Efficient sonogashira coupling reaction catalyzed by palladium(II) β-oxoiminatophosphane complexes under mild conditions

Article abstract of DOI:10.1055/s-2008-1077873

Palladium(II) β-oxoiminatophosphane complexes were used as catalysts for the Sonogashira coupling reaction. In the presence of very low amounts of the complexes, various aryl iodides and bromides were efficiently coupled with phenylacetylene even at room

Full text of DOI:10.1055/s-2008-1077873

LETTER
1657
Efficient Sonogashira Coupling Reaction Catalyzed by Palladium(II)
b-Oxoiminatophosphane Complexes under Mild Conditions
S
D
onogashira Coup
o
lingCatalyze
n
dby Pd(II)
b
g
-Oxoiminato
-
phospha
H
nes wan Lee, Huili Qiu, Min-Ho Cho, Ik-Mo Lee, Myung-Jong Jin*
Department of Chemical Engineering and Chemistry, Inha University, Incheon 402-751, Korea
Fax +82(32)8720959; E-mail: mjjin@inha.ac.kr
Received 26 March 2008
in the coupling reaction. In this context, we envisioned
Pd(II) b-oxoiminatophosphane complexes 2 as catalysts
for the Sonogashira coupling reaction. b-Oxoimines have
Abstract: Palladium(II) b-oxoiminatophosphane complexes were
used as catalysts for the Sonogashira coupling reaction. In the pres-
ence of very low amounts of the complexes, various aryl iodides and
bromides were efficiently coupled with phenylacetylene even at attracted much interest in coordination chemistry due to
6
room temperature. Furthermore, the catalysts were also effective for their resonance stability and unique structure. The com-
the reactions of aryl chlorides. It is noteworthy that this catalyst sys-
tem employed mild, efficient, aerobic, solvent-free, and copper-free
conditions.
plexes of b-oxoimines have often been used as precursors
7
for the metallo-organic chemical vapor deposition and
8
catalysts for olefin polymerization. We have recently
Key words: palladium, b-oxoiminatophosphane, catalysis, arylat-
ed alkynes, Sonogashira reaction
shown that palladium complexes 2 can be used as effec-
9
tive catalysts in the Heck reaction. It should be of interest
to explore their catalytic ability in other coupling reac-
tions. Herein, we wish to report the catalytic activities of
The palladium-catalyzed Sonogashira cross-coupling re- air-stable Pd(II) b-oxoiminatophosphane complexes 2 in
action of terminal alkynes with aryl halides is one of the the copper-free Sonogashira reaction.
most valuable methods for the synthesis of internal arylat-
The b-oxoimines 1a–d were easily prepared from com-
1
ed alkynes. This reaction is typically performed in the
mercially available 2,4-pentanedione and primary amines
presence of catalytic amounts of a palladium complex and
copper iodide with a base. Among many successful cata-
lysts, phosphane-based Pd complexes are prototypical
catalysts that have been widely employed in the Sono-
in a one-step synthesis (Scheme 1). Condensation of 2,4-
pentanedione and primary amines under microwave heat-
ing produced the corresponding b-oxoimines 1 in high
yields.10 Interestingly, this reaction was complete within
2
3
gashira coupling. Recently, cyclopalladated phosphanes
five minutes in the absence of solvents. Deprotonation of
with EtOTl in THF, followed by treatment with Pd (m-
4
and N-heterocyclic carbenes have attracted considerable
attention. However, most of the results have been ob-
tained with reactive aryl iodides or aryl bromides and the
procedures for this reactions require relatively high cata-
lyst loading of 1–5 mol%. There are only a few catalysts
with high efficiency on the coupling of less reactive aryl
chlorides under copper-free and mild conditions. There-
fore, the search for practical catalysts that exhibit high ac-
tivity and broad generality is a field of continuous interest
1
2
1
1
Cl) Me (PPh ) led to the formation of the correspond-
2
2
3 2
1
2
ing Pd complexes 2. The use of EtOTl instead of con-
ventional strong bases such as NaH, KH, MeONa, and
BuLi was found to be more effective for the formation of
the Pd complexes. Pd complexes 2 are very stable to air
oxygen and moisture. These were characterized by ele-
5
1
13
31
mental analysis, H NMR, C NMR, P NMR, and X-ray
crystallography.1
3,14
RNH2
1) EtOTl, THF, r.t.
O
NR
Me
microwave, 5 min
2 2 2 3 2
2) Pd (µ-Cl) Me (PPh )
O
O
O
NHR
Pd
Ph3P
1
2
O
O
O
N
O
N
O
N
O
N
Pd
Pd
Pd
Pd
Ph3P
Me
Ph3P
Me
Ph3P
Me
Ph3P
Me
2a
2b
2c
2d
Scheme 1 Synthesis of Pd(II) b-oxoiminatophosphane complexes 2
SYNLETT 2008, No. 11, pp 1657–1660
x
x
.x
x
.2
0
0
8
Advanced online publication: 11.06.2008
DOI: 10.1055/s-2008-1077873; Art ID: U02608ST
©
Georg Thieme Verlag Stuttgart · New York

1
658
D.-H. Lee et al.
LETTER
The catalytic activity of 2 was initially examined in cop- Table 1 Sonogashira Coupling of Iodobenzene with Phenylacety-
per-free Sonogashira coupling of iodobenzene with phe- lenea
15
nylacetylene in the presence of piperidine as a base. The
reaction was initially performed with catalyst loading as
low as 0.01 mol% of 2. Surprisingly, the reaction occurred
completely at 50 °C within one hour (entry 1). When the Entry
loading of 2a was decreased to 0.001 mol%, high conver-
2
I
+
base
2
(
Base
Temp
(°C)
Time
(h)
Yield
(%)a
mol%)
sion could be still maintained (entry 2). Furthermore, the
reaction was also performed well with ultralow catalyst
1
2a (0.01)
2a (0.001)
2a (0.0002)
2a (0.01)
2a (0.01)
2a (0.1)
piperidine
piperidine
piperidine
piperidine
piperidine
piperidine
piperidine
pyrrolidine
50
50
50
40
25
25
25
25
25
25
40
25
40
25
40
25
1
5
98 (95)
97
loading of 0.0002 mol%, in which TOF (high turnover
2
3
4
5
6
7
8
9
0
1
2
3
4
–
1
frequency) of 37 917 h was obtained for the coupling
entry 3). Further optimization of the reaction conditions
12
2
91 (87)
98
(
was not attempted to obtain higher TOF. Parameters of re-
action temperature and base were surveyed in the cou-
pling. The catalyst showed outstanding performance at
low temperature of 25–40 °C (entries 4–7). It is notewor-
thy that catalyst 2a shows outstanding performance even
at 25 °C (entry 6). The results are summarized in Table 1.
Among bases, piperidine gave the best result for the cou-
12
10
5
81 (76)
93
2a (1.0)
95
2a (0.1)
10
24
24
2.5
18
3
83 (75)
73
pling. Replacement of piperidine by pyrrolidine, Et N, or
3
2a (0.1)
Et N
3
(
(
i-Pr) EtN resulted in decreased activity of 2a at 25 °C
entries 8–10). Palladium complexes 2b–d were next ex-
2
1
1
1
1
1
2a (0.1)
(i-Pr) EtN
82 (75)
94 (90)
82
2
amined in the coupling of iodobenzene with phenylacety-
lene (entries 11–16). N-Bound substituents have a notable
effect on the activity. Complex 2b has a similar catalytic
activity to 2a (entry 11). In contrast with 2a and 2b having
alkoxyalkyl group on the N atom, complexes 2c and 2d
seem to be less effective at low temperature. This result
indicates that the alkoxyalkyl group has a beneficial effect
on the catalytic activity.
2b (0.01)
2b (0.1)
2c (0.01)
2c (0.1)
piperidine
piperidine
piperidine
piperidine
piperidine
piperidine
91 (86)
59
24
5
15
2d (0.01)
2d (0.1)
89 (83)
67
To further extend the scope of our catalytic system, we in- 16
24
vestigated the coupling of various aryl halides with phe-
nylacetylene in the presence of 0.01 mol% of 2a. As
shown in Table 2, outstanding catalytic activity was ob-
served in the coupling of deactivated aryl iodides such as
a
All reactions were performed with iodobenzene (1.0 mmol), phen-
ylacetylene (1.1 mmol), base (2.0 mmol), and a catalytic amount of 2.
b
The GC yield was determined using n-dodecane as an internal stan-
dard. Isolated yield is given in parenthesis.
4
-iodoanisole, 2-iodoanisole, 4-iodotoluene, and 2-iodo-
phenol (entries 2–5) as well as activated 1-iodo-4-ni-
trobenzene (entry 1). The coupling of different aryl
bromides could be also performed in high yields (entries
Encouraged by these results, we performed the coupling
of several aryl chlorides. It is well known that C–Cl acti-
vation is much more difficult compared to those of C–Br
and C–I. Chlorobenzene was found to react with phenyl-
acetylene, 1-octyne, and (triisopropylsilyl)acetylene at 40
6
–12). Regardless of the substituents, the reaction could
be completed within a short time. In the coupling of bro-
mobenzene, the catalyst was found to sustain high activity
even at 25 °C, although somewhat long reaction time was
required for the satisfactory conversion (entry 6). The cat-
alyst loading of only 0.01 mol% was sufficient to achieve
high TOF at 40 °C (entries 7–12). Deactivated aryl bro-
mides having electron-donating group showed a slight
drop in reactivity compared to those having electron-with-
drawing group. However, excellent yields were still ob-
tained at the catalyst loading of 0.01 mol% in four hours.
We extended the scope of the coupling to other alkynes
such as propargyl alcohol, (triisopropylsilyl)acetylene,
and 1-octyne (entries 13-16).
°
C (entries 17–19). The coupling of aryl chloride required
the presence of 0.1 mol% catalyst in order to obtain the
desired product in satisfactory yields. Activated 1-chloro-
4
4
-nitrobenzene reacted efficiently with phenylacetylene at
0 °C (entry 20). An attempt to couple deactivated aryl
chloride with phenylacetylene was also successful to
some extent. Deactivated 4-chloroanisole, 4-chlorotolu-
ene, and 2-chlorotoluene were smoothly converted into
the corresponding coupling products at 50 °C, resulting in
good yields (entries 21–23).
In summary, palladium(II) b-oxoiminatophosphane com-
plexes turned out to be highly efficient catalysts for the
Sonogashira coupling reaction. The coupling of various
aryl halides with terminal alkynes proceeded with excel-
lent yields under mild conditions. It is noteworthy that the
palladium complexes acted as active catalysts for the cou-
Similar reaction rates were also observed in the reaction
of aryl bromides with these alkynes under the same con-
ditions.
Synlett 2008, No. 11, 1657–1660 © Thieme Stuttgart · New York

LETTER
Sonogashira Coupling Catalyzed by Pd(II) b-Oxoiminatophosphanes
1659
Table 2 Sonogashira Coupling of Aryl Halides with Terminal
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(
2
a
R2
R2
(2) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
X
+
Lett. 1975, 44, 4467. (b) McGaffin, G.; Meijer, A. Synthesis
R1
piperidine
R1
1
994, 583. (c) Chow, H.-F.; Wan, C.-W.; Low, K.-H.;
X = I , Br , Cl
Yeung, Y.-Y. J. Org. Chem. 2001, 66, 1910. (d) Novák, Z.;
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Entry R1
X
R2
2a
Temp Time Yield
3327. (e) Adjabeng, G.; Brenstrum, T.; Frampton, C. S.;
b
(
mol%) (°C) (h)
(%)
Robertson, A. J.; Hillhous, J.; McNulty, J.; Capretta, A.
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Y. S.; Tong, Y. J. J. Organomet. Chem. 2004, 689, 1295.
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4-O N
I
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
0.01
0.01
0.01
0.01
0.01
0.1
40
40
40
40
40
25
40
40
40
40
40
40
40
40
40
40
40
40
40
40
50
50
50
1
2
97 (94)
97 (93)
94
2
(
(
4-MeO
I
2-MeO
4-Me
2-Me
H
I
3
I
2
96
I
3
94 (90)
90 (84)
93
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
12
3
H
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.1
4-O N
2
98
2
(
(
(
(
4-MeO
2-MeO
4-Me
2-Me
H
3
91
1
1
1
1
1
1
1
1
1
1
2
2
2
4
97 (95)
3.5 92
4
3
93
91
CH OH
2
(
c) Pasko, S.; Hubert-Pfalzgraf, L. G.; Richard, P.; Abrutis,
A. Inorg. Chem. Commun. 2005, 8, 483.
H
n-C H
3.5 90 (86)
3.5 95
6
13
7) (a) Studebaker, D. B.; Neumayer, D. A.; Marks, T. J. Inorg.
Chem. 2000, 39, 3148. (b) Lim, S. W.; Lee, J. C.; Shon, D.
S.; Lee, W. I.; Lee, I. M. Chem. Mater. 2002, 14, 1548.
(c) Lim, S. W.; Choi, B.; Min, Y. S.; Lee, S. S. J. Organomet.
Chem. 2004, 689, 224.
8) (a) Shultz, L. H.; Tempel, D. J.; Brookhart, M. J. Am. Chem.
Soc. 2001, 123, 11539. (b) Zhang, D.; Jin, G. X.; Hu, N.
Chem. Commun. 2002, 574. (c) He, X.; Yao, Y.; Luo, X.;
Zhang, J.; Zhang, L.; Wu, Q. Organometallics 2003, 22,
H
Si(i-Pr)3
Si(i-Pr)3
Ph
4-MeO
H
4
12
18
18
8
92
94 (91)
76
H
n-C H
0.1
6
13
H
Si(i-Pr)3
Ph
0.1
86
4952. (d) Gui, G.; Bao, F.; Gao, H.; Zhu, F.; Wu, Q. Appl.
4-O N
0.1
93
Organomet. Chem. 2005, 19, 627.
2
(
9) Lee, D.-H.; Cho, M.-H.; Kwon, I.-K.; Jun, I.-C.; Lee, I.-M.;
4-MeO
4-Me
Ph
0.1
12
12
18
89
Jin, M.-J. Catal. Commun. 2008, 9, 1517.
(
10) General Procedure for the Synthesis of b-Oxoimines
,4-Pentanedione (11.0 mmol) and primary amines (10.0
Ph
0.1
81 (75)
73
2
mmol) were placed in a 10 mL glass tube. The vessel was
sealed with a septum and placed into the microwave cavity.
The reaction temperature was raised from r.t. to 130 °C
under microwave irradiation of 150 W. The power was
maintained for 5 min. After allowing the mixture to cool to
r.t., the reaction mixture was vacuumed under reduced
pressure. The residue was purified by column chromato-
2
2-Me
Ph
0.1
a
All reactions were performed with aryl halide (1.0 mmol), alkyne
(
1.1 mmol), piperidine (2.0 mmol), and a catalytic amount of 2a.
The GC yield was determined using n-dodecane as an internal stan-
b
dard. Isolated yield is given in parenthesis.
graphy (SiO ; hexane–ethyl acetate, 4:1) to give b-oxoimine
2
1
as a liquid.
pling of aryl chlorides. Further studies of other coupling
reactions catalyzed by this system are in progress.
1
Compound 1a: light yellow liquid (2.11 g, 96%). H NMR
(
1
400 MHz, CDCl ): d = 11.11 (br s, 1 H), 7.23 (t, J = 8.0 Hz,
3
H), 7.17 (d, J = 8.0 Hz, 1 H), 6.91 (t, J = 7.6 Hz, 1 H), 6.85
(
3
d, J = 8.4 Hz, 1 H), 4.99 (s, 1 H), 4.41 (d, J = 6.8 Hz, 1 H),
.84 (s, 3 H), 2.00 (s, 3 H), 1.92 (s, 3 H). C NMR (100
References and Notes
1
3
(
1) For recent reviews, see: (a) Littke, A. F.; Fu, G. G. Angew.
Chem. Int. Ed. 2002, 41, 4176. (b) Negishi, E.; Anastasia, L.
Chem. Rev. 2003, 103, 1979. (c) Nicolaou, K. C.; Bulger, P.
MHz, CDCl ): d = 194.4, 162.7, 156.6, 128.4, 127.6, 126.0,
1
C H NO (219.28): C, 71.21; H, 7.81; N, 6.39. Found: C,
3
20.3, 110.0, 95.3, 55.1, 42.1, 28.7, 18.6. Anal. Calcd for
1
3
17
2
71.02; H, 7.54; N, 6.58.
Synlett 2008, No. 11, 1657–1660 © Thieme Stuttgart · New York

1
660
D.-H. Lee et al.
LETTER
1
Compound 1b: light yellow liquid (1.68 g, 98%). H NMR
400 MHz, CDCl ): d = 10.833 (br s), 4.96 (s, 1 H), 3.45 (t,
129.9, 127.8, 127.7, 127.1, 126.6, 120.3, 109.2, 97.6, 55.2,
49.0, 26.4, 23.3, 23.2, –0.8. P NMR (162 MHz, CDCl3):
3
1
(
3
J = 6.0 Hz, 2 H), 3.35 (s, 3 H), 3.32 (q, J = 8.0 Hz, 2 H), 2.00
d = 44.31. Anal. Calcd for C H NO PPd (602.01): C,
3
2
34
2
(
(
s, 3 H), 1.93 (s, 3 H), 1.83 (t, J = 6.0 Hz, 2 H). 13C NMR
63.84; H, 5.69; N, 2.33. Found: C, 63.74; H, 5.53; N, 2.10.
1
100 MHz, CDCl ): d = 193.6, 162.4, 94.5, 68.7, 58.0, 39.2,
9.7, 28.1, 18.1. Anal. Calcd for C H NO (171.24): C,
Compound 2b: light brown solid (0.39 g, 73%). H NMR
3
2
(400 MHz, CDCl ): d = 7.70–7.34 (m, 15 H), 4.70 (s, 1 H),
9
17
2
3
6
3.13; H, 10.01; N, 8.18. Found: C, 63.41; H, 10.18; N, 8.21.
3.60 (m, 2 H), 3.42 (t, J = 6.0 Hz, 2 H), 3.32 (s, 3 H), 2.01 (s,
3 H), 1.90 (t, J = 7.2 Hz, 2 H), 1.53 (s, 3 H), 0.10 (d, J = 3.6
1
Compound 1c: light yellow liquid (1.34 g, 95%). H NMR
1
3
(
400 MHz, CDCl ): d = 10.94 (br s, 1 H), 4.94 (s, 1 H), 3.20
q, J = 3.6 Hz, 2 H), 1.97 (s, 3 H), 1.91 (s, 3 H), 1.61 (q,
J = 3.6 Hz, 2 H), 0.98 (t, J = 7.2 Hz, 3 H). C NMR (100
MHz, CDCl ): d = 193.4, 162.2, 94.2, 44.0, 28.0, 22.8, 18.1,
0.7. Anal. Calcd for C H NO (141.21): C, 68.04; H, 10.71;
Hz, 3 H). C NMR (100 MHz, CDCl ): d = 162.7, 141.9,
3
3
(
129.0–126.5 (PPh 18C), 95.3, 63.8, 51.8, 50.2, 30.7, 26.4,
24.8, 11.9. P NMR (162 MHz, CDCl ): d = 43.76. Anal.
Calcd for C H NO PPd (553.97): C, 60.71; H, 6.19; N,
3
,
1
3
31
3
3
28 34
2
1
2.53. Found: C, 61.12; H, 6.00; N, 2.25.
Compound 2c: brown solid (0.37 g, 71%). H NMR (400
8
15
1
N, 9.02. Found: C, 68.17; H, 10.79; N, 9.32.
1
Compound 1d: light brown liquid (1.45 g, 93%). H NMR
MHz, CDCl ): d = 7.81–7.26 (m, 15 H), 4.69 (s, 1 H), 3.44
3
(
(
400 MHz, CDCl ): d = 11.24 (br s, 1 H), 4.87 (s, 1 H), 2.03
s, 3 H), 1.95 (s, 3 H), 1.37 (s, 9 H). C NMR (100 MHz,
(m, 2 H), 2.00 (s, 3 H), 1.62 (m, 2 H), 1.52 (s, 3 H), 0.91 (t,
3
1
3
13
J = 7.2 Hz, 3 H), 0.09 (d, J = 3.6 Hz, 3 H). C NMR (100
CDCl ): d = 193.6, 163.0, 96.1, 52.2, 30.7, 28.7, 20.4. Anal.
MHz, CDCl ): d = 176.8, 164.0, 134.8–127.7 (PPh , 18 C),
3
3
3
3
1
Calcd for C H NO (155.13): C, 69.63; H, 11.04; N, 9.02.
97.7, 52.1, 26.2, 25.7, 23.5, 22.6, 11.5, 11.5, 1.8. P NMR
9
17
Found: C, 69.54; H, 11.38; N, 9.13.
11) Ladipo, F. T.; Anderson, G. K. Organometallics 1994, 13,
03.
12) (a) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998.
b) Amisial, L. D.; Dai, X.; Kinney, R. A.; Warren, T. H.
(162 MHz, CDCl ): d = 43.75. Anal. Calcd for
3
(
(
C H NOPPd (523.94): C, 61.89; H, 6.16; N, 2.67. Found:
2
7
32
3
C, 61.66; H, 5.71; N, 3.06.
Compound 2d: brown solid (0.41 g, 76%). H NMR (400
1
(
MHz, CDCl ): d = 7.70–7.37 (m, 15 H), 5.27 (s, 1 H), 2.01
3
Inorg. Chem. 2004, 43, 6537.
(s, 3 H), 1.71 (s, 3 H), 1.15 (s, 9 H) 0.55 (t, 3 H, J = 2.0 Hz).
1
3
(
(
13) CCDC-608595 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/ data_request/cif.
C NMR (100 MHz, CDCl ): d = 158.1, 157.7, 152.1, 151.4,
3
150.6, 135.6, 132.5, 131.4, 127.5, 127.4, 123.6, 123.5,
123.0, 93.5, 50.1, 30.0, 24.8, 23.4, 19.2, 19.0, 0.8. 31P NMR
(162 MHz, CDCl ): d = 44.11. Anal. Calcd for
3
14) General Procedure for the Synthesis of Palladium(II)
b-Oxoiminatophosphanes 2
C H NOPPd (537.14): C, 62.51; H, 6.37; N, 2.60. Found:
2
8
34
C, 62.40; H, 5.78; N, 3.10.
A solution of EtOTl (0.3 g, 1.2 mmol) in THF (10 mL) was
added dropwise at r.t. to a solution of b-oxoimine 1 (1.0
mmol) in THF (10 mL). After being stirred at r.t. for 1 h, a
solution of Pd (m-Cl) Me (PPh ) (0.51 g, 0.6 mmol) in
(15) General Procedure for the Preparation of Sonogashira
Reaction
Reactions were carried out in a glass vial equipped with a
Teflon screw cap. Aryl halide (1.0 mmol), phenylacetylene
(1.1 mmol), piperidine (2.0 mmol), and a catalytic amount of
Pd complex 2 were mixed. The resulting mixture was stirred
at the appropriate temperature. The samples were withdrawn
periodically and analyzed by GC and GC-MS. The GC yield
was determined using n-dodecane as an internal standard
and based on the amount of aryl halide employed. The
reaction mixture was filtered and washed with H O and Et O
2
2
2
3 2
THF (5 mL) was added dropwise to the mixture. The
reaction mixture was stirred at r.t. for 1 h. The mixture was
then filtered through Celite on a frit. The solvent was
removed under reduced pressure and the residue was washed
with hexane (30 mL). Removal of the solvent gave Pd
complex 2.
1
Compound 2a: light brown solid (0.45 g, 75%). H NMR
2
2
(
400 MHz, CDCl ): d = 7.59–7.17 (m, 17 H), 6.92 (t, J = 4.0
several times. The organic phase was separated and dried
3
Hz, 1 H), 6.81 (d, J = 4.4 Hz, 1 H), 4.83 (s, 1 H), 4.72 (d,
J = 3.6 Hz, 2 H), 3.81 (s, 3 H), 1.89 (s, 3 H), 1.61 (s, 3 H),
–
over MgSO , and then the solvent was evaporated under
4
reduced pressure. The product was isolated by column
chromatography on SiO2.
1
3
0.08 (d, J = 3.2 Hz, 3 H). C NMR (100 MHz, CDCl ): d =
3
1
78.0, 166.7, 156.0, 134.9, 134.8, 131.4, 131.0, 129.9,
Synlett 2008, No. 11, 1657–1660 © Thieme Stuttgart · New York