Ytterbium(III) triflate-catalyzed stereoselective synthesis of β-lactams via [2 + 2] cyclocondensation in ionic liquid

Article abstract of DOI:10.1080/00397910600908843

Catalyzed by ytterbium(III) triflate [Yb(OTf)₃], β-lactams were stereoselectively synthesized from imines and acetyl chlorides in ionic liquid under mild conditions. The ionic liquid and catalyst could be recycled and reused as opposed to tradi

Full text of DOI:10.1080/00397910600908843

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Ytterbium(III) Triflate–Catalyzed
Stereoselective Synthesis of β‐Lactams via
[2+2] Cyclocondensation in Ionic Liquid
Rener Chen a , Bingjian Yang a & Weike Su a
a College of Pharmaceutical Sciences , Zhejiang Key Laboratory of
Pharmaceutical Engineering, Zhejiang University of Technology , Hangzhou,
China
Published online: 24 Nov 2006.
To cite this article: Rener Chen , Bingjian Yang & Weike Su (2006) Ytterbium(III) Triflate–Catalyzed
Stereoselective Synthesis of β‐Lactams via [2+2] Cyclocondensation in Ionic Liquid, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:21,
3167-3174, DOI: 10.1080/00397910600908843
To link to this article: http://dx.doi.org/10.1080/00397910600908843
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Synthetic Communicationsw, 36: 3167–3174, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600908843
Ytterbium(III) Triflate–Catalyzed
Stereoselective Synthesis of b-Lactams via
[2 1 2] Cyclocondensation in Ionic Liquid
Rener Chen, Bingjian Yang, and Weike Su
College of Pharmaceutical Sciences, Zhejiang Key Laboratory of
Pharmaceutical Engineering, Zhejiang University of Technology,
Hangzhou, China
Abstract: Catalyzed by ytterbium(III) triflate [Yb(OTf)3], b-lactams were stereo-
selectively synthesized from imines and acetyl chlorides in ionic liquid under mild
conditions. The ionic liquid and catalyst could be recycled and reused as opposed to
traditional solvent–catalyst systems.
Keywords: [2 þ 2] Cyclocondensation, ionic liquid, b-lactams, ytterbium triflate
INTRODUCTION
b-Lactams, as the key components of many biologically active compounds
such as penicillin and cephalosporin antibiotics, have already been intensively
studied.[1–3] There are many of methods for the construction of the azetidi-
none ring,[1] such as rhodium-catalyzed carbonylation of an aziridine,[4]
rhodium-catalyzed intramolecular insertion of an a-diazo amide into a C-H
bond,[5] copper-catalyzed coupling of an alkyne with nitrone,[6,7] amino-
ether-catalyzed reaction of ester enolates with imines,[8] ketene–imine
cycloaddition,[9–15] and so on.[16] Among them, the annelation of acetylchlor-
ides with imines has proven to be a versatile and convenient procedure.
However, the reaction requires more than a stoichiometric amount of
Received in R.O.C. February 16, 2006
Address correspondence to Weike Su, College of Pharmaceutical Sciences,
Zhejiang Key Laboratory of Pharmaceutical Engineering, Zhejiang University of
Technology, Hangzhou 310014, China. E-mail: suweike@zjut.edu.cn
3167

3168
R. Chen, B. Yang, and W. Su
triethylamine (about 3 equiv.), which cannot be recovered and reused after
the usual aqueous workup. Particularly, this reaction also needs a large
amount of organic solvents, which generate large amounts of waste in indus-
trial processes. Considering the ecological problems, an environmentally
acceptable process will be of high interest.
In recent years, ionic liquids (ILs) have emerged, which are organic salts
whose ions do not pack well and remain liquid at room temperature.[17,18]
They have unique properties such as a wide liquid range, good solvency,
tunable polarity, high thermal stability, negligible vapor pressure, and ease
of recyclability. In addition, ytterbium(III) triflate is a novel, recyclable
catalyst that has been applied in many organic reactions.[19–22] In this
article, we report that ytterbium(III) triflate catalyzed the synthesis of
b-lactams in ionic liquid [Nbupy][BF4] (4) (Scheme 1).
RESULTS AND DISCUSSION
The preparation of b-lactams involves two steps: (a) generation of ketenes in
situ by dehydrohalogenation of acetylchlorides in the presence of ILs and
Yb(OTf)3, and (b) [2 þ 2] cyclocondensation of imines with ketenes.
Initially, we want to synthesize N-substituted 1-aryl-3-oxo-1,2,3,4-tetrahy-
droisoquinolines, which, according to the work of Venkov and Mollov,[23]
were synthesized from arylacetyl chlorides and Schiff bases in the presence of
aluminum chloride as catalyst. However, when the reaction was catalyzed by
Yb(OTf)3 in [Nbupy][BF4], we gained not isoquinolines but b-lactams. Further-
more, the result of the use of ILs and Yb(OTf)3 was similar to that of only ILs, but
the yield of the former was higher than that of the latter. We assumed that one side
of pyridinium-based IL was alkaline, and ketene was generated in it; the other side
of the carbonyl of ketenes was activated by Yb(OTf)3 and the electronegativity of
a-carbon was increased to accelerate the reaction. The result is shown in Table 1.
The yield of b-lactams in the presence of IL and Yb(OTf)3 was more than
that of Et3N. Hence the Yb(OTf)3/IL system was selected. The results are
summarized in Table 2.
Scheme 1.

Stereoselective Synthesis of b-Lactams
3169
Table 1. Comparison of the reaction condition
Entry
1
Catalyst
AlCl3
Solvent
Product
Yield (%)
40[23]
ClCH2CH2Cl
2
Yb(OTf)3
ClCH2CH2Cl
12
3
4
5
None
[Nbupy][BF4]
[Nbupy][BF4]
34
Yb(OTf)3
Et3N
62
CH2Cl2/
benzene
30[13]
The reactions were stereoselective. When the reaction (Table 2, Entry 1)
was operated at room temperature (208C) for 24 h, the yield of b-lactams was
low. From the 1H NMR spectra of two hydrogens on the azetidinonic ring of
the azetidinone 3a, two diastereoisomers were formed: cis isomer (10%)
(5.01, 1H, d, J ¼ 6.0 Hz; 5.46, 1H, d, J ¼ 6.0 Hz), trans isomer (90%)
(4.28, 1H, d, J ¼ 2.4 Hz; 4.96, 1H, d, J ¼ 2.4 Hz). The coupling constant of
cis azetidinone was higher than that of the trans isomer.[15] Fortunately
when the temperature rose to 608C, we got better results with only the
trans-configuration. All products were determined as trans-configuration, as
shown in Table 2. We conjectured that the products were governed by
steric effects at 608C.[15]
As shown in Table 2, only when R1 were electron-withdrawing groups
and R3, R4 were electron-donor groups were the reactions achieved easily.
In other cases, only trace products could be obtained, and acylamides were
the main products. We assumed that the zwitterionic intermediate (6) was
more stable in IL when R1 was an electron-withdrawing group and R3, R4
were electron-donor groups (Scheme 2).
In addition, the reactions were slow with some aliphatic imines such as
N-benzylidene (phenyl) methanamine and N-benzylidene (4-methoxyphenyl)
methanamine. It was assumed that those zwitterionic intermediates were
unstable in ionic liquid.

3170
R. Chen, B. Yang, and W. Su
Table 2. Synthesis of b-lactams in the presence of [Nbupy][BF4] and Yb(OTf)3
Entry
R1
R2
R3
R4
Producta,b
Yield (%)c
1
2
H
H
H
H
H
H
H
H
H
H
H
H
H
3a
3b
3c
3d
3e
3f
62
73
H
CH3
H
3
NO2
NO2
H
75
72
4
CH3
H
5
CH3O
CH3O
CH3O
H
Trace
Trace
Trace
65
6
H
CH3
NO2
H
7
H
3g
3h
3i
8
H
Cl
C2H5
CH3
9
10
H
H
H
76
62
H
H
H
3j
1
aThe C3–C4 relative configuration was determined by H NMR spectroscopy; all
products have trans-stereochemistry and are racemic mixtures.
bAll compounds were identified by their physical properties and analytical data.
cReport yields after crystallization from EtOAc–cyclohexane. Purification of the
products by column chromatography, silica gel 200–300 mesh, eluent, EtOAc/
cyclohexane ¼ 1:4.
The reactions were also affected by the steric factors. When ketimine was
used, the reaction failed because of the steric constraints imposed by two sub-
stituted groups.
To evaluate the possibility of recycling the ILs and Yb(OTf)3 used for the
reaction, the recovered ILs were concentrated in vacuo. Then the reaction was
performed with recycled ILs and Yb(OTf)3, and the process was repeated four
times. There is little decline in the rate or yield of the reaction during each cycle.
For example, for the product 3a, the yields were 62%, 60%, 59%, and 59%.
Scheme 2.

Stereoselective Synthesis of b-Lactams
3171
In conclusion, the present method provided an environmentally friendly,
efficient, and convenient procedure to synthesize b-lactams in the presence of
[Nbupy][BF4] and Yb(OTf)3. All products are trans-configuration. The
[Nbupy][BF4] and Yb(OTf)3 can be recovered, avoiding the use of a large
amount of organic liquids.
EXPERIMENTAL
All reagents are commercially available. Yb(OTf)3 was prepared from ytterbium
oxide with trifluoromethanesulfonic acid in water according to the litera-
ture;[24] [Nbupy][BF4] was synthesized from pyridine, 1-bromobutane, and
NaBF4 by the literature procedure;[25] and imines and acetylchlorides were
prepared by the usual method. All reactions were performed under an atmos-
phere of dry nitrogen. Melting points were taken on a digital melting-point
1
apparatus WRS-1B and are uncorrected. H NMR and 13C NMR spectra
were recorded on a Varian 400-MHz instrument using CDCl3 as the solvent,
and chemical shifts are expressed in parts per million (ppm) using TMS as
an internal standard. IR measurements were carried out with Nicolet Aviatar
370 instrument. Mass spectra were measured with a Trace Finnigan DSQ.
Optical rotation were measured with an Autopol IV polarimeter (A7040-
12). Elemental analyses were recorded on a ThermoFinnigan Flash EA1112.
All spectral data of the products were identical to authentic samples.
General Procedure for the Preparation of b-Lactams
The imine (4 mmol) was added to dry pyridinium-based ionic liquid (4 mL), and
the mixture was stirred until the imine was dissolved in ionic liquid under N2
protection at room temperature. Then the arylacetyl chloride (4 mmol) was
injected. After 15 min, Yb(OTf)3 (0.2 mmol) was added, and the temperature
was raised to 608C. The mixture was stirred for 2 h. When the reaction
finished, the mixture was extracted with ether (6 ꢀ 10 mL). The organic
extract was washed with 5% Na2CO3 (40 mL) and water (40 mL), dried with
anhydrous magnesium sulfate, and evaporated in vacuo. The residual product
was purified by recrystallization from AcOEt/cyclohexane or by column chrom-
atography (silica gel, 200–300 mesh, eluent, cyclohexane/AcOEt ¼ 4 : 1).
Then the IL was concentrated in vacuo (10 Torr for 2 h at room temperature).
The IL and Yb(OTf)3 were recovered and could be used again.
Data
3a: White crystalline solid. Mp 130–1318C (lit.,[26] 129–1308C). IR (KBr)
1
y/cm21: 1753 (C55O), 1633, 1600, 1497, 753, 696. H NMR: d (ppm) 4.29

3172
R. Chen, B. Yang, and W. Su
(d, 1H, J ¼ 2.4 Hz), 4.96 (d, 1H, J ¼ 2.4 Hz), 7.06–7.42 (m, 15H). 13C NMR:
d (ppm) 63.7, 65.1, 117.2, 124.1, 125.9, 127.5, 127.9, 128.7, 129.0, 129.1,
129.3, 134.7, 137.4, 137.5, 165.6. M/Z (CIþ): 300 (M þ Hþ).
3b: White crystalline solid. Mp 178–1808C (lit.,[27] 176–1778C). IR (KBr)
1
y/cm21: 1743 (C55O), 1612, 1514, 1452, 817, 756, 698. H NMR: d (ppm)
2.27 (s, 3H), 4.26 (d, 1H, J ¼ 2.4 Hz), 4.92 (d, 1H, J ¼ 2.4 Hz), 7.06–7.08
(m, 2H), 7.24–7.40 (m, 12H). 13C NMR: d (ppm) 21.2, 63.9, 65.4, 117.5,
126.2, 128.2, 128.9, 129.1, 129.3, 129.6, 129.9, 134.0, 135.0, 135.2, 135.4,
137.9, 165.6. M/Z(CIþ): 314(M þ Hþ).
3c: Yellowish crystalline solid. Mp 123–1248C. IR (KBr) y/cm21: 1738
(C55O), 1601, 1532, 1497, 757, 743, 692. H NMR: d (ppm) 4.29 (d, 1H,
1
J ¼ 2.4 Hz), 5.06 (d, 1H, J ¼ 2.4 Hz), 7.08–7.42 (m, 10H), 7.58–7.62
(m, 1H), 7.72–7.76 (m, 1H), 8.21–8.28 (m, 2H). 13C NMR: d (ppm) 62.9,
65.5, 117.3, 121.4, 124.0, 124.8, 127.9, 128.5, 129.5, 129.6, 130.9, 131.9,
134.1, 137.2, 140.2, 149.2, 165.1. M/Z (CIþ): 345 (M þ Hþ); anal. calcd.
for C21H16N2O3: C, 73.24; H, 4.68; N, 8.13. Found: C, 73.27; H, 4.97; N, 7.76.
3d: Yellowish crystalline solid. Mp 126–1278C. IR (KBr) y/cm21: 1752
(C55O), 1618, 1522, 1454, 817, 738, 695. H NMR: d (ppm) 2.29 (s, 3H),
1
4.27 (d, 1H, J ¼ 2.4 Hz), 5.04 (d, 1H, J ¼ 2.4 Hz), 7.08–7.42 (m, 9H),
7.58–7.62 (m, 1H), 7.72–7.74 (m, 1H), 8.22–8.26 (m, 2H). 13C NMR: d
(ppm) 21.1, 62.8, 65.5, 117.3, 121.4, 123.9, 127.7, 128.5, 128.7, 129.4,
129.6, 129.7, 130.0, 130.8, 131.9, 134.2, 134.4, 134.7, 140.3, 149.1, 164.8.
M/Z (CIþ): 359 (M þ Hþ); anal. calcd. for C22H18N2O3: C, 73.73; H, 5.06;
N, 7.82. Found: C, 73.79; H, 5.28; N, 7.51.
3h: White crystalline solid. Mp 146–1478C (lit.,[28] 1438C). IR (KBr)
1
y/cm21: 1746 (C55O), 1596, 1492, 1455, 830, 753, 700. H NMR: d (ppm)
4.30 (d, 1H, J ¼ 2.4 Hz), 4.94 (d, 1H, J ¼ 2.4 Hz), 7.21–7.43 (m, 14H),
13C NMR: d (ppm) 63.7, 65.3, 118.3, 125.8, 127.3, 127.9, 128.8, 129.0,
129.1, 129.3, 134.37, 135.9, 137.0, 165.4; M/Z (CIþ): 334 (M þ Hþ).
3i: White crystalline solid. Mp 90–918C; IR (KBr) y/cm21: 1742 (C55O),
1602, 1493, 1455, 768, 700, 755; 1H NMR: d (ppm) 1.25–1.29 (t, 3H,
J ¼ 6.3 Hz), 2.77–2.88 (m, 2H, J ¼ 6.3 Hz), 4.38 (d, 1H, J ¼ 2.4 Hz),
5.11 (d, 1H, J ¼ 2.4 Hz), 7.15–7.40 (m, 14H); 13C NMR: d (ppm) 14.3,
25.0, 64.1, 66.0, 123.7, 126.4, 126.5, 126.92, 127.5, 127.8, 128.6, 129.0,
129.4, 133.9, 135.0, 137.7, 138.2, 166.6; M/Z (CIþ): 328 (M þ Hþ); anal.
calcd. for C23H21NO: C, 84.37; H, 6.46; N, 4.28. Found: C, 84.32; H,
6.23; N, 4.62.
3j: Yellowish crystalline solid. Mp 145–1468C (lit.,[29] 143–1448C); IR
1
(KBr) y/cm21: 1741 (C55O), 1602, 1496, 1456, 763, 756, 699; H NMR: d

Stereoselective Synthesis of b-Lactams
3173
(ppm) 2.47 (s, 3H), 4.35 (d, 1H, J ¼ 2.4 Hz), 5.14 (d, 1H, J ¼ 2.4 Hz), 7.09–
7.39 (m, 14H); 13C NMR: d (ppm) 26.9, 64.1, 65.5, 122.8, 126.3, 126.4, 126.5,
127.5, 127.8, 128.6, 129.0, 129.1, 131.5, 132.2, 134.8, 134.9, 137.8, 166.1; M/
Z (CIþ): 314 (M þ Hþ).
ACKNOWLEDGMENTS
The authors are grateful to the National Basic Research Program
(No. 2003CB114402) and National Natural Science Foundation of China
(Nos. 20276072 and 20476098) for financial support.
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