Synthesis and photolysis of 3-tert-butyl-4-oxy(mercapto)-1,4-dihydropyrazolo[5,1-c][1,2,4]triazines

Article abstract of DOI:10.1007/s11172-020-2825-4

Reactions of 3-tert-butyl- or 3,4-di-tert-butyl-substituted 8-methylpyrazolo[5,1-c][1,2,4]-triazines with trifluoroacetic anhydride afforded 1-(2,2,2-trifluoroacetyl)-1,4-dihydropyrazolo-[5,1-c][1,2,4]triazin-4-yl 2,2,2-trifluoroacetates. The treatment with H₂X and RXH (X = O or S; R = Me or Et) of covalent trifluoroacetate that does not contain the Bu^t group at the C(4) atom allowed us to synthesize 1-(3-tert-butyl-4-R-pyrazolo[5,1-c][1,2,4]triazin-1(4H)-yl)-2,2,2-trifluoroethan-1-ones. The structure of 4-ethylthio derivative was fully established by the single-crystal X-ray diffraction analysis. The UV irradiation of obtained 2,2,2-trifluoroethan-1-ones leads to the aromatization of triazine ring. The UV photolysis of 1-trifluoroacetyl-4-hydroxy derivative has been proposed as a novel method for the photogeneration of acidity. Antimicrobial and antifungal activities of the synthesized compounds were evaluated.

Full text of DOI:10.1007/s11172-020-2825-4

Russian Chemical Bulletin, International Edition, Vol. 69, No. 4, pp. 731—738, April, 2020
731
Synthesis and photolysis of
3-tert-butyl-4-oxy(mercapto)-1,4-dihydropyrazolo[5,1-c][1,2,4]triazines
S. M. Ivanov,a,b K. A. Lyssenko,c,d and V. F. Travenb
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5328. Е-mail: sergey13iv1@mail.ru
bD. I. Mendeleev University of Chemical Technology of Russia,
9 Miusskaya pl., 125047 Moscow, Russian Federation.
Fax: +7 (499) 978 8660
cA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085
dM. V. Lomonosov Moscow State University,
1 Leninskie Gory, 119991 Moscow, Russian Federation.
Fax: +7 (495) 939 0126
Reactions of 3-tert-butyl- or 3,4-di-tert-butyl-substituted 8-methylpyrazolo[5,1-с][1,2,4]-
triazines with trifluoroacetic anhydride afforded 1-(2,2,2-trifluoroacetyl)-1,4-dihydropyrazolo-
[5,1-c][1,2,4]triazin-4-yl 2,2,2-trifluoroacetates. The treatment with H2X and RXH (X = O or
S; R = Me or Et) of covalent trifluoroacetate that does not contain the But group at the C(4)
atom allowed us to synthesize 1-(3-tert-butyl-4-R-pyrazolo[5,1-c][1,2,4]triazin-1(4H)-yl)-
2,2,2-trifluoroethan-1-ones. The structure of 4-ethylthio derivative was fully established by the
single-crystal X-ray diffraction analysis. The UV irradiation of obtained 2,2,2-trifluoroethan-
1-ones leads to the aromatization of triazine ring. The UV photolysis of 1-trifluoroacetyl-4-
hydroxy derivative has been proposed as a novel method for the photogeneration of acidity.
Antimicrobial and antifungal activities of the synthesized compounds were evaluated.
Key words: pyrazolo[5,1-c][1,2,4]triazine, 1,2,4-triazine, acylation, photolysis, photo-
generation of acidity.
Among a variety of six-membered heterocycles, 1,2,4-tri-
azines excel in their diverse practical applications.1—3 This
is certainly related to specific chemical properties exhibited
by the compounds of this type.4—6 Recently, an application
of triazines as the framework in novel sensory systems has
been intensively investigated. Thus, highly structured
mesoporous carbon nitride, which is a sensor for vapors
of acids and bases, has been prepared via a polymerization
of 3-amino-1,2,4-triazine.7 A fluorescent indicator based
on the complex of iridium(III) with the anion of 3-(2-pyr-
idyl)-5,6-bis(4-sulfophenyl)-1,2,4-triazine was successfully
applied for the both qualitative and quantitative determi-
nations of albumin in human serum.8 The use of 6-azau-
ridine and its analogues as pH-dependent bioorthogonal
fluorescent labels for DNA9 and proteins10 was also reported.
Aryl- and hetaryl-substituted triazines have been proposed
as chromogenic reagents for the forensic examination of
traces formed by metal objects on tissue and skin.11
We have previously explored aromatic 8-alkylpyr-
azolo[5,1-c][1,2,4]triazines containing one12 or few13
tert-butyl substituents at the triazine core. Diastereomeri-
cally pure 3,4-dihydropyrazolo[5.1-c][1,2,4]triazine-3,4-
diyl diacetates were obtained via a bromination in the
presence of carboxylic acids.14 Treatment with N-halo-
genosuccinimides of compounds possessing a vacant posi-
tion at the C(8) atom leads to products of the electrophilic
heteroaromatic substitution.15 A deprotonation of the C(4)H
moiety in some 1,4-dihydro derivatives bearing the tri-
fluoroacetamide group allowed obtaining pyrazolo[1,5-a]-
[1,3,5]triazines.16 In the present work, we have for the
first time synthesized 1-trifluoroacetyl-1,4-dihydropyr-
azolo[5,1-c][1,2,4]triazin-4-yl trifluoroacetates, studied
their reactions, and considered the structure and UV
photolysis of oxygen and sulfur-containing products of
their solvolysis, as well as their biological activity.
We have found that the reaction of 8-methyl-3-tert-
butyl(3,4-di-tert-butyl)pyrazolo[5,1-c][1,2,4]triazines
1a,b with trifluoroacetic anhydride in aprotic solvents
within 1–3 min at room temperature leads to the forma-
tion of covalent trifluoroacetates 2a,b (Scheme 1). The
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 0731—0738, April, 2020.
1066-5285/20/6904-0731 © 2020 Springer Science+Business Media LLC

Ivanov et al.
732
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
Scheme 1
Compound 4
a
b
X
O
O
R2
H
Me
Compound 4
c
d
X
S
S
R2
H
Et
1, 2: R1 = H (a), But (b)
Reagents and conditions: i. (CF3CO)2O, 15 min, 20 C; ii. 1) R2XH, THF, 5 min—3 h, –35—0 C; 2) KHCO3, H2O, 5 min, 0 C.
subsequent treatment with an aqueous KHCO3 solution
results in a fast solvolysis of compound 2a to give 1-[3-(tert-
butyl)-4-hydroxy-8-methylpyrazolo[5,1-c][1,2,4]triazin-
1(4H)-yl]-2,2,2-trifluoroethan-1-one (4a). Structure of
the latter was confirmed by IR and NMR spectroscopy
and high resolution mass spectrometry data. Thus, the IR
spectrum of this compound contains characteristic bands
of the OH and C=O groups at 3101 and 1743 cm–1, respec-
tively. Its 19F NMR spectrum consists of a single signal at
 = –68.90 (Fig. 1). A presence of the C(4)H—OH group
was confirmed by the two doublets appeared at  = 6.32 and
7.65 with J = 8.5 Hz in the 1H NMR spectrum and by the
signal of C(4) atom at  = 70.1 in the 13C NMR spectrum.
3,4-Di-tert-butyl-8-methyl-1-(2,2,2-trifluoroacetyl)-
1,4-dihydropyrazolo[5,1-c][1,2,4]triazin-4-yl 2,2,2-tri-
fluoroacetate (2b) turned out to be stable upon a short-time
treatment with diluted solutions of the acids and bases. It
was isolated using flash chromatography (SiO2), but its
defragmentation occurred during the recording of mass
spectra, showing a fragmentation ion with its mass cor-
responding to starting triazine 1b. Compound 2b was
isolated as a solid crystalline substance with low melting
point, which was decomposed with resinification during
its storage for several days at room temperature. The
quantitative composition of trifluoroacetate 2b was deter-
mined by the elemental analysis performed for a freshly
prepared sample. The formation of unstable compound 2a
upon the dissolution of triazine 1a in a mixture of
(CF3CO)2O (13—14 equiv.) and CDCl3 has been con-
firmed by 1H and 19F NMR spectroscopy of the reaction
mixture. In particular, signals from all the protons were
shifted upfield by 0.3–0.5 ppm relative to the correspond-
ing signals of compound 1a in pure CDCl3, as well as
discoloration of the solution was observed, which may
indicate a broken aromaticity of the heterocycle. In the
19F NMR spectrum (see Fig. 1), there were two new sin-
glets at  = –76.79 and –70.80 with the same integral
intensities, assigned to the CF3CO2С(4) and CF3CON(1)
groups, in addition to an intense peak of trifluoroacetic
anhydride that was the cosolvent. In the case of trifluoro-
acetate 2b, its IR and 13C NMR spectra were also recorded.
Characteristic vibrations of the C=O and CF3 bonds ap-
peared as the narrow intense bands at 1802 (CF3C=O),
1737 (CF3C=ON), and 1171 (CF3) cm–1, while quadru-
plets of the two C atoms of CF3 groups were located at
 = 110.6—119.4 (J = 286—287 Hz). Signals at  = 151.4—
156.2 (J = 39—44 Hz) in the 13C NMR spectrum were
assigned to the carbonyl group.
2a + (CF3CO)2O
2b
4a
4b
4c
4d
The solvolysis of compound 2a was carried out not
only with H2O, but also with a large molar excess
(32—238 equiv.) of MeOH, EtSH, or H2S in a THF me-
–68 –69 –70 –71 –72 –73 –74 –75 –76 –77 –78

Fig. 1. 19F NMR spectra of compounds 2а,b and 4a—d.

Photolysis of pyrazolo[5,1-c][1,2,4]triazines
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
733
or methanol led only to the formation of solvolysis prod-
ucts 4a,b.
F(1)
C(18)
O(1)
F(3)
It should be noted that aromatic triazines 1a,b did not
interact with either acetic anhydride or di-tert-butyl di-
carbonate even at the boiling point of reagent (> 140 C,
only the starting compound was isolated). However,
1-Boc-protected 4-hydroxypyrazolo[5,1-c][1,2,4]triazine
5 was successfully converted into the corresponding
4-ethylthioderivative 7 via its sequential treatment with
trifluoroacetic anhydride and ethyl mercaptan at a de-
creased temperature (Scheme 2).*
C(10)
C(11)
C(8)
F(2)
N(1)
C(9)
C(7)
N(2)
C(13)
H(7)
C(3)
C(4)
N(5)
C(12)
N(6)
C(14)
Scheme 2
H(4)
S(1)
C(15)
C(16)
C(17)
Fig. 2. Molecular structure of compound 4d in its crystal, rep-
resented by ellipsoids of the thermal vibrations (p = 50%).
dia at the temperature ranging from –35 to 0 С, and
resulted in the formation of expected products 4b—d (see
Scheme 1). The structure 1-(3-tert-butyl-4-ethylthio-8-
methylpyrazolo[5,1-c][1,2,4]triazin-1(4H)-yl)-2,2,2-tri-
fluoroethane-1-one (4d) was proved by the single crystal
X-ray diffraction analysis (Fig. 2) and also confirmed by
IR and NMR spectroscopy and high resolution mass
spectrometry.
It has been found that compound 2a does not practi-
cally react either with 2-propanol or n-butanol. An SN2-
like mechanism has been previously proposed17 for the
nucleophilic substitution reactions of 3-nitro-4-trifluoro-
acetoxyazolo[5,1-c][1,2,4]triazines. It was assumed that
covalent 3-tert-butyl trifluoroacetate 2a dissociates in
a protic medium under the certain conditions (see Scheme 1)
due to the formation of hydrogen bonds between the sol-
vent and trifluoroacetate residue, proceeding with the
consequent solvolysis of intermediate pyrazolo[5,1-c]-
[1,2,4]triazin-1(5)-ium cations according to the SN1-like
mechanism (see Scheme 1). In the case of either bulky
molecules of a solvent or di-tert-butyl-substituted substrate
2b, the efficient formation of hydrogen bonds is hindered,
which leads to the slowed down dissociation step and to
the impossibility of nucleophilic substitution. In the re-
actions of trifluoroacetates 2a,b with such strong nucleo-
philes as MeO– or EtO–, AlkMgBr or AlkLi, small
amounts of initial triazines 1a,b were formed in addi-
tion to the resinification, which might be a result of
the attack at the carbonyl groups of side chain. Com-
pounds 2a,b were inert towards their treatment with NaN3
or NaCN in anhydrous hexamethylphosphoramide at
room temperature for few days. An addition of water
Reagents and conditions: i. (CF3CO)2O, THF, 10 min, –50 C;
ii. 1) EtSH, THF, 15 min, –(50—20) C; 2) KHCO3, H2O,
30 min, 0 C.
It has been experimentally found that compound 5 is
not protonated by trifluoroacetic acid at –20 C. Thus,
the 1H NMR spectrum of heterocycle 5 recorded in
a CDCl3—CF3CO2H mixture was not significantly different
from those recorded in either CDCl3 or DMSO-d6 media
(see Experimental). Apparently, the acylation of alcohol
group in aminal 5 with trifluoroacetic anhydride leads to
the formation of highly reactive trifluoroacetate 6, which
consequently undergoes the solvolysis in EtSH medium.
It is known that N-acyl-substituted derivatives of par-
tially hydrogenated hetarenes possess an increased pho-
tochemical sensitivity and are capable of aromatization
upon UV irradiation. Thus, the photolysis of [3,7-bis-
(diethylamino)-10H-phenoxazin-10-yl]methanones is
accompanied by the removal of acyl protection and by the
intense fluorescence of generated dye.18 Acyl derivatives
of dihydro form of Coumarin 6 upon an irradiation undergo
* Submitted to the Journal: L. M. Mironovich, S. M. Ivanov,
N. G. Kolotyrkina, Russ. J. Org. Chem., 2020.

Ivanov et al.
734
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
A
photoaromatization with the formation of laser dye that
has been applied in a design of the new medium for the
optical recording of information.19,20
238 nm
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
252 nm
274 nm
We have found that the UV irradiation of compounds
4a,b,d is also leading to their aromatization to give bright
yellow triazine 1a (Scheme 3). The irradiation of di-
luted solution of compound 4d with UV light ( = 240—
400 nm) results in the decreased absorption maxima at
246 and 284 nm and in the appearance of intense ab-
sorption band at 236 nm and wide band in the range of
320—450 nm (its maximum at 368 nm) (Fig. 3). Similar
changes in the spectra were observed during the photo-
lysis of compound 4b (Fig. 4).
250
300
350
400
450
/nm
Fig. 4. Absorption spectra of compound 4b dissolved in CH3CN
(С = 5.24•10–5 mol L–1) upon UV irradiation (A03 filter,
maximum exposure time of 300 s).
Scheme 3
A
234 nm
286 nm
0.35
0.30
0.25
0.20
0.15
0.10
0.05
250
300
350
400
450
/nm
Fig. 5. Absorption spectra of compound 4а dissolved in CH3CN
(С = 5.48•10–5 mol L–1) upon UV irradiation (A03 filter,
maximum exposure time of 40 s, the dashed line corresponds to
the absorption spectrum of compound 1а).
Compound 4a can also be readily photolysed upon the
UV irradiation, resulting in the gradual decrease in the
absorption maxima at 252 and 278 nm and in the shift of
long-wavelength maximum to 300 nm. Upon its UV ir-
radiation for 40 s, the optical density increases at 236 nm
and also in the range of 300—450 nm (Fig. 5), thus indi-
cating the formation of compound 1a.
According to the Scheme 3, the photoaromatization
of compound 4a should be concerted with the formation
of trifluoroacetic acid. This fact was confirmed by pH
measurements for the aqueous acetonitrile solution of
compound 4a (Fig. 6, curve 2). The pH value of this solu-
tion decreases upon UV irradiation much faster than that
in a control experiment without any irradiation (see Fig. 6,
curve 1). Thus, increases in the acidity were estimated as
2•10–3 pH s–1 upon the UV irradiation and as 7•10–4 pH s–1
without any irradiation.
The generation of acidity during the photolysis of
compound 4a is of undoubted interest. An irradiation of
the acidity photogenerators in a non-polar solvent (e.g.,
toluene) or in a polymethylmethacrylate film in the pres-
ence of lactone Rhodamine B is accompanied by the
appearance of intense fluorescence from the open form of
dye, which is applied in systems for the optical recording
of information. Moreover, data on the application of acid-
ity photogenerators to control biochemical reactions have
been reported in recent years.21,22
A
236 nm
1.8
1.6
1.4
284 nm
1.2
1.0
0.8
368 nm
0.6
0.4
0.2
250
300
350
400
450
/nm
Fig. 3. Changes in the absorption spectra of compound 4d dis-
solved in THF (С = 1.38•10–4 mol L–1) upon UV irradiation
(A03 filter, maximum exposure time of 120 s).
Note. Figures 3—5 are available in full color on the web page of
the journal (https://link.springer.com/journal/volumesAndIssues/
11172).

Photolysis of pyrazolo[5,1-c][1,2,4]triazines
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
735
Table 1. GC/MS data for the reaction mixture obtained via the photolysis of compound 4b (THF,  = 240—400 nm,
50 min)
tret (Irel (%))
MS (EI, 70 eV), m/z (Irel (%))
11.59 (0.5, 1a)
11.53 (100, 4b)
190 [M]+ (61), 175 [M – NH] (100), 148 [M – NH – HCN] (87), 67 (13), 41 [C2H3N]+ (14)
318 [M]+ (55), 287 [M – OCH3]+ (74), 191 (45), 178 (66), 166 (100), 148 (22), 138 (26),
69 [CF3]+ (35), 57 [But]+ (58)
Table 2. GC/MS data for the reaction mixture obtained via the photolysis of compound 4d (THF,  = 240—400 nm, 50 min)
tret (Irel (%))
MS (EI, 70 eV), m/z (Irel (%))
13.00 (100%, 4d)
348 [M]+ (1), 287 [M – EtS]+ (100), 189 [M – CF3CO – EtSH]+ (5), 175 [M – CF3CO – EtS – NH]+ (14),
148 [M – CF3CO – EtS – NH – HCN] (19), 69 [CF3]+ (5), 57 [But]+ (3)
11.57 (65%, 1a)
4.73 (34%, EtSSEt)
190 [M]+ (59), 175 [M – NH] (100), 148 [M – NH – HCN] (88), 67 (13), 41 [C2H3N]+ (12)
122 [M]+ (100), 94 (47), 86 (79), 66 (45), 56 (36), 42 (100), 28 (43)
pH
5.0
In the case of ethylthio derivative 4d, subequimolar
amounts of diethyl disulfide were formed in addition to
aromatic triazine 1a (see Table 2). In addition, we have
4.9
4.8
4.7
4.6
4.5
4.4
observed trace amounts of products of the free radical
degradation of solvent, as well as of unidentified com-
pounds bearing a CF3 moiety according to the mass spectra.
Pyrazolo[5,1-c][1,2,4]triazines 4a—c were screened
for their possible antimicrobial and antifungal activity
using five bacterial and two fungal cell lines (Table 3).
These compounds have demonstrated the weak (the inhi-
bition of 15—30%) antimicrobial and antifungal activities
1
2
at the concentration of 32 g mL–1
.
Therefore, we have investigated the reactions of aro-
matic pyrazolo[5,1-c][1,2,4]triazines with trifluoroacetic
anhydride, resulting in the formation of covalent
1-(2,2,2-trifluoroacetyl)-1,4-dihydropyrazolo[5,1-c][1,2,4]-
triazin-4-yl 2,2,2-trifluoroacetates. Their treatment with
water, methanol, ethyl mercaptan, or hydrogen sulfide
allowed us to synthesize a series of previously unknown
4-substituted 1-(3-tert-butyl-pyrazolo[5,1-c][1,2,4]tri-
azin-1(4H)-yl)-2,2,2-trifluoroethan-1-ones. The photo-
lysis of the latter upon UV irradiation leads to their aro-
matization and can be proposed as a novel method for the
photogeneration of acidity. The isolated compounds ex-
hibited weak both antimicrobial and antifungal activities.
50
100
150
200
250 /s
Fig. 6. Changes in the pH value of compound 4a dissolved
in CH3CN (the solvent was a СН3CN : Н2О mixture (1 : 5),
С = 5.48•10–5 mol L–1) without any exposure to UV radiation (1)
and UV-irradiated through an A03 light filter (2).
Beyond our expectations, GC/MS monitoring of the
reaction mixture upon UV irradiation of substrates dis-
solved in methanol, aqueous (95%) ethanol, absolute THF,
or diglyme has not confirmed the formation of any ap-
preciable amounts of methyl and ethyl thiotrifluoroacetates
from compounds 4b and 4d, respectively (Tables 1 and 2).
Table 3. Antimicrobial and antifungal activities of compounds 4а—d
Com-
pound
Growth inhibition (%)
Bacteria
S. aureus – MRSA E. coli K. pneumoniae — MDR P. aeruginosa
Fungi
A. baumannii
C. albicans
C. neoformans
4a
4b
4c
4d
12.26
18.39
29.78
19.26
12.74
14.32
17.69
10.15
21.58
14.39
24.31
18.84
19.41
20.44
18.89
14.32
12.69
16.81
19.48
15.09
4.76
1.37
3.05
2.38
3.51
11.62
12.29
18.18

Ivanov et al.
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Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
(s, 3 H, C(8)CH3); 1.52 (s, 9 H, But). UV (THF), max/nm (log):
236 (4.116), 368 (3.162), 384 (2.869). UV (CH3CN), max/nm
(log): 234 (2.905), 298 (1.626), 375 (1.802).
Experimental
UV spectra were recorded on an SF-104 spectrophotometer.
1
IR spectra were recorded neat or in KBr pellets using an Agilent
A colorless solution of compound 2a. H NMR (300 MHz,
1
Cary 660 FTIR spectrometer. Н, 13С (APT, attached proton
CDCl3 (0.5 mL)—(CF3CO2)O (0.2 mL)), : 7.71 (s, 1 H, C(7)H);
7.55 (s, 1 H, C(4)H); 2.25 (s, 3 H, C(8)CH3); 1.35 (s, 9 H, But).
19F NMR (282 MHz, CDCl3 (0.5 mL)—(CF3CO2)O (0.2 mL)),
: –70.80 (s, 3 F, CF3CON(1)); –76.79 (s, 3 F, CF3CO2C(4));
–77.13 (s, ~50 F, cosolvent (CF3CO2)O). UV (THF), max/nm
(log): 296 (2.945), 384 (2.204).
test), and 19F NMR spectra were recorded on Bruker AM–300,
Bruker DRX–500 and Bruker AV–600 instruments operating at
the frequencies of 300 MHz for 1Н, 75, 126, or 151 MHz for 13C,
and 282 MHz for 19F, respectively. Internal standards were
DMSO-d5 and CHCl3 (1H), DMSO-d6 and CDCl3 (13C), and
CF3CO2H (19F). GC/MS was performed on a TRACE GC
ULTRA chromatograph equipped with a TR-5MS capillary
column (length of 30 m, diameter of 0.25 mm, and phase thick-
ness of 0.25 m) and with a DSQ II quadrupole mass spectrometer
as the detector (electron impact, ionization energy of 70 eV),
and operating in the following temperature mode: 70 С (2 min) 
280 С (15 deg min–1)  280 С (10 min). High resolution
mass spectra (HRMS) were recorded on a Bruker micrOTOF II
instrument using electrospray ionization. The measurements
were carried out in the mode of positive ions (the voltage on the
capillary was 4500 V), and the solvent was methanol. Single
crystal X-ray diffraction analysis was performed on a Bruker
APEX II CCD diffractometer ((MoKa) = 0.71072 Å). Melting
points were determined using a Melting point SMP30 instrument
(STUART). The pH values of solutions were measured using
a PHS-3D instrument (Sancin). Merck Silica (60—200 mesh)
silica gel was used for the chromatography. Compounds 1a,b and
5 were prepared according to the known procedures.12,13 Dry
H2S was obtained via careful heating23 of a mixture of finely
dispersed S8 (10 g, 312 mmol), paraffin [CH2]n (10 g, 713 mmol),
and asbestos (20 g) at 300 C.
3,4-Di-tert-butyl-8-methyl-1-(2,2,2-trifluoroacetyl)-1,4-
dihydropyrazolo[5,1-c][1,2,4]triazin-4-yl 2,2,2-trifluoroacetate
(2b). Under an argon atmosphere, (CF3CO2)O (0.50 mL,
3.60 mmol) was added in one portion at ~20 C to a bright yellow
solution of compound 1b (0.25 g, 1.01 mmol) in THF (15 mL).
The resulting solution was almost colorless. It was stirred for
15 min and then added dropwise within 2 min to a cooled (0 C),
vigorously stirred two-phase mixture of CH2Cl2 (30 mL) and an
aqueous (100 mL) solution of KHCO3 (10 g, 0.1 mol). It was
vigorously stirred for additional 10 min at the same temperature.
The organic phase was separated, washed with water (50 mL),
dried over anhydrous MgSO4, and filtered. The filtrate was also
filtered through a thin layer of SiO2 and evaporated in vacuo at
40 C. Compound 2b was obtained as white crystals, m.p.
82—84.5 С (decomp.), the yield of 0.41 g (89%). IR (KBr),
/cm–1: 2985, 2938, 2884 (CH), 1802 (CF3C=OO), 1737
(CF3C=ON), 1592, 1495, 1400, 1359, 1233, 1203, 1171 (CF3),
1129, 1100, 1073, 1034, 1007, 967, 939, 892, 851, 789, 754, 726,
1
648, 567, 526. H NMR (300 MHz, CDCl3), : 1.11 (s, 9 H,
C(4)But); 1.43 (s, 9 H, C(3)But); 2.15 (s, 3 H, C(8)CH3); 7.41
(s, 1 H, C(7)H). 19F NMR (282 MHz, CDCl3), : –75.84 (s, 3 F,
CF3CO2C(4)); –69.86 (s, 3 F, CF3CON(1)). 13C NMR (APT,
126 MHz, CDCl3), : 10.41 (C(8)CH3); 25.40 (C(4)C(CH3)3);
31.21 (C(3)C(CH3)3); 40.76 (C(4)CMe3); 44.16 (C(3)CMe3);
93.72 (C(8)); 105.18 (C(4)); 110.62, 112.59, 115.17, 117.44
(q, CF3CON(1), J = 286.2 Hz); 112.89, 114.87, 117.15, 119.43
(q, CF3CO2C(4), J = 287.1 Hz); 128.63 (C(8a)); 142.14 (C(7)H);
151.37, 151.68, 151.99, 152.29 (q, CON(1), J = 38.7 Hz); 155.14,
155.49, 155.84, 156.19 (q, CO2C(4), J = 44.1 Hz); 160.94 (C(3)).
MS (EI, 70 eV), m/z (Irel (%)): 246 [1b]+ (100), 231 [1b – NH]+
(52), 189 [1b – But]+ (67), 175 [1b – NH – But]+ (17), 162
[1b – N2 – But + H]+ (31), 148 (13), 109 (13), 82 [4-methyl-
pyrazole]+ (18), 57 [But]+ (9), 41 [C2H3N]+ (12), 29 [N2H]+
(4). HRMS: found m/z (Irel (%)) 247.1908 (40), 376.2322 (100);
C18H22F6N4O3; calculated for 2b + Н = 457.1669, 1b + Н =
= 247.1917. Elemental analysis. Found (%): С, 47.29; Н, 5.03;
N, 12.23. C18H22F6N4O3. Calculated (%): C, 47.37; H, 4.86;
N, 12.28.
Photolysis of compounds 4a,b,d (general procedure). A portion
of the corresponding compound 4a,b,d (50 mg) was dissolved in
the solvent (50 mL, either MeOH, CH3CN, or THF). The result-
ing solution was diluted to the desired concentration. The photo-
lysis was carried out under an air atmosphere in a quartz flask
under stirring and irradiation with a Hamamatsu LC-4 source of
UV radiation equipped with an L8253 xenon lamp (power of
150 W) and an A03 filter (transmission range of 240—400 nm).
The reaction progress was monitored by changes in the absorption
spectra, as well as using GC/MS (see Tables 1 and 2).
Biological experiments were carried out according to the
methods reported previously.14 The inhibition percentage was
estimated via measuring the difference in the fluorescence in-
tensity of culture solutions in water without addition and in the
presence of studied compounds (C = 32 g mL–1) after incuba-
tion for 18 h at 37 C for bacteria (density of colony forming
units of 5•105 mL–1) and for 24 h at 35 C for fungi (density of
colony forming units of 2.5•103 mL–1). 7-Hydroxy-3H-phen-
oxazin-3-one-10-oxide was selected as the indicator of viability
of microorganisms.
Solvolysis of trifluoroacetate 2a (general step in the synthesis
of compounds 4а—d). Under an argon atmosphere, (CF3CO2)O
(0.50 mL, 3.60 mmol) was added in one portion at ~20 C to
a solution of compound 1a (0.25 g, 1.31 mmol) in THF (20 mL).
The resulting solution of compound 2a was almost colorless. It
was stirred for 15 min and then cooled down to –20 C (for the
synthesis of compounds 4a,b,d) or to –35 C (in the case of 4c,
external cooling with a heptane—liquid N2 mixture). Water
(5 mL, 0.28 mol), MeOH (2.5 mL, 61.8 mmol), or EtSH (3.0 mL,
41.6 mmol) precooled to 0 C were added dropwise within 1—3 min
in order to obtain compounds 4a, 4b, or 4d, respectively.
Compound 4c was synthesized via flushing the solution of com-
pound 2a with a stream of dry gaseous H2S (312 mmol, cooled
3-tert-Butyl-8-methyl-1-(2,2,2-trifluoroacetyl)-1,4-dihydro-
pyrazolo[5,1-c][1,2,4]triazin-4-yl 2,2,2-trifluoroacetate (2a).
Compound 1a (20 mg, 0.11 mmol) was dissolved in CDCl3
1
(0.5 mL) under an argon atmosphere. The H NMR spectrum
of obtained bright yellow solution was recorded. A portion of
(CF3CO2)O (0.20 mL, 1.44 mmol) was then added to the solution
at ~20 C. The discoloration was observed. The resulting solution
was mixed well, and 1H and 19F NMR spectra were recorded.
A bright yellow solution of compound 1a. 1H NMR (300 MHz,
CDCl3), : 8.33 (s, 1 H, C(7)H); 7.97 (s, 1 H, C(4)H); 2.59

Photolysis of pyrazolo[5,1-c][1,2,4]triazines
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
737
to a temperature below 0 C) for 30 min. The reaction mixture
was stirred at 0 C for 5 min (4a), 30 min (4b), or 2—3 h (4c,d).
Crystalline KHCO3 (5 g, 49.9 mmol, in small portions), EtOAc
(30 mL, to isolate compounds 4b—d), and cooled water (0 C,
50 mL) were subsequently added under vigorous stirring. The
reaction mixture was vigorously stirred at 0 C for 5 min, the
organic phase was then separated, and the mother liquor was
extracted with EtOAc (530 mL). The combined organic phases
were washed with cooled water (150 mL), dried over anhydrous
MgSO4, and filtered. The filtrate was evaporated in vacuo at 40 C.
To isolate compound 4a, THF was evaporated from the reac-
tion mixture at ~20 C. The formed precipitate was filtered off,
washed with water (3×20 mL), and dried in air. The residues
were purified using flash chromatography with the follow-
ing eluents: EtOAc—СHCl3 (1 : 2) for 4a; EtOAc—hexane
(1 : 30—1 : 10) for 4b; CHCl3—hexane (1 : 1—2 : 1) for 4c; and
CH2Cl2—hexane (1 : 3—2 : 1) for 4d. After that, the correspond-
ing product was washed with cooled hexane (0 C, 32 mL, to
remove small amounts of impurity 1a), dried in a stream of argon,
and compounds 4а—d were thus isolated.
m.p. 55—60 С (decomp.), the yield of 0.33 g (78%). IR (neat),
/cm–1: 3414 (SH), 3095, 2984, 2972, 2941, 2906, 2872 (CH),
2547, 1718 (CO), 1617, 1587 (C=N), 1505, 1478, 1466, 1458,
1430, 1397, 1370, 1362, 1258, 1235, 1204, 1171 (CF3), 1104,
1091, 1041, 1018, 997, 969, 941, 906, 853, 825, 786, 736, 724,
700, 658, 639, 619, 566, 531, 451. 1H NMR (300 MHz, CDCl3),
: 1.39 (s, 9 H, But); 2.21 (s, 3 H, C(8)CH3); 2.78 (d, 1 H, C(4)
SH, J = 7.5 Hz); 6.32 (d, 1 H, C(4)H, J = 7.5 Hz); 7.43 (s, 1 H,
C(7)–H). 19F NMR (282 MHz, CDCl3), : –70.22 (s, 3 F,
CF3CON(1)). 13C NMR (APT, 75 MHz, CDCl3, three compo-
nents of the CF3CO quadruplet were not observed), : 11.04
(C(8)CH3); 28.84 (C(CH3)3); 38.07 (C(CH3)3); 50.57 (C(4)H);
106.67 (C(8)); 110.49, 114.30, 118.10, 121.90 (q, CF3СON(1),
J = 286.7 Hz); 129.11 (C(8a)); 143.16 (C(7)H); 152.57
(q, CF3CON(1)); 162.33 (C(3)). MS: found m/z (Irel (%))
321.1005 [M + H] (100), 343.0826 [M + Na] (40). C12H15F3N4OS.
Calculated: М + Н = 321.0991, M + Na = 343.0811.
1-(3-tert-Butyl-4-ethylthio-8-methylpyrazolo[5,1-c][1,2,4]-
triazin-1(4H)-yl)-2,2,2-trifluoroethanone (4d). Colorless crystals,
m.p. 140—150 С (sublimation), the yield of 0.38 g (83%). UV
(THF), max/nm (log): 246 (3.940), 284 (3.764). IR (neat),
/cm–1: 2975, 2958, 2935, 2906, 2877 (CH), 1722 (CO), 1586,
1507, 1478, 1458, 1425, 1395, 1372, 1353, 1261, 1233, 1200,
1176 (CF3), 1088, 1041, 998, 917, 852, 825, 801, 768, 746, 726,
1-(3-tert-Butyl-4-hydroxy-8-methylpyrazolo[5,1-c][1,2,4]-
triazin-1(4H)-yl)-2,2,2-trifluoroethanone (4a). White powder,
m.p. 180—190 С (decomp.), the yield of 0.38 g (95%). UV
(MeOH), max/nm (lg): 252 (3.943), 278 (4.002). UV (CH3CN),
max/nm (lg): 242 (4.068), 278 (4.002). IR (KBr), /cm–1: 3101
(OH), 2977, 2938, 2872, 2835 (CH), 2709, 1743 (CO), 1633,
1585, 1518, 1477, 1453, 1412, 1399, 1363, 1309, 1260, 1239,
1207, 1176 (CF3), 1104, 1060, 1010, 921, 882, 857, 833, 790,
1
701, 657, 637, 613, 568, 532, 487, 451. H NMR (300 MHz,
CDCl3), : 1.25 (t, 3 H, SCH2CH3, J = 7.4 Hz); 1.38 (s, 9 H,
But); 2.19 (s, 3 H, C(8)CH3); 2.80—2.50 (diastereotopic m, 2 H,
SCH2CH3); 6.15 (s, 1 H, C(4)H); 7.40 (s, 1 H, C(7)H). 19F NMR
(282 MHz, CDCl3), : –70.07 (s, 3 F, CF3CON(1)). 13C NMR
(APT, 75 MHz, CDCl3, two components of the CF3CO quadru-
plet were not observed),: 10.91 (C(8)CH3); 13.96 (SCH2CH3);
25.57 (SCH2CH3); 28.96 (C(CH3)3); 37.98 (C(CH3)3); 56.39
(C(4)H); 106.53 (C(8)); 110.55, 114.35, 118.16, 121.95 (q,
CF3CON(1), J = 286.6 Hz); 129.15 (C(8a)); 142.35 (C(7)H);
152.90, 153.35 (q, CF3CON(1), J = 33.4 Hz); 161.52 (C(3)).
MS: found m/z (Irel (%)) 349.1297 [M + H] (100), 371.1112
[M + Na] (30). C14H19F3N4OS. Calculated: М + Н = 349.1304,
M + Na = 371.1124.
1
741, 706, 658, 632, 571, 530, 493, 445. H NMR (300 MHz,
DMSO-d6), : 1.27 (s, 9 Н, But); 2.09 (s, 3 Н, С(8)СН3); 6.32
(d, 1 Н, С(4)Н, J = 8.2 Hz); 7.52 (s, 1 H, C(7)H); 7.65 (d, 1 Н,
С(4)ОН, J = 8.5 Hz). 19F NMR (282 MHz, DMSO-d6), :
–68.90 (s, 3 F, CF3CON(1)). 13C NMR (APT, 75 MHz,
DMSO-d6), : 10.26 (C(8)CH3); 27.59 (С(СН3)3); 37.00
(С(СН3)3); 70.14 (C(4)H); 104.39 (C(8)); 110.15, 113.96, 117.76,
121.56 (q, CF3CON(1), J = 286.6 Hz); 128.54 (C(8a)); 141.24
(C(7)H); 150.77, 151.26, 151.76, 152.25 (q, CF3CON(1),
J = 37.7 Hz); 161.66 (С(3)). MS: found m/z (Irel (%)) 305.1224
[M + H] (100). C12H15F3N4O2. Calculated: М + Н = 305.1220.
1-(3-tert-Butyl-8-methyl-4-methoxypyrazolo[5,1-c][1,2,4]-
triazin-1(4H)-yl)-2,2,2-trifluoroethanone (4b). Colorless crystals,
m.p. 105—110 С (decomp.), the yield of 0.34 g (81%). UV
(CH3CN), max/nm (lg): 252 (4.021), 274 (4.156). IR (neat),
/cm–1: 2978, 2955, 2936, 2874, 2853, 2835 (CH), 1734 (CO),
1625, 1585, 1506, 1466, 1425, 1398, 1371, 1353, 1265, 1237,
1200, 1177 (CF3), 1111, 1094, 1075, 1044, 1022, 999, 953, 937,
856, 837, 792, 746, 732, 701, 667, 652, 635, 570, 549, 533, 473,
Single crystal X-ray diffraction analysis of compound 4d.
Crystals of compound 4d (C14H19F3N4OS, M = 348.39,
 = 0.233 mm–1, dсalc = 1.391 g cm–3) at 120 К triclinic, space
–
group P1, a = 9.0928(6), b = 9.9948(7), and c = 10.9181(8) Å,
 = 96.3559(13),  = 103.9402(13),  = 116.7910(12),
V = 831.65(10) Å3, Z = 2. The crystal was grown in ethyl acetate.
The intensities of 16754 reflections for the crystal of compound
4d were measured on a Bruker APEX II CCD diffractometer
((MoKa) = 0.71072 Å,  = 1.99—29.00), and 4430 independent
reflections (Rint = 0.0257) were used in the further refinement.
The structure was solved by the direct method and refined
by the least-squares method in the anisotropic full-matrix ap-
proximation on F2hkl. Positions of the hydrogen atoms were
geometrically calculated and refined in the isotropic approxima-
tion according to the riding model. Final values of the uncer-
tainty factors were wR2 = 0.0878 and GOOF = 1.014 for all the
independent reflections (R1 = 0.0315 was calculated on F for
3830 observed reflections with I > 2(I)). All the calculations were
performed using the SHELXTL software package.24 The coordin-
ates of atoms and full structural data were deposited with the Cam-
bridge Structural Databank (CCDC 1875748; www.ccdc.ac.uk).
tert-Butyl 3-tert-butyl-4-hydroxypyrazolo[5,1-c][1,2,4]tri-
1
452, 433. H NMR (300 MHz, CDCl3), : 1.31 (s, 9 Н, But);
2.20 (s, 3 Н, С(8)СН3); 3.41 (s, 3 H, OCH3); 5.93 (s, 1 Н, С(4)
Н); 7.44 (s, 1 H, C(7)H). 19F NMR (282 MHz, CDCl3), :
–69.88 (s, 3 F, CF3CON(1)). 13C NMR (APT, 75 MHz, CDCl3,
two components of the CF3CO quadruplet were not observed),
: 10.84 (C(8)CH3); 27.88 (C(CH3)3); 37.51 (C(CH3)3); 56.22
(OCH3); 77.46 (C(4)H); 105.81 (C(8)); 110.44, 114.24, 118.04,
121.84 (q, CF3CON(1), J = 286.8 Hz); 129.51 (C(8a)); 142.48
(C(7)H); 152.55, 153.06 (q, CF3CON(1), J = 37.9 Hz); 158.77
(C(3)). MS: found m/z (Irel (%)) 319.1382 [M + H] (100),
341.1196 [M + Na] (20). C13H17F3N4O2. Calculated: М + Н =
= 319.1376, M + Na = 341.1196.
1-(3-tert-Butyl-4-mercapto-8-methylpyrazolo[5,1-c][1,2,4]-
triazin-1(4H)-yl)-2,2,2-trifluoroethanone (4c). White powder,
1
azin-1(4H)-carboxylate (5). H NMR (300 MHz, DMSO-d6,

Ivanov et al.
738
Russ. Chem. Bull., Int. Ed., Vol. 69, No. 4, April, 2020
20 С), : 7.55 (d, 1 H, C(7)H, J = 1.9 Hz); 7.34 (d, 1 H, OH,
J = 7.9 Hz); 6.27—6.23 (m, 2 H, C(4)H + C(8)H); 1.55 (s, 9 H,
ButO); 1.27 (s, 9 H ButC(3)). 1H NMR (300 MHz, CDCl3,
20 С), : 7.57 (s, 1 H, C(7)H); 7.3—6.4 (br.s, 1 H, OH);
6.47—6.42 (m, 2 H, C(4)H + C(8)H); 1.65 (s, 9 H, ButO); 1.40
(s, 9 H, ButC(3)). 1H NMR (300 MHz, CF3CO2H (0.03 mL)—
CDCl3 (0.6 mL)—5 (20 mg), –20 С), : 11.00 (br.s, ~ 6 H,
CF3CO2H + OH); 8.02 (d, 1 H, C(7)H, J = 2.3 Hz); 6.82 (d, 1 H,
C(8)H, J = 2.0 Hz); 6.67 (s, 1 H, C(4)H); 1.67 (s, 9 H, ButO);
1.38 (s, 9 H, ButC(3)).
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tert-Butyl 3-tert-butyl-4-(ethylthio)pyrazolo[5,1-c][1,2,4]-
triazin-1(4H)-carboxylate (7). Under an argon atmosphere,
(CF3CO2)O (0.50 mL, 3.60 mmol) was added in one portion to
a cooled (to –(50—45) C, external cooling with a heptane—liq-
uid N2 mixture) solution of compound 5 (0.4 g, 1.36 mmol) in
THF (20 mL). The resulting mixture was stirred for 10 min at
the same temperature, and cooled EtSH (0 C, 3 mL, 41.6 mmol)
was added dropwise within 3 min under vigorous stirring. Then
the cooling bath was removed, and the reaction mixture was
stirred for 15 min at –50  –20 С. After that, crystalline KHCO3
(5 g, 50 mmol) was added in one portion, and the resulting
mixture was stirred for 30 min at –20 С  0 С. Then, EtOAc
(100 mL) and cooled water (0 C, 100 mL) were subsequently
added, and the mixture was vigorously stirred at 0 C for 5 min.
The organic phase was separated, and the mother liquor was
extracted with EtOAc (325 mL). The combined organic phases
were washed with cooled water (100 mL), dried over anhydrous
MgSO4, and filtered. The filtrate was evaporated in vacuo at
25 C. The residue was purified using flash chromatography
(eluent was EtOAc—hexane, 1 : 20—1 : 10). Compound 7 was
isolated as white powder, m.p. 77.6—78.1 С, the yield of 0.39 g
(85%). UV (H2O), max/nm (log): 255.0 (3.175). IR (neat),
/cm–1: 2960 (CH), 1751 (CO), 1543, 1499, 1453, 1408, 1368,
1327, 1276, 1230, 1146, 1109, 1058, 996, 912, 848, 789, 772,
681, 640, 619, 579. 1H NMR (300 MHz, CDCl3), : 1.21 (t, 3 H,
SCH2CH3, J = 7.4 Hz); 1.39 (s, 9 H ButC(3)); 1.63 (s, 9 H,
ButO); 2.75—2.50 (diastereotopic m, 2 H, SCH2CH3); 6.16
(s, 1 H, C(4)H); 6.31 (d, 1 H, C(8)H, J = 2.0 Hz); 7.52 (d, 1 H,
C(7)H, J = 2.0 Hz). 13C NMR (APT, 151 MHz, CDCl3), :
13.46 (SCH2CH3), 25.23 (SCH2CH3), 27.65, 28.74 (2 C(CH3)3),
36.94 (C(3)C(CH3)3), 54.97 (C(4)H), 83.35 (OC(CH3)3), 93.24
(C(8)H), 134.60 (C(8a)), 139.87 (C(7)H), 149.10 (C(3)), 152.69
(CO2But). MS: found m/z (Irel (%)) 339.1843 [М + Н] (25),
361.1662 [M + Na] (100), 377.1396 [M + K] (1). C16H26N4O2S.
Calculated: М + Н = 339.1849, M + Na = 361.1669, M + K =
= 377.1408.
This work was financially supported by the Russian
Foundation for Basic Research (project No. 18-33-00019
mol_a; S. M. Ivanov, in the parts of synthesis and bio-
logical activity) and the Russian Science Foundation
(project No. 17-13-01302; V. F. Traven, in the parts of
photolysis and UV spectroscopy).
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Received October 10, 2019;
in revised form December 6, 2019;
accepted December 23, 2019