Original approach to new derivatives of [1,3]oxazolo[4,5-d]pyrimidine

Article abstract of DOI:10.1055/s-2006-950221

Based on the readily available 4-dichloromethylene-2-phenyl-1,3-oxazol- 5(4H)-one, preparative methods for the synthesis of a set of functionalized derivatives of the poorly studied [1,3]oxazolo[4,5-d]pyrimidine system were elaborated. Georg Thieme Verlag

Full text of DOI:10.1055/s-2006-950221

3462
PAPER
Original Approach to New Derivatives of [1,3]Oxazolo[4,5-d]pyrimidine
N
ew
Derivatives
o
i
f
[1,3]O
t
xazol
a
o[4,5-
d
]pyr
l
a
Institute of Bioorganic Chemistry and Petrochemistry, National Academy of Sciences of Ukraine, Murmanskaya str. 1,
02094 Kyiv, Ukraine
Fax +380(44)5732561; E-mail: drach@bpci.kiev.ua
Oak Ridge National Laboratory Washington, P.O. Box 2008, Oak Ridge, TN 37381-6242, USA
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, Murmanskaya str. 5, 02094 Kyiv, Ukraine
b
c
Received 1 June 2006; revised 15 June 2006
ones 3a–c, which on heating in pyridine underwent cyclo-
condensation followed by recyclization affording the cor-
responding [1,3]oxazolo[4,5-d]pyrimidines 7a–c in 60–
70% yields (Scheme 1). Since the structure of one of the
intermediates 3a was reliably established earlier,15 the
structures of the similar compounds 3b and c were con-
Abstract: Based on the readily available 4-dichloromethylene-2-
phenyl-1,3-oxazol-5(4H)-one, preparative methods for the synthe-
sis of a set of functionalized derivatives of the poorly studied
[1,3]oxazolo[4,5-d]pyrimidine system were elaborated.
Key words: heterocycles, fused-ring systems, cyclizations, ox-
azoles, pyrimidines
1
firmed by H NMR and IR spectroscopy (Table 1). The
structure of compound 10, the end product of the cascade
transformation 1→3c→7c→10, was unequivocally
solved by X-ray diffraction (Figure 2 and Table 2).
Annelation of azole moieties at the d-side of the pyrimi-
dine nucleus leads to analogues structurally related to pu-
rine. Of these, derivatives of the [1,3]oxazolo[5,4-
d]pyrimidine fused system I (Figure 1) have been studied
the most comprehensively, and various methods for their
synthesis have been presented.1–9
In the solid state there are two symmetrically independent
molecules 10A and 10B with virtually the same geometry.
The central oxazolo[4,5-d]pyrimidine system N(1–
3)C(1–5)O(1) is planar within 0.03 Å. Notable is the quite
strong16 intramolecular hydrogen bond N(2)–H···O(3) ‘as-
sisted by resonance’17 forming a six-membered ring
O(3)C(12)N(4)C(5)N(2)H(2) as an interesting peculiarity
of the molecular structure of compound 10.
N
O
N
N
N
O
N
N
I
Since the presence of the [1,3]oxazolo[4,5-d]pyrimidine
system in key compound 10 was reliably established,
there are no doubts that the similar compounds 7a–c as
well as 8a,b and 9a,b are also derivatives of this system.
Furthermore, the melting points and spectral characteris-
tics of compounds 7a, 8a, and 9a differ from those of the
corresponding derivatives of the isomeric [1,3]oxazo-
lo[5,4-d]pyrimidine system prepared by known methods.6
This supports the correctness of the interpretation that the
products of the complex transformation of compounds
3a–c lead to derivatives of [1,3]oxazolo[4,5-d]pyrimi-
dine. The mechanism of their formation involves an im-
portant role played by intermediate spiro compounds 4a–
c, close analogues of which have been synthesized recent-
ly.18 Further transformations 4→5→6→7 seem quite
probable. Another way of looking at this is that structure
4 bears a chloromethylene group, which on treatment with
pyridine is turned into a carbene, whose skeletal rear-
rangement may result in structure 7.
II
Figure 1 [1,3]Oxazolo[5,4-d]- and [1,3]oxazolo[4,5-d]pyrimidines
In contrast, the isomeric [1,3]oxazolo[4,5-d]pyrimidines
II (Figure 1) are poorly studied. Some have been obtained
from 4-amino-5-hydroxypyrimidine and its derivatives by
condensations with ortho esters10,11 and benzoic anhy-
drides.12 These methods are obviously unsuitable for the
introduction of functional substituents at position 7 of the
[1,3]oxazolo[4,5-d]pyrimidine system. The formation of
2-phenyl-[1,3]oxazolo[4,5-d]pyrimidin-7-one by isomer-
ization of its isooxazole analogue13 under flash vacuum
pyrolysis at 700 °C is also more likely to be of scientific
than preparative interest.
We report here on the elaboration of a preparative method
for the synthesis of a series of mono- and difunctionalized
derivatives of fused system II based on readily available
4-dichloromethylene-2-phenyl-1,3-oxazol-5(4H)-one
(1)14 (Scheme 1).
In conclusion, it should be noted that the use of chloro-
containing 1,3-oxazol-5(4H)-ones for the preparative syn-
thesis of the poorly studied derivatives of system II
(Figure 1) is not limited to the transformations presented
in Scheme 1, and will be expanded upon in the near fu-
ture.
Treating 1,3-oxazol-5(4H)-one 1 with benzamidine hy-
drochloride (2a) or similar reagents 2b and c in the pres-
ence of triethylamine led to substituted imidazol-4(5)-
SYNTHESIS 2006, No. 20, pp 3462–3466
x
x.
x
x
.
2
0
0
6
Advanced online publication: 04.09.2006
DOI: 10.1055/s-2006-950221; Art ID: P06406SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
New Derivatives of [1,3]Oxazolo[4,5-d]pyrimidine
3463
H
Cl
Cl
Cl
O
Cl
O
H2N
N
Et3N
R
Ph
N
+
N
NH
O
Ph
+
H
H2N
X–
O
R
1
2a–c
3a–c
Py, ∆
. .
+
H
O
H
O
O
H
Cl
O
Ph
Ph
O
Py, ∆
– Py⋅HCl
O
N
N
N
Ph
-
N
N
N
N
NH
N
R
R
6a–c
R
4a–c
5a–c
Py
O
O
N
POCl3, Me2NPh
PhC(O)Cl, Py
R = NH2
NH
Ph
N
R
7a–c
Cl
SPh
O
O
N
O
N
O
N
PhSH, Et3N
N
N
NH
O
Ph
Ph
Ph
N
R
N
R
N
N
Ph
H
8a,b
9a,b
10
R = Ph (a), SMe (b), NH2 (c); X = Cl (2a,c), I (2b)
Scheme 1 Synthesis of substituted derivatives of [1,3]oxazolo[4,5-d]pyrimidine from 4-dichloromethylene-2-phenyl-1,3-oxazol-5(4H)-one
4(5)-(Benzoylamino)-4(5)-(dichloromethyl)-2-phenyl-4,5-dihy-
dro-1H-imidazol-5(4)-one (3a)
Compound 3a was prepared by a literature procedure.15
4(5)-(Benzoylamino)-4(5)-(dichloromethyl)-2-(methylsulfanyl)-
4,5-dihydro-1H-imidazol-5(4)-one (3b)
Compound 3b was prepared as described above in the general pro-
cedure, starting from 1 (9.68 g, 40 mmol), S-methylisothiouronium
iodide (2b; 8.72 g, 40 mmol), and Et3N (5.74 mL, 41 mmol). Re-
crystallization of the product from EtOH gave colorless crystals.
Yield: 11.83 g (89%); mp 197–198 °C.
Figure 2 Perspective view and labeling scheme of oxazolo[4,5-
Anal. Calcd for C12H11Cl2N3O2S: C, 43.39; H, 3.34; Cl, 21.34; N,
d]pyrimidine 10A (N,N-dimethylformamide not shown)
12.65; S, 9.65. Found: C, 43.51; H, 3.59; Cl, 21.27; N, 12.59; S,
9.63.
1H NMR spectra were recorded on a Varian Gemini-300 instru-
ment, and TMS was used as internal standard. IR spectra were mea-
sured on a Specord M-80 spectrometer of samples prepared as KBr
discs. The spectroscopic data of compounds 3 and 7–10 are present-
2-Amino-4(5)-(benzoylamino)-4(5)-(dichloromethyl)-4,5-dihy-
dro-1H-imidazol-5(4)-one (3c)
Compound 3c was prepared as described above in the general pro-
cedure, starting from 1 (9.68 g, 40 mmol) and guanidine hydrochlo-
ed in Table 1.
ride (2c; 3.82 g, 40 mmol). Recrystallization of the product (DMF–
EtOH, 2:1) gave colorless crystals.
4(5)-Benzoylamino-4(5)-dichloromethyl-4,5-dihydro-1H-
imidazol-5(4)-ones 3a–c; General Procedure
Yield: 7.23 g (60%); mp >300 °C.
To a soln of 1,3-oxazol-5(4H)-one 1 (9.68 g, 40 mmol), prepared by
a literature procedure,14 in dry THF (100 mL) was added compound
2 (40 mmol) followed by Et3N (5.74 mL, 41 mmol). The mixture
was stirred at r.t. for 72 h. The precipitate that formed was collected
by filtration, washed with H2O, dried, and recrystallized.
Anal. Calcd for C11H10Cl2N4O2: C, 43.87; H, 3.35; Cl, 23.55; N,
18.61. Found: C, 43.58; H, 3.60; Cl, 23.43; N, 18.69.
Synthesis 2006, No. 20, 3462–3466 © Thieme Stuttgart · New York

3464
V. M. Sviripa et al.
PAPER
2-Phenyl[1,3]oxazolo[4,5-d]-pyrimidin-7(6H)-ones 7a–c;
General Procedure
A soln of compound 3 (10 mmol) in pyridine (15 mL) was refluxed
for 10 h. The pyridine was removed in vacuo. The residue was treat-
ed with H2O, filtered, dried, and recrystallized.
2,5-Diphenyl-7-(phenylsulfanyl)[1,3]oxazolo[4,5-d]pyrimidine
(9a)
Compound 9a was prepared according to the above general proce-
dure starting from 8a (0.62 g, 2 mmol) and PhSH (0.22 g, 2 mmol).
Recrystallization (MeCN) of the product gave colorless crystals.
Yield: 0.63 g (83%); mp 185–186 °C.
2,5-Diphenyl[1,3]oxazolo[4,5-d]pyrimidin-7(6H)-one (7a)
Compound 7a was prepared according to the above general proce-
dure, starting from 3a (3.62 g, 10 mmol). Purification by recrystal-
lization (DMF) gave colorless crystals.
Anal. Calcd for C23H15N3OS: C, 72.42; C, 3.96; N, 11.02; S, 8.41.
Found: C, 72.23; H, 4.20; N, 10.92; S, 8.36.
2,5-Diphenyl-7-(phenylsulfanyl)[1,3]oxazolo[5,4-d]pyrimidine
The [5,4-d]pyrimidine isomer of 9a was prepared from the known
7-chloro-2,5-diphenyl[1,3]oxazolo[5,4-d]pyrimidine;6 mp 160–
161 °C (MeCN). A mixture of 9a and its isomer gives a marked de-
pression of the melting point.
Yield: 2.46 g (85%); mp >300 °C.
Anal. Calcd for C17H11N3O2: C, 70.58; H, 3.83; N, 14.53. Found: C,
70.35; H, 3.92; N, 14.80.
5-(Methylsulfanyl)-2-phenyl[1,3]oxazolo[4,5-d]pyrimidin-
7(6H)-one (7b)
Compound 7b was prepared according to the above general proce-
dure, starting from 3b (3.32 g, 10 mmol). Purification by recrystal-
lization (dioxane) gave colorless crystals.
Anal. Calcd for C23H15N3OS: C, 72.42; C, 3.96; N, 11.02; S, 8.41.
Found: C, 72.10; H, 4.15; N, 10.81; S, 8.25.
5-(Methylsulfanyl)-2-phenyl-7-(phenylsulfanyl)[1,3]oxazo-
lo[4,5-d]pyrimidine (9b)
Yield: 1.61 g (62%); mp >300 °C.
Compound 9b was prepared according to the above general proce-
dure starting from 8b (0.56 g, 2 mmol) and PhSH (0.22 g, 2 mmol).
Recrystallization (MeCN) of the product gave colorless crystals.
Anal. Calcd for C12H9N3O2S: C, 55.59; N, 3.50; N, 16.21; S, 12.37.
Found: C, 55.47; H, 3.70; N, 16.14; S, 12.43.
Yield: 0.56 g (80%); mp 163–165 °C.
5-Amino-2-phenyl[1,3]oxazolo[4,5-d]pyrimidin-7(6H)-one (7c)
Compound 7c was prepared according to the above general proce-
dure, starting from 3c (3.01 g, 10 mmol). Purification by recrystal-
lization (DMF) gave colorless crystals; yield: 1.60 g (70%); mp
>300 °C.
Anal. Calcd for C18H13N3OS2: C, 61.52; C, 3.73; N, 11.96; S, 18.25.
Found: C, 61.21; H, 3.96; N, 11.70; S, 18.09.
5-(Benzoylamino)-2-phenyl[1,3]oxazolo[4,5-d]pyrimidin-
7(6H)-one (10)
Anal. Calcd for C11H8N4O2: C, 57.89; N, 3.53; N, 24.55. Found: C,
57.58; H, 3.81; N, 24.39.
A mixture of 7c (1.14 g, 5 mmol) and PhCOCl (0.70 g, 5 mmol) in
pyridine (10 mL) was refluxed for 4 h. The pyridine was removed
in vacuo, the residue was washed with H2O (3 × 10 mL), recrystal-
lized from DMF, and dried in vacuo (0.1 Torr) at 80–90 °C.
7-Chloro-2-phenyl[1,3]oxazolo[4,5-d]pyrimidines 8a,b;
General Procedure
A mixture of compound 7a or 7b (10 mmol), POCl3 (30 mL), and
Me2NPh (2.42 g, 20 mmol) was refluxed for 3 h. Excess POCl3 was
removed in vacuo. The residue was recrystallized.
Colorless crystals; yield: 1.49 g (90%); mp 256–257 °C.
Anal. Calcd for C18H12N4O3: C, 65.06; H, 3.64; N, 16.86. Found: C,
64.81; C, 3.80; N, 16.61.
X-ray Structure Determination of 10·DMF
7-Chloro-2,5-diphenyl[1,3]oxazolo[4,5-d]pyrimidine (8a)
Compound 8a was prepared according to the above general proce-
dure, starting from 7a (2.89 g, 10 mmol). Recrystallization (diox-
ane) of the product gave colorless crystals.
Although crystals of compound 10 were obtained, they were not
suitable for X-ray structural analysis. Suitable 10·DMF solvate
crystals were obtained after recrystallization from DMF. Selected
bond lengths and bond angles of 10 are presented in Table 2.
Yield: 2.77 g (90%); mp 219–220 °C.
Crystal Data: C18H12N4O3·C3H7NO,
M
405.41, triclinic,
Anal. Calcd for C17H10ClN3O: C, 66.35; H, 3.28; Cl, 11.52; N,
13.65. Found: C, 66.14; H, 3.49; Cl, 11.32; N, 13.53.
a = 7.712(2), b = 12.070(4), c = 22.884(6) Å, a = 82.95(2),
b = 87.65(2), g = 71.40(3)°, V = 2003.7(11) Å3, Z = 4, d = 1.35 g
cm–3, space group P–1 (N 2), m = 10.9 cm–1, F(OOO) = 848, crystal
size ca. 0.16 × 0.31 × 0.37 mm.
7-Chloro-5-(methylsulfanyl)-2-phenyl[1,3]oxazolo[4,5-d]pyri-
midine (8b)
Data Collection: All crystallographic measurements were per-
formed at r.t. on a CAD-4 Enraf-Nonius diffractometer operating in
the w–2q scan mode (scanning rate ratio 2q/w = 1.2), qmax = 65° (0
≤ h ≤ 5, –13 ≤ k ≤ 14, –26 ≤ l ≤ 26) using Cu-Ka radiation
(l = 1.54178 Å). During the data collection the crystal had decom-
posed, and decay correction was applied. Intensities of 5430 reflec-
tions (4868 unique reflections, Rint 0.02) were measured. Data were
corrected for Lorentz and polarization effects, and an empirical ab-
sorption correction based on azimutal scan data was applied.19
Compound 8b was prepared according to the above general proce-
dure, starting from 7b (2.59 g, 10 mmol). Recrystallization (MeCN)
of the product gave colorless crystals.
Yield: 2.06 g (74%); mp 156–158 °C.
Anal. Calcd for C12H8ClN3OS: C, 51.90; H, 2.90; Cl, 12.77; N,
15.13; S, 11.54. Found: C, 51.69; H, 3.10; Cl, 12.68; N, 15.05; S,
11.49.
2-Phenyl-7-(phenylsulfanyl)[1,3]oxazolo[4,5-d]pyrimidines
9a,b; General Procedure
A mixture of 8a or 8b (2 mmol), PhSH (0.22 g, 2 mmol), and Et3N
(0.28 mL, 2 mmol) in dioxane (15 mL) was refluxed for 6 h. The di-
oxane was removed in vacuo, the residue was treated with H2O,
dried, and recrystallized.
Structure Solution and Refinement: The structure was solved by di-
rect methods and refined by full-matrix least-squares technique in
the anisotropic approximation using the CRYSTALS program
package.20 In the refinement, 3469 reflections with I ≥ 3s(I) were
used. More than 90% of the hydrogen atoms were located in the dif-
ference Fourier maps. Remaining hydrogen atoms were placed in
calculated positions. All hydrogen atoms were included in the final
refinements with fixed positional and thermal parameters (only H-
Synthesis 2006, No. 20, 3462–3466 © Thieme Stuttgart · New York

PAPER
New Derivatives of [1,3]Oxazolo[4,5-d]pyrimidine
d for 1H NMR (300 MHz, DMSO-d6)
3465
Table 1 Spectroscopic Data of Compounds 3 and 7–10
Compound IR (KBr) (cm–1)
3b
1650 (NC=O), 1780 (C=O)a, 3200–3400 (NH, assoc.)
2.49 (s, 3 H, CH3), 6.29 (s, 1 H, CH), 7.45–7.83 (m, 5 H, C6H5), 9.00
(s, 1 H, NH), 11.57 (s, 1 H, NH)
3c
1660 (NC=O), 1730 (C=O)a, 3050–3200 (NH, assoc.),
3360, 3430 (NH2)
6.37 (s, 1 H, CH), 7.04 (br s, 1 H, NH), 8.07 (br s, 1 H, NH), 7.43–7.82
(m, 5 H, C6H5), 8.42 (br s, 1 H, NH), 8.71 (s, 1 H, NH)
7ab,c
7b
1690 (C=O), 3050–3200 (NH, assoc.)
7.54–8.21 (m, 10 H, 2C6H5), 13.09 (s, 1 H, NH)
2.59 (s, 3 H, CH3), 7.62–8.14 (m, 5 H, C6H5), 13.04 (br s, 1 H, NH)
6.63 (s, 2 H, NH2), 7.58–8.13 (m, 5 H, C6H5), 11.16 (s, 1 H, NH)
7.55–8.41 (m, 10 H, 2C6H5)
1685 (C=O), 3050–3200 (NH, assoc.)
7c
1695 (C=O), 3100–3380 (NH, NH2, assoc.)
1620–1800, 3050–3500 (bands are absent)
1620–1800, 3050–3500 (bands are absent)
1620–1800, 3050–3500 (bands are absent)
1620–1800, 3050–3500 (bands are absent)
1675 (NC=O), 1700 (C=O),e 3050–3300 (NH, assoc.)
8a
8b
2.60 (s, 3 H, CH3), 7.68–8.29 (m, 5 H, C6H5)
9ad
9b
7.41–8.19 (m, 15 H, 3C6H5)
2.36 (s, 3 H, CH3), 7.54–7.95 (m, 10 H, 2C6H5)
7.51–8.18 (m, 10 H, 2C6H5), 12.18 (s, 1 H, NH), 12.76 (s, 1 H, NH)
10
a C=O group of imidazolone moiety.
b UV (DMSO): l = 265 nm (e = 27600).
c 13C NMR (75 MHz, DMSO-d6): d = 125.52, 127.42, 127.94, 128.57, 129.33, 131.50, 131.90, 132.02, 132.63, 152.10, 155.87, 158.58, 164.76.
d 13C NMR (100 MHz, DMSO-d6): d = 124.8, 125.8, 127.7, 128.0, 128.5, 129.3, 129.5, 130.2, 130.5, 133.5, 135.7, 136.4, 137.9, 151.6, 159.4,
161.2, 166.4.
e C=O group of pyrimidinone moiety.
2, H-4, H-5, and H-8 were refined isotropically). Convergence was
Acknowledgment
obtained at R = 0.055 and Rw = 0.058, GOF = 1.064 (557 refined
This research was supported in part by the IPP program through the
parameters; obsd/variable = 6.2; the largest minimal peaks in the fi-
nal difference map 0.313 and –0.294 e/Å3). The Chebushev weight-
ing scheme21 with parameters 1.20, –0.32, 0.39, and –0.57 was used.
Full crystallographic details have been deposited at the Cambridge
Crystallographic Data Center (CCDC). Any request to the CCDC
for this material should quote the full literature citation and refer-
ence number CCDC 292924.
Science and Technology Center in Ukraine (STCU). Oak Ridge Na-
tional Laboratory is managed by UT-Battelle, LLC, under contract
DE-AC05-00OR22725 for the U.S. Department of Energy. Contri-
bution 20, the Discovery Chemistry Project.
References
Table 2 Selected X-ray Bond Lengths (Å) and Bond Angles (°) of
10A
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O(1)–C(1) 1.370(3)
O(1)–C(3) 1.378(3)
N(1)–C(2) 1.363(4)
N(1)–C(5) 1.302(4)
N(2)–C(4) 1.406(4)
N(2)–C(5) 1.365(4)
N(3)–C(1) 1.313(4)
N(3)–C(2) 1.387(4)
C(3)–C(4) 1.401(4)
N(2)–H 0.92(4)
C(1)–O(1)–C(3) 103.0(2)
C(2)–N(1)–C(5) 112.5(2)
C(4)–N(2)–C(5) 124.4(3)
C(1)–N(3)–C(2) 103.8(2)
O(1)–C(1)–N(3) 114.8(3)
O(1)–C(3)–C(4) 128.1(3)
N(1)–C(2)–N(3) 124.9(2)
N(1)–C(5)–N(2) 125.0(3)
N(1)–C(2)–C(3) 125.6(3)
O(1)–C(3)–C(2) 108.9(3)
N(2)H···O(3) 135(2)
N(2)···O(3) 2.597(4)
H···O(3) 1.87(3)
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Synthesis 2006, No. 20, 3462–3466 © Thieme Stuttgart · New York

3466
V. M. Sviripa et al.
PAPER
(19) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta
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