High regioselective Diels-Alder reaction of myrcene with acrolein catalyzed by zinc-containing ionic liquids

Article abstract of DOI:10.1002/adsc.200404154

The ambient zinc-containing ionic liquids, MX-ZnCl₂, functioning as both Lewis acid catalyst and green solvent, are employed for a high regioselective Diels-Alder reaction of myrcene with acrolein for the first time, where MX is either 1-butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-methylimidazolium bromide (EmimBr), N-butylpyridinium bromide (BPyBr), or N-ethylpyridinium bromide (EtPyBr). Compared with the analogous reaction performed over a ZnCl₂ catalyst in the conventional solvent dichloromethane, higher regioselectivity of the 'para' cycloadduct and excellent yield were achieved at shorter reaction time in these ionic liquids with optimized molar compositions of MX and ZnCl₂. These moisture-insensitive ionic liquids can be easily separated from reaction products after simple washing with hexane, allowing their reuse with no obvious loss in activity.

Full text of DOI:10.1002/adsc.200404154

UPDATES
High Regioselective Diels---Alder Reaction of Myrcene with
Acrolein Catalyzed by Zinc-Containing Ionic Liquids
Donghong Yin,* Changzhi Li, Biaomo Li, Liang Tao, Dulin Yin
Institute of Fine Catalysis and Synthesis, and Key Laboratory of Chemical Biology and Traditional Chinese Medicine
Research (Ministry of Education), Hunan Normal University, Changsha 410081, P. R. China
Fax: (þ86)-731-8872531, e-mail: yindh@hunnu.edu.cn
Received: June 3, 2004; Accepted: November 9, 2004
Abstract: The ambient zinc-containing ionic liquids,
MX-ZnCl2, functioning as both Lewis acid catalyst
and green solvent, are employed for a high regiose-
lective Diels–Alder reaction of myrcene with acro-
lein for the first time, where MX is either 1-butyl-
3-methylimidazolium chloride (BmimCl), 1-ethyl-3-
methylimidazolium bromide (EmimBr), N-butylpyr-
idinium bromide (BPyBr), or N-ethylpyridinium
bromide (EtPyBr). Compared with the analogous re-
action performed over a ZnCl2 catalyst in the con-
ventional solvent dichloromethane, higher regiose-
lectivity of the ꢀparaꢁ cycloadduct and excellent yield
were achieved at shorter reaction time in these ionic
liquids with optimized molar compositions of MX
and ZnCl2. These moisture-insensitive ionic liquids
can be easily separated from reaction products after
simple washing with hexane, allowing their reuse
with no obvious loss in activity.
teresting question. In our previous reports, rate en-
hancement and the ꢀparaꢁ regioselectivity improvement
of the reaction has been achieved by the use of homoge-
neous and heterogeneous ZnCl2 Lewis acid catalysts.[3]
However, it is known that almost all homogeneous
Lewis acids have serious drawbacks, e.g., requirements
of large amount of catalyst and laborious work-up pro-
cedures, problems of environmentally hazardous
waste-streams and difficulty in their reuse. Heterogene-
ous Lewis acid catalysts can be recycled, but an addition-
al modification of the catalyst is usually required in its
preparation procedure; also, the catalytic reaction has
to be carried out in large volumes of toxic and volatile
organic solvents which may be hazardous to the environ-
ment.
Keywords: acrolein, Diels–Alder reaction, ionic liq-
uids (ILs), myrcene, zinc chloride
Introduction
The Diels–Alder reaction is one of the most useful car-
bon-carbon bond-forming reactions in organic synthesis
and is widely used in the chemical industry for the syn-
thesis of natural products in a regio- and stereo-control-
led way. Myrcene (7-methyl-3-methene-1,6-octadiene),
prepared by cracking b-pinene from turpentine oil, gives
rise to a variety of fragrant compounds and their inter-
mediates when reacted with conjugated alkylene car-
bonyl compounds.[1] For example, the Diels–Alder reac-
Scheme 1. The Diels–Alder reaction of myrcene with acro-
lein.
Recently, the application of room temperature ionic
tion of myrcene with acrolein can yield two regioselec- liquids (ILs) as green solvents and catalysts in different
tive cycloadducts, the ꢀpara-ꢁ and ꢀmetaꢁ-myrac aldehyde organic transformations has attracted much attention
(4- and 3-(4’-methyl-3’-pentyl)-D3-cyclohexenecarbox- due to their unique physicochemical properties, e.g.,
aldehyde) (Scheme 1). It is known that the ꢀparaꢁ-myrac non-volatility, non-inflammability, and facility in recy-
aldehyde has been found to be very useful in perfumes, cling. It is reported that some ionic liquids, such as 1-bu-
eau de colognes, cosmetic powders and dryer-added fab- tyl-3-methylimidazolium
trifluoromethanesulfonate
ric softener articles, as well as in cigarette and smoking [Bmim][OTf], 1-butyl-3-methylimidazolium hexafluoro-
tobacco articles.[2] How to synthesize the ꢀparaꢁ-myrac phosphate [Bmim][PF6], 1-butyl-3-methylimidazolium
aldehyde with high regioselectivity is therefore a very in- tetrafluoroborate [Bmim][BF4], and 1-ethyl-3-methyli-
Adv. Synth. Catal. 2005, 347, 137–142
DOI: 10.1002/adsc.200404154
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137

Donghong Yin et al.
UPDATES
Table 1. The results of Diels–Alder reactions of myrcene with acrolein obtained in ionic liquids at various molar ratios of
ZnCl2 to MX.[a]
Entry
MX
Molar ratio of ZnCl2 to MX
Yield [%]
Regioselectivity [p/m]
[b]
1
2
3
4
5
6
7
BmimCl
0: 1
4.3
21.5
26.8
47.4
62.3
97.3
96.3
75: 25
94: 6
94: 6
94: 6
94: 6
95: 5
95: 5
0.9: 1
1: 1
1.1: 1
1.5: 1
2: 1
3: 1
8
9
EmimBr
0: 1[b]
0.9: 1
1: 1
1.1: 1
1.5: 1
2: 1
4.1
18.6
22.8
45.9
66.5
97.4
97.5
74: 26
93: 7
93: 7
94: 6
94: 6
94: 6
94: 6
10
11
12
13
14
3: 1
15
16
17
18
19
20
21
BPyBr
0: 1[b]
0.9: 1
1: 1
1.1: 1
1.5: 1
2: 1
4.3
20.4
25.0
50.0
72.6
96.9
97.1
77: 23
94: 6
94: 6
94: 6
94: 6
95: 5
95: 5
3: 1
22
23
24
25
26
27
28
EtPyBr
0: 1[b]
0.9: 1
1: 1
1.1: 1
1.5: 1
2: 1
3.7
17.9
22.2
42.7
58.1
92.2
94.4
76: 24
94: 6
94: 6
95: 5
95: 5
95: 5
95: 5
3: 1
[a]
The reaction was performed at 158C for 2 h.
Reaction time was 4 h.
[b]
midazolium hexafluorophosphate [Emim][PF6] etc., Results and Discussion
were favorably reused to accelerate some Diels–Alder
reactions.[4,5] However, in the case of the Diels–Alder re-
action catalyzed by Lewis acid, these neutral ILs did not
show great advantages due to their weak Lewis acidity.
Chloroaluminate ionic liquids have been reported as
Effect of ZnCl2 Composition in Ionic Liquids on the
Reaction
both solvent and Lewis acid catalyst for several Diels– In Table 1 are listed the results of the Diels–Alder reac-
Alder reactions[6] but the widespread utilization of tion between myrcene and acrolein in different ionic liq-
chloroaluminate ionic liquids is rather limited due to uids at various molar ratios of ZnCl2 to MCl. Very poor
their moisture sensitivity. Therefore, it is very interest- yield and regioselectivity were obtained if the reaction
ing to find a new type of ionic liquids that possesses was performed in ZnCl2-free BmimCl, EmimBr, BPyBr,
both a suitable Lewis acid property and stability to mois- or EtPyBr ionic liquids (entries 1, 8, 15 and 22). Howev-
ture.[7] In this paper, the Diels–Alder reactions of myr- er, the reaction rate in zinc-containing ionic liquids was
cene with different carbonyl-containing dienophiles in very sensitive to the molar ratio of ZnCl2 to MX in the
ꢀ
ꢀ
ꢀ
BmimCl ZnCl2, EmimBr ZnCl2 BPyBr ZnCl2 and range from 0.9:1 to 2:1. For example, if the reaction
EtPyBr ZnCl2 ionic liquids has been examined for the was carried out in the zinc-containing ionic liquid with
ꢀ
first time. These ionic liquids are favorably moisture-in- a molar ratio of ZnCl2 to BmimCl of 0.9:1, the yield of
sensitive which allows their repeated use for at least five cycloadducts was only 21.5%, but the regioslectivity of
times, without noticeable decrease in their catalytic ac- p/m (94/6) increased significantly (Entry 2), as com-
tivities.
pared with that in pure BmimCl. With an increase of
the molar fraction of ZnCl2 from 1:1 to 2:1, the reaction
138
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Regioselective Diels–Alder Reaction of Myrcene with Acrolein
UPDATES
Table 2. The results of Diels–Alder reactions of myrcene with acrolein in dichloromethane and in ionic liquids.
Entry
Solvent
Catalyst
Time [h]
Yield [%]
Regioselectivity [p/m]
[a]
1
2
3
4
5
6
7
CH2Cl2[a]
CH2Cl2[a]
CH2Cl2
None
6
6
6
2
2
2
2
6.5
69.0
83.2
97.3
97.4
96.9
92.2
72: 28
92: 8
93: 7
95: 5
94: 6
95: 5
95: 5
ZnCl2
ZnCl2/NaY
None
None
None
[b]
ꢀ
BmimCl ZnCl2 [b]
ꢀ
EmimBr ZnCl2
BPyBr ZnCl2
[b]
ꢀ
[b]
ꢀ
EtPyBr ZnCl2
None
[a]
Ref.[3b], the reaction was performed at 308C.
The molar ratio of ZnCl2 to XCl was 2: 1 and the reaction was performed at 158C.
[b]
rate increased clearly (entries 2–6). Especially at a mo- the zinc-containing ionic liquids may play very impor-
lar ratio of ZnCl2 to BmimCl of 2:1, a yield of 97.3% for tant roles in the Diels–Alder reaction.
¼
the cycloadducts with high regioselectivity (p/m 95/5)
was obtained at 158C after only 2 h (entry 6). Both the
yield and regioselectivity (p/m ratio) remain almost un- Comparison among Different Carbonyl-Containing
changed with an increase of the molar ratio of ZnCl2 to Dienophiles
BmimCl up to 3:1 (entry 7). Similar reaction rules were
ꢀ
obtained
for
EmimBr ZnCl2
(entries 8–14), Further, we have carried out the Diels–Alder reactions
ꢀ
ꢀ
BPyBr ZnCl2 (entries 15–21) and EtPyBr ZnCl2 (en- of myrcene with different carbonyl-containing dieno-
tries 22–28). The enhancement of the reaction rate philes to examine the possibility of the wide use of this
and regioselectivity observed here could be attributed kind of ionic liquids. The results of the Diels–Alder reac-
ꢀ
to an increase in Lewis acidity as a result of the increase tion in the ionic liquid BmimCl ZnCl2 (molar ratio of
of the amount of ZnCl2 in each zinc-containing ionic liq- ZnCl2 to BmimCl¼2:1) and organic solvents at differ-
uid.
ent reaction temperatures are compared in Table 3. It
is observed that the yield and regioselectivity were
strongly influenced by the dienophiles and reaction tem-
perature. Compared with the reaction performed in the
solvent dichloromethane, the yield of the cycloadducts
and the regioselectivity of the ꢀparaꢁ cycloadduct in-
ꢀ
Comparison between the Reactions in
Dichloromethane and in Ionic Liquids
In Table 2 are summarized the results of the Diels–Al- creased significantly in the ionic liquid BmimCl ZnCl2
der reactions of myrcene with acrolein carried out in at 158C (entries 1 and 2, 3 and 4, 7 and 8, and 11 and
ionic liquids and in the solvent dichloromethane. The 12). By using the corresponding dienophile as solvent
thermal reaction rate in dichloromethane was very low at the reaction temperature of 758C, the reaction per-
with poor regioselectivity (entry 1). In our previous re- formed in the ionic liquid also gave a high yield and re-
port,[3] higher yield and regioselectivity of cycloadducts gioselectivity (entries 5 and 6, 9 and 10, and 13 and 14).
were obtained over the heterogeneous catalyst of ZnCl2/ The results indicated that the ionic liquid
ꢀ
NaY prepared by treatment with microware irradiation BmimCl ZnCl2 acted in the reactions as both Lewis
than that over the typical homogeneous Lewis acid of acid catalysis and polar solvent. Moreover, the reaction
ZnCl2 in the conventional solvent of dichloromethane temperature has a remarkable effect on the ꢀpara/metaꢁ
ꢀ
(entries 2 and 3). However, when the reaction was per- regioselectivity in BmimCl ZnCl2 ionic liquid. The re-
formed in the zinc-containing ionic liquids with different gioselectivity decreased significantly with increasing re-
cations, a significant enhancement in the reaction rate action temperature (entries 4–6, 8–10, and 12–14).
was observed. Typically, a higher yield of the cycload-
Both the electron density at the carbonyl of dieno-
ducts with higher regioselectivity was achieved in the phile and the steric crowding of the dienophile have
zinc-containing ionic liquids after a reaction period of great effects on the Diels–Alder reaction rate. It is
only 2 h at 158C (entries 4–7), whereas lower yield known that in the Diels–Alder reaction of myrcene cat-
and regioselectivity were obtained over homogeneous alyzed by Lewis acid the carbonyl of the dienophile is co-
and heterogenous ZnCl2 catalysts in the solvent of di- ordinated with the Lewis acid sites of Zn2þ to activate
chloromethane even after 6 h reaction at 308C (en- the dienophile,[3a] and therefore the electron density at
tries 1–3), indicating that the ionic liquid acted here the carbonyl of the dienophiles should be taken into ac-
not only as a Lewis acid catalyst but also as a green sol- count. Among the four dienophiles, acrolein has no elec-
vent with polar or ionic characteristics and high internal tron-donating group and the least steric crowding, which
pressure.[8] The mid-strength Lewis acid active sites in gives the highest yield of myrac aldehyde with highest
Adv. Synth. Catal. 2005, 347, 137–142
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139

Donghong Yin et al.
UPDATES
Table 3. The results of Diels–Alder reactions between myrcene and different dienophiles.[a]
Entry Dienophile
Solvent
Temperature [8C] Time [h] Yield [%] Regioselectivity [p/m]
[b]
1
2
CH2Cl2
15
15
6
2
6.5
97.3
72: 28
95: 5
ꢀ
BmimCl ZnCl2
acrolein
[b]
3
4
5
CH2Cl2
15
15
75
75
48
48
9
3.8
85.9
46.3
95.2
67: 33
93: 7
67: 33
79: 21
ꢀ
BmimCl ZnCl2
Methyl acrylate[c]
ꢀ
6
BmimCl ZnCl2
9
methyl acrylate
methyl methacrylate
mesityl oxide
[b]
7
8
9
CH2Cl2
15
15
48
48
9
1.7
69.9
10.9
83.1
67: 33
92: 8
67: 33
77: 23
ꢀ
BmimCl ZnCl2
methyl methacrylate[c] 75
ꢀ
10
BmimCl ZnCl2
75
9
[b]
11
12
13
14
CH2Cl2
15
15
75
75
48
48
9
2.2
31.0
6.4
56: 44
90: 10
70: 30
70: 30
ꢀ
BmimCl ZnCl2
mesityl oxide[c]
ꢀ
BmimCl ZnCl2
9
52.3
[a]
[b]
[c]
Unless otherwise noted, the reaction conditions were 2 mL of IL, 1.0 mmol of myrcene, and 1.1 mmol of dienophile.
CH2Cl2 (2 mL) was used as the only solvent.
10 mmol of methyl acrylate, methyl methacrylate and mesityl oxide were used as dienophile and solvent, respectively.
regioselectivity at a short reaction time (entry 2). How- air without special anti-moisture procedures, which
ever, methyl acrylate and methyl methacrylate with an shows that the zinc-containing ionic liquids are moisture
ꢀ
electron-donating group ( OCH3) gave poorer yields stable. The above procedure was repeated four times
when used as dienophiles even at a prolonged reaction and the results obtained in the ionic liquids are listed
time (entries 4 and 8). Mesityl oxide gave the lowest in Table 4. It was observed that there was no noticeable
yield of cycloadducts and lowest regioselectivity, which decrease in activity even up to 5 subsequent runs. The
is mainly attributed to the difficulty in activating it due simple experimental and product isolation procedures
to steric crowding (entries 12 and 14). Taking into ac- as wellas the easiness for recovery and reuse of the novel
count the electronic properties and steric hindrance of reaction medium may contribute to the development of
the dienophiles, the activity sequence for the four dien- a green strategy for the high regioselective synthesis of
ophiles is as follows: acrolein>methyl acrylate>meth- ꢀparaꢁ-myrac aldehyde.
yl methacrylate>mesityl oxide, which is in good agree-
ment with the yields of cycloadducts listed in Table 3.
Conclusions
Recycling of the Ionic Liquids in the Diels–Alder
Reaction of Myrcene with Acrolein
In summary, this paper has described that the zinc-con-
taining ionic liquids are novel reaction media as well as
Lewis acid catalysts for the highly regioselective Diels–
The Diels–Alder reaction catalyzed by the zinc-contain- Alder reaction of myrcene with acrolein. High yields of
ing ionic liquids is a two-phase reaction, namely, the di- myrac aldehyde with high regioselectivity were ach-
enophile acrolein dissolves in the ionic liquid, but the ieved in the zinc-containing ionic liquids at the molar ra-
less polar diene forms a separate phase. After comple- tio of ZnCl2 to MX of 2:1 and a short reaction time. The
tion of the reaction the products also form a separate experimental procedure here is quite simple and con-
phase on top of the ionic liquid and therefore are easily venient, the non-volatile and moisture-stable ionic liq-
separated by decanting or removing with a pipette. The uids can be conveniently recovered for recycled uses,
isolated products can be distilled under reduced pres- and thus the generation of wastes and safety-related
sure. The remaining ionic liquid is further washed with problems may be avoided. The novel reaction/catalytic
hexane and is recycled in subsequent runs. All the proce- media described here may serve as promising replace-
dures for recovering the ionic liquids are operated in the ments of homogeneous and heterogeneous ZnCl2 in tox-
140
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Regioselective Diels–Alder Reaction of Myrcene with Acrolein
UPDATES
Table 4. Reuse of ionic liquids in Diels–Alder reactions of myrcene with acrolein.[a]
Entry
MX-ZnCl2
Run time
Yield [%]
Regioselectivity [p/m]
ꢀ
1
2
3
4
5
BmimCl ZnCl2
1
2
3
4
5
97.3
96.8
97.9
95.7
97.3
95: 5
95: 5
94: 6
94: 6
94: 6
ꢀ
6
7
8
9
10
EmimBr ZnCl2
1
2
3
4
5
97.4
97.7
96.8
96.3
97.1
94: 6
94: 6
94: 6
94: 6
94: 6
ꢀ
11
12
13
14
15
BPyBr ZnCl2
1
2
3
4
5
96.9
95.4
98.4
96.0
98.2
95: 5
95: 5
94: 6
94: 6
94: 6
ꢀ
16
17
18
19
20
EtPyBr ZnCl2
1
2
3
4
5
92.2
93.5
94.1
90.0
91.6
95: 5
95: 5
95: 5
95: 5
94: 6
[a]
The reaction was performed at 158C for 2 h and the molar ratio of ZnCl2 to BmimCl was 2: 1.
ic organic solvents and are expected to find wider appli- Synthesis of BmimCl; Typical Procedure
cations in related organic synthesis.
To N-methylimidazole (20 mL, 0.25 mmol) as solvent an equal
molar amount of n-chlorobutane was added into a round-bot-
tomed flask and then the mixture was refluxed at 808C for 48 h
with stirring. After cooling the reaction mixture to room tem-
perature, the top phase was decanted and the bottom phase was
washed with ethyl acetate (5ꢁ40 mL) to ensure that any un-
Experimental Section
reacted starting material was removed from the bottom phase.
The volatile materials were evaporated under high vacuum to
obtain BmimCl as a slightly yellow liquid.
Chemicals
AnhydrousZnCl2 was preparedbyusingthionylchloride (SOCl2)
to dehydrate ZnCl2 ·2 H2O,[3] the remaining SOCl2 was then re-
ꢀ
Zinc-Containing Ionic Liquids BmimCl ZnCl2,
EmimBr ZnCl2, BPyBr ZnCl2 and EtPyBr ZnCl2
moved by distillation under vacuum, the obtained anhydrous
ZnCl2 was kept in a vacuum desiccator with potassium hydroxide
overnight prior to use. Acrolein (from Aldrich), methyl acrylate,
methyl methacrylate and mesityl oxide (from Fluka) were fresh
distilled before use. Myrcene, pyridine, and 1-methylimidazole
supplied by ACROS were used as received. 1-Chlorobutane, bro-
moethane, n-hexane, dichloromethane, and chloroethane were
ꢀ
ꢀ
ꢀ
These were prepared by mixing MX (20 mmol) and appropri-
ate amounts of anhydrous ZnCl2 followed by heating at
1008C for 12 h under nitrogen with vigorous stirring. The color
of the resulted ionic liquids depended on the kind of MX used.
˜
purchased from ABCR (AssociaÅao Brasileira de Captadores
de Recursos) and freshly distilled before use.
General Procedures for Diels–Alder Reactions
In a typical reaction, myrcene (1.0 mmol) was added slowly to a
solution of zinc-containing ionic liquid (2 mL) and acrolein
(1.1 mmol) under vigorous stirring for an appropriate time. Af-
ter the reaction was completed, the upper clear liquid was pi-
petted off for gas chromatographic analysis, the remaining ion-
ic liquid phase was washed with hexane (3ꢁ3 mL), purified un-
der vacuum and directly reused in the subsequent runs. The re-
action products was analyzed by gas chromatography on an
Agilent Technologies 6890N (FID, HP-5 column, 30 mꢁ
General Procedures for the Synthesis of Ionic Liquids
1-Butyl-3-methylimidazolium chloride (BmimCl), 1-ethyl-3-
methylimidazolium bromide (EmimBr), N-1-butylpyridinium
bromide (BPyBr) and N-1-ethylpyridinium bromide (EtPyBr)
were synthesized following the procedures described in the lit-
erature.[9]
Adv. Synth. Catal. 2005, 347, 137–142
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141

Donghong Yin et al.
UPDATES
0.32 mmꢁ0.25 mm), nitrogen as a carrier gas, injector temper-
ature 2208C, detector temperature 2008C, oven temperature
1508C, using trans-naphthane as an internal standard. The re-
tention times were: acrolein 2.58 min, myrcene 3.35 min,
ꢀmetaꢁ-myrac aldehyde 12.61 min, and ꢀparaꢁ-myrac aldehyde
13.89 min, respectively. After distillation and purification of
the upper clear liquid under vacuum, the product ꢀparaꢁ-myrac
aldehyde (bp 136–1388C/500 Pa) was identified by GC-MS,
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c) I. Meracz, T. Oh, Tetrahedron Lett. 2003, 44, 6465–
6468.
1
FT-IR (Nicolet Nexus FT-IR Spectrometer), H NMR and
13C NMR (INOVA-400-Varian) spectra, the data of which
were in good agreement with those of aþn authentic samplþe;
MS: m/z¼192 [Mþ peak], 177 [192 CH3] , 149 [177 CHO] ,
ꢀ
ꢀ
þ
þ
ꢀ
¼
ꢀ
[122 CHO] ,þ 69
þ
122
[192 CH2CH CHO] ,
93
þ
¼
¼
ꢀ
ꢀ
[(CH3)2 CHCH2] , 41 [CH2 CH CH2] , 29 [ CHO] ; IR:
n¼2820, 2720, 1730, 1390,1370, 1205, 805 cmꢀ1
;
1H NMR
(400 MHz, CDCl3): d¼1.60, 1.68 [2 s, 6 H (CH3)2], 1.92–2.10
ꢀ
ꢀ ꢀ
ꢀ
ꢀ ꢀ
ꢀ
¼
ꢀ
(m, CH2 CH2 CH2
,
,
CH2 ), 2.23 (m, CH CH2), 2.46
ꢀ
ꢀ
¼
ꢀ
¼
ꢀ
(m, CH ), 5.08 [m, CH C(CH3)2], 5.43 (m, CH CH2),
9.69 (s, CHO); 13C NMR (100 MHz, CDCl3): dC ¼17.66
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
( CH3), 22.53 ( CH3), 24.48 ( CH2), 25.68 ( CH2), 26.30
ꢀ
ꢀ
ꢀ
ꢀ
( CH2), 26.86 ( CH2), 37.58 ( CH2), 46.07 ( CH), 118.34
¼
¼
¼
¼
( CH), 124.04 ( CH), 131.57 ( C), 138.01 ( C), 204.82
ꢀ
( CHO).
Acknowledgements
The project was sponsored by the Scientific Research Founda-
tion for the Returned Overseas Chinese Scholars, Ministry of
Education [2001-498]. The authors also thank the Natural Sci-
ence Foundation of Hunan Province for financial support to
this project (01JJY3008).
[7] a) A. P. Abbott, G. Capper, D. L. Davies, H. L. Munro,
R. K. Rasheed, V. Tambyrajah, Chem. Commun. 2001,
19, 2010–2011; b) A. P. Abbott, G. Capper, D. L. Davies,
R. K. Rasheed, V. Tambyrajah, Green Chem. 2002, 4,
24–26.
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