Organocatalytic direct asymmetric aldol reactions of 3-isothiocyanato oxindoles to ketones: Stereocontrolled synthesis of spirooxindoles bearing highly congested contiguous tetrasubstituted stereocenters

Article abstract of DOI:10.1021/ol200724q

Chemical equations presented. The first example of a direct catalytic asymmetric intermolecular aldol reaction of 3-isothiocyanato oxindoles to simple ketones with bifunctional thiourea-tertiary amine as catalyst is reported. This strategy provides a promising approach for the asymmetric synthesis of a range of enantioenriched spirocyclic oxindoles bearing two highly congested contiguous tetrasubstituted carbon stereocenters. Versatile transformations of the spirocyclic oxindole products into other structurally diverse spirocyclic oxindoles have also been demonstrated.

Full text of DOI:10.1021/ol200724q

ORGANIC
LETTERS
2011
Vol. 13, No. 9
2472–2475
Organocatalytic Direct Asymmetric Aldol
Reactions of 3-Isothiocyanato Oxindoles to
Ketones: Stereocontrolled Synthesis of
Spirooxindoles Bearing Highly Congested
Contiguous Tetrasubstituted Stereocenters
Wen-Bing Chen,†,§ Zhi-Jun Wu,‡ Jing Hu,† Lin-Feng Cun,† Xiao-Mei Zhang,† and
Wei-Cheng Yuan*,†
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041,
China, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China,
and Graduate School of Chinese Academy of Sciences, Beijing 100049, China
yuanwc@cioc.ac.cn
Received March 17, 2011
ABSTRACT
The first example of a direct catalytic asymmetric intermolecular aldol reaction of 3-isothiocyanato oxindoles to simple ketones with bifunctional
thiourea-tertiary amine as catalyst is reported. This strategy provides a promising approach for the asymmetric synthesis of a range of
enantioenriched spirocyclic oxindoles bearing two highly congested contiguous tetrasubstituted carbon stereocenters. Versatile transformations
of the spirocyclic oxindole products into other structurally diverse spirocyclic oxindoles have also been demonstrated.
The catalytic asymmetric aldol reaction is one of the
most important methods for the asymmetric formation of
CꢀC bonds and has found widespread application in
organic synthesis.1 In this respect, in contrast to the remark-
able advances that have been made with aldehydes as
electrophiles,1,2 the development of aldol additions to ke-
tones was quite slow.3 This state is attributable, at least in
part, to the lower reactivity of ketones and the decreased
steric discrimination compared to aldehydes.4 Therefore,
the direct asymmetric ketoneꢀaldol reactions to date
mainly focus on the activated ketone electrophiles5 or
intramolecular aldol additions based on enamine catalysis.6
Furthermore, the intermolecular aldol reactions for simple
(4) (a) Guthrie, J. P. J. Am. Chem. Soc. 1991, 113, 7249. (b) Guthrie,
J. P.; Wang, X.-P. Can. J. Chem. 1992, 70, 1055.
† Chengdu Institute of Organic Chemistry.
‡ Chengdu Institute of Biology.
(5) For selected examples, see: (a) Bøgevig, A.; Kumaragurubaran, N.;
Jørgensen, K. A. Chem. Commun. 2002, 620. (b) Tokuda, O.; Kano, T.; Gao,
W.-G.; Ikemoto, T.; Maruoka, K. Org. Lett. 2005, 7, 5103. (c) Tang, Z.; Cun,
L.-F.; Cui, X.; Mi, A. Q.; Jiang, Y.-Z.; Gong, L.-Z. Org. Lett. 2006, 8, 1263.
(d) Liu, J.; Yang, Z.; Wang, Z.; Wang, F.; Chen, X.; Liu, X.; Feng, X.; Su, Z.;
Hu, C. J. Am. Chem. Soc. 2008, 130, 5654. (e) Hara, N.; Nakamura, S.;
Shibata, N.; Toru, T. Chem.;Eur. J. 2009, 15, 6790.
(6) For a relevant review on asymmetric enamine catalysis, see:
Mukherjee, S.; Yang, J. W.; Hoffmann, S.; List, B. Chem. Rev. 2007,
107, 5471.
(7) For selected examples, see: (a) Denmark, S. E.; Fan, Y. J. Am. Chem.
Soc. 2002, 124, 4233. (b) Oisaki, K.; Suto, Y.; Kanai, M.; Shibasaki, M.
J. Am. Chem. Soc. 2003, 125, 5644. (c) Denmark, S. E.; Fan, Y.; Eastgate,
M. D. J. Org. Chem. 2005, 70, 5235. (d) Moreau, X.; Bazan-Tejeda, B.;
Campagne, J.-M. J. Am. Chem. Soc. 2005, 127, 7288. (e) Oisaki, K.; Zhao,
D.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 7164.
§ Graduate School.
(1) For selected reviews on aldol reactions, see: (a) Modern Aldol
Reactions; Mahrwald, R., Ed.; Wiley-VCH: Weinheim, Germany, 2004;
Vol. 1, p 2. (b) Carreira, E. M. In Catalytic Asymmetric Synthesis, 2nd.;
Ojima, I., Ed.; Wiley-VCH: Weinheim, Germany, 2000; Chapter 8B2.C.
(2) For selected reviews, see: (a) Nelson, S. G. Tetrahedron: Asym-
metry 1998, 9, 357. (b) Palomo, C.; Oiarbide, M.; Garcia, J. M. Chem.
Soc. Rev. 2004, 33, 65. (c) Dean, S. M.; Greenberg, W. A.; Wong, C.-H.
Adv. Synth. Catal. 2007, 349, 1308. (d) Mlynarski, J.; Paradowska, J.
Chem. Soc. Rev. 2008, 37, 1502.
(3) For selected reviews on enantioselective addition to ketones, see:
(a) Riant, O.; Hannedouche, J. Org. Biomol. Chem. 2007, 5, 873. (b)
Hatano, M.; Ishihara, K. Synthesis 2008, 1647. (c) Shibasaki, M.; Kanai,
M. Chem. Rev 2008, 108, 2853. (d) Adachi, S.; Harada, T. Eur. J. Org.
Chem. 2009, 3661.
r
10.1021/ol200724q
Published on Web 04/07/2011
2011 American Chemical Society

ketones as electrophiles are closely tied to either preformed
enolates7 or in situ-generated enolates with stoichiometric
reducing reagents.8 In these conditions, the direct catalytic
asymmetric intermolecular aldol reactions of unmodified
carbonyl compounds to simple ketones for the construction
of optically active β-hydroxy (tertiary alcohols) carbonyl
compounds should pose a far more difficult challenge.9 To
our knowledge, there is only one method for the direct
asymmetric intermolecular aldol addition to simple ketones
that has been developed to date, by Shibasaki and co-
workers:10 Mg Schiff base complexes catalyzed the direct
aldol reaction/cyclization sequence of R-isothiocyanato es-
ters to simple ketones under proton-transfer conditions.
Herein, we report the first organocatalytic direct asymmetric
intermolecular aldol reactions of 3-isothiocyanato oxindoles
to simple ketones with bifunctional thiourea-tertiary amines
as catalysts; this reaction readily afforded a new family of
enantioenriched spirocyclic oxindoles bearing highly con-
gested contiguous tetrasubstituted carbon stereocenters in
up to 95% yield, 95:5 dr, and 98% ee.
exhibit potent antimicrobial, antitumor, and oviposition-
stimulant biological activities, are awakening intense interest
in the development of efficient methodologies for their synth-
esis and related biological studies.12,14 Very recently, a highly
efficient strategy for the construction of spirobrassinin oxazo-
line analogues (I) through the organocatalyzed asymmetric
synthesis of spirocyclic thiocarbamates was established by
Wang and co-workers (Scheme 1), and a promising antipyre-
tic activity was revealed by the preliminary biological evalua-
tion.13 Taking the correlation between the molecular struc-
tural diversities and the potential biological activities into
account, we notice that the synthesis of another family of
spirobrassinin oxazoline analogues (II) has not been realized
up until now (Scheme 1), let alone relevant further biological
studies of them. We envisioned that this type of spirobrassinin
oxazoline analogues (II) would be readily accessed via the
methylation of the corresponding spiro[thiocarbamate-3,30-
oxindole] precursors (Scheme 1).13
Scheme 2. Strategy for the Direct Asymmetric Intermolecular
Aldol Reaction of 3-Isothiocyanato Oxindoles 2 to Simple
Ketones with Chiral Bifunctional Thiourea
Scheme 1. Spirobrassinin and Its Related Analogues
On the basis of the above considerations and our recent
success in asymmetric organocatalysis,15 we reasoned that a
new class of nucleophilic 3-isothiocyanato oxindoles 2 would
be generated by installing an isothiocyanato group into the
On the other hand, spirocyclic oxindoles are important
subunits frequently found in biologically active compounds.11
Spirobrassinin and its related analogues (Scheme 1),12,13
particular class of spirocyclic oxindole-type phytoalexins that
a
(14) (a) Pedras, M. S. C.; Okanga, F. I.; Zaharia, I. L.; Khan, A. Q.
Phytochemistry 2000, 53, 161. (b) Gross, D. J. Plant. Dis. Prot. 1993, 100,
433.
(8) For selected examples, see: (a) Lam, H. W.; Joensuu, P. M. Org. Lett.
2005, 7, 4225. (b) Deschamp, J.; Chuzel, O.; Hannedouche, J.; Riant, O.
Angew. Chem., Int. Ed. 2006, 45, 1292. (c) Zhao, D.; Oisaki, K.; Kanai, M.;
Shibasaki, M. Tetrahedron Lett. 2006, 47, 1403. (d) Zhao, D.; Oisaki, K.;
Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 14440. (e) Oisaki, K.;
Zhao, D.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2007, 129, 7439.
(9) (a) Trost, B. M.; Jiang, C. Synthesis 2006, 369. (b) Cozzi, P. G.;
Hilgraf, R.; Zimmermann, N. Eur. J. Org. Chem. 2007, 5969.
(10) Yoshino, T.; Morimoto, H.; Lu, G.; Matsunaga, S.; Shibasaki,
M. J. Am. Chem. Soc. 2009, 131, 17082.
(15) (a) Liao, Y.-H.; Zhang, H.; Wu, Z.-J.; Cun, L.-F.; Zhang, X.-M.;
Yuan, W.-C. Tetrahedron: Asymmetry 2009, 20, 2397. (b) Liao, Y.-H.;
Chen, W.-B.; Wu, Z.-J.; Du, X.-L.; Cun, L.-F.; Zhang, X.-M.; Yuan,
W.-C. Adv. Synth. Catal. 2010, 352, 827. (c) Chen, W.-B.; Du, X.-L.;
Cun, L.-F.; Zhang, X.-M.; Yuan, W.-C. Tetrahedron 2010, 66, 1441. (d)
Liao, Y.-H.; Liu, X.-L.; Wu, Z.-J.; Cun, L.-F.; Zhang, X.-M.; Yuan,
W.-C. Org. Lett. 2010, 12, 2896. (e) Chen, W.-B.; Wu, Z.-J.; Pei, Q.-L.;
Cun, L.-F.; Zhang, X.-M.; Yuan, W.-C. Org. Lett. 2010, 12, 3132. (f)
Liu, X.-L.; Liao, Y.-H.; Wu, Z.-J.; Cun, L.-F.; Zhang, X.-M.; Yuan,
W.-C. J. Org. Chem. 2010, 75, 4872.
(11) (a) Williams, R. M.; Cox, R. J. Acc. Chem. Res. 2003, 36, 127. (b)
Lin, H.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2003, 42, 36. (c) Marti,
C.; Carreira, E. M. Eur. J. Org. Chem. 2003, 2209. (d) Galliford, C. V.;
Scheidt, K. A. Angew. Chem., Int. Ed. 2007, 46, 8748.
(16) For the synthesis of 3-isothiocyanato oxindoles 2, see the
Supporting Information.
(17) For the use of R-isothiocyanato imides as nucleophiles in the
reactions with aldehydes, aldimines, and R-ketoesters, see: (a) Willis,
M. C.; Cutting, G. A.; Piccio, V. J.-D.; Durbin, M. J.; John, M. P.
Angew. Chem., Int. Ed. 2005, 44, 1543. (b) Cutting, G. A.; Stainforth,
(12) (a) Takasugi, M.; Monde, K.; Katsui, N.; Shirata, A. Chem.
Lett. 1987, 1631. (b) Gross, D.; Porzel, A.; Schmidt, J. Z. Naturforsch.
1994, 49c, 281. (c) Monde, K.; Takasugi, M.; Shirata, A. Phytochemistry
1995, 39, 581. (d) Monde, K.; Harada, N.; Takasugi, M.; Kutschy, P.;
Suchy, M.; Dzurilla, M. J. Nat. Prod. 2000, 63, 1312. (e) Monde, K.;
€
N. E.; John, M. P.; Kociok-Kohn, G.; Willis, M. C. J. Am. Chem. Soc.
2007, 129, 10632. (c) Li, L.; Klauber, E. G.; Seidel, D. J. Am. Chem. Soc.
2008, 130, 12248. (d) Shi, Z.; Yu, P.; Chen, P. J.; Zhong, G. Adv. Synth.
Catal. 2009, 351, 2797. (e) Li, L.; Ganesh, M.; Seidel, D. J. Am. Chem.
Soc. 2009, 131, 11648. (f) Jiang, X.; Zhang, G.; Fu, D.; Cao, Y.; Shen, F.;
Wang, R. Org. Lett. 2010, 12, 1544. (g) Vecchione, M. K.; Li, L.; Seidel,
D. Chem. Commun. 2010, 46, 4604. (h) Chen, X.; Zhu, Y.; Qiao, Z.; Xie,
M.; Lin, L.; Liu, X.; Feng, X. Chem.ꢀEur. J. 2010, 16, 10124.
ꢀ
Osawa, S.; Harada, K.; Takasugi, M.; Suchy, M.; Kutschy, P.; Dzurilla,
ꢀ
ꢀ
M.; Balentova, E. Chem. Lett. 2000, 886. (f) Suchy, M.; Kutschy, P.;
Monde, K.; Goto, H.; Harada, N.; Takasugi, M.; Dzurilla, M.;
ꢀ
Balenova, E. J. Org. Chem. 2001, 66, 3940.
(13) Jiang, X.; Cao, Y.; Wang, Y.; Liu, L.; Shen, F.; Wang, R. J. Am.
Chem. Soc. 2010, 132, 15328.
Org. Lett., Vol. 13, No. 9, 2011
2473

3-position of oxindoles.16 Furthermore, we surmised that the
asymmetric intermolecular aldol reaction of 2 to simple
ketones would occur in the presence of chiral bifunctional
thiourea-tertiary amines, and the subsequent intramolecular
cyclization could yield a variety of spiro[thiocarbamate-3,30-
oxindoles] with two contiguous chiral quaternary carbon
centers (Scheme 2).17 To our delight, this working hypothesis
(Scheme 2) was demonstrated by the initial experiment
between 3-isothiocyanato oxindole 2a and acetophenone
(3a) catalyzed by DABCO to proceed smoothly for generat-
ing racemic 4aa in CH2Cl2 at room temperature (Table 1).
Among the chiral bifunctional thiourea-tertiary amine cata-
lysts with diverse structural scaffolds surveyed, catalyst 1
turned out to be the most effective catalyst (for details, see the
Supporting Information).
Afterward, our studies focused on a comprehensive
screen of other reaction conditions in the presence of cata-
lyst 1. As shown in Table 1, probing of solvents revealed that
mesitylene was better than other solvents in terms of enantios-
electivity (entry 1 vs 2ꢀ7). Further investigation of catalyst
loading in mesitylene was carried out; it was found that
20 mol % catalyst loading was beneficial to this process in
view of both reaction yield and enantioselectivity compared to
10 mol % and 5 mol % catalyst loading (entry 9 vs 1 and 8).
Lowering the reaction temperature resulted in a high, to 91%,
ee without sacrificing the yield, although with an extension of
Table 1. Optimization of Reaction Conditionsa
entry
solvent
x
time (h)
yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
mesitylene
toluene
EtOAc
CH2Cl2
CHCl3
10
10
10
10
10
10
10
5
20
20
20
20
20
16
16
16
16
16
16
16
16
16
28
28
48
108
88
88
65
86
90
70
90
82
93
92
90
90
90
74:26
71:29
73:27
70:30
70:30
43:57
60:40
73:27
71:29
84:16
84:16
86:14
87:13
87
77
45
60
71
56
61
87
THF
hexane
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
9
88
10
11
12
13
91e
91e,f
92e,g
93e,h
a Reaction was performed with 2a (0.1 mmol), 3a (0.2 mmol), and cata-
lyst 1 (specified loading) in 2.0 mL of solvent at 0 °C, unless otherwise noted.
b Isolated yields of both diastereoisomers. c Determined by HPLC. d Deter-
e
f
˚
mined by chiral HPLC for the major diastereoisomer. Run at ꢀ40 °C. 4 A
MS (30 mg) was used. g In 4 mL of solvent. h In 8 mL of solvent.
acetophenone 3l (entry 16) and propiophenone (3m) (entry
17), unfortunately, the corresponding products were ob-
served only in trace amount.
˚
the reaction time to 28 h (entry 10). Adding 4 A molecular
sieves (MS) as additive did not lead to any improvement to
this reaction (entry 11). In addition, the probe into the con-
centration effects of the reaction revealed that a low concen-
tration did not lead to an obvious increase in enantioselec-
tivity or to a detrimental effect on the diastereoselectivity and
yield (entries 12ꢀ13). Based on the comprehensive considera-
tion of reaction time, yield, diastereoselectivity, and enantios-
electivity, the optimal reaction conditions were established as
shown in Table 1, entry 12.
A large-scale experiment was conducted under the opti-
mized conditions to evaluate the applicability of our method.
The reaction between 2a and 3g proceeded cleanly in 28 h on
a 6.0 mmol scale (1.23 g of 2a) andafforded4ag with excellent
results (95% yield, 94:6 dr, and 97% ee) (Scheme 3). Mean-
while, the absolute configuration of the major stereoisomer
of product 4ag was unambiguously determined by X-ray
crystallography; it contains a (C7S, C10S) configuration.18
The absolute configuration for all other products was as-
signed by analogy.
Remarkably, the versatile transformations of 4ag into
other structurally diverse spirocyclic oxindoles, such as
compounds 5ꢀ9 bearing two contiguous tetrasubstituted
carbon stereocenters, were smoothly realized (for details,
see Supporting Information). As illustrated in Scheme 4,
the oxazolidinethione ring of 4ag was readily transformed
into the N-Boc oxazolidinone ring contained in spirocyclic
oxindole compound 5. With the removal of the Boc group in
5 under basic or acidic conditions, an N-unprotected oxazo-
lidinone ring formed in compound 6.19a Meanwhile, an alter-
native direct approach to the construction of 6 from 4ag
would be via oxidation reaction in a solution of 30% aqueous
hydrogen peroxide and formic acid in 61% yield. In addition,
the oxazolidinethione moiety of 4ag also could be smoothly
converted into a 2-alkylthio-oxazoline ring as in compounds
With optimized conditions in hand, the scope of the
reaction was investigated for 3-isothiocyanato oxindoles 2
and simple ketones 3. As shown in Table 2, various acet-
ophenone derivatives 3bꢀf with either an electron-donating
or electron-withdrawing group at the meta or para position
of the aromatic ring gave their corresponding spiro[thiocar-
bamate-3,30-oxindole] products in good yield, good dias-
tereoselectivity, and high enantioselectivity (entries 1ꢀ5).
Excellent results could be obtained smoothly by 2-aceto-
naphthone (3g) and its derivative 3h, respectively (entries 6, 7).
Aliphatic ketones 3i and 3j also proved to be amenable to
this procedure, with high yields and moderate ee values
(entries 8, 9). Additionally, the 3-isothiocyanato oxindole
core may also be modified. Thus, both the benzo moiety
(entries 10ꢀ14) and the N-protecting group may be changed
as well (entry 15). For example, the reactions of different
3-isothiocyanato oxindoles 2b and 2c with various aryl
methyl ketones proceeded smoothly to generate the desired
products in good yield, dr, and ee values (entries 10ꢀ14).
Furthermore, incorporating different protecting groups on
the N1 of 3-isothiocyanato oxindole led to different effects on
the reactivity and stereoselectivity (entry 15). However, when
we further expanded the substrate scope to ortho-substituted
(18) CCDC 810232 containsthesupplementarycrystallographicdatafor
this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(19) (a) Evans, D. A.; Weber, A. E. J. Am. Chem. Soc. 1987, 109,
7151. (b) Gueyrard, D.; Leoni, O.; Palmieri, S.; Rollin, P. Tetrahedron:
Asymmetry 2001, 12, 337.
2474
Org. Lett., Vol. 13, No. 9, 2011

Table 2. Scope of the Reactiona
entry
2
3
time (h)
4/yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2b
2b
2b
2c
2d
2a
2a
R0 = 4-MeC6H4, R00 = Me (3b)
R0 = 3-MeOC6H4, R00 = Me (3c)
R0 = 4-BrC6H4, R00 = Me (3d)
R0 = 4-FC6H4, R00 = Me (3e)
R0 = 3-F3CC6H4, R00 = Me (3f)
R0 = 2-naphthyl, R00 = Me (3g)
R0 = 6-MeO-2-naphthyl, R00 = Me (3h)
R0 = R00 = Me (3i)
cyclohexanone (3j)
R0 = Ph, R00 = Me (3a)
R0 = 4-MeC6H4, R00 = Me (3b)
R0 = 3-MeOC6H4, R00 = Me (3c)
R0 = 3-ClC6H4, R00 = Me (3k)
R0 = Ph, R00 = Me (3a)
48
28
28
28
24
28
48
48
24
48
45
48
48
48
28
60
60
4ab/75
4ac/81
4ad/95
4ae/80
4af/92
4ag/95
4ah/90
4ai/92
4aj/90
4ba/94
4bb/88
4bc/80
4bk/95
4ca/82
4da/92
trace
88:12
82:18
77:23
75:25
70:30
95:5
95
93
89
87
91
98
98
64
74
90
96
93
93
85
90
90:10
9
10
11
12
13
14
15
16
17
80:20
85:15
79:21
75:25
70:30
90:10
R0 = Ph, R00 = Me (3a)
R0 = 2-ClC6H4, R00 = Me (3l)
R0 = Ph, R00 = Et (3m)
trace
a Reaction was performed with 2 (0.15 mmol), 3 (0.30 mmol), and catalyst 1 (20 mol %) in 6.0 mL of mesitylene at ꢀ40 °C, unless otherwise noted.
b Isolated yields of both diastereoisomers. c Determined by 1H NMR. d Determined by chiral HPLC for the major diastereoisomer.
Scheme 3. Large-Scale Experiment for the Synthesis of 4ag
Scheme 4. Transformations of the Product 4ag to Other
Spirocyclic Oxindoles
7 and 8 according to a reported procedure,19b resulting in the
formation of another family of spirobrassinin oxazoline ana-
logues (II) (Scheme 1). Significantly, compound 8 could be
further transformed into the new spirooxindole product 9
bearing an oxazolidinimine moiety.19b It is worth noting that
no change takes place in the diastereoselectivity and enan-
tioselectivity during the various transformations.
In conclusion, we have developed a direct catalytic asym-
metric aldol reaction of 3-isothiocyanato oxindoles with
simple ketones by using a bifunctional thiourea-tertiary amine
as catalyst. This process provides a promising approach for
the asymmetric synthesis of a range of structurally complex
spirocyclic oxindoles bearing two highly congested contigu-
ous tetrasubstituted carbon stereocenters in up to 95% yield,
95:5 dr, and 98% ee. Significantly, the potential of construct-
ing more structurally diverse spirooxindoles has been revealed
by the transformation of the oxazolidinethione moiety con-
tained in the spirooxindole products into some other hetero-
cyclic structures. We believe that the availability of these
compounds will provide promising candidates for chemical
biology and drug discovery. The biological evaluation of these
compounds is currently underway in our laboratory.
Acknowledgment. We are grateful for financial support
from the National Natural Science Foundation of China
(No. 20802074) and the National Basic Research Program
of China (973 Program) (2010CB833300).
Supporting Information Available. Experimental de-
tails, characterization data for new compounds, X-ray
crystal structure, and a CIF file. This material is available
free of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 9, 2011
2475