Barbier-type reaction mediated with tin nano-particles in water

Article abstract of DOI:10.1016/j.tet.2004.12.063

Tin nano-particles are employed in the Barbier-type allylation reaction of carbonyl compounds in water to afford the corresponding homoallylic alcohols in good yields. The in situ generated allylation intermediates, allyltin(II) bromide and diallyltin dib

Full text of DOI:10.1016/j.tet.2004.12.063

Tetrahedron 61 (2005) 2521–2527
Barbier-type reaction mediated with tin nano-particles in water
*,†
Zhenggen Zha, Shu Qiao, Jiaoyang Jiang, Yusong Wang, Qian Miao and Zhiyong Wang
*
Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, University of Science and Technology of China,
Hefei 230026, China
Received 30 August 2004; revised 20 December 2004; accepted 21 December 2004
Abstract—Tin nano-particles are employed in the Barbier-type allylation reaction of carbonyl compounds in water to afford the
corresponding homoallylic alcohols in good yields. The in situ generated allylation intermediates, allyltin(II) bromide and diallyltin
1
dibromide, have been directly observed by using H NMR. A mechanism is proposed based on this observation.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
different sizes (Scheme 1). The details of this study are
described below.
The classical Barbier-type reaction has not been used as
extensively as Grignard-type reaction even though the latter
involves an extra step to prepare the organometallic
1
reagents. This is because many side reactions can also be
mediated by the metal at the same condition. In order to
extend the application of Barbier-type reactions and fully
take its advantage, Barbier-type reactions in aqueous media
2
were developed in the recent years. The importance of this
type of reaction has been gradually recognized not only
because the tedious protection–deprotection processes can
be simplified for certain functional groups containing acidic
hydrogen atoms, but also because there is a growing public
Scheme 1.
2. Results and discussion
3
interest in Green Chemistry. Many metal mediators as well
4
as their salts have been used in the Barbier-type allylation
reactions to enhance the reaction yield and improve the
2.1. TEM and XRD of 20-nm and 100-nm tin particles
5
stereoselectivity. The dimension of metal particles should
In order to further study the mechanism of Barbier-type
allylation reaction mediated by metal nano-particles and
the effect of the size of nano-particles on this reaction,
tin nano-particles with different sizes were prepared.
Tin nano-particles with an average diameter of 20-nm
were prepared by g-radiation (method A), and tin nano-
particles with an average diameter of 100-nm were
prepared conveniently by reduction of SnCl with KBH
in water at the presence of cetyl trimethylaminoium
bromide (CTAB) (method B). Both of the two kinds of
tin nano-particles were characterized by transmission
electron microscopy (TEM) and X-ray powder diffraction
affect on metal-mediated allylation of carbonyl compounds
in aqueous media because the key intermediate is believed
2
b
to be generated on the metal surface. However, the metal
particles smaller than the regular powder, for example,
6
nanometer-scale particles, have been not been well studied
7
as a reagent in organic reactions. As suggested by our
,8
7
previous study, the yield of the Barbier-type allylation
2
4
reaction can be improved by applying nano-scale metal
particles. In order to further test this idea and explore the
mechanism of Barbier-type reactions in water, we recently
studied the allylation mediated by tin nano-particles with
(XRD). The average size of two kinds of nano-particles
Keywords: Tin; Nano-particles; Barbier-type reaction; Mechanism; Water.
*
is illustrated by the TEM images (Fig. 1). As shown in
Figure 2, both 20- and 100-nm tin particles are polycrystal-
line and their XRD patterns are consistent with that of
Corresponding authors. Tel.: C86 551 3603185; fax: C86 551 3631760
Z.W.); e-mail: zwang3@ustc.edu.cn
(
†
Present address: Department of Chemistry, Columbia University, New
York, NY 10027, USA.
9
metallic tin.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.063

2522
Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
the aldehydes (entries 1–11 in Table 2) and ketones (entries
2–14 in Table 4) can be allylated in this reaction.
1
Furthermore, both aromatic (entries 1–9 and 14 in
Table 2) and aliphatic (entries 10–13 in Table 2) carbonyl
compounds are reactive. The ketone with hydroxyl group
is successfully employed in this reaction without
protection (entry 13 in Table 2), yielding the corresponding
homoallylic alcohol. Interestingly, the allylation of
4
-nitrobenzaldehyde mediated by 20 nm tin particles
generates the corresponding homoallylic alcohol in an
excellent yield (entry 9 in Table 2). In comparison,
the reaction between 4-nitrobenzaldehyde and allyl
1
0
bromide usually only yields N-alkylation products when
mediated by metals in aqueous media. 20-nm particles
usually give rise to a higher yield of the homoallylic
alcohol than 100-nm tin particles do. But the difference is
small.
Figure 1. (a) A TEM image of 20-nm tin particles prepared with method A;
b) A TEM image of 100-nm tin particles prepared with method B.
(
Figure 2. (a) XRD pattern of 20-nm tin particles; (b) XRD pattern of 100-nm tin particles.
2.2. Allylation reaction mediated by tin nano-particles in
water
2.3. Regioselectivity and diastereoselectivity of the
allylation reaction mediated by tin nano-particles
As shown in Table 1, tin nano-particles were found to be
more effective than regular tin in mediating the allylation
Additionally the regioselectivity and diastereoselectivity
were studied for the nanometer-sized tin mediated allylation
in the reaction of benzaldehyde and substituted allyl
bromide (Scheme 2). As indicated in Table 3, the reaction
between benzaldehyde and ethyl 4-bromo-2-butenate
exclusively affords g-adduct no matter whether tin is
regular or nanometer sized (entries 1–3 in Table 4). The
syn product is more favored by smaller tin particles. When
7
a
reaction. When 100-nm tin particles were employed the
corresponding homoallylic alcohol was obtained in a yield
of 98% (entry 7 in Table 1). When 20-nm tin nano-particles
were used, the corresponding homoallylic alcohol was
yielded quantitatively (entry 6 in Table 1).
2
0-nm tin particles mediates the allylation reaction, the ratio
Table 1. Allylation of benzaldehyde mediated by various metals in water
of syn to anti homoallylic alcohol is as high as 94:6. The
regular tin mediated reaction between benzaldehyde and
crotyl bromide favors a-adduct (entry 4 in Table 4). On the
other hand, the reaction mediated by tin nano-particles
affords g-adduct with a high selectivity (entries 5 and 6 in
Table 3, 90% for 100 nm tin particles and almost 100% for
20 nm tin particles). Interestingly, when 20-nm diameter tin
aggregates into bigger particles, both a- and g-products are
yielded in a ratio of 33:52 (entry 7 in Table 3).
a
Entry
Metal
Size
Yield (%) /
Time (h)
b
1
2
3
4
5
6
7
Fe
Mg
Al
Zn
Sn
Sn
Sn
Regular
Regular
Regular
Regular
Regular
20-nm
Polymerization
—
—
20/24
93/15
95/1.5 (100/6.0)
90/1.5 (98/9.0)
100-nm
a
1
Determined by H NMR.
The diameter of regular tin is about 80 mm.
b
2.4. The detection of the intermediates and a proposed
mechanism
Subsequently, tin nano-particles with average diameters of
0- and 100-nm were employed in the allylations of various
aldehydes and ketones. As summarized in Table 2, this
reaction usually generates the corresponding homoallylic
alcohol in a high yield (mostly higher than 90%). Both of
Different mechanisms have been proposed for the aqueous
Barbier type reactions involving the intermediates of a
radical, a radical anion, and an allylmetal species.
Direct observation of an intermediate is obviously important
to establish a specific mechanism. However, as we know, no
2
2
b
11
12

Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
Table 2. Allylation reactions mediated by tin nano-particles in water
2523
a
Yield (%) /Time (h)
Entries
1
Substrates
Products
Diameter of nano-Sn
20-nm
95/1.5
90/1.5
1
00-nm
2
3
4
5
6
20-nm
00-nm
99/3.0
96/3.0
1
20-nm
00-nm
99/2.0
95/2.0
1
20-nm
00-nm
99/2.0
95/2.0
1
20-nm
00-nm
96/3.0
89/3.0
1
20-nm
00-nm
96/3.0
92/3.0
1
7
20-nm
00-nm
99/3.0
96/3.0
1
8
9
20-nm
00-nm
95/3.0
81/3.0
1
20-nm
00-nm
95/3.0
86/3.0
1
10
20-nm
00-nm
90/1.5
90/1.5
1
1
1
1
1
2
3
n-C6H13CHO
20-nm
00-nm
99/1.5
99/1.5
1
20-nm
00-nm
85/1.5
81/1.5
1
20-nm
00-nm
81/3.0
79/3.0
1
14
20-nm
00-nm
65/4.0
59/4.0
1
a
1
The yield was determined by H NMR.
Scheme 2.

2524
Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
Table 3. Regioselectivity and diastereoselectivity of the allylation reaction mediated by tin nano-particles in water
a
b
Entry
R
1
R
2
R
Av size of Sn
Time (h)
Yield (%)
a(Z/E)
g(anti/syn)
1
2
3
4
5
6
7
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
H
H
COOCH
COOCH
COOCH
2
2
2
CH
CH
CH
3
3
3
Regular
100-nm
20-nm
Regular
100-nm
20-nm
24
12
12
24
12
12
24
58
55
61
76
70
80
85
—
—
—
54(53:47)
7(60:40)
Trace
33(74:26)
(26:74)
(19:81)
(6:94)
22(37:63)
63(49:51)
(44:56)
CH
CH
CH
CH
3
3
3
3
c
20-nm
52(33:67)
a
Isolated yield.
The ratio of syn isomer to anti isomer (E isomer to Z isomer) was determined by H NMR, C NMR and isolation.
The reaction was mediated by aggregated tin nano-particle.
b
c
1
13
12d
solution. In comparison, when regular tin was stirred with
Table 4. Allylation of carbonyl compounds by tin nano-particles in
different solvent
allyl bromide in D O at room temperature, neither 4 nor 5
2
1
was observed in H NMR spectrum. The absence of allyltin
intermediates suggests that the mechanism of allylation
reaction mediated by regular tin at neutral condition and
room temperature involves not an allyltin intermediate but a
Entry
Solvent
Metal
Yield (%)
1
2
3
4
5
6
7
8
9
—
—
—
Regular
20-nm
100-nm
20-nm
100-nm
20-nm
100-nm
20-nm
100-nm
0
0
0
0
0
0
0
95
90
2
b
radical or a radical anion. In fact, the allylation reaction
mediated by regular tin requires heat (or ultrasonic
irradiation) or the use of catalyst (HBr or Al) to promote
CH
CH
3
OH
OH
3
Ethyl ether
Ethyl ether
12d,13
the formation of covalent organometallic intermediates.
H
H
2
O
O
Compared with regular tin, nanometer sized tin particles
have a higher surface energy, which favors the formation of
allyltin intermediates.
2
1
Determined by H NMR.
It is noted that water plays an important role in the allylation
reaction. The reaction occurs only when water is used as
solvent or as a catalyst regardless of the presence of organic
solvent (entries 8 and 9 in Table 4). In anhydrous methanol
in situ allylation intermediate was reported to be observed in
distilled water at room temperature in the allylation reaction
mediated by metal tin. The following experiment was
designed to search for the intermediate. 100-nm tin particles
1
4
or diethyl ether, or in a solvent-free condition, no
allylation product is yielded (entries 1–7 in Table 4).
However, it is still not clear how water molecules
participate in the reaction.
(
178 mg, 1.5 mmol) and allyl bromide (0.18 mL, 2 mmol)
were mixed in water (2–4 mL) at room temperature. After
stirring for 15 min, the black mixture changed to milk-
white. Then benzaldehyde was added to this mixture and the
corresponding homoallylic alcohol was obtained in a good
yield. This result implies that the milk-white substance
should be the intermediate of the allylation. To identify the
structure of the white substance, allyl bromide and the tin
As shown in Figure 3B, when allyl bromide is replaced with
crotyl bromide, two peaks are observed at 2.16 and
1
.46 ppm, respectively in the H NMR spectrum. The two
2
peaks split into asymmetric multiplets rather than doublets,
nano-particles (20 or 100 nm) were stirred in D O at room
2
1
which are observed in the H NMR spectrum of the reaction
1
temperature and then monitored by using H NMR. In the
1
mixture of allyl bromide and tin nano-particles. This
indicates that a covalent bond is formed between tin atom
and g-C when crotyltin (II) bromide and dicrotyltin
H NMR spectrum two doublets at 2.16 and 2.48 ppm were
observed (Fig. 3A, a), which are assigned to be the signals
due to allyltin (II) bromide (4) and diallyltin dibromide (5),
1
2
15
respectively. The interconversion between allyltin (II)
bromide (4) and diallyltin dibromide (5) was further studied
by using tin nano-particles of different sizes. When a
mixture of 1 mmol of allyl bromide and 0.5 mmol of 20-nm
tin particles was stirred in D O at room temperature the
2
ratios of 4 to 5 were 42/58 (0.5 h), 28/72 (1 h), 16/84 (1.5 h)
and 8/92 (24 h), respectively (Fig. 3A, b–e). The ratio of 4 to
dibromide are generated.
As shown in Scheme 3, the formation of a g-C–Sn bond can
be explained by assuming that a p complex (I) is formed as
a transition state. In this mechanism, allyl bromide binds the
surface of tin nano-particles and generates the p-complex
(I) first. Then the p electrons delocalize toward the g-C and
give rise to the formation of g-C–Sn bond. Quantum
chemistry calculation also indicates that the p complex
5
decreased due to the transformation of 4 to 5. When
mmol of allyl bromide and 0.5 mmol of 100-nm tin were
1
stirred in D O at room temperature, two intermediates, 4
1
6
transition state (I) is favored in energy. Intermediate 4 can
be further converted to 5 in the presence allylic bromide,
and intermediate 5 can be converted to 4 in the presence of
excessive tin. Both intermediate 4 and 5 can react with a
carbonyl compound to give the corresponding homoallylic
alcohol.
2
and 5, were generated immediately as indicated by the
appearance of two doublets at 2.16 ppm and at 2.48 ppm in
1
the H NMR spectrum. The ratio of 4 to 5 were 1/99
(10 min) (Fig. 3A, f) and 0/100 (25 min) (Fig. 3A, g). After
addition of another 1 mmol of 100-nm tin particles, the
ratios of 4 to 5 were 55/45 (35 min), 36/64 (60 min) and
0
/100 (15 h), respectively, as shown in Figure 3A, h–j. This
indicates 5 can be converted to 4 by the addition of tin metal
and also suggests that 5 is more stable in the aqueous
Most of experimental results can be explained based on this
mechanism. For example, the allylation of 4-nitrobenzalde-
hyde yields reduction or polymerization products instead of

Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
2525
crotyltin (II) bromide intermediate (4 in Scheme 3), which
should be more stable than the a-C bonded crotyltin
bromide because the g-C is a secondary carbon. On the
other hand, a-addition product is the major in the allylation
reaction mediated by regular tin. This suggests again that
the mechanism of allylation reaction mediated by regular tin
at neutral condition and room temperature involves an
1
7
intermediate rather than allyltin. However, the allylation
of benzaldehyde with ethyl-4-bromo-2-butenate only yields
the g-addition products no matter whether regular tin or
nanometer-sized tin is used as a mediator (entries 1–3 in
Table 3). This implies that g-C bonded allyltin inter-
mediates are formed when allyl group is conjugated with an
electron-withdrawing group (such as an ester) regardless of
the dimension of metallic tin (regular powder or nano-
particles). The exclusive formation of g-C bonded allyltin
intermediates is due to the electron-withdrawing character
of ester group, which can stabilize the high electron density
1
8
on the Sn-bonded carbon.
3. Conclusion
In conclusion, the allylation reaction mediated with tin
nano-particles of different size in water has been system-
atically investigated. More importantly, the in situ generated
allylation intermediates (4 and 5 in Scheme 3) have been
1
directly observed using H NMR. A mechanism involving
the allyltin intermediates is proposed. Further research is in
progress in our laboratory to control the regioselectivity and
stereoselectivity in allylation reactions by adjusting the
dimension of the metal nano-particles.
4. Experimental
Analytical thin-layer chromatography (TLC) plates were
commercially available. Solvents were reagent grade unless
otherwise noted. Tin powder (150 mesh, 99.99%, 100 g
packing) was freshly opened for use. Carbonyl compounds
were further purified by redistillation or recrystallization
from commercial chemicals when necessary.
4.1. General method for preparation of 20-nm tin
particles
1
Figure 3. (A). Partial H NMR spectra for the mixture of allyl bromide and
nano-tin stirred in D O solution for different periods of time. (a–e) for
0-nm: (a) 1.5 h; (b) 0.5 h; (c) 1 h; (d) 1.5 h; (e) 24 h. (f–j) for 100-nm:
Method A. To 1L of a solution of 2-propanol in water
(2.0 mol/L) were added 2.25 g (0.01 mol) of SnCl $2H O
and 20 g (0.5 mol) of NaOH. The reaction mixture was
2
2
2
2
1
(
f) 15 min; (g) 25 min; (h) 35 min; (i) 60 min; (j) 15 h). (B) Partial H NMR
spectra of for the mixture of crotyl bromide and tin nano-particles stirred in
O solution for different periods of time. (a) 15 min; (b) 45 min; (c) 1.5 h;
d) 6 h.
stirred until SnCl and NaOH were dissolved. The solution
2
D
2
was bubbled with nitrogen over 20 min and then irradiated
(
6
0
4
with a g-ray ( Co) source for about 12 h (2.5!10 Gy).
The 20-nm tin particles were obtained from the solution by
centrifugation. The tin nano-particles were washed with
water and alcohol, respectively, and dried under vacuum.
1
0
homoallylic alcohol when mediated with regular tin since
the formation of allyltin intermediates is too slow to
compete with the side reactions. When mediated with tin
nano-particles, the allyltin intermediates are generated
faster. Therefore the corresponding homoallylic alcohol is
obtained in a high yield.
4.2. General method for preparation of 100-nm tin
particles
Method B. A solution of potassium borohydride (0.80,
15 mmol) in water (20 mL) was slowly added to a stirred
solution of SnCl $2H O (2.25 g, 10 mmol) and cetyl-
The selective formation of g-addition product in the
allylation reaction mediated with tin nano-particles (entries
2
2
5
and 6 in Table 3) can be explained by a g-C bonded
trimethylaminoium bromide (0.36 g, 2 mmol) in 100 mL

2526
Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
Scheme 3. A proposed mechanism for the allylation reaction mediated by tin nano-particles.
of water. The mixture was stirred at room temperature for
5 min and in-situ tin nano-particles were yielded. The
mixture of in-situ tin nano-particles was centrifuged to give
00-nm tin particles. The tin nano-particles were washed
with water and alcohol, respectively, and dried under
vacuum.
128.2, 127.9, 126.5, 120.5, 74.0, 60.9, 58.4, 14.0. HRMS
(EI) m/z calcd for C H O : 220.1099. Found: 220.1052.
1
1
3 16 3
1
4.5.2. anti-Ethyl-2-[hydroxy(phenyl)methyl]but-3-eno-
ate (entry 1, Table 3). IR(NaCl, cm ): 3450, 1731,
K1
1
635, 1304, 1176, 1035, 760, 700; H NMR (CDCl3,
00 MHz, ppm) d: 7.17–7.26 (m, 5H), 5.51–5.67 (m, 1H),
1
4
4
.3. Details of in situ NMR experiments
4.90–5.02 (m, 2H), 4.84 (d, JZ8.4 Hz, 1H), 4.10 (q, JZ
.2 Hz, 2H), 3.31–3.38 (m, 1H), 2.35–2.2.68 (br, 1H), 1.16
7
1
3
In a typical procedure, tin nano-particles (178 mg,
.5 mmol) and allyl bromide (0.14 mL, 1.5 mmol) were
added into 2 mL of D O at room temperature. After the
(t, JZ7.2 Hz, 3H). C NMR (CDCl , 100 MHz) d: 173.0,
3
1
141.2, 132.2, 128.4, 128.0, 126.7 119.5, 75.3, 61.1, 57.9,
14.1. HRMS (EI) m/z calcd for C H O : 202.0994
2
1
3 16 3
mixture was stirred for 10 min, the solution turned milky
white. The milky white solution was briefly evacuated,
purged with nitrogen, and transferred via cannula to a NMR
tube.
(
MK18). Found: 202.0996 (MK18).
4.5.3. Phenyl-3-penten-1-ol (mixture of Z and E) (entry 4,
Table 3). IR (film, cm ): 3432, 3083, 1644. H NMR
K1
1
(
(
(
CDCl , 400 MHz, ppm) d: 7.29–7.39 (m, 5H), 5.53–5.72
3
4.4. General method for allylation of carbonyl
compounds in aqueous media
m, 1H), 5.40–5.50 (m, 1H), 4.65–4.72 (m, 1H), 2.42–2.65
m, 3H), 1.61–1.73 (m, 3H). C NMR (CDCl , 100 MHz,
1
3
3
ppm) d 144.2, 144.1, 129.2, 128.4, 127.5, 127.4, 126.9,
26.0, 125.9, 125.8, 73.9, 73.6, 42.7, 36.9, 18.1, 13.0.
HRMS (EI) m/z calcd for C H O: 162.1045. Found:
To a mixture of carbonyl compound (1 mmol) in water
(
0
1
4 mL) was added tin nano-particles (0.118 g 1 mmol–
.177 g 1.5 mmol) and allyl bromide (0.14 mL 1.5 mmol) at
1
1 14
1
62.1049.
room temperature. The mixture was stirred for 0.5–4 h and
quenched with 1 N HCl (1 mL) solution. The mixture was
extracted with ether (3!10 mL), and the combined organic
4.5.4. 2-Methyl-1-phenyl-3-buten-1-ol (mixture of syn
and anti) (entry 4, Table 3). IR (film, cm ): 3417, 3080,
K1
layer was washed with saturated aqueous NaHCO solution
3
1
1
640. H NMR (CDCl , 400 MHz, ppm) d: 7.27–7.39
3
and dried over magnesium sulfate. The organic solvent was
evaporated, and the corresponding homoallylic alcohol was
yielded. The product was usually pure enough without
(
(
m, 5H), 5.64–5.88 (m, 1H), 5.13–5.23 (m, 2!0.37H)
anti), 4.99–5.09 (m, 2!0.63H) (syn), 4.57 (d, JZ5.84 Hz,
1
1
2
3
!0.63H) (syn), 4.32 (d JZ7.64 Hz, 1!0.37H) (anti),
.35–2.62 (m, 1H), 1.99–2.19 (br, 1H), 1.02 (q, JZ6.87 Hz,
further purification according to the H NMR spectrum, and
was further purified by flash chromatography on silica gel
only when necessary.
1
3
!0.63H) (syn), 0.90 (d, JZ6.83 Hz, 3!0.37H) (anti).
C
NMR (CDCl , 100 MHz, ppm) d: 14.1 (syn), 16.5 (anti),
3
4
1
4.6 (syn), 46.2 (anti), 77.1 (syn), 77.4 (anti), 115.5 (syn),
16.8 (anti), 126.5 (syn), 126.9 (anti), 127.4 (syn), 127.7
4
.5. Spectroscopic data
(
1
anti), 128.1 (syn), 128.3 (anti), 140.3 (syn), 140.6 (anti),
42.4 (anti), 142.6 (syn). HRMS (EI) m/z calcd for
C H O: 162.1045. Found: 162.1042.
1
IR (Perkin–Elmer, 2000FTIR), H NMR (CD Cl, 500 or
400 MHz), C NMR (CDCl , 125.7 or 100 MHz) and MS–
3
GC (HP 5890(II)/HP5972, EI.
3
1
3
1
1 14
4
(
.5.1. syn-Ethyl-2-[hydroxy(phenyl)methyl]but-3-enoate
entry 1, Table 3). IR(NaCl, cm ): 3482, 1728, 1638,
318, 1176, 1027, 765, 701. H NMR (CDCl , 400 MHz,
3
K1
Acknowledgements
1
1
ppm) d: 7.17–7.33 (m, 5H), 5.75–6.90 (m, 1H), 5.06–5.25
The authors are grateful to the National Natural Science
Foundation of China (No. 50073021 and 20472078) and
Science Foundation of Anhui Province ((No. 01046301) for
the support.
(
m, 2H), 4.91 (d, JZ6.4 Hz, 1H), 3.95 (q, JZ7.0 Hz, 2H),
.22–3.41 (m, 1H), 3.10 (s, 1H), 1.02 (t, JZ7.0 Hz, 3H). C
1
3
3
NMR (CDCl , 100 MHz, ppm) d: 172.5, 140.7, 132.0,
3

Z. Zha et al. / Tetrahedron 61 (2005) 2521–2527
2527
References and notes
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1
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H
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4
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