A novel bifunctional sulfonamide primary amine-catalyzed enantioselective conjugate addition of ketones to nitroolefins

Article abstract of DOI:10.1002/adsc.200800445

The enantioselective conjugate addition of a variety of ketones to nitroolefins has been developed. The process is efficiently catalyzed by a novel bifunctional sulfonamide primary amine in good yields and with good levels of enantioselectivity.

Full text of DOI:10.1002/adsc.200800445

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DOI: 10.1002/adsc.200800445
A Novel Bifunctional Sulfonamide Primary Amine-Catalyzed
Enantioselective Conjugate Addition of Ketones to Nitroolefins
Fei Xue,a Shilei Zhang,a,* Wenhu Duan,a,b,* and Wei Wanga,c,*
a
Department of Pharmaceutical Sciences, School of Pharmacy, East China University of Science & Technology, Shanghai
200237, Peopleꢀs Republic of China
E-mail: zlslo3@yahoo.com.cn, whduan@mail.shcnc.ac.cn
Shanghai Institute of Materia Medica, Shanghai Institutes of Biological Sciences, Chinese Academy of Sciences, Graduate
b
School of the Chinese Academy of Sciences, Shanghai 201203, Peopleꢀs Republic of China
Department of Chemistry and Chemical Biology, University of New Mexico, MSC03 2060, Albuqueruqe, NM 87131-0001,
c
USA
Fax : (+1)-505-277-2609; e-mail: wwang@unm.edu
Received: July 20, 2008; Revised: September 3, 2008; Published online: October 1, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800445.
Abstract: The enantioselective conjugate addition
of a variety of ketones to nitroolefins has been de-
veloped. The process is efficiently catalyzed by a
novel bifunctional sulfonamide primary amine in
good yields and with good levels of enantioselectivi-
ty.
addition reactions.[7] The notable feature of primary
amine catalysts is that they can activate aromatic ke-
tones[4c,e] and a,a-disubstituted aldehydes.[4d] Jacobsen
and co-workers have developed an elegant highly
enantioselective conjugate addition of acyclic keto-
nes[4c] and a,a-disubstituted aldehydes[4d] to nitroal-
kenes by applying a chiral primary amine-thiourea
catalyst. Herein we wish to report a new simple bi-
functional sulfonamide primary amine organocatalyst
that promotes Michael addition reactions while
achieving a respected level of enantioselectivity. Sig-
nificantly, the processes are applicable to a variety of
ketones as Michael donors.
Keywords: conjugate addition; ketones; nitroole-
fins; organocatalysis; sulfonamide primary amine
Organocatalytic asymmetric reactions have proved to
be efficient and powerful tools in organic synthesis.[1]
We have developed (S)-pyrrolidine trifluorometh-
anesulfonamide I as a general organocatalyst for pro-
A
Among them, Michael addition reactions, one of the moting a variety of asymmetric organic transforma-
most important and versatile processes for the synthe- tions with high efficiency (Figure 1).[8] As demonstrat-
À
À
sis of new C C and C X bonds, have been intensively ed, in many cases, the catalyst is superior to proline in
studied with organocatalysis.[2] The catalysts typically terms of catalytic activity and stereoselectivity toward
used for these processes are proline and its analogue- organic processes. It is believed that its high catalytic
derived secondary amines.[3] It is noted that, generally, efficiency is attributable to its unique activation
good enantio- and diastereoselectivity is obtained mode,[8d,g] arising from the trifluoromethanesulfon-
only for aldehydes and cyclohexanone as nucleo-
A
philes. However, poor catalytic activity and stereose- forces for activation of substrates and formation of a
lectivity are observed for acyclic ketones.[3] It is more well-controlled transition state. Inspired by the
problematic for aromatic ketones, and generally no proven ability of the sulfonamide group in highly ef-
reactions occur. Therefore, the development of new fective asymmetric conjugate addition reactions,[8d,g,i]
organocatalysts which are able to catalyze these diffi-
cult substrates is an important, but formidable and
challenging task in order to significantly expand the
scope of organocatalyzed conjugate addition reac-
tions.
Recently, primary amine catalysts have emerged to
be effective promoters for organic processes including
Michael addition,[4] aldol,[5] a-aminations[6] and cyclo- Figure 1. Bifunctional sulfonamide amine organocatalysts.
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Enantioselective Conjugate Addition of Ketones to Nitroolefins
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Table 1. Optimization of reaction conditions.
Entry
Solvent
Additive
Time [h]
Yield[a] [%]
ee[b] [%]
1
2
3
4
5
6
7
8
DMF
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
TEA
AcOH
H2O
30
18
63
34
53
72
53
68
63
0
93
92
84
92
87
58
93
À13
À24
25
17
10
DMSO
CH3CN
acetone
THF
iPrOH
EtOAc
H2O
CH2Cl2
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
156
144
240
84
240
72
156
72
72
72
54
54
72
À7
46
nd[c]
65
9
10
11
12
13
14[d]
15[e]
71
72
72
65
87
70
none
none
H2O
[a]
Isolated yield.
[b]
[c]
[d]
[e]
Determined by chiral HPLC analysis (Chiralpak AS-H).
Not determined.
Reaction was performed at 08C.
Reaction was performed at 0.5M of 2a.
we envisioned that the incorporation of the moiety additives revealed that they played a certain role in
into chiral trans-cyclohexane diamine scaffold governing reaction yields and enantioselectivity (en-
a
would result in a new bifunctional sulfonamide pri- tries 10–13). Interestingly, no matter what additives
mary amine organocatalyst II.[9] The catalyst could be were used, better outcomes (entries 10–12) were ob-
useful for effecting the Michael addition of ketones to tained than those without an additive (entry 13).
nitroolefins.
Water as an additive was beneficial to the process in
To probe the feasibility of II-catalyzed direct conju- terms of reaction yield and enantioselectivity
gate addition of ketones to nitroolefins, a model reac- (entry 12). It is believed that water can increase the
tion between acetone and trans-b-nitrostyrene was ex- catalyst turnover presumably due to facilitating the
plored (Table 1). Initially, the reaction was carried out release of the primary catalyst from the imine.[10] On
by using 15 mol% of catalyst II with triethylamine the other hand, based on our early studies,[8d,g] it was
(TEA) as an additive in various solvents. In polar found that water could enhance the enantioselectivity
aprotic solvents, such as DMF and DMSO (Table 1, of chiral trifluoromethanesulfonamide amine-cata-
entries 1 and 2), the process was completed in a rela- lyzed reactions because the trifluoromethanesulfon-
tively short time but with low or moderate yield and
A
poor enantioselectivity. It was also found that other activation of substrates and formation of a well-con-
solvents, such as CH3CN, acetone, THF, i-PrOH and trolled transition state. One of H-bond networks in-
EtOAc, were not suitable for the process (entries 3– volves water, the “O” of SO2 and the carbonyl group
7); in these cases a long reaction time was required to of the substrate.[8d,g] Lowering the reaction tempera-
obtain acceptable yields, alebit with unsatisfied ees ture led to an improvement of the enantioselectivity
(7–46%). Water as reaction medium was probed for but with a significant loss of yield (entry 14). These
the reaction, and no reaction took place (entry 8). To studies prompted us to select water as an additive and
our delight, the conjugate addition reaction proceed- room temperature as optimized reaction conditions to
ed well in high yields (> 90%) and moderate enantio- probe the scope of the conjugate addition processes.
selectivity (65% and 71% ee respectively, entries 9 The absolute stereoconfiguration of the product 3a
and 10) with CH2Cl2 and CHCl3 as solvent.
was determiend based on the comparision of the opti-
Further optimization of reaction conditions using cal rotation of its known counterpart.[11]
CHCl3 as solvent was carried out next. A survey of
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Fei Xue et al.
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Table 2. Scope of II-catalyzed conjugate addition of ketones
Table 2. (Continued)
to nitrostyrenes.[a]
Entry Product
Yield[b] [%] ee[c] [%]
11
12
88
86
71
Entry Product
1
Yield[b] [%] ee[c] [%]
78
75
92
72
13
94
2
3
4
5
84 (25)[d]
80 (93)[d]
14[e]
33[f]
96 (syn); 85 (anti)
15[g]
46
82
64
94
65
87
85
84
16[h]
33[i]
87 (syn)
[a]
Unless specified, see experimental section for reaction
conditions.
Isolated yield.
Determined by chiral HPLC analysis (Chiralcel AS-H,
AD-H and OD-H).
Reaction was carried out in an ice bath.
[b]
[c]
[d]
[e]
Conditions:
3-pentanone
(0.6 mL),
nitroolefin
(0.30 mmol), cat. (30 mol%), CHCl3 (1.2 mL) and water
(15 mol%), at room temperature for 72 h.
syn:anti=1:1, determined by H NMR analysis.
[f]
1
6
7
8
80
87
93
57
68
69
[g]
Conditions:
acetophenone
(0.2 mL),
nitroolefin
(0.20 mmol), cat. (15 mol%), CHCl3 (0.4 mL), at room
temperature for 72 h.
[h]
[i]
Conditions:
1-indanone
(0.2 mmol),
nitroolefin
(0.10 mmol), cat. (30 mol%), CHCl3 (0.2 mL), at room
temperature for 120 h.
1
syn : anti=3:1, determined by H NMR analysis.
As summarized in Table 2, generally, the II-cata-
lyzed Michael addition reactions proceeded smoothly
with a wide range of ketones and nitroalkenes. First,
the conjugate additions of acetone to a variety of ni-
troolefins were examined (Table 2, entries 1–13). It
was found that electronic nature of the substituents of
the nitroolefin had an effect on enantioselectivity.
High enantioselectivities (80–87% ee, entries 2–5)
were achieved with electron-rich substituents, while
those bearing electron-deficient substituents gave
only moderate enantioselectivities (57–69% ee, en-
tries 6–8). On the other hand, steric hindrance had
9
67
89
57
69
10
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Enantioselective Conjugate Addition of Ketones to Nitroolefins
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yield: 19 mg (92%); 72% ee, determined by HPLC (Chiral-
pak AS-H, i-PrOH/hexane=40/60, flow rate 1.0 mLminÀ1,
l=210 nm, 258C): tminor=8.2 min,
+183.3 (c 1.0 in CHCl3).
little effect on the conjugate addition processes. Sub-
stituents at the ortho-position of the aromatic systems
in nitroolefins had a limited impact on enantioselec-
tivity (57–71% ee, entries 9–11). Heteroaromatic ni-
troolefins were also good Michael acceptors and the
reactions provided high yields and good enantioselec-
tivities (entries 12 and 13).
Next, some challenging ketones were investigated
as Michael donors for the II-catalyzed conjugate addi-
tion reactions. It is realized that the addition of 3-pen-
t
major =10.4 min; [a]2D3:
Acknowledgements
This work was supported by Shanghai Basic Research Proj-
ect from the Shanghai Science and Technology Commission
tanone to nitrostyrene has puzzled chemists for a long (Grants 06JC14080) and start-up fund from School of Phar-
macy, East China University of Science & Technology.
time. Poor enantioselectivities were obtained by using
various pyrrolidine-based secondary amines as cata-
lysts. However, the reaction worked very well with
primary amine catalysts according to the results re-
ported by Jacobsen[4a] and Connon.[4g] We demonstrat-
ed that our catalytic system was also good for the con-
jugate addition reaction to give high enantioselectivi-
ties (96% and 85% ee for syn and anti isomers respec-
tively) in spite of poor diastereoselectivity (1:1 dr)
and yield (entry 14). The conjugate addition of aro-
matic ketones to nitrostyrene was another formidable
challenge to chemists, and until now, only two catalyt-
ic systems were efficient in this reaction.[4a,c] We were
glad to find that the catalyst II also enabled us to pro-
mote the addition of acetophenone to nitrostyrene in
moderate yield and high enantioselectivity (entry 15).
Encouraged by this result, we further first probed 1-
indanone as a substrate, which has not been explored
for the Michael addition reaction. To our delight, the
process took place efficiently and afforded the desired
product with moderate dr (3:1) and high ee
(entry 16).
References
[1] For recent reviews of asymmetric organocatalysis, see:
a) A. Berkessel, H. Groger, Asymmetric Organocataly-
sis-From Biomimetic Concepts to Applications in Asym-
metric Synthesis, Wiley-VCH, Weinheim, 2005; b) P. I.
Dalko, L. Moisan, Angew. Chem. Int. Ed. 2004, 43,
5138; c) Special Issue on Asymmetric Organocatalysis:
Acc. Chem. Res. 2004, 37, 487; d) G. Guillena, D. J.
Ramon, Tetrahedron: Asymmetry 2006, 17, 1465; e) T.
Marcelli, J. H. van Maarseveen, H. Hiemstra, Angew.
Chem. Int. Ed. 2006, 45, 7496; f) G. Guillena, C.
Najera, D. J. Ramon, Tetrahedron: Asymmetry 2007, 18,
2249; g) S. J. Connon, Org. Biomol. Chem. 2007, 5,
3407; h) N. Marion, S. Diez-Gonzalez, S. P. Nolan,
Angew. Chem. Int. Ed. 2007, 46, 2988; i) D. Enders, C.
Grondal, M. R. M. Huttl, Angew. Chem. Int. Ed. 2007,
46, 1570; j) Y. Takemoto, H. Miyabe, Chimia 2007, 61,
269; k) D. A. Longbottom, V. Franckevicius, S. V. Ley,
Chimia 2007, 61, 247; l) X.-H. Yu, W. Wang, Chem.
Asian J. 2008, 3, 516; m) X.-H. Yu, W. Wang. Org.
Biomol. Chem. 2008, 6, 2037; n) H. Kotsuki, H. Ikishi-
ma, A. Okuyama, Heterocycles, 2008, 75, 493; o) H.
Kotsuki, H. Ikishima, A. Okuyama, Heterocycles, 2008,
75, 757.
In conclusion, we have developed a novel bifunc-
tional chiral trans-cyclohexanediamine-based sulfon-
amide primary amine catalyst. The above studies have
demonstrated that the catalyst is capable of promot-
ing efficient conjugate additions of acetone to a wide
range of nitroolefins. Moreover, this catalytic system
can be applied to some challenging ketones such as 3-
pentanone, acetophenone and 1-indanone. Further ef-
forts on improvement of the enantio- and diastereose-
lectivity of the conjugate addition processes by modi-
fication of this class of catalysts and the exploration
of other valuable organic transformations are under
active investigation.
[2] a) D. Almas¸i, D. A. Alonso, C. Nµjera, Tetrahedron:
Asymmetry 2007, 18, 299; b) S. B. Tsogoeva, Eur. J.
Org. Chem. 2007, 1701; c) J. L. Vicario, D. Badía, L.
Carrillo, Synthesis 2007, 2065.
[3] For examples of organocatalytic conjugate additions of
ketones to nitroalkenes catalyzed by pyrrolidine deri-
vates, see: a) K. Sakthivel, W. Notz, T. Bui, C. F. Barba-
s III, J. Am. Chem. Soc. 2001, 123, 5260; b) B. List, P.
Pojarliev, H. J. Martin, Org. Lett. 2001, 3, 2423; c) D.
Enders, A. Seki, Synlett 2002, 26; d) A. Alexakis, O.
Andrey, Org. Lett. 2002, 4, 3611; e) O. Andrey, A.
Alexakis, G. Bernardinelli, Org. Lett. 2003, 5, 2559;
f) S. MossØ, A. Alexakis, Org. Lett. 2006, 8, 3577;
g) H. J. Martin, B. List, Synlett 2003, 1901; h) O.
Andrey, A. Alexakis, A. Tomassini, G. Bernardinelli,
Adv. Synth. Catal. 2004, 346, 1147; i) T. Ishii, S. Fujioka,
Y. Sekiguchi, H. Kotsuki, J. Am. Chem. Soc. 2004, 126,
9558; j) P. Kotrusz, S. Toma, H.-G. Schmalz, A. Adler,
Eur. J. Org. Chem. 2004, 1577; k) D. Terakado, M.
Takano, T. Oriyama, Chem. Lett. 2005, 34, 962;
l) C. E. T. Mitchell, A. J. A. Cobb, S. V. Ley, Synlett
2005, 611; m) S. B. Tsogoeva, D. A. Yalalov, M. J. Hate-
Experimental Section
Typical Procedure (Table 2, entry 1)
A mixture of trans-b-nitrostyrene 2a (0.10 mmol), acetone
(0.2 mL), catalyst II (15 mol%), CHCl3 (0.8 mL) and water
(15 mol%) was stirred at room temperature for 72 h. The re-
action mixture was concentrated un vacuum. The residue
was purified by flash silica gel chromatography (ethyl ace-
tate/hexane=1:6) to afford the adduct 3a as a white solid;
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Fei Xue et al.
COMMUNICATIONS
ley, C. Weckbecker, K. Huthmacher, Eur. J. Org.
Chem. 2005, 4995; n) Y. Xu, W. Zou, H. SundØn, I.
Ibrahem, A. Córdova, Adv. Synth. Catal. 2006, 348,
418; o) D. Xu, S. Luo, H. Yue, L. Wang, Y. Liu, Z. Xu,
Synlett 2006, 2569; p) N. Mase, K. Watanabe, H. Yoda,
K. Takabe, F. Tanaka, C. F. Barbas, III, J. Am. Chem.
Soc. 2006, 128, 4966; q) C.-L. Cao, M.-C. Ye, X.-L. Sun,
Y. Tang, Org. Lett. 2006, 8, 2901; r) S. Luo, X. Mi, L.
Zhang, S. Liu, H. Xu, J.-P. Cheng, Angew. Chem. Int.
Ed. 2006, 45, 3093; s) M. S. Rasalkar, M. K. Potdar,
S. S. Mohile, M. M. Salunkhe, J. Mol. Catal. A 2005,
235, 267; t) Y. Xu, A. Córdova, Chem. Commun. 2006,
460; u) M.-K. Zhu, L. F. Cun, A.-Q. Mi, Y.-Z. Jiang, L.-
Z. Gong, Tetrahedron: Asymmetry 2006, 17, 491;
v) S. V. Pansare, K. Pandya, J. Am. Chem. Soc. 2006,
128, 9624; w) S. Luo, X. Mi, S. Liu, H. Xu, J.-P. Cheng,
Chem. Commun. 2006, 3687; x) Y.-J. Cao, H. H. Lu, Y.-
Y. Lai, L.-Q. Lu, W.-J. Xiao, Synthesis 2006, 3795;
y) D. A. Yalalov, S. B. Tsogoeva, S. Schmatz, Adv.
Synth. Catal. 2006, 348, 826; z) D. Almas¸i, D. A.
Alonso, C. Nµjera, Tetrahedron: Asymmetry 2006, 17,
2064; aa) S. Luo, H. Xu, X. Mi, J. Li, X. Zheng, J.-P.
Cheng, J. Org. Chem. 2006, 71, 9244; ab) Z.-Y. Yan, Y.-
N. Niu, H.-L. Wei, L.-Y. Wu, Y.-B. Zhao, Y.-M. Liang,
Tetrahedron: Asymmetry 2006, 17, 3288; ac) Y.-J. Cao,
Y.-Y. Lai, X. Wang, Y.-J. Li, W.-J. Xiao, Tetrahedron
Lett. 2007, 48, 21; ad) D. Diez, M. J. Gil, R. F. Moro,
I. S. Marcos, P. García, P. Basabe, N. M. Garrido, H. B.
Broughton, J. G. Urones, Tetrahedron 2007, 63, 740;
ae) T. Mandal, C.-G. Zhao, Tetrahedron Lett. 2007, 48,
5803.
Yue, Y.-C. Chen, Y. Wu, J. Zhu, J.-G. Deng, Angew.
Chem. Int. Ed. 2007, 46, 389; i) S. H. McCooey, S. J.
Connon, Org. Lett. 2007, 9, 599; j) S. S. V. Ramasastry,
H. Zhang, F. Tanaka, C. F. Barbas, III J. Am. Chem.
Soc. 2007, 129, 288; k) P. Ricci, A. Carlone, G. Bartoli,
M. Bosco, L. Sambri, P. Melchiorre, Adv. Synth. Catal.
2008, 350, 49; l) R. P. Singh, K. Bartelson, Y. Wang, H.
Su, X. Lu, L. Deng J. Am. Chem. Soc. 2008, 130, 2422;
m) X. Wang, C. M. Reisinger, B. List J. Am. Chem. Soc.
2008, 130, 6070.
[5] S. Luo, H. Xu, J. Li, L. Zhang, J.-P. Cheng, J. Am.
Chem. Soc. 2007, 129, 3074.
[6] T.-Y. Liu, H.-L. Cui, Y. Zhang, K. Jiang, W. Du, Z.-Q.
He, Y.-C. Chen, Org. Lett. 2007, 9, 3671.
[7] W. Chen, W. Du, Y.-Z. Duan, Y. Wu, S.-Y. Yang, Y.-C.
Chen, Angew. Chem. Int. Ed. 2007, 46, 7667.
[8] For examples of (S)-pyrrolidine trifluoromethanesul-
A
Wang, H. Li, Tetrahedron Lett. 2004, 45, 7235; b) W.
Wang, J. Wang, H. Li, Tetrahedron Lett. 2004, 45, 7243;
c) W. Wang, H. Li, J. Wang, Tetrahedron Lett. 2004, 45,
8229; d) W. Wang, J. Wang, H. Li, Angew. Chem. Int.
Ed. 2005, 44, 1369; e) W. Wang, H. Li, J. Wang, Tetra-
hedron Lett. 2005, 46, 5077; f) J. Wang, H. Li, Y. Mei,
B. Lou, D. Xu, D. Xie, H. Guo, W. Wang, J. Org.
Chem. 2005, 70, 5678; g) J. Wang, H. Li, B. Lou, L.-S.
Zu, H. Guo, W. Wang, Eur. Chem. J. 2006, 12, 4321;
h) J. Wang, H. Li, L.-S. Zu, W. Wang, Adv. Synth.
Catal. 2006, 348, 425; i) L.-S. Zu, J. Wang, H. Li, X-H.
Yu, W. Wang, Org. Lett. 2006, 8, 3077; j) C.-L. Cao, X.-
L. Sun, J.-L. Zhou, Y. Tang, J. Org. Chem. 2007, 72,
4073; k) C.-L. Cao, X.-L. Sun, Y.-B. Kang, Y. Tang,
Org. Lett. 2007, 9, 4151.
[4] For a recent review of primary amine catalyzed reac-
tions, see: a) L.-W. Xu, Y.-X. Lu, Org. Biomol. Chem.
2008, 6, 2047; b) Y.-C. Chen, Synlett 2008, 1919; and ex-
amples see: c) H. Huang, E. N. Jacobsen, J. Am. Chem.
Soc. 2006, 128, 7170; d) M. P. Lalonde, Y. Chen, E. N.
Jacobsen, Angew. Chem. Int. Ed. 2006, 45, 6366; e) K.
Liu, H.-F. Cui, J. Nie, K.-Y. Dong, X.-J. Li, J.-A. Ma,
Org. Lett. 2007, 9, 923; f) S. B. Tsogoeva, S. Wei, Chem.
Commun. 2006, 1451; g) J.-W. Xie, L. Yue, W. Chen, W.
Du, J. Zhu, J.-G. Deng, Y.-C. Chen, Org. Lett. 2007, 9,
413; h) J.-W. Xie, W. Chen, R. Li, M. Zeng, W. Du, L.
[9] Catalyst II was prepared according to literature report-
ed method: a) B. Mohar, A. Valleix, J.-R. Desmurs, M.
Felemez, A. Wagner, C. Mioskowski, Chem. Commun.
2001, 2572; b) S. Luo, H. Xu, L. Chen, J.-P. Cheng Org.
Lett. 2008, 10, 1775.
[10] F. Peng, Z. Shao, J. Mol. Catal. A: Chem. 2008, 285, 1.
[11] J. Wang. H. Li, B. Lou, D. Xu, H. Guo, W. Wang,
Chem. Eur. J. 2006, 12, 4321.
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