Cinchona alkaloid amides/dialkylzinc catalyzed enantioselective desymmetrization of aziridines with phosphites

Article abstract of DOI:10.1021/ja309963w

The first highly enantioselective desymmetrization of aziridines with phosphites has been developed. Excellent yields and enantioselectivities were observed for the reaction with various aziridines using a new class of readily accessible chiral catalysts derived from 9-amino-9-deoxy-epi-cinchona alkaloids. In studies probing the reaction mechanism, we observed some complexes for the cinchona alkaloid amide-Zn(II) by ESI-MS analysis.

Full text of DOI:10.1021/ja309963w

Communication
pubs.acs.org/JACS
Cinchona Alkaloid Amides/Dialkylzinc Catalyzed Enantioselective
Desymmetrization of Aziridines with Phosphites
Masashi Hayashi, Noriyuki Shiomi, Yasuhiro Funahashi, and Shuichi Nakamura*
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology Gokiso, Showa-ku, Nagoya
466-8555, Japan
S
* Supporting Information
Initially, we examined the ring-opening reaction of aziridines
having various protecting groups on nitrogen using a
stoichiometric amount of Et2Zn (Table 1).
ABSTRACT: The first highly enantioselective desymmet-
rization of aziridines with phosphites has been developed.
Excellent yields and enantioselectivities were observed for
the reaction with various aziridines using a new class of
readily accessible chiral catalysts derived from 9-amino-9-
deoxy-epi-cinchona alkaloids. In studies probing the
reaction mechanism, we observed some complexes for
the cinchona alkaloid amide-Zn(II) by ESI-MS analysis.
a
Table 1. Screening of Protecting Groups on Aziridines
entry
PG
1
metal
Et2Zn
3
yield (%)
ptically active β-aminophosphonic acids and their
Oderivatives are very important compounds because of
their biological properties1 and their catalytic ability as chiral
ligands.2 Therefore, the stereoselective synthesis of chiral β-
aminophosphonic acids and their derivatives has received
considerable attention. One of the most efficient and direct
methods for the synthesis of chiral β-aminophosphonates is the
enantioselective desymmetrization of aziridines with phos-
phites.3 Although excellent methodologies have been developed
in the enantioselective hydrophosphonylation with various
electrophiles such as aldehydes, ketones, imines, α,β-unsatu-
rated carbonyl compounds, and nitroolefins,4 there is no report
on the enantioselective desymmetrization of aziridines with
phosphites due to their low reactivity.5 Yet, we recently
reported the first enantioselective hydrophosphonylation of
ketimines using cinchona alkaloid derivatives and Na2CO3 as an
activating reagent for phosphites.6 We hypothesized that the
activation of phosphites using some reagents would be effective
for the ring-opening reaction of less reactive aziridines. Herein,
our ongoing interest was extended to the enantioselective
desymmetrization of aziridines with phosphites using the
combination of novel cinchona alkaloid catalysts and dialkylzinc
(Figure 1).
1
Ts
Bz
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
−
−
17
68
−
−
−
−
2
Et2Zn
Et2Zn
Et2Zn
Et2Zn
Et2Zn
Et2Zn
nBuLi
Bu2Mg
Et3Al
3
SO2Py
4
COPy
5
3,5-(CF3)2Bz
3,5-(NO2)2Bz
4-OMeBz
COPy
6
7
1g
1d
1d
1d
1d
3g
3d
3d
3d
3d
8
b
9
COPy
<30
b
10
11
COPy
trace
COPy
Zn(OTf)2
−
a
Reaction conditions: aziridine 1 (0.2 mmol), metal (0.30 mmol),
diphenyl phosphite 2 (0.3 mmol), and toluene (1.5 mL) were used.
b
1
Determined by H and 31P NMR.
Although the reaction of N-tosylated or N-benzoylated
aziridines 1a,b did not afford products (entries 1 and 2), the
reaction of N-(2-pyridinesulfonyl)- or N-(2-picolinoyl)-
aziridines 1c,d did (entries 3 and 4). The reaction with various
aziridines (1c−f) having other benzoyl groups, such as 3,5-
bis(trifluoromethyl)benzoyl, 3,5-dinitrobenzoyl, or 4-methox-
ybenzoyl groups, failed to afford products (entries 5−7). The
reaction using other bases such as nBuLi, Bu2Mg, and Et3Al, or
Zn(OTf)2 as a Lewis acid, also did not afford products (entries
8−11). We next examined the catalytic desymmetrization of N-
(2-picolinoyl)aziridines with phosphites using Et2Zn as an
activating reagent for phosphites. The desymmetrization of
aziridines with diphenyl phosphite 2 was carried out in the
presence of 10 mol % of cinchona alkaloid derivatives/Et2Zn at
rt (Table 2).7
The reaction using cinchona alkaloid catalysts, such as
cinchonine, cinchonidine, quinidine, and quinine (4a−d), gave
the product 3d in moderate yield but with low enantiose-
Figure 1. Catalytic enantioselective desymmetrization of aziridines
with phosphites.
Received: October 9, 2012
Published: November 12, 2012
© 2012 American Chemical Society
19366
dx.doi.org/10.1021/ja309963w | J. Am. Chem. Soc. 2012, 134, 19366−19369

Journal of the American Chemical Society
Communication
summarized in Table 3. Catalyst loading could be reduced to 5
mol % without decreasing the yield and enantioselectivity
Table 2. Enantioselective Desymmetrization of Aziridines
1d,h−j with Diphenyl Phosphite 2
a
Table 3. Enantioselective Desymmetrization of Various
a
Aziridines 1d,k−q with Diphenyl Phosphite 2
b
entry
R (1)
3
4
time (h) yield (%)
ee (%)
1
2
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
H (1d)
Cl (1h)
NO2 (1i)
OMe (1j)
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3d
3h
3i
4a
4b
4c
4d
4e
4f
3
3
3
3
3
3
3
3
3
3
3
3
3
1
57
50
48
48
36
32
84
70
37
31
80
94
6
3 (S,S)
3 (S,S)
5 (S,S)
1 (S,S)
3 (S,S)
3
4
5
6
38 (R,R)
99 (R,R)
94 (S,S)
33 (R,R)
35 (S,S)
98 (S,S)
99 (R,R)
54 (R,R)
96 (R,R)
7
4g
4h
4i
8
9
10
4j
c
11
4h
4g
4g
4g
c
12
c
13
c
14
3j
91
a
Reaction conditions: aziridine 1 (0.2 mmol), Et2Zn (0.02 mmol), 4
(0.02 mmol), diphenyl phosphite 2 (0.3 mmol), MS 4 Å (40 mg), and
b
toluene (1.5 mL) were used. The absolute configuration of 3 is
provided in parentheses. Benzene was use as a solvent.
c
a
Reaction conditions: aziridine 1 (0.2 mmol), Et2Zn (0.02 mmol), 4
(0.02 mmol), MS 4 Å (40 mg), and benzene (1.5 mL) were used.
b
c
The absolute configuration of 3 is provided in parentheses. Toluene
d
lectivity (entries 1−4).8,9 Although the reaction using N-
benzoyl-9-amino-9-deoxy-epi-cinchonine 4e also afforded 3d
with low enantioselectivity, 2-pyridinesulfonamide 4f derived
from cinchonine gave 3d with 38% ee (entries 5−6). To our
delight, excellent enantioselectivity could be obtained in the
reaction using N-(2-picolinoyl)-9-amino-9-deoxy-epi-cincho-
nine 4g (entry 7).10,11 The reaction using N-(2-picolinoyl)-9-
amino-9-deoxy-epi-cinchonidine 4h gave 3d having a stereo-
chemistry opposite that obtained in the reaction using 4g
(entry 8). Moderate enantioselectivity was obtained in the
presence of picolinoylamides 4i,j derived from quinidine and
quinine (entries 9−10). After optimization of the solvent, the
reaction in benzene afforded the best enantioselectivity (entry
11). In order to improve the reactivity, we optimized the
structure of aziridine to add the substituent on the 4-position of
the pyridine ring, which can control the Lewis basicity of a
nitorgen atom. The reaction of a 4-chloro-2-picolinoyl
derivative 1h gave product 3h in good yield with excellent
enantioselectivity; however, 4-nitro derivative 1i gave 3i in low
yield (entries 12−13). The reactivity could be improved by the
reaction using electron-donating 4-methoxy-2-picolinoyl de-
rivative 1j with high enantioselectivity (entry 14). The absolute
configuration of product 3d was determined to be (1R,2R) by
X-ray crystallographic analysis (entry 7).
was used as a solvent. MS 5 Å was used instead of MS 4 Å, and
Na2CO3 (1.5 equiv) was added. Undesired phospha-Brook rearrange-
e
ment product was obtained.
(entry 2). In the case of the reaction using 2 mol % of catalyst,
changing the molecular sieves from 4 to 5 Å and the addition of
sodium carbonate were effective for improving reactivity (entry
3). The catalytic desymmetrization of other six-membered
aziridines 1k and 1l using 4g or 4h afforded both enantiomers
of products 3k and 3l with excellent enantioselectivity (entries
5−8). The reaction of five-membered aziridines 1m and 1n
afforded products 3m and 3n in moderate yield with high
enantioselectivity (entries 9−12). In the case of the reaction of
seven-membered aziridine 1o, the reaction hardly proceeded,
even with a high catalyst loading (entry 13). In the case of the
low reactive aziridines 1n,o, an undesired phospha-Brook
rearrangement product was obtained (entries 11−13). On the
other hand, nonbicyclic aziridines 1p,q were also good
substrates with respect to enantioselectivity and chemical
yield (entries 14−17). To the best of our knowledge, this result
is the first example of the enantioselective desymmetrization of
aziridines with phosphorus nucleophiles.
We next examined the transformation of product 3d to an
optically active β-aminophosphonic acid 6. After the trans-
esterification of optically active diphenyl β-amino phosphonate
3d to dimethyl phosphonate 5 using MeONa/MeOH,
hydrolysis under acidic conditions afforded the corresponding
Having established the optimized reaction conditions for the
desymmetrization of aziridines, we next examined the reaction
of various aziridines with diphenyl phosphite. The results are
19367
dx.doi.org/10.1021/ja309963w | J. Am. Chem. Soc. 2012, 134, 19366−19369

Journal of the American Chemical Society
Communication
β-aminophosphonic acid 6 in 97% yield without loss in
enantiopurity (Scheme 1).
undesired product. In order to clarify the assumed reaction
mechanism, we conducted some spectroscopic analyses. The
ESI-MS analysis of the mixture of 4g, Et2Zn, and diphenyl
phosphite in a 1:1:1 ratio in toluene showed complex A (cation
mode, calcd for C37H36N4O4PZn as complex A + H+: 695.2,
found: 695.1; see Supporting Information). We also observed
complex B in the case of the reaction using unreactive dimethyl
phosphite (cation mode, calcd for C37H39N6O2Zn as complex
Scheme 1. Synthesis of the Optically Active β-
Aminophosphonic Acid 6
−
B− OP(OR)2 : 663.2, found: 663.2). These signals support our
proposed reaction mechanism.14
From the above-mentioned consideration and absolute
stereochemistry of the product, the assumed structure of the
most reactive complex C is shown in Figure 3. Diphenyl
The enantioselective desymmetrization using N-(2-picolino-
yl)-9-amino-epi-9-deoxy-cinchonine 4g gave products 3 in high
yield with high enantioselectivity, although N-benzoyl-9-amino-
9-deoxy-epi-cinchonine 4e afforded an almost racemic product
(Table 2, entry 5 vs 7). On the other hand, the reaction of N-
benzoyl aziridine with diphenyl phosphite 2 did not afford
product 3 (Table 1, entry 2). These results imply that the
pyridyl groups in both aziridines and catalysts are nessesary to
give products in high yield with high stereoselectivity. The
proposed catalytic cycle for the enantioselective desymmetriza-
tion of aziridine is shown in Figure 2. The picolinoylamide
Figure 3. Proposed transition state for the reaction of 1d with
diphenyl phosphite 2 using 4g. H-atoms have been omitted for clarity.
phosphite approaches aziridine by coordinating to the
quinuclidine moiety in complex C; therefore the (1R,2R)-
isomer is a preferable form. Further studies are required to fully
elucidate the mechanistic details of the desymmetrization.
In conclusion, we have demonstrated the first enantiose-
lective desymmetrization of aziridines with phosphites using a
new class of readily accessible chiral catalysts derived from the
9-amino-9-deoxy-epi-cinchona alkaloid in combination with
Et2Zn. The catalytic desymmetrization of aziridines was
screened for a broad range of aziridines. This approach gives
us direct access to both enantiomers of optically active β-
aminophosphonates in high yields with high enantioselectiv-
ities. Further studies are in progress to study the potential of
these catalytic systems for other processes.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedure and characterization data including X-
ray crystallography analysis of 3d. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
S
Figure 2. Assumed reaction mechanism for the desymmetrization of
aziridines 1d with diphenyl phosphite 2.
AUTHOR INFORMATION
Corresponding Author
snakamur@nitech.ac.jp
Notes
■
catalyst 4g reacts with Et2Zn and diphenyl phosphite 2 to afford
4g-Zn(II)-phosphite (complex A).12 Picolinoyl aziridine 1d
coordinates to the zinc cation by N,O-bidantate chelation to
give complex B. Subsequently, diphenyl phosphite coordinates
to the quinuclidine moiety in complex B by H-bonding
(complex C). Therefore, the reactivities for aziridine 1b and
diphenyl phosphite are effectively enhanced by 4g-Zn(II)
catalysts.13 Phosphite attacks the aziridine carbon to give
complex D (path a), which affords product 3d by a proton
exchange reaction with diphenyl phosphite. Yet, in the case of
the reaction with less reactive aziridines such as 1n,o, the
phosphite attacks the picolinoyl carbon, which was also
activated by 4g-Zn(II) catalysts, to give α-ketophosphonates
(path b). Further nucleophilic addition of diphenyl phosphite
affords the phospha-Brook rearangement product as an
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by Commons from JST. We
thank Prof. Norio Shibata at Nagoya Institute of Technology
for unrestricted access to analytical facilities.
REFERENCES
■
(1) For reviews of the biological activity of β-amino phosphonic
acids, see: (a) Hildebrand, R. L.; Henderson, T. O. In The Role of
Phosphonates in Living Systems; Hildebrand, R. L., Eds.; CRC: Boca
19368
dx.doi.org/10.1021/ja309963w | J. Am. Chem. Soc. 2012, 134, 19366−19369

Journal of the American Chemical Society
Communication
Raton, FL, 1983; pp 5−29. (b) Horiguchi, M.; Kandatsu, M. Nature
1959, 184, 901−902. (c) Fields, S. C. Tetrahedron 1999, 55, 12237−
12273. (d) Kukhar, V. P.; Hudson, H. R., Eds. Aminophosphonic and
Aminophosphinic Acids: Chemistry and Biological Activity; Wiley: New
York, NY, 2000. (e) Palacios, F.; Alonso, C.; de Los Santos, J. M.
Chem. Rev. 2005, 105, 899−931.
(2) (a) Guiry, P. J.; Saunders, C. P. Adv. Synth. Catal. 2004, 346,
497−537. (b) Amoroso, D.; Graham, T. W.; Guo, R.; Tsang, C.-W.;
Abdur-Rashid, K. Aldrichimica Acta 2008, 41, 15−26 and references
therein.
(3) For reviews on enantioselective desymmetrization of aziridines
with various nucleophiles, see: (a) Pineschi, M. Eur. J. Org. Chem.
2006, 4979−4988. (b) Hu, X. E. Tetrahedron 2004, 60, 2701−2743.
(c) Schneider, C. Angew. Chem., Int. Ed. 2009, 48, 2082−2084.
(4) For reviews on enantioselective hydrophoshonylation, see:
(10) For N-benzoyl 9-amino-9-deoxy-epi-cinchona alkaloid catalysts,
see: (a) Taggi, A. E.; Hafez, A. M.; Wack, H.; Young, B.; Drury, W. J.,
III; Lectka, T. J. Am. Chem. Soc. 2000, 122, 7831−7832. (b) Brunner,
H.; Schmidt, P. Eur. J. Org. Chem. 2000, 2119−2133. (c) Brunner, H.;
Schmidt, P.; Prommesberger, M. Tetrahedron: Asymmetry 2000, 11,
1501−1512. (d) Taggi, A. E.; Hafez, A. M.; Wack, H.; Young, B.;
Ferraris, D.; Lectka, T. J. Am. Chem. Soc. 2002, 124, 6626−6635.
(e) Rogers, L. M.-A.; Rouden, J.; Lecomte, L.; Lasne, M.-C.
Tetrahedron Lett. 2003, 44, 3047−3050. (f) Brunner, H.; Baur, M.
A. Eur. J. Org. Chem. 2003, 2854−2862. (g) Baur, M. A.; Riahi, A.;
́
Henin, F.; Muzart, J. Tetrahedron: Asymmetry 2003, 14, 2755−2761.
(h) Seitz, T.; Baudoux, J.; Bekolo, H.; Cahard, D.; Plaquevent, J.-C.;
Lasne, M.-C.; Rouden, J. Tetrahedron 2006, 62, 6155−6165. (i) Cui,
H.-L.; Peng, J.; Feng, X.; Du, W.; Jiang, K.; Chen, Y.-C. Chem.Eur. J.
2009, 15, 1574−1577. (j) Xiong, X.-F.; Jia, Z.-J.; Du, W.; Jiang, K.; Liu,
T.-Y.; Chen, T. Y.-C. Chem. Commun. 2009, 6994−6996. (k) Zhang,
W.; Zheng, S.; Liu, N.; Werness, J. B.; Guzei, L. A.; Tang, W. J. Am.
Chem. Soc. 2010, 132, 3664−3665. (l) Rana, N. K.; Selvakumar, S.;
Singh, V. K. J. Org. Chem. 2010, 75, 2089−2091. (m) Sladojevich, F.;
Trabocchi, A.; Guarna, A.; Dixon, D. J. J. Am. Chem. Soc. 2011, 133,
1710−1713.
(a) Merino, P.; Marques
2008, 350, 1195−1208. (b) Ordon
́
-Lop
́
ez, E.; Herrera, R. P. Adv. Synth. Catal.
ez, M.; Rojas-Cabrera, H.; Cativiela,
́
̃
C. Tetrahedron 2009, 65, 17−49. (c) Zhao, D.; Wang, R. Chem. Soc.
Rev. 2012, 41, 2095−2108. For selected recent papers for
enantioselective desymmetrization of aziridines, see: (d) Nakamura,
S.; Hayashi, M.; Kamada, Y.; Sasaki, R.; Hiramatsu, Y.; Shibata, N.;
Toru, T. Tetrahedron Lett. 2010, 51, 3820−3823. (e) Xu, Y.; Lin, L.;
Kanai, M.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2011, 133,
5791−5793. (f) Ohmatsu, K.; Hamajima, Y.; Ooi, T. J. Am. Chem. Soc.
2012, 134, 8794−8797.
(11) For selected examples of chiral catalysts having a 2-picolinoyl
group, see: (a) Trost, B. M.; Hachiya, I. J. Am. Chem. Soc. 1998, 120,
1104−1105. (b) Soloshonok, V. A.; Ueki, H.; Tiwari, R.; Cai, C.;
Hruby, V. J. J. Org. Chem. 2004, 69, 4984−4990. (c) Trost, B. M.;
Zhang, Y. J. Am. Chem. Soc. 2006, 128, 4590−4591. (d) Onomura, O.;
Kouchi, Y.; Iwasaki, F.; Matsumura, Y. Tetrahedron Lett. 2006, 47,
3751−3754. (e) Zheng, H.-J.; Chen, W.-B.; Wu, Z.-J.; Deng, J.-G.; Lin,
W.-Q.; Yuan, W.-C.; Zhang, X.-M. Chem.Eur. J. 2008, 14, 9864−
9867. (f) Guizzetti, S.; Benaglia, M.; Celentano, G. Eur. J. Org. Chem.
2009, 3683−3687.
(5) For the synthesis of β-amino phosphonates through the ring-
opening reaction of aziridines with phosphites, see: (a) Duggan, M. E.;
Karanewsky, D. S. Tetrahedron Lett. 1983, 24, 2935−2938. For the
reaction of aziridines with phosphorus nucleophiles, see: (b) Katagiri,
T.; Takahashi, M.; Fujiwara, Y.; Ihara, H.; Uneyama, K. J. Org. Chem.
1999, 64, 7323−7329. (c) Caiazzo, A.; Dalili, S.; Yudin, A. K. Org. Lett.
2002, 4, 2597−2600.
1
(12) We also observed 4g-ZnEt species by H NMR analysis; see
Supporting Information.
(6) (a) Nakamura, S.; Hayashi, M.; Hiramatsu, Y.; Shibata, N.;
Funahashi, Y.; Toru, T. J. Am. Chem. Soc. 2009, 131, 18240−18241.
For related recent studies from our group, see: (b) Nakamura, S.;
Nakashima, H.; Yamamura, A.; Shibata, N.; Toru, T. Adv. Synth. Catal.
2008, 350, 1209−1212. (c) Ohara, M.; Nakamura, S.; Shibata, N. Adv.
Synth. Catal. 2011, 353, 3285−3289. (d) Hayashi, M.; Nakamura, S.
Angew. Chem., Int. Ed. 2011, 50, 2249−2252.
(13) For tridentate N-acyl 9-amino-9-deoxy-epi-cinchona alkaloids
and metal salts, see a review: (a) Stegbauer, L.; Sladojevich, F.; Dixon,
D. J. Chem. Sci. 2012, 3, 942−958. See also: (b) Casarotto, V.; Li, Z.;
Boucau, J.; Lin, Y.-M. Tetrahedron Lett. 2007, 48, 5561−5564. (c) Lin,
Y.-M.; Boucau, J.; Li, Z.; Casarotto, V.; Lin, J.; Nguyen, A. N.;
Ehrmantraut, J. Org. Lett. 2007, 9, 567−570. (d) Lin, Y.-M.; Li, Z.;
Boucau, J. Tetrahedron Lett. 2007, 48, 5275−5278. (e) Chidara, S.; Lin,
Y.-M. Synlett 2009, 1675−1679.
(14) Catalysts derived from 4i and 4j showed low reactivity and
enantioselectivity (Table 1, entries 9 and 10). We assumed that the
methoxy group in 4i and 4j enhanced the coordination ability of
nitrogen in the quinoline ring. These nitrogens would cause the
formation of a more complicated catalyst to give the product in low
yield with low enantioselectivity.
(7) For the enantioselective hydrophosphonylation of α,β-unsatu-
rated carbonyl compounds or nitroalkenes using phosphites and
Et2Zn, see: (a) Zhao, D.; Yuan, Y.; Chan, A. S. C.; Wang, R. Chem.
Eur. J. 2009, 15, 2738−2741. (b) Zhao, D.; Wang, Y.; Mao, L.; Wang,
R. Chem.Eur. J. 2009, 15, 10983−10987. For diastereoselective
addition of phosphite to nitroalkenes using Et2Zn, see: (c) Enders, D.;
Tedeschi, L.; Bats, J. W. Angew. Chem., Int. Ed. 2000, 39, 4605−4607.
For enantioselective hydrophosphonylation of aldehydes using
cinchona alkaloid−Lewis acid catalysts, see: (d) Yang, F.; Zhao, D.;
Lan, J.; Xi, P.; Yang, L.; Xiang, S.; You, J. Angew. Chem., Int. Ed. 2008,
47, 5646−5649. For enantioselective hydrophosphonylation of
aldehydes using a base, see: (e) Suyama, K.; Sakai, Y.; Matsumoto,
K.; Saito, B.; Katsuki, T. Angew. Chem., Int. Ed. 2010, 49, 797−799.
(8) The reaction without diethylzinc did not afford the product. We
also examined the reaction with various phosphites such as diethyl
phosphite, dibenzyl phosphite, and bis(2,2,2-trifluorethyl) phosphite;
however the enantioselectivity or yield was not good (see Supporting
Information).
(9) We also examined the desymmetrization of aziridines using
Trost’s dinuclear-Zn catalyst as a pioneering chiral zinc catalysts to
give the product in moderate yield with enantioselectivity (see
Supporting Information). For Trost’s dinuclear-Zn catalyst, see
selected examples: (a) Trost, B. M.; Ito, H. J. Am. Chem. Soc. 2000,
122, 12003−12004. (b) Trost, B. M.; Ito, H.; Silcoff, E. R. J. Am. Chem.
Soc. 2001, 123, 3367−3368. (c) Trost, B. M.; Silcoff, E. R.; Ito, H. Org.
Lett. 2001, 3, 2497−2500. (d) Trost, B. M.; Terrell, L. R. J. Am. Chem.
Soc. 2003, 125, 338−339. (e) Trost, B. M.; Lupton, D. W. Org. Lett.
2007, 9, 2023−2026. (f) Trost, B. M.; Hitce, J. J. Am. Chem. Soc. 2009,
131, 4572−4573.
19369
dx.doi.org/10.1021/ja309963w | J. Am. Chem. Soc. 2012, 134, 19366−19369