Erlenmeyer synthesis for azlactones catalyzed by ytterbium(III) triflate under solvent-free conditions

Article abstract of DOI:10.1080/00397910600941521

A series of 4-arylidene-2-phenyl-5(4)-oxazolones were synthesized from cyclodehydration-condensation of hippuric acid, aromatic aldehydes, and acetic anhydride catalyzed by ytterbium(III) triflate under mild conditions in excellent yields. Copyright Taylo

Full text of DOI:10.1080/00397910600941521

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Erlenmeyer Synthesis for Azlactones
Catalyzed by Ytterbium(III) Triflate under
Solvent‐Free Conditions
Chuanming Yu a , Baocheng Zhou a , Weike Su a & Zhenyuan Xu a
a Zhejiang Key Laboratory of Pharmaceutical Engineering , College of
Pharmaceutical Sciences, Zhejiang University of Technology , Hangzhou,
China
Published online: 23 Nov 2006.
To cite this article: Chuanming Yu , Baocheng Zhou , Weike Su & Zhenyuan Xu (2006) Erlenmeyer Synthesis
for Azlactones Catalyzed by Ytterbium(III) Triflate under Solvent‐Free Conditions, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:22, 3447-3453, DOI:
10.1080/00397910600941521
To link to this article: http://dx.doi.org/10.1080/00397910600941521
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Synthetic Communicationsw, 36: 3447–3453, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600941521
Erlenmeyer Synthesis for Azlactones
Catalyzed by Ytterbium(III) Triflate under
Solvent-Free Conditions
Chuanming Yu, Baocheng Zhou, Weike Su, and Zhenyuan Xu
Zhejiang Key Laboratory of Pharmaceutical Engineering, College of
Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou, China
Abstract: A series of 4-arylidene-2-phenyl-5(4)-oxazolones were synthesized from
cyclodehydration–condensation of hippuric acid, aromatic aldehydes, and acetic
anhydride catalyzed by ytterbium(III) triflate under mild conditions in excellent yields.
Keywords: Aromatic aldehyde, azlactone, hippuric acid, ytterbium triflate
4-Arylidene-2-phenyl-5(4)-oxazolones are important synthons for the
synthesis of several biologically active molecules.[1] They are also particularly
useful precursors for the synthesis of aminoacids,[2] peptides,[3] hetero-
cycles,[4] biosensors,[5] and antitumor[6] or anticancer[7] compounds. Develop-
ment of facile and environmentally friendly synthetic methods for azlactones
constitutes an active area of investigation.
The most well-known route to azlactones is the Erlenmeyer method,
which involves the direct condensation of aldehydes with hippuric acid in
the presence of stoichiometric amounts of fused anhydrous sodium acetate
as a basic catalyst in acetic anhydride.[8] Recently, some new reagents have
become available for the synthesis of azlactones, such as Al2O3-H3BO3,[9]
supported KF,[10] Bi(OAc)3,[11] Bi(OTf)3,[12] ZnCl2,[13] and Ca(OAc)2.[14]
Most of methods are suitable, but some need high temperature (reflux
Received in R.O.C. January 21, 2006
Address correspondence to Weike Su, Zhejiang Key Laboratory of Pharmaceutical
Engineering, College of Pharmaceutical Sciences, Zhejiang University of Technology,
Hangzhou 310014, China. E-mail: suweike@zjut.edu.cn
3447

3448
C. Yu et al.
temperature) and are difficult to handle. A simple, mild, environmentally
friendly, and easy method for azlactones synthesis is highly desirable.
Recently, ytterbium triflate [Yb(OTf)3] was used widely as a mild, recycl-
able, and environmentally friendly catalyst in organic reactions, such as the
Aldol reaction,[15] Mannich-type reaction,[16] Diels–Alder reaction,[17]
Friedel–Crafts acylation, and alkylation.[18] More recently, we have
reported that metal triflate can efficiently catalyze many organic synthetic
reactions.[19]
Because of our continued interest in the use of Yb(OTf)3 as an efficient
and environmentally friendly catalyst, we report that Yb(OTf)3 catalyzed
the Erlenmeyer reaction for synthesis of azlactones under solvent-free con-
ditions at 408C.
Our first attempt was performed with the benzaldehyde (1a) as substrate.
To investigate optimal conditions for the Erlenmeyer reaction (Scheme 1), a
series of experiments were tested (Table 1).
Compared to anhydrous sodium acetate,[8] Yb(OTf)3 employed here
shows high catalytic reactivity in terms of the amount of catalyst and yields
of the product (Table 1, entry 1). Moreover, the catalytic efficiency of other
metal triflates, such as Cu(OTf)2, Mg(OTf)2, Sr(OTf)2, and Zn(OTf)2were
tested (Table 1, entries 2–9). Under the same conditions, Yb(OTf)3 was
found to be the best catalyst in azlactones synthesis, Zn(OTf)2 was the
second, and Cu(OTf)2 was the worst.
Next, the amount of Yb(OTf)3 and reaction temperature were investi-
gated (Table 1, entries 10–15). Amounts of 5 mol%, 10 mol%, 20 mol% of
Yb(OTf)3 were used, and it was found that 10 mol% of Yb(OTf)3 was
enough to accomplish the reaction. Increasing the amount of catalyst did
not obviously improve the yield (Table 1, entries 10–12). Similarly,
reaction temperature was also studied. The yield was low (45%, Table 1,
entry 13) at room temperature for 3 h, but 82% yield was obtained at
408C, and increasing the temperature did not help to obviously improve the
yield (Table 1, entries 12–15). The catalyst can be recovered easily,[20] and
the yields were not reduced drastically when the recycled catalyst was used
(Table 1, entry 16). Moreover, the reaction could be performed on a large-
scale and gave satisfactory yield (Table 1, entry 17). Thus the following
reactions were performed with 10 mol% of Yb(OTf)3 at 408C.
To explore the scope of our method, a wide variety of aromatic aldehydes
smoothly gave the corresponding azlactones in good to excellent yields with
Scheme 1.

Erlenmeyer Synthesis of Azlactones
3449
Table 1. Preparation of azlactone (3a) under different conditions
Cat. amount
equiv.a
Temperature
(8C)
Reaction
time (h)
Yield
(%)b
Entry
1
Metal triflate
Anhydrous
sodium acetate
Cu(OTf)2
0.5
60
3
65c
2
3
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.2
0.1
0.1
0.1
0.1
0.1
40
100
40
3
NDd
NDd
60
Cu(OTf)2
Mg(OTf)2
MgOTf)2
Sr(OTf)2
Sr(OTf)2
Zn(OTf)2
Zn(OTf)2
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
3
4
3
5
100
40
3
63
50
6
3
7
100
40
3
51
70
8
3
9
100
40
3
73
73
10
11
12
13
14
15
16
3
40
40
0.5
0.5
3
83
82
25
60
45
85
0.5
0.5
0.5
100
40
83
76, 73,
70, 66e
80f
17
Yb(OTf)3
0.1
40
1
aBased on benzaldehyde.
bIsolated yields based on benzaldehyde.
c50 mol% anhydrous sodium acetate was used, and the procedure was performed
according to Ref. 8.
dNo product was detected.
eThe recovered catalyst was used.
f100 mmol benzaldehyde, 110 mmol hippuric acid, 10 mmol Yb(OTf)3 and 32 mL
acetic anhydride were used, and the reaction was carried out at 408C.
Yb(OTf)3 catalysis (Table 2). Moreover, all of benzaldehydes bearing
electron-withdrawing groups, as well as electron-donating groups, gave
good yields. Unfortunately, when aliphatic aldehyde was tested, no desired
products were detected (Table 2, entry 10).
In conclusion, an efficient, rapid, and environmentally benign method for
the preparation of azlactones has been developed in the presence of ytterbiu-
m(III) triflate under mild conditions. In addition, high yields and easy
operation make this method advantageous in comparison to other methods.
EXPERIMENTAL
Melting points were recorded on a digital melting-point apparatus, WRS-1B,
and are uncorrected. IR spectra were recorded using KBr pellets on an Avatar

3450
C. Yu et al.
Table 2. Preparation of azlactones from aldehydes catalyzed by Yb(OTf)3
Reaction
time (h)
Yield
(%)a
Entry
ArCHO
C6H5CHO
4-MeC6H4CHO
Azlactones
Mp (8C)
1
2
0.5
0.5
0.5
1
3a
3b
3c
3d
3e
3f
82
87
168–169
143–144
155–157
159–161
186–187
162–163
166–167
240–241
207–208
—
3
4-MeOC6H4CHO
2-ClC6H4CHO
4-ClC6H4CHO
84
91
4
5
1
87
82
6
2,4-Cl2C6H3CHO
3-NO2C6H4CHO
4-NO2C6H4CHO
Indole-3-carboxaldehyde
CH3CH2CH2CHO
1
7
1
3g
3h
3i
81
83
8
1
9
10
0.5
5
78
NDb
—
aIsolated yields based on aldehydes.
bNo product was detected.
370 FI-infrared spectrophotometer. 1H NMR spectra were recorded on a
Varian Mercury Plus-400 instrument using CDCl3 or DMSO as the solvent
with TMS as an internal standard. Mass spectra were measured with
Thermo Finnigan LCQ-Advantage (EI). Yb(OTf)3 was prepared from
ytterbium oxide and trifluoromethanesulfonic acid and recovered according
to the literature.[20]All reagents are commercially available and were used
without further purification.
General Procedure for the Preparation of Azlactones
A dry 50-ml flask was charged with aromatic aldehyde (2 mmol), hippuric
acid (2.2 mmol), acetic anhydride (6.6 mmol), and Yb(OTf)3 (0.2 mmol).
The mixture was stirred at 408C for the time indicated in Table 2. After the
reaction mixture was cooled to room temperature, 5 mL of 95% EtOH was
added, and a yellow solid was precipitated. The yellow solid was filtered off
and washed with hot water. The crude azlactone was purified by recrystalliza-
tion from acetone/water. The aqueous layer was washed by CH2Cl2 and was
concentrated in vacuo to give white crystals [Yb(H2O)9](OTf)3, which were
then heated at 1908C for 4 h in vacuo to give 113 mg of Yb(OTf)3 (91.2%).
Data
3a: Mp 168–1698C (lit.[21] mp 1688C); 1H NMR (400 MHz, CDCl3) d ¼ 7.27
(s, 1H, -CH55), 7.46–7.63 (m, 6H, ArH), 8.19–8.23 (m, 4H, ArH); MS: m/z
250 (M þ H)þ; IR (KBr) 3438, 1794, 1653 cm21
.

Erlenmeyer Synthesis of Azlactones
3451
1
3b: Mp 143–1448C (lit.[21] mp 141–1438C); H NMR (400 MHz, CDCl3)
d ¼ 2.42 (s, 3H, CH3), 7.21–7.40 (m, 3H, ArH and -CH55), 7.52–7.61
(m, 3H, ArH), 8.11–8.20 (m, 4H, ArH); MS: m/z 264 (M þ H)þ; IR (KBr)
3433, 2917, 1794, 1654, 1160 cm21
.
3c: Mp 155–1578C (lit.[21] mp 1568C); 1H NMR (400 MHz, DMSO) d ¼ 3.87
(s, 3H, CH3), 7.12 (d, 2H, J ¼ 8.4 Hz, ArH), 7.35 (s, 1H, -CH55), 7.62–7.74
(m, 3H, ArH), 8.12 (d, 2H, J ¼ 8.4 Hz, ArH), 8.32 (d, 2H, J ¼ 8.4 Hz, ArH);
MS: m/z 280 (M þ H)þ; IR (KBr) 3432, 2938, 1789, 1654, 1162 cm21
.
3d: Mp 159–1618C (lit.[21] mp 1598C); 1H NMR (400 MHz, CDCl3)
d ¼ 7.35–7.65 (m, 7H, ArH and -CH55), 8.19 (d, 2H, J ¼ 8.0 Hz, ArH),
8.92 (d, 1H, J ¼ 8.0 Hz, ArH); MS: m/z 285 (M þ H)þ; IR (KBr) 3441,
1793, 1652, 1170 cm21
.
3e: Mp 186–1878C (lit.[9] mp 1848C); 1H NMR (400 MHz, CDCl3) d ¼ 7.19
(s, 1H, -CH55), 7.45–7.66 (m, 5H, ArH), 8.15–8.20 (m, 4H, ArH); MS: m/z
285 (M þ H)þ; IR (KBr) 3433, 1796, 1655, 1160 cm21
.
3f: Mp 162–1638C (lit.[21] mp 1618C); 1H NMR (400 MHz, CDCl3) d ¼ 7.39
(d, 1H, J ¼ 7.6 Hz, ArH), 7.48 (s, 1H, -CH55), 7.53–7.66 (m, 4H, ArH), 8.17
(d, 2H, J ¼ 7.6 Hz, ArH), 8.90 (d, 1H, J ¼8.4 Hz, ArH); MS: m/z 319
(M þ H)þ; IR (KBr) 3444, 3091, 1798, 1656, 1170 cm21
.
3 g: Mp 166–1678C (lit.[12] mp 1648C); 1H NMR (400 MHz, DMSO)
d ¼ 7.54 (s, 1H, -CH55), 7.67–7.85 (m, 4H, ArH), 8.15 (d, 2H, J ¼ 7.6 Hz,
ArH), 8.31 (d, 1H, J ¼ 8.4 Hz, ArH), 8.68 (d, 1H, J ¼ 7.6 Hz, ArH), 9.22
(s, 1H, ArH); MS: m/z 295 (M þ H)þ; IR (KBr) 3443, 3104, 1797, 1655,
1163 cm21
.
3 h: Mp 240–2418C (lit.[22] mp 2418C); 1H NMR (400 MHz, CDCl3)
d ¼ 7.16 (t, 1H, J ¼ 8.0 Hz, ArH), 7.31–7.40 (m, 4H, ArH and -CH55),
7.49–7.63 (m, 3H, ArH), 8.12 (d, 2H, J ¼ 7.8 Hz, ArH); MS: m/z 295
(M þ H)þ; IR (KBr) 3440, 3093, 1790, 1658, 1169 cm21
.
3i: Mp 207–2088C (lit.[23]mp 2068C); 1H NMR (400 MHz, CDCl3)
d ¼ 7.27–7.28 (t, 2H, J ¼ 8.0 Hz, ArH), 7.53–7.72 (m, 5H, ArH
and -CH55), 8.14 (d, 2H, J ¼ 8.0 Hz, ArH), 8.40 (d, 1H, J ¼ 8.0 Hz, ArH),
8.64 (s, 1H, CH), 12.35 (s, 1H, NH); MS: m/z 289 (M þ H)þ; IR (KBr)
3440, 3229, 1729, 1642, 1169 cm21
.
ACKNOWLEDGMENTS
The authors are grateful to the National Basic Research Program (No.
2003CB114402) and National Natural Science Foundation of China (No.
20276072 and 20476098) for financial support.

3452
C. Yu et al.
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