Light-induced free radical oxidation of 2-oxo-1,2,3,4-tetrahydropyrimidines

Article abstract of DOI:10.1515/znb-2012-0313

A variety of 4-substituted 5-acetyl- and 5-carboethoxy-2-oxo-1,2,3,4- tetrahydropyrimidines were oxidized under UV irradiation in the presence or absence of benzoyl peroxide. The nature of the substituents on the 4- and 5-positions of the heterocyclic rin

Full text of DOI:10.1515/znb-2012-0313

Light-induced Free Radical Oxidation
of 2-Oxo-1,2,3,4-tetrahydropyrimidines
Hamid Reza Memariana, Leila Hejazia, and Asadallah Farhadib
a Catalysis Division, Department of Chemistry, Faculty of Science, University of Isfahan,
81746-73441 Isfahan, I. R. Iran
b Faculty of Petroleum Engineering, Petroleum University of Technology of Ahwaz, Ahwaz,
I. R. Iran
Reprint requests to Prof. Hamid R. Memarian. Fax: +98-311-6689732.
E-mail: memarian@sci.ui.ac.ir
Z. Naturforsch. 2012, 67b, 263 – 268; received January 15, 2012
A variety of 4-substituted 5-acetyl- and 5-carboethoxy-2-oxo-1,2,3,4-tetrahydropyrimidines were
oxidized under UV irradiation in the presence or absence of benzoyl peroxide. The nature of the
substituents on the 4- and 5-positions of the heterocyclic ring affects the rate of photo-oxidation, and
irradiation of these compounds in the presence of benzoyl peroxide decreases the time of reaction
drastically. Removal of 4-H by a benzoyloxy radical under formation of a trihydropyrimidinoyl radi-
cal intermediate occurs in the rate-determining step. The stability of this benzylic and allylic radical
intermediate is affected by the nature and the position of the additional substituent on the phenyl
group located at C-4.
Key words: Benzoyl Peroxide, Dihydropyrimidinones, Photo-oxidation, Substituent Effects,
Tetrahydropyrimidinones
Introduction
and sulfur [16] have been used for this purpose, but
none of these reagents is ideal, particularly due to their
2-Oxo-1,2,3,4-tetrahydropyrimidine
derivatives safety profile, lower yield and difficulty in product
(THPMs), also known as 3,4-dihydropyrimidin- isolation.
2(1H)-ones or Biginelli compounds, have received
considerable interest in recent times because of
their promising activities, as e. g. antihypertensive,
antitumor, antibacterial, anti-inflammatory agents,
and calcium channel blockers [1 – 6]. Moreover,
3-substituted 3H-pyrimidin-2(1H)-ones are phar-
macophores present in many biologically active
compounds [7]. There are a wide variety of methods
to synthesize pyrimidinones [8], but the efficient
synthesis of 1,2-dihydropyrimidinones by oxida-
tion of 1,2,3,4-tetrahydropyrimidinones has rarely
been explored [9 – 11]. In contrast to Hantzsch-type
1,4-dihydropyridines, whose aromatization to the
corresponding pyridines, either thermally [12] or
photochemically [13], is typically facile, the dehy-
drogenation of 1,2,3,4-tetrahydropyrimidinones is
known to be non-trivial [9, 11]. Various powerful and
mild oxidants such as chloranil, DDQ, KMnO4/clay,
MnO2, PCC, NaNO2/AcOH [9] or RuCl3/O2 in
AcOH [14], Co(NO3)2·6H2O/K2S2O8 [9], and also
dehydrogenating agents such as Pd/C [8], Br2 [15]
Benzoyl peroxide (BPO) is known as radical ini-
tiator and oxidizing agent in many organic reactions
[17 – 19]. This compound is very unstable and de-
grades to the benzyloxy radical in solution via a free
radical mechanism by applying heat or UV irradia-
tion [20]. This occurs because of the instability of the
O–O bond (bond energy ∼ 120 kJ mol−1) [21]. The
decomposition rate of BPO is dependent on the na-
ture of the solvent and also the presence of an oxy-
gen atmosphere. It has been shown that decomposition
of BPO in propan-2-ol is high among various alcohols
and ethers [22], and the presence of oxygen decreases
the rate of decomposition [23].
Numerous reports have been published on the
application of ultraviolet radiation in the oxida-
tion of organic compounds, but not for photo-
oxidation of the title compounds. Recently we have
reported the oxidation of various 4-arylsubstituted
5-acetyl- and 5-carboethoxy-2-oxo-1,2,3,4-tetrahydro-
pyrimidines (THPMs) under thermal [24] and sono-
thermal conditions [25, 26], and microwave irradia-
c
ꢀ 2012 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM

264
H. R. Memarian et al. · Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines
Table 1. The effect of the BPO / THPM ratio on the yield and
irradiation time in Ar-saturated CH3CN.
tion [27] by potassium peroxydisulfate (PPS) in aque-
ous acetonitrile, and also by exposing them to UV light
in chloroform solution [28, 29]. The results obtained
by the reaction in the presence of PPS indicate that
in situ formed hydroxyl radicals are responsible for
the removal of the 4-hydrogen in the rate-determining
step. Therefore due to steric and electronic effects,
the oxidation of 5-carboethoxy derivatives is faster
than that of the corresponding 5-acetyl compounds. In
contrast to these results, oxidation of 5-acetyl deriva-
tives under UV irradiation is faster than that of the
corresponding 5-carboethoxy compounds. This finding
is explained by better light absorption of the acetyl
derivatives compared to the carboethoxy derivatives.
Therefore, according to the proposed electron transfer-
induced photo-oxidation in chloroform (electron trans-
fer from excited THPMs to CHCl3), shorter irradiation
times are expected for the former compounds.
THPM
R1
BPO / THPM
Irradiation
time (h)
Conversion
(%)
1a
1a
1a
1a
1i
C6H5
C6H5
C6H5
C6H5
4-BrC6H4
4-BrC6H4
4-BrC6H4
1 : 1
2 : 1
3 : 1
4 : 1
4 : 1
5 : 1
6 : 1
47
33
31
17
16
8
40
∼ 100
∼ 100
∼ 100
100
100
100
1i
1i
5
Table 2. Solvent effect on the photooxidation of 5-acetyl-
THPM (2c, R1 = 4-CH3OC6H4).
BPO : 2c
4 : 1
4 : 1
Solvent
CH3CN
propan-2-ol
Irradiation time (h)
Conversion (%)
4
4
100
60
The results presented in Table 1 indicated that by in-
creasing the amount of BPO, as expected, the reaction
In continuation of these studies, we were interested time was shortened. Based on the light absorption by
to investigate the behavior of these compounds towards
the benzoyloxy radical obtained by UV irradiation of
BPO to elucidate
BPO and also THPMs under experimental conditions
(λ ≥ 280 nm), and a comparison of the molar extinc-
tion coefficients of THPMs and BPO at 310 nm (in-
1. the effect of the nature of the acetyl group ver- tensive emission line of mercury vapor), and also by
sus the carboethoxy group in 5-position on the rate of avoiding the saturation of the reaction medium by BPO
reaction;
as a radical precursor, it was found that selective exci-
2. the effect of the nature and position of the addi- tation of BPO was achieved by taking a ratio of 4 : 1 for
tional substituent on the phenyl group attached to C-4 BPO/THPMs. Since the light absorption by THPMs
of the heterocyclic ring;
cannot be totally excluded even in the presence of four
equivalents of BPO, photoreactions were carried out
simultaneously in the presence or the absence of BPO
under argon and oxygen atmosphere to determine also
the extent of photo-oxidation of THPMs in the absence
of BPO.
3. the possible competition between the hydro-
gen abstraction from 6-CH3 (as allylic hydrogen) and
4-H (as both allylic and benzylic hydrogen); and
finally
4. the effect of atmospheric oxygen and argon on
the photo-oxidation of tetrahydropyrimidinones.
In order to study the effect of the solvent in the
photo-oxidation of tetrahydropyrimidinones by BPO,
especially the effect of a hydrogen-donating solvent, a
mixture of 4 mM of BPO and 1 mM of compound 2c
as test substrate in propan-2-ol or acetonitrile were ir-
radiated simultaneously with a 400 W high-pressure
mercury lamp until maximum consumption of 2c. The
results presented in Table 2 show that the required ir-
radiation time in acetonitrile is short, and the output
is higher than in propan-2-ol. This retarding effect in
propan-2-olis probably due to the presence of an active
secondary hydrogen atom in this molecule. The ini-
tially formed benzoyloxy radical (BOR) abstracts this
easily accessible and active hydrogen atom and turns
to benzoic acid rather than removing hydrogen from
THPM to start oxidation. Therefore an increase of ir-
radiation time and an incomplete reaction is expected
Results and Discussion
Optimization of the BPO/THPM ratio
The optimization of the BPO/THPM ratio was car-
ried out by irradiation of compounds 1a and 1i in
acetonitrile as an inert solvent to observe the maxi-
mum consumption of these compounds (Scheme 1).
Scheme 1. See Table 3 for R1.
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM

H. R. Memarian et al. · Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines
265
THPMs R1
R2
Prod. BPO/Ar or (Ar)a BPO/O2 or (O2)a
Irrad. time (h) Conv. (%)
Table 3. Photo-oxidation of 5-
carboethoxy-THPMs (1a – 1n)
and 5-acetyl-THPMs (2a – 2o)
in dry acetonitrile in the pres-
ence or absence of BPO under
argon and oxygen atmosphere.
Irrad. time (h) Conv. (%).
1a
1b
1c
1d
1e
1f
1g
1h
1i
C6H5
C2H5O 3a
17 (50)
18 (20)
17 (22)
24 (24)
20 (20)
14 (18)
20 (20)
20 (20)
16 (20)
22 (22)
11 (20)
13 (17)
14 (16)
0.9 (3)
8 (22)
90 (60)
100 (80)
100 (90)
100 (80)
100 (80)
100 (60)
100 (70)
100 (60)
100 (40)
100 (60)
100 (90)
100 (90)
100 (60)
100 (100)
100 (90)
100 (50)
100 (100)
100 (80)
100 (80)
100 (90)
100 (80)
100 (100)
100 (100)
100 (80)
100 (70)
0 (0)
30 (50)
20 (20)
22 (22)
24 (24)
22 (22)
18 (18)
20 (20)
20 (20)
20 (20)
22 (22)
13 (20)
16 (17)
16 (16)
4 (7)
12 (22)
20 (20)
11 (14)
14 (18)
8 (10)
13 (13)
16 (20)
10 (16)
12 (14)
18 (18)
20 (20)
20 (20)
6 (11)
100 (40)
60 (40)
90 (20)
80 (20)
80 (20)
90 (40)
90 (40)
80 (30)
80 (20)
50 (10)
100 (70)
100 (50)
90 (40)
100 (100)
100 (60)
80 (40)
90 (90)
100 (60)
90 (60)
100 (80)
90 (70)
100 (100)
100 (70)
80 (50)
80 (60)
0 (0)
4-CH3C6H4
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
4-ClC6H4
3-ClC6H4
2-ClC6H4
=
=
=
=
=
=
=
=
=
=
=
=
=
CH3
=
=
=
=
=
=
=
=
=
=
=
=
=
=
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
4a
4-BrC6H4
1j
1k
1l
4-NO2C6H4
PhCH2CH2
CH3(Ph)CH
2-thienyl
4-Me2NC6H4
C6H5
4-CH3C6H4
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
4-ClC6H4
3-ClC6H4
2-ClC6H4
4-BrC6H4
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
CH3(Ph)CH
2-thienyl
1m
1n
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
4b 12 (20)
4c
4d
4e
4f
4 (8)
9 (18)
6 (10)
6 (13)
8 (20)
5 (12)
7 (14)
10 (18)
4g
4h
4i
4j
4k 16 (20)
4l
20 (20)
5 (11)
6 (8)
2m
2n
2o
4m
4n
4o
100 (100)
100 (70)
100 (100)
90 (70)
80 (50)
100 (100)
a The times and yields presented in
parentheses refer to the photoreac-
tions in the absence of BPO.
6 (8)
1.3 (3)
4-Me2NC6H4
0.5 (2)
by carrying out the reaction in propan-2-ol. It should by the benzoyloxy radical (BOR) was observed in our
be noted that due to the low solubility of THPMs in study.
many solvents and avoiding either the possible electron
For a better discussion of the output of this study, the
transfer from excited THPMs to electron-accepting
results presented in Table 3 should be divided in two
solvents (CCl4 or CHCl3) or hydrogen abstraction
categories: i) irradiation in the absence of BPO (photo-
from the solvent (propan-2-ol) by benzoyloxy radical,
oxidation), and ii) irradiation in the presence of BPO
as explained, the reaction is carried out in acetonitrile.
(free radical-induced photo-oxidation).
Under optimized reaction conditions (BPO : THPM
= 4 : 1), simultaneous irradiation of acetonitrile solu-
tions of each THPM (1a – 1n and 2a – 2o) in the pres-
Photo-oxidation
ence or the absence of BPO under argon and oxygen at-
mosphere was carried out until maximum progression
of the reaction (Scheme 2). The results are summarized
in Table 3.
The results show that the photochemical oxidation
of THPMs is faster in the presence of an argon as
compared to an oxygen atmosphere. The increase of
the required irradiation time in the presence of oxy-
The results presented in Table 3 indicate that the gen and also a failure of total quenching under oxygen
irradiation of THPMs (1a – 1n and 2a – 2o) in ace- are supporting the argument that both excited singlet
tonitrile either in the presence or in the absence and triplet states of THPMs are involved in the reac-
of BPO leads to the removal of 3- and 4-hydro- tion as we also observed by irradiation in CHCl3 solu-
gens under formation of the same oxidation prod- tion [29, 30]. The removal of 3- and 4-hydrogensin this
ucts, namely 2-oxo-1,2-dihydropyrimidines (3a – 3n study, namely photo-oxidation of THPMs in acetoni-
1
and 4a – 4o). IR, H NMR and MS spectral data con- trile solution, is probably a result of homolytic cleav-
firm the formation of the latter compounds. No hy- age of both C–H and N–H bonds. The results show that
drogen abstraction from 6-CH3 as an allylic hydrogen the nature and the position of the additional substituent
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM

266
H. R. Memarian et al. · Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines
on the phenyl ring in 4-position of the heterocyclic ring (DHPM). The observed effect of the nature of the sub-
affect the rate of the reaction; therefore the homolytic stituent in the 4-position of the heterocyclic ring on the
cleavage of the C–H bond is expected to happen at the rate of reaction supports the argument that the removal
beginning of the reaction.
of a hydrogen atom from the less polar C–H bond com-
pared with the more polar N–H bond is more facile.
Therefore, step 2 should be the rate-determining step.
In fact, the stability of a trihydropyrimidinoyl radical
intermediate (TrHPM·), which is both a benzylic and
an allylic radical, should lower the activation energy
for its formation. This influences the rate of step 2 as
the rate-determining step. This observation is possibly
the reason why the concurrent removal of a hydrogen
atom from the allylic position (6-CH3) is not observed
in the present study. It should also be noted that the
removal of a benzylic or secondary alkyl substituent
on the 4-position of the heterocyclic ring has not been
observed as it was found in the case of 1,4-dihydropy-
ridines [12, 13].
Free radical-induced photo-oxidation
A comparison of the results obtained by the reac-
tions in the absence and in the presence of BPO reveals
that photo-oxidation is faster using BPO. This indi-
cates the involvement of the initially formed benzoyl-
oxy radical (BOR) by photo-cleavage of BPO as an ac-
tive species for the removal of the less polar C(4)–H
bond rather than the more polar N(3)–H bond. The lat-
ter observation is supporting again the effect of the na-
ture of the aryl group in the 4-position on the required
time of irradiation. Whereas complete oxidation is ob-
served using BPO under an argon atmosphere, a longer
irradiation time is necessary, and incomplete oxida-
tion is observed in many cases under an oxygen atmo-
sphere. Again a comparison of the time of free rad-
ical photo-oxidation demonstrates that in contrast to
the effect of the electron-withdrawing substituent the
electron-donating substituents decrease the required ir-
radiation time. According to these results the follow-
ing mechanism is proposed for the free radical-induced
photo-oxidation (Scheme 2).
Selective excitation of BPO leads to the formation
of two benzoyloxy radicals (BOR) in the first step.
These reactive species abstract an easily removable 4-
hydrogen from the heterocyclic ring (4-H) under for-
mation of a trihydropyrimidinoyl radical intermediate
(TrHPM·) and benzoic acid. Removal of the second
hydrogen atom in the last step completes the reac-
tion under formation of 2-oxo-1,2-dihydropyrimidine
Conclusion
In the present study, we have investigated the
free radical-induced photodehydrogenation of 2-oxo-
1,2,3,4-tetrahydropyrimidines to the corresponding 2-
oxo-1,2-dihydropyrimidines using benzoyl peroxide in
acetonitrile under argon and oxygen atmospheres. The
reaction is faster under argon than under oxygen and is
influenced by the nature of the substituent on the 4- and
5-positions of the THPM ring. The results of this study,
especially the direct photo-oxidation of THPMs, show
the photosensitivity of tetrahydropyrimidines. Due to
their pharmaceutical activities, this is important for
drug designers and manufacturers.
Experimental Section
The title compounds were prepared through a three com-
ponents condensation using Co(HSO4)2 [30]. Melting points
were determined on a Stuart Scientific SMP2 apparatus and
are uncorrected. IR spectra were recorded using KBr discs
on a Shimadzu IR-435. 1H NMR spectra were obtained with
a Bruker DRX-300 Avance instrument. They are reported
as follows: chemical shifts δ in ppm (multiplicity, coupling
constants J in Hz, number of protons, and assignment). Mass
spectra were obtained on a Platform II Mass Spectrometer
from Micromass; EI mode at 70 eV. UV spectra (in CH3CN)
were taken with a Shimadzu UV-160 spectrometer.
A solution of a 2-oxo-1,2,3,4-tetrahydropyrimidine in dry
acetonitrile (c = 10−3 M) and a solution of a 2-oxo-1,2,3,4-
tetrahydropyrimidine and dibenzoyl peroxide with a molar
ratio of 1 : 4 in dry acetonitrile were prepared. Then two
equal volumes of each solution in a Pyrex tube were ir-
radiated simultaneously with a 400 W high-pressure mer-
Scheme 2.
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM

H. R. Memarian et al. · Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines
267
cury lamp while bubbling argon or oxygen through the solu- δ = 1.18 (m, 3H, PhCHCH3), 1.96 (s, 3H, 6-CH3), 2.16 (s,
tions during irradiation. The reaction was followed by thin- 3H, CH3CO), 2.79 (m, 1H, PhCHCH3), 4.25 (m, 1H, 4-H),
layer chromatography (TLC) until maximum consumption of 7.19 (mc, 6H, aromatic-H, 1-NH), 8.95 (s, 1H, 3-NH). –
THPMs. The products were purified with column and plate MS (EI, 70 eV): m/z (%) = 153 (100) [M–MeCHPh]+,
chromatography. The products were characterized by com- 110 (17) [M–MeCHPh–COCH3]+, 105 (30) [MeCHPh]+. –
parison of their physical and spectroscopic data with those UV/Vis (CH3CN): λmax(logε) = 291.6 (4.08), 211.2 nm
of authentic samples.
(4.06).
Ethyl 6-methyl-2-oxo-4-(1-phenylethyl)-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate (1k)
Ethyl 6-methyl-4-(1-phenylethyl)-2-oxo-1,2-dihydropyrimi-
dine-5-carboxylate (3k)
M. p.: 146 – 153 ◦C. – IR: ν = 3250 (NH), 1720
(COC2H5), 1700 (2-CO) cm−1. – 1H NMR (300 MHz,
[D6]DMSO): δ = 1.15 (mc, 6H, CHCH3, CH2CH3), 2.15
(s, 3H, 6-CH3), 2.84 (mc, 1H, CHCH3), 3.76 – 3.94 (two
m, 2H, CH2CH3), 4.20 (m, 1H, 4-H), 7.20 (mc, 6H,
aromatic-H, 1-NH), 9. 59 (s, 1H, 3-NH). – MS (EI,
70 eV): m/z (%) = 243 (17) [M–C2H5O]+, 183 (100)
[M–PhCHCH3]+, 155 (85) [M–PhCHCH3–C2H4]+, 137
(84) [M–PhCHCH3–C2H5OH]+, 110 (64) [M–PhCHCH3–
CO2C2H5]+, 105 (85) [PhCHCH3]+, 77 (82) [C6H5]+. –
UV/Vis (CH3CN): λmax(logε) = 277.2 (4), 241.6 (sh, 3.51),
209.6 nm (4.03).
M. p.: 204 – 209 ◦C. – IR: ν = 3300 (NH), 1710
(CO2C2H5), 1665 (2-CO), 1590 (C5=C6), 1420 (C=C), 1280
(C–O) cm−1. – 1H NMR (300 MHz, [D6]DMSO): δ =
1.15 (t, 3H, J = 7.06 Hz, CH2CH3), 1.42 – 1.44 (d, 3H,
J = 6.52 Hz, CHCH3), 2.23 (s, 3H, 6-CH3), 4.09 (q, 2H,
J = 6.56 Hz, CH2CH3), 4.45 – 4.47 (q, 1H, J = 6.81 Hz,
CHCH3), 7.22 (mc, 5H, aromatic-H), 12.18 (brd s, 1H,
NH). – MS (EI, 70 eV): m/z (%) = 286 (2) [M]+, 137
(100) [M–MeCHPh–C2H5O]+, 105 (52) [MeCHPh]+, 77
(29) [C6H5]+. – UV/Vis (CH3CN): λmax(logε) = 306 (3.70),
245 nm (4.14).
Acknowledgement
5-Acetyl-6-methyl-4-(1-phenylethyl)-1,2,3,4-tetrahydropy-
rimidin-2(1H)-one (2m)
We are thankful to the Center of Excellence (Chemistry)
and the Research Council and Office of Graduate Studies of
the University of Isfahan for their financial support.
M. p.: 183 – 185 ◦C. – IR: ν = 3250 (NH), 1710 (CH3CO),
1690 (2-CO) cm−1. – 1H NMR (300 MHz, [D6]DMSO):
[1] K. S. Atwal, G. C. Rovnyak, B. C. O’Reilly,
J. Schwartz, J. Org. Chem. 1989, 54, 5898 – 5907.
[2] G. C. Rovnyak, S. D. Kimball, B. Beyer, G. Cucinotta,
J. D. DiMacro, J. Gougoutas, A. Hedberg, M. Malley,
J. P. McCarthy, R. Zhang, S. Moreland, J. Med. Chem.
1995, 38, 119 – 129.
[3] C. O. Kappe, W. M. F. Fabian, M. A. Semones, Tetra-
hedron 1997, 53, 2803 – 2816.
[4] E. H. Hu, D. R. Sidler, U. H. Dolling, J. Org. Chem.
1998, 63, 3454 – 3457.
[5] K. S. Atwal, B. N. Swanson, S. E. Unger, D. M. Floyd,
S. Moreland, A. Hedberg, B. C. O’Reilly, J. Med.
Chem. 1991, 34, 806 – 811.
[6] G. J. Grover, S. Dzwonczyk, D. M. McMullen, D. E.
Normandin, C. S. Parham, P. G. Sleph, S. Moreland,
J. Cardiovasc. Pharmacol. 1995, 26, 289 – 294.
[7] J. U. Jeong, X. Chen, A. Rahman, D. S. Yamashita, J. I.
Luengo, Org. Lett. 2004, 6, 1013 – 1016.
[8] K. T. Wong, T. S. Hung, Y. Lin, C. C. Wu, G. H. Lee,
S. M. Peng, C. H. Chou, Y. O. Su, Org. Lett. 2002, 4,
513 – 516.
[9] J. J. Vanden Eynde, N. Audiart, V. Canonne, S. Michel,
Y. Van Haverbeke, C. O. Kappe, Heterocycles 1997,
45, 1967 – 1978.
[10] J. J. Vanden Eynde, N. Labuche, Y. Van Haverbeke,
L. Tietze, Arkivoc 2003, 22, 93 – 101.
[11] C. O. Kappe, Tetrahedron 1993, 49, 6937 – 6963.
[12] H. R. Memarian, M. Ghazaie, S. Kakhki Mehneh,
Z. Naturforsch. 2009, 64b, 1187 – 1192, and refs. cited
therein.
[13] H. R. Memarian, M. Abdoli-Senejani, D. Do¨pp,
J. Chin. Chem. Soc. 2007, 54, 131 – 139, and refs. cited
therein.
[14] S. H. Mashraqui, M. A. Karnik, Synthesis 1998, 5,
713 – 714.
[15] C. A. Snyder, M. A. Thorn, J. E. Klijanowicz, P. L.
Southwick, J. Heterocycl. Chem. 1982, 19, 603 – 607.
[16] R. S. Varma, D. Kumar, J. Chem. Soc., Perkin Trans.1
1999, 1755 – 1757.
[17] J. C. Bevington, S. W. Breuer, T. N. Huckerby, B. J.
Hunt, R. Jones, Eur. Polym. 1998, 34, 539 – 542.
[18] T. Tsubota, O. Hirabayashi, S. Ida, S. Nagaoka, M. Na-
gata, Y. Matsumoto, Diam. Rel. Mat. 2002, 11, 1374 –
1378.
[19] M. M. M. Sadeghi, H. R. Memarian, A. R. Momeni,
J. Sci. I. R. 2001, 3, 141 – 143.
[20] S. J. Dick, R. A. Annicelli (Eds.), Rubber technology,
2nd ed., Hanser, Cincinnati, 2009, p. 421.
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM

268
H. R. Memarian et al. · Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines
[21] O. L. Mageli, J. R. Kolczynski, Ind. Eng. Chem. 1966,
58, 25 – 32.
[26] H. R. Memarian, A. Farhadi, H. Sabzyan, Ultrason.
Sonochem. 2010, 17, 579 – 586.
[27] H. R. Memarian, H. Sabzyan, A. Farhadi, Z. Natur-
forsch. 2009, 64b, 532 – 540.
[22] P. D. Bartlett, K. Nozaki, J. Am. Chem. Soc. 1947, 69,
2299 – 2306.
[23] B. Barnett, W. E. Vaughan, J. Phys. Chem. 1947, 51,
942 – 955.
[28] H. R. Memarian, A. Farhadi, Monatsh. Chem. 2009,
140, 1217 – 1220.
[29] H. R. Memarian, A. Farhadi, H. Sabzyan, M. Soley-
mani, J. Photochem. Photobiol. A: Chem. 2010, 209,
95 – 103.
[24] H. R. Memarian, A. Farhadi, J. Iran. Chem. Soc. 2009,
6, 638 – 646.
[25] H. R. Memarian, A. Farhadi, Ultrason. Sonochem.
2008, 15, 1015 – 1018.
[30] H. R. Memarian, M. Ranjbar, J. Chin. Chem. Soc. 2011,
58, 522 – 527.
Brought to you by | University of Iowa Libraries
Authenticated
Download Date | 6/10/15 7:00 PM