Enantioselective organocatalytic Michael-Wittig-Michael-Michael reaction: Dichotomous construction of pentasubstituted cyclopentanecarbaldehydes and pentasubstituted cyclohexanecarbaldehydes

Article abstract of DOI:10.1021/ol1030487

Michael addition of carbethoxymethylenetriphenylphosphorane (a Wittig reagent) to nitroalkenes, followed by a reaction with ethyl formylformate and cinnamaldehydes, or formaldehyde and cinnamaldehydes, provided the respective pentasubstituted cyclopentanecarbaldehydes bearing a quaternary carbon center and pentasubstituted cyclohexanecarbaldehydes having five contiguous stereocenters with excellent enantioselectivities (up to >99% ee).

Full text of DOI:10.1021/ol1030487

ORGANIC
LETTERS
2011
Vol. 13, No. 6
1278–1281
Enantioselective Organocatalytic
Michael-Wittig-Michael-Michael Reaction:
Dichotomous Construction of
Pentasubstituted Cyclopentanecarbaldehydes
and Pentasubstituted
Cyclohexanecarbaldehydes
Bor-Cherng Hong,* Roshan Y. Nimje, Cheng-Wei Lin, and Ju-Hsiou Liao
Department of Chemistry and Biochemistry, National Chung Cheng University,
Chia-Yi, 621, Taiwan, R.O.C.
chebch@ccu.edu.tw
Received December 16, 2010
ABSTRACT
Michael addition of carbethoxymethylenetriphenylphosphorane (a Wittig reagent) to nitroalkenes, followed by a reaction with ethyl formylformate and
cinnamaldehydes, or formaldehyde and cinnamaldehydes, provided the respective pentasubstituted cyclopentanecarbaldehydes bearing a quaternary
carbon center and pentasubstituted cyclohexanecarbaldehydes having five contiguous stereocenters with excellent enantioselectivities (up to >99% ee).
The recent impressive progress made in organocata-
lysis via cascade, tandem, domino,1,2 or sequential reac-
tion sequences has driven synthetic chemists to develop
new procedures and methodologies for producing di-
verse arrays of compounds in an enantioselective and
efficient manner. Among the strategies demonstrated,
several are noteworthy, including multicomponent
reactions,3 stereoselective synthesis of quaternary car-
bons, and the construction of poly contiguous stereo-
centers in a stereoselective manner,4 especially of those
bearing a quaternary carbon center.5 Moreover, poly-
substituted cyclopentanes and cyclohexanes are preva-
lent in natural products that display biological activities,
and their derivatives have long served as important
intermediates in organic synthesis.6 Despite the suc-
cesses in the approaches to these systems, a diverse
synthesis providing the cyclopentane and cyclohexane
derivatives with multifunctionality, quaternary carbon
centers, and poly contiguous stereocenters with excellent
enantioselectivities remains a compelling area of inves-
tigation. As an extension of our efforts in developing
organocatalytic annulations,7 we envisaged a strategy
that involved an enantioselective organocatalytic Mi-
chael-Wittig-Michael-Michael reaction, Scheme 1,
which in turn would afford pentasubstituted cyclopenta-
necarbaldehydes bearing a quaternary carbon center and
pentasubstituted cyclohexanecarbaldehydes having five
(1) For recent reviews see: (a) Alba, A.-N.; Companyo, X.; Viciano,
M.; Rios, R. Curr. Org. Chem. 2009, 13, 1432. (b) Enders, D.; Grondal,
€
C.; Huttl, M. R. M. Angew. Chem., Int. Ed. 2007, 46, 1570. (c) Poisson, T.
Synlett 2008, 147. (d) Walji, A. M.; MacMillan, D. W. C. Synlett 2007,
1477.
(2) For selected recent examples of organocatalytic cascade/domino
reactions, see: (a) Wang, C.; Han, Z.-Y.; Luo, H.-W.; Gong, L.-Z. Org.
Lett. 2010, 12, 2266. (b) Zhang, F.-L.; Xu, A.-W.; Gong, Y.-F.; Wei,
ꢀ
M.-H.; Yang, X.-L. Chem.;Eur. J. 2009, 15, 6815. (c) Franzeen, J.;
Fisher, A. Angew. Chem., Int. Ed. 2009, 48, 787. (d) Zhao, G.-L.; Vesely,
J.; Rios, R.; Ibrahem, I.; Sunden, H.; Cordova, A. Adv. Synth. Catal.
2008, 350, 237. (e) Hazelard, D.; Ishikawa, H.; Hashizume, D.; Koshino,
H.; Hayashi, Y. Org. Lett. 2008, 10, 1445. (f) Penon, O.; Carlone, A.;
Mazzanti, A.; Locatelli, M.; Sambri, L.; Bartoli, G.; Melchiorre, P.
Chem.;Eur. J. 2008, 14, 4788. (g) Carlone, A.; Cabrera, S.; Marigo, M.;
Jørgenson, K. A. Angew. Chem., Int. Ed. 2007, 46, 1101. (h) Enders, D.;
€
Huttl, M. R. M.; Runsink, J.; Raabe, G.; Wendt, B. Angew. Chem., Int.
Ed. 2007, 46, 467. (i) Zhou, J.; List, B. J. Am. Chem. Soc. 2007, 129, 7498.
(j) Beeson, T. D.; Mastracchio, A.; Hong, J.-B.; Ashton, K.; MacMillan,
D. W. C. Science 2007, 316, 582.
r
10.1021/ol1030487
Published on Web 02/24/2011
2011 American Chemical Society

contiguous stereocenters, both with excellent enantioselec-
tivities.
On the other hand, the approach to the cyclohexane-
carboxylates from racemic 4a, prepared from (()-3a and
formaldehyde with cinnamaldehyde via double Michael
reactions, was conducted with catalyst I-HOAc in
CHCl3 at ambient temperature for 56 h, followed by
the reduction with NaBH4 in EtOH to give a 1:2 ratio of
syn-6a and anti-6a in 74% yields with 95% ee (Table 2,
entry 1). Owing to the 1:2 ratio of the diastereoisomeric
products, a moderate kinetic asymmetric transforma-
tion (KAT) took place in this reaction. In fact, the
occurrence of KAT was further supported by the fact
that an enantiomeric excess of 79% for the recovered
(R)-4a was observed after the reaction of (()-4a with 0.7
equiv of (E)-3-(4-bromophenyl)acrylaldehyde (5b).8 The
originally formed cyclohexanecarbaldehydes were
somewhat unstable during the isolation and purification
process; therefore, the reaction adducts were immedi-
ately reduced by NaBH4 to the corresponding alcohol,
and the ee values were determined on the major iso-
meric alcohol (anti-6a). The reaction in CH3CN was
completed in a short period of time (48 h) and with
higher yields (79%) with the same 1:2 isomeric ratio of
the alcohol products (Table 2, entry 2). The reactions in
other solvents (e.g., toluene, CH2Cl2, THF, DMF, and
EtOH) afforded lower yields and/or lower stereoselectivities
(Table 2, entries 3-7). Replacement of HOAc by another
acid (e.g., PNBA) gave a lower yield (Table 2, entry 8).
We began by embarking on the asymmetric domino-
Michael reactions toward racemic 7a, prepared from the
reaction of (()-3a and formylformate with cinnamalde-
hyde. To our delight, the reaction with the Jørgen-
sen-Hayashi catalyst I and HOAc in CHCl3 at ambient
temperature for 48 h provided a 1:1 diastereomeric ratio of
anti-8a and syn-8a in 94% yield, both with 99% ee (Table
1, entry 1). To optimize the reaction, we screened various
organocatalysts, solvents, additives, and reaction condi-
tions. The reaction in CH2Cl2 gave similar yield (91%) and
enantioselectivity (99% ee) but was somewhat slower to
reach completion (Table 1, entry 2). Conducting the same
reaction in other solvents (e.g., CH3CN, THF, EtOH,
DMF, and EtOAc) afforded lower yields and required
longer reaction times (Table 1, entries 3-8). Notably, the
reaction in EtOH did not need an acid additive (HOAc) for
the reaction and was completed in 83 h, however, with
somewhat lower yield (59%) (Table 1, entry 8). Further-
more, the reactions with catalyst III in the presence or the
absence of additive TEA afforded very low yields (Table 1,
entries 9 and 10). Reaction with pyrrolidine (II)-HOAc
gave lower yields (33%) but provided the racemic products
that served as suitable standards for HPLC analysis in
determining the ee values. Apparently, the original
procedure provides the best conditions for the reaction
(Table 1, entry 1). The structure of anti-8a was assigned by
the single-crystal X-ray analysis of the corresponding alcohol
(þ)-9 (Figure 1), obtained by the treatment of anti-8a with
NaBH4 (0 °C to rt, 6 h, 68% yield).
Table 1. Screening of Catalyst, Solvent, and Reaction Condi-
tions for the Synthesis of Pentasubstituted Cyclopentanecar-
baldehydesa
Scheme 1. Plan for the Dichotomous Synthesis
catalyst-
additive
time
(h)
yield
(%)b
ee
(%)c
entry
solvent
1
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I
CHCl3
CH2Cl2
toluene
CH3CN
THF
48
54
72
96
96
96
96
83
96
96
60
94
91
83
51
32
8
99, (99)d
99
2
3
nd
4
nd
5
nd
6
DMF
nd
7
EtOAc
EtOH
15
59
5
nd
8
nd
9
III
CH3CN
CH3CN
CHCl3
nd
10
11
III-TEA
II-AcOH
10
33
nd
0
a Unless otherwise noted, the reactions were performed in 0.5 M (()-
7a at 25 °C. b A 1:1 ratio of anti-8a:syn-8b was obtained in all cases, as
determined by 1H NMR analysis of the crude reaction mixture. c Unless
otherwise noted, the ee of anti-8a was determined by HPLC with a chiral
column (Chiralpak IC). d The ee of syn-8a in parentheses was determined
by HPLC with a chiral column (Chiralpak IC). nd: not determined.
Figure 1. Stereo plots of the X-ray crystal structures of (þ)-9: C,
gray; N, blue; O, red.
Org. Lett., Vol. 13, No. 6, 2011
1279

Substitution of HOAc with base (e.g., DABCO and DBU) in
the reaction preferentially provided the product anti-6a,
although with lower yields and ee value (for DABCO) or
much lower yields (for DBU and sparteine) (Table 2, entries
9-11). Reaction with pyrrolidine (II)-AcOH afforded the
products with excellent diastereoselectivity (9:1), (Table 2,
entry 12). As noted previously, the reactions with catalyst III
afforded very low yields (Table 2, entries 13 and 14).
Table 2. Screening of Catalyst, Solvent, and Reaction Condi-
tions for the Synthesis of Pentasubstituted Cyclohexanecar-
boxylatesa
Although the optimal reaction conditions for higher
yields and enantioselectivities for this dichotomous reaction
have been determined, we wanted to achieve better diaster-
eoselectivities to improve this asymmetric synthesis. Ac-
cordingly, use of the optically enriched 3a appeared to be a
solution. Enantioselective conjugate addition of stabilized
phosphorus ylides to nitroalkenes followed by proton
transfer to give chiral functionalized P-ylides had not been
previously disclosed until our recent observation in the
study of organocatalytic [1 þ 2 þ 3] annulation.9 After
extensive screening a series of organocatalysts in the en-
antioselective addition reaction, we observed the successful
reaction with catalyst IV (20 mol %, CH3CN, -40 °C, 4 d)
to afford 51% ee of (R)-3a.10 To our surprise, the reaction
with the monosubstituted thiourea catalyst V not only
provided the enantioselective 3a, but the product displayed
inverse enantioselectivity to give (S)-3a, as opposed to the
disubstituted thiourea catalyst IV.11 Various reaction con-
ditions were screened, and the best result was obtained with
60% ee by slow addition of the stabilized phosphorus ylides
2 to a mixture of nitroalkene 1a and catalyst VI (20 mol %)
in CHCl3 over 8 h at -40 °C, followed by the reaction at the
same temperature for an additional 72 h. Treatment of (R)-
3a (60% ee), prepared using the monothiourea catalyst VI,
with formylformate, followed by the addition of cinnamal-
dehyde (5a) and catalyst I-HOAc (20 mol %) in CHCl3,
provided anti-8a and syn-8a with much better dr (anti:syn =
3.7:1) in 94% total yields, (Table 3, entry 1). Having
established the optimal reaction conditions, we studied the
use of different R,β-unsaturated aldehydes to synthesize a
variety of pentasubstituted cyclopentanecarbaldehydes
(Table 3, entries 2-9). These adducts were in all cases
isolated in good yields (more than 90% yields in many
cyclopentanecarbaldehyde examples), good dr, and with
high enantioselectivities (>99% ee).12 On the other hand,
catalyst-
additive
time
(h)
yieldb
(%)
ee
entry
solvent
drc
1:2
(%)d
1
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I-AcOH
I
CHCl3
CH3CN
toluene
CH2Cl2
THF
56
48
60
50
60
72
72
72
56
72
72
48
96
96
74
79
57
78
58
9
95
96
nd
84
nd
nd
nd
nd
63
nd
nd
0
2
1:2
1:3
1:2
1:1
nd
3
4
5
6
DMF
7
EtOH
64
8
1:1
nd
8
I-PNAB
I-DABCO
I-DBU
I-sparteine CH3CN
II-AcOH
III
CH3CN
CHCl3
CH3CN
9
66
5
1:3.5
nd
10
11
12
13
14
23
76
5
nd
CH3CN
CH3CN
CH3CN
1:9
nd
nd
nd
III-Et3N
11
nd
a Unless otherwise noted, the reactions were performed in 0.5 M (()-
4a at 25 °C. b Total yields for the double Michael reactions and NaBH4
reduction. c syn:anti. Determined by 1H NMR analysis of the crude
reaction mixture. d The ee of anti-6a, determined by chiral column
(Chiralpak IC) of their corresponding alcohol. nd: not determined; na:
not available; DABCO: 1,4-diazabicyclo[2.2.2]octane; DBU: 1,8-
diazabicyclo[5.4.0]undec-7-ene; PNBA: 4-nitrobenzoic acid.
(3) For a review of natural product synthesis using multicomponent
ꢀ
reaction strategies, see: (a) Toure, B. B.; Hall, D. G. Chem. Rev. 2009,
109, 4439. Select examples: (b) Enders, D.; Narine, A. A. J. Org. Chem.
2008, 73, 7857. (c) Jiang, H.; Nielsen, J. B.; Nielsen, M.; Jørgensen, K. A.
Chem.;Eur. J. 2007, 13, 9068. (d) Zou, Y.; Wang, Q. R.; Goeke, A.
Chem.;Eur. J. 2008, 14, 5335. (e) Ramachary, D. B.; Reddy, Y. V.
J. Org. Chem. 2010, 75, 74.
(4) For recent examples of the construction of five contiguous
stereocenters via organocatalysis, see: (a) Reyes, E.; Jiang, H.; Milelli,
A.; Elsner, P.; Hazell, R. G.; Jørgensen, K. A. Angew. Chem., Int. Ed.
2007, 46, 9202. (b) Urushima, T.; Sakamoto, D.; Ishikawa, H.; Hayashi,
Y. Org. Lett. 2010, 12, 4588. (c) Imashiro, R.; Uehara, H.; Barbas, C. F.
Org. Lett. 2010, 12, 5250.
(5) (a) Wu, L. Y.; Bencivenni, G.; Mancinelli, M.; Mazzanti, A.;
Bartoli, G.; Melchiorre, P. Angew. Chem., Int. Ed. 2009, 48, 7196. (b)
Tan, B.; Shi, Z.; Chua, P. J.; Zhong, G. Org. Lett. 2008, 10, 3425. (c) Tan,
B.; Chua, P. J.; Zeng, X.; Lu, M.; Zhong, G. Org. Lett. 2008, 10, 3489. (d)
Tan, B.; Chu, J.; Li, Y.; Zhong, G. Org. Lett. 2008, 10, 2437. (e) Chen,
X. H.; Zhang, W. Q.; Gong, L. Z. J. Am. Chem. Soc. 2008, 130, 5652. (f)
Ishihara, K.; Nakano, K. J. Am. Chem. Soc. 2007, 129, 8930. (g)
Bencivenni, G.; Wu, L. Y.; Mazzanti, A.; Giannichi, B.; Pesciaioli, F.;
Song, M. P.; Bartoli, G.; Melchiorre, P. Angew. Chem., Int. Ed. 2009, 48,
7200.
(9) (a) See ref 7c. (b) Soon after our publication, a similar study of the
conjugate addition of the Wittig reagent to nitroalkenes was reported
that also afforded moderate ee; see: Allu, S.; Selvakumar, S.; Singh,
V. K. Tetrahedron Lett. 2010, 51, 446. (c) For an organocatalytic
enantioselective Mannich-type reaction of phosphorus ylides, see:
Zhang, Y.; Liu, Y.-K.; Kang, T.-R.; Hu, Z.-K.; Chen, Y.-C. J. Am.
Chem. Soc. 2008, 130, 2456.
(10) The absolute stereochemistry of (R)-3a was further elucidated
via the subsequent hydrogenation of (S)-4a, providing (2S,3R)-2-
methyl-4-nitro-3-phenylbutan-1-ol, and compared with the optical ro-
tation data of (2R,3S)-2-methyl-4-nitro-3-phenylbutanal, obtained
from the reaction of propionaldehyde and nitrostyrene catalyzed by
I-HOAc, via the literature procedure, see: Hayashi, Y.; Gotoh, H.;
Hayashi, T.; Shoji, M. Angew. Chem., Int. Ed. 2005, 44, 4212.
(11) For a recent review of unexpected inversions in asymmetric
(6) For reviews, see: (a) Heasley, B. Eur. J. Org. Chem. 2009, 1477. (b)
Schultz, A. G. Acc. Chem. Res. 1990, 23, 207.
(7) For our recent efforts in exploring new organocatalytic annula-
tions, see: (a) Hong, B.-C.; Kotame, P.; Liao, J.-H. Org. Biomol. Chem.
2011, 9, 382. (b) Hong, B.-C.; Dange, N. S.; Hsu, C.-S.; Liao, J.-H. Org.
Lett. 2010, 12, 4812. (c) Hong, B.-C.; Kotame, P.; Tsai, C.-W.; Liao,
J.-H. Org. Lett. 2010, 12, 776. (d) Hong, B.-C.; Jan, R.-H.; Tsai, C.-W.;
Nimje, R. Y.; Liao, J.-H.; Lee, G.-H. Org. Lett. 2009, 11, 5246. (e) Hong,
B.-C.; Nimje, R. Y.; Liao, J.-H. Org. Biomol. Chem. 2009, 7, 3095. (f)
Kotame, P.; Hong, B.-C.; Liao, J.-H. Tetrahedron Lett. 2009, 50, 704. (g)
Hong, B.-C.; Nimje, R. Y.; Sadani, A. A.; Liao, J.-H. Org. Lett. 2008, 10,
2345 and references cited therein.
ꢀ
reactions, see: Bartok, M. Chem. Rev. 2010, 110, 1663.
(8) 65% ee for the recovered (R)-4a in the reaction of (()-4a with 0.6
equiv of 5b and cat. I-HOAc (20 mol %), and 45% ee for the recovered
(R)-4a when (()-4a was reacted with 0.4 equiv of 5b and cat. I-HOAc
(20 mol %).
(12) Reaction of (()-7a with 5 gave 1:1 ratio of anti-8 and syn-8 with
the same high yields and enantioselectivities. The structure of syn-8 was
assigned on the basis of 1H and 13C NMR, COSY, DEPT, HMQC, and
NOESY analysis.
1280
Org. Lett., Vol. 13, No. 6, 2011

Table 3. Scope of the Organocatalytic Michael-
Wittig-Michael-Michael Reaction
Scheme 2. Plausible Dichotomous Reaction Mechanism of the
Succeeding Double-Michael Reaction with the Michael-Wittig
Adduct
number system, noted in Scheme 2).13 The relative topi-
cities were further demonstrated by the reaction with (S)-
enriched 7 to give anti-8, and by reaction with (S)-enriched
4 to give anti-6, respectively, as the major products.
In conclusion, we have discovered a sequential enantio-
selective organocatalytic Michael-Wittig-Michael-
Michael reaction. Remarkably, with the subtle variations
in the ester substituents on the enolates, this methodology
provides dichotomous construction of pentasubstituted
cyclopentanecarbaldehydes bearing a quaternary carbon
center and pentasubstituted cyclohexanecarbaldehydes
having five contiguous stereocenters with excellent enantios-
electivities (up to >99% ee). The production of five con-
tiguous chiral centers with high enantioselectivity is
especially noteworthy. An increase in diastereoselectivity
was achieved by the first organocatalytic asymmetric con-
jugate addition of a phosphonium ylide to nitrostyrenes
with the noncovalent thiourea catalyst VI. The unexpected
inversion in the asymmetric reactions obtained with mono-
thiourea V and dithiourea IV organocatalysts in the Mi-
chael reaction of the naked nucleophile (nonenolizable) 2 to
1a is notable. This observation may provide a useful clue in
the search for better organocatalysts for these types of
asymmetric conjugate addition reactions.
en-
try
time yield
ee
product
(h)a (%)b dr c (%)d
1
8a R1 = Ph; R2 = CO2Et; R3 = Ph
48 94 3.7:1 99,
(99)e
53 95 3.0:1 99
2
3
4
5
6
7
8
9
8b R1 = Ph; R2 = CO2Et; R3 = 4-BrC6H4
8cR1 = Ph; R2 = CO2Et; R3 = 4-OMeC6H4 61 93 3.0:1 99
8d R1 = Ph; R2 = CO2Et; R3 = 4-NO2C6H4 55 84 3.0:1 99f
8e R1 = Ph; R2 = CO2Et; R3 = 4-MeC6H4
8f R1 = Ph; R2 = CO2Et; R3 = 4-ClC6H4
8g R1 = 4-BrC6H4; R2 = CO2Et; R3 = Ph
60 89 2.7:1 95
56 92 3.0:1 99
55 94 3.2:1 97
8h R1 = 4-OMeC6H4; R2 = CO2Et; R3 = Ph 60 93 2.8:1 99
8i R1 = 4-FC6H4; R2 = CO2Et; R3 = Ph
51 91 3.1:1 99
48 79 6.3:1 96g
51 73 6.5:1 97g,h
59 76 5.4:1 93g,h
53 83 5.5:1 94g,h
10 6a R1 = Ph; R2 = H; R3 = Ph
11 6b R1 = Ph; R2 = H; R3 = 4-BrC6H4
12 6c R1 = Ph; R2 = H; R3 = 4-OMeC6H4
13 6d R1 = Ph; R2 = H; R3 = 4-NO2C6H4
a Reaction time for the double-Michael reaction step. b Combined
isolated yields of the isomers in the double-Michael reaction step.
c Determined by 1H NMR analysis of the crude reaction mixture.
d Unless otherwise noted, ee of the major stereoisomer (anti-isomer),
determined by HPLC with chiral column (Chiralpak IC). e The ee of the
minor stereoisomer is given in parentheses (syn-isomer). f Determined by
HPLC with chiral column (Chiralpak IA). g The ee of the corresponding
alcohol. h Determined by HPLC with chiral column (Chiralcel OD-H).
Acknowledgment. We acknowledge the financial sup-
port for this study from the National Science Council,
Taiwan, ROC. Thanks to the National Center for High-
Performance Computing (NCHC) for their assistance in
literature searching.
treatment of (R)-3a (60% ee), prepared with monothiourea
catalyst VI, with formaldehyde, followed by the addition of
R,β-unsaturated aldehydes 5and catalyst I-HOAc (20 mol %)
in CH3CN, provided anti-6 and syn-6 with good dr (e.g., anti-
6a:syn-6a = 6.3:1) and with high enantioselectivities, (Table 3,
entries 10-13).
To explain the stereochemistry of this transformation,
we propose a plausible mechanism, as shown in Scheme 2.
The reaction was initiated from the iminium activation of
the R,β-unsaturated aldehyde by catalyst I, followed by the
nitro-Michael addition of the nitroalkane nucleophile,
resulting from the aforementioned Michael-Wittig reac-
tion, from the Re face under the control of the catalyst to
give an intermediate with a 5-S configuration (irregular
Supporting Information Available. Experimental pro-
cedures and characterization data for the new compounds
and X-ray crystallographic data for compound (þ)-9 in
CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
(13) Thus, the stereoselectivity was similar to that observed in other
examples of organocatalysis; for examples, see: (a) Gotoh, H.; Okamura,
D.; Ishikawa, H.; Hayashi, Y. Org. Lett. 2009, 11, 4056. (b) Han, B.;
Xiao, Y.-C.; He, Z.-Q.; Chen, Y.-C. Org. Lett. 2009, 11, 4660. (c) Enders,
D.; Wang, C.; Bats, J. W. Synlett 2009, 1777.
Org. Lett., Vol. 13, No. 6, 2011
1281