A novel two step synthesis of 3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines

Article abstract of DOI:10.3987/COM-06-10788

A novel two step synthesis of 5,7-diaryl-3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines has been reported based on condensation of 2-thiobarbiturates with chalcones followed by reductive desulfurization of 5,7-diaryl-1,2,3,4-tetrahydro-4-oxo-2-thioxo-5H-py

Full text of DOI:10.3987/COM-06-10788

HETEROCYCLES, Vol. 68, No. 9, 2006
1885
HETEROCYCLES, Vol. 68, No. 9, 2006, pp. 1885 - 1892. © The Japan Institute of Heterocyclic Chemistry
Received, 18th May, 2006, Accepted, 3rd July, 2006, Published online, 4th July, 2006. COM-06-10788
A NOVEL TWO STEP SYNTHESIS OF 3,4-DIHYDRO-4-OXO-5H-
PYRANO[2,3-d]PYRIMIDINES
Jitender M. Khurana,* Arpita Agrawal, and Gagan Kukreja
Department of Chemistry, University of Delhi, Delhi-110007, INDIA
Abstract- A novel two step synthesis of 5,7-diaryl-3,4-dihydro-4-oxo-5H-
pyrano[2,3-d]pyrimidines has been reported based on condensation of 2-
thiobarbiturates with chalcones followed by reductive desulfurization of 5,7-
diaryl-1,2,3,4-tetrahydro-4-oxo-2-thioxo-5H-pyrano[2,3-d]pyrimidines with
nickel boride.
Pyrans1 and pyrimidines2 individually or in combination possess significant biological activity such as
antiallergic, analgesic, sedative and antiphlogistic. Some of the derivatives of pyrimidines also show
herbicidal and inseticidal properties. Different routes have been described in literature for the synthesis of
pyrano[2,3-d]pyrimidines3 as well as 2-oxo and 2-thioxo analogues of 1,2,3,4-tetrahydro-4-oxo-5H-
pyrano[2,3-d]pyrimidines.4,5 Unlike the synthesis of 2-oxo and 2-thioxo analogues, synthesis of 3,4-
dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines has not received any attention. Thus there is sufficient
scope for the development of new, efficient and practical protocols for the synthesis of 3,4-dihydro- 4-
oxo-5H-pyrano[2,3-d]pyrimidines. We have attempted to achieve the desired synthesis by addition of 2-
thiobarbiturates to different chalcones to give 5,7-diaryl-1,2,3,4-tetrahydro-4-oxo-2-thioxo-5H-
pyrano[2,3-d]pyrimidines followed by reductive desulfurization with nickel boride which has been
reported as a reducing6 and dethiating7 agent by our group.
We report herein a convenient and efficient synthesis of 3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines
(4a-f) and of 1,2,3,4-tetrahydro-4-oxo-5H-pyrano[2,3-d]pyrimidines (5g-h) in two steps. Thus 2-thioxo-5H-
pyrano[2,3-d]pyrimidines (3a-h) were prepared by the condensation of chalcones (1a-f) with 2-
thiobarbituric acids (2a-b) in high yields by a reported method.5 Though compounds (3a, 3b and 3g) are
reportedly known, a number of new compounds e.g. 3c-f and 3h were characterized by NMR, IR and MS
spectra. These results are listed in Table 1.
Selective desulfurization of 1,2,3,4-tetrahydro-4-oxo-2-thioxo-5H-pyrano[2,3-d]pyrimidines without
affecting the carbonyl group would result in the desired 3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines.

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HETEROCYCLES, Vol. 68, No. 9, 2006
Table 1: Preparation of 4-oxo-2-thioxo-5H-pyrano[2,3-d]pyrimidines
2-Thiobarbituric
acid (2)
2-Thioxo-5H-pyrano[2,3- Yield
S. No.
Chalcones (1)
d] pyrimidines (3)
(%)
1.
2.
3.
4.
5.
6.
7.
8.
1a
1b
1c
1d
1e
1f
2a
3a
3b
95
2a
2a
2a
2a
2a
2b
2b
94
3c
3d
3e
3f
80
86
89
85
80
68
1a
1c
3g
3h
We have already reported6,7 reductive desulfurization of 2-thiobarbiturates and 2-thioxo-4(3H)-
quinazolonones with nickel boride prepared in situ. Therefore reaction of 3a was carried out with nickel
boride in methanol by changing the molar ratio of substrate: nickel chloride: sodium borohydride. It was
observed that the reaction of 3a completed in 90 min using a molar ratio of 1:10:10 (S:NiCl2:NaBH4) in
dry methanol and gave 4a in good yield which was characterized by IR, MS and NMR spectra. IR
showed the absence of C=S stretch at 1560.56 cm-1 and appearance of C=N stretch at 1608.62 cm-1. FAB
MS spectra gives M++1 molecular ion peak at 303. NMR showed a new signal at δ 8.0646 due to H-2
proton which confirms the formation of –CH=N-. With lower molar ratios of substrate to nickel chloride
to sodium borohydride, the reaction was incomplete though it showed the formation of product (4a). The
reaction of 3a with nickel boride in dry THF, acetonitrile and ethanol was incomplete even after 24 h.
Reaction of 3a in dry DMF was complete in 1 h but produced a complex mixture of products as observed
by TLC. Removal of traces of DMF from the reaction mixture also posed some difficulties. Therefore
dry methanol was employed in all further reactions. There was very little reaction of 3a with NaBH4
alone even after 24 h and the starting material was recovered unchanged when treated with NiCl2 only.
The desulfurization is thus undoubtedly proceeding due to the involvement of nickel boride formed in situ.
Whereas 2-thioxo-5H-pyrano[2,3-d]pyrimidines (3b-f) underwent reductive desulfurization with nickel
boride to give 3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines (4b-f) as confirmed by spectral data 3g
and 3h yielded 1,2,3,4-tetrahydro-4-oxo-5H-pyrano[2,3-d]pyrimidines(5g and 5h) (Scheme I) respectively
as the desulphurised product because both the nitrogens are blocked with a phenyl group. This is evident
from disappearance of C=S streatching band in IR spectra and apperance of 2 protons of C-2 at ~ δ 5.17
and ~ δ 5.49 which coupled to each other (J=11.5). All the compounds are new products and have been
well characterized by NMR, IR and MS spectra. These results are summarized in Table 2. Nickel boride
shows high selectivity towards desulfurization and does not affect the amide carbonyl group. Nickel

HETEROCYCLES, Vol. 68, No. 9, 2006
1887
boride also did not affect the chloro and bromo groups in the halogenated substrates (3e and 3f).
O
C
R
N
O
S
R1
CH CH
1a-f
R2
+
2a-b
RN
a
b
R= H
R= Ph
R1
R2
O
AcOH, P2O5
-H2O
1 a
b
c
d
e
H
H
H
CH3O
CH3
Cl
CH3O
CH3O
H
R1
R
S
Cl
N
O
f
Br
H
RN
O
H
R1
R2
R
3 a
H
CH3O
CH3O CH3O
H
Cl
Br
H
H
H
H
H
H
H
H
H
Ph
R2
3a-h
b
c
d
e
f
anhyd. NiCl2, NaBH4
dry MeOH, rt
CH3
Cl
H
3g-h
3a-f
g
h
H
CH3O CH3O Ph
R1
R1
R
N
N
O
O
RN
RN
H
H
O
O
5g-h
R2
4a-f
R2
Scheme 1
Table 2: Preparation of 1,3,5,7-tetraaryl-3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines
S.
No.
2-Thioxo-5H-pyrano[2,3-d]
Molar ratio
S:NICL2:NaBH4
Time
(h)
Product (4)
Yield
(%)
pyrimidines (3)
1.
2.
3.
4.
5.
6.
3a
3b
3c
3d
3e
3f
1:10:10
1:10:10
1:10:10
1:10:10
1:15:15a
1:15:15a
1.5
1
4a
4b
4c
4d
4e
4f
89
86
82
84
86
86
1
2
1.5
1
7.
8.
3g
3h
1:25:25b
1:40:40c
8
6
5g
5h
72
69
a: Reaction was incomplete in 1:10:10 so carried out in 1:15:15.
b: Reaction started with 1:10:10 and gradually increased the ratio to 1:25:25
c: Reaction started with 1:10:10 and gradually increased the ratio to 1:40:40.

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We conclude that 5,7-diaryl-3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines and 1,3-diphenyl-5,7-diaryl-
1,2,3,4-tetrahydro-4-oxo-5H-pyrano[2,3-d]pyrimidines can be prepared by condensation of chalcones with
2-thiobarbiturates followed by reductive desulfurization with nickel boride in good yields.
EXPERIMENTAL
The melting points were determined on a Tropical Labequip apparatus and are uncorrected. IR spectra
were recorded on a Perkin Elmer FT-IR Spectrum 2000 and NMR spectra were recorded on Bruker
avance spectrospin dpx-300 MHz instrument, respectively. The FAB mass spectra were recorded on
JOEL SX 102/DA-6000 Spectrometer using Argon/Xenon as the FAB gas. Chalcones were prepared by
condensation of acetophenones with benzaldehydes by the general procedure of Kohler and Chadwell.8
1,3-Diphenyl-2-thiobarbituric acid was prepared by condensation of malonic acid and N, N'-
diphenylthiourea.9 2-Thiobarbituric acid (Merck) was used as such.
General Procedure for Preparation of 5,7-Diphenyl-1,2,3,4-tetrahydro-4-oxo-2-thioxo-5H-
pyrano[2,3-d]pyrimidines
In a typical procedure, a solution of chalcone (1a, 0.01 mol) and 2-thiobarbituric acid (2a, 0.01 mol) in
glacial acetic acid (30 mL) was taken in a 100 mL RB flask and P2O5 (10 g) was added to it. The contents
were refluxed for 30 min with constant stirring. The mixture was cooled and poured onto crushed ice.
The solid thus separated was filtered at pump, washed with dilute acetic acid, dried and crystallized from
EtOH to give 3a as white crystals. Yield: 95%. mp 290-291°C5; 1H-NMR (DMSO-d6): 4.4914-4.5071 (d,
J= 4.71, 1H, H-5), 5.9453-5.9609 (d, J= 4.68, 1H, H-6), 7.1970-7.7174 (m, 10H, H-Ar); IR (KBr) νmax
:
1567.76 (C=S), 1674.72 (C=O), 3433.88 (NH), 2840.28 (C–H), 1137.95 cm-1 (C–O–C); FAB MS:
335(M++1).
Compounds (3b-h) was obtained in a similar way. Spectroscopic data of all the new compounds are
reported here in.
3c: Crystallized from EtOH as light brown crystals. Yield: 80%. mp 248-250°C; 1H-NMR (DMSO-d6): δ
3.698-3.857 (2 x OCH3, 6H), 4.439-4.455 (d, J = 4.80 Hz, 1H, H-5), 5.658 (s, 1H, H-6), 6.798-6.919 (dd,
4H, 2′′,3′′,5′′ & 6′′), 7.205-7.231 (d, J=7.80, 2H, 3′ & 5′), 7.913-7.938 (d, J= 7.50, 2H, 2′ & 6′); IR (KBr)
ν
max: 1560.56 (C=S), 1668.23 (C=O), 3448.79 (NH), 2956.15 (C–H), 1034.65 (C–O–C), 833.53 cm-1 (p-
substituted aromatic); FAB MS: 395 (M++1); Anal. Calcd for C21H18N2O4S: C 63.94, H 4.59, N 7.10, S
8.13. Found: C 63.79, H 4.67, N 7.02, S 8.24.
1
3d: Crystallized from EtOH as white crystals. Yield: 86%. mp 272-274°C; H-NMR (DMSO-d6) : δ
2.279 (s, 3H, CH3), 4.496 (s, 1H, H-5), 5.755 (s, 1H, H-6), 7.086-7.368 (m, 11H, Ar′-H, Ar′′-H & 2N-H);

HETEROCYCLES, Vol. 68, No. 9, 2006
1889
IR (KBr) νmax: 3429.96 (NH), 2957.64 (C-H), 1666.63 (C=O), 1563.35 (C=S), 1054.12 (C–O–C), 859.37
cm-1 (p-substituted aromatic); FAB MS: 349 (M++1); Anal. Calcd for C20H16N2O2S: C 68.94, H 4.62, N
8.04, S 9.20. Found: C 68.76, H 4.67, N 7.95, S 9.24.
3e: Crystallized from EtOH as light yellow crystals. Yield: 89%. mp 280-285°C; 1H-NMR (DMSO-d6) :δ
4.537-4.552 (d, J = 4.50, 1H, H-5), 5.746-5.763 (d, J = 5.10, 1H, H-6), 7.235-7.637 (m, 10H, Ar′-H, Ar′′-
H & 2N-H); IR (KBr) νmax: 1672.12 (C=O), 1561.40 (C=S), 1092.53 (C–C1), 826.39 cm-1 (p-substituted
aromatic); FAB MS: 403 (M+); Anal. Calcd for C19H12N2O2Cl2S: C 56.58, H 2.99, N 6.94, S 7.95. Found:
C 56.60, H 3.02, N 6.82, S 7.97.
3f: Crystallized from EtOH as brick red powder. Yield: 84.7%. mp 290°C (decomp.); 1H-NMR (DMSO-
d6):δ 4.527-4.544 (d, J = 5.10, 1H, H-5), 5.821-5.838 (d, J = 5.10, 1H, H-6), 7.200-7.305 (m, 6H, Ar′′-H,
1N-H), 7.497-7.589 (m, 5H, Ar′-H, 3N-H); IR (KBr) νmax: 3435.11 (NH), 2955.07 (C-H), 1674.02 (C=O),
1568.70 (C=S), 845.31 cm-1 (p-substituted aromatic); FAB MS: 413 (M+), 415 (M++2); Anal. Calcd for
C19H13N2O2BrS: C 55.21, H 3.17, N 6.81, S 7.76. Found: C 55.33, H 3.24, N 6.74, S 7.85.
3h: Crystallized from EtOH as light brown powder. Yield: 68%. mp 295°C (decomp.); 1H-NMR (DMSO-
d6) :δ 4.629 (s, 1H, H-5), 5.697 (s, 1H, H-6), 3.777 (s, 2 x OCH3, 6H), 6.863 (s, 4H, 3′,5′,3′′ & 5′′ Ar-H),
7.145-7.478 (m, 4H, 2′, 6′, 2′′ & 6′′ Ar-H), 7.617 (m, 5H, N1Ar-H) 7.789-7.825 (m, 5H, N3Ar-H). . IR
(KBr) νmax: 1689.95 (C=O), 1511.41 (C=S), 839.86 cm-1 (p-substituted aromatic); FAB MS: 546 (M+);
Anal. Calcd for C33H26N2O4S: C 72.50, H 4.79, N 5.12, S 5.86. Found: C 72.42, H 4.89, N 5.17, S 5.90.
General Procedure for Preparation of 5,7-Diphenyl-3,4-dihydro-4-oxo-5H-pyrano[2,3-d]pyrimidines
In a typical procedure, 3a (0.2 g, 0.5988 mmol) and dry MeOH (10 mL) were placed in a 100 mL round
bottomed flask. The flask was mounted over a magnetic stirrer and the contents stirred. Anhyd. NiCl2
(0.7724 g, 5.9880 mmol) was added to the flask followed by NaBH4 (0.2275 g, 5.9880 mmol) at room
temperature. Addition of NaBH4 is highly exothermic. After complete disappearance of starting material
as monitored by TLC (90:10 AcOEt: benzene; v/v), the reaction mixture was filtered through a celite pad
(~ 1 inch). Nickel boride precipitate was washed with methanol (2 x 10 mL). The combined filtrate was
diluted with water (~ 40 mL) and extracted with AcOEt (3 x 60 mL). The combined AcOEt extract was
dried over anhyd. MgSO4, filtered and concentrated to afford 4a as white powder. Yield: 89%. mp 233°C;
1H-NMR (DMSO-d6) :δ 4.6306-4.6454 (d, J=4.44, 1H, H-5), 5.8757-5.8906 (d, J=4.47, 1H, H-6),
7.1818-7.6904 (m, 11H, Ar′-H, Ar′′-H & N-H), 8.0646 (s, 1H, H-2); IR (KBr) νmax: 1646.99 (C=O),
1608.62 (C=N); FAB MS: 303 (M++ 1); Anal. Calcd for C19H14N2O2: C 75.48, H 4.66, N 9.26. Found: C
75.43, H 4.59, N 9.37.

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HETEROCYCLES, Vol. 68, No. 9, 2006
Compounds (4b-f) and (5g-h) were obtained in similar way.
4b: White powder. Yield: 86%. mp 236-237°C; 1H-NMR (DMSO-d6): δ 3.7774 (s, 3 x O-CH3), 4.5856-
4.6001 (d, J = 4.35, 1H, H-5), 5.7842-5.7996 (d, J = 4.62, 1H, H-6), 6.9634-6.9916 (d, J= 8.46, 2H, 3′ &
5′-H), 7.1867-7.2688 (m, 5H, Ar′′-H), 7.5973-7.6254 (d, J= 8.43, 2H, 2′ & 6′-H), 8.0940 (s, 1H, H-2); IR
(Nujol) νmax: 1642.00 (C=O), 1607.75 (C=N); FAB MS: 333 (M++1); Anal. Calcd for C20H16N2O3: C
72.27, H 4.85, N 8.42. Found: C 72.42, H 4.79, N 8.37.
4c: Light yellow powder. Yield: 82%. mp 247-250°C; 1H-NMR (DMSO-d6) :δ 3.6994 (s, 3H, 4′-OCH3),
3.7808 (s, 3H, 4′′-OCH3), 4.5210-4.5358 (d, J = 4.44, 1H, H-5), 5.7573-5.7723 (d, J = 4.50, 1H, H-6),
6.8308-6.8576 (d, J= 8.05, 2H, 3′ & 5′-H), 6.9670-6.9943 (d, J= 8.19, 2H, 3′′ & 5′′-H), 7.1550-7.1818 (d,
J=8.04, 2H, 2′′ & 6′′-H), 7.5949-7.6224 (d, J=8.25, 2H, 2′ & 6′-H), 8.0751 (s, 1H, H-2); IR (KBr) νmax
:
1642.60 (C=O), 1608.71 (C=N); FAB MS: 363 (M++1); Anal. Calcd for C21H18N2O4: C 69.60, H 3.89, N
7.73. Found: C 69.49, H 3.91, N 7.57.
1
4d: White powder. Yield: 84%. mp 258-260°C; H-NMR (DMSO-d6) :δ 2.2837 (s, 3H, CH3), 4.6216-
4.6369 (d, J = 4.59, 1H, H-5), 5.7364-5.7524 (d, J = 4.80, 1H, H-6),7.0681-7.0930 (d, J= 7.47,2H, 2′′ &
6′′-H), 7.1856-7.2116 (d, J= 7.80, 2H, 5′′ &3′′-H), 7.3653-7.3892 (m, 3H, 3′,4′& 5′-H), 7.6534-7.6749 (d,
J=6.45, 2H, 2′ & 6′-H), 7.9444 (s, 1H, H-2); IR (KBr) νmax: 1667.96 (C=O), 1562.83 (C=N); FAB MS:
317 (M++1); Anal. Calcd for C20H16N2O2: C 75.93, H 5.09, N 8.85. Found: C 75.76, H 5.02, N 8.69.
1
4e: White powder. Yield: 86%. mp 205°C (decompose); H-NMR (DMSO-d6) :δ 4.7204-4.7359 (d, J =
4.65, 1H, H-5), 6.0709-6.0870 (d, J = 4.83, 1H, H-6), 7.3612-7.4496 ( q, 5H, 2′′, 3′′, 5′′, 6′′ & N3-H),
7.5626-7.5909 (d, J= 8.49, 2H, 3′ & 5′-H), 7.7758-7.8037 (d, J= 8.37, 2H, 2′ & 6′-H), 8.2028 (s, 1H. H-
2); IR (KBr) νmax: 1641.91 (C=O), 1607.04 (C=N); FAB MS: 371 (M+); Anal. Calcd for C19H12N2O2Cl2:
C 61.47, H 4.34, N 7.54. Found: C 61.65, H 4.29, N 7.64.
4f: Light yellow powder. Yield: 86%. mp 255°C (decompose); 1H-NMR (DMSO-d6): δ 4.6805-4.6941 (d,
J= 4.08,1H. H-5), 6.0937-6.1093 (d, J= 4.68, 1H, H-6), 7.2646-7.7025 (m, 10 H, 4 Ar′ -H, 5 Ar′′ -H, 1
N3-H), 8.1814 (s, 1H, H-2); IR (KBr) νmax: 1650.33 (C=O), 1610.88 (C=N); FAB MS: 381 (M++1); Anal.
Calcd for C19H13N2O2Br: C 60.02, H 3.44, N 7.36. Found: C 59.84, H 3.33, N 7.49.
1
5g: White powder. Yield: 72%. mp 192-194 °C; H-NMR (DMSO-d6): δ 4.8582-4.8761 (d, J= 5.37,1H,
H-5), 5.1757-5.2143 (d, J= 11.58, 1H, H-2), 5.5245-5.5630 (d, J= 11.55, 1H, H-2), 5.9033-5.9213 (d, J=
5.4, 1H, H-6), 7.1226-7.4303 (m, 20 H, 5 Ar′ -H, 5 Ar′′ -H, 10 N-Ar-H); IR (Nujol) νmax: 1614.68 (C=O);
FAB MS: 457 (M++1); Anal. Calcd for C31H24N2O2: C 81.55, H 5.29, N 6.13. Found: C 81.44, H 5.27, N 6.20.

HETEROCYCLES, Vol. 68, No. 9, 2006
1891
5h: Light brown powder. Yield: 69%. mp 186-189°C; 1H-NMR (DMSO-d6): δ 3.7670 (s, 6H, 2x OCH3),
4.7578-4.7756 (d, J= 5.34,1H. H-5), 5.1766-5.2155 (d, J= 11.67, 1H, H-2), 5.4969-5.5355(d, J= 11.58,
1H, H-2), 5.7448-5.7629 (d, J= 5.43, 1H, H-6), 7.0892-7.4526 (m, 18 H, 4 Ar′ -H, 4 Ar′′ -H, 10 N-Ar-H);
IR (KBr) νmax: 1653.54 (C=O); FAB MS: 516 (M++1); Anal. Calcd for C33H28N2O4: C 76.72, H 5.46, N
5.42. Found: C 76.81, H 5.39, N 5.35.
ACKNOWLEDGEMENT
The author AA is thankful to CSIR, New Delhi, India for providing JRF and SRF.
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