Asymmetric hydrogenation of α- Or β-acyloxy α,β- unsaturated phosphonates catalyzed by a Rh(i) complex of monodentate phosphoramidite

Article abstract of DOI:10.1039/c1ob06835k

The Rh(i) complex of a monodentate phosphoramidite bearing a primary amine moiety (DpenPhos) has been disclosed to be highly efficient for the asymmetric hydrogenation of a variety of α- or β-acyloxy α,β- unsaturated phosphonates, providing the corresponding biologically important chiral α- or β-hydroxy phosphonic acid derivatives with excellent enantioselectivities (90->99% ee).

Full text of DOI:10.1039/c1ob06835k

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PAPER
Asymmetric hydrogenation of α- or β-acyloxy α,β-unsaturated phosphonates
catalyzed by a Rh(I) complex of monodentate phosphoramidite†‡
Jinzhu Zhang, Kaiwu Dong, Zheng Wang and Kuiling Ding*
Received 1st November 2011, Accepted 18th November 2011
DOI: 10.1039/c1ob06835k
The Rh(I) complex of a monodentate phosphoramidite bearing a primary amine moiety (DpenPhos) has
been disclosed to be highly efficient for the asymmetric hydrogenation of a variety of α- or β-acyloxy
α,β-unsaturated phosphonates, providing the corresponding biologically important chiral α- or β-hydroxy
phosphonic acid derivatives with excellent enantioselectivities (90–>99% ee).
Chiral α- or β-hydroxy phosphonic acids are widely used as key
building blocks in pharmaceutical chemistry owing to their
potential biological activity.1 Intense efforts have been made in
their asymmetric synthesis based on chiral auxiliary strategies in
early works.2 In contrast, more and more reports of enantioselec-
tive synthesis via asymmetric catalysis have emerged in recent
years, which holds the potential of providing optically active pro-
ducts with high efficiency.2a Although a plethora of reports have
appeared on the synthesis of α-hydroxy phosphonates, including
asymmetric reduction of α-carbonyl phosphonates,3 oxidation of
vinyl phosphonates,4 hydrophosphonylation (Pudovik reaction),5
and others,6 metal complex catalyzed asymmetric hydrogenation
(AH) has received more and more attention due to its ideal atom
efficiency, high activity, clean process and easy manipulation.
Since the first report on the Rh(I)-catalyzed asymmetric hydro-
genation of α-acyloxy α,β-unsaturated phosphonates by Burk
phosphonates using Rh(I) complexes of monodentate phosphora-
midite ligands.
We have previously demonstrated that chiral monodentate
phosphoramidite DpenPhos ligands are highly efficient in Rh(I)-
catalyzed AH of various α,β-unsaturated carboxylates and α- or
β-enamido phosphonates, affording the corresponding optically
active acid derivatives with excellent ee values.15 It was dis-
closed that the presence of a P–N–H moiety in the relevant phos-
phoramidite ligand is critically important for the high activity of
Rh(I) catalyzed asymmetric hydrogenation, which may provide a
suitable platform for extending their application in the catalytic
asymmetric hydrogenation of challenging substrates.15b,c With
this concept in mind, we investigated the ligand effect in the
Rh(I)-catalyzed asymmetric hydrogenation of (E)-1-(dimethoxy-
phosphoryl)-2-phenylvinyl benzoate (1a) in the present work.
The monodentate phosphoramidite ligands L1,16 MonoPhos,17
and L3,15a were first examined for the catalysis. No reaction
occurred under the experimental conditions shown in Table 1
(entries 1–3), even though these ligands were known to be
efficient in the Rh(I) catalyzed hydrogenation of several bench-
mark unsaturated substrates. Gratifyingly, L2/Rh(I) complex cat-
alyzed the reaction smoothly, giving the desired product with
79% ee in full conversion of substrate (entry 4), which stimu-
lated us to use Dpen-derived ligand L4 to further improve the
enantioselectivity of the reaction. Excellent ee (98%) and cataly-
tic activity were obtained by using L4/Rh(I) complex (entry 5),
again indicating the critical importance of P–N–H moiety in the
ligands for the catalysis.15b,c,18 The ee of the product was further
increased to 99% with the decrease of hydrogen pressure to 1
atm (entry 7). This is the first time the asymmetric hydrogenation
of this kind of substrate has been realized at ambient pressure
with such a high enantioselectivity.
using privileged diphosphine ligands, such as DuPhos or BPE,7
8–12
various bidentate diphosphine ligands
have been disclosed
to be effective for the hydrogenation of this type of substrate. On
the other hand, the conceptually analogous AH of β-substituted
β-acyloxy α,β-unsaturated phosphonate derivatives for the prep-
aration of chiral β-hydroxyphosphonates has been less explored,
albeit that a leading work by Pizzano uses phosphine-phosphite
hybrid ligands.13 With the rapid development of monophosphine
ligands for their ready availability and excellent stereocontrol in
asymmetric catalyis,14 there has been no report on the AH of this
type of substrate with Rh(I) complexes of monophosphine
ligands to the best of our knowledge. Herein, we present a
highly efficient AH of various α- or β-acyloxy unsaturated
State Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai, 200032, P.R. China. E-mail: kding@mail.sioc.ac.cn;
Fax: +86 21 64166128
†Dedicated to Professor Christian Bruneau on the occasion of his 60th
birthday.
‡Electronic supplementary information (ESI) available: Experimental
details and characterization data for all new compounds. See DOI:
10.1039/c1ob06835k
With the leading results in hand, we investigated the adapta-
bility of the catalytic system by further extending the substrate
scope. The hydrogenation was carried out in CH2Cl2 at 1 atm of
H2 with 1 mol% of catalyst. As shown in Table 2, Rh/(S,S)-L4
catalyst turns out to be quite general for the catalysis and works
very well (>99% conversion in 1 h) for a wide range of
1598 | Org. Biomol. Chem., 2012, 10, 1598–1601
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Table 1 AH of α-benzoloxy α,β-unsaturated phosphonate (1a) with
Table 2 Rh(I)/(S,S)-L4 catalyzed AH of α-acyloxy β-substituted
α,β-unsaturated phosphonic acid dimethyl estersa
Rh(I) complexes of various monodentate phosphoramidite ligandsa
Entry
R in substrate
Ee (%)b
1
2
3
4
5
6
7
8
C6H5 (1a)
99(S)
>99(+)
99(+)
98(+)
98(S)
98(+)
97(+)
98(+)
>99(+)
>99(+)
>99(+)
98(+)
98(+)
98(+)
4-FC6H4 (1b)
4-ClC6H4 (1c)
4-BrC6H4 (1d)
4-MeOC6H4 (1e)
4-O2NC6H4 (1f)
2-ClC6H4 (1g)
2-BrC6H4 (1h)
2-MeOC6H4 (1i)
3,4-(MeO)2C6H3(1j)
3,4-Cl2C6H3 (1k)
3,4-(AcO)2C6H3 (1l)
Benzo[d][1,3]dioxol-5-yl (1m)
Naphtha-1-yl (1n)
Thien-2-yl (1o)
H (1p)
Entry
Ligand
PH (atm)
Conv. (%)b
Ee (%)c
2
1
2
3
4
5
6
7
8
L1
MonoPhos
20
20
20
20
20
5
—
—
—
>99
>99
>99
>99
31
—
—
—
79
98
98
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23d
L3
L2
L4
L4
L4
L2
1
1
>99
26
90(+)
>99(S)
>99(S)
>99(S)
>99(+)
>99(+)
98(+)
Me (1q)
MeO (1r)
EtO (1s)
iPrO (1t)
a Unless otherwise specified, all reactions were carried out with
0.25 mmol of substrate in 2.5 mL of CH2Cl2 for 16 h, in the presence of
1 mol% of catalyst prepared from [Rh(cod)2]BF4 and 2 equiv of ligand.
b Determined by 31P NMR analysis. c The ee values were determined by
chiral HPLC.
BnO (1u)
C6H5 (1v)c
96(+)
>99(S)
MeO (1r)
a Unless otherwise specified, all reactions were carried out under 1 atm
of hydrogen pressure with 0.25 mmol of substrate in 2.5 mL of CH2Cl2
in the presence of 1 mol% of catalyst generated in situ from [Rh(cod)2]
BF4 and 2 equiv of the ligand. b Determined by chiral HPLC. The
absolute configuration was determined by comparison of the specific
rotation with the literature value.10 c Benzoyl group (Bz) is replaced with
acetyl (Ac) in this substrate. d S/C = 500, 40 atm H2, rt, 16 h.
α-acyloxy β-substituted α,β-unsaturated phosphonic acid
dimethyl esters with various substitution patterns and electronic
properties. All substrates with para- or meta- substituted phenyl
rings either having an electron-donating or electron-withdrawing
substituent (1a–1i) afforded complete conversion (>99%) and
uniformly high enantioselectivities (97–99% ee), showing that
the electronic properties of the substrates have a negligible effect
on the enantioselectivity of the reaction (Table 2, entries 1–9).
The hydrogenation of the substrates (1j–m) having two electron-
donating or electron-withdrawing substituents at both the para-
and the meta- positions also achieved excellent enantiomeric
excesses in the corresponding products (entries 10–13). The
hydrogenation of 1-naphthyl or 2-thienyl substituted substrates
(1n–o) afforded the corresponding hydrogenation products
(2n–o) with 98% and 90% ee, respectively (entries 14–15), indi-
cating a slight negative impact of the 2-thienyl group on the
enantioselectivity of the catalysis, probably due to the competing
coordination of the 2-thienyl moiety in the substrate 1o. The
hydrogenation of β-H, β-alkyl, or β-alkoxy substituted α-acyloxy
α,β-unsaturated phosphonic acid dimethyl esters (1p–1u) pro-
ceeded smoothly with excellent enantiocontrol of the catalysis
(>98% ee) and complete conversion of the substrates (entries
16–21). The enantioselectivity decreased slightly in the hydro-
genation of the phosphonate 1v with an acetoxy group in com-
parison with that obtained in the hydrogenation of 1a having
benzoyloxy group (entries 22 vs. 1), showing the favorable
impact of the bulky chelating unit in the substrates. The
efficiency of the catalytic system was also studied at a lower cata-
lyst loading (0.2 mol%, entry 23), the yield and enantioselectiv-
ity of 2r remained unchanged although higher hydrogen pressure
(40 atm) and longer reaction time were needed to realize the
complete conversion of substrate.
We then decided to apply the present methodology to the
hydrogenation of β-acyloxy β-substituted α,β-unsaturated phos-
phonates for the synthesis of optically active β-hydroxy phos-
phonic acid derivatives, one type of key building block for
potential drug candidates.19 As can be seen from Table 3, the
substrates (3a–c) examined here were hydrogenated with good to
full conversion of starting material and excellent enantioselectiv-
ity (entries 1–3), providing an efficient approach to the optically
active β-hydroxy phosphonic acid derivatives (4a–c). The hydro-
genation of γ-phthalimide substituted substrate 3c proceeded
with a slight decrease of catalytic activity and enantioselectivity
in comparison with those of β-phenyl (3a) or β-methyl (3b) sub-
stituted α,β-unsaturated phosphonates.
In conclusion, the Rh(I) complex of a monodentate phosphora-
midite bearing a primary amine moiety (DpenPhos) has been
disclosed to be highly efficient for the asymmetric hydrogenation
of a variety of α- or β-acyloxy α,β-unsaturated phosphonates,
providing a convenient methodology for access to the corre-
sponding optically active α- or β-hydroxy phosphonic acid
derivatives with biological importance. This methodology rep-
resents the first use of monodentate chiral phosphine ligands in
the Rh(I)-catalyzed asymmetric hydrogenation of α- or
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Table 3 Asymmetric hydrogenation of β-acyloxy β-substituted
α,β-unsaturated phosphonic acid diethyl estersa
hydrogen. The reaction mixture was stirred for the specified
period of time. The hydrogen gas was released and the conver-
sion was determined by 31P NMR or 1H NMR analysis. The
reaction mixture was filtered through a short pad of silica gel and
eluted with EtOAc. The ee of the product was determined by
chiral HPLC.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (Nos. 21172237, 20821002, 21032007), the Major
Basic Research Development Program of China (Grant No.
2010CB833300), the Science and Technology Commission of
Shanghai Municipality, and the Chinese Academy of Sciences is
gratefully acknowledged.
Entry Substrate Ligand PH (atm) T (h) Conv. (%)b Ee (%)c
2
1
2
3
3a
3b
3c
L4
L5
L5
20
10
50
16
16
24
>99
>99
86
95(+)
96(−)
93(+)
a Unless otherwise specified, all reactions were carried out with
0.25 mmol of substrate in 2.5 mL of CH2Cl2 in the presence of 1 mol%
of catalyst generated in situ from [Rh(cod)2]BF4 and 2 equiv of the
ligand. b Determined by 31P NMR analysis. c Determined by chiral
HPLC.
Notes and references
1 (a) D. V. Patel, K. Riellygauvin and D. E. Ryono, Tetrahedron Lett.,
1990, 31, 5587; (b) D. V. Patel, K. Riellygauvin and D. E. Ryono, Tetra-
hedron Lett., 1990, 31, 5591.
2 For reviews, see: (a) O. I. Kolodiazhnyi, Tetrahedron: Asymmetry, 2005,
16, 3295; (b) S. Sobhani and Z. Tashrifi, Tetrahedron, 2010, 66, 1429;
(c) P. Merino, E. Marqués-López and R. P. Herrera, Adv. Synth. Catal.,
2008, 350, 1195.
3 (a) T. Gajda, Tetrahedron: Asymmetry, 1994, 5, 1965; (b) C. Meier and
W. H. G. Laux, Tetrahedron, 1996, 52, 589; (c) N. S. Goulioukina, G.
N. Bondarenko, A. V. Bogdanov, K. N. Gavrilov and I. P. Beletskaya,
Eur. J. Org. Chem., 2009, 510; (d) M. Kitamura, M. Tokunaga, T. Pham,
W. D. Lubell and R. Noyori, Tetrahedron Lett., 1995, 36, 5769;
(e) I. Gautier, V. RatovelomananaVidal, P. Savignac and J. P. Genêt, Tetra-
hedron Lett., 1996, 37, 7721; (f) V. V. Nesterov and O. I. Kolodiazhnyi,
Tetrahedron, 2007, 63, 6720.
β-acyloxy α,β-unsaturated phosphonates, and the results are
comparable or even superior to those obtained with Rh(I) com-
plexes of bidentate phosphine ligands. Further research on the
application of this type of ligand in the asymmetric catalysis of
other type of challenging substrates is undergoing in our
laboratory.
4 (a) T. Yokomatsu, Y. Yoshida, K. Suemune, T. Yamagishi and S. Shibuya,
Tetrahedron: Asymmetry, 1995, 6, 365; (b) T. Yokomatsu, T. Yamagishi,
T. Sada, K. Suemune and S. Shibuya, Tetrahedron, 1998, 54, 781.
5 (a) H. Wynberg and A. Smaardijk, Tetrahedron Lett., 1983, 24, 5899;
(b) A. A. Smaardijk, S. Noorda, F. Vanbolhuis and H. Wynberg, Tetrahe-
dron Lett., 1985, 26, 493; (c) T. Yokomatsu, T. Yamagishi and
S. Shibuya, Tetrahedron: Asymmetry, 1993, 4, 1779; (d) T. Yokomatsu,
T. Yamagishi and S. Shibuya, J. Chem. Soc., Perkin Trans. 1, 1997,
1527; (e) M. D. Groaning, B. J. Rowe and C. D. Spilling, Tetrahedron
Lett., 1998, 39, 5485; (f) H. Sasai, M. Bougauchi, T. Arai and
M. Shibasaki, Tetrahedron Lett., 1997, 38, 2717; (g) T. Arai,
M. Bougauchi, H. Sasai and M. Shibasaki, J. Org. Chem., 1996, 61,
2926; (h) J. P. Duxbury, A. Cawley, M. Thornton-Pett, L. Wantz,
J. N. D. Warne, R. Greatrex, D. Brown and T. P. Kee, Tetrahedron Lett.,
1999, 40, 4403; (i) B. Saito, H. Egami and T. Katsuki, J. Am. Chem.
Soc., 2007, 129, 1978; ( j) B. Saito and T. Katsuki, Angew. Chem., Int.
Ed., 2005, 44, 4600; (k) J. P. Abell and H. Yamamoto, J. Am. Chem.
Soc., 2008, 130, 10521; (l) S. H. Gou, X. Zhou, J. Wang, X. H. Liu and
X. M. Feng, Tetrahedron, 2008, 64, 2864; (m) X. Zhou, X. H. Liu,
X. Yang, D. J. Shang, J. G. Xin and X. M. Feng, Angew. Chem.,Int. Ed.,
2008, 47, 392; (n) X. Zhou, Y. L. Liu, L. Chang, J. N. Zhao, D. J. Shang,
X. H. Liu, L. L. Lin and X. M. Feng, Adv. Synth. Catal., 2009, 351,
2567; (o) D. Uraguchi, T. Ito and T. Ooi, J. Am. Chem. Soc., 2009, 131,
3836; (p) D. Uraguchi, T. Ito, S. Nakamura and T. Ooi, Chem. Sci., 2010,
1, 488; (q) K. Suyama, Y. Sakai, K. Matsumoto, B. Saito and T. Katsuki,
Angew. Chem., Int. Ed., 2010, 49, 797; (r) F. Yang, D. B. Zhao, J.
B. Lan, P. H. Xi, L. Yang, S. H. Xiang and J. S. You, Angew. Chem., Int.
Ed., 2008, 47, 5646.
Experimental
1. General procedure for hydrogenations under ambient
pressure
The chiral monodentate phosphoramidite ligand (5 μmol) and
Rh(cod)2BF4 (2.5 μmol; cod = 1,5-cyclooctadiene) were added
to CH2Cl2 (1.2 mL) in a Schlenk tube under argon atmosphere.
A hydrogen balloon was switched to the Schlenk tube to replace
the argon. The mixture was stirred for 10 min and then the sub-
strate 1 (0.25 mmol) in CH2Cl2 (1.3 mL) was added. The reac-
tion mixture was stirred for the specified period of time. The
hydrogen gas was released and the conversion was determined
by 31P NMR or H NMR analysis of an aliquot of the mixture.
The reaction mixture was filtered through a short pad of silica
gel and eluted with EtOAc. The ee of the product was deter-
mined by chiral HPLC.
1
2. General procedure for hydrogenations under pressures
higher than 1 atm
The chiral monodentate phosphoramidite ligand (5 μmol) and
Rh(cod)2BF4 (2.5 μmol; cod = 1,5-cyclooctadiene) were dis-
solved in 2.5 mL of solvent under argon atmosphere in a vial.
The vial was transferred into a Parr steel autoclave in a glove
box, where the substrate 3 (0.25 mmol) was added into the cata-
lyst solution. The autoclave was sealed and purged three times
with hydrogen and finally pressurized to a specified pressure of
6 (a) V. D. Pawar, S. Bettigeri, S. S. Weng, J. Q. Kao and C. T. Chen,
J. Am. Chem. Soc., 2006, 128, 6308; (b) S. F. Zhu, W. Q. Chen,
Q. Q. Zhang, H. X. Mao and Q. L. Zhou, Synlett, 2011, 919;
(c) J. L. Huang, J. Wang, X. H. Chen, Y. H. Wen, X. H. Liu and
X. M. Feng, Adv. Synth. Catal., 2008, 350, 287; (d) R. Dodda and
C. G. Zhao, Org. Lett., 2006, 8, 4911; (e) S. Samanta and C. G. Zhao, J.
Am. Chem. Soc., 2006, 128, 7442.
7 M. J. Burk, T. A. Stammers and J. A. Straub,, Org. Lett., 1999, 1, 387.
1600 | Org. Biomol. Chem., 2012, 10, 1598–1601
This journal is © The Royal Society of Chemistry 2012

View Article Online
8 (a) I. D. Gridnev, N. Higashi and T. Imamoto, J. Am. Chem. Soc., 2001,
123, 4631; (b) I. D. Gridnev, M. Yasutake, T. Imamoto and I.
P. Beletskaya, Proc. Natl. Acad. Sci. U. S. A., 2004, 101, 5385;
(c) I. D. Gridnev and T. Imamoto, Chem. Commun., 2009, 7447.
9 (a) M. Rubio, A. Suárez, E. Álvarez and A. Pizzano, Chem. Commun.,
2005, 628; (b) M. Rubio, S. Vargas, A. Suárez, E. Álvarez and
A. Pizzano, Chem.–Eur. J., 2007, 13, 1821.
10 (a) D. Y. Wang, X. P. Hu, J. D. Huang, J. Deng, S. B. Yu, Z. C. Duan, X.
F. Xu and Z. Zheng, Angew. Chem., Int. Ed., 2007, 46, 7810;
(b) D. Y. Wang, J. D. Huang, X. P. Hu, J. Deng, S. B. Yu, Z. C. Duan and
Z. Zheng, J. Org. Chem., 2008, 73, 2011; (c) M. Qiu, X. P. Hu, J.
D. Huang, D. Y. Wang, J. Deng, S. B. Yu, Z. C. Duan and Z. Zheng, Adv.
Synth. Catal., 2008, 350, 2683.
11 J. Wassenaar and J. N. H. Reek, J. Org. Chem., 2009, 74, 8403.
12 H. Fernández-Pérez, S. M. A. Donald, I. J. Munslow, J. Benet-Buchholz,
F. Maseras and A. Vidal-Ferran, Chem.–Eur. J., 2010, 16, 6495.
13 S. Vargas, A. Suárez, E. Álvarez and A. Pizzano, Chem.–Eur. J., 2008,
14, 9856.
14 (a) M. van den Berg, B. L. Feringa and A. J. Minnaard, in Handbook
of Homogeneous Hydrogenation, ed. J. G. de Vries, C. J. Elsevier, Wiley-
VCH, Weinheim, 2007, Vol. 2, p995; (b) G. Shang, W. Li and X. Zhang,
in Catalytic Asymmetric Synthesis, ed. I. Ojima, Wiley, Hoboken, 3rd
edn., 2010. p343; (c) A. J. Minnaard, B. L. Feringa, L. Lefort and
J. G. de Vries, Acc. Chem. Res., 2007, 40, 1267; (d) J. H. Xie and Q.
L. Zhou, Acc. Chem. Res., 2008, 41, 581; (e) L. Eberhardt, D. Armspach,
J. Harrowfield and D. Matt, Chem. Soc. Rev., 2008, 37, 839.
15 (a) Y. Liu and K. Ding, J. Am. Chem. Soc., 2005, 127, 10488; (b) Y. Liu,
C. A. Sandoval, Y. Yamaguchi, X. Zhang, Z. Wang, K. Kato and
K. Ding, J. Am. Chem. Soc., 2006, 128, 14212; (c) J. Zhang, Y. Li and
Z. Wang, K. Ding, Angew. Chem., Int. Ed., 2011, 50, 11743.
16 (a) E. Keller, J. Maurer, R. Naasz, T. Schader, A. Meetsma and
B. L. Feringa, Tetrahedron: Asymmetry, 1998, 9, 2409; (b) A. Alexakis,
J. Vastra, J. Burton, C. Benhaim and P. Mangeney, Tetrahedron Lett.,
1998, 39, 7869.
17 (a) R. Hulst, N. K. de Vries and B. L. Feringa, Tetrahedron: Asymmetry,
1994, 5, 699; (b) M. van den Berg, A. J. Minnaard, E. Schudde, A
A. H. M. de Vries, J. G. de Vries and B. L. Feringa, J. Am. Chem. Soc.,
2000, 122, 11539.
18 For an excellent review, see: J. Meeuwissen and J. N. H. Reek, Nat.
Chem., 2010, 2, 615.
19 (a) P. Cui, J. L. Tomsig, W. F. McCalmont, S. Lee, C. J. Becker, K.
R. Lynch and T. L. Macdonald, Bioorg. Med. Chem. Lett., 2007, 17,
1634; (b) J. F. Dellaria, R. G. Maki, H. H. Stein, J. Cohen, D. Whittern,
K. Marsh, D. J. Hoffman, J. J. Plattner and T. J. Perun, J. Med. Chem.,
1990, 33, 534; (c) A. E. Wróblewski and A. Halajewska-Wosik, Tetrahe-
dron: Asymmetry, 2004, 15, 3201.
This journal is © The Royal Society of Chemistry 2012
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