Rare earth perfluorooctanoate [RE(PFO)3] catalyzed one-pot synthesis of benzopyran derivatives

Article abstract of DOI:10.1016/j.jfluchem.2005.10.004

Rare earth perfluorooctanoate [RE(PFO)₃] catalyzes facile condensation of dimedone, aldehydes and malononitrile under mild conditions to afford the corresponding 5-oxo-5,6,7,8-tetrahydro-4H-benzo-[b]-pyran derivatives in good to high yields. Th

Full text of DOI:10.1016/j.jfluchem.2005.10.004

Journal of Fluorine Chemistry 127 (2006) 97–100
www.elsevier.com/locate/fluor
Rare earth perfluorooctanoate [RE(PFO)3] catalyzed
one-pot synthesis of benzopyran derivatives
a
a
a
a
Li-Min Wang a,b, , Jue-Hua Shao , He Tian , Yong-Hong Wang , Bo Liu
*
a Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology,
130 Meilong Lu, Shanghai 200237, PR China
b Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, PR China
Received 25 August 2005; received in revised form 14 October 2005; accepted 14 October 2005
Available online 28 November 2005
Abstract
Rare earth perfluorooctanoate [RE(PFO)3] catalyzes facile condensation of dimedone, aldehydes and malononitrile under mild conditions to
afford the corresponding 5-oxo-5,6,7,8-tetrahydro-4H-benzo-[b]-pyran derivatives in good to high yields. The catalyst can be easy to prepare,
stable and storable, easily recycled and reused for several cycles with consistent activity.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Rare earth perfluorooctanoate [RE(PFO)3]; Aldehydes; Dimedone; Malononitrile; 4H-benzopyran derivatives
1. Introduction
In spite of the wide range applications of 4H-benzopyran
compounds, an easy, industrial available method for the
preparation of these compounds has still remained a challen-
ging task. During the last decade, rare earth catalysts in a
variety of synthetic reactions have been recognized because of
their unique advantages [11]. Meanwhile, the use of surface-
active reagents that can form micelles’ [12] structures as
catalysts is widespread and has been investigated in detail for
different reactions in aqueous solutions [13,14]. Although the
surfactant-aided organic reactions are still at their preliminary
stages, having Lewis acid–surfactants combined catalysts
(LASCs) concept [15] in mind, we have developed the novel
catalyst rare earth perfluorooctanoate [RE(PFO)3], which have
been employed for organic synthesis. In our previous works
[16,17], we have reported that [RE(PFO)3] are versatile
catalysts for condensation reaction of indole with carbonyl
compounds and Mannich reaction, in which the catalyst showed
high catalytic activity and can be recovered and recycled
without significant loss of activity. In the course of our research
on RE(III) salts, we found that [RE(PFO)3] are easily to handle,
are readily available at low cost and fairly stable in the water
and air. In this article, we would like to report an effective
method to produce 2-amino-5,6,7,8-tetrahydron-7,7-dimethyl-
5-oxo-4-phenyl-4H-benzo-[b]-pyran-3-carbonitrile in good to
high yields catalyzed by [RE(PFO)3] under mild conditions.
In recent years, 4H-benzopyran compounds have attracted
much attention due to biological and pharmacological activities
they display. It has been reported that benzopyran derivatives
served as important regulators for potassium cation channel [1]
and some 2-amino-4H-benzopyran compounds have photoche-
mical activities [2] as well. Considering the importance of the
compounds, many methods for the synthesis of 5-oxo-5,6,7,8-
tetrahydro-4H-benzo-[b]-pyran derivatives have been reported
successively. The conventional synthesis involves the condensa-
tion of dimedone with aromatic aldehyde and malononitrile
under refluxing in acetic acid [3] or the bicomponent
condensation of dimedone with a-cyanocinnamonitriles in the
presence of ethanolic piperidine [4]. Other effective methods
include the use of microwave [5], ultrasonic irradiation [6] or
HTMAB [7], TEBA [8], DBSA [9], KF-alumina [10] as catalyst
in one-pot reaction. However, these procedures have drawbacks,
which involve reacting at high temperature, using of expensive
catalyst, the low yield or commercially unavailable.
* Corresponding author. Tel.: +86 21 64252288/64252756;
fax: +86 21 64252288/64252758.
E-mail addresses: wanglimin@ecust.edu.cn, wcathy@china.com
(L.-M. Wang).
0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2005.10.004

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L.-M. Wang et al. / Journal of Fluorine Chemistry 127 (2006) 97–100
Table 2
Effect of solvents in reaction of benzaldehyde, dimedone and malononitrile
a
catalyzed by Yb(PFO)3
Entry
Solvent
Yield (%)b
1
2
3
4
5
6
C2H5OH
CH2CN
CH3COOC2H5
Toluene
H2O
90
80
50
48
34
52
Scheme 1.
2. Results and discussion
None
a
All reactions were catalyzed by Yb(PFO)3 at 60 8C.
Isolated yields.
First of all, we began to examine the catalytic activities of
different catalysts to optimize the reaction condition of
benzaldehyde, dimedone with malononitrile (Scheme 1) as
model reaction mentioned above. Among these catalysts,
Yb(PFO)3 showed the best catalytic activity in this reaction
(Table 1). RE(PFO)3 have been regarded as new types of Lewis
acids because of the hard character of their rare earth metal
cations and the strong electron-withdrawing effect of the
perfluoroalkane chain ‘‘(–CF2–CF2–)’’ group. It is partly
possible that the unique characteristic of the catalyst such as
surface activity of its long perfluoroalkane chain made the rare
earth perfluorooctanoates disperse into the substrates more
efficiently to promote the desired process, and perfluoroalkane
chain refuses the approach of water molecules to the central
metal cation, thus maintaining its high Lewis acidity. It has also
been reported that perfluorooctanoic acid is an excellent
dispersant with favorable surfactivity. Therefore, we even tried
perfluorooctanoic acid (CF3(CF2)6COOH) itself (Table 1, entry
1), finding that the acid was less efficient for the reaction owing
to lack of the binding with RE cation, giving only trace amount
of the adduct. In addition, while adding different amount
catalyst of Yb(PFO)3, the speed of reaction and yield was
obviously increased. It was also found that 5 mol% is preferred
and increased the amount of catalyst has not produced a better
result. Meanwhile, further studies show that raising the
temperature can affect the reaction to some extent. When
reactions were carried out at room temperature, the yield was
not so satisfactory (82%). After raising temperature at 60 8C, it
was shown that the yield increased to 90%. However, while
increasing temperature continuously, the yield has not
obviously changed. When the reaction was completed, the
b
catalysts can be recycled several times without significant loss
of activity and with yields range from 88 to 90% (1st, 90%; 2nd,
90%; 3rd, 88% yield; respectively).
Then, we choose Yb(PFO)3 as catalyst in different organic
media to investigate the solvent effect in the model reaction. In
each case, the substrates were mixed together with 5 mol%
Yb(PFO)3 agitated with 2 ml solvent. The results are shown in
Table 2. It is noteworthy to mention that the polar solvents such
as ethanol or acetonitrile, which afford better yields than non-
polar ones, and ethanol is the most effective solvent. It could be
explained by the reason that the better solubility of the catalyst
and reagents in polar solvent form a homogeneous phase and
made the catalysis more effective. Although the yield in water
is not so satisfactory, the surface tension experiment suggest
that the surface tension of water can be reduced when
increasingly added the rare earth perfluorooctanoate salt
because of the surface activity of its long perfluoroalkane
chain. Therefore, how to make this catalysis take place
smoothly in water with higher yield is further researching now.
In order to show the general applicability of the method, the
reaction of structurally diverse aldehydes under similar
conditions was investigated. By this method, the three
component reaction was carried out easily in the presence of
5 mol% Yb(PFO)3 in ethanol at 60 8C (Scheme 2) to produce
4H-benzo-[b]-pyran derivatives in good to excellent yields. The
results of this study are tabulated in Table 3, which revealed that
both electron-rich and electron-deficient para-substitutes on
Table 3
Yb(PFO)3 catalyzed the preparation of 4H-benzo-[b]-pyran derivativesa
Table 1
Effect of different catalysts in reaction of benzaldehyde, dimedone and mal-
ononitrilea
Entry
R
Time (h)
Product (4)
Yield (%)b
1
C6H5
5
4
4a
4b
4c
4d
4e
4f
4g
4h
4i
90
90
83
84
80
81
87
92
28
29
54
86
Entry
Catalyst
Amount of
Time
(h)
Yield
(%)b
2
4-O2NC6H4
3-O2NC6H4
4-HOC6H4
4-BrC6H4
catalyst (mol%)
3
4
1
2
3
4
5
6
7
8
CF3(CF2)6COOH
Yb(OTf)3
5
12
5
Trace
4
6
5
78
5
6
Yb(PFO)3
Yb(PFO)3
Yb(PFO)3
La(PFO)3
1
5
80
6
2,4-Cl2C6H3
4-CH3OC6H4
4-(CH3)2NC6H3
CH3(CH2)3
CH3(CH2)2
CH2 CH
6
2.5
5
5
86
7
8
5
90
8
5
5
5
84
9
40
32
18
5
Sm(PFO)3
5
5
82
10
11
12
4j
4k
4l
c
Yb(PFO)3
5
5
90, 90, 88
C6H5CH CH
a
All reactions were carried out in EtOH at 60 8C.
Isolated yields.
b
c
a
All reactions were catalyzed by Yb(PFO)3 in EtOH at 60 8C.
Isolated yields.
b
Catalyst was reused in three times.

L.-M. Wang et al. / Journal of Fluorine Chemistry 127 (2006) 97–100
99
La = 11.0%, calcd. for La(PFO)3: La = 10.08%. 19F NMR
(500 MHz, DMSO): d = À81.17 (t, 9F, J = 0.02 Hz), À115.94
(t, 6F, J = 0.02 Hz), À112.3 (s, 6F), À122.74 (s, 12F,
J = 0.13 Hz), À123.51 (s, 6F), À126.73 (s, 6F). EDS (%): C,
7.12; O, 7.77; F, 68.88; La, 16.23. Yb(PFO)3: mp: 124–126 8C.
IR (KBr): 3400–3500 (m), 1650 (vs), 1450 (vs), 1200 (vs), 1150
(vs) cmÀ1. ICP: found: Yb = 12.10%, calcd. for Yb(PFO)3:
Yb = 12.25%. 19F NMR (500 MHz, D2O): d = À81.70 (t, 9F,
J = 0.02 Hz), À118.43 (t, 6F, J = 0.02 Hz), À122.73 (s, 6F),
À122.95 (s, 6F), À123.68 (s, 6F), À124.16 (s, 6F), À126.98 (s,
6F). EDS (%): C, 37.29; O, 10.73; F, 46.68; Yb, 5.30.
Scheme 2.
aromatic ring produced a satisfactory yield without distinct
difference. However, aliphatic aldehydes showed a relatively
lower yield even in the prolonged reaction time. In addition,
when the a,b-unsaturated aldehydes such as cinnamaldehyde
was used in this reaction, the reaction time is shorter than
acraldehyde and the yield is much higher than the latter.
4.4. General experiment
A mixture of aldehyde (2 mmol), dimedone (2 mmol),
malononitrile (2 mmol), ethanol (2 ml) was well stirred with
Yb(PFO)3 (5 mol%, 0.01 mmol) at 60 8C for appropriate time.
When the reaction was completed as indicated by TLC, the
crude product was filtered and recrystallized with ethanol to get
pure product. The residue was evaporated and methylene
chloride and sat. aq. NaCl solution were added, the catalyst was
deposited in the middle of two phases, and the catalyst recovery
was accomplished simply by filtration and drying in air or under
vacuum.
3. Conclusion
In summary, there is no doubt that lanthanide perfluor-
ooctanoates RE(PFO)3, especially Yb(PFO)3 is an effective
catalyst and provides a new and useful method to synthesis of
4H-benzopyran derivatives by condensation of aldehydes,
dimedone and malononitrile. The catalysts show environmental
friendly character, which can be easily prepared, stored,
recycled without obvious loss of activity. Due to water-resistant
and oil-resistant character of perfluorooctyl chains, the
catalysts can also form a middle layer between dichloro-
methane and the lower water layer when being extracted and be
reused more conveniently. Therefore, a simple work-up
procedure, mild reaction conditions and good yields make
our methodology a valid contribution to the existing processes
in the field of 4H-benzopyran derivatives.
4.4.1. Compound 4a
1
Mp: 227–229 8C (lit. [5]: 226–228 8C). H NMR (500 Hz,
CDCl3): d = 1.05 (s, 3H), 1.10 (s, 3H), 2.18 (d, J = 16.3 Hz,
1H), 2.24 (d, J = 16.3 Hz, 1H), 2.45 (s, 2H), 4.40 (s, 1H), 4.53
(s, 2H), 7.18–7.33 (m, 5H).
4.4.2. Compound 4b
4. Experimental
1
Mp: 171–172 8C (lit. [5]: 174–176 8C). H NMR (500 Hz,
CDCl3): d = 1.06 (s, 3H), 1.12 (s, 3H), 2.22 (d, J = 16.4 Hz,
1H), 2.27 (d, J = 16.4 Hz, 1H), 2.50 (s, 2H), 4.54 (s, 1H), 4.65
(s, 2H), 7.42 (d, J = 8.6 Hz, 2H), 8.17 (d, J = 8.6 Hz, 2H).
4.1. Methods and apparatus
1
Melting points were determined on a Kofler hot plate. H
NMR spectra were recorded at Bruker WP-500SY (500 MHz)
in CDCl3 using TMS as internal standard. Mass spectra were
determined on a Micromass GCT spectrometer.
4.4.3. Compound 4c
1
Mp: 204–205 8C (lit. [5]: 210–212 8C). H NMR (500 Hz,
CDCl3): d = 1.05 (s, 3H), 1.14 (s, 3H), 2.19 (d, J = 16.4 Hz,
1H), 2.27 (d, J = 16.4 Hz, 1H), 2.50 (s, 2H), 4.56 (s, 1H), 4.67
(s, 2H), 7.48–8.09 (m, 4H).
4.2. Catalyst preparation
The RE(PFO)3 catalyst can be prepared from either rare earth
oxides or chlorides by stirring them with perfluorooctanoic acid,
then thegained solid was collected, washed and dried in the air or
invacuum for several hours. For example, to a stirring solution of
perfluorooctanoic acid (PFOA, 2.5 g, 0.6 mmol) in H2O (5 ml),
RE2O3 (0.1 mmol) was added and the mixture was stirred for
12 h under reflux. The aqueous layer was decanted and washed
with H2O to get gelatin-like solid. The solid was dried invacuum
at r.t. for 2 h to give white sheet solid.
4.4.4. Compound 4d
1
Mp: 208–210 8C (lit. [6]: 206–208 8C). H NMR (500 Hz,
CDCl3): d = 1.05 (s, 3H,), 1.13 (s, 3H), 2.15 (d, J = 16.3 Hz,
1H), 2.24 (d, J = 16.3 Hz, 1H), 2.45 (s, 2H), 4.30 (s, 1H), 4.49
(s, 2H), 4.75 (s, 1H), 6.70 (d, J = 8.4 Hz, 2H), 7.18 (d,
J = 8.4 Hz, 2H).
4.4.5. Compound 4e
Mp: 201–203 8C (lit. [6]: 196–198 8C). 1H NMR (500 Hz,
CDCl3): d = 1.03 (s, 3H), 1.12 (s, 3H), 2.15 (d, J = 16.4 Hz,
2H), 2.29 (d, J = 16.4 Hz, 2H), 2.45 (s, 2H), 4.35 (s, 1H),
4.64 (s, 2H), 7.13 (d, J = 8.4 Hz, 2H), 7.44 (d, J = 8.4 Hz,
2H).
4.3. Catalyst characterization
La(PFO)3: mp: 138–140 8C. IR (KBr): 3400–3500 (m),
1650 (m), 1620 (vs), 1200 (vs), 1150 (vs) cmÀ1. ICP: found:

100
L.-M. Wang et al. / Journal of Fluorine Chemistry 127 (2006) 97–100
4.4.6. Compound 4f
Acknowledgements
1
Mp: 115–117 8C H NMR (500 Hz, CDCl3): d = 1.06 (s,
3H), 1.12 (s, 3H), 2.16 (d, J = 16.3 Hz, 1H), 2.29 (d,
J = 16.3 Hz, 1H), 2.45 (s, 2H), 4.55 (s, 1H), 4.86 (s, 2H),
7.14–7.37 (m, 3H); MS: m/z (%) = 260 (6.56), 245 (100), 230
(4.63); anal. calcd. for C18H16Cl2N2O2: C, 59.52; H, 4.44; N,
7.71. Found: C, 59.35; H, 4.32; N, 7.69.
This research was financially supported by the Ministry of
Education of China and Education Committee of Shanghai and
Key Laboratory of Organofluorine Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences.
4.4.7. Compound 4g
References
1
Mp: 186–188 8C (lit. [5]: 192–194 8C). H NMR (500 Hz,
[1] H.B. Sun, W.Y. Hua, L. Chen, S.X. Peng, T. Wang, G.Q. Liu, Chem. J.
Chin. Univ. 18 (5) (1997) 730–733.
CDCl3): d = 1.05 (s, 3H), 1.11 (s, 3H), 2.11 (d, J = 16.4 Hz,
1H), 2.23 (d, J = 16.4 Hz, 1H), 2.45 (s, 2H), 3.78 (s, 3H), 4.36
(s, 1H), 4.50 (s, 2H), 6.83 (d, J = 8.6 Hz, 2H), 7.15 (d,
J = 8.6 Hz, 2H).
[2] D. Armetso, W.M. Horspool, N. Martin, A. Ramos, C. Seaone, J. Org.
Chem. 54 (1989) 3069–3072.
[3] S. Kamaljit, S. Jasbir, S. Harjit, Tetrahedron 52 (1996) 14273–14280.
[4] A.A. Hassanien, M.A. Zahrran, M.S.A. El-Gaby, M.M. Ghorab, J. Indian
Chem. Soc. 76 (1999) 350–354.
4.4.8. Compound 4h
1
[5] J.F. Zhou, S.J. Tu, Y. Gao, M. Ji, Chin. J. Org. Chem. 21 (2001) 742–745.
[6] S.J. Tu, H. Jiang, Q.Y. Zhuang, C.B. Miu, D.Q. Shi, X.S. Wang, Y. Gao,
Chin. J. Org. Chem. 23 (2003) 488–490.
Mp: 202–204 8C (lit. [6]: 208–210 8C). H NMR (500 Hz,
CDCl3): d = 1.03 (s, 3H), 1.10 (s, 3H), 2.18 (d, J = 16.4 Hz,
1H), 2.28 (d, J = 16.4 Hz, 1H), 2.44 (s, 2H), 2.85 (s, 6H), 4.55
(s, 1H), 4.88 (s, 2H), 6.64 (d, J = 7.7 Hz, 2H), 7.12 (d,
J = 7.7 Hz, 2H).
[7] T.S. Jin, A.Q. Wang, X. Wang, J.S. Zhang, T.S. Li, Synlett (2004) 871–
873.
[8] D.Q. Shi, S. Zhang, Q.Y. Zhuang, S.J. Tu, H.W. Hu, Chin. J. Org. Chem. 23
(2003) 877–879.
[9] T.S. Jin, A.Q. Wang, J.S. Zhang, F.S. Zhang, T.S. Li, Chin. J. Org. Chem.
24 (2004) 1598–1600.
4.4.9. Compound 4i
1
Mp: 165–167 8C H NMR (500 Hz, CDCl3): d = 0.88 (t,
[10] X.S. Wang, D.Q. Shi, S.J. Tu, C.S. Yao, Synth. Commun. 33 (2003) 119–
126.
J = 7.3 Hz, 3H), 1.08 (s, 3H), 1.12 (s, 3H), 1.15–1.63 (m, 6H),
2.26 (d, J = 16.3 Hz, 1H), 2.29 (d, J = 16.3 Hz, 1H), 2.36 (s,
2H), 3.40 (s, 1H), 4.43 (s, 2H); MS: m/z (%) = 273 (0.16), 217
(100), 161 (25); anal. calcd. for C16H22N2O2: C, 70.04; H, 8.08;
N, 10.21. Found: C, 69.91; H, 8.10; N, 10.38.
[11] (a) S. Kobayashi, M. Sugiura, H. Kitagawa, W.W. Lam, Chem. Rev. 102
(2002) 2227–2302;
(b) L.M. Wang, J.J. Xia, H. Tian, F. Qin, C. Qian, J. Sun, Synthesis 8
(2003) 1241–1247;
(c) L.M. Wang, C. Qian, H. Tian, Y. Ma, Synth. Commun. 33 (2003)
1459–1468;
(d) L.M. Wang, J.J. Liu, H. Tian, C. Qian, Synth. Commun. 34 (2004)
1349–1357;
4.4.10. Compound 4j
1
Mp: 172–174 8C H NMR (500 Hz, CDCl3): d = 0.90 (t,
(e) L.M. Wang, J.J. Liu, H. Tian, C. Qian, J. Sun, Adv. Synth. Catal. 347
(2005) 689–694;
J = 7.3 Hz, 3H), 1.08 (s, 3H), 1.12 (s, 3H), 1.15–1.63 (m, 4H),
2.27 (d, J = 16.3 Hz, 1H), 2.33 (d, J = 16.3 Hz, 1H), 2.36 (s,
2H), 3.40 (s, 1H), 4.43 (s, 2H); MS: m/z (%) = 260 (M+, 37%),
217 (100), 161 (22.15); anal. calcd. for C15H20N2O2: C, 69.20;
H, 7.74; N, 10.76. Found: C, 69.00; H, 7.60; N, 10.69.
(f) L.M. Wang, S. Jia, L. Zhang, J. Han, Z. Fan, H. Tian, Tetrahedron 61
(2005) 1539–1543.
[12] K. Manabe, X.M. Sun, S. Kobayashi, J. Am. Chem. Soc. 123 (2001)
10101–10102.
[13] (a) I.V. Berezin, K. Martinek, A.K. Yatsimirskii, Russ. Chem. Rev. 42
(1973) 787–802;
4.4.11. Compound 4k
1
(b) S. Tascioglu, Tetrahedron 52 (1996) 11113–11152.
[14] (a) F.M. Menger, J.U. Rhee, H.K. Rhee, J. Org. Chem. 40 (1975) 3803–
3805;
Mp: 170–172 8C H NMR (500 Hz, CDCl3): d = 0.98 (s,
3H), 1.05 (s, 3H), 2.16 (d, J = 16.3 Hz, 1H), 2.24 (d,
J = 16.3 Hz, 1H), 2.35 (s, 2H), 3.68 (s, 1H), 4.25 (s, 2H),
5.64–5.78 (m, 2H), 6.34–6.46 (m, 1H); MS: m/z (%) = 244 (M+,
100%), 245 (13.23%), 246 (2.35%); anal. calcd. for
C14H16N2O2: C, 68.83; H, 6.60; N, 11.47. Found: C, 68.97;
H, 6.47; N, 11.54.
(b) I. Grassert, U. Schmidt, S. Ziegler, C. Fischer, G. Oehme, Tetrahe-
dron: Asymm. 9 (1998) 4193–4202;
(c) R. Selke, J. Holz, A. Riepe, A. Borner, Chem. Eur. J. 4 (1998) 769–
771;
(d) K. Yonehara, T. Hashizume, K. Mori, K. Ohe, S. Uemura, J. Org.
Chem. 64 (1999) 5593–5598;
(e) K. Yonehara, K. Ohe, S. Uemura, J. Org. Chem. 64 (1999) 9381–9385;
(f) M.S. Goedheijt, B.E. Hanson, J.N.H. Reek, P.C.J. Kamer, P.W.N.M.
van Leeuwen, J. Am. Chem. Soc. 122 (2000) 1650–1657.
[15] (a) S. Kobayashi, T. Wakabayashi, Tetrahedron Lett. 39 (1998) 5839–
5840;
4.4.12. Compound 4l
1
Mp: 182–184 8C H NMR (500 Hz, CDCl3): d = 1.08 (s,
3H), 1.12 (s, 3H), 2.28(d, J = 16.3 Hz, 1H), 2.35 (d,
J = 16.3 Hz, 1H), 2.40 (s, 2H), 4.09 (s, 1H), 4.53 (s, 2H),
6.12 (m, 1H), 6.50 (d, J = 16.3 Hz, 1H), 7.18–7.36 (m, 5H);
MS: m/z (%) = 320.2 (M+, 100%), 321.2 (18.99%), 322.2
(3.28%); anal. calcd. for C20H20N2O2: C, 74.98; H, 6.29; N,
8.74. Found: C, 74.88; H, 6.44; N, 8.56.
(b) K. Manabe, Y. Mori, T. Wakabayashi, S. Nagayama, S. Kobayashi, J.
Am. Chem. Soc. 122 (2000) 7202–7207.
[16] L.M. Wang, J. Han, H. Tian, J. Sheng, Z. Fan, P. Tang, Synlett (2005) 337–
339.
[17] L.M. Wang, J. Han, J. Sheng, H. Tian, Z. Fan, Z. Y. Catal. Commun. 6
(2005) 201–204.