Organocatalytic enantioselective cascade michael-michael-wittig reactions of phosphorus ylides: One-pot synthesis of the all-cis trisubstituted cyclohexenecarboxylates via the [1+2 + 3] annulation

Article abstract of DOI:10.1021/ol9021832

The stereoselective synthesis of all-cis 5-nitro-4,6-diphenylcyclohex-1- enecarboxylic ester has been achieved by an organocatalytic asymmetric MichaelMichael-Wittig cascade reaction of phosphorus ylides, nitroolefins, and α,β-unsaturated aldehydes with e

Full text of DOI:10.1021/ol9021832

ORGANIC
LETTERS
2009
Vol. 11, No. 22
5246-5249
Organocatalytic Enantioselective
Cascade Michael-Michael-Wittig
Reactions of Phosphorus Ylides:
One-Pot Synthesis of the all-cis
Trisubstituted Cyclohexenecarboxylates
via the [1 + 2 + 3] Annulation
Bor-Cherng Hong,*,† Rei-Hau Jan,† Chih-Wei Tsai,† Roshan Y. Nimje,†
Ju-Hsiou Liao,† and Gene-Hsiang Lee‡
Department of Chemistry and Biochemistry, National Chung Cheng UniVersity,
Chia-Yi, 621, Taiwan, R.O.C., and Instrumentation Center, National Taiwan
UniVersity, Taipei, 106, Taiwan, R.O.C.
chebch@ccu.edu.tw
Received September 21, 2009
ABSTRACT
The stereoselective synthesis of all-cis 5-nitro-4,6-diphenylcyclohex-1-enecarboxylic ester has been achieved by an organocatalytic asymmetric Michael-
Michael-Wittig cascade reaction of phosphorus ylides, nitroolefins, and r,ꢀ-unsaturated aldehydes with excellent enantioselectivities (up to >99% ee).
Nitroalkenes, known as chemical chameleons for the ease of
their functional group transformation, have received extensive
attention in organic chemistry.1 Of these transformations,
asymmetric conjugate addition reactions of carbon nucleophiles
(especially aldehydes and ketones) to nitrostyrenes are of great
interest.2,3 Apart from carbon nucleophiles, enantioselective
Michael additions of phosphorus nucleophiles, such as
diphenylphosphine oxide, dialkyl phosphite, and benzylphos-
phonate, to nitroalkenes have only of late been successful.4,5
Recently, a cascade organocatalytic conjugate addition of
sulfur ylides to nitroolefins for the synthesis of oxazolidin-
2-ones was accomplished.6 However, the efficient organo-
catalytic and enantioselective conjugate addition of phos-
phorus ylides to nitroalkenes remains elusive.7 Moreover,
conjugate additions of nitroalkanes to electron-poor alkenes
(Michael acceptors) giving the corresponding 1,4-adducts
with high stereoselectivity are of great interest to synthetic
chemists.8 Despite many examples of organocatalytic con-
jugate addition of nitroalkanes to R,ꢀ-unsaturated ketones9
and esters,10 reactions with R,ꢀ-unsaturated aldehydes have
been far less successful, mainly because the 1,2-addition
toward the enal could interfere with the desired 1,4-addition
(3) For recent examples, see: (a) Wiesner, M.; Revell, J. D.; Wennemers,
H. Angew. Chem., Int. Ed. 2008, 47, 1871. (b) Luo, S.; Li, J.; Zhang, L.;
Xu, H.; Cheng, J.-P. Chem.sEur. J. 2008, 4, 1273. (c) Zhu, S.; Yu, S.;
Ma, D. Angew. Chem., Int. Ed. 2008, 47, 545. (d) Hayashi, Y.; Itoh, T.;
Ohkubo, M.; Ishikawa, H. Angew. Chem., Int. Ed. 2008, 47, 4722. (e) Garcia-
Garcia, P.; Ladepeche, A.; Halder, R.; List, B. Angew. Chem., Int. Ed. 2008,
47, 4719. (f) Enders, D.; Wang, C.; Bats, J. W. Angew. Chem., Int. Ed.
2008, 47, 7539. (g) Mandal, T.; Zhao, C.-G. Angew. Chem., Int. Ed. 2008,
47, 7714. (h) Han, B.; Xiao, Z.-Q.; Chen, Y.-C. Org. Lett. 2009, 11, 4660.
† National Chung Cheng University.
‡ National Taiwan University.
(1) Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH: New
York, 2001.
(2) For reviews, see: (a) Berner, B. J.; Tedeschi, L.; Enders, D. Eur. J.
Org. Chem. 2002, 1877. (b) Sulzer-Mosse´, S.; Alexandre Alexakis, A. Chem.
Commun. 2007, 3123
.
10.1021/ol9021832 CCC: $40.75
Published on Web 10/23/2009
 2009 American Chemical Society

Scheme 1
process.11 A few examples of the catalytic asymmetric
derivatives, is rare, because synthesis of contiguous cis
stereocenters is a persistent challenge in organic synthesis
due to steric hindrance. Taking into account the above
observations and considering domino reactions14 for devel-
oping organocatalytic annulation, especially for multicom-
ponent reactions,15 we envisioned that a suitable substrate,
such as a stabilized Wittig reagent, may permit the objective
of organocatalytic dynamic kinetic asymmetric transforma-
tion (DYKAT)16,17 as illustrated in Scheme 1. Conjugate
addition of stabilized Wittig reagent 2 to nitrostyrene 1a
would generate racemic 3a (another stabilized Wittig re-
agent), which could decompose back to 1a and 2,18 and
provide a vehicle for their interconversion. Consequently, if
the racemization process is fast relative to the subsequent
nitro-Michael and Wittig reactions, an effective DYKAT
results, followed by organocatalytic enantioselective conju-
gate addition of the in situ generated nitroalkane to the R,ꢀ-
unsaturated aldehyde, with the subsequent Wittig reaction,
giving the [1 + 2 + 3] annulation adducts. Although general
addition of nitroalkanes to R,ꢀ-unsaturated aldehydes have
been recently reported, but those protocols usually gave low
selectivity (syn/anti ratios) and provided mixtures of trans-
and cis-adducts, sometimes in nearly 1:1 ratios.12 In the few
reported cases of intramolecular conjugate addition of
nitroalkanes to R,ꢀ-unsaturated aldehydes, the anti-Michael
addition predominated and furnished the trans-adducts.13
Consequently, the highly enantioselective syn-Michael ad-
dition of nitroalkanes to R,ꢀ-unsaturated aldehydes, yielding
the three all-cis consecutive stereocenters in the cyclohexene
(4) (a) Rai, V.; Namboothiri, I. N. N. Tetrahedron: Asymmtery 2008, 19,
2335. (b) Fu, X.; Jiang, Z.; Tan, C.-H. Chem. Commun. 2007, 5058. (c)
Bartoli, G.; Bosco, M.; Carlone, A.; Locatelli, M.; Mazzanti, A.; Sambri,
L.; Melchiorre, P. Chem. Commun. 2007, 722. (d) Wang, J.; Heikkinen,
L. D.; Li, H.; Zu, L.; Jiang, W.; Xie, H.; Wang, W. AdV. Synth. Catal. 2007,
349, 1052.
(5) Ishikawa, H.; Suzuki, T.; Hayashi, Y. Angew. Chem., Int. Ed. 2009,
48, 1304.
(6) Lu, L.-Q.; Cao, Y.-J.; Liu, X.-P.; An, J.; Yao, C.-J.; Ming, Z.-H.;
Xiao, W.-J. J. J. Am. Chem. Soc. 2008, 130, 6946.
(7) For other reactions, see: (a) Zhang, Y.; Liu, Y.-K.; Kang, T.-R.;
Hu, Z.-K.; Chen, Y.-C. J. Am. Chem. Soc. 2008, 130, 2456. (b) Liu, Y.-K.;
Ma, C.; Jiang, K.; Liu, T.-Y.; Chen, Y.-C. Org. Lett. 2009, 11, 2848.
(8) Ballini, R.; Bosica, G.; Fiorini, D.; Palmieri, A.; Petrini, M. Chem.
ReV. 2005, 105, 933.
(13) (a) Zhao, G.-L.; Ibrahem, I.; Dziedzic, P.; Sun, J.; Bonneau, C.;
Co´rdova, A. Chem.sEur. J. 2008, 14, 10007. (b) Enders, D.; Narine, A.;
Benninghaus, T. R.; Raabe, G. Synlett 2007, 1667. (c) Palomo, C.; Landa,
A.; Mielgo, A.; Oiarbide, M.; Puente, A.; Vera, S. Angew. Chem., Int. Ed.
2007, 46, 8431. (d) Enders, D.; Hu¨ttl, M. R.; Grondal, C.; Raabe, G. Nature
2006, 441, 861. (e) Kraus, G. A.; Goronga, T. Synthesis 2007, 1765.
(14) For a review, see: Enders, D.; Grondal, C.; Hu¨ttl, M. R. M. Angew.
Chem., Int. Ed. 2007, 46, 1570.
(9) (a) Hanessian, S.; Shao, Z.; Warrier, J. S. Org. Lett. 2006, 8, 4787.
(b) Mitchell, C. E. T.; Brenner, S. E.; Garcia-Fortanet, J.; Ley, S. V. Org.
Biomol. Chem. 2006, 4, 2039. (c) Tsogoeva, S. B.; Jagtap, S. B.;
Ardemasova, Z. A. Tetrahedron: Asymmetry 2006, 17, 989. (d) Vakulya,
B.; Varga, S.; Csampai, A.; Soos, T. Org. Lett. 2005, 7, 1967. (e) Prieto,
A.; Halland, N.; Jørgensen, K. A. Org. Lett. 2005, 7, 3897. (f) Mitchell,
C. E. T.; Brenner, S. E.; Ley, S. V. Chem. Commun. 2005, 5346.
(10) Ooi, T.; Fujioka, S.; Maruoka, K. J. Am. Chem. Soc. 2004, 126,
11790.
(15) For our previous efforts in exploring new organocatalytic annula-
tions, see: (a) Hong, B.-C.; Nimje, R. Y.; Liao, J.-H. Org. Biomol. Chem.
2009, 7, 3095. (b) Kotame, P.; Hong, B.-C.; Liao, J.-H. Tetrahedron Lett.
2009, 50, 704. (c) Hong, B.-C.; Nimje, R. Y.; Sadani, A. A.; Liao, J.-H.
Org. Lett. 2008, 10, 2345. (d) Hong, B.-C.; Nimje, R. Y.; Wu, M.-F.; Sadani,
A. A. Eur. J. Org. Chem. 2008, 1449, and references cited therein.
(16) For reviews, see: (a) Steinreiber, J.; Faber, K.; Herfried Grieng, H.
Chem.sEur. J. 2008, 14, 8060. (b) Pellissier, H. Tetrahedron 2008, 64, 1563.
(17) For recent examples, see: (a) Malkov, A. V.; Stonc˘ius, S.; Vrankova´,
K.; Arndt, M.; Koc˘ovsky´, P. Chem.sEur. J. 2008, 14, 8082. (b) Mu¨ller,
C. E.; Wanka, L.; Jewell, K.; Schreiner, P. R. Angew. Chem., Int. Ed. 2008,
47, 6180. (c) Peschiulli, A.; Quigley, C.; Tallon, S.; Gun’ko, Y. K.; Connon,
S. J. J. Org. Chem. 2008, 73, 6409. (d) Wang, J.; Xie, H.; Li, H.; Zu, L.;
Wang, W. Angew. Chem., Int. Ed. 2008, 47, 4177. (e) Yang, J.; Wang, T.;
Ding, Z.; Shen, Z.; Zhang, Y. Org. Biomol. Chem. 2009, 7, 2208.
(11) (a) Tsogoeva, S. B. Eur. J. Org. Chem. 2007, 1701. (b) Vicario,
J. L.; Bad´ıa, D.; Carrillo, L. Synthesis 2007, 2065. (c) Reyes, E.; Jiang, H.;
Milelli, A.; Elsner, P.; Hazell, R. G.; Jørgensen, K. A. Angew. Chem., Int.
Ed. 2007, 46, 9202.
(12) (a) Hojabri, L.; Hartikka, A.; Moghaddam, F. M.; Arvidsson, P. I.
AdV. Synth. Catal. 2007, 349, 740. (b) Gotoh, H.; Ishikawa, H.; Hayashi,
Y. Org. Lett. 2007, 9, 5307. (c) Zu, L.; Xie, H.; Li, H.; Wang, J.; Wang,
W. AdV. Synth. Catal. 2007, 349, 2660. (d) Wang, Y.; Li, P.; Liang, X.;
Zhang, T. Y.; Ye, J. Chem. Commun. 2008, 1232.
Org. Lett., Vol. 11, No. 22, 2009
5247

studies of the facile racemization of R-branched carbonyl
compounds have been reported, the nitro phosphonium ylide
has never been applied in an organocatalytic DYKAT. In
this paper, we explore the feasibility of such an idea and
demonstrate herein an unusual stereoselective synthesis of
all-cis substituted cyclohexene derivatives.
Table 1. Screening of Catalysts and Conditions for the Reaction
of 1a, 2, and Cinnamaldehyde (4a)a
additive
yield
drc
5:8:9:7:6
Figure 1. Stereoplots for X-ray crystal structures of (()-9a, (+)-
entry cat.
solvent
CHCl3
CH2Cl2
CH3CN
DMF
(20 mol %) (%)b
5a, (+)-5b, and (-)-7b: C gray, N blue, O red, and Br purple
8:0:1:3:1d e
,
1
2
I
I
HOAc
HOAc
HOAc
HOAc
HOAc
HOAc
82
78
61
33
45
35
87
34
9
11
5
9
81
79
82
82
80
20:0:1:5:7
7:1:1:2:3
3:0:1:2:5
8:0:1:4:2
4:0:1:1:2
28:1:5:11:0f
3:1:1:1:2
1:0:0:0:10
1:0:0:0:6
1:0:0:0:9
1:0:0:0:8
7:0:1:3:2
7:0:1:4:3
5:0:1:4:2
0:6:1:2:1h
1:3:1:2:1
3
I
To assess the feasibility of this strategy, a solution of
nitrostyrene (1a), (carbethoxymethylene)triphenylphosphorane
(2), and catalyst I (20 mol %)19 in CHCl3 (0.4 M) was stirred
at ambient temperature for 3 h, affording 3a. Following the
addition of HOAc (20 mol %) and cinnamaldehyde (4a), the
solution was stirred at room temperature for 3 days until
completion of the reaction, monitored by TLC and 1H NMR,20
and afforded an 82% yield of 5a, 9a, and 7a in a ratio of 8:1:3
(Table 1, entry 1). Notably, 5a is the all-cis trisubstituted
cyclohexenecarboxylate, and 8a was not observed in the reaction
mixtures, probably due to the steric hindrance of the anti-
Michael approach of (R)-3a to cinnamaldehyde (4a), Scheme
1. The relative structures of 5a and 9a were assigned unam-
biguously by single-crystal X-ray analysis (Figure 1). We note
that no epimerization occurred during purification. The diaster-
4
I
5
I
toluene
THF
6
I
7
I
CH2Cl2-MeOHg HOAc
8
9
II
III
IV
V
VI
I
I
I
I
I
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
HOAc
HOAc
HOAc
HOAc
HOAc
PhCO2H
PNBA
DNBA
DABCO
DBU
10
11
12
13
14
15
16
17
a Unless otherwise noted, the reactions were performed in 0.4 M 2 with
a ratio 1.2/1/1.2 of 1a/2/4a at 25 °C for 60-72 h. b Isolated yields of the
adducts (5a-9a), not including 6. c Determined by 1H NMR analysis of
the crude reaction mixture. d Determined by HPLC with a chiral column
(Chiracel OD-H). e Ee: 95% for 5a; 90% for 7a. f 85% for 5a. g 0.1 mL of
MeOH was added before the addition of cinnamaldehyde. h Ee: 92% for
8a. PNBA: p-nitrobenzoic acid. DNBA: 3,5-dinitrobenzoic acid.
1
eomeric ratio determined by H NMR spectroscopic analysis
of the crude reaction mixture was maintained after isolation of
the single isomers, and high enantioselectivities were observed
(95% ee for 5a and >99% ee after a single recrystallization).
The absolute configurations of the stereoisomers were assigned
by single-crystal X-ray analysis of (+)-5b and (-)-7b,21
prepared from 4-bromo-ꢀ-nitrostyrene, vide infra. The structures
indicate that the conjugate addition of nitroalkane (3b) to the enal
takes place from the Si face through the control of the catalyst I.
We evaluated a series of solvents for this reaction. In CH2Cl2,
5a was the major product, but with somewhat more 6, the
parasitic dead end product of the cascade reaction (entry 2, Table
1). Conducting the same reaction in other solvents (e.g., CH3CN,
DMF, toluene, and THF) afforded lower yields and lower
stereoselectivity (entries 3-6, Table 1). The solubility of 3a in
pure MeOH was very low, but addition of a small amount of
MeOH in CH2Cl2 (1:10) slightly increased the yields with no
observable 6, although 5a was obtained with lower ee, 85%
vs. 95%, (entry 7, Table 1). Catalyst I was found to be the
most promising candidate for the transformation among a series
of potential organocatalysts (entries 8-12, Table 1). Reaction
with pyrrolidine (II)-HOAc afforded lower overall yields along
with substantial quantities of 6 and other diastereomers.
Nevertheless, this racemic product was a suitable standard for
HPLC analysis in determining the ee of 5a in Table 1. The
reactions with other catalysts (III-VI) predominately gave 6,
and were not suitable for this process. Replacement of HOAc
by other acids under the same reaction conditions gave similar
yields and selectivities (entries 13-15, Table 1). Replacement
of the acid additive by a base (e.g., DABCO, DBU) favored
formation of the adduct 8a (entries 16 and 17, Table 1).22
Notably, when 8a was observed as the major product, 5a was
not obtained in the reaction with the additive DABCO.
Presumably, 8a was produced from the isomerization of the
initially formed 5a under basic conditions. This assumption was
confirmed by the treatment of 5a (99% ee) with DABCO in
(18) (a) Asunskis, J.; Schechter, H. J. Org. Chem. 1968, 33, 1164. (b)
Connor, D. T.; Strandtmann, M. V. J. Org. Chem. 1973, 38, 1047.
(19) For a review of chiral diarylprolinol silyl ether catalyzed reactions, see:
Mielgo, A.; Palomo, C. Chem. Asian J. 2008, 3, 922. For a review of
diarylprolinol in organocatalysis, see: Lattanzi, A. Chem. Commun. 2009, 1452.
(20) The reaction at 0.1 M of 2 took nearly 30 days for completion.
(21) 99% ee for 5b, and 96% for 7b.
(22) For a review of organocatalysis by chiral Brønsted bases, see:
Palomo, C.; Oiarbide, M.; Lo´pez, R. Chem. Soc. ReV. 2009, 38, 632.
5248
Org. Lett., Vol. 11, No. 22, 2009

CHCl3 to give 8a predominately with a similar enantioselectivity
(>99% ee). Having established the optimal reaction conditions,
we studied the use of different nitroolefins as well as different
R,ꢀ-unsaturated aldehydes to synthesize a variety of trisubsti-
tuted cyclohexene derivatives. The major isomers (5b-j) were,
in all cases, isolated in good yields and with high enantiose-
lectivities (entries 2-10, Table 2). For the trinitro derivative,
however, 5h and 8h were observed in a ratio of 2:1. The
isomerization of 5h to 8h during the regular reaction and
workup conditions may be attributed to the highly acidic nature
of the CHNO2 group in the polysubstituted nitro derivative.
equilibrium between 2 and 3a as well as to obtain improved
diastereoselectivity of 5a, an enantiomerically enriched 3a was
needed. Unfortunately, unlike the previous examples of phos-
phorus and enolizable carbon nucleophiles, vide infra, enanti-
oselective addition of 2 to the nitroalkene (1a) remained elusive.
While many examples of organocatalysis rely on covalent
attachment of the catalyst (e.g., proline catalysis via enamine/
iminium intermediates), noncovalent organocatalysis (e.g.,
thiourea derivatives and cinchonidine acting as general acids
and general bases) provides alternative catalytic strategies.23
Taking this concept into account, we screened a series of
thiourea catalysts for the enantioselective formation of 3a. After
numerous attempts at the enantioselective addition of the naked
nucleophile (nonenolizable) 2 to 1a, the successful reaction with
catalyst VII afforded 51% ee of (R)-3a.24 Treatment of (R)-3a
(51% ee) under standard reaction conditions (catalyst I-HOAc)
with 4a provided 5a with the same 4S,5R,6S configuration and
with much better dr (5:9:7:8 ) 6:0:1:0). In addition, the same
reaction conditions conducted in the absence of 4a afforded
the racemization of 3a, further demonstrating the occurrence
of DYKAT in the process. On the other hand, addition of 4a to
the reaction mixture of 3a and thiourea catalyst VII in the absence
of catalyst I gave no reaction for 4 days. Therefore, the two-step
reaction (with noncovalent and covalent catalyst) was able to
function independently and proceeded in a one-pot manner.
In conclusion, we have discovered an unprecedented orga-
nocatalytic enantioselective cascade nitro-Michael-Michael-
Wittig reaction with certain evidence of dynamic kinetic
asymmetric transformation. It is remarkable that this annulation
provides a simple and direct protocol for the stereoselective
construction of trisubstituted cyclohexenecarboxylates in a
multicomponent one-pot operation; the presence of three
contiguous chiral centers, with all-cis stereochemistry in the
product and with high enantioselectivity, is especially notewor-
thy. An increase in diastereoselectivity was achieved by the first
organocatalytic asymmetric conjugate addition of phosphonium
ylide to nitrostyrenes with the noncovalent thiourea catalyst.
Further work is underway to gain more insight into this annulation
as well as the exploration of its synthetic applications.25
Table 2. Scope of the Organocatalytic [1 + 2 + 3] Annulations
drb
ee
entry
R1
R2
yield (%)a 5:9:7:8 (%)c
5a: 55 (83) 6:1:2:0d 95e
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
4-BrC6H4
4-OMeC6H4
4-FC6H4
5b: 41 (82) 4:1:3:0
5c: 50 (90) 5:1:3:0
5d: 52 (91) 4:1:2:0
5e: 39 (71) 5:1:3:0
99f
99
96
99
95
97
99
92
95
3,4-OCH2OC6H3 Ph
Ph
Ph
Ph
Ph
Ph
4-OMeC6H4 5f: 47 (73) 9:2:3:0
4-BrC6H4 5g: 59 (82) 5:1:1:0
4-NO2C6H4 5h: 34 (85) 4:1:3:2
4-MeC6H4
4-ClC6H4
5i: 48 (76) 5:1:2:0
5j: 51 (77) 8:1:3:0
10
a Isolated yields of the major diastereomers (5a-j), combined isolated yields
of the isomers in parentheses. b Determined by 1H NMR analysis of the crude
reaction mixture. c Ee of the major stereoisomer (5a-j), determined by HPLC
with a chiral column (Chiracel OD-H). d 5:9:7:8 ) 6:0:1:0 from 51% ee of
(R)-3a, prepared in situ from 1a and 2 with the thiourea catalyst VII. e >99%
ee, following a single recrystallization. f 96% for 7b.
Acknowledgment. We acknowledge the financial support
for this study from the National Science Council, R.O.C. The
authors thank the National Center for High-Performance
Computing (NCHC) for their assistance in literature searching.
The successful transformation that gave 55% isolated yields
of 5a (83% for all isomers) with high enantioselectivity
suggested that the process may follow a novel dynamic kinetic
resolution (DKR)-mediated reaction, as we proposed. The
initially formed intermediate 3a would undergo a DKR process
in the presence of base (the secondary amine catalyst), whereby
deprotonation of the highly acidic nitroalkane proton of 3a
would lead to a reversible process and the subsequent
Michael-Wittig reaction would favor the reaction of (R)-3a to
give the diastereoselectivity, even though the kR/kS is ∼2.
Compound 3a recovered from the reaction during 50% conver-
sion showed 0% ee. This outcome would result from the rapid
racemization or because no enantioselective addition of 2 to
1a occurred in the presence of catalyst I. To shed light on the
Supporting Information Available: Experimental pro-
cedures and characterization data for the new compounds
and X-ray crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL9021832
(24) Structure of VII:
(25) The reaction with ꢀ-alkyl-substituted enals was stopped at the
second-step conjugate addition. Attempts at activating the reaction are
underway. The reaction of ꢀ-alkyl-substituted nitroolefins with compound
2 is feasible and actively being investigated.
(23) For reviews, see: (a) Connon, S. J. Synlett 2009, 354. (b) Connon,
S. J. Chem. Commun. 2008, 2499. (c) Zhang, Z.; Schreiner, P. R. Chem.
Soc. ReV. 2009, 38, 1187.
Org. Lett., Vol. 11, No. 22, 2009
5249