An efficient aerobic oxidative aromatization of Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines

Article abstract of DOI:10.1016/j.tet.2005.12.056

4-Substituted Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines were oxidized to the corresponding pyridines and pyrazoles, respectively, in high yields by molecular oxygen in the presence of catalytic amount of N-hydroxyphthalimide (NHPI

Full text of DOI:10.1016/j.tet.2005.12.056

Tetrahedron 62 (2006) 2492–2496
An efficient aerobic oxidative aromatization of Hantzsch
1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines
*
Bing Han, Zhengang Liu, Qiang Liu, Li Yang, Zhong-Li Liu and Wei Yu
*
National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, People’s Republic of China
Received 15 October 2005; revised 22 December 2005; accepted 26 December 2005
Available online 26 January 2006
Abstract—4-Substituted Hantzsch 1,4-dihydropyridines and 1,3,5-trisubstituted pyrazolines were oxidized to the corresponding pyridines
and pyrazoles, respectively, in high yields by molecular oxygen in the presence of catalytic amount of N-hydroxyphthalimide (NHPI) and
Co(OAc)2 in acetonitrile at room temperature.
q 2006 Elsevier Ltd. All rights reserved.
1. Introduction
antiinflammatory, antiarrhythmic, antidiabetic and antibac-
terial activities.9 For this oxidative conversion of pyrazo-
lines, various methods have been reported, which employed
reagents such as Pb(OAc)4,10 MnO2,11 KMnO4,12
Zr(NO3)4,13 iodobenzene diacetate,14 silver nitrate,15 Pd/C8b
and activated carbon.8c
Hantzsch 1,4-dihydropyridines (DHPs), a class of model
compounds of NADH coenzyme,1 have been extensively
studied in view of the biological pertinence of these
compounds to the NADH redox process,2 and their
therapeutic functions for treatment of a variety of diseases,3
such as cardiovascular disorders,3a cancer3b and AIDS.3c
The oxidation of DHPs to the corresponding pyridine
derivatives constitutes the principal metabolic route in
biological systems,1–2 as well as a facile access to the
corresponding pyridine derivatives, which show antihy-
poxic and antiischemic activities,4 from the easily available
DHPs.5 Therefore, oxidative aromatization of DHPs has
attracted continuing interests of organic and medicinal
chemists and a plethora of protocols has been developed.6–8
Early works mostly used strong oxidants, such as HNO3,6b
KMnO4,6c or CAN6d and I2–MeOH.6e Recently, attention
has been paid to more efficient and environmentally benign
methods, such as electrochemical oxidation7 and catalytic
aerobic oxidation using RuCl3,8a Pd/C,8b activated carbon8c
On a continuance of our interest in the DHP based
chemistry, we reported a photochemical approach for the
aromatization of DHPs.16 Very recently, we found that
N-hydroxyphthalimide (NHPI) could effectively catalyze
the aerobic oxidative aromatization of DHPs in CH3CN at
refluxing temperature (Scheme 1).17 This NHPI–O2 system,
which was first used by Ishii et al. for oxygenation of
hydrocarbons, provides a simple, mild, and highly efficient
approach for the aromatization of DHPs. Subsequent
investigation showed that the capacity of this approach
could be enhanced significantly in the presence of catalytic
amount of Co(OAc)2. This method is also applicable to the
oxidative aromatization of 1,3,5-trisubstituted pyrazolines.
Herein, we wish to report this work in detail.
8d
or Fe(ClO4)3 as the catalyst.
H
R
R
EtO2C
CO2Et
EtO2C
CO2Et
1,3,5-Trisubstituted pyrazolines are important five-mem-
bered heterocyclic compounds, which can be easily
prepared from phenylhydrazine and chalcone derivatives.
The oxidative aromatization of these dihydroheteroaro-
matics provides the corresponding pyrazoles, which are
known to possess diverse biological activities, including
NHPI (20 mol%)
O2, CH3CN, reflux.
N
H
N
1
2
Scheme 1.
Keywords: Hantzsch 1,4-dihydropyridines; 1,3,5-Trisubstituted pyrazo-
lines; Aerobic oxidative aromatization; N-Hydroxyphthalimide; Cobalt
diacetate.
2. Results and discussion
As demonstrated by Ishii et al., NHPI is a very effective
organic catalyst for the functionalization of hydrocarbons
*
Corresponding authors. Fax: C86 931 8625657;
e-mail addresses: liuzl@lzu.edu.cn; yuwei@lzu.edu.cn
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.12.056

B. Han et al. / Tetrahedron 62 (2006) 2492–2496
2493
Table 1. Catalytic aerobic aromatization of 1,4-dihydropyridines (1) by
NHPI or NHPI–Co(OAc)2
the strong electron-withdrawing nitro substituent renders 1g
unreactive.
Entry Methoda Substrate
R
Productb Time Yield
(h)
(%)c
It was found by Ishii et al. that the presence of a small
amount of transition metals, such as Mn2C and Co2C, could
significantly enhance the oxidizing capacity of the
NHPI–O2 system.19 Accordingly, it was expected that
the same effect could also be observed in our case for the
oxidation of DHPs. Indeed, stirring substrate 1 with
10 mol% of NHPI and 0.5 mol% of Co(OAc)2 in
acetonitrile at room temperature for several hours led to
the clean formation of pyridine product 2 in excellent yields
(Scheme 2, method B in Table 1). By comparison, no
appreciable reaction took place in the absence of Co(OAc)2
under the otherwise same reaction conditions. Most
significantly, even substrate 1g, which was resistant to
oxidation under the previous conditions,17 was smoothly
transformed to 2g in 98% yield by this new treatment,
demonstrating the high effectiveness of this
NHPI–Co(OAc)2–O2 system (entry 14 in Table 1). In the
case of 1f, a small amount of C–C cleavaged product 2a was
formed along with the normal aromatization product 2f
(entry 12 in Table 1). This phenomenon was also observed
in the previously reported aromatization of DHPs under
other oxidative conditions.16
1
2
3
4
5
6
7
8
A
B
A
B
A
B
A
B
A
B
A
B
A
B
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
1f
H
H
Me
Me
Ph
Ph
2a8c
2a
0.5
0.5
3
4
4
4
1.5
3
5
4
99
99
98
98
99
99
98
99
96
98
93
91, 8
NR
98
2b8c
2b
2c8c,13
2c
4-MeOC6H4 2d6e
4-MeOC6H4 2d
9
4-ClC6H4
4-ClC6H4
2-Furyl
2-Furyl
4-NO2C6H4
4-NO2C6H4
2e13
2e
10
11
12
13
14
2f6e
7
3
10
1f
1g
1g
2f, 2a
—
2g8c,13
5
a Method A: an CH3CN solution (3 mL) of Hantzsch 1,4-dihydropyridine 1
(1 mmol) and NHPI (0.2 mmol) was refluxed under stirring and oxygen
atmosphere (1 atm). Method B: a mixture of 1,4-dihydropyridine 1
(1 mmol), NHPI (0.1 mmol) and Co(OAc)2$4H2O (0.005 mmol) in
acetonitrile (3 mL) was stirred under an oxygen atmosphere (1 atm) at
room temperature.
b The products were identified by comparing their 1H, 13C NMR and EI-MS
spectral data and melting points with those reported in the cited
references.
c Isolated yield.
under O2, NO, or NO2 atmosphere to give oxygen-
containing compounds, such as alcohols, ketones, car-
boxylic acids and nitroalkanes.18 This process is believed to
be via a NHPI mediated free radical mechanism. We
envisioned that NHPI could also be used to catalyze the
aerobic oxidation of DHPs. Indeed, when DHPs (1) was
refluxed in acetonitrile under O2 atmosphere in the presence
of 20 mol% NHPI, the corresponding pyridine derivatives 2
was formed in excellent yields (Scheme 1, method A in
Table 1). The only exception was 1g, which gave no product
after prolonged reflux (entry 13 in Table 1). Apparently,
The NHPI catalyzed aerobic oxidation of DHPs was
supposed to be following a free radical chain process,17
similar to that proposed previously by Ishii et al.18a The
initiation step was the generation of phthalimide-N-oxyl
radical (PINO) by the hydrogen transfer from NHPI to O2.
Co2C could accelerate this step by binding with O2 to form a
Co3C–oxygen complex, which can abstract the hydrogen
from NHPI much more effectively than oxygen
(Scheme 3).18a Consequently, the whole process was
remarkably accelerated in the presence of Co(OAc)2, as
demonstrated by the present result, as well as those observed
H
R
R
NHPI (10 mol%),
Co(OAc)2 (0.5 mol%)
EtO2C
CO2Et
EtO2C
CO2Et
O2, CH3CN, rt
N
H
N
1
2
Scheme 2.
H
R
O
EtO2C
CO2Et
.
LnCoIIIOOH
NO
N
H
O
1
PINO
O
R
CO2Et
EtO2C
.
O2
LnCoII
NOH
LnCoIIIOO
.
LnCoIIIOOH
N
H
O
NHPI
3
O
R
EtO2C
CO2Et
.
LnCoIIIOOH
NO
LnCoIIIOO
.
O
N
PINO
NHPI
2
Scheme 3.

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B. Han et al. / Tetrahedron 62 (2006) 2492–2496
by others.18 In the subsequent propagation step, PINO
abstracted hydrogen from DHP to produce radical 3.
Generally, alkyl radicals would react with oxygen to form
alkyl peroxyl radicals, which in turn, would produce
oxygenated products. In the present case, however, the
strong driving force of aromatization made the second
with TMS as the internal standard in CDCl3. EI-MS spectra
were measured on an HP 5988A spectrometer by direct inlet
at 70 eV. The high resolution mass spectra (HRMS) were
measured on a Bruker Daltonics APEX II 47e spectrometer
by ESI.
hydrogen abstraction from radical 3 by PINO and/or Co3C
–
3.1. A typical procedure for the aromatization of
4-Hantzsch 1,4-dihydropyridines (1)
oxygen complex very effective. Therefore, the pyridine
derivative formed exclusively rather than the oxygenated
products.
A mixture of 1,4-dihydropyridine 1a (253 mg, 1.00 mmol),
NHPI (16 mg, 0.10 mmol) and Co(OAc)2$4H2O (1 mg,
0.005 mmol) in acetonitrile (3 mL) was stirred under an
oxygen atmosphere at room temperature for 4 h. After
removal of the solvent under reduced pressure, the residue
was column chromatographed (over silica gel) to afford the
corresponding pyridine derivative 2a 249 mg. Yield: 99%.
Having successfully achieved the aromatization of DHPs,
we applied this method to the oxidative aromatization of
1,3,5-trisubstituted pyrazolines (Scheme 4, Table 2). As
shown in Table 2, treatment of 1,3,5-trisubstituted pyr-
azolines (4) with the above mentioned procedure led to the
formation of the corresponding pyrazoles (5) in high yields.
3.1.1. Diethyl 2,6-dimethyl-3,5-pyridinedicarboxylate
(2a). Pale yellow solid; mp 70–71 8C (lit.8c 71–72 8C).
R1
R2
R1
R2
NHPI (10 mol%),
Co(OAc)2 (0.5 mol%)
1H NMR (300 MHz, CDCl3): dZ1.42 (t, 6H, JZ7.2 Hz),
2.85 (s, 6H), 4.40 (q, 4H, JZ7.2 Hz), 8.68 (s, 1H).
N
N
N
N
O2, CH3CN, rt
Ph
Ph
4
13C NMR (75.5 MHz, CDCl3): dZ14.2, 24.8, 61.3, 123.0,
140.8, 162.1, 165.8.
5
Scheme 4.
EI-MS: m/z (rel int., %)Z251 (39.8), 206 (100), 195 (19.6),
178 (53.8), 150 (29.0), 106 (21.6).
Table 2. Catalytic aerobic aromatization of 1,3,5-trisubstituted pyrazolines
with NHPI–Co(OAc)2
a
R1
R2
Time Productb
(h)
Yield
(%)c
3.1.2. Diethyl 2,4,6-trimethyl-3,5-pyridinedicarboxylate
(2b). Pale yellow oil (lit.8c).
Entry Substrate
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
Ph
Ph
7
9
9
7
6
6
5a8c,13
5b8c,13
5c8c,13
5d8c,13
5e
90
95
88
91
89
91
1H NMR (300 MHz, CDCl3): dZ1.39 (t, 6H, JZ7.2 Hz),
2.27 (s, 3H), 2.52 (s, 6H), 4.41 (q, 4H, JZ7.2 Hz).
4-MeOC6H4 Ph
4-ClC6H4 Ph
4-NO2C6H4 Ph
Ph Me
4-MeOC6H4 Me
13C NMR (75.5 MHz, CDCl3): dZ14.1, 16.9, 22.8, 61.5,
127.5, 142.0, 154.8, 168.3.
5f
a Reaction condition: a mixture of 1,3,5-trisubstituted pyrazolines 4
(1 mmol), NHPI (0.1 mmol) and Co(OAc)2$4H2O (0.005 mmol) in
acetonitrile (5 mL) was stirred under an oxygen atmosphere (1 atm) at
room temperature.
EI-MS: m/z (rel int., %)Z265 (31.4), 236 (45.9), 220 (100),
208 (43.2), 192 (25.9), 77 (34.5).
b Compounds 5a–5d were identified by comparing their 1H, 13C NMR and
EI-MS spectral data and melting points with those reported in the cited
references. 5e and 5f were two new compounds characterized by 1H, 13
NMR, EI-MS and HRMS spectra.
C
3.1.3. Diethyl 4-phenyl-2,6-dimethyl-3,5-pyridinedicar-
boxylate (2c). Pale yellow solid; mp 61–62 8C (lit.8c 60–
61 8C).
c Isolated yields.
1H NMR (300 MHz, CDCl3): dZ0.90 (t, 6H, JZ7.2 Hz),
2.60 (s, 6H), 4.00 (q, 4H, JZ7.2 Hz), 7.2–7.4 (m, 5H).
In conclusion, the oxidative aromatization of substituted
Hantzsch dihydropyridines and pyrazolines was achieved
efficiently by using molecular oxygen as the terminal
oxidant with NHPI and Co(OAc)2 as the co-catalysts at
room temperature. Extension of this method to the
preparation of other heterocyclic compounds is under way
in this laboratory.
13C NMR (75.5 MHz, CDCl3): dZ13.4, 22.8, 61.2, 126.8,
127.9, 128.0, 128.3, 136.4, 146.0, 155.3, 167.7.
EI-MS: m/z (rel int., %)Z327 (71.2), 282 (48.1), 254 (42.4),
236 (100), 209 (29.4), 139 (33.8).
3. Experimental
3.1.4. Diethyl 4-(4-methoxyphenyl)-2,6-dimethyl-3,
5-pyridinedicarboxylate (2d). Pale yellow solid; mp 49–
50 8C (lit.6e 51–53 8C).
N-Hydroxyphthalimide (NHPI) (purityO98%) was pur-
chased from ALDRICH.
1H NMR (300 MHz, CDCl3): dZ0.89 (t, 6H, JZ7.2 Hz),
2.50 (s, 6H), 3.72 (s, 3H), 3.96 (q, 4H, JZ7.2 Hz), 6.80 (d,
2H, JZ6.9 Hz), 7.10 (d, 2H, JZ6.9 Hz).
1H and 13C NMR spectra (300 and 75.5 MHz, respectively)
were recorded on a Varian Mercury plus-300 spectrometer

B. Han et al. / Tetrahedron 62 (2006) 2492–2496
2495
13C NMR (75.5 MHz, CDCl3): dZ13.8, 22.9, 55.4, 61.4,
113.7, 127.4, 128.8, 129.6, 146.0, 155.3, 160.0, 168.2.
13C NMR (75.5 MHz, CDCl3): dZ105.2, 125.3, 125.8,
127.4, 128.0, 128.3, 128.4, 128.6, 128.7, 128.9, 130.5,
133.0, 140.0, 144.3, 151.9.
EI-MS: m/z (rel int., %)Z357 (100), 312 (25.1), 282 (27.9),
266 (83.5), 135 (31.8), 84 (51.1).
EI-MS m/z (rel int., %): 296 (48.6), 86 (64.2), 84 (100), 77
(91.5), 51 (55.2).
3.1.5. Diethyl 4-(4-chlorophenyl)-2,6-dimethyl-3,5-pyri-
dinedicarboxylate (2e). Pale yellow solid; mp 65–66 8C
(lit.13 65–66 8C).
3.2.2. 1,3-Diphenyl-5-(4-methoxyphenyl)pyrazole (5b).
Pale yellow solid; mp 79–80 8C (lit.13 78 8C).
1H NMR (300 MHz, CDCl3): dZ0.96 (t, 6H, JZ7.2 Hz),
2.59 (s, 6H), 4.03 (q, 4H, JZ7.2 Hz), 7.19 (d, 2H, JZ
8.4 Hz), 7.35 (d, 2H, JZ8.4 Hz).
1H NMR (300 MHz, CDCl3): dZ3.81 (s, 3H), 6.78 (s, 1H),
6.85 (d, 2H, JZ9.0 Hz), 7.21 (d, 2H, JZ9.0 Hz), 7.3–7.5
(m, 8H), 7.93 (d, 2H, JZ7.5 Hz).
13C NMR (75.5 MHz, CDCl3): dZ13.5, 22.8, 61.4, 126.7,
128.2, 129.5, 134.6, 134.9, 144.7, 155.5, 167.5.
13C NMR (75.5 MHz, CDCl3): dZ55.2, 104.7, 113.9,
122.9, 125.3, 125.7, 127.3, 127.9, 128.6, 128.9, 130.0,
133.0, 140.2, 144.2, 151.8, 159.5.
EI-MS: m/z (rel int., %)Z363 (36.2), 361 (100), 316 (49.3),
288 (32.9), 270 (34.3), 139 (24.5), 43 (19.1).
EI-MS m/z (rel int., %): 326 (23.1), 325 (4.1), 152 (14.5),
105 (61.5), 86 (100), 77 (36.6), 49 (61.4).
3.1.6. Diethyl 4-(2-furyl)-2,6-dimethyl-3,5-pyridinedi-
carboxylate (2f). Pale yellow oil (lit.6e).
3.2.3. 5-(4-Chlorophenyl)-1,3-diphenylpyrazole (5c).
Pale yellow solid; mp 113–114 8C (lit.13 114–115 8C).
1H NMR (300 MHz, CDCl3): dZ1.12 (t, 6H, JZ6.9 Hz),
2.49 (s, 6H), 4.18 (q, 4H, JZ6.9 Hz), 6.39 (d, 1H, JZ
3.3 Hz), 6.54 (d, 1H, JZ3.3 Hz), 7.42 (br s, 1H).
1H NMR (300 MHz, CDCl3): dZ6.83 (s, 1H), 7.2–7.5 (m,
12H), 7.91 (d, 2H, JZ8.1 Hz).
13C NMR (75.5 MHz, CDCl3): dZ13.7, 22.5, 61.5, 111.6,
111.7, 124.5, 133.5, 143.7, 147.8, 155.4, 167.9.
13C NMR (75.5 MHz, CDCl3): dZ105.2, 125.2, 125.7,
127.6, 128.0, 128.6, 128.7, 128.9, 129.0, 129.9, 132.7,
134.3, 139.8, 143.1, 151.9.
EI-MS: m/z (rel int., %)Z317 (71.5), 272 (52.3), 243 (38.1),
214 (100), 95 (45.3).
EI-MS m/z (rel int., %): 332 (13.0), 330 (38.8), 329 (20.7),
105 (27.4) 84 (71.4), 77 (100), 51 (57.7).
3.1.7. Diethyl 4-(4-nitrophenyl)-2,6-dimethyl-3,5-pyridi-
nedicarboxylate (2g). Pale yellow solid; mp 114–115 8C
(lit.8c 115–116 8C).
3.2.4. 1,3-Diphenyl-5-(4-nitrophenyl)pyrazole (5d). Pale
yellow solid; mp 142–144 8C (lit.13 142–143 8C).
1H NMR (300 MHz, CDCl3): dZ1.00 (t, 6H, JZ6.9 Hz),
2.64 (s, 6H), 4.05 (q, 4H, JZ6.9 Hz), 7.47 (d, 2H, JZ
9.0 Hz), 8.27 (d, 2H, JZ9.0 Hz).
1H NMR (300 MHz, CDCl3): dZ6.94 (s, 1H), 7.3–7.5 (m,
10H), 7.92 (d, 2H, JZ8.4 Hz), 8.18 (d, 2H, JZ8.4 Hz).
13C NMR (75.5 MHz, CDCl3): dZ106.2, 123.8, 125.4,
125.8, 128.2, 128.3, 128.7, 129.2, 129.3, 132.4, 136.7,
139.5, 141.9, 147.2, 152.4.
13C NMR (75.5 MHz, CDCl3): dZ13.6, 23.0, 61.6, 123.1,
126.1, 129.3, 143.3, 143.9, 147.7, 156.1, 167.0.
EI-MS: m/z (rel int., %)Z372 (48.8), 355 (29.5), 327 (100),
299 (39.5), 281 (32.5), 139 (23.2).
EI-MS m/z (rel int.): 341 (20), 294 (30.1), 191 (11.5), 163
(13.2), 105 (63.8), 77 (100), 51 (21.8).
3.2. A typical procedure for the aromatization
of 1,3,5-trisubstituted pyrazolines (4)
3.2.5. 1,5-Diphenyl-3-methyl-pyrazole (5e). Pale yellow
oil.
A mixture of 1,3,5-triphenylpyrazoline (4a) (298 mg,
1.00 mmol), NHPI (16 mg, 0.10 mmol) and Co(OAc)2$4-
H2O (1 mg, 0.005 mmol) in acetonitrile (5 mL) was stirred
under an oxygen atmosphere at room temperature for 7 h.
After removal of the solvent under reduced pressure, the
residue was column chromatographed (over silica gel) to
afford 1,3,5-triphenylpyrazole (5a) 267 mg. Yield: 90%.
1H NMR (300 MHz, CDCl3): dZ2.38 (s, 3H), 6.31 (s, 1H),
7.1–7.3 (m, 10H).
13C NMR (75.5 MHz, CDCl3): dZ13.5, 107.7, 125.0,
127.0, 128.0, 128.3, 128.5, 128.8, 130.6, 140.0, 143.6,
149.4.
3.2.1. 1,3,5-Triphenylpyrazole (5a). Pale yellow solid; mp
141–142 8C (lit.13 139–140 8C).
EI-MS m/z (rel int., %): 234 (100), 218 (14.3), 192 (13.8),
77 (31.5), 51 (20.5).
1H NMR (300 MHz, CDCl3): dZ6.83 (s, 1H), 7.3–7.5 (m,
13H), 7.93 (d, 2H, JZ7.7 Hz).
ESI-HRMS: m/z Calcd for C16H14N2CHC: 235.1230;
found: 235.1228.

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B. Han et al. / Tetrahedron 62 (2006) 2492–2496
689. (e) Yadav, J. S.; Subba Reddy, B. V.; Sabitha, G.; Kiran
Kumar Reddy, G. S. Synthesis 2000, 11, 1532. (f) Itoh, T.;
Nagata, K.; Matsuya, Y.; Miyazaki, M.; Ohsawa, A. J. Org.
Chem. 1997, 62, 3582. (g) Mashraqui, S. H.; Karnik, M. A.
Synthesis 1998, 713. (h) Varma, R. S.; Kumar, D. Tetrahedron
Lett. 1999, 40, 21. (i) Ko, K.-Y.; Kim, J.-Y. Tetrahedron Lett.
1999, 40, 3207. (j) Anniyappan, M.; Muralidharan, D.;
Perumal, P. T. Tetrahedron 2002, 58, 5069. (k) Zhu, X.-Q.;
Zhao, B.-J.; Cheng, J.-P. J. Org. Chem. 2000, 65, 8158.
7. (a) Lopez-Alarcon, C.; Nunez-Vergara, L. J.; Sturm, J. C.;
Squella, J. A. Electrochim. Acta 2003, 48, 505. (b) Arguello, J.;
Nunez-Vergara, L. J.; Sturm, J. C.; Squella, J. A. Electrochim.
Acta 2004, 49, 4849.
3.2.6. 1-Phenyl-3-methyl-5-(4-methoxyphenyl)pyrazole
(5f). Pale yellow solid; mp 86–88 8C.
1H NMR (300 MHz, CDCl3): dZ2.37 (s, 3H), 3.77 (s, 3H),
6.25 (s, 1H), 6.80 (d, 2H, JZ8.7 Hz), 7.13 (d, 2H, JZ
8.7 Hz), 7.2–7.4 (m, 5H).
13C NMR (75.5 MHz, CDCl3): dZ13.5, 55.1, 107.1, 113.8,
123.1, 125.0, 126.9, 128.7, 129.8, 140.2, 143.4, 149.3,
159.3.
MS m/z (rel int., %): 264 (100), 249 (39.5), 115 (18.4), 77
(58.1), 51 (28.7).
8. (a) Mashraqui, S. H.; Karnik, M. A. Tetrahedron Lett. 1998,
39, 4895. (b) Nakamichi, N.; Kawashita, Y.; Hayashi, M. Org.
Lett. 2002, 4, 3955. (c) Nakamichi, N.; Kawashita, Y.;
Hayashi, M. Synthesis 2004, 1015. (d) Heravi, M. M.;
Behbahani, F. K.; Oskooie, H. A.; Shoar, R. H. Tetrahedron
Lett. 2005, 46, 2775.
ESI-HRMS: m/z Calcd for C17H16N2OCHC: 265.1335;
found: 265.1335.
Acknowledgements
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(grant 20372030) for financial support.
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