Copper(I) chloride mediated cyclization of optically active 2,3-allenoic acids or 1:1 salts of optically active 2,3-allenoic acids with chiral amines

Article abstract of DOI:10.1055/s-2006-950293

Copper(I) chloride mediated cyclization of optically active 2,3-allenoic acids or 1:1 salts of optically active 2,3-allenoic acids with chiral amines was developed. The reaction of optically active 2,3-allenoic acids in methanol requires only a catalytic

Full text of DOI:10.1055/s-2006-950293

PAPER
3711
Copper(I) Chloride Mediated Cyclization of Optically Active 2,3-Allenoic
Acids or 1:1 Salts of Optically Active 2,3-Allenoic Acids with Chiral Amines
C
opper(I) Chl
h
oride Mediat
e
ed
C
yclizat
n
ion of
O
ptica
g
llyActive
2
m
,3-Allenoic
A
cids ing Ma,* Zhanqian Yu
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Lu, Shanghai 200032, P. R. of China
E-mail: masm@mail.sioc.ac.cn
Received 1 May 2006; revised 26 June 2006
Since the optically active 2,3-allenoic acids were easily
Abstract: Copper(I) chloride mediated cyclization of optically ac-
prepared by the resolution of racemic 2,3-allenoic acids
tive 2,3-allenoic acids or 1:1 salts of optically active 2,3-allenoic ac-
ids with chiral amines was developed. The reaction of optically
active 2,3-allenoic acids in methanol requires only a catalytic
with optically active amines,5 we explored the possibility
of using the 1:1 acid–chiral amine salts directly as the
amount of copper(I) chloride, while the reaction of the 1:1 salts of starting point. However, when the same reaction condi-
optically active 2,3-allenoic acids with chiral amines needs one
equivalent of copper(I) chloride and should be carried out in dichlo-
romethane to ensure high efficiency of chirality transfer.
tions were applied to the 1:1 salt of (S)-2-methyl-4-phe-
nylbuta-2,3-dienoic acid with the naturally occurring
cinchonidine (3a) (Table 2), product 2a was obtained in
71% yield, but only 4.3% ee, although two equivalents
Key words: chirality transfer, cyclization, copper(I) chloride,
amines, 2,3-allenoic acids
copper(I) chloride was used (Table 2, entry 1). Poor re-
sults were also obtained in ethanol and ethyl acetate
(Table 2, entries 2 and 3). Fortunately, when the reaction
Butenolides make up a class of interesting compounds
with potential biological activities,1 and thus, much atten-
tion has been paid to the development of new synthetic
methodologies to prepare these compounds.2 Among
these, cyclization involving 2,3-allenoic acids and their
derivatives is one of the most efficient pathways.3,4 In this
paper, we wish to report the cycloisomerization of optical-
ly active 2,3-allenoic acids and 1:1 salts of optically active
2,3-allenoic acids with chiral amines.5
Under the catalysis of copper(I) chloride (4 mol%),6 cy-
cloisomerization of optically active 2,3-allenoic acids af-
forded the optically active butenolides 2a and 2b with
very highly efficient chirality transfer. Four examples are
listed in Table 1.
was conducted in tetrahydrofuran, the enantiopurity of
(R)-(+)-2a was much higher (Table 2, entry 4). The best
results were obtained when dichloromethane was used as
the solvent: the reaction of 3a at ambient temperature af-
forded (R)-(+)-2a without obvious loss of the chirality
(Table 2, entries 5–9). On the basis of these studies, we
concluded that the best results were obtained when the re-
action was conducted at 25 °C with one equivalent of cop-
per(I) chloride as the mediator and dichloromethane as the
solvent (Table 2, entry 10). The requirement of one equiv-
alent of copper(I) chloride may be due to the coordination
ability of cinchonidine with copper(I).
With the established reaction conditions, differently sub-
stituted optically active butenolides 2 were prepared in
moderate to excellent yields in high enantiopurity
(Table 3). The stereochemical outcome of this transfor-
Table 1 Cycloisomerization of Optically Active 2,3-Allenoic Acids Catalyzed by Copper(I) Chloride
R3
H
R1
R2
R3
CuCl (4 mol%)
R2
R1
MeOH, 60 °C, 2 h
O
COOH
O
1
2
Entry
R1
Ph
H
R2
H
R3
Starting material 1 ee of 1 (%)
Product 2
(R)-(+)-2a
(S)-(–)-2a
(R)-(+)-2b
(S)-(–)-2b
Yield (%)
ee of 2a (%)
98.6
1
2
3
4
Me
Me
Pr
(S)-(+)-1b
(R)-(–)-1b
(S)-(+)-1a
(R)-(–)-1a
>97.3
>94.3
>97.4
>97.2
94
83
85
90
Ph
H
97.6
Ph
H
99.8
Ph
Pr
96.5
a Determined by HPLC analysis with chiral columns.
SYNTHESIS 2006, No. 21, pp 3711–3714
x
x.
x
x
.
2
0
0
6
Advanced online publication: 09.10.2006
DOI: 10.1055/s-2006-950293; Art ID: F06706SS
© Georg Thieme Verlag Stuttgart · New York

3712
S. Ma, Z. Yu
PAPER
Table 2 Copper(I) Chloride Mediated Cyclization of the 1:1 Salt of (S)-2-Methyl-4-phenylbuta-2,3-dienoic Acid with L-(–)-Cinchonidine
H
Me
Me
Ph
H
CuCl
Ph
H
O
Solvent
CO2
O
(L)-cinchonidine-H
2a
3a
Entry
ee of 3aa (%)
Solvent
MeOH
EtOH
CuCl (equiv)
Temp (°C)
Time (h)
3.6
Yield of 2ab (%)
ee of 2ac (%)
1
2
97.7
97.7
97.7
89.2
89.2
89.2
89.2
89.2
89.2
97.7
2
2
70
70
70
60
reflux
30
0
71
73
80
82
82
80
67
62
34
78
4.3
1
11
3
EtOAc
THF
2
13.5
12
9
4
2
81.9
87.5
90.1
88.5
88.6
58.5
97.1
5
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
2
4.5
6
2
4.5
7
2
5
8
1
0
10.5
12
9
10%
1
25
25
10
10
a Determined by HPLC analysis with chiral columns after converting 3a into its ester.
b Isolated yield.
c Determined by HPLC analysis with chiral columns.
Table 3 Copper(I) Chloride Mediated Cyclization Reaction of 1:1 Salts of Optically Active 2,3-Allenoic Acids with Chiral Amines
Entry
3
R1
R2
H
R3
Basea
A
ee of 3b (%) Time (h)
2
Yield (%) ee of 2c (%)
1
2
3
4
5
6
7
8
9
3a
3b
3c
3d
3e
3f
3g
3h
3i
Ph
Me
Me
Pr
>97.7
>94.3
>97.2
>97.4
>97.6
>97.8
>98.3
>96.7
>99.0
10
10
10
10
10
10
12.5
10
10
(R)-(+)-2a
(S)-(–)-2a
(R)-(+)-2b
(S)-(–)-2b
(R)-(+)-2c
(S)-(–)-2c
(R)-(+)-2d
(S)-(–)-2d
(R)-(+)-2e
78
90
94
94
96
93
81
95
87
97.1
97.3
99.9
99.1
96.9
98.5
98.2
99.3
98.0
H
Ph
H
C
Ph
B
H
Ph
H
Pr
C
Ph
allyl
allyl
Me
Me
Pr
B
H
Ph
H
C
1-Naph
H
B
1-Naph
H
C
4-BrC6H4
A
a A = L-(–)-cinchonidine, B = L-(–)-(1-phenylethyl)amine, C = D-(+)-a-(1-phenylethyl)amine.
b Determined by HPLC analysis with chiral columns after converting 3 into its ester.
c Determined by HPLC analysis with chiral columns.
mation was established by the X-ray diffraction study of noic acids and cyclization of 1:1 salts of 2,3-allenoic acids
(R)-(+)-2e (Figure 1).7 By comparing the absolute config- with chiral amines. The reactions of optically active 2,3-
urations of (R)-(+)-2e with that of 3i (Table 3, entry 9), it allenoic acids in methanol require only a catalytic amount
was concluded that the reaction proceeds via anti-oxymet- of copper(I) chloride, while the reactions of the optically
alation.8 All the absolute configurations of 2 are tentative- active 1:1 salts of optically active 2,3-allenoic acids with
ly assigned on the basis of this study.
chiral amines in dichloromethane need one equivalent of
copper(I) chloride. Further studies in this area are being
conducted in our laboratory.
In conclusion, we have developed a copper(I) chloride
mediated cycloisomerization of optically active 2,3-alle-
Synthesis 2006, No. 21, 3711–3714 © Thieme Stuttgart · New York

PAPER
Copper(I) Chloride Mediated Cyclization of Optically Active 2,3-Allenoic Acids
3713
From 3c: Under N2, a soln of the (S)-(+)-1a–L-(–)-(1-phenyleth-
yl)amine salt (3c; 100 mg, 0.31 mmol; >97.2% ee) and CuCl (31
mg, 0.31 mmol) in CH2Cl2 (3 mL) was stirred at r.t. for 10 h.
Yield: 59 mg (94%); 99% ee.
HPLC (AS column, rate = 0.7 mL/min, hexane–i-PrOH, 80:20);
[a]D20 +163.5 (c 0.955, EtOH; 99.8% ee).
1H NMR (300 MHz, CDCl3): d = 7.46–7.19 (m, 5 H), 7.09 (s, 1 H),
5.87 (s, 1 H), 2.4–2.28 (br, 2 H), 1.70–1.55 (br, 2 H), 1.08–0.91 (br,
3 H).
(S)-(–)-5-Phenyl-3-propylfuran-2-(5H)-one [(S)-(–)-2b]5
From (R)-(–)-1a: Under N2, a soln of (R)-(–)-1a (81 mg, 0.4 mmol;
>97.4% ee) and CuCl (1.6 mg, 0.016 mmol) in MeOH (4 mL) was
stirred at 60 °C for 2 h. After evaporation, the residue was purified
by flash chromatography (silica gel, PE–EtOAc, 15:1).
Yield: 73 mg (90%); 96.5% ee; [a]D20–162.4 (c 1.04, EtOH).
From 3d: Under N2, a soln of the (R)-(–)-1a–D-(+)-(1-phenyleth-
yl)amine salt (3d; 100 mg, 0.31 mmol; >97.4% ee) and CuCl (31
mg, 0.31 mmol) in CH2Cl2 (3 mL) was stirred at r.t. for 10 h.
Figure 1 ORTEP representation of butenolide (R)-(+)-2e
Yield: 59 mg (94%); 99.1% ee.
(R)-(+)-3-Allyl-5-phenylfuran-2(5H)-one [(R)-(+)-2c]5
Under N2, a soln of the (S)-2-allyl-4-phenylbuta-2,3-dienoic acid–
L-(–)-(1-phenylethyl)amine salt (3e; 102 mg, 0.31 mmol; >97.6%
ee) and CuCl (31 mg, 0.31 mmol) in CH2Cl2 (3 mL) was stirred at
r.t. for 10 h.
NMR spectra were recorded on a Varian Mercury-300 or Bruker
AM-300 spectrometer with chemical shifts reported in ppm relative
to TMS as internal standard. IR spectra were measured by using an
Avatar-360 spectrometer. EI mass spectra were determined with a
HP 5989A mass spectrometer.
Yield: 61 mg (96%); 96.9% ee.
R)-(+)-3-Methyl-5-phenylfuran-2(5H)-one [(R)-(+)-2a]5
From (S)-(+)-1b: Under N2, a soln of (S)-(+)-1b (69 mg, 0.4 mmol;
>97.3% ee) and CuCl (1.6 mg, 0.016 mmol) in MeOH (4 mL) was
stirred at 60 °C for 2 h. After evaporation, the residues were purified
by flash chromatography (silica gel, PE–EtOAc, 10:1). Yield: 65
mg (94%); 98.6% ee.
HPLC (AS column, rate = 0.7 mL/min, hexane–i-PrOH, 75:25);
[a]D20 +126 (c 2.40, EtOH).
1H NMR (300 MHz, CDCl3): d = 7.45–7.22 (m, 5 H), 7.15–7.11 (m,
1 H), 5.82–6.00 (m, 2 H), 5.26–5.13 (m, 2 H), 3.11 (d, J = 5.4 Hz, 2
H).
From 3a: Under N2, a soln of the (S)-(+)-1b–L-(–)-cinchonidine salt
(3a; 117 mg, 0.25 mmol, >97.7% ee) and CuCl (26 mg, 0.26 mmol)
in CH2Cl2 (8 mL) was stirred at r.t. for 12 h. Yield: 34 mg (78%);
97.1% ee.
(S)-(–)-3-Allyl-5-phenylfuran-2(5H)-one [(S)-(–)-2c]
Under N2, a soln of the (R)-2-allyl-4-phenylbuta-2,3-dienoic acid–
D-(+)-(1-phenylethyl)amine salt (3f; 103 mg, 0.32 mmol; >97.8%
ee) and CuCl (31 mg, 0.31 mmol) in CH2Cl2 (3 mL) was stirred at
r.t. for 10 h.
HPLC (AS column, rate = 0.7 mL/min, hexane–i-PrOH, 90:10);
[a]D20 226.7 (c 1.025, EtOH, 98.6% ee); mp 57–58 °C (EtOAc, PE).
Yield: 60 mg (93%); 98.5% ee; [a]D20 –128 (c 2.45, EtOH).5
1H NMR (300 MHz, CDCl3): d = 7.40–7.25 (m, 5 H), 7.13 (d,
J = 1.6 Hz, 1 H), 5.87 (s, 1 H), 1.99 (d, J = 1.5 Hz, 3 H).
(R)-(+)-3-Methyl-5-(1-naphthyl)furan-2(5H)-one [(R)-(+)-2d]
Under N2, a soln of the (S)-2-methyl-4-naphthylbuta-2,3-dienoic
acid–L-(–)-(1-phenylethyl)amine salt (3g; 87 mg, 0.252 mmol;
>98.3% ee) and CuCl (27 mg, 0.27 mmol) in CH2Cl2 (3 mL) was
stirred at r.t. for 10 h.
(S)-(–)-3-Methyl-5-phenylfuran-2(5H)-one [(S)-(–)-2a]5
From (R)-(–)-1b: Under N2, a soln of (R)-(–)-1b (70 mg, 0.4 mmol;
>94.3% ee) and CuCl (1.6 mg, 0.016 mmol) in MeOH (4 mL) was
stirred at 60 °C for 2.5 h. After evaporation, the residues were puri-
fied by flash chromatography (silica gel, PE–EtOAc, 10:1).
Yield: 46 mg (81%); 98.2% ee.
HPLC (AS column, rate = 0.7 mL/min, hexane–i-PrOH, 80:20); mp
95–97 °C; [a]D20 +94 (c 0.975, EtOH).
Yield: 58 mg (83%); 97.6% ee; [a]D20 –211.8 (c = 1.05, EtOH).
From 3b: Under N2, a soln of the (R)-(–)-1b–D-(+)-(1-phenyleth-
yl)amine salt (3b; 60 mg, 0.2 mmol; >94.3% ee) and CuCl (21 mg,
0.21 mmol) in CH2Cl2 (2 mL) was stirred at r.t. for 10 h.
1H NMR (300 MHz, CDCl3): d = 8.06 (d, J = 8.0 Hz, 1 H), 7.96–
7.82 (m, 2 H), 7.68–7.50 (m, 2 H), 7.50–7.38 (m, 3 H), 6.65 (s, 1 H),
2.02 (s, 3 H).
Yield: 32 mg (90%); 97.3% ee.
(S)-(–)-3-Methyl-5-(1-naphthyl)furan-2(5H)-one [(S)-(–)-2d]5
Under N2, a soln of the (R)-2-methyl-4-naphthylbuta-2,3-dienoic
acid–D-(+)-(1-phenylethyl)amine salt (3h; 140 mg, 0.406 mmol,
>96.7% ee) and CuCl (41 mg, 0.41 mmol) in CH2Cl2 (4 mL) was
stirred at r.t. for 10 h.
(R)-(+)-5-Phenyl-3-propylfuran-2(5H)-one [(R)-(+)-2b]5
From (S)-(+)-1a: Under N2, (S)-(+)-1a (82 mg, 0.4 mmol; >97.2%
ee) and CuCl (1.6 mg, 0.016 mmol) in MeOH (4 mL) was stirred at
60 °C for 3 h. After evaporation, the residues were purified by flash
chromatography (silica gel, PE–EtOAc, 15: 1).
Yield: 86 mg (95%); 99.3% ee.
Yield: 70 mg (85%); 99.8% ee.
Synthesis 2006, No. 21, 3711–3714 © Thieme Stuttgart · New York

3714
S. Ma, Z. Yu
PAPER
(R)-(+)-5-(4-Bromophenyl)-3-propylfuran-2(5H)-one [(R)-(+)-
2e]
Under N2, a soln of the (S)-4-(4-bromophenyl)-2-propylbuta-2,3-di-
enoic acid–D-(–)-(1-phenylethyl)amine salt (3i; 115 mg, 0.2 mmol;
>97.9% ee) and CuCl (20 mg, 0.2 mmol) in CH2Cl2 (2 mL) was
stirred at r.t. for 10 h.
Phytochemistry 1998, 49, 2493. (e) Otsuka, H.; Kotani, K.;
Bando, M.; Kido, M.; Takeda, Y. Chem. Pharm. Bull. 1998,
46, 1180. (f) Guo, S.; Wang, L.; Chen, D. Indian J. Chem.,
Sect. B: Org. Chem. Incl. Med. Chem. 1997, 36, 339.
(g) Evidente, A.; Sparapano, L. J. Nat. Prod. 1994, 57,
1720. (h) Damtoft, S.; Jensen, S. R. Phytochemistry 1995,
40, 157. (i) Estevez-Reyes, R.; Estevez-Braun, A.;
Gonzalez, A. G. J. Nat. Prod. 1993, 56, 1177. (j) Claydon,
N.; Hanson, J. R.; Truneh, A.; Avent, A. G. Phytochemistry
1991, 30, 3802. (k) Seki, T.; Satake, M.; Mackenzie, L.;
Kaspar, H. F.; Yasumoto, T. Tetrahedron Lett. 1995, 36,
7093. (l) Marshall, J. A.; Wolf, M. A.; Wallace, E. M. J.
Org. Chem. 1997, 62, 367.
Yield: 49 mg (87%); 98.0% ee.
HPLC (AS column, rate = 0.7 mL/min, hexane–i-PrOH, 80:20);
[a]D20 124 (c 0.795, EtOH); mp 79–80 °C (EtOAc, PE).
IR (KBr): 1741, 1649, 1590, 1489 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.49 (d, J = 7.4 Hz, 2 H), 7.12 (d,
J = 7.4 Hz, 2 H), 7.06 (s, 1 H), 5.82 (t, J = 1.8 Hz, 1 H), 2.31 (t,
J = 7.7 Hz, 2 H), 1.68–1.52 (m, 2 H), 0.96 (t, J = 7.4 Hz, 3 H).
13C NMR (75.4 MHz, CDCl3): d = 13.6, 20.6, 27.1, 81.4, 123.0,
128.0, 132.0, 134.1, 134.2, 147.2, 173.7.
MS (EI, 70 eV): m/z (%) = 280 [M(79Br)]+, 66.84, 282 [M(81Br)]+,
65.80, 183 (100).
(3) (a) Basabe, P.; Delgado, S.; Marcos, I. S.; Diez, D.; Diego,
A.; De Roman, M.; Urones, J. G. J. Org. Chem. 2005, 70,
9480. (b) Naito, S.; Escobar, M.; Kym, P. R.; Liras, S.;
Martin, S. F. J. Org. Chem. 2002, 67, 4200. (c) Jiang, S.;
Liu, Z.-H.; Sheng, G.; Zeng, B.-B.; Cheng, X.-G.; Wu, Y.-
L.; Yao, Z.-J. J. Org. Chem. 2002, 67, 3404. (d) Busqué, F.;
March, P.; Figueredo, M.; Font, J.; Sanfeliu, E. Tetrahedron
Lett. 2002, 43, 5583. (e) Yoneda, E.; Kaneko, T.; Zhang, S.;
Onitsuka, K.; Takahashi, S. Org. Lett. 2000, 2, 441.
(f) Brown, S. P.; Goodwin, N. C.; MacMillan, D. W. C. J.
Am. Chem. Soc. 2003, 125, 1192.
Anal. Calcd for C13H13BrO2: C, 55.54; H, 4.66. Found: C, 55.65; H,
4.93.
Acknowledgment
(4) (a) Ma, S.; Shi, Z.; Yu, Z. Tetrahedron Lett. 1999, 40, 2393.
(b) Ma, S.; Shi, Z.; Yu, Z. Tetrahedron 1999, 40, 12137.
(c) Fu, C.; Ma, S. Eur. J. Org. Chem. 2005, 3942. (d) Ma,
S.; Yu, Z. Angew. Chem. Int. Ed. 2002, 41, 1775. (e) Ma,
S.; Yu, Z. Chem. Eur. J. 2004, 10, 2078. (f) Ma, S.; Yu, Z.
Org. Lett. 2003, 5, 1507; corrigendum Org. Lett. 2003, 5,
2581. (g) Ma, S.; Yu, Z. J. Org. Chem. 2003, 68, 6149.
(h) Ma, S.; Gu, Z. J. Am. Chem. Soc. 2005, 127, 6182.
(i) Ma, S.; Yu, Z.; Gu, Z. Chem. Eur. J. 2005, 11, 2351.
(5) For copper(I) chloride catalyzed cycloisomerization of
racemic 2,3-allenoic acids, see: Ma, S.; Yu, Z.; Wu, S.
Tetrahedron 2001, 57, 1585.
Financial support from the National Natural Science Foundation of
China (20121202 and 20332060) and the Shanghai Municipal Com-
mittee of Science and Technology is greatly appreciated.
References
(1) (a) Larock, R. D.; Riefling, B.; Fellows, C. A. J. Org. Chem.
1978, 43, 131; and references cited therein. (b) Brima, T. S.
US 4968817, 1990; Chem. Abstr. 1991, 114, 185246y.
(c) Tanabe, A. Jpn. Kokai Tokyo Koho JP 63211276
[88211276], 1988; Chem. Abstr. 1989, 110, 94978q.
(d) Lee, G. C. M. Eur. Pat. Appl. EP 372940, 1990; Chem.
Abstr. 1990, 113, 191137j. (e) Ducharme, Y.; Gauthier, J.
Y.; Prasit, P.; Leblanc, Y.; Wang, Z.; Leger, S.; Therien, M.
PCT Int. Appl. WO 95 00501, 1995; Chem. Abstr. 1996,
124, 55954y. (f) Lee Gary, C. M.; Garst, M. E. PCT Int.
Appl. WO 91 16055, 1991; Chem. Abstr. 1992, 116,
59197m.
(2) For some recent examples, see: (a) Chia, Y.; Chang, F.; Wu,
Y. Tetrahedron Lett. 1999, 40, 7513. (b) Takahashi, S.;
Maeda, K.; Hirota, S.; Nakata, T. Org. Lett. 1999, 1, 2025.
(c) Siddiqui, B. S.; Afshan, F.; Ghiasuddin; Faizi, S.; Naqvi,
S. N.-H.; Tariq, R. M. J. Chem. Soc., Perkin Trans. 1 1999,
2367. (d) Cortez, D. A. G.; Fernandes, J. B.; Vieria, P. C.;
Silva, M. F. G. F.; Ferreira, A. G.; Cass, Q. B.; Pirani, J. R.
(6) (a) Ma, S.; Shi, Z. Chem. Commun. 2002, 540. (b) Ma, S.;
Wu, S. Chem. Commun. 2001, 441.
(7) Crystal data for compound (R)-(+)-2e: C13H13BrO2,
M = 281.14, monoclinic, space group P2 (1), Mo Ka, final R
indices [I > 2s(I)], R1 = 0.0657, wR2 = 0.1570, a = 4.3321
(19) Å, b = 10.916 (5) Å, c = 13.117 (6) Å, a = 90°,
b = 92.942 (7)°, g = 90°, V = 619.5 (5) Å3, T = 293 (2) K,
Z = 2, reflections collected/unique: 3518:2213
(Rint = 0.1593), no observation [I > 2s(I)] 957, parameters
155; CCDC 602980 contains the data.
(8) (a) Ma, S.; Yu, Z. J. Org. Chem. 2003, 68, 6149. (b)Ma,S.;
Yu, Z. Angew. Chem. Int. Ed. 2003, 42, 1955. (c) Ma, S.;
Yu, Z.; Gu, Z. Chem. Eur. J. 2005, 11, 2351.
Synthesis 2006, No. 21, 3711–3714 © Thieme Stuttgart · New York