5-phenyl-2-(pyrazol-4-yl)-1,3,4-thiadiazoles

Article abstract of DOI:10.1007/s10593-005-0201-9

By recyclization of 2-R-6-ethyl-7-hydroxy(methoxy)-3-(5-phenyl-1,3,4- thiadiazol-2-yl)chromones when treated with hydrazine hydrate and phenylhydrazine, we synthesized 5-phenyl-2-(3-R-5-R1-1H-pyrazol-4-yl)-1H-1,3,4- thiadiazoles and 2-(3-R-5-R1-1-phenyl-1

Full text of DOI:10.1007/s10593-005-0201-9

Chemistry of Heterocyclic Compounds, Vol. 41, No. 5, 2005
5-PHENYL-2-(PYRAZOL-4-YL)-1,3,4-THIADIAZOLES
T. V. Shokol, V. V. Semenyuchenko, and V. P. Khilya
By recyclization of 2-R-6-ethyl-7-hydroxy(methoxy)-3-(5-phenyl-1,3,4-thiadiazol-2-yl)chromones when
treated with hydrazine hydrate and phenylhydrazine, we synthesized 5-phenyl-2-(3-R-5-R1-1H-pyrazol-
4-yl)-1H-1,3,4-thiadiazoles
and
2-(3-R-5-R1-1-phenyl-1H-pyrazol-4-yl)-5-phenyl-1H-1,3,4-thia-
diazoles. We confirmed the structure of the latter from 1H NMR spectra.
Keywords: pyrazoles, thiadiazoles, chromones.
Pyrazole derivatives have been widely used as dyes [1] and drugs [2]. Derivatives of 1,3,4-thiadiazole
also are characterized by a broad spectrum of useful properties: they are carboanhydrase inhibitors and drugs for
treatment of gastrointestinal infections, and are used for photometric determination of some metals [3].
Combining the rings of these two heterocycles in the same molecule may lead to products with
interesting pharmacological effects. Among pyrazolyl-1,3,4-thiadiazoles, compounds have already been found
that are hypoglycemic [4, 5], antiinflammatory [6], bactericidal [7-9], and fungicidal [8, 10]. We note that all the
known procedures for synthesis of such compounds are based on adding a thiadiazole ring onto an already
constructed pyrazole ring [4-11]. We used an alternative route for synthesis of 5-phenyl-2-(pyrazol-4-yl)-1,3,4-
thiadiazoles: formation of a pyrazole ring from compounds containing a thiadiazole moiety by recyclization of 3-
hetarylchromones when treated with hydrazine and its derivatives [12, 13]. The starting compounds in this
reaction are 2-R-6-ethyl-7-hydroxy-3-(5-phenyl-1,3,4-thiadiazol-2-yl)chromones 1a-f [14] and their 7-methoxy
derivatives 2a-c, obtained by alkylation of the corresponding products 1a-c with dimethyl sulfate in dimethyl
formamide in the presence of potassium carbonate.
In the 1H NMR spectra of compounds 2a-c, recorded in DMSO-d6, there is no downfield signal from the
OH-7 group which is typical of the starting products 1a-c, and instead we see a three-proton singlet for the
CH3-7 group in the 3.84-3.98 ppm region. Signals for the rest of the protons of the corresponding products 1a-c
and 2a-c are found in the same regions.
When suspensions of compounds 1a-f and 2a-c are boiled in alcohol with excess hydrazine hydrate,
practically instantaneously a solution is formed from which chromatographically pure pyrazoles 3a-f and 4a-c
precipitate or are precipitated by water in quantitative yields. Boiling chromone 1f for a longer time (10 min)
with hydrazine hydrate leads to a mixture of the ester 3f and the hydrazide 3g.
The pure hydrazide 3g was obtained by boiling a mixture of the products 3f and 3g with hydrazine
hydrate in DMF. Recyclization of chromones 1a-e and 2a to form N-phenylpyrazoles 5a-e, 6 when treated with
phenylhydrazine in alcohol occurs with a much longer reaction time (1.5-11 h). For compounds 1b,c, under
these conditions the reaction does not go to completion, and boiling in DMF for 2-4.5 h is required for the
reaction to finish. Pyrazoles 3-6 readily dissolve in a 2N solution of sodium hydroxide, which indicates the
presence of a free phenol hydroxyl in their molecules. The products 3 and 4 form blue-green chelates with an
__________________________________________________________________________________________
Taras Shevchenko Kiev National University, Kiev 01033, Ukraine; e-mail: vkhilya@univ.kiev.ua.
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 781-787, May, 2005. Original article
submitted March 11, 2003.
0009-3122/05/4105-0673©2005 Springer Science+Business Media, Inc.
673

674

alcoholic solution of ferric chloride, due to the presence of a phenol hydroxyl that is favorably located sterically
relative to the nitrogen atom in the pyrazole ring. The chelate structure of pyrazoles 3 and 4 allows us to explain
1
the 1.0-1.2 ppm upfield shift of the signal for the H-6 proton (of the phenyl ring) in the H NMR spectra (in
DMSO-d6) compared with the position of the H-5 proton peak for the starting chromones 1 and 2. Evidence for
the pyrazole structure of derivatives 3a and 4a also comes from the broadened signal for the H-5 proton of the
pyrazole ring, which experiences spin–spin coupling with the proton at the nitrogen atom.
1
In the H NMR spectra of compounds 3, downfield we see a considerably broadened one-proton singlet
at 12.6-13.4 ppm and two close broadened singlets in the 9.2-9.5 ppm region. In the spectra of
N-phenylpyrazoles 5a-d and 6, we also observe two singlets in the 9.3-9.6 ppm region, while the signal in the
13 ppm region is missing. These data allow us to assign the signal furthest downfield in the spectra of pyrazoles
3 to the N–H proton, and we can assign the signals in the 9 ppm region to the hydroxy group protons.
In the spectrum of methoxy derivative 4b, we also see the broadened singlet for the N–H proton furthest
downfield at 12.80 ppm and only one broadened one-proton singlet for the OH-2 group at 9.48 ppm, and a three-
proton singlet from the CH3O-4 group also appears in the 2.57 ppm region. In the spectra of the methoxy
derivatives 4a,c in the region where mobile protons absorb, we observe twice the number of peaks assigned to
the protons of the OH-2 groups, the protons of the N–H and C–H (for compound 4a) of the pyrazole ring, which
TABLE 1. Characteristics of Compounds 2-6
Found, %
Calculated, %
Com-
pound
Empirical
formula
Solvent for
recrystallization
Yield,
%
——————
mp, °C
S
N
8.76
8.80
8.31
8.47
7.86
7.69
7.65
7.40
2a
2b
С20H16N2O3S
223
219
DMF
DMF
85
87
C21H18N2O3S
2c
3a
3b
3c
3d
3e
3f
C22H20N2O3S
C19H16N4O2S
C20H18N4O2S
C21H20N4O2S
C20H15F3N4O2S
C22H20N4O4S
C22H20N4O4S
C20H18N6O3S
C20H18N4O2S
C21H20N4O2S
C22H22N4O2S
C25H20N4O2S
C26H22N4O2S
C27H24N4O2S
C26H19F3N4O2S
C28H24N4O4S
C26H22N4O2S
8.19
8.17
8.90
8.80
8.47
8.47
7.87
8.17
8.26
8.01
7.20
7.35
7.12
7.35
7.66
7.59
8.66
8.47
7.91
8.17
8.13
7.89
7.46
7.29
6.98
7.05
7.11
6.84
6.07
6.31
6.48
6.26
6.90
7.14
15.62
15.38
14.60
14.81
14.05
14.28
14.10
13.99
13.10
12.84
12.69
12.84
20.12
19.89
15.06
14.81
14.54
14.28
13.80
13.78
13.09
12.72
12.58
12.33
12.21
11.96
11.24
11.02
11.07
10.93
206
260
246
243
226
299
114
>300
221
246
196
234
244
218
231
273
233
DMF
71
88
92
90
82
91
91
87
92
77
98
61
60
81
79
83
74
Ethanol
Aqueous alcohol
Aqueous alcohol
Aqueous alcohol
Aqueous alcohol
Aqueous alcohol
Aqueous alcohol
Ethanol
3g
4a
4b
4c
5a
5b
5c
5d
5e
6a
Ethanol
Aqueous alcohol
Ethyl acetate
Ethyl acetate
Toluene
Toluene
Methanol
7.22
7.05
12.46
12.33
Toluene
675

TABLE 2. 1H NMR Spectra of Compounds 2-6
Com-
pound
Chemical shifts, δ, ppm (J, Hz)
1
2
2a
1.26 (3H, t, J = 7.5, CH3CH2-6); 2.72 (2H, q, J = 7.5, CH3CH2-6); 3.99 (3H, s, CH3O-7);
7.23 (1H, s, H-8); 7.53 (3H, s, HPh-3',4',5'); 7.94 (1H, s, H-5); 8.04 (2H, m, HPh-2',6');
9.38 (1H, s, H-2)
2b
2c
3a
3b
3c
1.23 (3H, t, J = 7.5, CH3CH2); 2.69 (2H, q, J = 7.5, CH3CH2-6); 3.05 (3H, s, CH3-2);
3.96 (3H, s, CH3O-7); 7.12 (1H, s, H-8); 7.53 (3H, m, HPh-3',4',5'); 7.87 (1H, s, H-5);
8.03 (2H, m, HPh-2',6')
1.24 (3H, t, J = 7.5, CH3CH2-6); 1.47 (3H, t, J = 7.5, CH3CH2-2); 2.70 (2H, q, CH3CH2-6);
3.45 (2H, q, CH3CH2-2); 3.98 (3H, s, CH3O-7); 7.12 (1H, s, H-8);
7.53 (3H, m, HPh-3',4',5'); 7.87 (1H, s, H-5); 8.02 (2H, m, HPh-2',6')
1.14 (3H, t, J = 7.5, CH3); 2.47 (2H, q, J = 7.5, CH2); 6.49 (1H, s, H-3); 6.92 (1H, s, H-6);
7.47 (3H, m, HPh-3',4',5'); 7.82 (2H, m, HPh-2',6'); 8.03 (1H, s, Hpyrazole-5);
9.41 (1H, br. s, OH-4); 9.45 (1H, br. s, OH-2); 13.09 (1H, br. s, N–H)
1.12 (3H, t, J = 7.5, CH3); 2.46 (2H, q, J = 7.5, CH2); 2.55 (3H, s, CH3 pyrazole);
6.43 (1H, s, H-3); 6.85 (1H, s, H-6); 7.46 (3H, m, HPh-3',4',5'); 7.81 (2H, m, HPh-2',6');
9.26 (1H, br. s, OH-4); 9.34 (1H, s, OH-2); 12.70 (1H, br. s, N–H)
1.12 (3H, t, J = 7.5, CH3); 1.31 (3H, t, J = 7.5, CH3 pyrazole); 2.46 (2H, q, J = 7.5, CH2);
2.98 (2H, q, J = 7.5, CH2 pyrazole); 6.44 (1H, s, H-3); 6.85 (1H, s, H-6); 7.47 (3H, m,
HPh-3',4',5'); 7.80 (2H, m, HPh-2',6'); 9.25 (1H, br. s, OH-4); 9.36 (1H, br. s, OH-2);
12.67 (1H, br. s, N–H)
3d
3e
3f
1.08 (3H, t, J = 7.5, CH3); 2.42 (2H, q, J = 7.5, CH2); 6.45 (1H, s, H-3); 6.89 (1H, s, H-6);
7.51 (3H, m, HPh-3',4',5'); 7.86 (2H, m, HPh-2',6'); 9.55 (1H, s, OH-4); 9.56 (1H, s, OH-2),
13.82 (1H, s, N–H)
1.11 (3H, t, J = 7.5, CH3); 2.48 (2H, q, J = 7.5, CH2); 2.63 (2H, t, J = 8, CH2CH2COOH);
3.20 (2H, t, J = 8, CH2CH2COOH); 6.46 (1H, s, H-3); 6.84 (1H, s, H-6); 7.46 (3H, m,
HPh-3',4',5'); 7.80 (2H, m, HPh-2',6'); remaining protons exchanged with D2O
1.03 (3H, t, J = 7.5, CH3); 1.25 (3H, t, J = 7.5, CH3 pyrazole); 2.40 (2H, q, J = 7.5, CH2);
4.24 (2H, q, J = 7.5, CO2CH2); 6.39 (1H, s, H-3); 6.84 (1H, s, H-6); 7.51 (3H, m,
HPh-3',4',5'); 7.92 (2H, m, HPh-2',6'); 9.39 (1H, s, OH-4); 9.46 (1H, s, OH-2);
13.58 (1H, s, N–H)
3g
4a
1.09 (3H, t, J = 7.5, CH3); 2.46 (2H, q, J = 7.5, CH2); 4.49 (2H, br. s, NH2);
6.41 (1H, s, H-3); 6.87 (1H, s, H-6); 7.50 (3H, m, HPh-3',4',5'); 7.87 (2H, m, HPh-2',6');
9.40 (2H, br. s, OH-2 + OH-4); 10.24 (1H, br. s, NH–NH2); 13.45 (1H, br. s, N–Hpyrazole
1.14 (3H, t, J = 7.5, CH3CH2); 2.54 (2H, q, J = 7.5, CH3CH2); 3.84 (3H, s, CH3O);
6.55 (1H, s, H-3); 6.99 (1H, s, H-6); 7.46 (3H, m, HPh-3',4',5'); 7.83 (2H, m, HPh-2',6');
8.02 (br. s, Hpyrazole-5 (B)); 8.33 (br. s, Hpyrazole-3 (A)); 9.42 (br. s, O H-2 (A));
9.65 (br. s, OH-2 (B)); 13.14 (br. s, N–H (B)); 13.33 (br. s, N–H (A))
)
4b
4c
1.11 (3H, t, J = 7.5, CH3CH2); 2.53 (2H, q, J = 7.5, CH3CH2); 2.57 (3H, s, CH3 pyrazole-5);
3.82 (3H, s, CH3O); 6.50 (1H, s, H-3); 6.94 (1H, s, H-6); 7.46 (3H, m, HPh-3',4',5');
7.81 (2H, m, HPh-2',6'); 9.48 (1H, br. s, OH-2); 12.80 (1H, br. s, N–H)
1.11 (3H, t, J = 7.5, CH3CH2); 1.31 (3H, t, J = 7.5, CH3CH2 pyrazole); 2.50 (2H, q, J = 7.5,
CH3CH2); 3.00 (2H, q, J = 7.5, CH3CH2 pyrazole); 3.82 (3H, s, CH3O); 6.47 (1H, s, H-3);
6.92 (1H, s, H-6); 7.46 (3H, m, HPh-3',4',5'); 7.81 (2H, m, HPh-2',6');
9.36 (br. s, OH-2 (A)); 9.51 (br. s, OH-2 (B)); 12.74 (br. s, N–H (B));
13.07 (br. s, N–H (A))
5a
5b
5c
1.03 (3H, t, J = 7.5, CH3); 2.43 (2H, q, J = 7.5, CH2); 6.46 (1H, s, H-3); 6.71 (1H, s,
H-6); 7.36 (5H, s, N–Ph); 7.46 (3H, m, HPh-3',4',5'); 7.78 (2H, m, HPh-2',6'); 8.31 (1H, s,
Hpyrazole-3); 9.39 (1H, s, OH-4); 9.49 (1H, s, OH-2)
1.01 (3H, t, J = 7.5, CH3CH2); 2.42 (2H, q, J = 7.5, CH3CH2); 2.67 (3H, s, CH3 pyrazole-3);
6.46 (1H, s, H-3); 6.68 (1H, s, H-6); 7.34 (5H, s, N–Ph); 7.47 (3H, m, HPh-3',4',5');
7.76 (2H, m, HPh-2',6'); 9.41 (1H, s, OH-2); 9.53 (1H, s, OH-4)
1.01 (3H, t, J = 7.5, CH3); 1.39 (3H, t, J = 7.5, CH3 pyrazole); 2.39 (2H, q, J = 7.5, CH2);
3.16 (2H, q, J = 7.5, CH2 pyrazole-3); 6.45 (1H, s, H-3); 6.68 (1H, s, H-6);
7.34 (5H, s, N–Ph); 7.47 (3H, m, HPh-3',4',5'); 7.77 (2H, m, HPh-2',6'); 9.39 (1H, s, OH-4);
9.51 (1H, s, OH-2)
676

TABLE 2 (continued)
1
2
5d
0.98 (3H, t, J = 7.5, CH3); 2.38 (2H, q, J = 7.5, CH2); 6.40 (1H, s, H-3); 6.72 (1H, s, H-6);
7.40 (5H, s, N–Ph); 7.51 (3H, m, HPh-3',4',5'); 7.82 (2H, m, HPh-2',6'); 9.55 (1H, s, OH-4);
9.60 (1H, s, OH-2)
5e
1.01 (3H, t, J = 7.5, CH3); 2.42 (2H, q, J = 7.5, CH2CH3); 2.79 (2H, t, J = 8,
CH2CH2COOH); 3.35 (2H, t, J = 8, CH2CH2COOH); 6.46 (1H, s, H-3); 6.68 (1H, s, H-6);
7.34 (5H, s, N–Ph); 7.46 (3H, m, HPh-3',4',5'); 7.77 (2H, m, HPh-2',6'); 9.41 (1H, br. s,
OH-4); 9.52 (1H, br. s, OH-2); 11.92 (1H, br. s, COOH)
6a
1.02 (3H, t, J = 7.5, CH3CH2); 2.46 (2H, q, J = 7.5, CH3CH2); 3.83 (3H, s, CH3O);
6.51 (1H, s, H-3); 6.80 (1H, s, H-6); 7.35 (5H, s, N–Ph); 7.47 (3H, m, HPh-3',4',5');
7.79 (2H, m, HPh-2',6'); 8.33 (1H, s, Hpyrazole-3); 9.68 (1H, s, OH-2)
allowed us to hypothesize that in this case, we are observing the prototropism phenomenon due to the existence
of two tautomeric forms A and B in 1:3 ratio. We identified the tautomers A and B based on data taking into
account the chemical shifts of the protons and the integrated intensity of the peaks. We assign the downfield
signal in the pair of peaks for the N–H proton to tautomer A, since in this case it takes part in formation of the
intramolecular hydrogen bond with the oxygen atom of the hydroxy group, which is always accompanied by a
paramagnetic shift. In the pair of peaks for the signal from the C–H proton of the pyrazole ring in compound 4a,
the H-3 proton of tautomer A absorbs downfield since it is close to the pyridine nitrogen atom, while the H-5
proton of tautomer B is close to the pyrrole nitrogen atom. From the integrated intensities of the peaks it follows
that the signal from the OH-2 group of tautomer B is located downfield while that signal for tautomer A is
located upfield.
As we go from pyrazoles 3 and 4 (containing hydrogen atoms at the nitrogen atoms) to
N-phenylpyrazoles 5 and 6, formation of two isomers is also possible, but chromatographic analysis showed that
only one isomer was present in the reaction mixture. We decided in favor of 2-{3-R-5-[5-ethyl-2-hydroxy-4-(R1-
oxy)phenyl)-1-phenyl]pyrazol-4-yl}-5-phenyl-1,3,4-thiadiazoles (5a-e and 6) based on consideration of the
1
chemical properties and the H NMR spectra. Products 5a-e and 6 do not yield a colored complex with an
alcoholic solution of ferric chloride. The phenol hydroxyls appear in the 1H NMR spectra of compounds 5a-e as
two narrow peaks in the 9.3-9.6 pm region, which indicates the absence of intramolecular interaction between
the OH-2 group and the nitrogen atom of the pyrazole. The signal from the protons of the N-phenyl group of the
pyrazole is a five-proton singlet at 7.34-7.40 ppm, in contrast to the phenyl signals in the thiadiazole ring where
we observe three-proton and two-proton multiplets at 7.45-7.50 ppm and 7.77-7.81 ppm. The absence of
splitting for the signal from the N-phenyl group indicates that this benzene ring deviates from the plane of the
pyrazole ring. Evidence for this also comes from the 0.1-0.2 ppm upfield shift of the H-6 proton signal for the
phenol substituent compared with signals from the same protons of pyrazoles 3 and 4, since this proton is within
the region shielded by the ring currents of the N-phenyl ring.
Thus recyclization of 3-(1,3,4-thiadiazolyl)chromones when treated with hydrazine and phenylhydrazine
is a simple and convenient method for synthesis of 2-(pyrazol-4-yl)-1,3,4-thiadiazoles with different substituents
at the 3 and 5 positions of the pyrazole ring.
EXPERIMENTAL
The homogeneity of the synthesized compounds was monitored using TLC (Silufol UV-254, 9:1
chloroform–methanol). The 1H NMR spectra were recorded in DMSO-d6 on a Varian Mercury 400 spectrometer
(400 MHz). Internal standard: TMS.
1
The characteristics of compounds 2-6 are given in Table 1; the H NMR spectral data are given in
Table 2.
677

2-R-6-Ethyl-7-methoxy-3-(5-phenyl-1,3,4-thiadiazol-2-yl)chromones (2a-c). Freshly calcined
pulverized potassium carbonate (0.83 g, 6 mmol) was added to a solution of 2-R-6-ethyl-7-hydroxy-3-(5-phenyl-
1,3,4-thiadiazol-2-yl)chromone (1a-c) (3 mmol) in DMF (10 ml) and then dimethylsulfate (1.14 g, 9 mmol) was
added dropwise. The reaction mixture was stirred with heating for 2 h and then cooled down and added to water
(100 ml). The precipitate was filtered out.
5-[R-3-(5-Ethyl-2,4-dihydroxyphenyl)pyrazol-4-yl]-5-phenyl-1,3,4-thiadiazoles (3a-f) and 2-[5-R-3-
(5-Ethyl-2-hydroxy-4-methoxyphenyl)pyrazol-4-yl]-5-phenyl-1,3,4-thiadiazoles (4a-c). Hydrazine hydrate (1
g, 30 mmol) was added to a suspension of the corresponding 2-R-6-ethyl-7-hydroxy(methoxy)-3-(5-phenyl-
1,3,4-thiadiazol-2-yl)chromone 1a-f or 2a-c (1 mmol) in ethanol (10 ml) and boiled until dissolved (0.5-5 min).
The reaction mixture was cooled down and the precipitate was filtered out (in the case of products 2a and 4b) or
else the mixture was poured into water (100 ml) and then the precipitate was filtered out.
3-(5-Ethyl-2,4-dihydroxyphenyl)-4-(5-phenyl-1,3,4-thiadiazol-2-yl)pyrazol-5-ylcarboxylic Acid
Hydrazide (3g). A solution of a mixture of the ethyl ester 3f and the hydrazide 3g of 3-(2,4-dihydroxy-5-
ethylphenyl)-4-(5-phenyl-1,3,4-thiadiazol-2-yl)pyrazol-5-ylcarboxylic acid (0.35 g) and hydrazine hydrate
(1 ml) in DMF (2 ml) was boiled for 5 min and then poured into water (50 ml), and then the precipitate was
filtered out.
2-[3-R-5-(5-Ethyl-2,4-dihydroxyphenyl)-1-phenylpyrazol-4-yl]-5-phenyl-1,3,4-thiadiazoles (5a-e).
A suspension of the corresponding 2-R-7-hydroxy-3-(5-phenyl-1,3,4-thiadiazol-2-yl)-6-ethylchromone 1a,d,e
(1 mmol) and phenylhydrazine (0.32 g, 3 mmol) was boiled in ethanol (20 ml) for 1.5-11 h until the precipitate
dissolved and then was poured into water (50 ml) and the precipitate was filtered out. The reaction with
chromones 2b,c was carried out with heating in DMF (3 ml) for 2-4.5 h.
2-[5-(5-Ethyl-2-hydroxy-4-methoxyphenyl)-1-phenylpyrazol-4-yl]-5-phenyl-1,3,4-thiadiazole
(6)
was synthesized by the preceding procedure, with boiling in ethanol for 10 h.
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