Vanadium-catalyzed asymmetric oxidation of sulfides using Schiff base ligands derived from ss-amino alcohols with two stereogenic centers

Article abstract of DOI:10.1002/ejoc.200900289

Novel Schiff base ligands derived from ss-amino alcohols with two stereogenic centers were prepared and. used in the preparation of optically pure sulfoxides by using aqueous hydrogen peroxide as the oxidant. A variety of sulfides were smoothly converted into the corresponding sulfoxides cata-lyzed by the chiral vanadium-Schiff base complex. Good yields (>80%) with excellent: enantioselectivities (>99%ee) were obtained in most cases.

Full text of DOI:10.1002/ejoc.200900289

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900289
Vanadium-Catalyzed Asymmetric Oxidation of Sulfides Using Schiff Base
Ligands Derived from β-Amino Alcohols with Two Stereogenic Centers
Yinuo Wu,[a] Juntao Liu,[a] Xingshu Li,*[a] and Albert S. C. Chan[a]
Keywords: Asymmetric catalysis / Vanadium / Schiff bases / Oxidation / Sulfur
Novel Schiff base ligands derived from β-amino alcohols with
two stereogenic centers were prepared and used in the prep-
aration of optically pure sulfoxides by using aqueous hydro-
gen peroxide as the oxidant. A variety of sulfides were
smoothly converted into the corresponding sulfoxides cata-
lyzed by the chiral vanadium–Schiff base complex. Good
yields (Ͼ80%) with excellent enantioselectivities (Ͼ99%ee)
were obtained in most cases.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
In recent years, some other metal catalysts were also in-
vestigated for asymmetric sulfoxidation. Bolm found that
an iron catalyst formed in situ by mixing Fe(acac)3 and
Schiff bases could efficiently catalyze this reaction.[10] By
adding some acids or carboxylates into the catalyst system,
the enantioselectivity increased to 66–96%.[11] Strukul re-
ported the chiral platinum diphosphane complex catalyzed
oxidation of sulfides in water with surfactants.[12] Very re-
cently, Katsuki et al. performed highly enantioselective sulf-
oxidation (up to 96%ee) in water by using a Fe(salalen)
complex.[13] They also carried out the reaction by using a
chiral Al(salalen) complex in the presence of a phosphate
buffer. Very high ee values were obtained for aryl methyl
sulfides (99%ee), but relatively lower enantioselectivities
were also observed for phenyl ethyl sulfide (90%ee) and
benzyl methyl sulfide (80% ee).[14]
Kinetic resolution of racemic sulfoxides or the oxidation/
kinetic resolution of prochiral sulfides for the preparation
of enantiomerically pure sulfoxides were also developed.
Uemura et al. demonstrated an accompanying kinetic reso-
lution of the sulfoxide enantiomers following the asymmet-
ric oxidation of sulfides in 1993.[15] Scettri et al. reported
the preparation of sulfoxides by the method of Uemura
with furyl hydroperoxide as the oxidant.[16] Chan et al. de-
scribed an improved method in which initial oxidation at
0 °C followed by resolution at 25 °C allowed the isolation
of sulfoxides in excellent enantioselectivities.[17] Jackson et
al. successfully developed vanadium-catalyzed sulfur oxi-
dation/kinetic resolution for the synthesis of chiral sulfox-
ides in chloroform.[18] Zeng et al. combined enantioselective
sulfoxidation with concomitant kinetic resolution to pro-
duce chiral sulfoxides with vanadium–Schiff base complexes
derived from phenylalaninol or isoleucinol as the catalyst;
moderate yields (40.6%) and up to 99%ee were obtained
by using H2O2 (2.0 equiv.) as the oxidant.[19]
Introduction
Enantiopure sulfoxides are valuable chiral auxiliaries
that are useful in organic synthesis as well as in the prepara-
tion of biologically active compounds.[1] The catalytic asym-
metric oxidation of sulfides is a convenient method for the
preparation of these valuable compounds.[2] Among all the
oxidants that have been used in asymmetric oxidation,
aqueous hydrogen peroxide (H2O2) is the most attractive,
as it is both economical and environmentally benign. These
unique features have stimulated the development of practi-
cal procedures for the asymmetric oxidation of sulfides with
hydrogen peroxide as the oxidant in recent years.[3] In 1995,
Bolm et al. revealed that catalysts prepared in situ from
VO(acac)2 and Schiff bases effectively catalyzed the oxi-
dation of sulfides to sulfoxides by using hydrogen peroxide
as the oxidant with good enantioselectivity.[4,5] The low cat-
alyst loading (1% equiv.) and the use of inexpensive hydro-
gen peroxide as the oxidant made it particularly attractive
for large-scale industrial applications. Many ligands and im-
proved methods were also developed on the basis of the
protocol outlined by Bolm. Berkessel et al. developed (S)-
t-leuciol-based Schiff base ligands from chiral salicylic alde-
hydes.[6] Skarzewski et al. introduced Schiff bases with a
phenyl substituent at C3 of salicylaldehyde.[7] Anson et al.
reported that a vanadium–chiral Schiff base complex de-
rived from 3,5-diiodosalicylaldehyde and (S)-t-leucinol was
effective in the vanadium-catalyzed oxidation of alkyl aryl
sulfides.[8] Ahn described the application of sterically hin-
dered chiral Schiff base ligands that were prepared from
chiral t-leucinol and aldehydes derived from binol for the
oxidation of sulfides.[9]
[a] School of pharmaceutical science, Sun Yat-Sen University,
Guangzhou 510006, P. R. China
Fax: +86-20-3994-3050
E-mail: lixsh@mail.sysu.edu.cn
Although a great deal of progress has been made in
asymmetric sulfoxidation, effective catalyst systems and
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2009, 2607–2610
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2607

Y. Wu, J. Liu, X. Li, A. S. C. Chan
SHORT COMMUNICATION
practical procedures providing high yields and excellent pared ligand 4 (Scheme 1), a diastereomer of ligand 3i, ac-
enantioselectivities are still highly desirable. Herein, we re- cording to a literature method.[27] Under the same reaction
port a highly enantioselective vanadium–Schiff base cata- conditions, ligand 4 afforded the product in moderate yield
lyst system that consists of VO(acac)2 and Schiff bases de- but with poor enantioselectivity (Table 1, Entry 12; 78%
rived from β-amino alcohols with two stereogenic centers yield, 5%ee).
for the preparation of optically pure sulfoxides with the use
of aqueous hydrogen peroxide as the oxidant.
Chiral β-amino alcohols and their derivatives are impor-
tant ligands in asymmetric synthesis including the enantio-
selective catalytic borane reduction of prochiral ketones,[20]
enantioselective addition of dialkylzinc,[21] asymmetric hy-
drogen transfer from alcohols to ketones,[22] and other
asymmetric reactions.[23] Generally, there are many differ-
ences in activity and enantioselectivity between the various
amino alcohols. Gau et al. prepared a series of N-sulfo-
nylated β-amino alcohols with one and two stereogenic cen-
ters and examined their application in asymmetric trialkyl-
aluminum additions, diethylzinc additions, and trimethyl-
silylcyanation to aldehydes.[24] Their results showed that li-
Scheme 1. Ligands for asymmetric oxidation of sulfides.
gands with two stereogenic centers are better than those
with only one stereogenic center. With regard to modifica-
Table 1. Influence of chiral ligands on the enantioselectivity of the
asymmetric oxidation of thioanisole.[a]
tions of Schiff base ligands derived from amino alcohols
for the enantioselective oxidation of sulfides, previous work
mainly focused on different β-amino alcohols with one
stereogenic center or on substitutions of the salicylaldehyde
moiety.[8] We wish to investigate the structural influence of
the β-amino alcohol moiety, especially those with two ste-
reogenic centers, to find a highly efficient catalyst system
for this reaction. Because phenylalanine is a very cheap,
commercially available material that can be modified to dif-
ferent target products with routine procedures,[25,26] we
chose it as the starting material for the screening of the
amino alcohols with two stereogenic centers for the enan-
tioselective oxidation of sulfides.
Entry
Ligand
Yield [%][b]
ee [%][c] and configuration[d]
1
2
3
4
5
6
7
8
1
83
85
97
87
97
80
59
98
54
97
98
78
43 (S)[d]
3 (S)
61 (S)
53 (S)
47 (S)
30 (S)
6 (S)
38 (S)
8 (S)
44 (S)
67 (S)
5 (R)
2
3a
3b
3c
3d
3e
3f
3g
3h
3i
9
10
11
12
Results and Discussion
4
In a preliminary screening of Schiff base ligands with two
stereogenic centers, we carried out the vanadium-catalyzed
oxidation of thioanisole in CH2Cl2 with H2O2 (1.1 equiv.)
as the oxidant. When Schiff base 1, derived from a commer-
cially available amino alcohol, was used as the ligand, good
yield with moderate enantioselectivity was obtained (83%
yield, 43%ee). With the expectation that the enantio-
[a] All reactions were carried out with 30% H2O2 (1.1 equiv.) for
24 h under ambient conditions. H2O2 (30.34%) was titrated by
using an iodometric method. [b] Isolated yield. [c] Determined by
HPLC analysis with a chiralcel OD-H column. [d] Absolute config-
uration determined by comparison of the optical rotation with the
literature value.
The result indicated that syn diastereomer 3i (S,R) ex-
selectivity might be substantially influenced by structural erted a stronger chiral discrimination effect than anti dia-
changes in the ligands, we prepared Schiff base ligand 2 and stereomer 4 (S,S) in the reaction. The solvent effect of the
ligands 3a–i (Scheme 1) according to a literature method VO(acac)2/3i system was examined. Methanol and THF
for evaluation.[25,26] Ligand 2, derived from a β-amino gave the product with very low ee values (5 and 6%ee,
alcohol with one stereogenic center, afforded poor enantio- respectively), and toluene afforded a moderate yield and
selectivity (3%ee). In contrast, ligand 3a, derived from a β- enantioselectivity. Because chloroform was another good
amino alcohol with two stereogenic centers, gave the desired solvent for this reaction and there were some examples
product with enhanced enantioselectivity (61%ee) at room showing more efficient kinetic resolution in this solvent,[18]
temperature. Inspired by this result, we investigated other ligands 3a–i were also examined in chloroform at 0 °C. The
ligands of this family (i.e., 3b–i) and found that ligand 3i results showed that ligand 3i was still the best ligand under
afforded nearly quantitative yield with 67%ee. To investi- these reaction conditions (82% yield and 84%ee were ob-
gate the effect of the stereochemistry of the Schiff base li- tained by using 1.1 equiv. 30% H2O2 as the oxidant). As
gands on the enantioselectivity of the product, we also pre- some thioanisole (starting material) still remained and no
2608
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2607–2610

Vanadium-Catalyzed Asymmetric Oxidation of Sulfides
sulfone was found in the reaction system, it appeared that selectivities were higher than 99% (Table 3, Entries 1–9).
no kinetic resolution took place under these reaction condi- Interestingly, phenyl ethyl sulfide, which usually gave rela-
tions.
tively lower enantioselectivities than methyl aryl sulfides in
We also investigated the kinetic resolution of racemic other excellent catalyst systems,[15] gave 84% yield with
phenyl methyl sulfoxide in chloroform with the use of 99%ee. The results of other alkyl aryl sulfides were also
0.65 equiv. of H2O2 as the oxidant and VO(acac)2/3i as the examined. Even in the case of naphthyl butyl sulfide, which
catalyst (Scheme 2). When the reaction was carried out at contains a larger alkyl group, an excellent ee value of 98%
0 °C for 48 h, 50% conversion with 73%ee was obtained (S was achieved.
= 13.8). The result implied that a higher ratio of hydrogen
peroxide in the asymmetric oxidation of sulfide would be
favorable for the ee values. We then adjusted the ratio of
Table 3. Substrate generality for VO(acac)2/3i-catalyzed asymmetric
oxidation of sulfides in chloroform.[a]
hydrogen peroxide/sulfide to 1.20, and obtained 91% yield
with 89%ee under the same reaction conditions. Good
yield (81%) with very high enantioselectivity (up to 99%ee)
were obtained with 1.35 equiv. of H2O2 as the oxidant. The
results showed that chloroform was the best choice of sol-
vent with VO(acac)2/3i as catalyst at 0 °C, and the optimal
ratio of hydrogen peroxide to sulfide was 1.35. It could be
concluded that the highly efficient asymmetric oxidation
and the following kinetic resolution gave good chemical
yield and excellent enantioselectivity when the oxidant was
used in excess. In contrast, it was found that a lower tem-
perature was favorable for higher chemical yields and ee
values. When the reaction temperature was lowered to
–10 °C, the ee value of the product was found to further
increase to 95% (Table 2, Entry 5).
Entry
Ar
R
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
10
Ph
Ph
Me
Et
81
84
83
85
81
84
85
81
80
81
99 (S)[d]
99 (S)
99 (S)
99 (S)
Ͼ99 (S)
Ͼ99 (S)
99 (S)
Ͼ99 (S)
Ͼ99
p-ClC6H5
p-CH3C6H5
p-BrC6H5
p-MeOC6H5
2-naphthyl
2-naphthyl
2-naphthyl
2-naphthyl
Me
Me
Me
Me
Me
Et
nPr
nBu
98
[a] All reactions were carried out with VO(acac)2 (1 mol-%) and
ligand 3i (1 mol-%) for 48 h under ambient conditions. [b] Isolated
yields. [c] Determined by HPLC analysis with a chiralcel OD-H
column. [d] Absolute configuration determined by comparison of
the optical rotation with the literature value.
To the best of our knowledge, among all the catalyst sys-
tems used for the asymmetric oxidation of sulfides with hy-
drogen peroxide as the oxidant, this catalyst system is the
best in terms of good yield and excellent enantioselectivity.
Scheme 2. Kinetic resolution of racemic sulfides.
Table 2. Optimization of reaction conditions for the VO(acac)2/3i-
catalyzed asymmetric oxidation of thioanisole.[a]
Conclusions
Novel Schiff bases derived from β-amino alcohols with
two stereogenic centers have been established as highly ef-
fective chiral ligands for the production of chiral sulfoxides.
A variety of sulfides were converted into the corresponding
sulfoxides smoothly with good yields and excellent enantio-
selectivities. The easy preparation of the ligands from natu-
ral amino acids and their excellent performance make them
good choices for practical applications. Further studies on
the scope of this reaction are under way.
Entry
Solvent
T (°C)
H2O2 [equiv.] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
CHCl3
CH2Cl2
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
r.t.
0
0
0
–10
0
1.1
1.1
1.1
1.2
1.2
1.3
1.35
89
93
82
93
91
86
81
66
54
84
89
95
97
99
0
[a] All reactions were carried out with VO(acac)2 (1 mol-%) and
ligand 3i (1 mol-%) for 48 h. [b] Isolated yields. [c] Determined by
HPLC analysis with a chiralcel OD-H column. The absolute con-
figuration (S) was determined by comparison of the optical rota-
tion with the literature value.
Experimental Section
General Procedure for Asymmetric Oxidation of Sulfides: A test
tube was charged with VO(acac)2 (0.01 mmol), chiral ligand 3i
(0.015 mmol) and CHCl3 (2 mL). The mixture was stirred for 0.5 h
at room temperature. The sulfide substrate (1 mmol) was then
added, and the mixture was stirred for another 10 min. After the
solution was cooled to 0 °C, 30 % H2O2 (1.1 equiv.) was slowly
added. The mixture was stirred smoothly (200 rpm) at 0 °C for
Under the optimal reaction conditions, expanding the
scope of the catalyst system to other alkyl aryl sulfides was
performed, and the results are listed in Table 3. All sub-
strates gave high yields, and in most cases the enantio- 48 h, and the reaction was monitored by TLC. After the reaction
Eur. J. Org. Chem. 2009, 2607–2610
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2609

Y. Wu, J. Liu, X. Li, A. S. C. Chan
SHORT COMMUNICATION
arz, R. Siedlecka, M. Zielinska-Blajet, B. Pisarski, J. Chem.
Res. 2001, 263–264.
[8] B. Pelotier, M. S. Anson, I. B. Campbell, S. J. F. Macdonald,
G. Priem, R. F. W. Jackson, Synlett 2002, 1055–1060.
[9] Y. C. Jeong, S. Choi, Y. D. Hwang, K. H. Ahn, Tetrahedron
Lett. 2004, 45, 9249–9252.
[10] J. Legros, C. Bolm, Angew. Chem. 2004, 116, 4321–4324; An-
gew. Chem. Int. Ed. 2004, 43, 4225–4228.
[11] J. Legros, C. Bolm, Angew. Chem. 2003, 115, 5645–5647; An-
gew. Chem. Int. Ed. 2003, 42, 5487–5489.
[12] A. Scarso, G. Struku, Adv. Synth. Catal. 2005, 347, 1227–1234.
[13] H. Egami, T. Katsuki, J. Am. Chem. Soc. 2007, 129, 8940–8941.
[14] T. Yamaguchi, K. Matsumoto, B. Saito, T. Katsuki, Angew.
Chem. Int. Ed. 2007, 46, 4729–4731.
was complete, the aqueous layer was separated off, and the solvent
was removed under reduced pressure. The residue was purified by
flash column chromatography on silica gel (30% ethyl acetate in
petroleum ether) to give the desired product. The enantiomeric ex-
cess was determined by chiral HPLC on a chiralcel OD-H column
[hexane/isopropyl alcohol (9:1); 0.5 mLmin–1, 254 nm].
Supporting Information (see footnote on the first page of this arti-
cle): Preparative methods and characterization of the chiral ligands;
procedures for asymmetric oxidation of sulfides; NMR spectra and
HPLC behavior of the products; [α]D values and absolute configu-
rations.
[15] N. Komatsu, M. Hashizume, T. Sugita, S. Uemura, J. Org.
Chem. 1993, 58, 7624–7626.
[16] A. Massa, F. R. Siniscalchi, V. Bugatti, A. Lattanzi, A. Scettri,
Tetrahedron: Asymmetry 2002, 13, 1277–1283.
[17] X. Jia, X. Li, L. Xu, Y. Li, Q. Shi, T. Au-Yeung, C. Yip, X.
Yao, A. Chan, Adv. Synth. Catal. 2004, 346, 723–726.
[18] C. Drago, L. Caggiano, R. F. W. Jackson, Angew. Chem. Int.
Ed. 2005, 44, 7221–7223.
Acknowledgments
We are grateful for the financial support of the National Science
Foundation of China (20472116) and the Science and Technology
Foundation of Guangzhou (07A8206031).
[19] Q. Zeng, H. Wang, T. Wang, Y. Cai, W. Weng, Y. Zhao, Adv.
Synth. Catal. 2005, 347, 1933–1936.
[1] a) H. Pellissier, Tetrahedron 2006, 62, 5559–5601; b) I.
Fernández, N. Khiar, Chem. Rev. 2003, 103, 3651–3706; c) S. A.
Blum, R. G. Bergman, J. A. Ellman, J. Org. Chem. 2003, 68,
150–155; d) S. Renfrey, J. Featherstone, Nat. Rev. Drug Discov.
2002, 1, 175; e) P. Renaud, M. Gerster, Angew. Chem. Int. Ed.
1998, 37, 2562–2579; f) M. C. Carreño, Chem. Rev. 1995, 95,
1717–1760.
[2] a) H. B. Kagan, T. O. Luukas, “Catalytic Asymmetric Sulfide
Oxidations” in Transition Metals for Organic Synthesis (Eds.:
M. Beller, C. Bolm), 2nd ed., Wiley-VCH, Weinheim, Ger-
many, 2004, vol. 2, pp. 479–495; b) C. Bolm, Coord. Chem. Rev.
2003, 237, 245–256.
[20] a) S. Itsuno, M. Nakano, K. Miyazaki, H. Masuda, K. Ito, A.
Hirao, S. J. Nakahama, J. Chem. Soc. Perkin Trans. 1 1985,
2039; b) E. J. Corey, R. K. Bakshi, S. Shibata, J. Am. Chem.
Soc. 1987, 109, 5551–5553; c) E. J. Corey, M. Azimioata, S.
Sarshar, Tetrahedron Lett. 1992, 33, 3435–3438; d) L. Deloux,
M. Screbnik, Chem. Rev. 1993, 93, 763–784; e) D. J. Ager, I.
Prakash, D. R. Schaad, Chem. Rev. 1996, 96, 835–875; f) J.
Martens, I. Reiners, Tetrahedron: Asymmetry 1997, 8, 27–28;
g) E. J. Corey, C. J. Helal, Angew. Chem. Int. Ed. 1998, 37,
1986–2012.
[21] a) M. Kitamura, S. Suga, K. Kawai, R. Noyori, J. Am. Chem.
Soc. 1986, 108, 6071–6072; b) M. Kossenjans, J. Martens, Tet-
rahedron: Asymmetry 1998, 9, 1409–1417.
[22] a) J. Takehara, S. Hashiguch, A. Fuji, S. I. Inoue, T. Ikariya,
R. Noyori, Chem. Commun. 1996, 2, 233–234; b) K. Soai, T.
Hayasaka, S. Ugajin, J. Chem. Soc., Chem. Commun. 1989, 8,
516–517.
[23] a) K. Tomioka, M. Shindo, K. Koga, Chem. Pharm. Bull. 1989,
34, 1120–1122; b) A. Ando, T. Shoir, J. Chem. Soc., Chem.
Commun. 1987, 9, 656–658.
[24] a) J. You, H. Gau, M. C. K. Choi, Chem. Commun. 2000, 19,
1963–1964; b) J. You, S. Hsieh, H. Gau, Chem. Commun. 2001,
17, 1546–1547; c) J. You, M. Shao, H. Gau, Tetrahedron: Asym-
metry 2001, 12, 2971–2975.
[25] G. Jalce, M. Seck, X. Franck, R. Hocquemiller, B. Figadere, J.
Org. Chem. 2004, 69, 3240–3241.
[26] M. T. Reetz, M. W. Drewes, A. Schmitz, Angew. Chem. 1987,
99, 1186–1188; Angew. Chem. Int. Ed. Engl. 1987, 26, 1141–
1143.
[3] For some examples of asymmetric sulfoxidation with the use
of H2O2 as terminal oxidant, see: a) C. Drago, L. Caggiano,
R. F. W. Jackson, Angew. Chem. Int. Ed. 2005, 44, 7221–7223;
b) J. Legros, C. Bolm, Chem. Eur. J. 2005, 11, 1086–1092; c) J.
Sun, C. Zhu, Z. Dai, M. Yang, Y. Pan, H. Hu, J. Org. Chem.
2004, 69, 8500–8503; d) D. J. Weix, J. A. Ellman, Org. Lett.
2003, 5, 1317–1320; e) V. V. Thakur, A. Sudalai, Tetrahedron:
Asymmetry 2003, 14, 407–410; f) T. Miyazaki, T. Katsuki, Syn-
lett 2003, 1046–1048; g) C. Ohta, H. Shimizu, A. Kondo, T.
Katsuki, Synlett 2002, 161–163; h) B. Saito, T. Katsuki, Tetra-
hedron Lett. 2001, 42, 3873–3876; i) P. B. Page, J. P. Heer, D.
Bethell, B. L. Lund, Phosphorus Sulfur Silicon Relat. Elem.
1999, 153–154; P. B. Page, J. P. Heer, D. Bethell, B. L. Lund,
Phosphorus Sulfur Silicon Relat. Elem. 1999, 247–258.
[4] C. Bolm, F. Bienewald, Angew. Chem. 1995, 107, 2883–2885;
Angew. Chem. Int. Ed. Engl. 1995, 34, 2640–2642.
[5] C. Bolm, G. Schlingloff, F. Bienewald, J. Mol. Catal. A 1997,
117, 347–350.
[6] A. H. Vetter, A. Berkessel, Tetrahedron Lett. 1998, 39, 1741–
1744.
[7] a) J. Skarzewski, E. Ostrycharz, R. Siedlecka, Tetrahedron:
Asymmetry 1999, 10, 3457–3461; b) J. Skarzewski, E. Ostrych-
[27] J. M. Andres, R. Barrio, M. A. Martinez, R. Pedrosa, A. Perez-
Encabo, J. Org. Chem. 1996, 61, 4210–4213.
Received: March 18, 2009
Published Online: April 20, 2009
2610
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 2607–2610