Asymmetric Catalytic 1,2-Hydroperoxidation of Isatin-Derived Ketimine with Hydrogen Peroxide in the Crowding Environment of PEGs

Article abstract of DOI:10.1021/acs.orglett.7b00032

The first enantioselective catalytic 1,2-hydroperoxidation has been achieved in the presence of PEG-600 using an acid-base bifunctional chiral squaramide as the organocatalyst, affording a range of enantioenriched α-N-substituted hydroperoxides bearing an

Full text of DOI:10.1021/acs.orglett.7b00032

Letter
pubs.acs.org/OrgLett
Asymmetric Catalytic 1,2-Hydroperoxidation of Isatin-Derived
Ketimine with Hydrogen Peroxide in the Crowding Environment of
PEGs
Wengang Guo, Yan Liu,* and Can Li*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
S
* Supporting Information
ABSTRACT: The first enantioselective catalytic 1,2-hydro-
peroxidation has been achieved in the presence of PEG-600
using an acid−base bifunctional chiral squaramide as the
organocatalyst, affording a range of enantioenriched α-N-
substituted hydroperoxides bearing an oxindole moiety with
excellent stereoselectivities (up to 99% ee).
Scheme 1. Organocatalytic Additions to Double Bonds
Using Hydrogen Peroxide as Nucleophile
he chiral hydroperoxides and peroxides occur as essential
Tstructural motifs in a large class of natural products and
bioactive unnatural compounds which display antitumor,
anticancer, and antiparasite activities.1 Moreover, many hydro-
peroxides have been used in asymmetric oxidations mimicking
biological oxidant hydroperoxyflavin, an α-N-substituted hydro-
peroxide.2 As a result, the development of methods for the
synthesis of structurally distinct classes of chiral peroxides and
hydroperoxides has become a attractive sustainable field in
synthetic and medicinal chemistry.3 Since the first successful
formations of chiral peroxides (ROOR′) were reported by the
Deng group4 and List group5 independently in 2008, significant
progress has been achieved in developing asymmetric catalytic
methodologies for this purpose.6 In contrast, the synthesis of
chiral hydroperoxides (ROOH) has captured much less
attention. Current approaches for the enantioselective synthesis
of hydroperoxides were mainly based on the stoichiometric use
of chiral starting materials7a−c or via optical resolution of
racemic hydroperoxides.7d−f To the best of our knowledge, to
date there is still no approach available for direct catalytic
synthesis of chiral hydroperoxide.8
We envisioned that enantioenriched hydroperoxide could be
accessed directly via nucleophilic addition to the CN double
bond using H2O2 as the donor in the presence of an
appropriate chiral organocatalyst. In the past, several organo-
catalytic systems for efficient asymmetric epoxidation,9 Baeyer−
Villiger reactions,10 peroxidation,5,6d and other oxidations11
have been successfully developed by using H2O2 as the terminal
oxidant, thanks to the pioneering works of Jørgenson, Miller,
List, Ding, Ooi, Arai, and co-workers (Scheme 1). Nevertheless,
none of these processes has involved the isolation of
hydroperoxide. One of the major difficulties may be attributed
to the inherent high reactivity of the relevant hydroperoxides,
which can readily undergo subsequent intra- or intermolecular
reactions to form more stable epoxides,9 peroxides,5 etc. It thus
became our goal to exploit new strategies for the development
of asymmetric hydroperoxidation.
Polyethylene glycols (PEGs), polyethers with monomeric
structure, are widely used as crowding reagents in the structural
study of biological macromolecules such as proteins, nucleic
acids, etc.12 As acceptors, PEGs can bring about strong
hydrogen-bonding (HB) interactions with biological molecules,
which play key roles in such research systems. Inspired by the
utilization of PEG in biology and recent advances in
supramolecular catalysis,13 we targeted the asymmetric syn-
thesis of hydroperoxides and found that an intermolecular HB
crowding environment generated between the PEG-600 and
−OOH group can suppress the reactivity of hydroperoxides.
This strategy allowed us to develop the first organocatalyzed
asymmetric 1,2-hydroperoxidation of isatin-derived ketimines 1
using H2O2 as nucleophile (Scheme 1) to afford a variety of α-
N-substituted oxindole-3-hydroperoxides with excellent ee (up
to 99%).
Received: January 5, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00032
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
For the asymmetric hydroperoxidation using H2O2 as the
oxidant, isatin-derived ketimine 1a was examined as the
electrophile with regard to its high reactivity and the structural
relevance of this type of compounds in bioactivity studies.14 At
the outset of this investigation, organic base catalysis, as our
ongoing interest,15 was tested in the model reaction. We
speculated that base catalysts with HB donors16 might be
plausible catalysts for hydroperoxidation via electrophilic
activation of the ketimine with HB interaction and nucleophilic
activation of H2O2 with tertiary amine. However, the dimeric
peroxide 3a was isolated in high yield (81%) (Table 1, entry 1)
hydroperoxide 2a as major product. Moreover, we also found
that the ratios of 2a were enhanced significantly with increasing
amounts of additives (Table 1, entries 4−9). These results
suggested that crowding HB can indeed suppress the
dimerization and enhance the proportion of hydroperoxide
2a. Our persistence was finally rewarded when the same
reaction was carried out in the presence of PEG-600. We were
pleased to find that the reaction proceeded smoothly at −10 °C
to give the hydroperoxide 2a as the major product in 80% yield
and 95% ee after 2 h (Table 1, entry 9). Interestingly, the use of
PEGs with different molecular weights as additives led to
different enantioselective control for the 1,2-hydroperoxida-
tions (Table 1, entries 9−14). Only PEG-600 and -800 could
afford hydroperoxide 2a with excellent enantioselectivities.
These results indicated that PEGs with appropriate degrees of
polymerization could provide favorable microenvironments for
this catalytic reaction. Further screening of squaramide catalysts
4a−c proved that 4a was the optimal catalyst in terms of
isolated yield and enantioselectivity (Table 1, entry 9 versus
entries 15 and 16). To further explore the efficiency of this
catalytic system, the model reaction was carried out with a
reduced catalyst loading (2 mol %), affording 2a with the
enantioselectivity as high as 95% after prolonged reaction time
to 24 h (Table 1, entry 17).
With the optimized reaction conditions in hand (Table 1,
entry 9), we examined the substrate scope of this hydro-
peroxidation reaction. A wide range of isatin-derived ketimines
with different substitution patterns were found to be viable
substrates for this reaction, affording the corresponding
hydroperoxides 2a−n in excellent enantioselectivities (91−
99% ee) (Scheme 2).
Notably, the reaction system tolerates a variety of functional
groups on the aryl ring, including F, Cl, Br, OCF3, CF3, CH3,
and OMe substituents 2b−m. On the other hand, the
electronic nature of the substituents on the aryl ring of the
ketimines exhibited a significant influence on the product
a
Table 1. Optimization of Reaction Conditions
b
c
d
entry
cat.
additive
no
2/3
2 or 3, yield (%)
ee (%)
1
2
3
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4a
1:20
1:8
3a, 81
3a, 60
2a, 68
ND
ND
ND
77
e
ether
e
THF
6:1
h
4
1,4-dioxane
1,4-dioxane
1,4-dioxane
PEG-600
PEG-600
PEG-600
PEG-200
PEG-300
PEG-400
PEG-800
PEG-1000
PEG-600
PEG-600
PEG-600
1:1
ND
ND
84
i
5
j
2:1
ND
6
6:1
73/2a
ND
7
2:1
ND
ND
95
8
6:1
ND
9
10:1
ND
ND
ND
ND
ND
7:1
2a, 80
2a, 51
2a, 69
2a, 74
2a, 60
2a, 70
2a, 59
2a, 15
2a, 61
10
11
12
13
14
15
80
76
73
94
67
64
f
16
g
10:1
ND
90
17
95
a
Unless otherwise indicated, all reactions were carried out on a 0.1
mmol scale with 2.5 equiv of H2O2, the corresponding PEG (200 mg),
Scheme 2. Substrate Scope
and 10 mol % of (R,R)-4 in CH2Cl2 (0.5 mL) for 2 h. PEG =
b
polyethylene glycol. ND: not determined. The ratio of 2a and 3a was
c
1
determined by crude H NMR analysis. Isolated yield based on 1a
d
purified by silica gel column chromatography. Determined by HPLC
analysis using a Daicel AD-H column. As solvent, 0.5 mL,
respectively. The reaction is very sluggish. 2 mol % of catalyst,
reaction time: 24 h. 50 μL of 1,4-dioxane. 100 μL of 1,4-dioxane.
e
f
g
h
i
j
200 μL of 1,4-dioxane.
with squaramide bifunctional organocatalyst 4a,17 probably due
to the good nucleophilicity of hydroperoxide 2a. We envisioned
that nucleophilicity of hydroperoxide may be depressed to
some extent through weak HB interaction with an appropriate
acceptor. Such a HB acceptor can inhibit the further
deprotonation of hydroperoxide, thus allowing for effective
trapping and isolation of the generally reactive hydroperoxide.
To verify this idea, several additives and solvents bearing HB
acceptor groups were tested to reduce the dimer formation
(Table 1, entries 2−9). We were glad to find that these
additives and solvents could promote the generation of
B
DOI: 10.1021/acs.orglett.7b00032
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
yields. In general, electron-withdrawing groups furnished the
hydroperoxide products 2b−h,k−m in higher yields than those
with electron-donating substituents 2i,j, albeit with similarly
high ee values. Changing the substituent on the nitrogen in the
imino group from the Boc group to the 1-adamantyloxycar-
bonyl group (Adoc) also showed high enantioselectivity (2n, ee
= 94%).18 The absolute configuration of 2m was unambigu-
ously established as S on the basis of the X-ray crystallographic
analysis, and those of others were assigned by analogy.
To explore the practicality of the current methodology, a
gram-scale synthesis of 2m was carried out under optimized
conditions with catalyst 4a. The corresponding product 2m was
obtained with 71% yield and 96% ee (Scheme 3). Furthermore,
HRMS analysis revealed that 4a was not oxidized by H2O2 or 2
and could be recovered in 73% yield.
that there may exist some intermolecular hydrogen-bonding
interactions between PEG and 2m.19
Control experiments were conducted to investigate the
generation of dimer 3 (Scheme 4b). Using CH2Cl2 as the
solvent, hydroperoxide 2a reacted with ketimine 1a via catalysis
of 4a to afford the dimer 3a with 65% yield in just 5 min. In
contrast, there was no further reaction between the dimer 3a
and aqueous H2O2 in the presence of 4a and PEG-600. These
results demonstrate that dimer 3 is very likely formed via the
intermediacy of hydroperoxides 2.20
In summary, the first acid−base bifunctional organocatalyzed
asymmetric 1,2-hydroperoxidation has been developed, thus
providing a direct and highly efficient approach for the
synthesis of enantioenriched α-N-substituted hydroperoxide
bearing an oxindole motif. In all cases, excellent enantiose-
lectivities (91−99% ee) were achieved. Mechanistic studies
revealed that the crowding intermolecular HB between the
additive (PEG-600) and −OOH group plays an important role
in the control of the chemoselectivity. This strategy offers a
potential opportunity for the discovery of biomimetic
asymmetric oxidation systems. Further work will be devoted
to the development of new hydroperoxidations and biomimetic
asymmetric oxidation.
Scheme 3. Gram-Scale Synthesis
In the studies of the reactions between H2O2 and ketimines
1, we found that α-N-substituted peroxide 3, the dimeric
products, are unstable and decomposed quickly during NMR
measurements or HPLC analysis. In contrast, α-N-substituted
hydroperoxides 2 exhibit good stabilities (Scheme 4a). We
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00032.
■
S
Scheme 4. (a) Thermal Stability of Hydroperoxide 2m; (b)
Control Experiments
Experimental procedures, characterization of products,
NMR spectra, HPLC traces, and X-ray data for 2m
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*Email: yanliu503@dicp.ac.cn.
*Email: canli@dicp.ac.cn.
ORCID
Yan Liu: 0000-0001-5788-3236
Can Li: 0000-0002-9301-7850
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Natural Science
Foundation of China (21322202, 21472187). W.G. thanks Dr.
Shengmei Lu (DICP) for helpful discussions.
reasoned that the stability of hydroperoxides 2 might result
from intramolecular hydrogen bonding between the −OOH
1
REFERENCES
moiety and the oxindolyl carbonyl. H NMR spectra of tert-
■
butyl hydroperoxide exhibited that the chemical shift of the
(1) (a) Posner, G. H.; O’Neill, P. M. Acc. Chem. Res. 2004, 37, 397.
(b) Meshnick, S. R.; Thomas, A.; Ranz, A.; Xu, C. M.; Pan, H. Z. Mol.
Biochem. Parasitol. 1991, 49, 181. (c) Posner, G. H.; Oh, C. H. J. Am.
Chem. Soc. 1992, 114, 8328.
(2) For reviews, see: (a) Adam, W.; Richter, M. J. Acc. Chem. Res.
1994, 27, 57. (b) Gelalcha, F. G. Chem. Rev. 2007, 107, 3338. For
selected examples, see: (c) Hamann, H.-J.; Hoft, E.; Chmielewski, M.;
Maciejewski, S. Chirality 1993, 5, 338. (d) Hamann, H.-J.; Hoft, E.;
Mostowicz, D.; Mishnev, A.; Urbanczyk-Lipkowska, Z.; Chmielewski,
M. Tetrahedron 1997, 53, 185. (e) Adam, W.; Korb, M. N.
Tetrahedron: Asymmetry 1997, 8, 1131. (f) Adam, W.; Rao, P. B.;
1
−OOH moiety is around 7.4 ppm. In comparison, the H
NMR signal of the −OOH moiety in 2m is around 10.0 ppm.
Upon addition of CD3OD (CD3OD/CDCl3, 1:20), H/D
exchange of the −OOH proton in 2m (15 min, 59%) was much
slower than that in tert-butyl hydroperoxide (2 min, 100%)
under the same conditions. These results are consistent with
the existence of intramolecular hydrogen bonding in the
structure of 2. Moreover, upon addition of CD3OD, 2m in
deuterated CDCl3/PEG-600 shows a much slower rate of H/D
exchange (15 min, 32%) than 2m in CDCl3, which suggests
Degen, H.-G.; Saha-Moller, C. R. J. Am. Chem. Soc. 2000, 122, 5654.
̈
C
DOI: 10.1021/acs.orglett.7b00032
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(g) Aoki, M.; Seebach, D. Helv. Chim. Acta 2001, 84, 187. (h) Adam,
W.; Alsters, P. L.; Neumann, R.; Saha-Moller, C. R.; Seebach, D.;
Zhang, R. Org. Lett. 2003, 5, 725.
(3) For reviews, see: (a) Casteel, D. A. Nat. Prod. Rep. 1999, 16, 55.
(b) Kitching, W.; Rahm, F.; Hayes, P. Y. Heterocycles 2004, 64, 523.
(c) Della Sala, G. D.; Lattanzi, A. ACS Catal. 2014, 4, 1234.
(4) Lu, X.; Liu, Y.; Sun, B.; Cindric, B.; Deng, L. J. Am. Chem. Soc.
2008, 130, 8134.
(14) For selected examples about isatin-derived N-Boc-ketimines as
electrophiles in asymmetric synthesis, see: (a) Yan, W. J.; Wang, D.;
Feng, J. C.; Li, P.; Zhao, D. P.; Wang, R. Org. Lett. 2012, 14, 2512.
(b) Hara, N.; Nakamura, S.; Sano, M.; Tamura, R.; Funahashi, Y.;
Shibata, N. Chem. - Eur. J. 2012, 18, 9276. (c) Lv, H.; Tiwari, B.; Mo, J.
M.; Xing, C.; Chi, Y. R. Org. Lett. 2012, 14, 5412. (d) Nakamura, S.;
Hyodo, K.; Nakamura, M.; Nakane, D.; Masuda, H. Chem. - Eur. J.
2013, 19, 7304. (e) Liu, Y. L.; Zhou, J. Chem. Commun. 2013, 49,
4421. (f) Arai, T.; Matsumura, E.; Masu, H. Org. Lett. 2014, 16, 2768.
(5) Reisinger, C. M.; Wang, X.; List, B. Angew. Chem., Int. Ed. 2008,
47, 8112.
(g) Beceno, C.; Chauhan, P.; Rembiak, A.; Wang, A.; Enders, D. Adv.
̃
Synth. Catal. 2015, 357, 672.
(6) (a) Zheng, W.; Wojtas, L.; Antilla, J. C. Angew. Chem., Int. Ed.
2010, 49, 6589. (b) Feng, X.; Yuan, Y.-Q.; Cui, H.-L.; Jiang, K.; Chen,
Y.-C. Org. Biomol. Chem. 2009, 7, 3660. (c) Rubush, D. M.; Morges,
M. A.; Rose, B. J.; Thamm, D. H.; Rovis, T. J. Am. Chem. Soc. 2012,
134, 13554. (d) Lifchits, O.; Mahlau, M.; Reisinger, C. M.; Lee, A.;
(15) (a) Guo, W.; Wu, B.; Zhou, X.; Chen, P.; Wang, X.; Zhou, Y.-
G.; Liu, Y.; Li, C. Angew. Chem., Int. Ed. 2015, 54, 4522. (b) Guo, W.;
Wang, X.; Zhang, B.; Shen, S.; Zhou, X.; Wang, P.; Liu, Y.; Li, C.
Chem. - Eur. J. 2014, 20, 8545. (c) Guo, W.; Wei, J.; Liu, Y.; Li, C.
Tetrahedron 2014, 70, 6561. (d) Wei, J.; Guo, W.; Zhang, B.; Liu, Y.;
Du, X.; Li, C. Chin. J. Catal. 2014, 35, 1008. (e) Wei, J.; Guo, W.;
Zhou, X.; Liu, Y.; Li, C. Chin. J. Chem. 2014, 32, 985. (f) Cheng, P.;
Guo, W.; Chen, P.; Liu, Y.; Du, X.; Li, C. Chem. Commun. 2016, 52,
3418.
(16) For reviews, see: (a) Tian, S.-K.; Chen, Y.; Hang, J.; Tang, L.;
McDaid, P.; Deng, L. Acc. Chem. Res. 2004, 37, 621. (b) Doyle, A. G.;
Jacobsen, E. N. Chem. Rev. 2007, 107, 5713. (c) Taylor, M. S.;
Jacobsen, E. N. Angew. Chem., Int. Ed. 2006, 45, 1520.
̀
Fares, C.; Polyak, I.; Gopakumar, G.; Thiel, W.; List, B. J. Am. Chem.
Soc. 2013, 135, 6677. (e) Russo, A.; Lattanzi, A. Adv. Synth. Catal.
2008, 350, 1991. (f) Lu, X.; Deng, L. Org. Lett. 2014, 16, 2358.
(g) Arai, T.; Tsuchiya, K.; Matsumura, E. Org. Lett. 2015, 17, 2416.
(7) (a) Kienle, M.; Argyrakis, W.; Baro, A.; Laschat, S. Tetrahedron
Lett. 2008, 49, 1971. (b) Chmielewski, M.; Jurczak, J.; Maciejewski, S.
Carbohydr. Res. 1987, 165, 111. (c) Dussault, P.; Lee, I.; Kreifels, S. J.
Org. Chem. 1991, 56, 4087. (d) Hoft, E.; Hamann, H. J.; Kunath, A.
Tetrahedron: Asymmetry 1992, 3, 507. (e) Dussault, P.; Porter, N. A. J.
Am. Chem. Soc. 1988, 110, 6276. (f) Haring, D.; Adam, W.; Saha-
(17) The first example of squaramides as bifunctional organocatalysts
was reported by Rawal. See: Malerich, J. P.; Hagihara, K.; Rawal, V. H.
J. Am. Chem. Soc. 2008, 130, 14416. For a selected review about the
Moller, C. R.; Schreier, P. J. Org. Chem. 1999, 64, 832.
̈
(8) (a) Jiang and Tan have reported pentanidium-catalyzed α-
hydroxylation of 3-substituted oxindoles, affording hydroperoxides as
minor products. Only one hydroperoxide as major product was shown
with 85% yield and 80% ee when adequate oxygen was used. See:
Yang, Y.; Moinodeen, F.; Chin, W.; Ma, T.; Jiang, Z.; Tan, C.-H. Org.
Lett. 2012, 14, 4762. (b) By using masked hydroperoxide as
nucleophile, Nakamura reported asymmetric catalytic peroxidation of
isatin-derived ketimines. The corresponding hydroperoxides were
obtained after deprotection of product; see: Nakamura, S.; Takahashi,
S. Org. Lett. 2015, 17, 2590.
(9) (a) Marigo, M.; Franzen, J.; Poulsen, T. B.; Zhuang, W.;
Jørgensen, K. A. J. Am. Chem. Soc. 2005, 127, 6964. (b) Sunden, H.;
Ibrahem, I.; Cordova, A. Tetrahedron Lett. 2006, 47, 99. (c) Peris, G.;
Jakobsche, C. E.; Miller, S. J. J. Am. Chem. Soc. 2007, 129, 8710.
(d) Lifchits, O.; Reisinger, C. M.; List, B. J. Am. Chem. Soc. 2010, 132,
10227. (e) Wang, X.; Reisinger, C. M.; List, B. J. Am. Chem. Soc. 2008,
130, 6070. (f) Uraguchi, D.; Tsutsumi, R.; Ooi, T. J. Am. Chem. Soc.
2013, 135, 8161. (g) Arai, S.; Tsuge, H.; Shioiri, T. Tetrahedron Lett.
1998, 39, 7563. (h) Wu, S.; Pan, D.; Cao, C.; Wang, Q.; Chen, F.-X.
Adv. Synth. Catal. 2013, 355, 1917.
́
application of squaramides organocatalysts Aleman, J.; Parra, A.; Jiang,
H.; Jørgensen, K. A. Chem. - Eur. J. 2011, 17, 6890.
(18) When N-Me-isatin-derived ketimine was used as substrate, the
desired product and corresponding dimer 3 formed an inseparable
mixture that could not be purified by column chromatography. The
NMR yield is about 65%.
(19) The NMR analysis revealed the existence of interaction between
catalyst 4a and PEGs. For details, See S-Figure 2 in the Supporting
Information.
(20) Control experiments were performed for further demonstration
of the role of the PEG on chemoselectivity. For details, see S-Scheme 4
in the Supporting Information.
(10) (a) Xu, S.; Wang, Z.; Zhang, X.; Zhang, X.; Ding, K. Angew.
Chem., Int. Ed. 2008, 47, 2840. (b) Xu, S.; Wang, Z.; Li, Y.; Zhang, X.;
Wang, H.; Ding, K. Chem. - Eur. J. 2010, 16, 3021.
(11) (a) Liao, S.; Coric, I.; Wang, Q.; List, B. J. Am. Chem. Soc. 2012,
́
134, 10765. (b) Liu, Z.-M.; Zhao, H.; Li, M.-Q.; Lan, Y.-B.; Yao, Q.-B.;
Tao, J.-C.; Wang, X.-W. Adv. Synth. Catal. 2012, 354, 1012.
(12) (a) Nakano, S.-I.; Miyoshi, D.; Sugimoto, N. Chem. Rev. 2014,
114, 2733. (b) Minton, A. P. J. Biol. Chem. 2001, 276, 10577.
(13) For recent reviews on supramolecular catalysis, see:
(a) Meeuwissen, J.; Reek, J. Nat. Chem. 2010, 2, 615. (b) Li, Y.;
Wang, Z.; He, Y.; Fan, Q. Chin. J. Chem. 1991, 30, 338. For selected
examples about asymmetric supramolecular catalysis, see: (c) Li, Y.;
Feng, Y.; He, Y.; Chen, F.; Pan, J.; Fan, Q. Tetrahedron Lett. 2008, 49,
2878. (d) Xu, L.; Lam, K.; Ji, J.; Wu, J.; Fan, Q.; Lo, W.; Chan, A.
Chem. Commun. 2005, 1390. (e) Li, Y.; He, Y.; Li, Z.; Zhang, F.; Fan,
Q. Org. Biomol. Chem. 2009, 7, 1890. (f) Li, Y.; Ma, B.; He, Y.; Zhang,
F.; Fan, Q. Chem. - Asian J. 2010, 5, 2454. (g) Zhou, H.; Fan, Q.; Tang,
W.; Xu, L.; He, Y.; Deng, G.; Zhao, L.; Gu, L.; Chan, A. Adv. Synth.
Catal. 2006, 348, 2172. (h) Wang, Y.; Yu, T.; Zhang, H.; Luo, Y.; Xu,
P. Angew. Chem., Int. Ed. 2012, 51, 12339. (i) Mandal, T.; Zhao, C.
Angew. Chem., Int. Ed. 2008, 47, 7714. (j) Xia, A.; Xu, D.; Wu, C.;
Zhao, L.; Xu, Z. Chem. - Eur. J. 2012, 18, 1055.
D
DOI: 10.1021/acs.orglett.7b00032
Org. Lett. XXXX, XXX, XXX−XXX