Mild Mn(OAc)3-Mediated Aerobic Oxidative Decarboxylative Coupling of Arylboronic Acids and Arylpropiolic Acids: Direct Access to Diaryl 1,2-Diketones

Article abstract of DOI:10.1021/acs.orglett.5b01265

A simple and efficient method for the synthesis of diaryl 1,2-diketones has been developed. The reaction represents the first example of diaryl 1,2-diketones that are synthesized directly from arylboronic acids and arylpropiolic acids by a radical pathway in moderate to good yields. This reaction proceeds under mild reaction conditions and with good tolerance of a variety of functional groups. Preliminary mechanistic studies were conducted.

Full text of DOI:10.1021/acs.orglett.5b01265

Letter
pubs.acs.org/OrgLett
Mild Mn(OAc)3‑Mediated Aerobic Oxidative Decarboxylative
Coupling of Arylboronic Acids and Arylpropiolic Acids: Direct Access
to Diaryl 1,2-Diketones
Wen-Xin Lv, Yao-Fu Zeng, Shang-Shi Zhang, Qingjiang Li,* and Honggen Wang*
School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou, 510006, China
S
* Supporting Information
ABSTRACT: A simple and efficient method for the synthesis
of diaryl 1,2-diketones has been developed. The reaction
represents the first example of diaryl 1,2-diketones that are
synthesized directly from arylboronic acids and arylpropiolic
acids by a radical pathway in moderate to good yields. This reaction proceeds under mild reaction conditions and with good
tolerance of a variety of functional groups. Preliminary mechanistic studies were conducted.
1,2-Diketones are one of the most important skeletons, since
they are not only found in many natural products and bioactive
molecules1 but also broadly used as versatile building blocks in
the construction of a variety of other chemicals,2 especially
biologically active heterocyclic compounds, such as imidazoles,
quinoxalines, and indolone-N-oxide. Among them, benzil
derivatives also exhibit bioactivities in antitumor applications.3
Accordingly, a variety of traditional and modern methods for
the preparation of 1,2-diketones have been developed.4 Scheme
1 schematically presents some of the well-established protocols:
(c) the need for harsh reaction conditions. Therefore, it is still
of great interest to uncover new synthetic methods for the
realization of these important molecules.
Over the past 30 years, Mn(III)-mediated oxidative radical
coupling reactions have proven to be excellent and efficient
methods for formation of C−C and C−heteroatom bonds.8 In
this regard, the use of arylboronic acids as the aryl radical
precursors has attracted considerable attention and found
numerous applications. This is partly due to the fact that
arylboronic acids are easily available and are stable under
atmospheric and aqueous conditions.9 Furthermore, it is known
that arylboronic acids are easily decomposed to aryl radicals
when oxidants are present.10 Herein, we report a simple and
efficient method for the synthesis of diaryl 1,2-dikeones via a
Mn(OAc)3·2H2O11-mediated oxidative decarboxylative cou-
pling of arylpropiolic acids with arylboronic acids (Scheme 1d).
Initially, phenylpropiolic acid (1a) and phenylboronic acid
(2a) were chosen as the model substrates to optimize the
reaction conditions. To our delight, the reaction of 1a and 2a
(1:3 ratio) in the presence of Mn(OAc)3·2H2O (4.0 equiv) as
an oxidant in toluene at room temperature under air afforded
the desired benzil 3aa, albeit in a low yield of 17% (Table 1,
entry 1). Based on this promising result, a wide variety of
reaction parameters (oxidants, solvents, and additives) were
examined, and some of the representative results are shown in
Table 1. Mn(OAc)3·2H2O turned out to be the best choice of
oxidant (entries 1−3). The solvent also played a very important
role in this reaction (entries 4−6). It was found that
cyclohexane gave a yield comparable to that when using
toluene (entry 6). Additionally, the use of H2O as a cosolvent
improved the yields remarkably to 40% and 32%, respectively
(entries 7 and 8). Water might increase the solubility of the
oxidants in this reaction. Further studies showed that the
addition of a base such as KOAc was beneficial for the reaction
Scheme 1. Different Approaches toward 1,2-Diketones
(1) the oxidation of ketone derivatives, including α-halo and α-
hydroxyl ketones, with different oxidants (Scheme 1a);5 (2) the
oxidation of olefins and internal alkynes with various oxidants,
such as selenium dioxide, iodine, potassium permanganate, and
others (Scheme 1b).6 Recently, Lee reported a straightforward
method to synthesize 1,2-diketones directly from aryl halides
and alkynyl carboxylic acids through a decarboxylative
Sonogashira-type coupling and oxidation procedure (Scheme
1c).7 Despite the many advances made in recent years, there are
still several drawbacks: (a) the use of expensive transition-metal
catalysts; (b) the tedious preparation of the starting materials;
Received: April 29, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01265
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope of Arylpropiolic Acids
b
entry
oxidant
solvent
toluene
additive yield (%)
1
Mn(OAc)3·2H2O
FeCl3
−
17
0
2
toluene
−
3
Cu(OAc)2
toluene
−
0
4
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
THF
−
−
−
−
0
5
DMF
0
6
cyclohexane
cyclohexane/H2O
toluene/H2O
cyclohexane/H2O
cyclohexane/H2O
cyclohexane/H2O
cyclohexane/H2O
17
40
32
55
47
26
75
(73)
62
68
7
8
−
9
KOAc
K2CO3
Et3N
KOAc
10
11
c
12
cd
,
13
14
Mn(OAc)3·2H2O
Mn(OAc)3·2H2O
cyclohexane/H2O
cyclohexane/H2O
KOAc
KOAc
ce
,
a
General reaction conditions: 1a (0.1 mmol, 1.0 equiv), 2a (0.3 mmol,
3.0 equiv), oxidant (0.4 mmol, 4.0 equiv), additive (0.4 mmol, 4.0
equiv), solvent (2 mL; if water was added, the ratio is 10:1), 25 °C, 20
b
h. Yields determined by 1H NMR using p-iodoanisole as the internal
c
standard; isolated yield in parentheses. Oxidant (5.0 equiv), 40 h; 2.0
a
equiv of 2a were added; 12 h later, another portion of 2a (1.0 equiv)
Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol), Mn(OAc)3·2H2O
d
e
was added. KOAc (2.0 equiv) was employed. 2a (2.0 equiv) was
(1.0 mmol), KOAc (0.8 mmol), cyclohexane/H2O (4.4 mL, 10:1), rt,
40 h under air, isolated yield. At 40 °C. Cyclohexane/H2O (3.3 mL,
b
c
employed.
10:1) was employed.
(entries 9−11). Gratifyingly, a good yield of 75% was obtained
when 5.0 equiv of oxidant were applied with the addition of 2a
in two portions (entry 12). The decrease of the loading of both
base (KOAc) and phenylboronic acid gave diminished 62% and
68% yields, respectively (entries 13 and 14).
a
Scheme 3. Substrate Scope of Arylboronic Acids
With the optimized conditions in hand (Table 1, entry 12),
we next explored the scope of the reaction. A variety of
arylpropiolic acids bearing different substituents were tested,
and the results are summarized in Scheme 2. It was found that
the transformation was very general, both electron-withdrawing
and -donating functional groups such as acetyl (3ba), nitro
(3ca), trifluoromethyl (3da), ester (3ea), fluoro (3fa), chloro
(3ga, 3ha), bromo (3ia), methoxyl (3ja, 3ka, 3la), alkyl (3na,
3oa, 3pa), and phenyl (3ma) were well tolerated, giving the
corresponding products in moderate to good yields. Notably, 3-
(thiophen-2-yl)propiolic acid (3qa) was also compatible with
the reaction conditions, which represented an interesting
outcome given the utility of these substructures in medicinal
chemistry. It is worthy of note that the halide substituents such
as Cl and Br (3ga, 3ha, 3ia) provide opportunities for further
functionalization. Unfortunately, the current reaction condi-
tions were not compatible with the alkylpropiolic acid. For
example, the use of oct-2-ynoic acid gave no trace of the desired
product (3ra).
a
Reaction conditions: 1a (0.3 mmol), 2 (0.9 mmol), Mn(OAc)3·2H2O
(1.5 mmol), KOAc (1.2 mmol), cyclohexane/H2O (4.4 mL, 10:1), 40
°C, 40 h, under air, isolated yield. At 50 °C. At rt.
b
c
As revealed in Scheme 3, a broad range of arylboronic acids
were also suitable for this reaction. Ester (3ea), alkyl (3ab),
chlorides (3ga, 3ad, 3ae), and trifluoromethyl (3ac) are
tolerated, affording the corresponding products in moderate
yields. ortho-Substituents did not hamper the reactivity (3ae).
The use of formyl-substituted phenylboronic acid led to a low
yield (18%, 3af). It was found that the reaction is sensitive to
the electronic property of the arylboronic acids. Electron-rich
arylboronic acids (3ja, 3ag) gave inferior yields compared to
the electron-deficient ones.
To elucidate the mechanism of the reaction, a number of
experiments were carried out. When phenylacetylene 4, instead
of 1a, was subjected to the reaction conditions, no desired
product 3aa was observed (eq 1). In addition, it is well-known
that 1,2-diphenylethyne 5 could be easily oxidized to benzil 3aa
B
DOI: 10.1021/acs.orglett.5b01265
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Possible Mechanism
Mn(OAc)3, to form a α-styrenyl radical C. This intermediate is
trapped by molecular oxygen to deliver peroxyl radical D.
Thereafter, a decarboxylation takes place to furnish alkyl
manganese species E.14 The cyclization of intermediate E
produces intermediate F, with the concomitant release of the
reduced MnII. The desired 1,2-diketone 3aa is formed by a
subsequent fragmentation of F.15
In conclusion, we have developed a simple and efficient
method for the synthesis of diaryl 1,2-dikeones using a
Mn(OAc)3-mediated oxidative decarboxylative coupling of
arylpropiolic acids with arylboronic acids. The reaction
proceeds under mild reaction conditions and with good
tolerance to a variety of functional groups. Preliminary
mechanistic studies were carried out, and the two oxygen
atoms of the 1,2-diketone were found out to be originated from
the molecular oxygen. Because of the ready availability of the
starting materials and the value of the products, we believe this
method should be useful in organic synthesis and medicinal
chemistry.
under various oxidative reaction conditions.6d−g,12 Thus,
diphenylethyne 5, a possible intermediate, was subjected to
the standard conditions (eq 2). Again, none of the desired
product 3aa was obtained. These results rule out the possible
intermediacy of 4 and 5. When a radical scavenger, 2,2,6,6-
tetramethylpiperidineoxy (TEMPO), was introduced to the
reaction mixture, no 1,2-diketone was formed, indicating a
possible radical pathway involved in the mechanism (eq 3).
When the reaction was performed under argon, a low yield
(15%) of 3aa was obtained, demonstrating the importance of
O2 in this transformation (eq 4). To further elucidate the role
of O2 and H2O for the reaction, isotopic labeling experiments
were also conducted. Under an 18O2 atmosphere, two 18O
labeled benzil 3aa-di18O was obtained, along with a significant
amount of 3aa and 3aa-mono18O, which were suspected to be
generated through the 18O atom exchange with water under the
reaction conditions (eq 5). Indeed, when 3aa was treated with
Mn(OAc)3, and KOAc, in cyclohexane/H2O18, significant
incorporation of 18O was found in the presence of either
phenylpropiolic acid (1a) or phenylboronic acid (2a) (eq 6).
And when the reaction was conducted in the presence of air
and H218O, the 18O labeling products were observed (3aa/
mono18O/di18O = 32:52:16) (eq 7 vs eq 5). Taken together,
these experiments clearly suggested that the two oxygen atoms
of the 1,2-diketone originate from the molecular oxygen.
On the basis of the above observations and literature
precedent,13 a proposed mechanism is depicted in Scheme 4.
Phenylboronic acid (2a) is oxidized by a MnIII salt to generate
the phenyl radical A, which undergoes intermolecular attack to
manganese salt B, generated in situ from arylpropiolic acid and
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and full analytical data. The
Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b01265.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: wanghg3@mail.sysu.edu.cn.
*E-mail: liqingj3@mail.sysu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the support of this work by "1000-Youth
Talents Plan", a Start-up Grant from Sun Yat-sen University
and the National Natural Science Foundation of China
(81402794 and 21472250).
REFERENCES
■
(1) For selected examples, see: (a) Rozwadowska, M. D.;
Chrzanowska, M. Tetrahedron 1985, 41, 2885. (b) Angelstro, M. R.;
Mehdi, S.; Burkhart, J. P.; Peet, N. P.; Bey, P. J. Med. Chem. 1990, 33,
11. (c) Ngadjui, B. T.; Kouam, S. F.; Dongo, E.; Kapche, G. W. F.;
Abegaz, B. M. Phytochemistry 2000, 55, 915. (d) Maurya, R.; Singh, R.;
C
DOI: 10.1021/acs.orglett.5b01265
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Deepak, M.; Handa, S. S.; Yadav, P. P.; Mishra, P. K. Phytochemistry
2004, 65, 915. (e) Mahabusarakam, W.; Deachathai, S.; Phongpaichit,
S.; Jansakul, C.; Taylor, W. C. Phytochemistry 2004, 65, 1185.
(f) Nicolaou, K. C.; Gray, D. L. F.; Tae, J. J. Am. Chem. Soc. 2004, 126,
613. (g) Wadkins, R. M.; Hyatt, J. L.; Wei, X.; Yoon, K. J. P.; Wierdl,
M.; Edwards, C. C.; Morton, C. L.; Obenauer, J. C.; Damodaran, K.;
Beroza, P.; Danks, M. K.; Potter, P. M. J. Med. Chem. 2005, 48, 2906.
(2) (a) De Kimpe, N.; Stanoeva, E.; Boeykens, M. Synthesis 1994,
427 and references cited therein. For recent examples, see:
(b) Wolkenberg, S. E.; Wisnoski, D. D.; Leister, W. H.; Wang, Y.;
Zhao, Z.; Lindsley, C. W. Org. Lett. 2004, 6, 1453. (c) Shipe, W. D.;
Yang, F.; Zhao, Z.; Wolkenberg, S. E.; Nolt, M. B.; Lindsley, C. W.
Heterocycles 2006, 70, 655. (d) Deng, X.; Mani, N. Org. Lett. 2006, 8,
269. (e) Held, I.; Xu, S. J.; Zipse, H. Synthesis 2007, 1185. (f) Rong, F.;
Chow, S.; Yan, S.; Larson, G.; Hong, Z.; Wu, J. Bioorg. Med. Chem. Lett.
2012, 68, 4950. (g) Sawama, Y.; Takubo, M.; Mori, S.; Monguchi, Y.;
Sajiki, H. Eur. J. Org. Chem. 2011, 3361. (h) Barta, T. E.; Stealey, M.
A.; Collins, P. W.; Weier, R. M. Bioorg. Med. Chem. Lett. 1998, 8, 3443.
(7) Min, H.; Palani, T.; Park, K.; Hwang, J.; Lee, S. J. Org. Chem.
2014, 79, 6279.
(8) (a) Curran, D. P. Synthesis 1988, 417, 489. (b) Yamada, T.;
Iwahara, Y.; Nishino, H.; Kurosawa, K. J. Chem. Soc., Perkin Trans. 1
2002, 609. (c) Snider, B. B.; Patricia, J. J. J. Org. Chem. 1989, 54, 38.
(d) Demir, A. S.; Emrullahoglu, M. Curr. Org. Synth. 2007, 4, 223.
(e) Snider, B. B. Chem. Rev. 1996, 96, 339. (f) Jasperse, C. P.; Curran,
D. P. Chem. Rev. 1991, 91, 1237. (g) Giese, B. Radicals in Organic
Synthesis: Formation of Carbon−Carbon Bonds; Pergamon Press:
Oxford, 1986. (h) Radikale, C. In Houben−Weyl Methoden der
Organischen Chimie; Regitz, M., Giese, B., Eds.; Thieme: Stuttgart,
1989; Vol. E, p 19A.
(9) Suzuki, A. Angew. Chem., Int. Ed. 2011, 50, 6722.
́ ́
2007, 17, 1663. (g) Herrera, A. J.; Rondon, M.; Suarez, E. J. Org.
(10) (a) Riant, O.; Samuel, O.; Flessner, T.; Taudien, S.; Kagan, H.
B. J. Org. Chem. 1997, 62, 6733. (b) Guchhait, S. K.; Kashyap, M.;
Saraf, S. Synthesis 2010, 1166. (c) Sikora, A.; Zielonka, J.; Lopez, M.;
Dybala-Defratyka, A.; Joseph, J.; Marcinek, A.; Kalyanaraman, B. Chem.
Res. Toxicol. 2011, 24, 687.
Chem. 2008, 73, 3384. (h) Boyce, G. R.; Johnson, J. S. Angew. Chem.,
Int. Ed. 2010, 49, 8930. (i) McKenna, J. M.; Halley, F.; Souness, J. E.;
McLay, I. M.; Pickett, S. D.; Collis, A. J.; Page, K.; Ahmed, I. J. Med.
Chem. 2002, 45, 2173. (j) Braibante, M. E. F.; Braibante, H. T. S.;
Uliana, M. P.; Costa, C. C.; Spenazzatto, M. J. Braz. Chem. Soc. 2008,
19, 909. (k) Hui, X.; Desrivot, J.; Bories, C.; Loiseau, P. M.; Franck, X.;
(11) The price of Mn(OAc)3·2H2O purchased directly from the
supplier is about $6 USD per gram.
̀
Hocquemiller, R.; Figadere, B. Bioorg. Med. Chem. 2006, 16, 815.
(12) (a) Wolfe, S.; Pilgrim, W. R.; Garrad, T. F.; Chamberlain, P.
Can. J. Chem. 1971, 49, 1099. (b) Niu, M.; Fu, H.; Jiang, Y.; Zhao, Y.
Synthesis 2008, 2879. (c) Tingoli, M.; Mazzella, M.; Panunzi, B.; Tuzi,
A. Eur. J. Org. Chem. 2011, 399. (d) Lee, D. G.; Chang, V. S.;
Chandler, W. D. J. Org. Chem. 1985, 50, 4306. (e) Trosien, S.;
Waldvogel, S. R. Org. Lett. 2012, 14, 2976. (f) Zhu, Z.; Espenson, J. H.
J. Org. Chem. 1995, 60, 7728.
(l) Bhosale, R. S.; Sarda, S. R.; Ardhapure, S. S.; Jadhav, W. N.;
Bhusare, S. R.; Pawar, R. P. Tetrahedron Lett. 2005, 46, 7183.
(m) Prasad, K. R. K.; Joshi, N. N. J. Org. Chem. 1996, 61, 3888.
(3) (a) Mousset, C.; Giraud, A.; Provot, O.; Hamze, A.; Bignon, J.;
Liu, J.-M.; Thoret, S.; Dubois, J.; Brion, J.-D.; Alami, M. Bioorg. Med.
Chem. Lett. 2008, 18, 3266. (b) Ganapaty, S.; Srilakshmi, G. V. K.;
Pannakal, S. T.; Rahman, H.; Laatsch, H.; Brun, R. Phytochemistry
2009, 70, 95. (c) Al-kahraman, Y. M. S. A.; Yasinzai, M.; Singh, G. S.
Arch. Pharm. Res. 2012, 35, 1009.
(13) (a) Tobisu, M.; Koh, K.; Furukawa, T.; Chatani, N. Angew.
̈
Chem., Int. Ed. 2012, 51, 11363. (b) Demir, A. S.; Reis, O.;
Emrullahoglu, M. J. Org. Chem. 2003, 68, 578.
(4) (a) Muzart, J. J. Mol. Catal. A: Chem. 2011, 338, 7. (b) Xu, C.-F.;
Xu, M.; Jia, Y.-X.; Li, C.-Y. Org. Lett. 2011, 13, 1556. (c) Ren, W.; Liu,
J.-F.; Chen, L.; Wan, X.-B. Adv. Synth. Catal. 2010, 352, 1424. (d) Ren,
W.; Xia, Y.-Z.; Ji, S.-J.; Zhang, Y.; Wan, X.-B.; Zhao, J. Org. Lett. 2009,
8, 1841. (e) Chu, J.-H.; Chen, Y.-J.; Wu, M.-J. Synthesis 2009, 13, 2155.
(f) Mousset, C.; Provot, O.; Hamze, A.; Bignonb, J.; Brion, J.-D.;
Alami, M. Tetrahedron 2008, 64, 4287. (g) Wan, Z.; Jones, C. D.;
Mitchell, D.; Pu, J. Y.; Zhang, T. Y. J. Org. Chem. 2006, 71, 826.
(h) Giraud, A.; Provot, O.; Peyrat, J.-F.; Alami, M.; Brion, J.-D.
Tetrahedron 2006, 62, 7667. (i) Katritzky, A. R.; Zhang, D.;
Kirichenko, K. J. Org. Chem. 2005, 70, 3271. (j) Antoniotti, S.;
Dunach, E. Eur. J. Org. Chem. 2004, 3459. (k) Chang, C.-L.; Kumar,
M.-P.; Liu, R.-S. J. Org. Chem. 2004, 69, 2793. (l) Khurana, J. M.;
Kandpal, B. M. Tetrahedron Lett. 2003, 44, 4909. (m) Yusubov, M. S.;
Filimonov, V. D.; Chi, K.-W. Russ. Chem. Bull. 2001, 50, 649.
(n) Dayan, S.; Ben-David, I.; Rozen, S. J. Org. Chem. 2000, 65, 8816.
(o) Che, C.-M.; Yu, W.-Y.; Chan, P.-M.; Cheng, W.-C.; Peng, S.-M.;
Lau, K.-C.; Li, W.-K. J. Am. Chem. Soc. 2000, 122, 11380.
(14) Anderson, J. M.; Kochi, J. K. J. Am. Chem. Soc. 1970, 92, 2450.
(15) Zhang, C.; Jiao, N. J. Am. Chem. Soc. 2010, 132, 28 and
references cited therein.
(5) (a) Lian, M.; Li, Q.; Zhu, Y.; Yin, G.; Wu, A. Tetrahedron 2012,
68, 9598. (b) Qi, C.; Jiang, H.; Huang, L.; Chen, Z.; Chen, H. Synthesis
2011, 387. (c) Su, Y.; Zhang, L.; Jiao, N. Org. Lett. 2011, 13, 2168.
(d) Chang, H. S.; Woo, J. C.; Lee, K. M.; Ko, Y. K.; Moon, S.-S.; Kim,
D.-W. Synth. Commun. 2002, 32, 31. (e) Ogata, Y. J. Am. Chem. Soc.
́ ́ ́
1975, 97, 6983. (f) Baranac-Stojanovic, M.; Markovic, R.; Stojanovic,
M. Tetrahedron 2011, 67, 8000. (g) Muthupandi, P.; Sekar, G.
Tetrahedron Lett. 2011, 52, 692. (h) Urgoitia, G.; SanMartin, R.;
Herrero, M. T.; Domínguez, E. Green Chem. 2011, 13, 2161. (i) Lee, J.
C.; Park, H.-J.; Park, J. Y. Tetrahedron Lett. 2002, 43, 5661.
(j) Katritzky, A. R.; Lang, H.; Wang, Z.; Zhang, Z.; Song, H. J. Org.
Chem. 1995, 60, 7619.
(6) (a) Wang, Z.; Jiang, H.; Li, X. J. Org. Chem. 2011, 76, 6958.
(b) Buehler, C. A.; Harris, J. O.; Arendale, W. F. J. Am. Chem. Soc.
1950, 72, 4953. (c) Chen, S.; Liu, Z.; Shi, E.; Chen, L.; Wei, W.; Li, H.;
Cheng, Y.; Wan, X. Org. Lett. 2011, 13, 2274. (d) Xu, Y.; Wan, X.
Tetrahedron Lett. 2013, 54, 642. (e) Sheu, C.; Richert, S. A.; Cofre, P.;
Ross, B., Jr.; Sobkowiak, A.; Sawyer, D. T.; Kanofsky, J. R. J. Am. Chem.
Soc. 1990, 112, 1936. (f) Gao, A.; Yang, F.; Li, J.; Wu, Y. Tetrahedron
D
DOI: 10.1021/acs.orglett.5b01265
Org. Lett. XXXX, XXX, XXX−XXX