PEG-N2O4 system as an efficient reagent both for the rapid oxidation of urazoles and 1,4-dihydropyridines under nonaqueous conditions

Article abstract of DOI:10.1002/jccs.200800106

N₂O₄ was easily impregnated on polyethyleneglycol to give a stable reagent. Polyethyleneglycol-N₂O₄ (PEG-N ₂O₄) system was used as an effective oxidizing agent for the oxidation of urazoles and bis-urazoles to their corresponding triazolinediones and also for the aromatization of 1,4-dihydropyridines into the corresponding pyridine derivatives under mild conditions at room temperature with good to excellent yields.

Full text of DOI:10.1002/jccs.200800106

7
04
Journal of the Chinese Chemical Society, 2008, 55, 704-711
Note
PEG-N O System as an Efficient Reagent both for the Rapid Oxidation of
2
4
Urazoles and 1,4-Dihydropyridines under Nonaqueous Conditions
a,
Mohammad Ali Zolfigol, * Ezat Ghaemi, Elaheh Madrakian and Khodabakhsh Niknam
a
a
b
a
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, 6517838683, Iran
b
Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Iran
N
2
O
4
was easily impregnated on polyethyleneglycol to give a stable reagent. Polyethyleneglycol-
N
2
O
4
2 4
(PEG-N O ) system was used as an effective oxidizing agent for the oxidation of urazoles and
bis-urazoles to their corresponding triazolinediones and also for the aromatization of 1,4-dihydropyri-
dines into the corresponding pyridine derivatives under mild conditions at room temperature with good to
excellent yields.
Keywords: Dinitrogen tetroxide; Polyethyleneglycol; Oxidation; Urazoles; Bis-urazoles;
1
,4-Dihydropyridines.
INTRODUCTION
to avoid these problems the reactions are usually performed
at low temperature and careful control of reaction condi-
Polymer-supported reagents and catalysts have been
widely applied in organic transformations. Immobilization
of reactive species on a polymer-support could provide
many important advantages over analogous homogeneous
systems; for example, separation of the support from the re-
action mixture can be achieved by simple filtration aiding
isolation and purification procedures. Also, excess of poly-
meric reagent can be readily employed without incurring
an increase in work-up. Transition metal complexes and
optically active catalysts might be efficiently retained for
re-use, and noxious or toxic species might be encapsulated
when bound to a macromolecule, with obvious advantages
in environmental terms. Also these reagents have many
other advantages such as simple experimental procedures,
stability, easy handling and minimal chemical wastes com-
4
tions is needed. In recent years some reports were pub-
lished on the use of N
2 4
O complexes of organic and inor-
5
, poly-
7
, activated char-
ganic compounds such as 18-crown-6 ether/N
6
, M(NO
2
O
4
vinylpyrolidone/N
8
, N
2
O
4
3
)
x
.yN
2
O
4
9
, silica acetate-N
12
10
coal/N
2
O
4
2
O
4
/SiO
2
2
O
4
,
calix[4]-
11
arene/N
2
O
4
,
DMF-NO
2
,
and (tributyl phosphate-NO ,
2
1
3
polyethylene glycol-NO
2
, dioxane-NO
2
)
as useful re-
agents that solve many difficulties in handling and increase
selectivity of organic reactions. In continuation of our in-
vestigation on the preparation of N
2 4
O complexes, we now
report on the application of N O impregnated polyethyl-
4
2
eneglycol as a cheap reagent made from available sources.
Polyethyleneglycols (PEGs) are important components
1
of crown ethers, cryptands, and other ion receptors and
4
1
-3
pared to their traditional homogeneous reagents.
their capability of binding with different ions have been
1
5
Dinitrogen tetroxide as an important industrial chem-
ical is commercially supplied at low price in a cylinder, and
can be used directly or in the form of liquid (b.p. 21 ºC) col-
lected by transfer distillation into an ice-cooled vessel.
Apart from difficulties in handling the poisonous and cor-
rosive dinitrogen tetroxide, the biggest disadvantage of uti-
widely investigated. This reagent polyethyleneglycol-
(PEG-N ) was easily prepared by bubbling N
gas through a solution of polyethylene glycol in cold
CH Cl (Scheme I).
Herein, we wish to report the use of polyethylene-
glycol-N complex as an efficient oxidizing agent both
N
2
O
4
2
O
4
2 4
O
2
2
2
O
4
lizing gaseous N
2
O
4
in organic reactions is its high reactiv-
for the oxidation of urazoles and bis-urazoles to their corre-
sponding triazolinediones and aromatization of 1,4-dihy-
ity which usually causes undesired side reactions. In order
*
Corresponding author. Fax: +98 811 8257407; E-mail: zolfi@basu.ac.ir and mzolfigol@yahoo.com (Mohammad Ali Zolfigol);
E-mail: niknam@pgu.ac.ir and khniknam@gmail.com (Khodabakhsh Niknam)

Oxidation of Urazoles and 1,4-Dihydropyridines
J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 705
Scheme I
N O
2
N O
2 4
4
O
O
O
O
n
N O
2
N O
2
4
4
PEG-N O
2
4
dropyridines into corresponding pyridine derivatives in
nonaqueous conditions.
wish to report a simple, cheap and convenient method for
the effective oxidation of urazoles (1) and bis-urazoles (3)
to their corresponding triazolinediones (2, 4) by using
RESULTS AND DISCUSSION
PEG-N
2 4
O as an oxidizing agent (Scheme II).
Oxidation of Urazoles
Scheme II
4
-Substituted-1,2,4-triazole-3,5-diones (TADs) have
H
R1
O
been used both as substrates and reagents in various or-
ganic reactions. For example, they have been used in Diels-
Alder, ene or [2+2] cycloadditions, dehydrogenation reac-
tions, electrophilic aromatic substitution, condensation of
dicarbonyl compounds, and oxidation of alcohols to alde-
N
N
N
N
H
O
R1
O
N
N
O
N
N
O
O
O
O
N
N
PEG-N O
2
4
or
R2
N
O
N
O
or
R2
N
CH2Cl2, rt.
R2
R2
N
O
O
1
2
N
N
1
hydes and ketones. Very recently, aromatization of 1,4-
6
N
N
H
R1
4
dihydropyridines and pyrazolines as well as oxidation of
3
1
thiols with TADs have been reported. The unusual reac-
7
tivity of TADs (2, 4) makes them interesting and also hard
1
8
to prepare and purify. It is interesting to note that 4-phen-
yl-1,2,4-triazoline-3,5-dione (2f) is an extremely reactive
dienophile and enophile which is at least 1000 times more
reactive than tetracyanoethylene in the Diels-Alder reac-
tion with 2-chlorobutadiene and 2000 times more reactive
1
6
than maleic anhydride. All known methods of synthesis
of these compounds (1,2,4-triazolidine-3,5-diones) require
oxidation of the corresponding urazoles (1, 3). Although a
variety of reagents are capable of efficient oxidations of
urazoles (1, 3) to TADs, this transformation is not easy be-
cause these compounds are very sensitive to the oxidizing
agents and reaction conditions. In addition, most of the re-
ported reagents produce by-products which either destroy
or are difficult to remove from the sensitive triazolinedi-
ones. Another major drawback of the older procedures is
their use of reagents which are either highly toxic or impart
A good range of urazoles (1) and bis-urazoles (3)
were subjected to the oxidation reaction in the presence of
the oxidizing agents PEG-N in dichloromethane. All
1
9-23
serious disposal problems (or both).
Recently, we among
2 4
O
many others, have demonstrated that application of hetero-
geneous systems, for the above reported oxidations, has
many advantages over their liquid phase counterparts such
as simple experimental procedures, mild reaction condi-
oxidation reactions were performed under mild and com-
pletely heterogeneous conditions, at room temperature
with good to excellent yields (Table 1).
The present oxidation reaction can be readily carried
out only by placing a suspension of urazoles (1) or bis-
5
b,24
tions and minimization of chemical wastes.
Herein, we

7
06 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008
Zolfigol et al.
Table 1. Oxidation of urazoles (1) and bis-urazoles (3) to their
corresponding triazolinediones (2, 4) with PEG-N O in
Aromatization of 1,4-Dihydropyridines
2
4
Amlodepine besylate, nifedepine, and related dihy-
2+
dropyridines are Ca channel blockers and are rapidly
a
dichloromethane at room temperature
b
Entry Urazole or (bis) Time (Min) Yield %
Product
emerging as one of the most important classes of drugs for
the treatment of cardiovascular diseases, including hyper-
tension. In the human body, it has been observed that these
compounds undergo oxidation to form pyridine deriva-
tives. These oxidized compounds are largely devoid of the
pharmacological activity of the parent compounds. Addi-
tionally, dihydropyridines are often produced in a synthetic
1
2
3
4
5
6
7
8
9
1
1a
1b
1c
1d
1e
1f
30
30
30
30
30
30
30
30
30
30
60
60
40
78
80
84
87
90
95
97
85
90
85
82
81
82
2a
2b
2c
2d
2e
2f
1g
1h
1i
2g
2h
2i
2
5
sequence, which have to be oxidized to pyridines.
0
1j
2j
The oxidation of dihydropyridines (5) is an old reac-
tion in general organic chemistry. Even in recent years, sev-
eral groups have reported new methods for aromatization,
including oxidation with ferric or cupric nitrates on a solid
11
1k
1l
2k
2l
1
1
2
3
3a
4a
a
Urazole or bis-urazole (1 mmol), PEG-N O (1 mmol).
2
4
26-27
28
ceric ammonium nitrate, clay-supported cu-
b
support,
Isolated yields.
2
pric nitrate accompanied by ultrasound-promotion, io-
9
3
dine, tetrabutylphosphonium dichromate, photoinduced
0
31
3
2
33
2 4 2 2
urazoles (3), PEG-N O in CH Cl and efficiently stirring
4
aromatization, and KHSO supported onto Bentonite.
the resulting heterogeneous mixture at room temperature.
The triazolinediones (2) and bis-triazolinediones (4) are
obtained by simple filtration and evaporation of the sol-
vent. As reported above the oxidation reactions are hetero-
geneous because urazoles and bis-urazoles [(1, 3) as white
solids] are insoluble in dichloromethane, whereas all of the
triazolinediones and bis-triazolinediones [(2, 4), red, pink
or brown solids] are very soluble in dichloromethane. The
Ohsawa et al. reported an excellent procedure for this trans-
formation. They have demonstrated the remarkably practi-
cal use of NO gas as a clean and efficient oxidation for this
3
4
purpose. We decided to seek a completely homogeneous
system for dihydropyridine oxidation, and we have investi-
gated a number of different reaction conditions based upon
+
the in situ generation of NO . In continuation of our study
3
5
23,36
about the applications of NOX,
N
2
O
4
,
metal nitrate
3
dinitrogen tetroxide complexes and complexation of tran-
5
sition metals with microcyclic ethers, herein we report a
7
2 4 2 2
PEG-N O in CH Cl can act as a relatively efficient re-
+
agent as a source for the delivery of nitrosonium ion (NO )
5
under mild conditions. The following mechanism for the
+
oxidation reaction via in situ generation of NO may be
simple and convenient method for the effective conversion
of 1,4-dihydropyridines (5) to their corresponding pyridine
suggested (Scheme III).
2 4
derivatives (6) by using PEG-N O as an oxidizing agent
under mild and homogeneous conditions (Scheme IV).
Scheme III
Different types of dihydropyridines (5) were sub-
2 4
jected to oxidation reaction in the presence of PEG-N O
O
O
O
in dichloromethane (Scheme IV). The oxidation reactions
were performed under mild and completely homogeneous
conditions at room temperature with good to excellent
yields. Aryl-, alkyl-, and heterocyclic-substituted 1,4-dihy-
dropyridines 5d-h oxidized into the corresponding pyri-
dine derivatives (Table 2).
+
-
O
N
O
N
NO NO
3
N
N
O
O
O
H
H
H
NO
N
N
N
N
N
N
+
NO
-HNO
O
O
O
O
O
O
N
N
-
HNO3
N
In addition, we used this reagent for the oxidation of
other heterocyclic compounds, such as pyrazoline and ox-
azoline for example (Scheme V). As shown in Scheme V,
both pyrazoline and oxazoline were oxidazed by reacting 1
R2
R2
R2
2
HNO
H O + N O
2 2

Oxidation of Urazoles and 1,4-Dihydropyridines
J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 707
Scheme IV
Table 2. Aromatization of 1,4-dihydropyridines (5), (1 mmol) in
the presence of PEG-N O (1 mmol) in dichloro-
2
4
methane at room temperature
a
Time (Min) Yield %
Entry
R
Product
1
2
3
4
5
6
7
H
Me
6a
6b
6c
6d
6e
6f
60
60
55
55
55
50
50
93
88
90
83
86
93
82
Ph
4-NO -C H
2
6
5
3-NO -C H
2
6
5
2-MeO-C H5
6
2-Furyl
6g
a
Isolated yields.
son with older reported procedures in the literature, Table 3
2
gas, [NO .
34
and NO, with respect to reaction
2
+
compares some of our results with N
-
2
O
4
5b,c
3 2
crown.H(NO ) ],
Scheme V
times and yields of obtained products.
In conclusion, a practical, efficient and convenient
method for the oxidation of urazoles, bis-urazoles and 1,4-
dihydropyridines is described. We suggest that these sys-
tems could be used for the oxidation of a wide variety of
urazole derivatives, 1,4-dihydropyridines, and also pyrazo-
line and oxazolines under mild and safe conditions.
Ph
Ph
N
N
PEG-N O
2
4
N
Ph
N
Ph
CH Cl , rt.
2
2
9
0 min, 90%
Ph
Ph
N
O
N
O
PEG-N O
2
4
Et
Et
EXPERIMENTAL SECTION
CH Cl , rt.
2
2
General
6
0 min, 87%
Chemicals were purchased from Fluka, Merck and
Aldrich chemical companies. Polyethylene glycol 2000
(Mw 1800-2200 g/mol) was purchased from Merck. Yields
refer to isolated pure products. The oxidation products
were characterized by comparison of their spectral (IR and
mmol of these heterocyclic compounds, and PEG-N
2 4
O (1
mmol) in dichloromethane (5 mL) at room temperature in
6
0 to 90 min in high yields.
2 4
To show the efficiency of the PEG-N O in compari-
2
2
+
-
Table 3. Comparison of the results PEG-N O (I) with N O (gas) (II), and [NO .crown.H(NO ) ]
2
4
2
4
3 2
5c
5
b
+
-
(
III), in the oxidation of urazoles and PEG-N O (I) with [NO .crown.H(NO ) ] (III),
2 4 3 2
3
4
and NO (IV) in the oxidation 1,4-hydropyridines
a
Yield %
Time (Min)
Entry
1
Substrate
Product
I
II III IV
I
II III IV
H
O
H
N
N
N
N
O
O
O
N
30 90 imid.
-
95 94
-
-
-
N
Ph
Ph
Ph
Ph
H
EtOOC
Me
COOEt
Me
EtOOC
Me
COOEt
Me
2
55
-
imid. 240 90
-
76
N
N
H
a
Isolated yields.

7
08 J. Chin. Chem. Soc., Vol. 55, No. 3, 2008
Zolfigol et al.
1
H-NMR) and physical data with those of authentic sam-
and washed with CH
2
Cl
2
(2 × 10 mL). Dichloromethane
3
8
was removed by water-bath (40–50 °C) and simple distil-
ples which were produced by other reported proce-
5
dures.
b,24,34-40
lation. The yield was 0.1681 g (95%) of crystalline red
2
solid (2f), Mp 171-175 °C, (lit. Mp 170-178 °C).
2
Caution: NO
2
-N
2
O
4
gas is exceedingly toxic. Thus,
2 4
O must be conducted in an ef-
all operations, involving N
ficient hood.
Spectral data
2a: 4-Methyl-1,2,4-triazoline-3,5-dione: pink crys-
1
8
1
Preparation of polyethyleneglycol-N
2
O
4
tals, mp 97-98 °C, Lit. 98-98.5 °C. H NMR (90 MHz,
CDCl ): d ppm 3.26 (s, 3H).
2b: 4-Eethyl-1,2,4-triazoline-3,5-dione: pink crys-
Into a 250-mL one necked round-bottomed flask was
charged 45.0 g of lead(II) nitrate. (The lead(II) nitrate had
been crushed to a fine powder with mortar and pestle and
then dried in an oven at 120 ºC for three days). The flask
was equipped with a 35-cm column and was connected to a
3
1
9
1
tals, mp 54-55 °C, Lit. 53 °C. H NMR (90 MHz, CDCl
3
):
1
d ppm 1.32 (t, 3H), 3.69 (q, 2H). C NMR (CDCl
3
3
): d ppm
12.710, 36.702, 159.19.
2
trap through a polyethylene tube. The trap was connected
2
2c: 4-n-Propyl-1,2,4-triazoline-3,5-Dione: pink crys-
2
3
1
to the gas inlet tube.
tals, mp 40-42 °C, Lit. 44 °C. H NMR (90 MHz, CDCl
3
):
d ppm 0.94 (t, 3H), 1.68 (m, 2H), 3.63 (t, 2H). C NMR
(CDCl ): d ppm 10.668, 20.598, 42.884, 159.336.
2d: 4-n-Butyl-1,2,4-triazoline-3,5-dione: pink crys-
1
3
The lead(II) nitrate was heated with a bunsen burner.
Brownish red NO
2
-N
2
O
4
gas evolved, and it was bubbled
3
slowly through the cold polyethylene glycol (10.0 g) in
methylene chloride solution (25 mL) for 30 min. After a
few minutes, a deep-red color appeared. The temperature
was kept below 0 ºC. The red solution was allowed to stir
1
8
1
tals, mp 42-43 °C, Lit. 44-44.5 °C. H NMR (90 MHz,
CDCl ): d ppm 0.86 (t, 3H), 1.21-1.53 (m, 4H), 3.5 (t, 2H).
C NMR (CDCl ): d ppm 36.372, 42.928, 52.572, 64.572,
168.324.
2e: 4-Cycloyl-1,2,4-triazoline-3,5-Dione: pink crys-
3
1
3
3
another 60 min, and then dry N
the solution in order to extrude the excess of NO
then the solvent was evaporated. The obtained solid was
dried under vacuum to give PEG supported N as white
cream powder (14.0 g). The capacity of the reagent was de-
termined to be 2.5 mmol of N per gram of the polymer.
2
gas was bubbled through
2
gas and
1
tals, mp 95-97 °C, Lit. 95-96 °C. H NMR (90 MHz,
8
1
1
3
2
O
4
CDCl
(CDCl
2f: 4-Phenyl-1,2,4-triazoline-3,5-dione: red crystals,
3
): d ppm 1.26-1.8 (m, 10H), 3.90 (m, 1H). C NMR
3
): d ppm 24.207, 24.963, 28.729, 53.936, 158.775.
2
O
4
2
mp 171-175 °C, Lit. 170-178 °C. H NMR (90 MHz,
2
1
The reagent could be stored in the refrigerator for several
months without losing its weight or activity.
1
3
CDCl
124.274, 129.611, 129.900, 157.971.
2g: 4-(4-Chloro phenyl)-1,2,4-triazoline-3,5-dione:
3 3
): d ppm 7.48 (S, 5H). C NMR (CDCl ): d ppm
Oxidation of 4,4¢-(4,4¢-diphenylmethylene)-bis-urazole
(
3a) to bis(p-3,5-dioxo-1,2,4-triazoline-4-ylphenyl)-
2
red crystals, mp 132-135 °C, Lit. 130-132 °C. H NMR
1
1
methane (4a): A general procedure
1
3
A suspension of compound 3a (0.366 g, 1 mmol) and
(90 MHz, CDCl
d ppm 125.100, 130.237, 191.466.
2h: 4-(3,4-Dichloro phenyl)-1,2,4-triazoline-3,5-
3 3
): d ppm 7.41 (S, 5H). C NMR (CDCl ):
PEG-N
stirred for the specified time (Table 1). Then the reaction
mixture was filtered and washed with CH Cl (2 × 10 mL).
Dichloromethane was removed by water-bath (40–50 °C)
and simple distillation. A good yield of crystalline pink
2 4 2 2
O (0.1 g) were added into CH Cl (10 mL) and
3
dione: red crystals, mp 111-113 °C, Lit. 113-115 °C. C
9
13
2
2
3
8
NMR (22.5 MHz, CDCl ): dppm 122.809, 123.588, 126.657,
3
128.783, 131.626, 134.239, 157.121.
2
solid (4a) Mp 182-185 ºC, and (lit. Mp 185 °C) was
1
2i: 4-(4-Nitro phenyl)-1,2,4-triazoline-3,5-dione: red
1
8
crystals, mp 125-126 °C, Lit. 128-129 °C. H NMR (90
1
obtained.
1
3
Oxidation of 4-phenyl urazole (1f) to 4-phenyl-1,2,4-
triazoline-3,5-dione (2f): A typical procedure
A suspension of compound 1f (0.177 g, 1 mmol) and
MHz, CDCl
d ppm 124.152, 125.164, 125.478, 193.894.
2j: 4-(4-Methoxyphenyl)-4H-1,2,4-triazole-3,5-di-
3 3
): d ppm 7.89-8.40 (m, 5H). C NMR (CDCl ):
4
0
one: dark red crystal, Mp 89-93°C, Lit. 89-93 °C, IR
PEG-N
mL) and the suspension was vigorously stirred for 0.5 h
Table 1, Entry 6). Then the reaction mixture was filtered
2
O
4
(0.1 g) were added into dichloromethane (10
-
1 1
(KBr): 2968, 1773, 1515, 1258, 1173 cm , H NMR (90
MHz, CDCl ): d 3.85 (s, 3H), 6.97-7.38 (m, 4H).
(
3

Oxidation of Urazoles and 1,4-Dihydropyridines
J. Chin. Chem. Soc., Vol. 55, No. 3, 2008 709
2
k: 4-(4-tert-Butylphenyl)-4H-1,2,4-triazole-3,5-di-
ACKNOWLEDGMENT
4
one: red crystal, Mp 122-126 ºC, Lit. 122-126 ºC, IR
0
Financial support for this work by the Research Coun-
cil of Bu-Ali Sina University, Hamedan, Iran, is gratefully
acknowledged.
-
1 1
(
(
KBr): 2962, 1788, 1714, 1518, 1407, 1185 cm , H NMR
90 MHz, CDCl ): d 1.28 (s, 9H), d 7.33 (m, 4H).
l: 4-(Naphthalen-1-yl)-4H-1,2,4-triazole-3,5-dione:
3
2
4
0
brown crystal, Mp 109-111 ºC, Lit. 109-111 ºC, IR (KBr):
Received November 26, 2007.
-1 1
923, 1715, 1417, 770 cm , H NMR (90 MHz, CDCl
2
6
3
): d
REFERENCES
.94-7.83 (m, 7H).
1
. Sherrington, D. C.; Hodge, P. Synthesis and Separation Us-
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Publishers: London, 1995; Chapter 6, pp 141-196.
. Hodge, P. Polymer-supported Asymmetric Organic Synthe-
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SPCC: UK, 1990; pp 273-292.
4
a: Bis(para-3,5-dioxo-1,2,4-triazoline-4-eyl-phen-
9
3
yl)methane: pink crystals, mp 180-185 °C (dec), Lit. 185
C (dec).
2
°
General procedure for the aromatization of 1,4-di-
hydropyridines
3
To a solution of compound 5 (1 mmol) and CH
2 2
Cl (5
mL), PEG-N O (0.2 mL, 1 mmol) was added and the mix-
4
2
ture was stirred for the appropriate time at room tempera-
ture (Table 2). After completion of the reaction, the mixture
was filtered, and evaporation of the solvent on a rotary
evaporator afforded a residue, which was passed through a
short pad of silica gel using a mixture of ethyl acetate and
n-hexane as eluent to afford the highly pure product.
Spectral data
4. (a) Shiri, M. Synlett 2006, 1789. (b) Ono, N. The Nitro
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3
-1
a: Mp 70-71 ºC, Lit. 70 ºC, IR (KBr): u (cm ) =
6
1
725 (C=O); H NMR (90 MHz, CDCl
1
2
3
): d ppm 1.4 (t, 6H),
.8 (s, 6H), 4.3 (q, 4H), 8.7 (s, 1H).
3
b: Mp Oil, Lit. Oil, IR (KBr): u (cm ) = 1725
3
-1
6
1
C=O); H NMR (90 MHz, CDCl
(
(
3
): d ppm 1.4 (t, 6H), 2.3
6
7
s, 3H), 2.5 (s, 6H), 4.37 (q, 4H).
2
003, 1591. (b) Iranpoor, N.; Firouzabadi, H.; Pourali, A.
3
c: Mp 62-64 ºC, Lit. 61 ºC, IR (KBr): u (cm ) =
3
-1
6
Tetrahedron 2002, 58, 5179.
1
722 (C=O); H NMR (90 MHz, CDCl
1
2
3
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4
d: Mp 114-115 ºC, Lit. 114-115 ºC, IR (KBr): u
1
6
-
1
1
(
cm ) 1724 (C=O), 1552, 1524 (N=O); H NMR (90 MHz,
CDCl ): 0.9 (t, 6H), 2.6 (s, 6H), 4.0 (q, 4H), 7.4 (d, 2H), 8.2
d, 2H).
e: Mp 61-62 ºC, Lit. 60-62 ºC, IR (KBr): u (cm ) =
3
8
. Iranpoor, N.; Firouzabadi, H.; Pourali, A. Synlett 2004, 347.
(
3
3
-1
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1
725 (C=O); H NMR (90 MHz, CDCl
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2
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3
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f: Mp 56-58 ºC, Lit. 56-58 ºC, IR (KBr): u (cm ) =
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-1
6
1
718 (C=O); H NMR (90 MHz, CDCl
1
2
3
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.6 (s, 6H), 3.7 (s, 3H), 4.0 (q, 4H), 6.9-7.3 (m, 4H).
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