2-R-7-methyl[1,2,4]triazolo[2,3-a]pyrimidines: Synthesis and structures

Article abstract of DOI:10.1007/s11172-008-0163-z

Reactions of 5-R-3-amino-1,2,4-triazoles with ethoxymethylideneacetylacetone and ethyl ethoxymethylideneacetoacetate proceeded regioselectively, giving 2-R-7-methyl[1,2,4]triazolo[2,3-a]-pyrimidines in good yields. The compounds obtained were characterized by elemental analysis, ¹H NMR spectroscopy, and X-ray diffraction analysis (for ethyl 2-ethylthio-7-methyl[1,2,4]triazolo[2,3-a]pyrimidine-6-carboxylate). The frontier orbitals, the molecular electrostatic potential, and the geometries of the reagent molecules were calculated by the DFT method (B3LYP/6-31G* *).

Full text of DOI:10.1007/s11172-008-0163-z

Russian Chemical Bulletin, International Edition, Vol. 57, No. 6, pp. 1268—1272, June, 2008
1268
2ꢀRꢀ7ꢀMethyl[1,2,4]triazolo[2,3ꢀa]pyrimidines: synthesis and structures
Kh. S. Shikhaliev,a D. V. Krylski,a A. Yu. Potapov,a S. E. Nefedov,b and O. E. Sidorenkoa
aVoronezh State University,
1 Universitetskaya pl., 394006 Voronezh, Russian Federation.
Eꢀmail: chocd261@chem.vsu.ru
bN. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences,
31 Leninsky prosp., 119991 Moscow, Russian Federation
Reactions of 5ꢀRꢀ3ꢀaminoꢀ1,2,4ꢀtriazoles with ethoxymethylideneacetylacetone and ethyl
ethoxymethylideneacetoacetate proceeded regioselectively, giving 2ꢀRꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]ꢀ
pyrimidines in good yields. The compounds obtained were characterized by elemental analysis,
1H NMR spectroscopy, and Xꢀray diffraction analysis (for ethyl 2ꢀethylthioꢀ7ꢀmethylꢀ
[1,2,4]triazolo[2,3ꢀa]pyrimidineꢀ6ꢀcarboxylate). The frontier orbitals, the molecular electrostatic
potential, and the geometries of the reagent molecules were calculated by the DFT method
(B3LYP/6ꢀ31G**).
Key words: 5ꢀRꢀ3ꢀaminoꢀ1,2,4ꢀtriazole, ethoxymethylideneacetylacetone, ethyl ethoxyꢀ
methylideneacetoacetate, 2ꢀRꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyrimidines, synthesis, structures.
Condensation of aminoazoles (e.g., 3ꢀaminoꢀ1,2,4ꢀtriꢀ
and acetoacetanilides).2,24—26 These reactions proceed
through the formation of an ylidene derivative of
the active methylene component, which was proved by an
independent synthesis.
An attempt to extend the earlier27 discovered threeꢀ
component condensation with hetarylguanidines, linear
βꢀdicarbonyl compounds, and triethyl orthoformate to
3ꢀaminoꢀ1,2,4ꢀtriazole was unsuccessful. Instead of
the expected triazolopyrimidines 8, we isolated products
of condensation of compound 1 with the orthoester.
Compounds 8 were obtained through the use of ethoxyꢀ
methylidene derivatives 9 (Scheme 2).
In the present work, we studied reactions of 5ꢀRꢀ3ꢀamiꢀ
noꢀ1,2,4ꢀtriazoles with ethoxymethylideneacetylacetone
and ethyl ethoxymethylideneacetoacetate by using quanꢀ
tumꢀchemical molecular calculations, preparatively isoꢀ
lated the resulting compounds, and examined their
structures.
azole and its derivative 1) with bielectrophiles provides a
general route to fused heterocyclic systems. With unsymꢀ
metrical binucleophiles such as aminoazoles,
a regioselectivity problem arises in their reactions with
unsymmetrical electrophiles.1—5 Structures, reactivity,
and regioselectivity in reactions of 3ꢀaminoꢀ1,2,4ꢀtriꢀ
azole have been extensively studied by physicochemical,
spectroscopic, Xꢀray diffraction, and quantumꢀchemical
methods.1—3,6—13
Condensation of 3ꢀaminoꢀ1,2,4ꢀtriazole with ethyl
acetoacetate and its homologs proceeds regioselectively
to give triazolopyrimidinones 2,1,3,14,15 while cyclizaꢀ
tion with unsymmetrical diketones leads to a mixture of
regioisomeric triazolopyrimidines 3 and 3´ (Scheme 1).
Their relative yields depend on the steric factor.16
A reaction of aminotriazole and its derivatives with
βꢀethoxymethylidenecarbonyl compounds, which is usuꢀ
ally carried out in acetic acid, starts with nucleophilic
substitution of the ethoxy group17—19 with participation
of the exoꢀamino group of the azole. This is followed by
intramolecular cyclization involving the "pyrroleꢀtype"
ring N atom and ketone carbonyl, ester, or cyano groups
(compounds 4—6). In the case of ethyl ethoxymethꢀ
ylidenecyanoacetate, chemoselectivity takes place.20—22
Use of ethoxymethylidenecyanamide for the synthesis of
5ꢀamino[1,2,4]triazolo[2,3ꢀa][1,3,5]triazines (5ꢀazaꢀ
adenines) 7 is described in Ref. 23.
Results and Discussion
We found that reflux of compounds 1a—f with ethoxyꢀ
methylideneacetylacetone or ethyl ethoxymethylideneꢀ
acetoacetate in acetic acid for 40—60 min gives, regardꢀ
less of the substituent R, the corresponding 2ꢀRꢀ7ꢀmeꢀ
thyl[1,2,4]triazolo[2,3ꢀa]pyrimidines 8 in high yields
rather than regioisomeric compounds 8´. The plauꢀ
sible reaction scheme involves a nucleophilic attack
of the exoꢀamino group of the aminoazole on the C
atom of the ethoxymethylidene fragment with elimiꢀ
Aminotriazoles can also be involved in threeꢀcompoꢀ
nent condensations with aldehydes and active methylene
dicarbonyl compounds (dimedone, ethyl acetoacetate,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1244—1248, June, 2008.
1066ꢀ5285/08/5706ꢀ01268 © 2008 Springer Science+Business Media, Inc.

[1,2,4]Triazolo[2,3ꢀa]pyrimidines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 6, June, 2008
1269
Scheme 1
R = H, Me, MeS
nation of an ethanol molecule and subsequent cycliꢀ
zation through the endocyclic N atom and the carbonꢀ
yl group.
The resulting 2ꢀRꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyriꢀ
midines were characterized by 1H NMR spectra; the strucꢀ
ture of ethyl 2ꢀethylthioꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀ
Scheme 2
1: R = H (a), Me (b), CF3 (c), MeS (d), PhCH2S (e), EtS (f); 8: R = H, R´ = Me (a), R = R´ = Me (b), R = CF3, R´ = Me (c),
R = H, R´ = OEt (d), R = Me, R´ = OEt (e), R = CF3, R´ = OEt (f), R = MeS, R´ = Me (g), R = PhCH2S, R´ = Me (h),
R = EtS, R´ = OEt (i); 9: R´ = Me (a), OEt (b)

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 6, June, 2008
Shikhaliev et al.
N(4)
C(5)
N(1)
C(4)
S(1)
C(3)
C(11)
O(2)
N(3)
C(7)
C(10)
C(6)
C(2)
C(1)
N(2)
C(9)
C(8)
O(1)
Fig. 1. Structure of ethyl 2ꢀethylthioꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyrimidineꢀ6ꢀcarboxylate (8i).
a]pyrimidineꢀ6ꢀcarboxylate (8i) was confirmed by Xꢀray
diffraction data.
N(3)—C(7) 1.351 Å, N(3)—C(4) 1.350 Å, N(1)—C(4)
1.326 Å, N(4)—C(4) 1.356 Å, N(4)—C(5) 1.325 Å,
C(5)—C(6) 1.396 Å, and C(6)—C(7) 1.352 Å), which
shows a similar bond distribution in the heterocycle.30
To estimate the reactivities of the reagents, we perꢀ
formed quantumꢀchemical DFT calculations with the
exchangeꢀcorrelation functional B3LYP and the basis set
6ꢀ31G**.
According to our quantumꢀchemical calculations, the
amplitude of the HOMO of aminotriazole 1a is higher
for the exoꢀamino group. In structures 9a,b, the LUMO is
most greatly contributed by the Cβ atom at the double
bond (Fig. 2).
The shapes of the frontier orbitals in the reactant
molecules are consistent with the concept of the orbital
control of the reaction in question. At the same time, the
molecular electrostatic potential (MEP) map (Fig. 3)
shows that the negative potential in 3ꢀaminoꢀ1,2,4ꢀtriꢀ
azole concentrates near the N(2) and N(4) atoms. This is
an unfavorable factor in the context of the assumed reacꢀ
According to Xꢀray diffraction data, the presence of
the substituents 2ꢀEtS (S(1)—C(3) 1.743(2) Å) and
6ꢀEtOOC (C(6)—C(9) 1.496(3) Å, C(9)—O(1) 1.200(3)
Å, and C(9)—O(2) 1.324(3) Å) in structure 8i (Fig. 1)
substantially changes the bond lengths in the planar triꢀ
azolopyrimidine fragment (N(1)—C(3) 1.355(3) Å,
N(2)—C(3) 1.331(3) Å, N(2)—N(3) 1.370(3) Å,
N(1)—C(4) 1.322(3) Å, N(3)—C(4) 1.387(3) Å,
N(4)—C(4) 1.345(3) Å, N(3)—C(7) 1.364(3) Å,
N(4)—C(5) 1.312(3) Å, C(5)—C(6) 1.416(3) Å,
C(6)—C(7) 1.378(3) Å, and C(7)—C(8) 1.494(3) Å)
compared to the average distances in the known pyrimiꢀ
dineꢀcontaining molecules (C—N 1.333—1.342 Å,
N—N 1.287—1.326 Å, and C—C 1.379—1.400 Å),28 as
well as compared to the sole structurally characterized29
(1R,3R,4R)ꢀ1,7,7ꢀtrimethylꢀ3ꢀ(1,2,4ꢀtriazolo[2,3ꢀa]pyꢀ
rimidinꢀ2ꢀyl)bicyclo[2.2.1]heptanꢀ2ꢀone (N(1)—C(3)
1.362 Å, C(3)—N(2) 1.321 Å, N(2)—N(3) 1.363 Å,
a
b
Fig. 2. B3LYP/6ꢀ31G**ꢀcalculated structure of the HOMO of 3ꢀaminoꢀ1,2,4ꢀtriazole (a) and the LUMO of ethoxymethylideneacetylacetone (b).

[1,2,4]Triazolo[2,3ꢀa]pyrimidines
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 6, June, 2008
1271
a
b
0.05149
0.04624
0.04099
0.03547
0.03049
0.02524
0.01999
0.04705
0.04081
0.03458
0.02835
0.02212
0.01589
0.00966
0.01474
0.00343
0.00949
0.00424
–0.00101
–0.00626
–0.01151
–0.01676
–0.02201
–0.02726
–0.03251
–0.03776
–0.04301
–0.04826
–0.05351
–0.00280
–0.00903
–0.01526
–0.02149
–0.02772
–0.03395
–0.04018
–0.04641
–0.05264
–0.05887
–0.06510
–0.07134
–0.07757
Fig. 3. B3LYP/6ꢀ31G**ꢀcalculated MEP map for 3ꢀaminoꢀ1,2,4ꢀtriazole (a) and ethoxymethylideneacetylacetone (b).
1H NMR, δ: 1.40 (t, 3 H, OCH2CH3, J = 7.1 Hz); 3.12 (s, 3 H, Me);
4.42 (q, 2 H, OCH2CH3, J = 7.1 Hz); 8.81 (s, 1 H, CH, triazole);
9.20 (s, 1 H, CH, pyrimidine).
Ethyl 2,7ꢀdimethyl[1,2,4]triazolo[2,3ꢀa]pyrimidineꢀ6ꢀcarboꢀ
xylate (8e). Yield 62%, m.p. 114—116 °C. Found (%): C, 54.56;
H, 5.41; N, 25.56. C10H12N4O2. Calculated (%): C, 54.54; H, 5.49;
N, 25.44. 1H NMR, δ: 1.34 (t, 3 H, OCH2CH3, J = 7.1 Hz); 2.62,
3.05 (both s, 3 H each, Me); 4.26 (q, 2 H, OCH2CH3, J = 7.1 Hz);
9.54 (s, 1 H, CH, pyrimidine).
tion mechanism. In ethoxymethylideneacetylacetone,
the Cβ atom of the ethoxymethylidene group is in
the positive potential area, which favors this reaction
scheme.
Therefore, the reaction is orbitalꢀcontrolled.
Experimental
The course of the reactions was monitored and the purity of the
compounds obtained was checked by TLC on Silufol UVꢀ254
plates with chloroform—ethyl acetate (1 : 2) as an eluent. 1H NMR
spectra were recorded on a Bruker ACꢀ300 instrument (300 MHz)
in DMSOꢀd6 with Me4Si as the internal standard.
Ethyl 7ꢀmethylꢀ2ꢀtrifluoromethyl[1,2,4]triazolo[2,3ꢀa]pyriꢀ
midineꢀ6ꢀcarboxylate (8f). Yield 64%, m.p. 135—137 °C. Found (%):
C, 43.69; H, 3.42; N, 20.46. C10H9F3N4O2. Calculated (%):
C, 43.80; H, 3.31; N, 20.43. 1H NMR, δ: 1.45 (t, 3 H, OCH2CH3,
J = 7.1 Hz); 3.20 (s, 3 H, Me); 4.32 (q, 2 H, OCH2CH3, J = 7.1 Hz);
9.66 (s, 1 H, CH, pyrimidine).
1ꢀ(7ꢀMethylꢀ2ꢀmethylthio[1,2,4]triazolo[2,3ꢀa]pyrimidinꢀ6ꢀ
yl)ethanone (8g). Yield 69%, m.p. 141—143 °C. Found (%):
C, 48.58; H, 4.44; N, 25.36. C9H10N4OS. Calculated (%): C, 48.63;
H, 4.53; N, 25.21. 1H NMR, δ: 2.66, 3.00 (both s, 3 H each, Me);
3.08 (s, 3 H, SMe); 9.21 (s, 1 H, CH, pyrimidine).
1ꢀ(2ꢀBenzylthioꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyrimidinꢀ6ꢀ
yl)ethanone (8h). Yield 60%, m.p. 130—131 °C. Found (%):
C, 60.49; H, 4.92; N, 18.66. C15H14N4OS. Calculated (%):
C, 60.38; H, 4.73; N, 18.78. 1H NMR, δ: 2.70, 2.98 (both s, 3 H each,
Me); 4.50 (s, 2 H, SCH2); 7.21—7.49 (m, 5 H, H arom.); 9.22
(s, 1 H, CH, pyrimidine).
Ethyl 2ꢀethylthioꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyrimidineꢀ
6ꢀcarboxylate (8i). Yield 66%, m.p. 150—152 °C. Found (%):
C, 49.59; H, 5.22; N, 21.16. C11H14N4O2S. Calculated (%):
C, 49.61; H, 5.30; N, 21.04. 1H NMR, δ: 1.45 (t, 3 H, OCH2CH3,
J = 7.1 Hz); 1.64 (t, 3 H, SCH2CH3, J = 6.9 Hz); 3.22 (s, 3 H, Me);
3.11 (q, 2 H, SCH2CH3, J = 6.9 Hz); 4.34 (q, 2 H, OCH2CH3,
J = 7.1 Hz); 9.72 (s, 1 H, CH, pyrimidine).
Quantumꢀchemical calculations were performed with the
GAUSSIANꢀ98 program package.31 For graphical representation
of the results obtained, the MOLEKEL program was used.32,33
1ꢀ(2ꢀRꢀ7ꢀMethyl[1,2,4]triazolo[2,3ꢀa]pyrimidinꢀ6ꢀyl)ethanꢀ
ones 8a—c,g,h and ethyl 2ꢀRꢀ7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]ꢀ
pyrimidineꢀ6ꢀcarboxylates 8d—f,i (general procedure). A mixture of
an appropriate aminotriazole (0.01 mol) and ethoxymethylideneꢀ
acetylacetone or ethyl ethoxymethylideneacetoacetate (0.011 mol)
in glacial acetic acid (5 mL) was refluxed for 40—60 min. On cooling,
the precipitate that formed was recrystallized from DMF.
1ꢀ(7ꢀMethyl[1,2,4]triazolo[2,3ꢀa]pyrimidinꢀ6ꢀyl)ethanone
(8a). Yield 75%, m.p. 155—157 oC. Found (%): C, 54.65; H, 4.44;
N, 31.86. C8H8N4O. Calculated (%): C, 54.54; H, 4.58; N, 31.80.
1H NMR, δ: 2.70, 3.08 (both s, 3 H each, Me); 7.86 (s, 1 H, CH,
triazole); 9.55 (s, 1 H, CH, pyrimidine).
1ꢀ(2,7ꢀDimethyl[1,2,4]triazolo[2,3ꢀa]pyrimidinꢀ6ꢀyl)ethanꢀ
one (8b). Yield 67%, m.p. 126—128 °C. Found (%): C, 56.76;
H, 5.47; N, 29.80. C9H10N4O. Calculated (%): C, 56.83; H, 5.30;
N, 29.46. 1H NMR, δ: 2.59, 2.73, 3.08 (all s, 3 H each, Me); 9.20
(s, 1 H, pyrimidine).
1ꢀ(7ꢀMethylꢀ2ꢀtrifluoromethyl[1,2,4]triazolo[2,3ꢀa]pyriꢀ
midinꢀ6ꢀyl)ethanone (8c). Yield 83%, m.p. 144—145 °C. Found (%):
C, 44.49; H, 2.76; N, 23.06. C9H7F3N4O. Calculated (%):
C, 44.27; H, 2.89; N, 22.95. 1H NMR, δ: 2.78, 3.12 (both s,
3 H each, Me); 9.43 (s, 1 H, CH, pyrimidine).
Xꢀray diffraction analysis was carried out on a Bruker P4
diffractometer (λꢀMo radiation, graphite monochromator, ω scan
mode, 2θ
= 50°). For 8i: molecular formula C11H14N4O2S,
max
M
= 266.32, space group P2(1)/c, a = 9.9130(10) Å,
b = 11.1460(10) Å, c = 11.668(2) Å, α = 90°, β = 98.430(10)°,
γ = 90° (295 K), V = 1275.3(3) Å3, Z = 4, the number of measured
reflections is 2213, the number of independent reflections with
Ethyl 7ꢀmethyl[1,2,4]triazolo[2,3ꢀa]pyrimidineꢀ6ꢀcarboxylate
(8d). Yield 57%, m.p. 98—99 °C. Found (%): C, 52.44; H, 4.67; N,
27.35. C9H10N4O2. Calculated (%): C, 52.42; H, 4.89; N, 27.17.
F 2 > 2σ(I) is 1740, ρ
= 1.387 g cm–3, μ = 0.254 cm–1
,
calc
R1 = 0.0430, wR2 = 0.1026. Calculations were performed with the

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Shikhaliev et al.
SHELXTL PLUS program package (PC version).34,35 The strucꢀ
ture was refined with the SHELXTLꢀ97 program. Crystallographic
parameters for compound 8i have been deposited with the
Cambridge Crystallographic Data Center (CCDC 641 944) and
can be made available from www.ccdc.cam.ac.uk/data_request/
cifwww.ccdc.cam.ac.uk/data_request/cif.
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19. C. Romano, E. Cuesta, C. Avendano, Heterocycles, 1990,
31, 267.
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Received July 14, 2006;
in revised form July 27, 2007