A mesoporous-silica-immobilized oxovanadium cocatalyst for the lipase-catalyzed dynamic kinetic resolution of racemic alcohols

Article abstract of DOI:10.1002/anie.201208988

V for resolution: A new oxovanadium catalyst (V-MPS; see scheme) immobilized in the pores of mesoporous silica has been developed. The combined use of V-MPS and lipases achieved the dynamic kinetic resolution of a wide range of racemic alcohols (1 or 2) t

Full text of DOI:10.1002/anie.201208988

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DOI: 10.1002/anie.201208988
Heterogeneous Catalysis
A Mesoporous-Silica-Immobilized Oxovanadium Cocatalyst for the
Lipase-Catalyzed Dynamic Kinetic Resolution of Racemic Alcohols**
Masahiro Egi, Koji Sugiyama, Moriaki Saneto, Ryosuke Hanada, Katsuya Kato, and Shuji Akai*
In the last decade, cooperative catalysis has received consid-
erable attention as a powerful synthetic method.[1] Two or
more catalysts function simultaneously or sequentially in
a single reaction vessel to construct complicated molecules,
which provides a means to perform unprecedented syntheses
that cannot be achieved by a single catalyst. Various catalytic
combinations involving transition metals, organocatalysts,
and biocatalysts have been developed thus far.[2]
the hydroxy group of 1 or 2, while the lipases effected chemo-
and enantioselective esterification. This is significantly differ-
ent from the above-mentioned ruthenium-catalyzed DKRs
and offered a synthetic advantage in that both (Æ)-1 and (Æ)-
2 were available as equivalent substrates. However, this
method required further improvement in both catalytic
activity and compatibility of the oxovanadium catalysts with
the lipases.[7,8] Herein, we report the preparation of a novel
oxovanadium catalyst (V-MPS) immobilized inside meso-
porous silica (MPS) with pores of approximately 3 nm in
diameter, which enabled a complete division of the racemi-
zation site and the enzymatic reaction site. The combined
lipase–V-MPS catalyst is reusable and achieved DKR of
a wide range of racemic alcohols with excellent chemical and
optical yields (Scheme 1).
A typical example is the combined use of lipases and
transition metals to attain the dynamic kinetic resolution
(DKR) of racemic secondary alcohols for producing single
enantiomer products in up to 100% yields,[3] in contrast to the
use of lipases alone, which can only achieve maximum yields
of 50%. In this DKR process, the enzymatic enantioselective
esterification of racemic alcohols is combined with the
transition-metal-catalyzed continuous racemization of opti-
cally active alcohols, which remain intact during the enzy-
matic reaction, through a redox process. However, such
cooperative cocatalysis often encounters crucial issues of low
compatibility between the lipases and the transition metals.
Although intense efforts have been devoted to developing
highly active racemization catalysts,[4,5] only a few ruthenium
complexes have met both the requirement of sufficient
compatibility with lipases and high racemization activity.[5]
We recently reported that a combination of oxovanadium
compounds (4 or 5) with lipases accomplished the efficient
and direct conversion of racemic allylic alcohols (Æ)-1 and
(Æ)-2 into optically active allyl esters (R)-3.[6] This method
featured a unique racemization process wherein 4 (or 5)
catalyzed the racemization of (S)-1 with 1,3-transposition of
Scheme 1. Basic concept for DKR by a combination of lipases and
oxovanadium compounds.
The immobilization of oxovanadium species inside a solid
carrier with microsized pores or multilayered structures[9]
enables the minimization of interactions between the oxo-
vanadium species and lipases while maintaining easy access of
substrate molecules to the metal center. The solid carrier
should be neutral and non-charged in order to exert little
adverse effect on the lipases. Among the various potential
solid carriers,[10,11] MPS, which is comprised of amorphous
silica and has a rigid well-ordered hexagonal structure with
uniform pore size,[12] is thought to be one of the best
candidates, in particular, MPS with a pore size of 2.7 nm.[13]
We therefore prepared a novel vanadium catalyst (V-MPS) by
=
4 and MPS (pore diameter
[*] Dr. M. Egi, K. Sugiyama, M. Saneto, R. Hanada, Prof. Dr. S. Akai
School of Pharmaceutical Sciences, University of Shizuoka
52-1, Yada, Suruga-ku, Shizuoka, Shizuoka 422-8526 (Japan)
E-mail: akai@u-shizuoka-ken.ac.jp
Dr. K. Kato
National Institute of Advanced Industrial Science and Technology
(AIST), 2266-98 Anagahora, Shimo-shidami, Moriyama-ku, Nagoya,
Aichi 463-8560 (Japan)
[**] We are grateful to Prof. Hironao Sajiki of Gifu Pharmaceutical
University (Japan) for his useful advice in clarifying the structure of
V-MPS. This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Organic Synthesis Based on
Reaction Integration” and a Grant-in-Aid for Scientific Research (B).
We acknowledge Amano Enzyme, Inc. and Roche Diagnostics K.K.
for supplying the lipases, and Taiyo Kagaku Co. Ltd. and Fuji Silysia,
Chemical Ltd. for providing MPS and MacroPSs. We are also
grateful to Prof. Masahiro Sakata and Dr. Satoshi Mitsunobu of the
University of Shizuoka (Japan) and the N.E. Chemcat Corporation
for measuring ICP, and to JEOL for measuring XPS.
combining O V(OSiPh3)3
2.7 nm)[14] in refluxing benzene for 8 h to anchor vanadium
species on the inner surface of the MPS pores.[15,16] The
resulting structure was characterized using several techniques
and deduced to be as shown in Figure 1, where the oxovana-
dium moiety covalently bound to the inner surface of MPS
possessed a single triphenylsiloxy group on average (see the
Supporting Information for details). The loading of the
vanadium component onto MPS was determined to be 0.20–
0.22 mmolgÀ1 by ICP analysis. For comparison, two other
immobilized oxovanadium catalysts, V-MacroPS(100) and
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208988.
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Chemie
Figure 1. Deduced structure of V-MPS.
V-MacroPS(400), were prepared using macroporous silica
(MacroPS), which possesses larger pore sizes of 100 and
400 nm, respectively.
The racemization activities of the immobilized catalysts
were evaluated using (R)-1a (> 99% ee) in MeCN at 358C
(Figure 2a) to find that the use of either 1 or 2 mol% of
V-MPS completed the racemization within 2 h, whereas
2 mol% of V-MacroPS(100) or V-MacroPS(400) was much
less reactive. Moreover, the known catalyst V(3)-MPS, which
=
was prepared by the reaction of O V(OiC3H7)3 and MPS
(2.7 nm), and in which the oxovanadium is immobilized
through three V OSi covalent bonds,[17] was less reactive than
À
V-MPS.[18] These results demonstrate that the mesoporous
environment seems to enhance the catalyst activity and that
the use of 4 as a precursor is another critical point in
producing a highly active catalyst. Furthermore, the racemi-
zation activity of V-MPS (1 mol%) was greater than that of 4
and 5 (10 mol% each; Figure 2b). Despite such high activity,
V-MPS did not cause the adverse side reaction of 1a
dimerization.[6b,19] The V-MPS-catalyzed racemization pro-
ceeded smoothly, even in less polar solvents such as heptane
(Figure 2b), which indicates that the V-MPS–lipase cocata-
lyzed DKR could be conducted in a variety of solvents.
V-MPS also showed excellent catalytic activity in DKR
reactions, thus demonstrating its excellent compatibility with
lipases. The reaction of (Æ)-2a with commercially available
immobilized Candida antarctica lipase B (CAL-B; 3.0 w/w),
V-MPS (1.0 mol%), and vinyl acetate (2.0 equiv) in MeCN at
358C, completed within 12 h. After centrifugation, (R)-3a
(99% yield, 99% ee) was obtained from the supernatant. As
triphenylsilanol was not detected in the crude reaction
product, V-MPS appeared to retain its structure after the
reaction. The centrifuged deposit containing a mixture of two
catalysts was dried under reduced pressure for 10 h at room
temperature and reused in the next run.[20] The high catalytic
performance of the mixed catalysts was completely sustained
up to the sixth run, affording (R)-3a in quantitative chemical
and optical yields. In the seventh run, the DKR reaction
provided (R)-3a (85% yield, 99% ee) and (S)-4-phenyl-3-
buten-2-ol (13% yield, 67% ee), which meant that V-MPS
Figure 2. Racemization of (R)-1a (>99% ee) by oxovanadium cata-
*
lysts. a) Comparison of immobilized catalysts. V-MPS (1 mol%; ),
&
*
V-MPS (2 mol%; ), V-MacroPS(100) (2 mol%; ), V-MacroPS(400)
~
(2 mol%; ), V(3)-MPS (2 mol%; ꢀ). b) Comparison of V-MPS, 4, and
*
5, and that of solvents. V-MPS (1 mol%; ) in MeCN, V-MPS
~
^
(1 mol%; ) in heptane, 4 (10 mol%; &) in MeCN, 5 (10 mol%; ) in
MeCN.
Scheme 2. Reusability of the V-MPS-lipase combo catalyst.
lost part of its activity (Scheme 2). The leaching of any
vanadium components was evaluated to be < 0.0003% by ICP
spectroscopy of the crude products, which indicates that the
binding between the vanadium moiety and the silanol of MPS
is sufficiently strong.
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Table 1: Asymmetric synthesis of optically active esters 3 from (Æ)-1 or (Æ)-2 through lipase–V-MPS cocatalysis.[a]
Entry
Substrate
Lipase/Solvent[b]
Product
Yield [%]
ee [%]
1
(Æ)-2a: R=H
A
(R)-3a: R=H
99
99
>99[c]
94[d]
98[e]
99
98[c]
71[d]
79[e]
100
64[d]
64[e]
96
2
3
(Æ)-2b: R=F
(Æ)-2c: R=Cl
A
A
(R)-3b: R=F
(R)-3c: R=Cl
93[d]
80[e]
97
52[d]
74[e]
98[d]
94[e]
92
99
4
(Æ)-2d
A
(R)-3d
(E/Z=7:1)
5[f,g]
6[g]
7[g]
8
(Æ)-1e: R1 =R2 =H
B
B
B
B
(S)-3e: R1 =R2 =H, R4 =nC9H19
93
96
97
98
>99
>99
99
(Æ)-1 f: R1 =H, R2 =OMe
(Æ)-1g: R1 =R2 =OMe
(S)-3 f: R1 =H, R2 =OMe, R4 =nC9H19
(S)-3g: R1 =R2 =OMe, R4 =nC9H19
(S)-3h: R1 =OMe, R2 =OTBS, R4 =Me
(Æ)-1h: R1 =OMe, R2 =OTBS
99
9[g]
(Æ)-1i
B
B
(S)-3i
(R)-3j
84
98
>99
10
(Æ)-1j
96
96
99
11[f]
12
(Æ)-2k
(Æ)-1l
A
A
(R)-3k
(R)-3l
68[e]
95[e]
94
95
78[d]
92[d]
13[h]
14[h]
(Æ)-2m: R1 =nC4H9
(Æ)-2n: R1 =nC11H23
A
A
(R)-3m: R1 =nC4H9, R4 =nC3H7
(R)-3n: R1 =nC11H23, R4 =nC3H7
82
>99
>99
96[e]
99
85
65[e]
95
15
16
17
(Æ)-2o: R1 =Ph
A
B
A
(R)-3o: R1 =Ph, R4 =Me
78[d]
97[d]
(Æ)-1p
(R)-3p
(R)-3q
90
97
100
97[c]
54[d]
99
>99[c]
97[d]
(Æ)-1q
18[g]
19[g]
(Æ)-1r
(Æ)-1s
A
A
(R)-3r
(R)-3s
98
96
97
99
[a] Unless otherwise noted, reactions were carried out using 0.20–1.2 mmol of substrates. [b] A: CAL-B in MeCN was used, B: PS-IM in heptane was
used. [c] Performed using 20 mmol of (Æ)-2a or (Æ)-1q for 12 h (entry 1) or for 10 h (entry 17). [d] 4 (10 mol%) was used instead of V-MPS. [e] 5
(10 mol%) was used instead of V-MPS. [f] Conducted at 508C. [g] Vinyl decanoate was used instead of vinyl acetate. [h] Vinyl butyrate was used
instead of vinyl acetate. TBS=tert-butyldimethylsilyl.
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The V-MPS-lipase combo catalysis was applied to allylic
alcohols (Æ)-1 and (Æ)-2, which were treated with V-MPS
(1.0 mol%), an immobilized lipase such as CAL-B or
Burkholderia cepacia lipase (PS-IM), and vinyl acetate in an
organic solvent such as MeCN or heptane, at 358C for 24 h to
give optically active allylic acetates 3. The results in Table 1
demonstrate that this method allowed for excellent substrate
generality and delivered high product yields and enantio-
selectivities (3a–p; entries 1–16), as well as bringing about
significant improvements in the results obtained under our
previous conditions using 4 or 5[6] (entries 1–3, 11, 12, 14, and
15). This method was also applied to a 20 mmol scale reaction
without any loss in reactivity or yield (entries 1 and 17). In
some cases, the use of acyl donors with a longer alkyl chain,
such as vinyl butyrate and vinyl decanoate, produced higher
enantioselectivities (entries 5–7, 9, 13, and 14). In the DKRs
of 1e–j, our method provided synthetically useful products,
(S)-3e–i and (R)-3j containing functional groups as R3
(entries 5–10). Optically pure (R)-3n, the key synthetic
intermediate of (+)-tanikolide, which employed the less
expensive cyclohexenone as the starting material, was pro-
duced in twice the previously obtained yield (entry 14).[21] The
kinetic resolution of (Æ)-1p using immobilized Pseudomonas
cepacia (recently classified as Burkholderia cepacia) lipase
gave (R)-3p (49%, > 99% ee) and (S)-1p (48%, > 99% ee),
which are useful chiral building blocks for the preparation of
bioactive aza-sugars.[22] Our DKR method converted (Æ)-1p
into (R)-3p in 90% yield and 97% ee (entry 16). Moreover, it
was applicable to benzyl 1q, furyl 1r, and propargyl alcohols
1s to give 3q–s in excellent yield and enantioselectivity,
whereas the previous conditions using 4 or 5[6] were relatively
ineffective (entries 17–19). In particular, neither rearrange-
ment nor oxidation was observed in the reaction of 1s, in
contrast to related reactions using other oxometal catalysts.[23]
The efficiency of this method was further demonstrated by
the asymmetric synthesis of (R)-imperanene (Scheme 3).[24]
The combined use of V-MPS and PS-IM for (Æ)-8, which was
through the use of different kinds of hydrolases is now in
progress.
In conclusion, we have prepared a new oxovanadium
catalyst (V-MPS) immobilized inside mesoporous silica with
pores of approximately 3 nm in diameter. V-MPS divides the
racemization site and the enzymatic reaction site to achieve
excellent compatibility with lipases and effective catalyst
sustainability. Furthermore, V-MPS exhibited higher racemi-
zation activity on optically active allyl and benzylic alcohols
than other oxovanadium compounds (4 and 5) or the related
oxovanadium catalysts V-MacroPS(100), V-MacroPS(400),
and V(3)-MPS. The combination of V-MPS and lipases served
as a powerful catalyst for the DKR of various racemic
alcohols to give optically active esters in excellent yield and
enantioselectivity. The application of this catalyst combina-
tion to a wider range of alcohols and the use of other
hydrolases as cocatalysts are under investigation in our
laboratory.
Received: November 9, 2012
Revised: January 18, 2013
Published online: February 19, 2013
Keywords: alcohols · dynamic kinetic resolution · lipases ·
.
mesoporous silica · vanadium
[1] a) M. Rueping, R. M. Koenigs, I. Atodiresei, Chem. Eur. J. 2010,
16, 9350; b) C. Zhong, X. Shi, Eur. J. Org. Chem. 2010, 2999;
c) A. E. Allen, D. W. C. MacMillan, Chem. Sci. 2012, 3, 633;
d) N. T. Patil, V. S. Shinde, B. Gajula, Org. Biomol. Chem. 2012,
10, 211.
[2] a) K. Baer, M. Kraußer, E. Burda, W. Hummel, A. Berkessel, H.
Grçger, Angew. Chem. 2009, 121, 9519; Angew. Chem. Int. Ed.
2009, 48, 9355; b) B. Westermann, M. Ayaz, S. S. van Berkel,
Angew. Chem. 2010, 122, 858; Angew. Chem. Int. Ed. 2010, 49,
846; c) D. E. A. Raup, B. Cardinal-David, D. Holte, K. A.
Scheidt, Nat. Chem. 2010, 2, 766; d) B. M. Trost, X. Luan, Y.
Miller, J. Am. Chem. Soc. 2011, 133, 12824; e) S. Zhou, S.
Fleischer, K. Junge, M. Beller, Angew. Chem. 2011, 123, 5226;
Angew. Chem. Int. Ed. 2011, 50, 5120, and references therein.
[3] a) A. Kamal, M. A. Azhar, T. Krishnaji, M. S. Malik, S. Azeeza,
Coord. Chem. Rev. 2008, 252, 569; b) J. H. Lee, K. Han, M.-J.
Kim, J. Park, Eur. J. Org. Chem. 2010, 999; c) M. C. Warner, C. P.
Casey, J.-E. Bꢀckvall, Top. Organomet. Chem. 2011, 37, 85.
[4] a) J.-E. Bꢀckvall, J. Organomet. Chem. 2002, 652, 105; b) S.
Wuyts, D. E. De Vos, F. Verpoort, D. Depla, R. De Gryse, P. A.
Jacobs, J. Catal. 2003, 219, 417; c) C. Roengpithya, D. A.
Patterson, E. J. Gibbins, P. C. Taylor, A. G. Livingston, Ind.
Eng. Chem. Res. 2006, 45, 7101; d) Y. Ahn, S.-B. Ko, M.-J. Kim, J.
Park, Coord. Chem. Rev. 2008, 252, 647.
[5] a) L. K. Thalꢁn, A. Sumic, K. Bogꢂr, J. Norinder, A. K. ꢃ.
Persson, J.-E. Bꢀckvall, J. Org. Chem. 2010, 75, 6842; b) A. Trꢀff,
R. Lihammer, J.-E. Bꢀckvall, J. Org. Chem. 2011, 76, 3917; c) H.
Kim, Y. K. Choi, J. Lee, E. Lee, J. Park, M.-J. Kim, Angew. Chem.
2011, 123, 11136; Angew. Chem. Int. Ed. 2011, 50, 10944.
[6] a) S. Akai, K. Tanimoto, Y. Kanao, M. Egi, T. Yamamoto, Y.
Kita, Angew. Chem. 2006, 118, 2654; Angew. Chem. Int. Ed.
2006, 45, 2592; b) S. Akai, R. Hanada, N. Fujiwara, Y. Kita, M.
Egi, Org. Lett. 2010, 12, 4900.
Scheme 3. Application of DKR to the synthesis of (R)-imperanene.
synthesized from vanillin in 95% overall yield, produced (S)-
9 in 88% yield, which was converted into (S)-10. Treatment of
(S)-10 with Grignard reagent 11 at À808C in Et2O led to
regio- and stereoselective nucleophilic addition to the oxir-
ane, thus forming (R)-12. Final deprotection by KOSiMe3
gave optically pure (R)-imperanene (62% overall yield from
vanillin). The preparation of the naturally occurring S isomer
[7] In some cases, the DKR yields were not sufficient, mainly due to
very slow racemization. Further prolonging the reaction time
and/or the use of more active oxovanadium catalysts resulted in
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a loss of catalytic activity in both the lipases and vanadium
catalysts because of catalyst interaction.[6b]
V-MPS with different properties, and the use of this alternative
V-MPS for DKR led to the formation of unidentified side
products. X-ray photoelectron spectroscopy (XPS) analysis of
the alternative V-MPS revealed the contamination of at least
three different vanadium components.
[8] Complex reactor devices have been used for the DKR of
alcohols, in which lipases and racemization catalysts were
mechanically separated, see: a) P. ꢄdman, L. A. Wessjohann,
U. T. Bornscheuer, J. Org. Chem. 2005, 70, 9551; b) S. Wuyts, J.
Wahlen, P. A. Jacobs, D. E. De Vos, Green Chem. 2007, 9, 1104.
[9] The concept of site isolation has been proposed for avoiding the
undesired interaction of two reactive reagents, such as acids and
bases, by spatial separation, see: a) B. Voit, Angew. Chem. 2006,
118, 4344; Angew. Chem. Int. Ed. 2006, 45, 4238, and references
therein; b) F. Gelman, J. Blum, D. Avnir, J. Am. Chem. Soc. 2002,
124, 14460; c) R. K. Zeidan, S.-J. Hwang, M. E. Davis, Angew.
Chem. 2006, 118, 6480; Angew. Chem. Int. Ed. 2006, 45, 6332.
[10] For the development of the covalent immobilization of homo-
geneous catalysts onto insoluble supports thus far, see: a) Q.-H.
Fan, Y.-M. Li, A. S. C. Chan, Chem. Rev. 2002, 102, 3385; b) A. F.
Trindade, P. M. P. Gois, C. A. M. Afonso, Chem. Rev. 2009, 109,
418; c) M. J. Climent, A. Corma, S. Iborra, Chem. Rev. 2011, 111,
1072.
=
[16] Our preliminary studies revealed that O V(OiC3H7)3 was less
reactive for racemization than 4, and therefore, we chose 4 as the
vanadium precursor. The difference in their activity is somewhat
attributed to the pKa value of each ligand (17 for iC3H7OH, 13.5
for Ph3SiOH).
[17] G. Du, S. Lim, M. Pinault, C. Wang, F. Fang, L. Pfefferle, G. L.
Haller, J. Catal. 2008, 253, 74.
[18] The effect of V-MPS, V-MacroPS(100), V-MacroPS(400), and
V(3)-MPS was compared by the DKR of (Æ)-1a to find that
V-MPS performed the best (see the Supporting Information for
details).
[19] The increase in catalytic activity for the 1,3-isomerization of
allylic alcohols was accompanied by the formation of biallyl
ethers, see: C. Morrill, G. L. Beutner, R. H. Grubbs, J. Org.
Chem. 2006, 71, 7813.
[11] Lipase-catalyzed DKR using zeolite as a racemization catalyst
has been reported, albeit using only 1-phenylethanol. This
method was partly accompanied by the formation of styrene
during the racemization, see: Y. Zhu, K.-L. Fow, G.-K. Chuah, S.
Jaenicke, Chem. Eur. J. 2007, 13, 541.
[20] As another experiment, the recovered mixture of V-MPS and
lipase was used for the next run without any treatment to afford
(R)-3a in high yield and ee (see the Supporting Information).
However, slightly better results were obtained by drying the
recovered catalysts.
[12] a) Y. Wan, D. Zhao, Chem. Rev. 2007, 107, 2821; b) N. Linares, E.
Serrano, M. Rico, A. M. Balu, E. Losada, R. Luque, J. Garcꢅa-
Martꢅnez, Chem. Commun. 2011, 47, 9024.
[13] For the good compatibility of lipases and MPS, see: K. Kato, R.
Irimescu, T. Saito, Y. Yokogawa, H. Takahashi, Biosci. Biotech-
nol. Biochem. 2003, 67, 203.
[14] At the beginning of this project, we synthesized MPS according
to a previous report, see: K. Kato, M. Suzuki, M. Tanemura, T.
Saito, J. Ceram. Soc. Jpn. 2010, 118, 410. Later, it was provided
by Taiyo Kagaku Co. Ltd.
[15] A similar reaction for 24 h did not change the catalytic activity of
V-MPS. In contrast, reaction in refluxing toluene generated
[21] H. Fujioka, S. Matsuda, M. Horai, E. Fujii, M. Morishita, N.
Nishiguchi, K. Hata, Y. Kita, Chem. Eur. J. 2007, 13, 5238.
[22] H. Ouchi, Y. Mihara, H. Takahata, J. Org. Chem. 2005, 70, 5207.
[23] V. Cadierno, P. Crochet, S. E. Garcꢅa-Garrido, J. Gimeno, Dalton
Trans. 2010, 39, 4015.
[24] For asymmetric total syntheses of imperanene, see: a) J. C.
Shattuck, C. M. Shreve, S. E. Solomon, Org. Lett. 2001, 3, 3021;
b) M. P. Doyle, W. Hu, M. V. Valenzuela, J. Org. Chem. 2002, 67,
2954; c) H. M. L. Davies, Q. Jin, Tetrahedron: Asymmetry 2003,
14, 941; d) J. A. Carr, K. S. Bisht, Org. Lett. 2004, 6, 3297; e) Y.
Takashima, Y. Kobayashi, J. Org. Chem. 2009, 74, 5920.
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