Aqueous Wittig reactions of aldehydes with in situ formed semistabilized phosphorus ylides

Article abstract of DOI:10.1080/00397910500213054

The Wittig reactions of three semistabilized phosphorus ylides generated in situ from the corresponding phosphonium salts with aldehydes in water without any organic cosolvent were investigated. Most of the olefination reactions completed within between 5 min and 4.5 h in refluxing water containing 1.5 equiv of LiOH and 1.4M LiCl to afford the products in 65-100% yields. The E:Z selectivity depended not only on the substituent attached to the benzene ring of the ylides but also on the substituent bound to the aromatic aldehydes. LiCl promotes the aqueous Wittig reactions and suppresses decomposition of the ylide or the corresponding phosphonium salt. Copyright Taylor & Francis, Inc.

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Aqueous Wittig Reactions of
Aldehydes with In Situ Formed
Semistabilized Phosphorus
Ylides
Jinlong Wu a , Dan Li a & Da Zhang a
a Laboratory of Asymmetric Catalysis and Synthesis,
Department of Chemistry, Zhejiang University,
Hangzhou, China
Version of record first published: 18 Aug 2006.
To cite this article: Jinlong Wu, Dan Li & Da Zhang (2005): Aqueous Wittig Reactions
of Aldehydes with In Situ Formed Semistabilized Phosphorus Ylides, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 35:19, 2543-2551
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Synthetic Communicationsw, 35: 2543–2551, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500213054
Aqueous Wittig Reactions of Aldehydes with
In Situ Formed Semistabilized Phosphorus
Ylides
Jinlong Wu, Dan Li, and Da Zhang
Laboratory of Asymmetric Catalysis and Synthesis, Department
of Chemistry, Zhejiang University, Hangzhou, China
Abstract: The Wittig reactions of three semistabilized phosphorus ylides generated
in situ from the corresponding phosphonium salts with aldehydes in water without
any organic cosolvent were investigated. Most of the olefination reactions completed
within between 5 min and 4.5 h in refluxing water containing 1.5 equiv of LiOH
and 1.4 M LiCl to afford the products in 65–100% yields. The E : Z selectivity
depended not only on the substituent attached to the benzene ring of the ylides but
also on the substituent bound to the aromatic aldehydes. LiCl promotes the aqueous
Wittig reactions and suppresses decomposition of the ylide or the corresponding
phosphonium salt.
Keywords: Aldehydes, aqueous reaction, phosphorus ylide, Wittig
Aqueous organic reactions have attracted increasing attention in recent years in
the search of environmentally benign chemical processes.[1] Some
reactions involving organometallic species, which had previously been
thought unlikely to proceed in aqueous media, have been now successfully
performed in water. The Wittig reaction is one of the most versatile
synthetic methods for preparation of olefins from carbonyl compounds, and
it is classically carried out using a hydride or organometallic base in
Received in Japan March 22, 2005
Address correspondence to Jinlong Wu, Laboratory of Asymmetric Catalysis and
Synthesis, Department of Chemistry, Zhejiang University, Hangzhou 310027, China.
E-mail: wyz@zju.edu.cn
2543

2544
J. Wu, D. Li, and D. Zhang
anhydrous aprotic solvents under an inert atmosphere.[2] The reactions of
semistabilized phosphorus ylides with water-soluble formaldehyde have
been investigated in water in the absence of organic solvents.[3] However, it
was found that other aldehydes did not successfully react with semistabilized
phosphorus ylides to give the products because of hydrolysis of the ylides.
Industrial synthesis of vitamin A acetate is a successful example of an
aqueous Wittig reaction involving a semistabilized phosphorus ylide.[4] The
Hornerr–Wadsworth–Emmons (HWE) reactions of triethyl phosphono-
acetate and diethyl 2-oxoalkanephosphonates with aldehydes were reported
to take place in highly concentrated (6–10 M) aqueous solutions of
potassium carbonate or potassium hydrogen carbonate.[5] However, decompo-
sition of phosphonium salts and phosphorus ylides caused by aqueous bases
was reported.[3b,6] For this reason, the Wittig reactions have been carried
out under heterogeneous conditions with inorganic bases, including liquid–
liquid,[7] solid–liquid,[8] and solid–solid phases.[9]
Recently, Russel and Warren[10a,b] took a different approach and reported
the Wittig reactions of water-soluble semistabilized phosphorous ylides with
aldehydes. In this approach the presence of an oxido or –COO2 in the phos-
phonium salts may inhibit hydrolysis in basic solution, but the scope of the
aldehde substrates was quite limited. Sieber et al.[10c] reported a water-
soluble PEG-supported phosphonium salt used for aqueous Wittig reactions
via the in situ formed semistabilized phosphorous ylide in the presence of
1 M NaOH. Various 4-substituted benzaldehydes were reacted to form the
diaryl alkenes in 38–95% yields and in ca. 1 : 1–3 : 1 E : Z ratios, providing
that the rate of reaction of the ylide with the aldehydes is much faster than
its decomposition. In these two cases, preparation of the water-soluble or
PEG-supported phosphonium salts was required. In connection with our
efforts in developing the Wittig reactions under unconventional conditions
by using microwave irradiation[11] and the asymmetric variations,[12] we
have developed aqueous Wittig reactions of unmodified stabilized phosphorus
ylides with aldehydes in the presence of LiCl.[13,14] We report here a simple
procedure for carrying out heterogeneous aqueous Wittig reactions by using
substituted benzyltriphenylphosphonium halides with aldehydes without
using any organic cosolvent. The acceleration effect of LiCl is also examined.
Reactions of some stabilized phosphorus ylides (for example, triphenyl-
phosphoranylideneacetonitrile) with aldehydes take place at room temperature
within several minutes in water in the presence of LiCl.[13] Because semista-
bilized phosphorus ylides have higher reactivity than stabilized ylides, it is
expected that semistabilized ylides can react with benzaldehyde (2c) under
the similar reaction conditions. However, the reaction was found to be not
as fast as expected when benzyltriphenylphosphorane, formed in situ from
benzyltriphenylphosphonium bromide (1b) and NaOH, reacted with 4-nitro-
benzaldehyde (2a) in water in the presence of LiCl at room temperature
(Table 1, entry 1). 4-Nitrostilbene 3 (R ¼ NO2) was obtained in 65% yield
with an E : Z ratio of 78 : 22. A similar yield was obtained for the reaction

Aqueous Wittig Reactions
2545
Table 1. Effect of reaction tempratures on olefination of 1b with aldehydes 2a
Temperature
(8C)
Time
(h)
3: Yieldc
(%); E : Zd
Entry
2:b R
1
2
3
4
5
6
2a: NO2
2a
rt
2.5
1.5
2
65; 78 : 22
75; 61 : 39
65; 46 : 54
84; 45 : 55
34; 37 : 63
58; 48 : 52
Reflux
rt
2c: H
2c
Reflux
rt
Reflux
0.25
19
2d: OMe
2d
3
aCarried out in H2O with 1.2 : 1 ratio of 1 and 2 in the presences of 1.5 equiv. of
NaOH and 1.4 M LiCl.
bMp: 105–1088C for 2a. Bp: 178–1798C for 2c, 2488C for 2d.
cIsolated yield of both isomers.
1
dDetermined by H NMR.
of 1b with benzaldehyde (2c), but a nearly 1 : 1 ratio was observed for the
olefin product (Table 1, entry 3). Moreover, a much slower reaction was
found for a less reactive aldehyde, 4-methoxybenzaldehyde (2d); after
reacting for 19 h at room temperature, only 34% yield of the product 3
(R ¼ OMe) was obtained with a reversed E : Z ratio of 37 : 63 (Table 1,
entry 5). To enhance the reactivity of the semistabilized ylide, the olefination
was performed in refluxing water and a significant temperature effect was
observed (Table 1, entries 2, 4, and 6). All three aldehydes afforded syntheti-
cally useful chemical yields of 58–84% within 3 h with a slightly diminished
diastereoselectivity as compared to that of the room temperature reactions. In
particular, the reaction time of 4-methoxybenzaldehyde (2d) shortened from
19 h to 3 h in refluxing water (Table 1, entry 5 vs. entry 6). Conversely, the
reactive aldehyde, 4-nitrobenzaldehyde (2a), showed a little temperature
effect, presumably because of the phase difference in the heterogeneous
reactions. The preliminary results given in Table 1 clearly demonstrate that
it is feasible to use conventional reagents such as the phosphonium salt 1b
and aldehydes 2 for aqueous Wittig reactions in the presence of LiCl.[13,14]
It is a remarkable extension of the early work that failed with aqueous
reactions of semistabilized phosphorus ylides with non-water-soluble
aldehydes.[3] The increased hydrophobic effect of LiCl[14] may significantly
promote the olefination while suppressing the decomposition of the phos-
phonium salt or the corresponding ylide in water.
Next, we investigated the scope of the aqueous Wittig reactions by using
three semistabilized phosphorus ylides formed in situ from the corresponding

2546
J. Wu, D. Li, and D. Zhang
phosphonium salts 1a–c with a collection of four representative aromatic
aldehydes 2a–d (Table 2). The selected ylides and aromatic aldehydes
cover the substrates with electronic-withdrawing or electronic-donating
groups. All reactions were performed in refluxing water in the presence of
LiCl (1.4 M) and NaOH (1.5 equiv.) (method A) or LiOH (1.5 equiv.)
Table 2. Aqueous Wittig reactions of phosphonium salts 1 with aldehydes 2 in the
presence of LiCl and NaOH or LiOHa
Time
(h)
3: Yieldd
(%); E : Ze
Entry
1:b R1, X
2:c R2
Base
1
2
1a: NO2, Br
1a
2a: NO2
2a
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
NaOH
LiOH
5
60; 20 : 80
74; 7 : 93
4.5
5.5
1.5
6
3
1a
1a
2b: Cl
2b
70; 89 : 11
65; 89 : 11
86; 100 : 0
79; 95 : 5
4
5
1a
1a
2c: H
2c
6
3.5
9
7
1a
1a
2d: OMe
2d
2a
54; 100 : 0
70; 75 : 25
75; 61 : 39
79; 77 : 23
79; 50 : 50
98; 47 : 53
84; 45 : 55
85; 47 : 53
58; 48 : 52
84; 46 : 54
100; 58 : 42
87; 62 : 38
63; 17 : 83
98; 11 : 89
81; 46 : 54
93; 40 : 60
46; 21 : 79
100; 16 : 84
8
2.5
1.5
0.92
0.58
0.33
0.25
0.22
3
9
1b: H, Br
1b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2a
2b
1b
1b
2b
2c
1b
1b
2c
2d
1b
1b
2d
2a
0.5
0.5
0.5
1
1c: OMe, Cl
1c
1c
1c
1c
1c
1c
1c
2a
2b
2b
2c
0.08
1.5
0.08
2 f
2c
2d
2d
2
aCarried out in refluxing H2O with a 1.2 : 1 ratio of 1 and 2 in the presence of LiCl
(1.4 M) and 1.5 equiv. of NaOH (method A) or LiOH (method B).
bMp: 2758C (dec.) for 1a, 295–2988C for 1b, and 245–2478C for 1c.
cMp: 105–1088C for 2a and 47–508C for 2b. Bp: 178–1798C for 2c and 2488C for 2d.
dIsolated yield of both isomers.
1
eDetermined by H NMR.
fReaction was not completed.

Aqueous Wittig Reactions
2547
(method B) as the base. We were delighted to find that all reactions in Table 2
proceeded smoothly to afford the desired olefin products in good to excellent
yields. In general, the reactions with LiOH as the base were faster than those
with NaOH; the difference in reaction time was especially much more
profound for the less reactive aldehyde 2d (Table 2, entries 7, 15, and 23 vs.
entries 8, 16, and 24). The influence of counterion on the reaction time
might be accounted for by the different hydrophobic effect of cations on
aqueous reactions.[14] Therefore, the chemical yields were generally higher
when LiOH was used as the base. Moreover, LiOH did not significantly
alter diastereoselectivity of the aqueous Wittig reactions except for one case
where a decreased E : Z ratio was noted (Table 2, entry 7 vs. entry 8). We
also examined the aqueous reaction of 4-nitrobenzyltriphenylphosphonium
bromide (1a) with unprotected 4-hydroxybenzaldehyde in the presence of
LiCl and NaOH under refluxing conditions. The desired product, (E)-4-
hydroxy-40-nitrostilbene, was detected but was isolated in only l4% yield
along with 4-nitrotoluene (90%), arising from decomposition of the phos-
phonium salt. It is reasonable to assume that under the basic conditions
4-hydroxybenzaldehyde was converted into the corresponding phenoxide,
which should have much lower reactivity toward the ylide, resulting in sub-
stantial decomposition of the ylide or its phosphonium salt. These results
confirm that the olefination and decomposition are the two competing
pathways in water for semistabilized phosphorus ylides.
It is interesting to examine the E and Z diastereoselectivity in relation with
the reactivity of both ylides and aldehydes. In the case of the ylide generated
from 1a, which possesses a strong electron-withdrawing NO2 group, its
reaction with the reactive 4-nitrobenzaldehyde (2a) afforded a greater pro-
portion of the Z alkene,[15] whereas nearly complete E selectivity was
observed for the reactions of aldehydes 2b–d (Table 2, entries 1–8). The coun-
terion of the base LiOH enhanced the Z selectivity for 4-nitrobenzaldehyde
(2a) but reduced the E preference for benzaldehyde (2c) and 4-methoxybenzal-
dehyde (2d). A reversed selectivity was found for the reactions of 2a with the
ylides formed from 1b and 1c; in these cases, E-olefins were formed as the
major isomer although to a much smaller extent (Table 2, entries 9, 10, 17,
and 18). For the reactions of 1b with aldehydes 2b–d, about 50 : 50
mixtures of the products were recorded. However, for the reactions of lc
with aldehydes 2b–d, the Z-olefins were generally formed as the dominant
isomers although the selectivity was not great for the reaction of benzaldehyde
(2c) with 1c (Table 2, entries 19–24). Therefore, it can be concluded that
aromatic aldehydes, except for the very reactive 4-nitrobenzaldehyde (2a),
prefer to form E-olefins with the semistabilized phosphorus ylide possessing
a strong electron-withdrawing NO2 group and Z-olefins with the semistabilized
phosphorus ylide possessing a strong electron-donating OMe group.
In summary, we have established a general protocol for aqueous Wittig
reactions of semistabilized phosphorus ylides generated in situ from the cor-
responding phosphonium salts with aromatic aldehydes in the presence of

2548
J. Wu, D. Li, and D. Zhang
LiOH (1.5 equiv) and LiCl (1.4 M) at refluxing temperature. The hetero-
geneous olefination reactions of representative aromatic aldehydes complete
within between 5 min and 4.5 h to furnish the products in 65–100% isolated
yields. The E : Z selectivity depends not only on the substituent attached to
the phenyl ring of the semistabilized phosphorus ylides but also on the substi-
tuent bound to the aromatic aldehydes. As the consequence of the hydrophobic
effect of LiCl, the semistabilized phosphorus ylides or the phosphonium salts
do not suffer from decomposition in water. Therefore, conventional reagents
can be used without extra structural modification in simple and high-
yielding olefination reactions.
EXPERIMENTAL
All reagents were obtained from commercial sources and used as received. All
reactions were carried out in open air in a round-bottom flask charged with a
water-cooling condenser and were monitored by TLC. The olefin products are
known compounds and were characterized by 1H NMR on a 500 MHz instru-
ment with comparison to the reported data.
Representative, Procedure for Aqueous Wittig Reaction
A mixture of the phosphonium salt 1 (0.4mmol) and the aldehyde 2 (0.35 mmol)
.
in H2O (10 mL) containing 1.5 equiv of NaOH or LiOH and LiCl H2O (846mg,
14.0mmol) was refluxed in air for the indicated time. After cooling to room
temperature, the reaction mixture was extracted with EtOAc (10 mL ꢀ 3) and
the combined organic layer was washed with brine, dried over Na2SO4,
filtered, and condensed under reduced pressure. The residue was then purified
by column chromatography (silica gel, eluting with 10–15% EtOAc in
hexane) to give the olefin product 3. The results are found in Table 2.
(E)- and (Z)-4,40-Dinitrostilbene (3, Rl 5 R2 5 NO2): As a 20 : 80 mixture
E : Z isomers. (E)-4,40-Dinitrostilbene : 1H NMR (500 MHz, CDCl3)[16,17]
d 8.27 (d, J ¼ 8.7 Hz, 4H), 7.69 (d, J ¼ 8.7 Hz, 4H), 7.30 (s, 2H). (Z)-4,40-
Dinitrostilbene: 1H NMR (500 MHz, CDCl3)[16,17] d 8.12 (d, J ¼ 8.7 Hz,
4H), 7.35 (d, J ¼ 8.7 Hz, 4H), 6.84 (s, 2H).
(E)-4-Chloro-40-nitrostilbene (3, R1 5 NO2, R2 5 Cl): As an 89 : 11
mixture of E : Z isomers. 1H NMR (500 MHz, CDCl3)[17] d 8.23
(d, J ¼ 8.7 Hz, 2H), 7.63 (d, J ¼ 8.7 Hz, 2H), 7.48 (d, J ¼ 8.4 Hz, 2H), 7.37
(d, J ¼ 8.4 Hz, 2H), 7.22 (d, J ¼ 16.0 Hz, 1H), 7.12 (d, J ¼ 16.0 Hz, 1H).
(E)- and (Z)-4-Nitrostilbene (3, Rl 5 H, R2 5 NO2 or Rl 5 NO2, R2 5 H):
As a 61 : 39 mixture of E : Z isomers. (E)-4-Nitrostilbene: 1H NMR (500 MHz,
CDCl3)[5a–c] d 8.24 (d, J ¼ 8.7 Hz, 2H), 7.65 (d, J ¼ 8.7 Hz, 2H), 7.57

Aqueous Wittig Reactions
2549
(d, J ¼ 7.5 Hz, 2H), 7.38–7.41 (m, 2H), 7.32–7.35 (m, 1H), 7.28
(d, J ¼ 16.3 Hz, 1H), 7.15 (d, J ¼ 16.3 Hz, 1H). (Z)-4-Nitrostilbene:
1H NMR (500 MHz, CDCl3)[5a–c] d 8.07 (d, J ¼ 7.5 Hz, 2H), 7.37
(d, J ¼ 7.5 Hz, 2 H), 7.25–7.26 (m, 3H), 7.19–7.21 (m, 2H), 6.82
(d, J ¼ 12.0 Hz, 1H), 6.61 (d, J ¼ 12.0 Hz, 1H).
(E)- and (Z)-4-Methoxy-40-nitrostilbene (3, R1 5 NO2, R2 5 OMe or
R1 5 OMe, R2 5 NO2): (E)-4-Methoxy-40-nitrostiblene: 1H NMR (500MHz,
CDCl3)[15] d 8.20 (d, J ¼ 8.6 Hz, 2H), 7.60 (d, J ¼ 8.6 Hz, 2H), 7.50
(d, J ¼ 8.8 Hz, 2H), 7.21 (d, J ¼ 14.8Hz, 1H), 7.02 (d, J ¼ 14.8Hz, 1H), 6.94
(d, J ¼ 8.8 Hz, 2H), 3.86 (s, 3H). (Z)-4-Methoxy-40-nitrostilbene: As a 58: 42
mixture of E : Z isomers. 1H NMR (500 MHz, CDCl3)[15] d 8.08 (d, J ¼ 8.7 Hz,
2H), 7.40 (d, J ¼ 8.7 Hz, 2H), 7.14 (d, J ¼ 8.7 Hz, 2H), 6.78 (d, J ¼ 8.7 Hz,
2H), 6.74 (d, J ¼ 12.2 Hz, 1H), 6.51 (d, J ¼ 12.2 Hz, 1H), 3.80 (s, 3H).
(E)- and (Z)-4-Chlorostilbene (3, R1 5 H, R2 5 Cl): As a 47 : 53 mixture of
1
E : Z isomers. (E)-4-Chlorostilbene: H NMR (500 MHz, CDCl3)[5a–c,18,19]
d 7.51 (d, J ¼ 7.9 Hz, 2H), 7.45 (d, J ¼ 8.5 Hz, 2H), 7.37–7.40 (m, 2H),
7.34 (d, J ¼ 8.5 Hz, 2H), 7.28–7.31 (m, 1H), 7.10 (d, J ¼ 16.5 Hz, 1H),
7.06 (d, J ¼ 16.5 Hz, l H). (Z)-4-Chlorostilbene: 1H NMR (500 MHz,
CDCl3)[5a–c,18,19] d 7.18–7.28 (m, 9H), 6.63 (d, J ¼ 12.2 Hz, 1H), 6.53
(d, J ¼ 12.2 Hz, 1H).
(E)- and (Z)-Stilbene(3, R1 5 R2 5 H): As a 45 : 55 mixture of E : Z isomers.
(E)-Stilbene: 1H NMR (500 MHz, CDCl2)[5a–c,20] d 7.52 (d, J ¼ 7.8 Hz, 4H),
1
7.36 (t, J ¼ 7.8 Hz, 4H), 7.19–7.26 (m, 2H), 7.11 (s, 2H). (Z)-Stilbene: H
NMR (500 MHz, CDCl3)[5a–c,20] d 7.19–7.26 (m, 10H), 6.60 (s, 2H).
(E)- and (Z)-4-Methoxystilbene (3, Rl 5 H, R2 5 OMe or R1 5 OMe,
1
R2 5 H): As a 48 : 52 mixture of E : Z isomers. (E)-4-Methoxystilbene: H
NMR (500 MHz, CDCl3)[5a–c,18–20] d 7.49 (d, J ¼ 7.4 Hz, 2H), 7.45
(d, J ¼ 8.6 Hz, 2H), 7.34 (t, J ¼ 7.4 Hz, 2H), 7.21–7.25 (m, 1H), 7.07
(d, J ¼ 16.6 Hz, 1H), 6.97 (d, J ¼ 16.6 Hz, 1H), 6.90 (d, J ¼ 8.6 Hz, 2H),
1
3.83 (s, 3H). (Z)-4-Methoxystilbene: H NMR (500 MHz, CDCl3)[5a–c,18–20]
d 7.17–7.27 (m, 5H), 7.18 (d, J ¼ 8.6 Hz, 2H), 6.75 (d, J ¼ 8.6 Hz, 2H),
6.53 (d, J ¼ 13.2 Hz, 1H), 6.50 (d, J ¼ 13.2 Hz, 1H), 3.78 (s, 3H).
(E)- and (Z)-4-Chloro-40-methoxystilbene (3, R1 5 OMe, R2 5 Cl): As a
1
17 : 83 mixture of E : Z isomers. (E)-4-Chloro-40-methoxystilbene: H NMR
(500 MHz, CDCl3)[20] d 7.44 (d, J ¼ 7.4 Hz, 2H), 7.41 (d, J ¼ 7.6 Hz, 2H),
7.30 (d, J ¼ 7.6 Hz, 2H), 7.03 (d, J ¼ 16.0 Hz, 1H), 6.91 (d, J ¼ 16.0 Hz,
1H, 6.90 (d, J ¼ 7.4 Hz, 2H), 3.83 (s, 3H. (Z)-4-Chloro-40-methoxystilbene:
1H NMR (500 MHz, CDCl3)[20] d 7.19 (br, s, 4H), 7.16 (d, J ¼ 7.4 Hz, 2H),
6.77 (d, J ¼ 7.4 Hz, 2H), 6.55 (d, J ¼ 12.4 Hz, 1H), 6.43 (d, J ¼ 12.4 Hz,
1H), 3.79 (s, 3H).

2550
J. Wu, D. Li, and D. Zhang
(E)- and (Z)-4,40-Dimethoxystilbene (3, R1 5 R2 5 OMe): As a 21 : 79
1
mixture of E : Z isomers. (E)-4,40-Dimethoxystilbene: H NMR (500 MHz,
CDCl3)[9,21] d 7.43 (d, J ¼ 8.5 Hz, 4H), 6.93 (s, 2H), 6.89 (d, J ¼ 8.5 Hz,
4H), 3.83 (s, 3H). (Z)-4,40-Dimethoxystilbene: 1H NMR (500 MHz,
CDCl3)[9,21] d 7.2 (d, J ¼ 8.5 Hz, 4H), 6.77 (d, J ¼ 8.5 Hz, 4H), 6.44
(s, 2H), 3.79 (s, 3H).
ACKNOWLEDGMENT
This work was supported in part by a research grant to J. Wu from Zhejiang
Natural Science Fund. The authors also thank Wei-Min Dai for his encourage-
ment and discussion during the course of this study.
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