When acid 2a is heated to its melting point decarboxylation occurs to give 1,2,4-triazol-5-one 3a. It was
interesting to note that refluxing the N-phenylimidazolone 1b in dilute hydrochloric acid gave the 4-phenyl-
1
,2,4-triazole 3b as the decarboxylation product of the intermediate acid 2b. Its structure was confirmed by the
known reaction of semicarbazide with orthoformate [4].
1
13
H and C NMR spectra were measured on a Bruker DRX-400 instrument (400 and 100 MHz
respectively) using DMSO-d with TMS as internal standard. Mass spectra for the compound prepared were
6
taken on a Varian MAT 311A spectrometer with an accelerating voltage of 3 kV and ionization energy of 70 eV
and with direct introduction of the sample into the ion source.
The synthesis of the 3-(R-benzylideneamino)imidazolidine-2,4,5-triones 1a,b was carried out by the
method reported in [3].
5
-Oxo-4,5-dihydro-1,2,4-triazole-3-carboxylic Acid (2a). A suspension of compound 1a (0.088 mg,
0
.4 mmol) in water (10 ml) and conc. HCl (1 ml) were refluxed with distillation of water and benzaldehyde as an
azeotrope to full solution of the precipitate (25-30 min). The reaction product was concentrated in vacuo. The
1
residue was filtered off. Yield 0.035 mg (67%); mp 234-235ºC (decomp.) (mp 235-237ºC [1]). H NMR
1
3
spectrum, , ppm: 13.50 (1H, br. s, OH); 11.92 (2H, s, (NH) ). C NMR spectrum, , ppm: 158.1 (COOH);
2
+
+
1
55.0 (C-3); 138.9 (C-5). Mass spectrum, m/z (I , %): 129 [M] (21), 85 [M-CO ] (100). Found, %: C 27.5;
r
e
l
2
H 2.5; N 32.9. C H N O . Calculated, %: C 27.9; H 2.3; N 32.5.
3
3
3
3
Compound 2b was prepared from compound 1b.
,4-Dihydro-1,2,4-triazol-3-one (3a). Compound 2a (0.129 g, 1.0 mmol) was heated at 235-240ºC for
2
1
3
-5 min. Cooling gave compound 3a (0.052 g, 63%); mp 234-235ºC. H NMR spectrum, , ppm: 11.18 (1H,
br. s, NH); 7.50 (1H, s, H-5). Mass spectrum, m/z (I , %): 85 [M] (52). Found, %: C 28.0; H 3.6; N 49.3.
+
rel
C H N O. Calculated, %: C 28.2; H 3.5; N 49.4
2
3
3
1
Phenyl-1H-1,2,4-triazol-5-one (3b) was prepared from compound 1b. Yield 35%; mp 179-181ºC. H
NMR spectrum, , ppm (J, Hz): 11.88 (1H, s, NH); 8.28 (1H, d, J = 1.5, H-5); 7.68 (2H, d, J = 8.3, H Ar); 7.46
+
(
[
2H, t, J = 7.9, H Ar); 7.31 (1H, t, J = 7.5, ArH). Mass spectrum (EI), m/z (I , %): 119 [M] (21), 119
rel
+
M-OCN] (21). Found, %: C 59.2; H 4.2; N 25.8. C H N O. Calculated, %: C 59.6; H 4.3; N 26.0.
8
7
3
This work was carried out with the financial support of the Russian Fund for Basic Research (08-03-
0376a and 08-03-92208a).
0
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1
2
.
.
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(
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4
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535