A route to 5-substituted dibenzofurans by anionic cycloaromatization of 2-(6-substituted 3-hexen-1,5-diynyl)phenyl tert-butyldimethyl ethers and related molecules

Article abstract of DOI:10.1002/1521-3773(20021104)41:21<4077::AID-ANIE4077>3.0.CO;2-Z

An anionic cycloaromatization reaction provides an efficient method for the synthesis of the dibenzofurans 2 by treatment of the tert-butyldimethylsilyl ethers 1 with sodium methoxide or potassium carbonate in methanol at reflux for 16 h, and gives 50-94%

Full text of DOI:10.1002/1521-3773(20021104)41:21<4077::AID-ANIE4077>3.0.CO;2-Z

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TMS
A Route to 5-Substituted Dibenzofurans by
Anionic Cycloaromatization of 2-(6-substituted
3-hexen-1,5-diynyl)phenyl tert-butyldimethyl
ethers and Related Molecules**
Cl
Cl
[Pd(PPh3)4], CuI
[Pd(PPh3)4], CuI
+
nBuNH2, Et2O
nBuNH2, Et2O
Cl
4
5 (51%)
3
Ming-Jung Wu,* Chia-Ying Lee, and Chi-Fong Lin
I
[Pd(PPh3)4], CuI
It is well-known that in a certain period of the life cycle of
lichens or living-plant wood tissues a number of unique
antimicrobial compounds are produced. The production of
such compounds is thought to be a result of the secondary
metabolism or as a part of their dynamic defense systems.[1]
The biphenyl- or dibenzofuran-containing phytoalexins show
manifold biological activities and have attracted much
attention for chemical syntheses and biological studies.[2]
The dibenzofuran skeletons were usually generated by an
acid-catalyzed ring-closure reaction of 2,2’-dihydroxybiphenyl
compounds[3] or by cyclization of diaryl ethers with palla-
dium.[4] Recently, we reported the anionic cycloaromatization
of enediynes promoted by nucleophilic addition to prepare a
variety of aromatic compounds.[5] We believe that 2-(6-
substituted 3(Z)-hexen-1,5-diynyl)phenols (1) could undergo
intramolecular anionic cycloaromatization to give dibenzo-
furans (2) under alkaline conditions. [Eq. (1)]
+
nBuNH2, Et2O
OH
R
O
6 R=TMS (71%)
8
K2CO3
MeOH
9 (7%)
7a R=H (86%)
Scheme 1. Synthesis of the benzofuran 9 froma dichloroethene precursor.
TMS ¼ trimethylsilyl.
We anticipated that the protection of the phenolic proton,
following the treatment of the product with a base would
allow us to synthesize dibenzofurans. Thus, 2-(3(Z)-decen-1,5-
diynyl)phenyl tert-butyldimethylsilyl ether (11a; Scheme 2)
was prepared by palladium-catalyzed coupling reaction of
(Z)-3-decen-1,5-diyne (7a) with 2-iodophenyl tert-butyldime-
thylsilyl ether (10) in 57% yield. Treatment of compound 11a
R
[Pd(PPh3)4]
R
I
I
CuI
Et3N
base, heat
+
TBSCl
+ 7a
(1)
nBuNH2
CH2Cl2
OH
O Si
Et2O
O
8
OH
10 (80%)
2
1
An attempt to synthesize compound 1 was carried out by
using cis-1,2-dichloroethene (3; Scheme 1) as a starting
material. The palladium-catalyzed coupling reaction of 3 with
1-hexyne (4) under Sonogashira reaction conditions[6] gave
the vinyl chloride 5 in 51% yield. Compound 5 was then
coupled with trimethylsilylacetylene under the same reaction
conditions to give the enediyne 6 in 71% yield. Desilylation of
6 was carried out by treatment of 6 with potassiumcarbonate
in dry methanol to form 7a in 86% yield. Finally, the
enediyne 7a was coupled with 2-iodophenol (8) using tetra-
kis(triphenylphosphane)palladiumas a catalyst to give the
benzofuran 9 in 7% yield and a recovered 50% yield of the
starting phenol 8 (Scheme 1). The failure to obtain the desired
2-(3(Z)-decen-1,5-diynyl)phenol (1a) may be attributed to
the acidic proton of the phenol, which could cause the acid-
catalyzed cyclization to give the benzofuran 9 instead of the
dibenzofuran 2a.
Na, MeOH
reflux
O
O Si
2a (60%)
11a (57%)
Scheme 2. Palladium-catalyzed coupling of 10 and 7a, and subsequent
cyclization of the product 11a (see Table 1).
with sodium methoxide methanol heated under reflux for 16 h
gave, after column chromatography, 5-butyldibenzofuran (2a)
in 60% yield.
After the synthesis of dibenzofuran by the described
method, we turned our attention to testing the generality of
this cyclization reaction. Thus, various enediynylphenyl tert-
butyldimethylsilyl ethers 11b j (Table 1) were prepared.[7]
Treatment of 11b j with sodium methoxide under the same
reaction conditions gave compounds 2b j in 50 94% yields.
Other reaction conditions have also been examined for this
cyclization reaction to synthesize dibenzofurans. The results
show that treatment of 11b and 11e with potassiumcarbonate
in refluxing methanol gave the dibenzofurans 2b and 2e,
respectively, in yields comparable to those obtained by
Method A.
[*] M.-J. Wu, C.-Y. Lee, C.-F. Lin
School of Chemistry
Kaohsiung Medical University
Kaohsiung (Taiwan)
Fax : (þ 886)7-312-5339
E-mail: mijuwu@cc.kmu.edu.tw
Compound 11k was prepared from p-tert-butylphenol.[8]
Treatment of 11k with sodiummethoxide in refluxing
[**] We would like to thank the National Science Council of the Republic
of China for financial support.
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Table 1. Generation of 5-substituted dibenzofurans and related molecules.
Tetrahedron Lett. 1976, 42, 3801; c) T. M. Konjin, J. G. C. van de Meene,
J. T. Bonner, D. S. Barkley, Biochemistry 1967, 58, 1152; d) S. Wang,
J. E. Hall, F. A. Tanious, W. D. Wilson, D. A. Partick, D. R. McCurdy,
B. C. Bender, R. R. Tidwell, Eur. J. Med. Chem. 1999, 34, 215.
[3] a) T. Yamato, C. Hideshima, G. K. S. Prakach, G. A. Olah, J. Org.
Chem. 1991, 56, 3192; b) O. Kruber, Ber. Dtsch. Chem. Ges. A 1932, 65,
1413; c) H. Gilman, J. Swiss, L. C. Cheney, J. Am. Chem. Soc. 1940, 62,
1963; d) A. P. Gray, V. M. Dipinto, I. J. Solomon, J. Org. Chem. 1976,
41, 2428.
R
X
R
methods
X
X
reflux 16 h
X
O
Si
O
Conditions
Yield
[4] a) R. van Helden, G. Verberg, B. Balder, Recl. Trav. Chim. Pays-Bas
1965, 84, 1263; b) J. M. Davidson, C. Triggs, J. Org. Chem. Soc. A, 1968,
1324; c) H. Iataaki, H. Yoshimoto, J. Org. Chem. 1973, 38, 76; d) F. R. S.
Clarke, R. P. C. Norman, C. B. Thomas, J. S. Wilson, J. Chem. Soc.
Perkin Trans. 1 1974, 38, 1289; e) B. Akermark, L. Eberson, E. Jonsson,
E. Petterson, J. Org. Chem. 1975, 40, 1365; f) H. Hagelin, J. D. Oslob, B.
Akermark, Chem. Eur. J. 1999, 5, 2413; g) S. D. Lombaert, L.
Blanchard, L. B. Stamford, J. Tan, E. M. Wallace, Y. Satoh, J. Fitt, D.
Hoyer, D. Simonsbergen, J. Moliterni, N. Marcopoulos, J. Med. Chem.
2000, 43, 488; h) M. V. Sargent, P. O. Stransky, J. Chem. Soc. Perkin
Trans. 1 1982, 7, 1605; i) J. A. Elix, J. L. Parker, Aust. J. Chem. 1987, 40,
187 192; j) J. A. Elix, R. Naidu, J. R. Laundon, Aust. J. Chem. 1992, 45,
785.
Method A: Na, MeOH
X, X ¼
H, H
11b
R ¼ C5H11
2b (56%)
2c (50%)
2d (57%)
2e (93%)
2 f (94%)
2g (92%)
2h (91%)
2i (75%)
2j (75%)
11c
11d
11e
11 f
11g
11h
11i
R ¼ C7H15
R ¼ C3H6OTHP
R ¼ C4H9
X, X ¼
R ¼ C5H11
R ¼ C7H15
R ¼ C3H6OTHP
R ¼ C4H9
X, X ¼
11j
R ¼ C5H11
Method B: K2CO3, MeOH
[5] a) M. J. Wu, L. J. Chang, L. M. Wei, C. F. Lin, Tetrahedron 1999, 55,
13193; b) M. J. Wu, C. F. Lin, S. H. Chen, Org. Lett. 1999, 1, 767.
[6] K. Sonogashira, T. Yatake, Y. Tohda, S. Takahashi, N. Hagihara, Chem.
Commun. 1977, 291.
11b
11e
2b (56%)
2e (93%)
THP ¼ tetrahydropyran
[7] Characterization data: 2a: 1H NMR (CDCl3, 200 MHz): d ¼ 7.98 (dd,
J ¼ 7.4, 1.8 Hz, 1H), 7.51 7.32 (m, 4H), 7.13 (dd, J ¼ 6.6, 1.6 Hz, 1H),
3.15 (t, J ¼ 7.4 Hz, 2H), 1.84 1.72 (m, 2H), 1.57 1.26 (m, 2H), 1.00 ppm
(t, J ¼ 7.2 Hz, 3H); 13C NMR (CDCl3, 50 MHz): d ¼ 156.3, 156.0, 138.6,
126.9, 126.4, 124.3, 123.0, 122.6, 122.3, 122.2, 111.5, 109.0, 33.5, 31.9, 22.7,
14.0 ppm; HRMS (EI) calcd for C16H16O 224.1201, found 224.1207. 2b:
1H NMR (CDCl3, 200 MHz): d ¼ 7.98 (dd, J ¼ 7.8, 2.0 Hz, 1H), 7.60 (dd,
J ¼ 7.8, 1.6 Hz, 1H), 7.50 7.32 (m, 4H), 7.13 (dd, J ¼ 7.0, 1.8 Hz, 1H),
3.13 (t, J ¼ 7.6 Hz, 2H), 1.85 1.77 (m, 2H), 1.56 1.38 (m, 4H), 0.92 ppm
(t, J ¼ 7.6 Hz, 3H); 13C NMR (CDCl3, 50 MHz): d ¼ 156.2, 156.0, 138.7,
126.9, 126.5, 124.6, 123.0, 122.6, 122.3, 122.1, 111.5, 109.1, 33.8, 31.9, 29.5,
22.6, 14.1 ppm; HRMS (EI) calcd for C17H18O 238.1358, found
238.1360. 2c: 1H NMR (CDCl3, 200 MHz): d ¼ 8.01 (dd, J ¼ 8.0,
1.2 Hz, 1H), 7.71 (d, J ¼ 7.6 Hz, 1H), 7.63 (dd, J ¼ 7.6 Hz, 1H), 7.56
7.41 (m, 2H), 7.32 7.21 (m, 2H), 4.61 (t, J ¼ 3.2 Hz, 1H), 3.96 3.85 (m,
2H), 3.61 3.47 (m, 2H), 2.69 (t, J ¼ 6.4 Hz, 2H), 2.04 1.84 (m, 2H),
1.75 1.48 ppm(m, 6H); 13C NMR (CDCl3, 50 MHz): d ¼ 156.4, 156.0,
138.7, 126.9, 126.4, 124.3, 123.0, 122.6, 122.3, 122.2, 111.5, 109.0, 33.8,
31.9, 29.7, 29.6, 22.6, 21.0, 14.2 ppm; HRMS (EI) calcd for C20H22O3
310.1569, found 310.1570. 2d: 1H NMR (CDCl3, 200 MHz): d ¼ 7.96
(dd, J ¼ 7.8, 1.0 Hz, 1H), 7.59 (dt, J ¼ 7.6, 1.0 Hz, 1H), 7.57 7.32 (m, 4H),
7.13 (dd, J ¼ 7.6, 1.4 Hz, 1H), 3.13 (t, J ¼ 7.6 Hz, 2H), 1.85 1.73 (m, 2H),
1.53 1.23 (m, 8H), 0.90 ppm (t, J ¼ 7.8 Hz, 3H); HRMS (EI) calcd for
C19H22O 266.1671, found 266.1672. 2e: 1H NMR (CDCl3, 400 MHz):
d ¼ 8.02 (dd, J ¼ 8.0, 1.2 Hz, 1H), 7.73 (d, J ¼ 1.2 Hz, 1H), 7.62 (dd, J ¼
7.6, 0.8 Hz, 1H), 7.55 7.51 (m, 2H), 7.36 (td, J ¼ 8.0, 1.6 Hz, 1H), 7.31
7.22 (m, 3H), 2.57 (t, J ¼ 6.8 Hz, 2H), 1.73 1.67 (m, 2H), 1.66 1.50 (m,
2H), 0.99 ppm(t, J ¼ 7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz): d ¼
154.1, 154.0, 134.0, 131.1, 129.2, 127., 127.6, 126.4, 124.5, 122.8, 121.2,
120.5, 111.0, 105.4, 96.1, 80.5, 30.5, 22.0, 19.4, 13.6 ppm; HRMS (EI)
calcd for C20H18O 274.1358, found 274.1346. 2 f: 1H NMR (CDCl3,
200 MHz): d ¼ 8.06 (dd, J ¼ 7.0, 0.8 Hz, 1H), 7.78 (s, 1H), 7.67 7.54 (m,
3H), 7.46 7.23 (m, 4H), 2.58 (t, J ¼ 7.0 Hz, 3H), 1.78 1.61 (m, 2H),
1.61 1.28 (m, 4H), 0.96 ppm (t, J ¼ 7.4 Hz, 3H); 13C NMR (CDCl3,
50 MHz): d ¼ 154.1, 154.0, 134.0, 131.1, 129.2, 127.7, 127.6, 126.4, 124.5,
122.7, 121.1, 120.5, 110.9, 105.4, 96.2, 80.5, 31.2, 28.2, 22.2, 19.8,
13.9 ppm; HRMS (EI) calcd for C21H20O 288.1515, found 288.1513. 2g:
1H NMR (CDCl3, 200 MHz): d ¼ 8.03 (dd, J ¼ 7.2, 1.2 Hz, 1H), 7.75 (s,
1H), 7.74 7.52 (m, 4H), 7.44 7.21 (m, 4H), 2.56 (t, J ¼ 7.0 Hz, 2H), 1.76
1.68 (m, 2H), 1.56 1.27 (m, 8H), 0.90 (t, J ¼ 7.0 Hz, 3H); 13C NMR
(CDCl3, 50 MHz): d ¼ 154.2, 154.1, 134.0, 131.1, 129.2, 127.8, 127.6,
126.4, 124.5, 122.7, 121.2, 120.6, 111.0, 105.4, 96.2, 80.5, 31.7, 29.0, 28.9,
28.5, 22.6, 19.8, 14.0 ppm; HRMS (EI) calcd for C23H24O 316.1828,
methanol gave 2k in 51% yield. On the other hand, treatment
of 11k with potassiumcarbonate in refluxing methanol gave
2k in 65% yield. [Eq. (2)]
methods
(2)
Si
O
O
2k (51%, by Method A)
2k (65%, by Method B)
11k
In conclusion, we have presented an efficient method for
the synthesis of a series of 5-substituted dibenzofurans and
related molecules in moderate to good yields by anionic
cycloaromatization of enediynes.
Experimental Section
General procedure for methanolysis of 11 (Method A): Freshly cut sodium
metal (5 mmol) was added to a solution of 2-(6-substituted 3(Z)-hexen-1,5-
diynyl)phenyl tert-butyldimethylsilyl ethers (1 mmol) in 10 mL of meth-
anol, the solution was heated to reflux and stirred for 16 h. After cooling to
room temperature, the methanol was removed in vacuum. Saturated
NaCl(aq.) was added to the residue, and extracted with EtOAc. The
combined organic layer was dried over anhydrous MgSO4(s). After
filtration and removal of solvent, the residue was purified by column
chromatography to give the separated products. Method B: The reaction
conditions were the same as described in Method A, but the sodium was
replaced with potassium carbonate (5 mmol).
Received: May 7, 2002 [Z19258]
[1] a) H. Miyagawa, M. Yamashita, T. Veno, N. Hamada, Phytochemistry
1997, 46, 1289; b) T. Kokubun, J. B. Harbone, Phytochemistry 1995, 40,
1649; c) T. Kokubun, J. B. Harbone, J. Eagles, P. G. Waterman,
Phytochemistry 1995, 39, 1033.
[2] a) H. R. Morris, G. W. Taylor, M. S. Masento, K. A. Jermyn, R. R. Kay,
Nature 1987, 328, 811; b) H. Abe, M. Vchiyama, Y. Tanaka, H. Saito,
1
found 316.1823. 2h: H NMR (CDCl3, 400 MHz): d ¼ 8.42 (dt, J ¼ 8.0,
1.2 Hz, 1H), 8.21 (dd, J ¼ 8.0, 0.8 Hz, 1H), 7.91 (dd, J ¼ 7.6, 1.2 Hz, 1H),
7.73 (dt, J ¼ 8.0, 0.8 Hz, 1H), 7.61 7.37 (m, 5H), 4.68 (t, J ¼ 4.0 Hz, 1H),
4078
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3.98 3.91 (m, 2H), 3.60 3.53 (m, 2H), 3.40 3.35 (m, 2H), 2.21 2.14 (m,
2H), 1.93 1.57 ppm(m, 6H); 13C NMR (CDCl3, 100 MHz): d ¼ 155.9,
152.2, 135.3, 133.0, 127.6, 126.1, 125.7, 125.5, 124.8, 122.9, 122.2, 121.9,
120.8, 120.0, 118.4, 111.7, 98.8, 66.8, 62.1, 30.9, 30.6, 29.6, 25.4, 19.5 ppm;
HRMS (EI) calcd for C24H24O3 224.1201, found 224.1207. 2i: 1H NMR
(CDCl3, 200 MHz): d ¼ 8.49 (s, 1H), 8.06 (s, 1H), 7.93 7.87 (m, 1H), 7.82
(s, 1H), 7.79 7.46 (m, 5H), 7.36 7.24 (m, 2H), 2.60 (t, J ¼ 6.8 Hz, 2H),
1.77 1.37 (m, 4H), 1.00 ppm (t, J ¼ 7.0 Hz, 3H); 13C NMR (CDCl3,
50 MHz): d ¼ 154.5, 154.2, 134.0, 132.4, 132.3, 132.0, 131.3, 128.3, 127.9,
127.2, 127.1, 126.8, 126.8, 125.9, 124.6, 122.8, 121.2, 118.3, 110.9, 105.8,
30.6, 29.7, 19.5, 13.6 ppm; HRMS (EI) calcd for C24H20O 324.1515,
found 324.1512. 2j: 1H NMR (CDCl3, 200 MHz): d ¼ 8.50 (s, 1H), 8.07
(s, 1H), 7.84 (s, 1H), 7.80 7.29 (m, 8H), 2.60 (t, J ¼ 6.8 Hz, 2H), 1.83
1.22 (m, 6H), 0.95 (t, J ¼ 6.8 Hz, 3H); 13C NMR (CDCl3, 50 MHz): d ¼
154.3, 154.1, 134.1, 132.4, 132.3, 132.0, 131.4, 128.3, 127.3, 127.1, 126.9,
126.8, 126.5, 125.8, 124.6, 122.8, 121.2, 119.0, 111.0, 105.8, 31.3, 29.6,
22.3, 19.8, 14.0 ppm; HRMS (EI) calcd for C25H22O 338.1671, found
338.1667. 2k: 1H NMR (CDCl3, 200 MHz): d ¼ 7.99 (t, J ¼ 1.2 Hz, 1H),
7.52 (d, J ¼ 1.4 Hz, 2H), 7.44 7.31 (m, 2H), 7.11 (dd, J ¼ 6.8, 1.4 Hz, 1H),
3.15 (t, J ¼ 7.6 Hz, 2H), 1.89 1.74 (m, 2H), 1.65 1.50 (m, 2H), 1.45 (s,
9H), 1.03 ppm(t, J ¼ 7.6 Hz, 3H); 13C NMR (CDCl3, 50 MHz): d ¼
156.7, 154.2, 145.6, 138.4, 126.6, 124.1, 123.9, 122.9, 122.5, 118.7, 110.7,
109.0, 34.7, 33.8, 32.0, 31.8, 22.9, 14.0 ppm; HRMS (EI) calcd for
C20H24O 280.1828, found 280.1816.
The HfCl4-Mediated Diels Alder Reaction of
Furan**
Yujiro Hayashi,* Masahiko Nakamura,
Shigehiro Nakao, Tae Inoue, and Mitsuru Shoji
7-Oxabicyclo[2.2.1]hept-2-ene derivatives are useful inter-
mediates for the synthesis of natural products such as
carbohydrates and prostaglandins.[1] One of the most straight-
forward methods for the construction of the 7-oxabicy-
clo[2.2.1]hept-2-ene skeleton is the Diels Alder reaction
between furan and appropriate dienophiles. However, the
facile retro-Diels Alder reaction and the low reactivity of
furan as a diene, as a result of its aromatic character, make the
Diels Alder reaction of furan one of the most difficult
cycloadditions.[2] In addition to the use of highly reactive
dienophiles in the Diels Alder reaction,[3] several methods
have been developed to overcome these difficulties, such as
the use of high pressure[4] or Lewis acid mediated reactions.[5]
Although several Lewis acids have been reported to promote
the reaction efficiently, there are problems in terms of
generality. For example, BF3¥OEt2 is a good catalyst for
methyl acrylate but a poor promoter for other dienopliles,[5c]
ZnI2 is suitable for acrylonitrile but not for a,b-unsaturated
esters,[5a] while methyl vinyl ketone and acrylonitrile are
activated by BiCl3.[5l] Some Lewis acids supported on silica gel
have also been utilized for the promotion of a particular
dienophile with furan.[5e,g,i,j] However, low endo/exo selectivity
is generally obtained because of the facile retro-Diels Alder
reaction. Herein we report the endo-selective Diels Alder
reaction of furan with a,b-unsaturated esters catalyzed by
HfCl4.
[8] Treatment of p-tert-butylphenol with bis(pyridium)iodonium(i)tetra-
fluoroborate (Ipy2BF4) gave 2-iodo-4-tert-butylphenol in 31% yield.
Compound 12k was treated with tert-butyldimethylsilyl chloride using
imidazole as a base to give 13k in 91% yield. Finally, compound 13k
Cl Si
IPy2BF4
I
I
HBF4
CH2Cl2
imidazole
CH2Cl2
OH
OTBS
OH
12k (31%)
13k ( 91%)
7a
[Pd(PPh3)4]
nBuNH2
First, we looked for an appropriate Lewis acid using the
reaction of furan and dimethyl maleate as a model and
employing furan as the solvent (40 equiv). The reaction was
performed in the presence of an equimolar amount of Lewis
acid at roomtemperature for 15 h. Of the several Lewis acids
screened,[6] HfCl4 was found to have suitable Lewis acidity to
promote the Diels Alder reaction in moderate yield (60%).[7]
Although most of the reported Lewis acids lose their Lewis
acidity by coordination with furan, which acts as a Lewis base,
HfCl4 still activates a,b-unsaturated esters efficiently even in
the presence of an excess amount of furan. Next, the use of a
solvent was examined, and CH2Cl2 was found to be the best
with respect to both yield and endo/exo selectivity.[8] For
example, the Diels Alder reaction of dimethyl maleate and
furan proceeds in CH2Cl2 at ꢀ208C within 5 h to afford the
cycloadduct in good yield (91%) and high diastereoselectivity
CuI, Et2O
OTBS
11k (38%)
was coupled with 7a using palladiumas a catalyst to give 11k (TBS ¼
tert-butyldimethylsilyl) in 38% yield.
[*] Prof. Dr. Y. Hayashi, M. Nakamura, S. Nakao, T. Inoue, Dr. M. Shoji
Department of Industrial Chemistry
Faculty of Engineering
Tokyo University of Science
Kagurazaka, Shinjuku-ku, Tokyo 162-8601 (Japan)
Fax : (þ 81)3-5261-4631
E-mail: hayashi@ci.kagu.tus.ac.jp
[**] This research was supported by the Asahi Glass Foundation and a
Grant-in-Aid for Scientific Research on Priority Areas (A) ™Exploi-
tation of Multi-Element Cyclic Molecules∫ from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
Angew. Chem. Int. Ed. 2002, 41, No. 21
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