Reactions of 3,4-dichloro-N-R-maleimides with substituted 2-thiouracils

Article abstract of DOI:10.1023/B:COHC.0000023775.37226.f1

Reactions of 3,4-dichloro-N-R-maleimides with substituted thiouracils at 40°C gave a 1:1 mixture of isomers of pyrrolothiazolopyrimidinetriones. Under conditions of thermodynamic control (100°C, 5 h) only pyrrolo[3′,4′:4,5]thiazolo[3,2-a]pyrimidine-4,6,8-triones were formed, hydrolysis of which followed by decarboxylation gave 5-oxo-5H-thiazolo[3,2-a]pyrimidine-2-carboxamide. The structure of N ²-phenyl-6-methyl-5-oxo-5H-thiazolo[3,2-a]pyrimidine-2-carboxamide was confirmed by X-ray crystallography. Analogous cyclization of 3,4-dichloro-N-R-maleimides with 2-thioxoquinazol-4-one also gave a mixture of two isomers which were successfully separated by fractional crystallization.

Full text of DOI:10.1023/B:COHC.0000023775.37226.f1

Chemistry of Heterocyclic Compounds, Vol. 40, No. 1, 2004
REACTIONS OF 3,4-DICHLORO-
N-R-MALEIMIDES WITH
SUBSTITUTED 2-THIOURACILS
Yu. M. Volovenko1, G. G. Dubnina1, and A. N. Chernega2
Reactions of 3,4-dichloro-N-R-maleimides with substituted thiouracils at 40°C gave a 1:1 mixture of
isomers of pyrrolothiazolopyrimidinetriones. Under conditions of thermodynamic control (100°C, 5 h)
only pyrrolo[3',4':4,5]thiazolo[3,2-a]pyrimidine-4,6,8-triones were formed, hydrolysis of which
followed by decarboxylation gave 5-oxo-5H-thiazolo[3,2-a]pyrimidine-2-carboxamide. The structure of
N2-phenyl-6-methyl-5-oxo-5H-thiazolo[3,2-a]pyrimidine-2-carboxamide was confirmed by X-ray
crystallography. Analogous cyclization of 3,4-dichloro-N-R-maleimides with 2-thioxoquinazol-4-one
also gave a mixture of two isomers which were successfully separated by fractional crystallization.
Keywords:
7-R-2-R2-3-R1-7,8-dihydro-4H,6H-pyrrolo[3',4':4.5][1,3]thiazolo[3,2-a]pyrimidine-4,6,8-
triones, N2-R-6-R1-5-oxo-5H[1,3]thiazolo[3,2-a]pyrimidine-2-carboxamide, nucleophilic substitution in
3,4-dichloro-N-R-maleimides.
In 1989-1990 Katritzky [1,2] described the formation of the pyrrolo-[3,4-d]thiazole ring as a result of the
reaction 3,4-dichloro-N-phenylmaleimide with thioamides (thiourea, thoacetamide, dithiooxamide). Japanese
authors later reported other products from the reaction of substituted maleimides with dithiooxamide –
tetrahydro-1H,5H-[1,4]dithiino[2,3-b:6,5-b']dipyrrole [3], but in 1993 Katritzky showed that pyrrolo[3,4-d]-
thiazoles were only obtained in negligible quantities from the maleimides with thioamides, while the main
products of the reactions are tetrahydro-1H,5H-[1,4]dithiino[2,3-b;6,5-b']dipyrroles [4].
In the present work we have studied the interaction of 3,4-dichloro-N-R-maleimides 1 with 2-thiouracils
2 which contain three nucleophilic centers: the sulfur atom and the nitrogen atoms at positions 1 and 3.
In mild conditions (30-40°C) the reaction did not stop at the replacement of just one chlorine atom of the
imide 1 but proceeded to cyclization as a result of which a mixture of the isomers 3 and 4 was formed in
approximately equal amounts (to judge from the 1H NMR spectrum) (Scheme 1).
When the reactions were carried out under more vigorous conditions (100°C, 5 h) only the isomers 3a-m
were formed. We did not succeed to isolate isomers 4 in pure form. Calculation of the energies of the isomers
with structures 3 and 4 was carried out using the AM1 method (Hyper Chem 5.0, grad = 0.01 kcal/mol, suite of
programs) showed that isomer 3 has a lower energy than isomer 4 (∆E = 7-10 kcal/mol). Evidently in the boiling
reaction mixture isomers 4 are converted to the more thermodynamically stable isomers 3 which is similar to
what occurs in the Dimroth rearrangement [5].
__________________________________________________________________________________________
1
Taras Shevchenko Kiev National University, Kiev 01033, Ukraine; e-mail: otava@carrier.kiev.ua.
2 Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Kiev 02094, Ukraine. Translated
from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 100-106, January, 2004. Original article submitted
June 6, 2001.
94
0009-3122/04/4001-0094©2004 Plenum Publishing Corporation

Scheme 1
O
O
N
R1
R2
Cl
Cl
HN
O
O
100 oC
;
Et3N
+
R
S
N
H
2
30–40 oC
O
R2
1
O
O
R1
O
N
R1
R
R
N
N
N
+
O
2
R
N
N
S
S
O
O
3a–m
4
1
1
2
2
R = CH Ph, Ar;
R = H, Me, R , R = CH –CH –CH ;
R = H, Me, n-Pr
2
2
2
2
The IR spectra of compounds 3a-m contain two carbonyl stretching bands of the maleimide ring at
1785-1775 (as) and 1730-1715 (s) cm-1. The carbonyl stretching vibration of the pyrimidine ring is observed at
1715-1685 cm-1, which corresponds to data [6] characteristic for isomers of type 3.
2-Thioxo-1,2,3,4-tetrahydro-4-quinazolone (5), a benzo-annelated derivative of 2-thiouracil, also reacted
with dichloromaleimides 1 ( R = 4-MeC6H4 or 4-MeOC6H4) under mild conditions to form a mixture of isomers
6 and 7 in equal amounts (according to the 1H NMR spectra).
O
NEt3
HN
1
+
O
O
S
N
H
5
O
O
O
R
+
N
N
R
N
N
O
N
N
O
S
O
S
1 : 1
6a,b
7a,b
6, 7 a R = 4-MeC6H4; b R = 4-MeOC6H4
However in this case heating the reaction mixture at 100°C for 5h gave only a negligible increase in the
quantity of isomer 7. Compounds 6b, 7a,b were isolated in pure form by fractional crystallization. The 1H NMR
spectrum of isomer 6b permitted the elucidation of the positions of the resonances of the protons of isomer 6a by
1
difference from the H NMR spectra of the mixture of isomers (Table 1). Isomers 6a,b are less soluble in
dioxane than isomers 7a,b, they are higher melting, and are chromatographically less mobile (Rf (6) = 7.8;
Rf (7) = 8.8).
As a result of the angular structure of the planar molecules of isomers 6a,b, the proton in position 1 is in
1
the same plane as the carbonyl groups of the maleimide ring. In the H NMR spectra of compounds 6a,b the
deshielding effect of the carbonyl groups cause a considerable shift of the proton of this signal to weak field:
9.11 (6a) and 9.08 ppm (6b) (1H, d). In the 1H NMR spectra compounds 7a,b the proton at position 9 appears at
weakest field at 8.31 and 8.29 ppm (1H, d). In the IR spectra of compounds 6b, 7a,b the two bands
corresponding to the carbonyl groups of the maleimide ring are observed at 1785-1770 (as) and
95

1720-1715 (s) cm-1. Vibrations of the carbonyl group of the quinazoline nucleus were observed at 1700 for
isomers 7a,b and at 1650 cm-1 for isomer 6b. Refluxing compounds 3 in aqueous dioxane in he presence of an
equimolar amount of triethylamine led to hydrolysis and opening of the maleimide ring with subsequent
decarboxylation to give compounds 8a-c.
O
R1
R2
62–85%
H
;
R
O
O
;
H2O
NEt3
N
N
N
3
S
O
– CO2
8a–c
1
The HNMR spectra of 8a-c include a singlet for the proton of the thiazole ring in the region of
8.99-9.14 ppm. The signal of the amide proton, which disappears on the addition of D2O, is observed in the
10.39-10.71 ppm region. The signals of the protons of the substituents R, R1, and R2 are cited in Table 1.
The carbonyl stretching frequencies of the maleimide ring are absent from the IR spectra of compounds
8a-c. The stretching vibrations of the amide carbonyl group and the carbonyl group of the pyrimidine ring
appear in the 1650-1640 cm-1 region. The N–H stretching vibrations are in the region of 3290-3280 cm-1and the
band for the C–H bond of the thiazole ring is in the 3080-3060 cm-1 region.
The structure of compound 8b was confirmed by X-ray crystallography (Fig. 1, Table 2) using the
CAMERON method [7]. The bicyclic system S(1)N(1)N(2)C(1-6) is planar within 0.026 A, and the interfacial angle
between the S(1)N(1)C(1-3) and N(1)N(2)C(3-6) rings is only 1.9°. The geometric parameters of the cyclic systems
indicate considerable electron density delocalization [8,9]. The exocyclic carboxamide fragment
C(1)C(8)O(2)N(3)C(9) is also almost planar (torsion angles C(1)-C(8)-N(3)-C(9) and O(2)–C(8)–N(3)–C(9) are 174.5 and
4.2°) and forms an interfacial angle with the bicyclic plane S(1)N(1)N(2)C(1-6) of 7.8°Atom N(3) has a planar trigonal
configuration of bonds: the sum of the angles is 360°. The benzene ring C(9-14) is rotated relative to the plane
C(1)C(8)O(2)N(3)C(9) by 28.1°. The molecules of compound 8b are connected into a zigzag chain in the crystal and
form intermolecular bonds N(3)–H(3)···O(1) with the following parameters: N(3)···O(1) 2.877(3), H(3)···O(1) 2.12(3),
N(3)–H(3) 0.81(3) A, N(3)H(3)O(1) 157(2)°.
Fig. 1. Molecular structure of compound 8b.
96

TABLE 1. Characteristics of Compounds 3a-m, 6a,b, 7a,b, 8a-c
Found, %
Calculated, %
——————
Com-
pound
Empirical
formula
R*
2
1Н NMR spectrum, δ, ppm (J, Hz)
mp, °C
Yield, %
N
4
S
5
1
3a
3
6
7
8
CH2Ph
С15H9N3O3S
13.42
13.50
10.23
10.30
4.78 (2Н, s, СН2); 6.42 (1H, d, J = 6.0, C(3)-H);
7.22-7.39 (5H, m, HAr); 8.03 (1Н, d, J = 6.0, C(2)-H)
179-180
65
3b
3-MeC6H4
С15H9N3O3S
С15H9N3O4S
13.59
13.50
10.10
10.30
2.38 (3Н, s, СН3); 6.50 (1H, d, J = 6.0, C(3)-H);
7.19-7.22 (2H, m, 2'- and 4'-HAr); 7.30 (1H, d, J = 9.0, 6'-HAr);
7.44 (1H, t, J = 7.8, 5'-HAr); 8.04 (1Н, d, J = 6.0, C(2)-H)
212-213
189-190
58
60
3c
4-MeOC6H4
13.07
12.84
9.56
9.79
3.80 (3Н, s, OСН3); 6.43 (1H, d, J = 6.0, C(3)-H);
7.08 (2H, d, J = 8.7, 3'- and 5'-HAr);
7.32 (2H, d, J = 8.7, 2'- and 6'-HAr); 8.05 (1Н, d, J = 6.0, C(2)-H)
3d
3e
3f
CH2Ph
Ph
С16H11N3O3S
С15H9N3O3S
С17H13N3O3S
С18H15N3O3S
13.04
12.92
9.99
9.85
2.30 (3Н, s, СН3); 4.78 (2Н, s, СН2); 6.32 (1H, s, C(3)-H);
7.2-7.3 (5H, m, HAr)
185-186
193-194
191-192
151-152
61
53
62
58
13.32
13.50
10.17
10.30
2.19 (3Н, s, СН3); 7.2-7.4 (3H, m, 3'-, 4'- and 5'-HAr);
7.72 (2H, d, J = 7.5, 2'- and 6'-HAr); 8.03 (1H, s, C(2)-H)
Ph
12.50
12.38
9.37
9.45
0.95 (3H, t, J = 7.2, CH3); 1.68 (2H, m, CH2-CH2-CH3); 2.58 (2H, t,
J = 7.2, CH2-CH2-CH3); 6.36 (1H, s, C(3)-H); 7.3–7.6 (5H, m, HAr)
3g
CH2Ph
12.09
11.89
9.28
9.07
0.93 (3H, t, J = 7.2, CH3); 1.65 (2H, m, CH2-CH2-CH3);
2.53 (2H, t, J = 7.0, CH2-CH2-CH3); 4.78 (2Н, s, C6H5СН2);
6.30 (1H, s, C(3)-H); 7.2-7.4 (5H, m, HAr)
3h
2-MeC6H4
С18H15N3O3S
12.12
11.89
9.31
9.07
0.92 (3H, t, J = 7.3, CH3); 1.67 (2H, m, CH2-CH2-CH3);
2.19 (3H, s, CH3); 2.58 (2H, t, J = 7.2, CH2-CH2-CH3);
6.38 (1H, s, C(3)-H); 7.30-7.45 (4H, m, HAr)
214-215
59
3i
Ph
С17H11N3O3S
С18H13N3O3S
С18H13N3O3S
12.38
12.46
9.48
9.50
2.08 (2H, m, C(7)-H); 2.76 (2H, t, J = 7.6, C(6)-H);
2.90 (2H, t, J = 7.6, C(8)-H); 7.4-7.55 (5H, m, HAr)
233–234
222-223
209-210
69
55
67
3j
3k
2-MeC6H4
4-MeC6H4
12.05
11.96
9.18
9.12
2.09 (2H, m, C(7)-H); 2.18 (3H, s, CH3); 2.76 (2H, t, J = 7.5, C(6)-H);
2.91 (2H, t, J = 7.5, C(8)-H); 7.3-7.45 (4H, m, HAr)
11.88
11.96
9.15
9.12
2.08 (2H, m, C(7)-H); 2.37 (3H, s, CH3); 2.75 (2H, t, J = 7.6, C(6)-H),
2.90 (2H, t, J = 7.6, C(8)-H); 7.26 (2H, d, J = 8.4, 3'- and 5'-HAr);
7.34 (2H, d, J = 8.4, 2'- and 6'-HAr)

TABLE 1 (continued)
1
2
3
4
5
6
7
8
3l*2
4-ClC6H4
С17H10ClN3O3S
11.44
11.30
8.70
8.62
2.08 (2H, m, C(7)-H); 2.75 (2H, t, J = 7.6, C(6)-H);
2.90 (2H, t, J = 7.6, C(8)-H); 7.43 (2H, d, J = 8.7, 3'- and 5'-HAr);
7.61 (2H, d, J = 8.7, 2'- and 6'-HAr)
228-29
53
3m*3
6а
2,3-Cl2C6H3
4-MeC6H4
С17H9Cl2N3O3S
С19H11N3O3S
10.42
10.34
8.08
7.89
2.10 (2H, m, C(7)-H); 2.77 (2H, t, J = 7.6, C(6)-H);
2.91 (2H, t, J = 7.6, C(8)-H); 7.58 (2H, m, 4'- and 5'-HAr);
7.87 (1H, d, J = 9.0, 6'-HAr)
2.40 (3Н, s, СН3); 7.26 (2Н, d, J = 8.2, 3'- and 5'-HAr);
7.37 (2Н, d, J = 7.8, 2'- and 6'-HAr); 7.75 (1H, t, J = 8.1, C(3)-H);
8.0 (1H, t, J = 8.1, С(2)-Н); 8.26 (1H, d, J = 7.8, C(4)-H);
9.11 (1H, d, J = 9.0, C(1)-H)
232-233
58
11.63
8.87
6b
7a
7b
4-MeOC6H4
4-MeC6H4
С19H11N3O4S
С19H11N3O3S
С19H11N3O4S
11.19
11.13
8.54
8.50
3.82 (3Н, s, OСН3); 7.13 (2Н, d, J = 8.4, 3'- and 5'-H);
7.39 (2Н, d, J = 7.8, 2'- and 6'-HAr); 7.72 (1H, t, J = 8.1, C(3)-H);
7.98 (1H, t, J = 8.4, С(2)-Н); 8.22 (1H, d, J = 8.1, C(4)-H);
9.08 (1H, d, J = 8.1, C(1)-H)
279-280
245-246
234-235
11
23
26
11.57
11.63
8.68
8.87
2.38 (3Н, s, СН3); 7.29 (2Н, d, J = 8.1, 3'- and 5'-HAr);
7.35 (2Н, d, J = 7.8, 2'- and 6'-HAr); 7.62 (1Н, t, J = 8.1, С(8)-H);
7.72 (1Н, d, J = 8.1, C(6)-H); 7.95 (1Н, t, J = 8.1, С(7)-H);
8.31 (1Н, d, J = 8.1, C(9)-H)
4-MeOC6H4
11.25
11.13
8.46
8.50
3.82 (3Н, s, OСН3); 7.09 (2Н, d, J = 9.0, 3'- and 5'-HAr);
7.34 (2Н, d, J = 8.7, 2'- and 6'-HAr); 7.61 (1Н, t, J = 8.1, С(8)-H);
7.70 (1Н, d, J = 7.8, C(6)-H); 7.94 (1Н, t, J = 7.8, С(7)-H);
8.29 (1Н, d, J = 8.1, C(9)-H)
8a
8b
8с
4-MeOC6H4
Ph
С14H11N3O3S
С14H11N3O2S
С17H15N3O2S
14.05
13.95
10.57
10.64
3.75 (3H, s, OCH3); 6.33 (1H, d, J = 6.0, C(6)-H);
6.95 (2Н, d, J = 8.0, 3'- and 5'-HAr); 7.61 (2Н, d, J = 8.0, 2'- and 6'-HAr);
8.06 (1H, d, J = 6.0, C(7)-H); 9.09 (1H, s, C(3)-H); 10.57 (1H, s, NH)
276-277
242-243
253-254
77
85
62
14.81
14.73
11.10
11.24
2.06 (3H, s, CH3); 7.16 (1H, t, J = 7.5, 4'-HAr);
7.39 (2H, t, 3'- and 5'-HAr); 7.71 (2H, d, J = 7.3, 2'- and 6'-HAr);
8.02 (1H, s, C(7)-H); 9.14 (1H, s, C(3)-H); 10.71 (1H, s, NH)
4-MeC6H4
13.08
2.91
10.00
9.85
2.03 (2H, m, C(7)-H); 2.28 (3H, s, CH3); 2.75 (2H, t, J = 7.8, C(8)-H);
2.85 (2H, t, J = 7.8, C(6)-H); 7.17 (2H, d, J = 8.4, 3'- and 5'-HAr);
7.54 (2H, d, J = 8.4, 2'- and 6'-HAr); 8.99 (1H, s, C(3)-H);
10.39 (1H, s, NH)
_______
* 3a-d,f-h, 8a R1 = H; 3e, 8b R1 = Me; 3i-m, 8c R1+R2 = (CH2)3; 3a-s,e, 8a,b R2 = H; 3d R2 = Me; 3f-h R2 = n-Pr.
*2 Found, %: Cl 9.62; calculated, %: Cl 9.54
*3 Found, %: Cl 17.42; calculated, %: Cl 17.45.

TABLE 2. Bond Lengths (d) and Bond Angles (ω) in the Molecule of
Compound 8b
Bond
d, Å
Angle
ω, deg.
S(1)–C(1)
S(1)–C(3)
O(1)–C(6)
O(2)–C(8)
N(1)–C(2)
N(1)–C(3)
N(1)–C(6)
N(2)–C(3)
N(2)–C(4)
C(1)–C(2)
C(1)–C(8)
C(4)–C(5)
C(5)–C(6)
N(3)–C(9)
N(3)–C(8)
1.740(3)
1.731(3)
1.226(3)
1.220(3)
1.395(3)
1.373(3)
1.413(3)
1.304(4)
1.362(4)
1.337(4)
1.491(4)
1.354(4)
1.428(4)
1.419(4)
1.341(4)
S(1)–C(3)–N(1)
N(1)–C(3)–N(2)
N(2)–C(4)–C(5)
C(4)–C(5)–C(6)
N(1)–C(6)–C(5)
O(2)–C(8)–N(3)
O(2)–C(8)–C(1)
N(3)–C(8)–C(1)
N(1)–C(2)–C(1)
C(3)–N(1)–C(6)
C(3)–N(2)–C(4)
C(8)–N(3)–C(9)
C(2)–N(1)–C(3)
C(1)–S(1)–C(3)
S(1)–C(1)–C(2)
110.7(2)
124.7(3)
126.5(3)
119.4(3)
113.0(2)
125.9(3)
118.6(2)
115.4(2)
112.5(2)
122.1(2)
114.3(2)
127.2(2)
113.8(2)
90.31(14)
112.6(2)
EXPERIMENTAL
The course of reactions and the purity of the synthesized compounds were monitored by TLC (Silufol
UV-254m eluent 9:1 chloroform–methanol). IR spectra of KBr discs were recorded with UR-20, Specord IR-75,
1
and Pye-Unicam instruments, H NMR spectra of DMSO-d6 solutions with TMS as internal standard were
recorded with a Varian machine (300 MHz).
The X-ray Crystallographic Study of a monocrystal (0.25 × 0.31 × 0.59 mm) of compound 8b was
carried out at room temperature with an automatic four circle Enraf-Nonius CAD-4 diffractometer (MoKα
radiation, relative rate of scanning 2θ/ω = 1.2, θmax = 65°, segment of the sphere 0 < h < 12, 0 < k < 8,
-17 < l < 17). 2306 reflexions were collected of which 2006 were symmetrically independent (Rint = 0.015).
Crystals of compound 8b are monoclinic, C14H11N3O2S; M = 285.32; a = 11.476(4), b = 7.471(4), c = 15.395(7) Å;
β = 100.62(3)°; V = 1297.3 A3; Z = 4; d = 1.46 g/cm3; space group P21/n; µ = 2.42 cm-1; F(000) = 593. The
structure was solved by direct methods and refined by least squares in the complete matrix anisotropic
approximation with the CRYSTALS suite of programs [10]. For the refinement 1748 reflexions with I > 3σ(I)
were used. All hydrogen atoms were found from difference syntheses of the electron density and were included
in the conclusion in fixed positions and thermal parameters (only atom H(3) was refined isotropically).
Calculation of absorption in the crystal was carried out using azimuthal scanning [11]. The Chebyshev weighting
scheme [12] with parameters of 1.61, 0.90, 1.51, -0.22, and 0.38 was used in the refinement. The final residual
factors were R = 0.041, Rw = 0.046, GoF 1.137. The complete set of crystallographic data, including atomic
coordinates and their anisotropic thermal parameters have been deposited in the Cambridge Crystal Structure
Bank (CCDC 155191).
7-R-2-R2-3-R1-7,8-Dihydro-4H,6H-pyrrolo[3',4':4,5]thiazolo[3,2-a]pyrimidine-1,3,9-triones 3i-m. A
solution of the corresponding dichloromaleimide 1 (10 mmol) ( R = 4-MeC6H4, 4-MeOC6H4) and
2-thioquinazolone 2 (1.78 g, 10 mmol), and triethylamine (1.52 g, 15 mmol) in dry dioxane (20 ml) was refluxed
for 5h. Ethanol (10 ml) was added to the cooled solution. The precipitate was filtered off, washed with ethanol,
and recrystallized from dry dioxane.
Mixtures of isomers 3 and 4 were prepared in milder conditions analogous to the method used to prepare
the mixtures of isomers 6a + 7a and 6b + 7b.
99

9-R,9,10-Dihydro-5H,8H-pyrrolo[3',4':4,5]thiazolo[3,2-a]quinazoline-5,8,10-triones 6a,b; 2-R-2,3-Di-
hydro-1H,10H-pyrrolo[3',4':4,5]thiazolo[2,3-b]quinazoline-1,3,10-triones 7a,b. To a solution of the
corresponding dichloromaleimide 1 (10 mmol) ( R = 4-MeC6H4, 4-MeOC6H4) and 2-thioquinazolone 2 (1.78 g,
10 mmol) in dry dioxane (25 ml), triethylamine (1.52 g, 15 mmol) was added dropwise at 30-40°C. The mixture
was then stirred for 4 h at 35-45°C and then kept at room temperature for 8h. The precipitate was filtered off and
washed with a small amount of dioxane and water. The precipitate was a mixture of approximately equal
amounts of isomers 6 and 7. The overall yields of the mixtures of isomers were 55 (6a + 7a) and 67% (6b + 7b).
Compounds 6b (11%), 7a (23%), and 6a (26%) were obtained in pure form by fractional crystallization from dry
dioxane.
N2-R-6-R1-5-Oxo-5H-thiazolo[3,2-a]pyrimidine-2-carboxamides 8a,b; N2(4-methylphenyl-5-oxo-
5,6,7,8-tetrahydrocyclopenta[f]thiazolo[3,2-a]pyrimidine-2-carboxamide (8c) (Table 1). A solution of the
corresponding compound 3 (3 mmol), triethylamine (0.303 g, 3 mmol), and water (1 ml) in dioxane (10 ml) was
boiled for 1 h and then the reaction mixture was kept at room temperature for 8 h. The precipitate was filtered off
and washed with a small amount of dioxane and water. It was recrystallized from DMF.
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