Mild and efficient oxidative aromatization of 4-substituted-1,4- dihydropyrimidines using (diacetoxyiodo)benzene

Article abstract of DOI:10.1002/jhet.389

(Chemical Equation Presented) 4-Alkyl or aryl-1,4-dihydropyrimidines were readily oxidized by (diacetoxyiodo)benzene under mild reaction conditions to the corresponding pyrimidine derivatives in good to excellent yields.

Full text of DOI:10.1002/jhet.389

740
Mild and Efficient Oxidative Aromatization of 4-Substituted-
1,4-dihydropyrimidines Using (Diacetoxyiodo)benzene
Vol 47
Nandkishor N. Karade,* Sumit V. Gampawar, Nilesh P. Tale,
and Sanjay B. Kedar
Department of Chemistry, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur,
Maharashtra 440 033, India
*E-mail: nnkarade@gmail.com
Additional Supporting Information may be found in the online version of this article.
Received June 28, 2009
DOI 10.1002/jhet.389
Published online 3 May 2010 in Wiley InterScience (www.interscience.wiley.com).
4-Alkyl or aryl-1,4-dihydropyrimidines were readily oxidized by (diacetoxyiodo)benzene under mild
reaction conditions to the corresponding pyrimidine derivatives in good to excellent yields.
J. Heterocyclic Chem., 47, 740 (2010).
INTRODUCTION
4-Aryl-3,4-dihydropyrimidin-2(1H)-ones
tion metal based oxidizing agents such as Mn(OAc)3
[8], MnO2 [6], (NH4)2Ce(NO3)6 [6], and CuCl2/Na2CO3/
tert-BuOOH [9]. The optimized conditions for the oxida-
tive aromatization of 2-methylthio-1,4-dihydropyrimi-
dine requires four or five equivalents of Mn(OAc)3 or
MnO2 respectively under different reaction conditions.
Thus, there is a need for the development of an efficient
and general method for the oxidative aromatization of
4-aryl or alkyl-1,4-dihydropyrimidine.
(Biginelli
compounds, DHPMs) represent an azaheterocyclic sys-
tem of remarkable pharmacological profile [1]. It was
investigated during 1980s and 1990s that DHPMs ex-
hibit a similar pharmacological profile to the Hantzsch’s
1,4-dihydropyridine calcium channel modulators of the
nifedipine type drugs [2]. In particular, the 2-heterosub-
stituted 4-aryl-1,4-dihydro-6-methyl-5-pyrimidinecarbox-
ylic acid esters were investigated by Atwal and co-
workers as potent mimics of 1,4-dihydropyridine [3].
The metabolism of these drugs involves an oxidative
dehydrogenation of 1,4-dihydro ring nucleus to the cor-
responding aromatic derivatives, which is catalyzed in
the liver by cytochrome P-450 [4]. The chemical oxida-
tion of 4-substituted-1,4-dihydropyrimidine also provides
an easy access to multi-substituted pyrimidine deriva-
tives, which are further known to exhibit anti-anoxic
and anti-lipid peroxidation activities [5]. Recently, the
S-alkylation of 4-aryl-3,4-dihydropyrimidin-2(1H)-thione
followed by oxidative aromatization has been demon-
strated for the generation of a variety of 2-substituted
pyrimidines via displacement of the reactive sulfonyl
group with nitrogen, oxygen, sulfur, and carbon nucleo-
philes [6]. In contrast to Hantzsch 1,4-dihydropyridine
[7], the chemical oxidative aromatization of 1,4-dihydro-
pyrimidine is relatively less investigated reaction. All
the reported methods for the chemical oxidation of 4-
substituted-1,4-dihydropyrimidine utilize mainly transi-
Hypervalent iodine reagents are used extensively in
organic synthesis as a mild, safe, and economical alter-
native to heavy metal reagents [10]. A literature survey
showed that phenyliodine(III) bis(trifluoroacetate)
[PhI(OCOCF3)2] can be used for the solid state oxida-
tion of Hantzsch’s 1,4-dihydropyridines under micro-
wave irradiation conditions [11]. Similarly, (diacetoxyio-
do)benzene has been reported for the oxidative aromati-
zation of 1,3,5-trisubstituted pyrazolines [12] and 2-imi-
dazolines [13]. Recently, we have reported a clean and
efficient oxidative dehydrogenation of 3,4-dihydropyri-
midin-2(1H)-ones to 1,2-dihydropyrimidines using a
combination of (diacetoxyiodo)benzene and tert-butylhy-
droperoxide in CH2Cl2 [14]. The application of hyperva-
lent iodine reagents for the oxidative aromatization of
1,4-dihydropyrimidines is hitherto unknown in the litera-
ture. Herein, we wish to report a simple and highly effi-
cient oxidative dehydrogenation of 4-substituted-1,4-
dihydropyrimidine to afford the multi-substituted pyrim-
idine derivatives by using (diacetoxyiodo)benzene (DIB)
as the mild oxidizing agent (Scheme 1).
C
V
2010 HeteroCorporation

May 2010
Mild and Efficient Oxidative Aromatization of 4-Substituted-
1,4-dihydropyrimidines Using (Diacetoxyiodo)benzene
741
Scheme 1
Scheme 2. Preparation of 1,4-dihydropyrimidine.
RESULTS AND DISCUSSION
Variously substituted 4-aryl or alkyl-1,4-dihydropyri-
midines precursor were prepared via two steps: (a) the
Biginelli reaction involving three component condensa-
tion of ethyl acetoacetate, aldehyde and thiourea under
reflux conditions in ethanol using a catalytic quantity of
HCl to afford 3,4-dihydropyrimidin-2(1H)-thiones and
(b) the S-methylation reaction of the resulting 3,4-dihy-
dropyrimidin-2(1H)-thiones using MeI/K2CO3/acetone
system (Scheme 2). The structures of 4-aryl or alkyl-
2a–k except 2a are new compounds in the literature and
the structures of these products were confirmed from IR,
1H, and C13 NMR spectroscopy and LCMS analysis.
1
1
1,4-dihydropyrimidines were confirmed by IR, H NMR,
and LCMS spectra.
The H NMR of the precursor 4-aryl-1,4-dihydropyrimi-
dine showed characteristic peak around d 5.82 due to C-
4 proton and a broad peak around d 6.36 due to N-H
proton. These two signals were found to be absent in
The oxidative aromatization of 1a (R ¼ C6H5) was
selected as a model reaction using different iodine based
oxidizing agents such as I2, I2/K2CO3, KIO3, PhI(OAc)2,
and Dess-Martin periodinane and the results are sum-
marized in Table 1. The oxidative aromatization using
molecular iodine in MeOH was previously reported for
the oxidative aromatization of Hantzsch’s 1,4-dihydro-
pyridine [15]. It is interesting to mention that molecular
iodine and I2/K2CO3 (Table 1, entry 1 and 2) did not
produce oxidative aromatization of 1a in satisfactory
yields. The oxidation of 1a to 2a took place in 63%
yield using the pentavalent hypervalent iodine reagent,
Dess-Martin periodinane (DMP) but with a relatively
long reaction time of 12 h. The trivalent hypervalent io-
dine reagent, PhI(OAc)2 was found to be the most effec-
tive reagent to produce the aromatized product 2a in
89% yield within 1 h.
1
the H NMR spectra of the aromatized product 2. The
methyl group at 6-position of 4-aryl-1,4-dihydropyrimi-
dine appears at d 2.37 which is shifted downfield to d
2.55 in the case of aromatized product.
A tentative reaction mechanism is shown in Scheme
3. The oxidative aromatization takes place around the
coordination sphere of trivalent iodine, DIB 3. The
ligand exchange reaction between 1,4-dihydropyrimidine
1 and DIB 3 forms a resonance stabilized carbocation 4
which is subsequently deprotonated to form 5. The con-
comitant reductive elimination of iodobenzene from 5
leads to form aromatized product 2.
Table 1
Optimization of the reaction conditions.
With optimal conditions in hand, the reaction of dif-
ferent 4-substituted-1,4-dihydropyrimidines with DIB
was examined to explore the scope of the reaction (Ta-
ble 2). In all the cases, the expected products 2a–l were
obtained in excellent yields. All the reactions were car-
ried out at room temperature using stochiometric use of
DIB. 4-Aryl-1,4-dihydropyrimidine containing either an
electron-withdrawing group or an electron-donating
group all afforded the corresponding products smoothly
(Table 2, entries 2a–h). The oxidative aromatization of
4-alkyl-1,4-dihydropyridine is most frequently accompa-
nied by the dealkylation reaction. The side product for-
mation due to the dealkylation reaction was also
observed in the DIB mediated oxidative aromatization
of 4-alkyl-1,4-dihydropyrimidine (Table 2). Dealkylation
was a major pathway in the case of i-propyl at 4-posi-
tion of 1,4-dihydropyrimidine. The aromatized products
Oxidizing
agent
Reaction time
and conditions
Yield of
2a (%)a
Entry
Solvent
1
2
3
4
5
6
I2
MeOH
MeOH
MeOH
CH2Cl2
MeOH
CH2Cl2
12 h, reflux
12 h, reflux
12 h, reflux
12 h, r.t.
09
22
00
63
73
89
I2/K2CO3
KIO3
DMP
PhI(OAc)2
PhI(OAc)2
12 h, r.t.
1 h, r.t.
a Isolated yields.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

742
N. N. Karade, S. V. Gampawar, N. P. Tale, and S. B. Kedar
Vol 47
Table 2
Oxidative aromatization of 4-substituted-1,4-dihydropyrimidine using (diacetoxyiodo)benzene.
Yield (%)a,b
Entry
Substrate, 1 R
2
3
a
b
c
d
e
f
C6H5
4ACH3C6H4
3ACH3OC6H4
4ACH3OC6H4
3,4A(CH3O)2C6H3
3,4,5A(CH3O)3C6H2
4AClC6H4
81
75
78
79
74
77
76
73
–
–
–
–
–
–
–
g
h
i
–
–
3ANO2C6H4
iAC3H7
(CH3)2CHCH2
69
–
–
j
k
66
63
CH3(CH2)5
a Isolated yields after chromatography.
b All the reactions were carried out at room temperature stirring of 1 h.
uct in excellent purity. The products were characterized by the
comparison of melting points with the literature value.
General procedure for the preparation of 4-substituted-1,
4-dihydropyrimidines. A suspension of 4-substituted-3,4-dihy-
dropyrimidin-2(1H)-thione (3 mmol) in acetone (15 mL) was
treated with finely ground potassium carbonate (1.0 g, 7.25
mmol) and methyl iodide (3.5 mmol). The reaction was
allowed to stir at room temperature overnight and was diluted
with ethyl acetate. It was filtered and the filtrate was washed
with water and brine and dried over magnesium sulfate. The
solvent was evaporated and the residue was crystallized from
ether-hexanes to provide 4-substituted-1,4-dihydropyrimidine
as a colorless solid.
CONCLUSION
In summary we have developed a general and practi-
cal route for the oxidative aromatization of 4-substi-
tuted-1,4-dihydropyrimidine using (diacetoxyiodo)ben-
zene as the safe oxidizing agent. The salient features of
this methodology are: (a) mild reaction conditions, (b)
transition metal-free protocol, (c) no excessive use of
the oxidant, (d) short reaction time, and (e) an easy ex-
perimental procedure.
General procedure for the oxidation/aromatization of
4-substituted-1,4-dihydro-(2-methylthio)-pyrimidines. To
EXPERIMENTAL
a
stirred solution of appropriate 4-substituted-1,4-dihydro-(2-
methylthio)-pyrimidine (2 mmol) in CH2Cl2 (10 mL) was
added (diacetoxyiodo)benzene (0.644 g, 2 mmol) at room tem-
perature. The reaction mixture was allowed to stir at room
temperature for 1 h. The progress of the reaction was moni-
tored by TLC. After the completion of reaction, the solvent
was removed under vacuum and the crude product was puri-
fied using column chromatography (silica gel, petroleum ether-
ethyl acetate) to give the corresponding aromatized product in
good yield.
General. All melting points are uncorrected. The glassware
was routinely oven-dried at 110ꢀC for a minimum of 4 h. Col-
umn chromatography was performed on silica gel 70–230
mesh. 1H and 13C NMR spectra were recorded on a Bruker
13
Advance 400 DPX spectrometer (1H at 400 MHz and
100 MHz) in CDCl3 with TMS as the internal standard. FTIR
C at
spectra were determined on
spectrometer.
a PerkinElmer 100 FTIR
General procedure for the preparation of 4-substituted-3,
4-dihydropyrimidin-2(1H)-thione. A mixture of appropriate
aldehyde (20 mmol), ethyl acetoacetate (20 mmol), thiourea
(22 mmol), and HCl (2 mL) in EtOH (40 mL) was refluxed
for 24 h. The reaction was monitored by TLC. After comple-
tion of reaction, the mixture was slowly poured to crushed ice
and the resulting solid was filtered off. The crude Biginelli
compound was recrystallized from ethanol to afford the prod-
Ethyl 4-methyl-2-(methylthio)-6-phenylpyrimidine-5-carboxy-
late (2a). IR (KBr): mmax ¼ 2926, 1722, 1582, 1534, 1225,
1
1080, 753, 698 cmꢁ1. H NMR (400 MHz, CDCl3): d 1.04 (t,
J ¼ 9.2 Hz, 3H), 2.57 (s, 3H), 2.62 (s, 3H), 4.18 (q, J ¼ 9.2
Hz, 2H), 7.46 (m, 3H), 7.62 (m, 2H). 13C NMR (100
MHz,CDCl3): d 13.55, 14.13, 22.57, 61.64, 120.89, 128.27,
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

May 2010
Mild and Efficient Oxidative Aromatization of 4-Substituted-
1,4-dihydropyrimidines Using (Diacetoxyiodo)benzene
743
Scheme 3. Reaction mechanism.
128.39, 130.03, 137.72, 163.54, 165.42, 168.09, 172.45.
LCMS (Mþ1) ¼ 289.
(s, 3H), 2.62 (s, 3H), 3.88 (s, 3H), 3.94 (s, 3H), 4.21 (q, J ¼
7.08 Hz, 2H), 6.91 (d, J ¼ 8.32 Hz, 1H) 7.27 (m, 2H). 13C
NMR (100 MHz,CDCl3): d 13.49, 14.21, 19.99, 55.74, 59.77,
109.81, 110.35, 110.86, 118.72, 137.42, 148.15, 148.64,
166.77. LCMS (Mþ1) ¼ 349.
Ethyl 4-methyl-2-(methylthio)-6-p-tolylpyrimidine-5-carboxy-
late (2b). IR (KBr): mmax ¼ 2925, 2853, 1722, 1613, 1574,
1532, 1224, 1078, 864, 793 cmꢁ1 1H NMR (400 MHz,
.
CDCl3): d 1.09 (t, J ¼ 7.00 Hz, 3H), 2.35 (s, 3H), 2.54 (s,
3H), 2.60 (s, 3H), 4.18 (q, J ¼ 7.00 Hz, 2H), 7.23 (d, J ¼
8.04 Hz, 2H), 7.56 (d, J ¼ 8.28 Hz, 2H). 13C NMR (100
MHz,CDCl3): d 13.72, 14.20, 21.44, 22.62, 61.74, 120.77,
128.34, 129.21, 134.83, 140.49, 163.39, 165.28, 168.40,
172.35. LCMS (Mþ1) ¼ 303.
Ethyl 4-(3,4,5-trimethoxyphenyl)-6-methyl-2-(methylthio)pyr-
imidine-5-carboxylate (2f). IR (KBr): mmax ¼ 2935, 2841,
1722, 1587, 1536, 1504, 1415, 1225, 1127, 1006, 797, 710
cmꢁ1
.
1H NMR (400 MHz, CDCl3): d 1.11 (t, J ¼ 7.2 Hz,
3H), 2.55 (s, 3H), 2.71 (s, 3H), 3.89 (s, 9H), 4.19 (q, J ¼ 7.2
Hz, 2H), 6.90 (s, 2H). 13C NMR (100 MHz,CDCl3): d 13.78,
14.17, 22.50, 29.68, 56.19, 60.94, 61.83, 105.67, 120.94,
133.00, 139.81, 153.26, 163.06, 165.26, 168.37, 172.37.
LCMS (Mþ1) ¼ 379.
Ethyl
4-(3-methoxyphenyl)-6-methyl-2-(methylthio)pyrimi-
dine-5-carboxylate (2c). IR (KBr): mmax ¼ 2928, 2851, 1723,
1
1601, 1536, 1429, 1223, 1122, 1046, 783, 703 cmꢁ1. H NMR
(400 MHz, CDCl3): d 1.07 (t, J ¼ 7.08 Hz, 3H), 2.55 (s, 3H),
2.61 (s, 3H), 3.84 (s, 3H), 4.19 (q, J ¼ 7.08 Hz, 2H), 7.01 (m,
1H), 7.19 (m, 2H), 7.33 (t, J ¼ 7.88 Hz, 1H). 13C NMR (100
MHz,CDCl3): d 13.67, 14.21, 22.63, 55.38, 61.77, 113.55,
116.08, 120.68, 121.06, 129.53, 139.03, 159.65, 163.37,
165.43, 168.15, 172.48. LCMS (Mþ1) ¼ 319.
Ethyl 4-methyl-2-(methylthio)-6-(3-nitrophenyl)pyrimidine-
1
5-carboxylate (2h). H NMR (400 MHz, CDCl3): d 1.13 (t, J
¼ 7.2 Hz, 3H), 2.60 (s, 3H), 2.62 (s, 3H), 4.25 (q, J ¼ 7.2 Hz,
2H), 7.65 (t, J ¼ 7.92 Hz, 1H), 7.19 (m, 1H), 8.34 (m, 1H),
8.53 (s, 1H). 13C NMR (100 MHz, CDCl3):d 13.75, 14.24,
22.85, 29.69, 62.12, 120.92, 123.46, 124.69, 129.58, 134.33,
139.28, 148.23, 161.00, 166.21, 167.40, 173.19. LCMS (Mþ1)
¼ 334.
Ethyl
4-(4-methoxyphenyl)-6-methyl-2-(methylthio)pyrimi-
dine-5-carboxylate (2d). IR (KBr): mmax ¼ 2928, 1721, 1608,
1
1580, 1531, 1509, 1402, 1255, 1224, 1079, 865, 797 cmꢁ1. H
Ethyl
4-methyl-2-(methylthio)pyrimidine-5-carboxylate
NMR (400 MHz, CDCl3): d 1.13 (t, J ¼ 9.6 Hz, 3H), 2.54 (s,
3H), 2.61 (s, 3H), 3.86 (s, 3H), 4.24 (q, J ¼ 9.6 Hz, 2H), 6.96 (d,
J ¼ 9.2 Hz, 2H), 7.65 (d, J ¼ 9.2 Hz, 2H). 13C NMR (100
MHz,CDCl3): d 13.72, 14.07, 22.47, 55.31, 61.63, 113.83,
114.15, 129.98, 161.33, 162.59, 165.09, 168.48, 172.11. LCMS
(Mþ1) ¼ 319.
(2i). IR (KBr): mmax ¼ 2925, 1724, 1641, 1564, 1528, 1403,
1326, 1281, 1200, 1091, 1045 cmꢁ1 1H NMR (400 MHz,
.
CDCl3): d 1.41 (t, J ¼ 7.12 Hz, 3H), 2.59 (s, 3H), 2.84 (s,
3H), 4.39 (q, J ¼ 7.24 Hz, 2H), 8.94 (s, 1H). LCMS (Mþ1) ¼
255.
Ethyl 4-isobutyl-6-methyl-2-(methylthio)pyrimidine-5-carbox-
ylate (2j). IR (KBr): mmax ¼ 2958, 2928, 2869, 1725, 1542,
Ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-(methylthio)pyri-
midine-5-carboxylate (2e). IR (KBr): mmax ¼ 2931, 2836,
1430, 1274, 1240, 1184, 1100 cmꢁ1 1H NMR (400 MHz,
.
1696, 1603, 1513, 1260, 1233, 1141, 1095, 795 cmꢁ1
.
1H
CDCl3): d 0.93 (d, J ¼ 5.96 Hz, 6H), 1.39 (t, J ¼ 7.1 Hz,
NMR (400 MHz, CDCl3): d ¼ 1.15 (t, J ¼ 7.08 Hz, 3H), 2.54
3H), 2.17 (m, 1H), 2.46 (s, 3H), 2.56 (s, 3H), 2.62 (d, J ¼
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

744
N. N. Karade, S. V. Gampawar, N. P. Tale, and S. B. Kedar
Vol 47
7.12 Hz, 3H), 4.41 (q, J ¼ 7.1 Hz, 2H). 13C NMR (100
MHz,CDCl3): d 14.05, 14.16, 22.48, 22.87, 28.33, 44.23,
61.62, 122.22, 164.52, 167.19, 167.77, 171.97. LCMS (Mþ1)
¼ 269.
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Baarnhielm, C.; Berntsson, P. J Med Chem 1991, 34, 1838.
[5]
(a) Kuno, A.; Sugiyama, Y.; Katsuta, K.; Kamitani, T.;
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Sugiyama, Y.; Katsuta, K.; Kamitani, T.; Takasugi, H. Chem Pharm
Bull 1992, 9, 2423.
Ethyl 4-hexyl-6-methyl-2-(methylthio)pyrimidine-5-carboxy-
late (2k). IR (KBr): mmax ¼ 2956, 2928, 2856, 1725, 1652,
1541, 1402, 1228, 1102, 1078 cmꢁ1 1H NMR (400 MHz,
.
[6] Matloobi, M.; Kappe, C. O. J Comb Chem 2007, 9, 275.
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CDCl3): d 0.90 (t, 3H), 1.29 (m, 8H), 1.36 (t, J ¼ 7.16 Hz, 3H),
2.46 (s, 3H), 2.56 (s, 3H), 2.71 (t, J ¼ 6.4 Hz, 2H), 4.37 (q, J ¼
7.16 Hz, 2H). LCMS (Mþ1) ¼ 297.
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Acknowledgment. The authors are thankful to the Department
of Science and Technology (No. SR/FTP/CS-77/2005) and the
University Grants Commission, New Delhi, India (No. MRP 32-
245/2006 SR), for the financial support.
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DOI 10.1002/jhet