Ru-Catalyzed Asymmetric Hydrogenative/Transfer Hydrogenative Desymmetrization of Meso-Epoxy Diketones

Article abstract of DOI:10.1021/acs.orglett.6b01073

Via a strategy of asymmetric reductive desymmetrization, chiral cis-epoxy naphthoquinols with multiple contiguous stereocenters and functional groups were synthesized with excellent enantioselectivities (96-99% ee) and diastereoselectivities (8/1-15/1). A combined asymmetric hydrogenation/transfer hydrogenation mechanism was proposed based on experimental results.

Full text of DOI:10.1021/acs.orglett.6b01073

Letter
pubs.acs.org/OrgLett
Ru-Catalyzed Asymmetric Hydrogenative/Transfer Hydrogenative
Desymmetrization of Meso-Epoxy Diketones
Yuping Hong,† Jianzhong Chen,‡ Zhenfeng Zhang,*,† Yangang Liu,† and Wanbin Zhang*,†,‡
‡
†School of Pharmacy and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road,
Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: Via a strategy of asymmetric reductive
desymmetrization, chiral cis-epoxy naphthoquinols with multi-
ple contiguous stereocenters and functional groups were
synthesized with excellent enantioselectivities (96−99% ee)
and diastereoselectivities (8/1−15/1). A combined asymmet-
ric hydrogenation/transfer hydrogenation mechanism was
proposed based on experimental results.
Scheme 1. Enantioselective Hydrogenative/Transfer
Hydrogenative Desymmetrization of meso-Substrates
n the field of asymmetric catalysis, transition-metal-catalyzed
Ienantioselective hydrogenation/transfer hydrogenation is
considered to be one of the most efficient and practical reduction
methodologies.1 Most chiral compounds, including cyclic and
acyclic chiral amines, alcohols, carboxylic derivatives, and alkanes
can be prepared via a simple reduction of related CC, CN,
and CO bonds.2 However, normally, enantioselective hydro-
genation/transfer hydrogenation only generates one or at most
two stereocenters; the preparation of compounds bearing
multiple and contiguous stereocenters using this methodology
is difficult. Such reactions are of particular interest because
structural motifs bearing contiguous stereocenters are abundant
in natural products and pharmaceuticals.3 To overcome this
drawback, a strategy of asymmetric desymmetrization of meso-
substrates can be employed,4 although this methodology has
been insufficiently studied. Some impressive research has been
reported independently by Ikariya, Zhang, and Bergens on the
enantioselective hydrogenative desymmetrization of meso-cyclic
imides and anhydrides for the preparation of chiral products
bearing two or more stereocenters.5 On the other hand, only one
example concerning the enantioselective transfer hydrogenative
desymmetrization of meso-cyclic diketones has been realized,
even though the hydroxyl ketones are considered to be more
versatile products. Using HCOOH as a hydrogen source, a sole
alkyl−alkyl diketone substrate was reduced with a high
enantioselectivity of 99% ee but a low yield of 44%.6 Related
hydrogenative methodology using H2 as the hydrogen source still
remains elusive, and a more efficient process with a wider
substrate scope is highly desired. Herein, we present an
asymmetric desymmetrization of meso-cyclic aryl−alkyl dike-
tones utilizing a combined hydrogenation/transfer hydro-
genation process. Using H2 and EtOH as the hydrogen source,
several meso-epoxy naphthoquinones with different substituents
were desymmetrized via an effective asymmetric reduction
(Scheme 1). The cis-epoxy naphthoquinol products, the
structural motifs of which are found in various bioactive
compounds,7 can be simply derivatized via manipulation of
both the epoxy and carbonyl groups for the synthesis of other
polyfunctional compounds with multiple stereocenters.
Inspired by some excellent research concerning the asym-
metric hydrogenation of ketones (especially that of 1-tetralone
and its analogues),8 we envisaged that the asymmetric hydro-
genative desymmetrization of meso-2,3-epoxy-tetrahydro-
naphthalene-1,4-diones could be realized using a similar
Received: April 13, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01073
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ruthenium catalyst. First, a unique RuPHOX−Ru complex with a
C2 symmetric phosphine/oxazoline ligand, developed within our
group and successfully applied to the asymmetric hydrogenation
and transfer hydrogenation of aryl−alkyl ketones,8j,9 was used for
the hydrogenation of the model substrate 1a. No reaction
occurred under the reported conditions8j or under normal
hydrogenation conditions (Table 1, entries 1−2). The well-
(14%), which are produced by further hydrogenation of 2a, were
detected with the structures being determined via NMR
spectroscopy (entry 12).11 When the H2 pressure was increased,
conversion increased but with a slight decrease in ee (entries 13−
14 vs 5). Elevating the temperature from rt to 50 °C reduced both
the conversion and enantioselectivity (entry 15). A large amount
of starting material was recovered (55%), and a small amount of
open loop epoxy propane byproducts 2ac (16%) and 2ad (12%)
were observed in the NMR spectra.11 When the reaction was
conducted in the absence of H2, no product was detected (entry
16).
a
Table 1. Ligand and Solvent Screening
In order to further increase the conversion and enantiose-
lectivity, additives, especially bases which have been frequently
used in Ru-catalyzed asymmetric hydrogenation and transfer
hydrogenation of ketones, were applied to the reactions (Table
2). Addition of Cs2CO3 (10 mol %) resulted complete
a
Table 2. Base Screening
b
c
entry
ligand
solvent
conv/% (cis/trans)
ee/% (cis)
d
1
RuPHOX
RuPHOX
BINAP
iPrOH
MeOH
MeOH
MeOH
EtOH
iPrOH
acetone
DCM
0
−
−
−
2
0
b
c
3
0
entry
base
temp/°C time/h conv/% (cis/trans)
ee/% (cis)
4
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
TsDPEN
52/5
63/5
48/5
32/3
30/3
10/1
9/1
5/0
46/0
80/7
88/8
15/2
0
92
96
92
91
89
86
87
82
99
95
95
92
−
1
2
3
4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
−
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
Li2CO3
KHCO3
NaHCO3
K2HPO4
K3PO4
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
0
24
8
3
1
1
1
1
1
3
3
3
3
3
3
3
3
3
3
90/10
91/9
92/8
81/6
48/4
0
88
97
98
98
95
−
5
6
7
d
8
5
de
,
9
THF
6
7
8
9
df
,
10
11
EtOAc
toluene
EtOH
EtOH
EtOH
EtOH
EtOH
21/3
0
97
dg
,
−
e
12
93/7
89/11
83/7
60/6
92/8
91/9
91/9
92/8
92/8
91/9
96
98
96
97
98
98
99
>99
>99
>99
f
13
10
11
12
13
14
15
16
17
18
g
14
h
15
i
16
a
Conditions: 1a (0.1 mmol), ligand-Ru (1 mol %), H2 (5 atm),
b
solvent (1.5 mL), rt (20−25 °C), 24 h, unless otherwise noted. The
conversions of cis-2a and trans-2a were calculated from 1H NMR
K3PO4
c
h
i
spectra. The ee values of cis-2a were determined by HPLC using a
chiral column. tBuOK (10 mol %). Cl is replaced by OTf in the
catalyst. H2 (10 atm). H2 (20 atm). 50 °C. In the absence of H2.
K3PO4
0
d
e
K3PO4
0
f
g
h
i
a
Conditions: 1a (0.1 mmol), (R,R)-TsDPEN-Ru (1 mol %), H2 (5
atm), base (10 mol %), EtOH (1.5 mL), rt (20−25 °C), unless
b
otherwise noted. The conversions of cis-2a and trans-2a were
known catalyst, BINAP-Ru, used often for the hydrogenation of
ketones, also showed no activity in this reaction (entry 3).
Finally, the TsDPEN−Ru complex, which is commercially
available and widely used for both hydrogenation and transfer
hydrogenation of aryl−alkyl ketones,8c,d,10 was examined in this
reaction using normal hydrogenation conditions. The desired
product was obtained in 52% conversion and 92% ee, together
with 5% of its diastereoisomer (entry 4). Other solvents were
examined (entries 5−11). The conversions in the protic solvents
were much higher than those in nonprotic solvents. EtOH was
discovered to be the best solvent with regards to both conversion
(63% for product with 5% for its diastereoisomer) and
enantioselectivity (96% ee) (entry 5). To improve the activity,
a cationic catalyst Ru(OTf)((R,R)-TsDPEN), preprepared from
RuCl((R,R)-TsDPEN) and AgOTf, was used in the hydro-
genation. The substrate was completely converted to give the
desired product only in 46% conversion with excellent diastereo-
and enantioselectivity. However, byproducts 2aa (40%) and 2ab
c
calculated from 1H NMR spectra. The ee values of cis-2a were
d
e
determined by HPLC using chiral column. In the absence of H2. In
f
g
h
the absence of base. iPrOH as solvent. MeOH as solvent. K3PO4 (5
i
mol %). K3PO4 (1 mol %).
conversion to the desired product (dr = 90/10) with a reduced
enantioselectivity of 88% (entry 1). Interestingly, when reducing
the hydrogenation time to 8 and 3 h, the reaction proceeds well
with an increase in ee (entries 2−3). Further reducing the
reaction time to 1 h resulted in incomplete conversion but similar
enantioselectivity (98%) and an even higher diastereoselectivity
(81/6) (entry 4). The conversion and enantioselectivity were
dramatically reduced in the absence of H2 when the same
reaction time of 1 h was used (entry 5). No product was detected
when the Cs2CO3 additive was removed (entry 6). Based on the
above-mentioned results, a combined hydrogenation/transfer
hydrogenation mechanism can be proposed.8c,d The product cis-
B
DOI: 10.1021/acs.orglett.6b01073
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
2a was obtained with low conversion (21%) and 97% ee using i-
PrOH as the hydrogen resource and solvent in the absence of H2,
while no reaction occurred using MeOH (entries 7−8). Other
weak bases were tested, with K3PO4 providing slightly better
enantioselectivity (entries 9−15). Further reducing the temper-
ature to 0 °C increased the enantioselectivity of the product to
above 99% ee (entry 16). Changing the loading amount of
K3PO4 made no obvious difference (entries 17−18).
Scheme 3. Substrate Scope
To gain a better understanding of the reaction mechanism,
deuterium labeling experiments using D2 and ethanol-D6 were
performed (Scheme 2). When the reaction was conducted with
Scheme 2. Deuterium Labeling Experiments
a
Conditions: 1 (0.10 mmol), (R,R)-TsDPEN-Ru (1 mol %), K3PO4 (5
mol %), H2 (5 atm), EtOH (1.5 mL), 0 °C. The yields were for the
mixture of cis/trans-isomers isolated by flash chromatography. The dr
1
values were calculated from H NMR spectra. The ee values were
b
D2 and normal ethanol, 1a was converted to 2a in 42%
conversion and 12/1 dr. The main product cis-2a was obtained in
94% ee with a H/D ratio of 9/1. When the reductants were
changed to H2 and ethanol-D6, the conversion was 33% with an
identical dr of 12/1. Compound cis-2a was produced in 95% ee
and with a H/D ratio of 1/3. Based on these results, it can be
envisaged that this transformation proceeds via a combined
asymmetric hydrogenation/transfer hydrogenation mode both
catalyzed by TsDPEN−Ru and transfer hydrogenation plays a
dominant role.
determined by HPLC using a chiral column. Reacted at room
c
temperature. Reacted under 10 atm of H2.
Scheme 4. Derivatizations on Epoxy and Carbonyl Groups
With the optimized reaction conditions in hand, the meso-
epoxy naphthoquinone substrate scope was investigated
(Scheme 3). The substrates 1b and 1c bearing the methyl
groups substituted at different positions gave their corresponding
products with an identical enantioselectivity of 99% ee and with
10/1 and 13/1 dr, respectively. The substrates 1d−1h bearing
electron-donating alkyl groups with acyclic or cyclic structures
were all converted to their related products with excellent
enantioselectivities (98−99%) and diastereoselectivities (10/1−
13/1). Substrate 1i with a naphthyl skeleton also gave its
corresponding product with 99% ee and a better dr of 15/1.
However, reductions of other substrates 1j, 1k, and 1l bearing
electron-withdrawing COOMe, Br, and Cl groups were less
reactive. Complete conversions were obtained under a high
temperature and/or high H2 pressure. The methoxycarbonyl
group substituted 1j was converted to 2j with the lowest
enantioselectivity (96% ee) and with a dr of 11/1. The remaining
two substrates 1k and 1l were converted to their respective
products both with 99% ee and 8/1 dr.
In conclusion, we have described the first Ru-catalyzed
asymmetric reductive desymmetrization of meso-epoxy naph-
thoquinones for the synthesis of a series of chiral cis-epoxy
naphthoquinols with three contiguous stereocenters. Excellent
enantioselectivities (96−99% ee) and diastereoselectivities (8/
1−15/1) were obtained, and a combined asymmetric hydro-
genation/transfer hydrogenation mechanism was proposed
based on experimental results. This methodology was further
applied to the synthesis of several multifunctionalized com-
pounds via simple derivatizations.
ASSOCIATED CONTENT
* Supporting Information
■
To demonstrate the practicality of this methodology, the
epoxy and carbonyl groups were converted to other function-
alities according to the reported conditions involving similar
transformations (Scheme 4).12,13 The epoxy ring of cis-2a
underwent a reductive opening to give the hydroxyl-substituted
ketone 3 in 78% yield. The carbonyl group was simply converted
to hydrozone 4 in 86% yield.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01073.
Synthetic details for substrates, procedures for hydro-
genation reactions, spectra of NMR and HPLC data
(PDF)
C
DOI: 10.1021/acs.orglett.6b01073
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(7) (a) Uchiyama, M.; Kimura, Y.; Ohta, A. Tetrahedron Lett. 2000, 41,
10013−10017. (b) Miyashita, K.; Sakai, T.; Imanishi, T. Org. Lett. 2003,
5, 2683−2686. (c) Yu, M.; Danishefsky, S. J. J. Am. Chem. Soc. 2008, 130,
2783−2785. (d) Mehta, G.; Kumar, Y. C. S.; Das, M. Tetrahedron Lett.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: wanbin@sjtu.edu.cn.
*E-mail: zhenfeng@sjtu.edu.cn.
2011, 52, 3505−3508. (e) Husain, S. M.; Schatzle, M. A.; Ludeke, S.;
̈
̈
Muller, M. Angew. Chem., Int. Ed. 2014, 53, 9806−9811.
̈
Notes
(8) (a) Ohkuma, T.; Hattori, T.; Ooka, H.; Inoue, T.; Noyori, R. Org.
Lett. 2004, 6, 2681−2683. (b) Huang, H.; Okuno, T.; Tsuda, K.;
Yoshimura, M.; Kitamura, M. J. Am. Chem. Soc. 2006, 128, 8716−8717.
(c) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.;
Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724−8725. (d) Sandoval, C. A.;
Li, Y.; Ding, K.; Noyori, R. Chem. - Asian J. 2008, 3, 1801−1810. (e) Li,
Y.; Zhou, Y.; Shi, Q.; Ding, K.; Noyori, R.; Sandoval, C. A. Adv. Synth.
Catal. 2011, 353, 495−500. (f) Zhu, Q.; Shi, D.; Xia, C.; Huang, H.
Chem. - Eur. J. 2011, 17, 7760−7763. (g) Matsumura, K.; Arai, N.; Hori,
K.; Saito, T.; Sayo, N.; Ohkuma, T. J. Am. Chem. Soc. 2011, 133, 10696−
10699. (h) Touge, T.; Hakamata, T.; Nara, H.; Kobayashi, T.; Sayo, N.;
Saito, T.; Kayaki, Y.; Ikariya, T. J. Am. Chem. Soc. 2011, 133, 14960−
14963. (i) Jolley, K. E.; Zanotti-Gerosa, A.; Hancock, F.; Dyke, A.;
Grainger, D. M.; Medlock, J. A.; Nedden, H. G.; Le Paih, J. J. M.;
Roseblade, S. J.; Seger, A.; Sivakumar, V.; Prokes, I.; Morris, D. J.; Wills,
M. Adv. Synth. Catal. 2012, 354, 2545−2555. (j) Guo, H.; Liu, D.; Butt,
N. A.; Liu, Y.; Zhang, W. Tetrahedron 2012, 68, 3295−3299.
(k) Patchett, R.; Magpantay, I.; Saudan, L.; Schotes, C.; Mezzetti, A.;
Santoro, F. Angew. Chem., Int. Ed. 2013, 52, 10352−10355. (l) Zhou, H.;
Huang, H. ChemCatChem 2013, 5, 2253−2257. (m) Rodríguez, S.; Qu,
B.; Fandrick, K. R.; Buono, F.; Haddad, N.; Xu, Y.; Herbage, M. A.; Zeng,
X.; Ma, S.; Grinberg, N.; Lee, H.; Han, Z. S.; Yee, N. K.; Senanayake, C.
H. Adv. Synth. Catal. 2014, 356, 301−307.
(9) (a) Wang, J.; Liu, D.; Liu, Y.; Zhang, W. Org. Biomol. Chem. 2013,
11, 3855−3861. (b) Wang, Y.; Wang, J.; Liu, D.; Zhang, W. Youji Huaxue
2014, 34, 1766−1772. (c) Wang, J.; Wang, Y.; Liu, D.; Zhang, W. Adv.
Synth. Catal. 2015, 357, 3262−3272.
(10) (a) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc. 1995, 117, 7562−7563. (b) Noyori, R.; Hashiguchi, S.
Acc. Chem. Res. 1997, 30, 97−102. (c) Sandoval, C. A.; Ohkuma, T.;
Utsumi, N.; Tsutsumi, K.; Murata, K.; Noyori, R. Chem. - Asian J. 2006,
1, 102−110. (d) Ohkuma, T.; Tsutsumi, K.; Utsumi, N.; Arai, N.;
Noyori, R.; Murata, K. Org. Lett. 2007, 9, 255−257. (e) Arai, N.; Satoh,
H.; Utsumi, N.; Murata, K.; Tsutsumi, K.; Ohkuma, T. Org. Lett. 2013,
15, 3030−3033.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by the National Natural
Science Foundation of China (Nos. 21232004, 21472123, and
21572131), Science and Technology Commission of Shanghai
Municipality (No. 14XD1402300), and China Postdoctoral
Science Foundation (No. 2014M551397). We thank the
Instrumental Analysis Center of Shanghai Jiao Tong University
(SJTU) for characterization.
REFERENCES
■
(1) Selected reviews for hydrogenation: (a) Handbook of Homogeneous
Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH:
Weinheim, Germany, 2007. (b) Asymmetric Catalysis on Industrial Scale,
2nd ed.; Blaser, H.-U., Federsel, H.-J.; Eds.; Wiley-VCH: Weinheim,
Germany, 2010. (c) Xie, J.-H.; Zhu, S.-F.; Zhou, Q.-L. Chem. Rev. 2011,
111, 1713−1760. (d) Wang, D.-S.; Chen, Q.-A.; Lu, S.-M.; Zhou, Y.-G.
Chem. Rev. 2012, 112, 2557−2590. (e) Xie, J.-H.; Zhou, Q.-L. Huaxue
Xuebao 2012, 70, 1427−1438. (f) Etayo, P.; Vidal-Ferran, A. Chem. Soc.
̀ ́
Rev. 2013, 42, 728−754. (g) Verendel, J. J.; Pamies, O.; Dieguez, M.;
Andersson, P. G. Chem. Rev. 2014, 114, 2130−2169. Selected reviews
for transfer hydrogenation: (h) Gladiali, S.; Alberico, E. Chem. Soc. Rev.
2006, 35, 226−236. (i) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007,
40, 1300−1308. (j) Wang, C.; Wu, X.; Xiao, J. Chem. - Asian J. 2008, 3,
1750−1770. (k) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282−2291.
(l) Li, Y.-Y.; Yu, S.-L.; Shen, W.-Y.; Gao, J.-X. Acc. Chem. Res. 2015, 48,
2587−2598.
(2) Recent representative papers for hydrogenation: (a) Guo, C.; Sun,
D.-W.; Yang, S.; Mao, S.-J.; Xu, X.-H.; Zhu, S.-F.; Zhou, Q.-L. J. Am.
Chem. Soc. 2015, 137, 90−93. (b) Molinaro, C.; Scott, J. P.; Shevlin, M.;
́
Wise, C.; Menard, A.; Gibb, A.; Junker, E. M.; Lieberman, D. J. Am.
(11) The structures of 2aa−2ad were shown as follows: 2aa and 2ab:
(a) Rashid, A.; Read, G. J. Chem. Soc. C 1969, 2053−2058. 2ac: (b)
Reference 7e. 2ad: (c) Allingham, M. T.; Bennett, E. L.; Davies, D. H.;
Harper, P. M.; Howard-Jones, A.; Mehdar, Y. T. H.; Murphy, P. J.;
Thomas, D. A.; Caulkett, P. W. R.; Potter, D.; Lam, C. M.; O’Donoghue,
A. C. Tetrahedron 2016, 72, 496−503.
Chem. Soc. 2015, 137, 999−1006. (c) Yan, Q.; Kong, D.; Li, M.; Hou, G.;
Zi, G. J. Am. Chem. Soc. 2015, 137, 10177−10181. (d) Gao, M.; Meng, J.-
J.; Lv, H.; Zhang, X. Angew. Chem., Int. Ed. 2015, 54, 1885−1887.
(e) Kuwano, R.; Hashiguchi, Y.; Ikeda, R.; Ishizuka, K. Angew. Chem., Int.
Ed. 2015, 54, 2393−2396. (f) Zhang, J.; Chen, F.; He, Y.-M.; Fan, Q.-H.
Angew. Chem., Int. Ed. 2015, 54, 4622−4625. (g) Li, W.; Schlepphorst,
C.; Daniliuc, C.; Glorius, F. Angew. Chem., Int. Ed. 2016, 55, 3300−3303.
Recent representative papers for transfer hydrogenation: (h) Yang, P.;
Xu, H.; Zhou, J. Angew. Chem., Int. Ed. 2014, 53, 12210−12213. (i) Xu,
H.; Yang, P.; Chuanprasit, P.; Hirao, H.; Zhou, J. Angew. Chem., Int. Ed.
2015, 54, 5112−5116. (j) Bigler, R.; Huber, R.; Mezzetti, A. Angew.
Chem., Int. Ed. 2015, 54, 5171−5174.
(12) Nicolaou, K. C.; Bunnage, M. E.; McGarry, D. G.; Shi, S.; Somers,
P. K.; Wallace, P. A.; Chu, X.-J.; Agrios, K. A.; Gunzner, J. L.; Yang, Z.
Chem. - Eur. J. 1999, 5, 599−617.
(13) (a) Cranwell, P. B.; Russell, A. T.; Smith, C. D. Synlett 2015, 27,
131−135. (b) Cranwell, P. B.; Russell, A. T. J. Chem. Educ. 2016, 93,
949−952.
(3) (a) Enders, D.; Huttl, M. R. M.; Grondal, C.; Raabe, G. Nature
̈
2006, 441, 861−863. (b) Reyes, E.; Jiang, H.; Milelli, A.; Elsner, P.;
Hazell, R. G.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2007, 46, 9202−
9205. (c) Arai, T.; Yokoyama, N. Angew. Chem., Int. Ed. 2008, 47, 4989−
4992. (d) Guo, S.; Xie, Y.; Hu, X.; Xia, C.; Huang, H. Angew. Chem., Int.
Ed. 2010, 49, 2728−2731. (e) Watson, C. G.; Balanta, A.; Elford, T. G.;
Essafi, S.; Harvey, J. N.; Aggarwal, V. K. J. Am. Chem. Soc. 2014, 136,
17370−17373.
́ ́
́
(4) For review: Fernandez-Perez, H.; Etayo, P.; Lao, J. R.; Nunez-Rico,
̃
J. L.; Vidal-Ferran, A. Chem. Commun. 2013, 49, 10666−10675.
(5) (a) Ito, M.; Kobayashi, C.; Himizu, A.; Ikariya, T. J. Am. Chem. Soc.
2010, 132, 11414−11415. (b) Takebayashi, S.; John, J. M.; Bergens, S.
H. J. Am. Chem. Soc. 2010, 132, 12832−12834. (c) Liu, T.-L.; Li, W.;
Geng, H.; Wang, C.-J.; Zhang, X. Org. Lett. 2013, 15, 1740−1743.
(6) Clay, D. R.; Rosenberg, A. G.; McIntosh, M. C. Tetrahedron:
Asymmetry 2011, 22, 713−716.
D
DOI: 10.1021/acs.orglett.6b01073
Org. Lett. XXXX, XXX, XXX−XXX