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M. Mojzych et al. / Journal of Molecular Structure 1067 (2014) 147–153
derivatives of pyrazolo[4,3-e] tetrazolo[4,5-b][1,2,4]triazine ring
system 5 and 6 (Scheme 1), their 1H NMR spectra, X-ray analysis
and theoretical calculations in order to elucidate the exact struc-
ture of the obtained products in gaseous phase, polar and non-po-
lar solutions and crystalline state due to possibility of existing
valence tautomerism between azido and tricycle forms. It should
be noted, that tricyclic fused system can exist in linear (a) and
angular (b) fused isomers (Scheme 1). It is known from our previ-
ously study [9,10], that similar azido-pyrazolo-triazine compounds
leads in solution to an equilibrium between azido and tricycle tau-
tomeric forms with the latter crystallization to X-ray confirmed
linearly fused solid products. However, we did not find direct
experimental evidence that the angular fused form cannot be
formed in solution, therefore, we investigated the tautomeric equi-
librium of azido and both tricyclic linear and angular forms using
theoretical calculation at ab initio DFT level.
Synthesis of 5-azido-1,3-dimethyl-1H-pyrazolo[4,3-e][1,2,4]triazine
(3)
The solution of 1 (108 mg, 0.6 mmol) in CH3COOH/H2O mixture
(1:1, 4 mL) was cooled to 0–5 °C and aqueous NaNO2 (81 mg in
2 mL of water) was added dropwise so that the temperature of
reaction mixture was kept below 5 °C. After additional stirring
for 15 min at 0–5 °C the yellow precipitate was filtered off, washed
with water and dried at room temperature to give 108 mg (95%) of
3. Mp 90 °C. 1H NMR (CDCl3) d: 2.63 (s, 3H); 4.27 (s, 3H). IR (KBr)
cmÀ1: 2941, 2139 (N3), 1596, 1535, 1262, 1179, 978. HRMS (EI,
m/z) 190.07074, Calcd for C6H6N8 [M+] 190.07154.
Synthesis of 5-azido-1-benzyl-3-methyl-1H-pyrazolo[4,3-
e][1,2,4]triazine (4)
The compound was prepared according to the procedure de-
scribed for derivative 3 in a 95% yield. 1H NMR (CDCl3) d: 2.60 (s,
3H); 5.74 (s, 2H); 7.29–7.44 (m, 5H). IR (KBr) cmÀ1: 2961, 2923,
2853, 2225, 2140 (N3), 1425, 1261, 1100; HRMS (EI, m/z)
266.10180, Calcd for C12H10N8 [M+] 266.10284.
Experimental
General methods
Melting points were determined in open capillaries and are
uncorrected. 1H NMR spectra were recorded on a Varian Gemini
200 MHz spectrometer using tetramethylsilane as the internal
standard. The IR spectra were measured with a Magna IR-760 spec-
trophotometer in KBr pellets. Mass spectra were measured on AMD
604 spectrometer (electron impact, 70 eV). Elemental analyses
were obtained on Perkin–Elmer 2400-CHN analyzer and the results
for the indicated elements were within 0.3% of the calculated val-
ues. Starting compounds 1 and 2 were synthesized according to lit-
erature procedure [6,11]. Spectroscopic data for derivative 1 are
described in literature [6]. Compounds 3 and 4 were used as crude
products.
Synthesis of 5,7-dimethyl-5H-pyrazolo[4,3-e]tetrazolo[4,5-
b][1,2,4]triazine (5)
The azido compound 3 (105 mg, 0.55 mmol) was dissolved in
boiling ethanol (4 mL) and the resulting mixture was left to slow
crystallization at room temperature. The crystals and dry residue
after evaporation of the alcohol were combined and purified on
column chromatography using chloroform/ethanol mixture 30:1
as eluent to give 5 (97 mg, 0.51 mmol, 93%) as a red solid. Mp
193–195 °C. 1H NMR (CDCl3) d: 2.79 (s, 3H), 4.18 (s, 3H). MS (EI
70 eV, m/z, %): 190 (44) [M+], 185 (12), 115 (16), 93 (86), 78 (25),
67 (55), 41 (100). HRMS (EI, m/z) 190.07074, Calcd for
C6H6N8[M+] 190.07154.
1-Benzyl-5-hydrazine-3-methyl-1H-pyrazolo[4,3-e][1,2,4]triazine (2)
Synthesis of 5-benzyl-7-methyl-1H-pyrazolo[4,3-e]tetrazolo[4,5-
b][1,2,4]triazine (6)
Mp 98 °C. 1H NMR (DMSO) d: 2.43 (s, 3H); 5.68 (s, 2H); 7.25–
7.34 (m, 5H). 13C NMR (DMSO) d: 10.74, 51.02, 127.60, 127.73,
128.61, 133.87, 136.80, 138.27, 145.95, 163.26. IR (KBr) cmÀ1
:
The compound was prepared according to the procedure de-
scribed for derivative 5 in a 90% yield. Mp 135–136 °C. 1H NMR
(CDCl3) d: 2.76 (s, 3H); 5.61 (s, 2H); 7.28–7.31 (m, 3H); 7.40–7.45
(m, 2H). MS (EI 70 eV, m/z, %): 266 (12) [M+], 169 (6), 142 (8), 91
(100), 65 (14). HRMS (EI, m/z) 266.10180, Calcd for C12H10N8
[M+] 266.10284.
3278, 1526, 1467, 1110, 699. MS (EI 70 eV, m/z, %): 255 (100)
[M+], 226 (14), 210 (21), 196 (11), 91 (76), 65 (17). HRMS (EI, m/
z) 255.12293, Calcd for C12H13N7[M+] 255.12324.
X-ray structure analysis
Yellow plate crystals of 5 and 6 suitable for X-ray diffraction
analysis were grown by slow evaporation of an ethanol solution.
X-ray data of 5 were collected on the Enraf–Nonius MACH3 diffrac-
tometer; crystal size 0.42 Â 0.32 Â 0.10 mm,
x-2h scans, psi-scan
absorption correction [12], room temperature. Data collection for
6 was performed on the Bruker SMART APEXII CCD diffractometer;
crystal size 0.25 Â 0.25 Â 0.10 mm,
u and x scans, room tempera-
ture. The structures were solved by direct methods using SIR92
[13] and refined by full-matrix least-squares with SHELXL97 [14].
The H atoms were positioned geometrically and treated as riding
on their parent C atoms with C–H distances of 0.96 Å (CH3),
0.97 Å (CH2), and 0.93 Å (aromatic). All H atoms were refined with
isotropic displacement parameters taken as 1.5 times those of the
respective parent atoms. The structure solution of 5 in the space
group Pbnm (=Pnma) reveals special position of the molecule on
the crystallographic mirror plane dividing the molecule on two
symmetrical parts and generating pseudosymmetry inconsistent
to expected molecular structure of 5 (Fig. 1). This pseudosymmetry
Scheme 1.