1.7 mmol) in absolute ethanol (5 ml). The instantly formed crystals were filtered
off after 10 min, dissolved in a minimum amount of water, and acidified using dilute HCl solution to pH 6. The
solution was cooled to a negative temperature. After 1 h the crystals formed were filtered off and washed with
acetone to give yellowish crystals of compound 8 (0.18 g). Mass spectrum, m/z, (Irel, %): 178 [M]+ (100),
163 (13), 162 (66), 161 (22), 149 (20), 134 (12), 107 (16), 94 (34), 67 (17), 55 (16), 43 (34).
7-Methyl-2-phenylpyrazolo[1,5-a]pyrimidine (9). A. The acetyl derivative 7 (0.8 g, 0.4 mmol) was
dissolved in an aqueous alcoholic solution (1:1) (10 ml) of KOH (1.1 g, 2 mmol) and the product was refluxed
for 20 h. At the end of the reaction the solvent was evaporated to dryness and the residue was washed twice with
benzene. After some time crystals of compound 9 precipitated (100 mg) with mp 94-95°C. Mass spectrum,
m/z (Irel, %): 209 [M]+ (18), 208 (100), 207 (21), 194 (6), 94 (6), 84 (21), 82 (13).
B. Similarly to the above using 10 ml of an aqueous alcohol solution of KOH (1.4 g) and the carbethoxy
derivative 2 (0.28 g, 1 mmol) with refluxing for 20 h. The solvent was removed and the residue was treated with
benzene to give compound 9 (0.12 g). The melting point and chromatographic mobility were the same as for the
sample prepared by the counter method using the acetyl derivative 7.
490

Preparation of 6-Carbethoxy-4,7-dimethyl-2-phenylpyrazolo[1,5-a]pyrimidinium (5) and
4,7-dimethyl-2-phenylpyrazolo[1,5-a]pyrimidinium (10) Iodides. The corresponding pyrazolo[1,5-a]-
pyrimidine 2 or 9 (2.5 mmol) and methyl iodide (3 ml) were heated in a sealed ampul on a water bath. After 10 h
the ampul was opened, the solvent was evaporated to dryness, and the residue was washed with hexane.
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