Imidazo[1,5-a]- and thiazolo[3,4-a]-Quinoxalines based on 3-(α-thiocyano-benzyl)quinoxalin-2(1H)-one

Article abstract of DOI:10.1023/A:1021269602024

When 3-(α-thiocyanobenzyl-2(1H)-one is heated, competing processes of [a]-annelation of the imidazole or thiazole rings occurs with formation of imidazo[1,5-a]- and thiazolo[3,4-a]quinoxalin-4(5H)-ones.

Full text of DOI:10.1023/A:1021269602024

Chemistry of Heterocyclic Compounds, Vol. 38, No. 9, 2002
a
a
IMIDAZO[1,5- ]- AND THIAZOLO[3,4- ]-
QUINOXALINES BASED ON 3-(α-THIOCYANO-
BENZYL)QUINOXALIN-2(1H)-ONE
V. A. Mamedov, A. A. Kalinin, I. Kh. Rizvanov, N. M. Azancheev, Yu. Ya. Efremov,
and Ya. A. Levin
When 3-(α-thiocyanobenzyl-2(1H)-one is heated, competing processes of [a]-annelation of the
imidazole or thiazole rings occurs with formation of imidazo[1,5-a]- and thiazolo[3,4-a]quinoxalin-
4(5H)-ones.
Keywords: imidazo[1,5-a]quinoxalines, thiazolo[3,4-a]quinoxalines, quinoxalines, Kornblum reaction.
The fact that 3-(α-thiocyanobenzyl)quinoxalin-2(1H)-one (1) [1] has endocyclic imine, carbamoyl, and
exocyclic benzylthiocyanate functional groups capable to tautomerism and also the reality of thiocyano–
isothiocyano isomerism makes it possible to obtain various condensed systems based on it. We showed earlier
that in acid medium, the quinoxalinone 1 is isomerized to the tricyclic 1-imino-3-phenylthiazolo[3,4-a]-
quinoxalin-4(5H)-one (2) [1]. This work is devoted to realization of another direction for conversions of
compound 1: to derivatives of the tricyclic imidazo[1,5-a]quinoxaline system.
At the melting point of thiocyanate 1 (205°C), rapid crystallization of the melt occurs with formation of
a high-melting compound that is difficultly soluble even in hot DMSO and DMF. The latter compound is not
identical to compound 2; but judging from elemental analysis and mass spectra (Table 1), it is isomeric to 2 and
also to the starting quinoxaline 1. We can explain these isomeric relationships by the ability of organic
thiocyanates to be easily isomerized to isothiocyanates [2] and we can suggest the next conversions on heating
compound 1, including isomerization of thiocyanate 1 to isothiocyanate 3 and cyclization of the latter as a result
of nucleophilic attack by the N(4) of the quinoxaline ring on the carbon atom of the isothiocyano group. Further
stabilization of the zwitterion intermediate 4 formed occurs by migration of a benzyl proton to the nucleophilic
centers, the N(2) or S atoms, with formation of the end products 5 and/or 6.
13
13
In the C{1H} NMR and C NMR spectra of the solution of the isolated product in DMSO (Table 2),
the signal furthest downfield at 160.33 ppm can be assigned to the carbon atom of the dyad C=N of the
imidazole ring of tautomer 5, since the carbon atom of the N–C(=S)–N moiety in tautomer 6 usually resonates
significantly further downfield (190-210 ppm) [3]. Thus we can choose between tautomeric structures 5 and 6
1
more likely in favor of the former. The H NMR spectrum of the compound obtained in the same solvent
(Table 1) does not contradict this conclusion, since the presence in the spectrum of a singlet peak at 13.72 ppm
(1H) more likely suggests the presence of an SH group rather than the NH group of the thione tautomer: the
latter usually appears in the 8-11 ppm region for solutions of related tautomeric systems in DMSO, while the SH
group of the thiolactim tautomer usually appears near 14 ppm [4-6].
__________________________________________________________________________________________
A. E. Abruzov Institute of Organic and Physical Chemistry, Kazan Science Center, Russian Academy of
Sciences, Kazan 420088, Russia; e-mail: mamedov@iopc.kcn.ru. Translated from Khimiya Geterotsiklicheskikh
Soedinenii, No. 9, pp. 1279-1288, September, 2002. Original article submitted March 27, 2000.
0009-3122/02/3809-1121$27.00©2002 Plenum Publishing Corporation
1121

TABLE 1. Physicochemical and Spectral Characteristics of Synthesized Compounds
Found, %
1
——————
δ
Com-
pound
Empirical
formula
Found, M
H NMR spectrum (DMSO-d6), , ppm
-1
ν
mp*, °C
>360
Yield, %
——————
IR spectrum, , cm
Calculated, %
(spin–spin coupling constant, J, Hz)
Calculated, M
C
H
N
S
C16H11N3OS
293.06015
293.06229
65.85
65.51
4.08
3.78
14.36
14.32
10.88
10.93
1670 (С=О),
1600 (C=C),
1630 (C=N),
7.14-7.83 (8H, m, 5HPh, 6-, 8-H);
10.48 (1H, d, J = 8.25, 9-H);
90 (А);
5
45 (B)
11.34 (1H, s, NH); 13.72 (1H, s, SH)
2680-3210 (NH, SH)
C31H21N5O2S
C23H17N3SO
527.13959
527.14160
69.82
70.57
4.22
4.01
13.87
13.27
5.34
6.08
275-276
285-287
1610 (C=N),
7.02 (1H, s, CHCl); 7.25-7.59 (18H, m, 62 (А);
8
1670 (C=O),
10HPh, 6-, 9-H, 5'-, 8'-H);
30 (B);
2570-3210 (NH)
11.39 (1H, s, NH); 12.72 (1H, s, N'H)
68 (C)
71.53
72.04
4.67
4.47
10.65
10.96
8.67
8.36
1610 (C=N),
4.74 (2H, s, CH2); 7.17-7.52 (11H, m,
5HPh, 6-, 8-H, 2m-H3-Ph, 1p-H3-Ph);
8.23 (2H, br. d, 2o-H3-Ph); 8.30 (1H, d,
J = 8.36, 9-H); 11.30 (1H, br. s, NH)
50
70
15
11
1657 (C=O),
2600-3220 (NH)
C15H10N2O2
71.96
71.99
3.66
4.03
11.02
11.19
—
276-278
1605 (C=N),
1660 (C=O),
1690 (C=O),
2680-3180 (NH)
7.45-8.08 (9H, m, 5HPh, 5-, 8-H);
13
12.92 (1H, s, NH)
C16H10N2O2S
C16H11N3O
294.04575
294.04630
64.93
65.29
3.62
3.42
9.50
9.52
11.02
10.89
312-315
>360
1675 (C=O),
7.17-7.66 (8H, m, 5HPh, 6-, 8-H);
8.90 (1H, d, J = 8.25, 9-H);
11.39 (1H, s, NH)
14
15
2600-3220 (NH)
261.08850
261.09021
73.82
73.55
4.02
4.24
16.19
16.08
—
1620 (C=N),
7.33-7.38 (6H, m, 6-, 8-H, 1p-H3-Ph
,
59 (А);
1660 (C=O),
2m-H3-Ph); 8.31 (2H, br. d, J = 8.80,
2о-H3-Ph); 8.32 (1H, d, J = 8.00, 9-H);
9.22 (1H, s, 1-H); 11.49 (1H, br. s, NH)
11 (C)
2650-3240 (NH)
_______
* Solvent for crystallization: DMF (8, 11), Ac (13), DMSO (14), MeCN–DMSO, 3:2 (15).

TABLE 2. 13C NMR Spectra of Compounds 5 and 15 (DMSO–acetone-d6, 10:1)
δ
Chemical shifts, , ppm (spin–spin coupling constant, J, Hz)
Com-
pound
C=O, s
155.05
C=N
C(3)
C(3a)
C(6), C(9)
С(7), С(8)
C(5a), C(9a)
Ph
160.33
(br. s)
133.37 (s)
130.86 (s) 128,15 (dd, J = 162.96, 6.61); 118.53 (dm, J = 162.52);
122.49 (dd, J = 165.16; 8.81) 116.03 (dm, J = 158.55)
124.85 (m);
116.03 (m)
131.30 (dt, J = 160.75, 6.61, Сp);
130.99 (dt, J = 162.96, 8.15, Со);
128.92 (dd, J = 160.75, 6.61, Сm);
127.71 (t, J = 8.81, Ci)
5
159.04
136.55
137.14
147.85
131.14 (dd, J = 163.65; 7.63); 126.73 (dd, J = 164.80;
132.84 (t, J = 6.87); 130.06 (dt, J = 160.98, 6.86, Со);
120.25 (t, J = 3.82) 131.96 (dt, J = 160.98, 7.63, Сp);
131.73 (dd, J = 160.22, 7.63, Сm);
15
(d, J = 15.15) (t, J = 7.63) (t, J = 4.58)
7.63); 119.25 (d. br. d,
120.88 (dd, J = 163.65; 7.63)
J = 164.41; 7.63)
131.73 (dd, J = 160.22, 7.63, Сm)
TABLE 3. 13C NMR Spectra of Compound 8 (DMSO–acetone-d6, 10:1)
δ
Chemical shifts, , ppm (spin–spin coupling constant, J, Hz)
Com-
C(6), C(9), C(5')
,
pound
C=O
C=N
C(3)
C(3a)
C(7), C(8), C(6'), C(7')
Ci1 or Ci2
PhCH
Ph
C(8')
153.11 (d,
157.85 (br. d,
142.96 (t, 119.07 (d,
114.94 (dd,
116.40 (dm, J = 164.3); 131.78 (m); 51.60 (dt,
128.81 (dt, J = 161.40; 7.10, Со);
8
J = 1.70, C(2)); J = 5.30, C(3')); J = 4.40) J = 14.30) J = 165.5, 7.6); 122.91 (ddd, J = 163.9, 130.79 (m); J = 149.11; 128.49 (dt, J = 159.50; 7.00, Со);
154.07 (s, C(4)) 139.96 (d,
115.93 (dd,
8.3, 1.8);
128.98 (m); 2.60)
128.05 (dd, J = 160.70; 7.70, Сm);
127.10 (dd, J = 159.50; 7.60, Сm);
127.64 (dt, J = 61.50; 7.70, Сp);
127.61 (dt, J = 160.10; 8.00, Сp);
136.35 (br. q, J = 7.20, Сi);
J = 3.20, C(1)
)
J = 163.1; 7.5); 127.91 (dm, J = 162.5); 120.97 (m)
121.78 (dd,
129.87 (dm, J = 153.7)
J = 161.2; 7.0);
126.44 (dd,
J = 160.3; 7.6)
132.10 (t, J = 7.40, Сi)

_
S
NCS
S
N
C
N
N
H
Ph
H
+
N
Ph
205–225 oC
N
Ph
O
O
O
N
H
N
H
N
H
1
3
4
HCl
S
HN
S
H
HS
N
N
Ph
N
N
N
Ph
Ph
O
N
H
O
O
N
H
N
H
2
5
6
Under relatively mild conditions (in DMF solution at 140±5°C), thiocyanate 1 undergoes a different
conversion than at higher temperatures that does not amount to isomerization and cyclization with formation of
compounds 2 or 5. The major reaction product is a compound having the empirical formula C31H21N5O2S;
according to elemental microanalysis and mass spectral determination of the molecular weight, it is composed
of two moieties: C15H11N2O and C16H10N13OS, i.e., of benzylquinoxalinyl and thiocyano- or
isothiocyanobenzylquinoxalinyl, imidazoquinoxalinylthiyl or thiazoloquinoxalinyliminyl residues. Based on
this, a structure of one of the possible isomeric compounds 1-3, 5, 6 may be assigned to the product obtained.
The absence of absorption bands in its IR spectrum from thiocyanate and isothiocyanate groups eliminates
derivatives of bicyclic compounds 1 and 3 from consideration. Since the presence of two one-proton singlets in
1
the H NMR spectrum corresponding to two different types of NH lactam groups and the presence of two
signals from carbon atoms of the C=O group in the 13C spectrum eliminates structures 5, 6 and indicates that the
lactam group of the tricyclic structure remains untouched and there is an additional benzylquinoxalinone
moiety, we consider structures 7-9 next.
A
S
A N
A
S
S
N
N
N
N
N
Ph
Ph
Ph
N
H
7
O
N
H
9
O
N
H
O
8
Ph
N
3'
A =
N
O
H 1'
1124

Ph
N
Cl
N
H
O
10
8
PhCH2S
5
N
N
Ph
PhCH2Cl
N
H
O
11
The absence of a signal for the thiocarbonyl group in the 13C NMR spectrum in the 190-210 ppm region
allows us to eliminate structure 9 from consideration. We cannot choose between structures 7 and 8 based on
spectral data. But making a choice is quite possible based on purely chemical data. Thus as a result of
thermolysis of compound 8 at 280±5°C, the imidazoquinoxaline 5 is formed in good yield, which is evidence in
favor of the imidazoquinoxaline nature of the compound under study, i.e., in favor of structure 8. And finally,
unambiguous proof for such a structure for the product obtained comes from an alternate synthesis of the
product: reaction of imidazoquinoxaline 5 with 3-(α-chlorobenzyl)quinoxalin-2(1H)-one (10) leads to formation
of compound 8, identical to the product formed from thiocyanate 1 in DMF.
Formation of specifically an S-benzylated product on benzylation of imidazo[3,4-a]quinoxaline 5 is also
confirmed by reaction of the latter with unsubstituted benzyl chloride, which leads to the benzylthio compound
11 in high yield. The structure of product 11 is established on the basis of spectral data, like compound 8 there
13
is a signal in its C NMR spectrum from the carbon atom of the C=N group at 154.83 ppm and there is no
signal from the 13C atom of the C=S group in the 190-210 ppm region.
We can explain as follows the fact that when thiocyanate 1 is heated in DMF, instead of the expected
imidazoquinoxaline 5 we obtained its derivative 8:
1
3
5
8 + [HSCN]
Imidazoquinoxaline 5 evidently is formed from compound 1 not only at temperatures higher than 200°C
but also at 140°C (in DMF), but at this lower temperature the thiocyanate substituent in compound 1 begins to
exhibit the ability to play the usually uncharacteristic [2] role of a pseudohalogen, completely analogous to the
role of the chlorine atom in chloride 10 when the latter is used to obtain compound 8 from imidazoquinoxaline
5. This is possible if the activation energy for nucleophilic substitution in thiocyanate 1 with
imidazoquinoxaline 5 is appreciably lower than the activation energy for the overall process of isomerization of
thiocyanate 1 to imidazoquinoxaline 5. Finally we must note that nucleophilic substitution of the thiocyanate
anion in the organic thiocyanate 1 probably is reversible, since if we heat it in DMF as described above but in
the presence of potassium thiocyanate, we can detect intermediate mercapto compound 5 in the reaction mixture
in addition to the starting compound 1 and the end product 8, while if the salt is not added to the solution then
we do not observe intermediate 5. This may be a consequence of slower consumption of the indicated
intermediate as a result of the occurrence of the reverse reaction 8 + -SCN → 1 + 5.
1125

The thiocyanic acid obtained in the course of conversion of thiocyanate 1 to compound 8 should form
salts with the latter and other nitrogen-containing heterocycles found in the reaction mixture, but alkalinization
is not required in isolation of the end products. We know that thiocyanates of weak bases dissociate to an
appreciable degree on heating to form free HSCN, tending toward oligomerization, the product of which is
isolated as a gum.
While annelation of the thiazole ring [1] to a quinoxaline ring occurs when thiocyanate 1 is treated with
acid but annelation of the imidazole ring occurs in neutral medium, in the presence of bases the reaction also is
directed toward imidazoquinoxaline 5: when compound 1 is heated in toluene in the presence of pyridine, it is
specifically this compound that is formed.
We should note that in the mass spectrum of unrecrystallized product 8, when the vaporization
temperature of the sample is raised from 200 to 240°C, in addition to its molecular ion M+ peak (m/z 527) we
observe the appearance of a new peak with m/z 468.1586 of composition C30H20N4O2, which corresponds to
twice the mass of the product of cleavage of HSCN from thiocyanate 1 (the same peak appears under these
conditions in the mass spectral study of thiocyanate 1 itself). It can be considered as the molecular ion of
compound 12, formed by thermal self-condensation of unreacted starting thiocyanate 1 as a result of double
nucleophilic substitution, in which the role of the electrophile is played by the thiocyanobenzyl group, and the
role of the nucleophile is played by the imine nitrogen atom of the quinoxaline part of the molecule (yet another
example of the thiocyano group in compound 1 behaving as a pseudohalogen).
H
O
N
NCS
H
N
N
O
N
..
Ph
Ph
Ph
N
O
–2HSCN
..
N
SCN
Ph
N
H
N
H
O
1
12
When thiocyanate 1 is heated in DMSO under the same conditions as for the heating in DMF considered
above, the direction of the reaction is fundamentally changed. The major product in this case is
3-benzoylquinoxalin-2(1H)-one (13), the formation of which probably can be explained by the occurrence of a
reaction analogous to Kornblum oxidation of organic halides, in which the role of the halogen is played by
thiocyanate 1 and the role of the base is played by the quinoxaline moiety of the same molecule (Scheme 1).
The results of elemental analysis and spectral data (Tables 1 and 4) clearly prove the structure of the
ketone 13 formed, and it is a lactam tautomer both in a 1:5 DMSO–acetone mixture (according to NMR data)
and in the crystalline state (according to IR spectroscopy). The same ketone, obtained by oxidation of
3-benzylquinoxalin-2(1H)-one by chromic anhydride, had a melting point 20°C lower than the one we
determined [7].
We isolate from the reaction mixture, in addition to compound 13, two more minor tricyclic condensed
products 14 and 15, the structure of which has been established from the combined results of elemental analysis,
IR and mass, and also NMR spectra (Tables 1 and 2). Their formation may be explained on the basis of the
above-considered rearrangement of thiocyanate 1 to isothiocyanate 3 and isomerization of compounds 1 and 3 to
the tricyclic condensed systems 2 and 5 respectively, occurring when thiocyanate 1 is heated. As an
intermediate, thiazole quinoxaline 2 can lead to thiazolinoquinoxaline end product 14 both through preliminary
nucleophilic addition of DMSO to the imino group with intermediate formation of compound 16, and as a result
1126

Scheme 1
_
Ph
O
SCN
Ph
O
Ph
O
H
SCN
H
O
N
N
+
N
SMe2
O
DMSO
+
–Me2S
–[HSCN]
N
H
1
N
H
N
H
+
–H
13
_
–SCN
_
Ph
Ph
_
Me
H
CH2
N
+
S
N
+
S
O
Me
O
Me
N
H
O
N
H
O
–Me2S
of hydrolysis of the same imino derivative 2 by uncontrollable water impurity to the oxo derivative of
heterocyclic system 17. Taking isothiocyanate 3 as the starting point, the tricyclic imidazoquinoxaline system
can lose the sulfur-containing functional group by oxidation of mercapto derivative 5 with the help of DMSO to
form sulfinic acid 18, followed by cleavage of SO2. A similar loss of a sulfur atom under oxidation conditions
by heterocyclic compounds capable of thiolactam/thiolactim tautomerism (oxidative dethionation, essentially
leading to the reduction product) is known for a number of such systems when they are treated with various
oxidants [8]. This assumption is completely confirmed by the fact that heating compound 5 in DMSO at 140°C
indeed leads to their loss of sulfur and conversion to compound 15.
HO2S
N
N
N
N
Ph
Ph
2Me2SO
–2Me2S
5
–SO2
N
H
O
N
H
O
1
3
18
15
H2O
2
Me2SO
Me
..
+
Me
S
N
H
O
O
S
S
N
Ph
N
Ph
N
H
16
O
[Me2SNH]
N
H
14
O
1127

TABLE 4. 13C NMR Spectrum of Benzoylquinoxaline 13 (DMSO–
acetone-d6, 10:1)
Chemical shifts, δ, ppm, (spin-spin coupling constant, J, Hz)
C=N C(5), C(8), dd C(7), C(6), dd C(i), C(i2), m
Com-
pound
C=O
Ph
191.79
(br. s,
155.74 131.09 128.50 130.65;
129.03 (dt, J = 161.9;
6.50, Со); 133.95 (dt,
J = 163.2; 7.20, Сp);
128.39 (dd, J = 163.3;
3.74, Сm); 134.02
13
(br. s)
(J = 161.10; (J = 163.40; 132.12
PhC=O);
152.77
(s, C2)
8.10);
8.20);
123.17
115.31
(J = 163.40; (J = 164.00;
8.20)
8.10)
(t, J = 7.11, Ci)
EXPERIMENTAL
The melting points were determined on a Boetius stage. The IR spectra were taken on a UR-20
spectrometer (vaseline mulls). 1H NMR spectra were recorded on a Bruker MCL-250 spectrometer (250 MHz).
The 13C NMR spectra were taken on a Bruker WV-400 (100 MHz) spectrometer. The chemical shifts were
given on a δ scale relative to DMSO-d6. The molecular weights of the ions (electron impact) were recorded on
an MX-1310 mass spectrometer with resolution R = 10 000. Sample injection was done using an SVP-5 direct
injection system for an ion source temperature of 160°C; U = 60 eV.
3-(α-Thiocyanobenzyl)quinoxalin-2(1H)-one (1) was synthesized from 3-(α-chlorobenzyl)quinoxalin-
2(1H)-one and potassium thiocyanate [1].
3-(α-Chlorobenzyl)-2(1H)-one (10) was obtained using the familiar procedure in [9] from the methyl
ester of phenylchloropyruvic acid [10] and o-phenylenediamine.
When the same compound was synthesized by different methods, the match between their physical and
spectral properties and also the absence of a depression of the melting point for a mixed sample established that
the products were identical.
a
1-Mercapto-3-phenylimidazo[1,5- ]quinoxalin-4(5H)-one (5).
1
A. Thiocyanate (0.50 g, 1.70 mmol)
was heated up to 225°C in a test tube and held at that temperature for 5 min. At 205°C, the compound melts and
then crystallizes. After cooling, the crystals of product 5 were triturated with CF3COOH (2 ml), filtered out and
washed with acetone (2 × 3 ml).
B. Compound 8 (0.12 g, 0.23 mmol) was heated to 290°C in a test tube and then cooled; 5 ml of dioxane
was added, it was brought to the boiling point, then product 5 was filtered out and washed with acetone
(2 × 3 ml).
a
1-(α-2-Oxo-1,2-dihydro-3-quinoxalinyl)benzylthio-3-phenylimidazo[1,5- ]quinoxalin-4(5H)-one (8).
A. A solution of thiocyanate 1 (2.70 g, 9.2 mmol) in DMF (20 ml) was held for 3 h at 135°C to 145°C. The
crystals that precipitated on cooling were filtered out, washed with 2-propanol (2 × 5 ml), and recrystallized
from DMF. To remove residues of DMF, the product 8 obtained was held for 3 h under an oil pump vacuum at
160-200°C.
B. A solution of thiocyanate 1 (0.30 g, 1.0 mmol) in a mixture of toluene (3 ml) and pyridine (0.5 ml)
was refluxed for 3 h. The reaction mixture was cooled, the crystals of product 8 were filtered out and washed
with 2-propanol (2 × 3 ml).
C. A solution of compound 5 (0.20 g, 0.68 mmol) and chloride 10 (0.20 g, 7.4 mmol) in DMF (15 ml)
was held at 100-110°C for 1 h 30 min, cooled and held for ~16 h at room temperature. The crystals of product 8
that precipitated were filtered out and washed with acetone (2 × 5 ml).
1128

a
1-Benzylthio-3-phenylimidazo[1,5- ]quinoxalin-4(5H)-one (11).
5
A solution of compound (0.20 g,
0.68 mmol) and benzyl chloride (1 ml) in DMF (10 ml) was held for 1 h 30 min at 100-110°C. The solution was
cooled and held for ~16 h at room temperature. The crystals of product 11 that precipitated were filtered out and
washed with 2-propanol (2 × 5 ml). Water was added to the filtrate, the crystals of product 11 formed were
filtered out and washed with 2-propanol.
a
3-Phenylimidazo[1,5- ]quinoxalin-4(5H)-one (15).
5
A. A solution of compound (0.50 g) in DMSO
(7 ml) was held for 1 h at 135-155°C, cooled and poured into water (20 ml). The crystals of product 15 that
precipitated were filtered out and then washed with water and 2-propanol.
Reaction of 3-(α-Thiocyanobenzyl)quinoxalin-2(1H)-one (1) with DMSO. A solution of compound 1
(1.60 g, 5.50 mmol) [1] in DMSO (25 ml) were held for 1 h at 135-145°C. The solution was cooled and poured
into 50 ml water. The crystalline mixture of reaction products was filtered out, dried, and recrystallized from a
2:3 DMSO–MeCN mixture. The mixture of compounds 14 and 15 was filtered out. The filtrate was poured into
water; then crystals of 3-benzoylquinoxaline-2(1H)-one (13) precipitated. The crystalline mixture of compounds
14 and 15 was separated by recrystallization from DMSO; 3-phenylthiazolo[3,4-a]quinoxaline-1,4(5H)-dione
(14) precipitated under these conditions, while compound 15 was precipitated from the DMSO mother liquor by
water.
REFERENCES
1.
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