Synthesis of N1-substituted 1,2,3,6-tetrahydropyrimidin-2-ones via an unexpected reaction of thiazolo[3,2-a]-pyrimidine derivatives and nitrile oxide

Article abstract of DOI:10.1002/cjoc.201090042

A new class of N1-substituted 1,2,3,6-tetrahydropyrimidin-2-ones was prepared in moderate yields by the reaction of nitrile oxide with thiazolo[3,2-a]pyrimidine derivatives via a domino 1,3-dipolar cycloaddition/ring-opening/ substitution process. The structures of the products were characterized thoroughly by IR, elemental analysis, MS, NMR together with X-ray crystallographic analysis.

Full text of DOI:10.1002/cjoc.201090042

FULL PAPER
Synthesis of N1-Substituted 1,2,3,6-Tetrahydropyrimidin-
2-ones via an Unexpected Reaction of Thiazolo[3,2-a]-
pyrimidine Derivatives and Nitrile Oxide
Li, Xiaofang(李筱芳)
Yi, Pinggui*(易平贵)
Yu, Xianyong*(于贤勇)
Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan Province
College Key Laboratory of QSAR/QSPR, School of Chemistry and Chemical Engineering, Hunan University of
Science and Technology, Xiangtan, Hunan 411201, China
A new class of N1-substituted 1,2,3,6-tetrahydropyrimidin-2-ones was prepared in moderate yields by the reac-
tion of nitrile oxide with thiazolo[3,2-a]pyrimidine derivatives via a domino 1,3-dipolar cycloaddition/ring-opening/
substitution process. The structures of the products were characterized thoroughly by IR, elemental analysis, MS,
NMR together with X-ray crystallographic analysis.
Keywords 1,2,3,6-tetrahydropyrimidin-2-one, cycloaddition, thiazolo[3,2-a]pyrimidine, nitrile oxide, synthesis
Introduction
bonylation, and alkylation) of N1 position of THPM is
of considerable importance for the preparation of bio-
logically important THPM.6
Tetrahydropyrimidone (THPM) is an important type
of heterocyclic compounds,1 which have wide range
biological responses and can be used as calcium channel
modulators, antihypertensive agents, mitotic kinesin
Eg5 inhibitors, and melanin concentrating hormone re-
ceptor (MCH1-R) as antagonists.2-5 Some classical
compounds among them are shown in Figure 1. On the
other hand, the functionalization (acylation, alkoxycar-
Spiro-compounds also are an important class of or-
ganic compounds because of their biological activi-
ties.7,8 One of the most widely used methods for the
synthesis of these compounds is via 1,3-dipolar
cycloaddition reaction to exocyclic double bonds.9,10 In
the present work, we report the results of our attempt to
apply the 1,3-dipolar cycloaddition reaction involving
ethyl 2-[(methoxycarbonyl)methylidene]-2,3-dihydro-6-
methyl-3-oxo-4-[(un)substituted phenyl]-5H-thiazolo-
[3,2-a]pyrimidine-5-carboxylate (1) and nitrile oxide (2).
Such an approach, however, resulted in a synthesis of
various N1-substituted 1,2,3,6-tetrahydropyrimidin-2-
ones 4 instead of the expected spiroisoxazoline 3, as
shown in Scheme 1.
What is more, substances containing isoxazole
building blocks are frequently used in drug discovery as
an important bioisostere to improve pharmacokinetic
properties of drug candidates.11,12 Therefore, we believe
that our present results should be useful for the synthe-
sizing bioactive complex N1-substituted 1,2,3,6-tetra-
hydropyrimidin-2-ones.
Experimental
Materials and instruments
113 and 214 were prepared according to the reported
procedures. All NMR spectra were recorded on a
Figure 1 Biologically active THPM.3
*
E-mail: fine_chem@163.com
Received February 6, 2009; revised June 20, 2009; accepted September 3, 2009.
Project supported by the National Natural Science Foundation of China (Nos. 20772027, 20803020, 20971041), the Scientific Research Fund of
Hunan Provincial Education Department (Nos. 09k081, 1330907), and Doctoral Science Foundation of Hunan University of Science and Technology
(No. E50839).
Chin. J. Chem. 2010, 28, 97— 101
© 2010 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
97

Li et al.
FULL PAPER
Scheme 1
Bruker AV-II 500 MHz NMR spectrometer, operating
(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-tetrahydropy-
rimidine-1-carbonyl]isoxazole-4-carboxylate (4a) was
performed on a Bruker SMART CCD diffractometer at
113 K. Crystal data and details of the data collection
and refinement are collected in Table 1. The structure
was solved using the direct method implemented in the
program SHELXS97. All non-hydrogen atoms were
refined using a full-matrix least-square procedure on F2.
Anisotropic displacement parameters were assigned to
non-hydrogen atoms.
1
at 500 MHz for H, and 125 MHz for 13C. TMS was
1
used as an internal reference for H and 13C chemical
shifts and CDCl3 was as solvent. Elemental analysis was
measured by an Elementar analyzer (varioEL II). MS
was conducted by a Finnigan LCQ Advantage MAX
mass spectrometer. IR spectra were recorded on a
Perkin-Elmer spectrometer (Spectrum One). Melting
points were measured by a Yanaco MP500 melting
point apparatus and uncorrected.
Crystal structure determination
General
procedure
for
the
synthesis
of
N1-substituted 1,2,3,6-tetrahydropyrimidin-2-ones
Determination of the unit cell and data collection for
methyl 3-(2,6-dichlorophenyl)-5-[6-(4-chlorophenyl)-5-
A mixture of 1 (1 mmol) and 2 (2 mmol) in dioxane
(30 mL) were refluxed overnight. The solvent was
evaporated in vacuum. The residue was purified by
column chromatography on silica gel using petroleum
ether and ethyl acetate (5∶1, V∶V) as eluent to afford
the corresponding 4a—4h.
Table 1 Crystal data and structure refinement for compound 4a
Empirical formula
Formula weight
Temperature
C27H21Cl6N3O7
712.17
113 K
Methyl 3-(2,6-dichlorophenyl)-5-[6-(4-chlorophenyl)-
5-(ethoxcabonyl)-4-methyl-2-oxo-1,2,3,6-tetrahydro-
pyrimidine-1-carbonyl]isoxazole-4-carboxylate (4a)
Wavelength
0.071073 nm
triclinic, P-1
Crystal system, space group
1
White solid, yield 88%; m.p. 192—193 ℃; H NMR
Unit cell dimensions,
(CDCl3, 500 MHz) δ: 1.25 (t, J＝7 Hz, 3H), 2.32 (s,
3H), 3.46 (s, 3H), 4.16—4.21 (m, 2H), 6.61 (s, 1H),
7.33 (d, J＝8.5 Hz, 2H), 7.38—7.41 (m, 3H), 7.44 (d,
J＝8.5 Hz, 2H), 8.63 (s, 1H); 13C NMR (CDCl3, 125
MHz) δ: 14.17, 18.16, 52.24, 54.68, 61.03, 105.14,
110.30, 126.15, 128.08, 128.88, 129.01, 131.70, 134.58,
135.25, 135.33, 136.80, 145.13, 150.74, 157.85, 159.54,
164.29, 166.82; IR (KBr) ν: 3427.3, 1732.8, 1713－.6,
a/nm
b/nm
c/nm
α/(°)
0.81813(16)
1.3847(3)
1.4366(3)
95.29(3)
β/(°)
γ/(°)
Cell volume/nm3
100.68(3)
105.09(3)
1.5272(5)
Z
2
1
1686.3, 1653.3, 1397.2, 1235.4,＋1092.9, 784.0 cm ;
Calculated density/(g•cm－
)
1.549
3
ESI-MS m/z: 592 [M ＋ H] . Anal. calcd for
C26H20Cl3N3O7: C 52.68, H 3.40, N 7.09; found C 52.59,
H 3.36, N 7.18.
µ/mm－
0.613
1
Crystal size/mm3
0.18×0.16×0.12
Methyl 3-(2,6-dichlorophenyl)-5-[6-(4-methoxy-
phenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-
tetrahydropyrimidine-1-carbonyl]isoxazole-4-carbox-
ylate (4b) White solid, yield 68%; m.p. 195—196 ℃;
1H NMR (CDCl3, 500 MHz) δ: 1.24 (t, J＝7 Hz, 3H),
2.33 (s, 3H), 3.44 (s, 3H), 3.80 (s, 3H), 4.16 (q, J＝7 Hz,
2H), 6.61 (s, 1H), 6.87 (d, J＝8.5 Hz, 2H), 7.36—7.39
(m, 3H), 7.42 (d, J＝8.5 Hz, 2H), 8.50 (s, 1H); 13C
NMR (CDCl3, 125 MHz) δ: 14.17, 18.11, 52.16, 54.79,
θ range for data collection/(°)
2.0－27.9
－10≤h≤10,
Limiting indices
－18≤k≤18,
－18≤l≤12
Reflections collected/unique
7128/4030 [R(int)＝0.074]
Final R indices [I＞2σ(I0)]
R1＝0.057, wR2＝0.152
Largest diff. peak and hole/(e•nm－ 3) 620 and －500
98
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Chin. J. Chem. 2010, 28, 97— 101

Synthesis of N1-Substituted 1,2,3,6-Tetrahydropyrimidin-2-ones
55.32, 60.85, 105.67, 110.01, 114.05, 126.25, 128.03,
128.81, 130.40, 131.60, 135.30, 135.39, 144.57, 150.79,
157.87, 159.58, 159.70, 164.49, 167.13; IR (KBr) ν:
1H NMR (CDCl3, 500 MHz) δ: 1.14 (t, J＝7 Hz, 3H),
2.23 (s, 3H), 3.48 (s, 3H), 4.16 (q, J＝7 Hz, 2H), 7.04 (s,
1H), 7.18—7.21 (m, 1H), 7.34—7.41 (m, 5H), 8.70 (s,
1H); 13C NMR (CDCl3, 125 MHz) δ: 14.18, 18.55,
52.08, 56.10, 60.63, 100.08, 109.05, 126.30, 127.94,
129.43, 129.59, 131.54, 133.17, 135.38, 144.07, 149.88,
157.70, 158.83, 159.65, 164.36, 167.49; IR (KBr) ν:
3418.3, 1732.7, 1706.6, 1683.5, 1651.9, 1396.6,＋1254.4,
3416.1, 1740.1, 1731.7, 1709.3, 1685.6, 166－2.3, 1606.8,
1
1513.9, 1391.3, 1＋246.7, 1095.1, 783.1 cm ; ESI-MS
m/z: 588 [M＋H] . Anal. calcd for C27H23Cl2N3O8: C
55.11, H 3.94, N 7.14; found C 55.21, H 3.79, N 7.08.
Methyl 3-(2,6-dichlorophenyl)-5-[6-phenyl-5-
(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-tetrahydro-
pyrimidine-1-carbonyl]isoxazole-4-carboxylate (4c)
－1
1099.2, 783.7 cm ; ESI-MS m/z: 626 [M＋H] . Anal.
calcd for C26H19Cl4N3O7: C 49.78, H 3.05, N 6.70;
found C 49.70, H 3.12, N 6.63.
1
White solid, yield 72%; m.p. 209—210 ℃; H NMR
(CDCl3, 500 MHz) δ: 1.25 (t, J＝5 Hz, 3H), 2.33 (s,
3H), 3.39 (s, 3H), 4.16—4.20 (m, 2H), 6.66 (s, 1H),
7.33—7.39 (m, 4H), 7.42—7.45 (m, 4H), 8.53 (s, 1H);
13C NMR (CDCl3, 125 MHz) δ: 14.16, 18.14, 52.11,
55.23, 60.90, 105.49, 110.10, 126.20, 127.41, 128.03,
128.69, 128.84, 131.62, 135.28, 135.41, 138.35, 144.88,
150.80, 157.87, 159.54, 164.47, 167.05; IR (KBr) ν:
3422.7, 1741.6, 1732.5, 1714.0, 16－88.1, 1657.4, 1387.9,
Methyl 3-(2,6-dichlorophenyl)-5-[6-(3,4-difluoro-
phenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-
tetrahydropyrimidine-1-carbonyl]isoxazole-4-carb-
oxylate (4g) White solid, yield 65%; m.p. 238—239
1
℃; H NMR (CDCl3, 500 MHz) δ: 1.27 (t, J＝5 Hz,
3H), 2.38 (s, 3H), 3.53 (s, 3H), 4.19—4.24 (m, 2H),
6.61 (s, 1H), 7.15—7.18 (m, 2H), 7.28—7.30 (m, 1H),
7.38—7.45 (m, 3H), 8.36 (s, 1H); 13C NMR (CDCl3,
125 MHz) δ: 14.18, 18.29, 52.27, 54.16, 61.13, 104.99,
110.50, 116.56, 117.58, 123.59, 126.18, 128.03, 131.66,
135.30, 145.27, 150.14, 157.87, 159.55, 164.18, 166.63;
IR (KBr) ν: 3412.6, 1730.3, 1710.3, 1647.8, 1517.4,
1
1296.7, ＋1238.7, 1096.9, 782.0 cm ; ESI-MS m/z: 558
[M＋H] . Anal. calcd for C26H21Cl2N3O7: C 55.93, H
3.79, N 7.53; found C 55.88, H 3.82, N 7.51.
Methyl 3-(2,6-dichlorophenyl)-5-[6-(2,4-dichloro-
phenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-
tetrahydropyrimidine-1-carbonyl]isoxazole-4-car-
boxylate (4d) White solid, yield 78%; m.p. 210—211
－1
1397.3, ＋1255.3, 1093.9, 786.8 cm ; ESI-MS m/z: 594
[M＋H] . Anal. calcd for C26H19Cl2F2N3O7: C 52.54, H
3.22, N 7.07; found C 52.60, H 3.16, N 7.15.
1
℃; H NMR (CDCl3, 500 MHz) δ: 1.25 (t, J＝5 Hz,
Methyl 3-(2,6-dichlorophenyl)-5-[6-(2,4-difluoro-
phenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-
tetrahydropyrimidine-1-carbonyl]isoxazole-4-carb-
oxylate (4h) White solid, yield 72%; m.p. 229—230
3H), 2.30 (s, 3H), 3.44 (s, 3H), 4.13—4.17 (m, 2H),
6.73 (s, 1H), 7.25 (d, J＝10 Hz, 1H), 7.36—7.43 (m,
4H), 7.49 (d, J＝10 Hz, 1H), 8.54 (s, 1H); 13C NMR
(CDCl3, 125 MHz) δ: 14.22, 18.33, 52.14, 55.46, 60.99,
103.43, 109.72, 126.20, 127.38, 128.02, 130.12, 131.62,
132.35, 134.26, 135.06, 135.19, 135.31, 144.09, 150.11,
157.74, 158.17, 159.57, 164.11, 167.09; IR (KBr) ν:
3429.2, 1732.9, 1696.6, 1683.4, 1662.0, 1396.0,＋1255.1,
1
℃; H NMR (CDCl3, 500 MHz) δ: 1.25 (t, J＝7 Hz,
3H), 2.29 (s, 3H), 3.44 (s, 3H), 4.15—4.17 (m, 2H),
6.60 (s, 1H), 6.82—6.90 (m, 2H), 7.35—7.42 (m, 3H),
7.50—7.55 (m, 1H), 8.67 (s, 1H); 13C NMR (CDCl3,
125 MHz) δ: 14.07, 18.25, 52.13, 52.98, 60.89, 102.80,
104.38, 109.89, 111.28, 122.18, 126.20, 128.00, 131.60,
135.30, 144.48, 150.15, 157.73, 157.98, 159.58, 164.21,
167.10; IR (KBr) ν: 3428.1, 1742.3, 1732.3, 1699.0,
1683.3, 1662.3, 1504.8, 1396.7, 1257.4, 1102.3, 786.3
－1
1103.7, 786.2 cm ; ESI-MS m/z: 626 [M＋H] . Anal.
calcd for C26H19Cl4N3O7: C 49.78, H 3.05, N 6.70;
found C 49.74, H 3.02, N 6.62.
Methyl 3-(2,6-dichlorophenyl)-5-[6-(3,4,5-trime-
thoxyphenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,
3,6-tetrahydropyrimidine-1-carbonyl]isoxazole-4-car-
boxylate (4e) White solid, yield 71%; m.p. 207—208
＋
－1
cm ; ESI-MS m/z: 594 [M＋H] . Anal. calcd for
C26H19Cl2F2N3O7: C 52.54, H 3.22, N 7.07; found C
52.44, H 3.26, N 7.01.
1
℃; H NMR (CDCl3, 500 MHz) δ: 1.27 (t, J＝5 Hz,
3H), 2.34 (s, 3H), 3.47 (s, 3H), 3.82 (s, 3H), 3.84 (s, 6H),
4.18—4.24 (m, 2H), 6.63 (s, 1H), 6.66 (s, 2H), 7.37—
7.42 (m, 3H), 8.37 (s, 1H); 13C NMR (CDCl3, 125 MHz)
δ: 14.27, 18.17, 52.16, 55.05, 56.19, 60.80, 66.97,
104.36, 105.40, 110.36, 126.22, 127.98, 128.04, 131.67,
133.73, 135.23, 135.36, 138.17, 144.89, 150.72, 153.32,
157.77, 157.86, 159.60, 164.52, 167.09; IR (KBr) ν:
3431.6, 1729.6, 1709.3, 1689.2, 1652.8, 1607.1, 1583.4,
Results and discussion
The structures of compounds 4a— 4h were estab-
lished by different spectroscopic techniques (IR, NMR,
and MS) and elemental analysis. The IR spectrum of 4a
－1
displayed ν
at 1732.8, 1713.6 and 1686.3 cm . The
C＝
O
mass spectrum of 4a showed a molecular ion peak at
m/z 592 ([M＋H]＋), which can exclude the existence of
1
－1
3a. The H NMR spectrum of 4a revealed a singlet at
1392.3, ＋1246.3, 1091.2, 783.5 cm ; ESI MS m/z: 648
δ 2.32 resulting from methyl, a triplet at δ 1.25 for
CH3CH2O protons, a multiplet in the range of δ 4.16—
4.21 for CH3CH2O protons, a singlet at δ 3.46 for CH3O
protons, a singlet at δ 6.61 for benzylic proton, several
multiplets in the range of δ 7.33—7.44 for aromatic
protons and a singlet at δ 8.63 resulting from NH. On
[M＋H] . Anal. calcd for C29H27Cl2N3O10: C 53.71, H
4.20, N 6.48; found C 53.68, H 4.18, N 6.43.
Methyl 3-(2,6-dichlorophenyl)-5-[6-(2,6-dichloro-
phenyl)-5-(ethoxycarbonyl)-4-methyl-2-oxo-1,2,3,6-
tetrahydropyrimidine-1-carbonyl]isoxazole-4-carbox-
ylate (4f) White solid, yield 69%; m.p. 225—226 ℃;
Chin. J. Chem. 2010, 28, 97— 101
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99

Li et al.
FULL PAPER
the other hand, the structure was established to be 4
rather than 3 based on the characterized peaks on their
1H NMR spectra. The absence of any signal between
δ 4.5—6.5 that is assignable for the methine proton of 3
excluded the presence of product 3.15-17
4, shown in Scheme 2. What’s more, when we increase
the molar ratios of 1 and 2 (from 1∶2 to 1∶1, and 2∶
1 finally), no other product except 1 and product 4 is
observed.
The 13C NMR spectrum of the product 4a exhibits
the presence of methyl carbon at 18.16, ethyl carbons at
δ 14.17 and 61.03, OCH3 at δ 52.24, benzylic carbon at
δ 54.68, and carbonyl carbons at δ 164.29 and 166.82.
Further, the structure of the product was confirmed by
X-ray diffraction analysis of 4a, as shown in Figure 2.
Mechanistically, the 1,3-dipolar cycloaddition reac-
tion of 1 with 2 results in the formation of intermediate
3, which undergoes a subsequent ring-opening to the
corresponding intermediate 3' (or 3"), finally, the S of 3'
(or 3") was substituted by O in the existence of nitrile
oxide 2,18-19 which lead to the formation of title product
Conclusion
A novel synthesis of N1-substituted 1,2,3,6-tetrahy-
dropyrimidin-2-ones was accomplished by the reaction
of nitrile oxide with thiazolo[3,2-a]pyrimidine deriva-
tives in moderate yields. The domino 1,3-dipolar
cycloaddition/ring-opening/substitution process offers
an
easy
way
to
synthesize
N1-substituted
1,2,3,6-tetrahydropyrimidin-2-ones. Therefore, we be-
lieve that our present results would enrich the chemistry
of tetrahydropyrimidin-2-one.
Figure 2 ORTEP diagram of 4a (solvent and partial H atoms have been omitted for clarity).
Scheme 2
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