Metal-Free One-Pot Synthesis of 3-Phosphinoylbenzofurans via Phospha-Michael Addition/Cyclization of H-Phosphine Oxides and in Situ Generated ortho-Quinone Methides

Article abstract of DOI:10.1021/acs.orglett.7b03863

A novel metal-free one-pot protocol for the effective and efficient synthesis of 3-phosphinoylbenzofurans via a phospha-Michael addition/cyclization of H-phosphine oxides and in situ generated ortho-quinone methides is described. Based on the expeditious

Full text of DOI:10.1021/acs.orglett.7b03863

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal-Free One-Pot Synthesis of 3‑Phosphinoylbenzofurans via
Phospha-Michael Addition/Cyclization of H‑Phosphine Oxides and in
Situ Generated ortho-Quinone Methides
Ji-Yuan Du,*,† Yan-Hua Ma,† Rui-Qing Yuan,† Nana Xin,† Shao-Zhen Nie,† Chun-Lin Ma,†
Chen-Zhong Li,†,‡ and Chang-Qiu Zhao†
†College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, China
‡Florida International University, Biomedical Engineering Department EC2671, 10555 West Flagler Street, Miami, Florida 33174,
United States
S
* Supporting Information
ABSTRACT: A novel metal-free one-pot protocol for the
effective and efficient synthesis of 3-phosphinoylbenzofurans
via a phospha-Michael addition/cyclization of H-phosphine
oxides and in situ generated ortho-quinone methides is
described. Based on the expeditious construction of C(sp2)−
P bonds, asymmetric synthesis of optically pure 3-phosphinoyl-
benzofurans containing chiral P-stereogenic center has also
been probed by using chiral RP-(−)-menthyl phenylphosphine
oxide.
Scheme 1. Previous Methods for C(sp2)−P Bond
romatic organophosphorus compounds and their deriva-
Construction and Our Design for the Synthesis of 3-
Atives are of particular interest in organic chemistry due to
their biological properties and synthetic value.1 For example,
Phosphinoylbenzofurans
some structurally related organophosphorus compounds have
been used as biologically active molecules in medicinal
chemistry,1b,d as phosphorus ligands in asymmetric catalysis,1a,f
and as buildings blocks in organic synthesis.1c In recent
decades, focusing on the synthesis of phosphorylated hetero-
cycles through C(sp2)−P bond formation, several method-
ologies have been established on the basis of classical reactions
of halophosphine electrophiles with active carbon nucleophiles
such as organometallic reagents,2 transition-metal-catalyzed
cross-coupling reactions of phosphines with aryl (pseudo)
halides,3,4 Friedel−Crafts reaction,5 and P-centered radicals
addition to unsaturated systems.6 Despite significant progress in
the construction of C(sp2)−P bonds, only a few approaches
(Scheme 1) have been developed for the synthesis of 3-
phosphinoylbenzofurans.7 In approach (a), for example,
Tsvetkov and Griffiths have developed the protocols for the
synthesis of 3-phosphinoylbenzofurans using bisphosphine
oxides and dialkyl benzoylphosphonates through Wittig−
Horner and rearrangement reactions, respectively.7a,c In 2012,
Swamy and co-workers reported a three-step synthetic
procedure using allenylphosphine oxides as key intermedia-
tes.7b,d Among the above-mentioned approaches, notably, the
hols.7e However, the above-mentioned methodologies generally
either require expensive metal catalysts or suffer from multistep
preparation of the reaction precursors in some cases.
C(sp2)−P bonds were constructed through traditional
substitution reactions using water-sensitive PIII reagents.
Recently, Liang and co-workers have reported a novel synthesis
of 3-phosphinoylbenzofuran derivatives via a copper(II)
catalyzed intermolecular cascade annulation reaction of
nucleophilic diphenylphosphine oxide and propargylic alco-
Received: December 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03863
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Recently, metal-free C(sp2)−P bond construction reactions
have attracted much attention as appealing alternatives to the
metal-mediated and environmentally friendly processes.8 For
example, Miura et al. developed a metal-free approach to
phosphorylated heterocycles using Tf2O through electrophilic
phosphination/cyclization of alkynes.8h Despite these elegant
improvements, there is still high demand for novel method-
ologies to efficiently and effectively construct C(sp2)−P bonds
for the synthesis of 3-phosphinoylbenzofuran compounds.
ortho-Quinone methides (o-QMs) are highly reactive as well
as ephemeral intermediates which have been employed for
decades in the synthesis of natural products and bioactive
molecules.9 Due to their intrinsic electrophilic reactivity, o-QMs
have been generally used in a series of 1,4-conjugate addition,
[4 + n]-cycloaddition, intramolecular [5 + 2]-cycloaddition, and
oxa-6π-electrocyclization.10 Very recently, the Kang group
developed a novel organocatalytic phosphonylation through
1,4-conjugate addition of in situ formed o-QMs with
trialkylphosphites for the construction of diaryl phospho-
nates.11 To our knowledge, however, there are few reports on
the reaction of o-QMs as nucleophile acceptors with H-
phosphine oxides. In light of the importance of 3-phosphinoyl-
benzofurans compounds and our interest in the development of
transformations with o-QMs, herein we report an effective one-
pot metal-free method for C(sp2)−P bond formation through
intermolecular phospha-Michael addition/intramolecular cycli-
zation reaction (Scheme 1), wherein we envision that both
nucleophilic P-center and electrophilic o-QMs would be
generated in situ from H-phosphine oxides and chemically
stable o-hydroxyl-benzyl alcohols (o-HBAs) under metal-free
conditions, respectively.12
obtained, despite low yields (entries 1−3). By considering the
acidic instability of in situ formed o-QM intermediates, this
model reaction was examined without an acid additive at
increased temperature (80 °C). Surprisingly, the titled reaction
proceeded smoothly and gave an improved yield of 82%, clearly
demonstrating that the temperature enhancement could
positively promote the formation of o-QMs (entry 4). In an
attempt to further improve the yield, several solvents were
examined (entries 5−9). With exception of polar DMF (entry
9), the desired product 3aa could be obtained in good yields
using toluene, EtOAc, (ClCH2)2, CH3CN, and THF, in which
(ClCH2)2 as the optimal solvent afforded 3aa in 92% yield
(entry 5).
With the optimal reaction conditions in hand, the scope of in
situ generated o-QMs in our metal-free, one-pot reaction was
first examined. A series of o-HBAs as o-QM precursors were
tested in the presence of diphenylphosphine oxide (2a)
(Scheme 2). The substrates bearing electron-donating sub-
Scheme 2. Scope of o-Hydroxyl Benzyl Alcohols (o-
ab
,
HBAs)
With the above-mentioned consideration, we initiated our
screenings for the model reaction of o-hydroxyl-benzyl alcohol
(o-HBA) 1a and diphenylphosphine oxide 2a under metal-free
reaction conditions (Table 1). Initially, several acids such as
TsOH, AcOH, and TFA were chosen for this transformation at
ambient temperature in CH3CN to facilitate the generation of
o-QM intermediates. Pleasingly, the desired product 3aa was
a
Table 1. Optimization of the Reaction Conditions
a
Unless otherwise specified, reactions were carried out using 1 (0.2
mmol) and 2a (0.3 mmol) in (ClCH2)2 (2 mL) at 80 °C until 1
disappeared, and then K2CO3 (0.6 mmol) was added. Times listed
b
entry
additive
solvent
temp (°C)
yield (%)
b
1
2
3
4
5
6
7
8
9
TsOH
AcOH
TFA
−
−
−
−
−
−
CH3CN
CH3CN
CH3CN
CH3CN
(ClCH2)2
toluene
EtOAc
25
25
25
80
80
80
80
80
80
14
29
42
82
92
75
76
79
0
were for the whole one-pot process.
stituents including methyl (3ca, 3ja), tert-butyl (3da, 3ia), and
methoxy (3ba) groups at ring A afforded the corresponding
products in 65−97% yields. The nitro group and halogen (F,
Cl, and Br) containing substrates at ring A gave 3ea−3ha in
moderate yields under the standard reaction conditions,
providing possibilities for late-stage chemical transformations
of the products. Besides, aryl and alkyl acetylene based
substrates were also screened, affording products (3ka−3ra)
in good to excellent yields. Substrates with different aryl (e.g.,
4-MeC6H4, 4-ClC6H4, 4-n-amyl-C6H4, 3-FC6H4) substituted
acetylene motifs were further investigated, and the desired 3-
phosphinoylbenzofurans 3ka−3na were afforded in 59−96%
yields. Besides, the substrates having alkyl (e.g., tert-butyl,
THF
DMF
a
A Schlenk tube (15 mL) charging with a magnetic stirring bar and 1a
(0.2 mmol, 1.0 equiv), 2a (0.3 mmol, 1.5 equiv), solvent (2 mL) and
additive (0.1 mmol, 0.5 equiv) (if applicable) was sealed without
degassing and heated at the indicated temperature until 1a was
disappeared; and then K2CO3 as base (0.6 mmol, 3.0 equiv) was added
to the resulting mixture and stirred at the indicated temperature.
b
Yield of isolated product. TFA = trifluoroacetic acid.
B
DOI: 10.1021/acs.orglett.7b03863
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
cyclopropyl, and n-amyl) substituted acetylene units were also
tolerable under standard reaction conditions, giving the desired
products 3oa (78%), 3pa (75%), and 3qa (88%), respectively.
Additionally, one example using the symmetric o-HBA
substrate 1r was conducted, and interestingly bis-3-
phosphinoylbenzofuran product 3ra was smoothly delivered
in 73% yield. To test the synthetic potential of this protocol, we
carried out a gram-scale reaction with 5 mmol (1.12 g) of 1a,
and the titled product 3aa was obtained with analogous
reactivity in good yield (91%, 1.85 g).
Scheme 4. Stereoselective Synthesis of Chiral 3-
Menthylphenylphosphinoylbenzofurans
a b c
, ,
Next, we explored the scope with respect to the H-phosphine
oxides (Scheme 3). Gratifyingly, the reaction of H-phosphine
a b
,
Scheme 3. Scope of H-Phosphine Oxides
a
Unless otherwise specified, reactions were carried out using 1 (0.2
mmol) and 4 (0.3 mmol) in (ClCH2)2 (2 mL) at 80 °C until 1
b
disappeared, and then K2CO3 (0.6 mmol) was added. Times listed
c
were for the whole one-pot process. The diastereomeric ratio were
detected by 31P NMR spectroscopy.
unambiguously established by X-ray crystallographic analysis
(CCDC 1568383).
Based on the above results and previous reports, a plausible
mechanism was proposed as shown in Scheme 5. The thermal
a
Unless otherwise specified, reactions were carried out using 1a (0.2
mmol) and 2 (0.3 mmol) in (ClCH2)2 (2 mL) at 80 °C until 1a
Scheme 5. Proposed Mechanism
b
disappeared, and then K2CO3 (0.6 mmol) was added. Times listed
were for the whole one-pot process.
oxides bearing the electron-donating groups including methyl
(2b, 2c) and methoxy (2d) at the para or ortho position of the
phenyl ring with o-HBA (1a) under optimal conditions readily
gave products 3ab−3ad in 83%, 92%, and 70% yields,
respectively. The substrate 2e with electron-withdrawing
chlorine at the phenyl ring could also proceed smoothly,
affording the desired product 3ae in 97% yield. While
di(benzyl)phosphineoxide (2f) was introduced as the reactant,
the product 3af was obtained in good yield (80%). Moreover,
the unsymmetrical phosphine oxide (2g) was also effective for
this transformation, giving the desired product 3ag in 82%
yield. Unexpectedly, diethyl phosphonate (2h) was ineffective
in this reaction following the decomposition of the starting
materials.
Recently, optically pure RP-(−)-menthyl phenylphosphine
oxide 4 was used as an excellent building block for the
intramolecular rearrangement reaction and intermolecular
addition reaction in our lab, leading to various optically pure
tertiary phosphine oxides compounds.13 Considering the
importance of chiral P-stereogenic phosphorus compounds in
organic synthesis, the reactions using the chiral H-phosphine
oxide were further explored in the asymmetric synthesis of
chiral 3-phosphinoylbenzofurans through phospha-Michael
addition/cyclization reactions. As shown in Scheme 4, various
o-HBAs were examined in this metal-free, one-pot stereo-
selective reaction. Generally, all the reactions gave the
corresponding chiral 3-menthylphenylphosphinoylbenzofurans
(5a−5h) with very high diastereoselectivity in good yields (up
to 92% yield). The absolute configuration of 5a was
dehydration of o-HBA 1a first resulted in the generation of o-
QM intermediate, and then an intermolecular phospha-Michael
addition of nucleophilic phosphinous acid 2a′ tautomerized
from 2a produced the alkynylphosphine oxide intermediate
3aa′ (CCDC 1568384). Following the isomerization of 3aa′, in
situ formed allenylphosphine oxide intermediate A quickly
underwent an intramolecular 1,4-addition/isomerization to
afford the final product 3-phosphinoylbenzofuran 3aa.
In summary, we have developed a novel metal-free one-pot
protocol featuring phospha-Michael addition/cyclization of H-
phosphine oxides with in situ generated o-QMs, leading to an
effective method for the construction of C(sp2)−P bonds in
various structurally interesting 3-phosphinoylbenzofurans.
Importantly, asymmetric synthesis of 3-phosphinoyl-
benzofurans containing chiral P-stereogenic centers has been
explored by using optically pure RP-(−)-menthylphenyl-
phosphine oxide. Further investigations on the development
of phosphorus-directed methodologies initiated by such type of
metal-free phospha-Michael addition are currently underway in
our group.
C
DOI: 10.1021/acs.orglett.7b03863
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(4) (a) Li, C.; Yano, T.; Ishida, N.; Murakami, M. Angew. Chem., Int.
Ed. 2013, 52, 9801. (b) Chen, Y.-R.; Duan, W.-L. J. Am. Chem. Soc.
2013, 135, 16754. (c) Feng, C.-G.; Ye, M.; Xiao, K.-J.; Li, S.; Yu, J.-Q.
J. Am. Chem. Soc. 2013, 135, 9322. (d) Mi, X.; Huang, M.; Zhang, J.;
Wang, C.; Wu, Y. Org. Lett. 2013, 15, 6266. (e) Yang, B.; Yang, T.-T.;
Li, X.-A.; Wang, J.-J.; Yang, S.-D. Org. Lett. 2013, 15, 5024. (f) Wu, B.;
Santra, M.; Yoshikai, N. Angew. Chem., Int. Ed. 2014, 53, 7543. (g) Hu,
G.; Gao, Y.; Zhao, Y. Org. Lett. 2014, 16, 4464. (h) Unoh, Y.; Satoh,
T.; Hirano, K.; Miura, M. ACS Catal. 2015, 5, 6634. (i) Xiong, B.;
Feng, X.; Zhu, L.; Chen, T.; Zhou, Y.; Au, C.-T.; Yin, S.-F. ACS Catal.
2015, 5, 537. (j) Jeon, W. H.; Son, J.-Y.; Kim, S.-E.; Lee, P. H. Adv.
Synth. Catal. 2015, 357, 811. (k) Gao, Y.; Deng, H.; Zhang, S.; Xue,
W.; Wu, Y.; Qiao, H.; Xu, P.; Zhao, Y. J. Org. Chem. 2015, 80, 1192.
(l) Yang, J.; Chen, T.; Han, L.-B. J. Am. Chem. Soc. 2015, 137, 1782.
(5) (a) Diaz, A. A.; Young, J. D.; Khan, M. A.; Wehmschulte, R. J.
Inorg. Chem. 2006, 45, 5568. (b) Hashimoto, S.; Nakatsuka, S.;
Nakamura, M.; Hatakeyama, T. Angew. Chem., Int. Ed. 2014, 53,
14074.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03863.
Experimental procedure and spectra data (PDF)
Accession Codes
CCDC 1568383−1568384 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
(6) (a) Leca, D.; Fensterbank, L.; Lacote, E.; Malacria, M. Chem. Soc.
■
Rev. 2005, 34, 858. (b) Hari, D. P.; Konig, B. Org. Lett. 2011, 13, 3852.
̈
Corresponding Author
*E-mail: dujy2015@163.com.
ORCID
(c) Wille, U. Chem. Rev. 2013, 113, 813. (d) Zhang, H.; Gu, Z.; Li, Z.;
Pan, C.; Li, W.; Hu, H.; Zhu, C. J. Org. Chem. 2016, 81, 2122.
(e) Quint, V.; Morlet-Savary, F.; Lohier, J.-F.; Lalevee, J.; Gaumont,
́
A.-C.; Lakhdar, S. J. Am. Chem. Soc. 2016, 138, 7436. (f) Gao, Y.; Lu,
G.; Zhang, P.; Zhang, L.; Tang, G.; Zhao, Y. Org. Lett. 2016, 18, 1242.
(g) Hua, H.-L.; Zhang, B.-S.; He, Y.-T.; Qiu, Y.-F.; Wu, X.-X.; Xu, P.-
F.; Liang, Y.-M. Org. Lett. 2016, 18, 216. (h) Li, C.-X.; Tu, D.-S.; Yao,
R.; Yan, H.; Lu, C.-S. Org. Lett. 2016, 18, 4928. (i) Peng, P.; Lu, Q.;
Peng, L.; Liu, C.; Wang, G.; Lei, A. Chem. Commun. 2016, 52, 12338.
(j) Wu, Z.-G.; Liang, X.; Zhou, J.; Yu, L.; Wang, Y.; Zheng, Y.-X.; Li,
Y.-F.; Zuo, J.-L.; Pan, Y. Chem. Commun. 2017, 53, 6637. (k) Gao, Y.;
Tang, G.; Zhao, Y. Phosphorus, Sulfur Silicon Relat. Elem. 2017, 192,
589.
(7) (a) Bovin, A. N.; Yarkevich, A. N.; Kharitonov, A. V.; Tsvetkov,
E. N. J. Chem. Soc., Chem. Commun. 1994, 973. (b) Chakravarty, M.;
Kumara Swamy, K. C. J. Org. Chem. 2006, 71, 9128. (c) Cheong, Y.-K.;
Duncanson, P.; Griffiths, D. V. Tetrahedron 2008, 64, 2329. (d) Sajna,
K. V.; Kumara Swamy, K. C. J. Org. Chem. 2012, 77, 5345. (e) Li, X.-S.;
Han, Y.-P.; Zhu, X.-Y.; Li, M.; Wei, W.-X.; Liang, Y.-M. J. Org. Chem.
2017, 82, 11636.
(8) (a) Hirai, T.; Han, L.-B. Org. Lett. 2007, 9, 53. (b) Wang, H.; Cui,
X.; Pei, Y.; Zhang, Q.; Bai, J.; Wei, D.; Wu, Y. Chem. Commun. 2014,
50, 14409. (c) Ma, D.; Chen, W.; Hu, G.; Zhang, Y.; Gao, Y.; Yin, Y.;
Zhao, Y. Green Chem. 2016, 18, 3522. (d) Wang, S.; Qiu, D.; Mo, F.;
Zhang, Y.; Wang, J. J. Org. Chem. 2016, 81, 11603. (e) Zhu, Y.-L.;
Wang, D.-C.; Jiang, B.; Hao, W.-J.; Wei, P.; Wang, A.-F.; Qiu, J.-K.; Tu,
S.-J. Org. Chem. Front. 2016, 3, 385. (f) Luo, K.; Chen, Y.-Z.; Chen, L.-
X.; Wu, L. J. Org. Chem. 2016, 81, 4682. (g) Xie, P.; Wang, J.; Fan, J.;
Liu, Y.; Wo, X.; Loh, T.-P. Green Chem. 2017, 19, 2135. (h) Unoh, Y.;
Hirano, K.; Miura, M. J. Am. Chem. Soc. 2017, 139, 6106.
(9) (a) Van De Water, R. W.; Pettus, T. R. R. Tetrahedron 2002, 58,
5367. (b) Willis, N. J.; Bray, C. D. Chem. - Eur. J. 2012, 18, 9160.
(10) (a) Bai, W.-J.; David, J. G.; Feng, Z.-G.; Weaver, M. G.; Wu, K.-
L.; Pettus, T. R. R. Acc. Chem. Res. 2014, 47, 3655. (b) Singh, M. S.;
Nagaraju, A.; Anand, N.; Chowdhury, S. RSC Adv. 2014, 4, 55924.
(c) Caruana, L.; Fochi, M.; Bernardi, L. Molecules 2015, 20, 11733.
(d) Jaworski, A. A.; Scheidt, K. A. J. Org. Chem. 2016, 81, 10145.
(11) Huang, H.; Kang, J. Y. Org. Lett. 2017, 19, 5988.
Ji-Yuan Du: 0000-0002-3059-9830
Nana Xin: 0000-0001-5025-2589
Chang-Qiu Zhao: 0000-0002-9016-8151
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from NSFC (21602094,
21502084), Natural Science Foundation of Shandong Province
(ZR2016BB11), and this work was supported by “Tai-Shan
Scholar Research Fund of Shandong Province”.
REFERENCES
■
(1) (a) Tang, W.; Zhang, X. Chem. Rev. 2003, 103, 3029. (b) George,
A.; Veis, A. Chem. Rev. 2008, 108, 4670. (c) Queffel
́
ec, C.; Petit, M.;
Janvier, P.; Knight, D. A.; Bujoli, B. Chem. Rev. 2012, 112, 3777.
(d) Hussain, H.; Al-Harrasi, A.; Al-Rawahi, A.; Green, I. R.; Gibbons,
S. Chem. Rev. 2014, 114, 10369. (e) Montchamp, J.-L. Acc. Chem. Res.
2014, 47, 77. (f) Ma, Y.-N.; Li, S.-X.; Yang, S.-D. Acc. Chem. Res. 2017,
50, 1480.
(2) (a) Redmore, D. Chem. Rev. 1971, 71, 315. (b) Coudray, L.;
Montchamp, J.-L. Eur. J. Org. Chem. 2008, 2008, 3601. (c) Zhao, D.;
Mao, L.; Wang, Y.; Yang, D.; Zhang, Q.; Wang, R. Org. Lett. 2010, 12,
1880. (d) Hatano, M.; Horibe, T.; Ishihara, K. Angew. Chem., Int. Ed.
2013, 52, 4549. (e) Das, D.; Seidel, D. Org. Lett. 2013, 15, 4358.
(f) Sobkowski, M.; Kraszewski, A.; Stawinski, J. Recent Advances in H-
Phosphonate Chemistry. Part 1. H-Phosphonate Esters: Synthesis and
Basic Reactions. In Phosphorus Chemistry II: Synthetic Methods;
Montchamp, J.-L., Ed.; Springer International Publishing: Cham,
2015; p 137.
(3) (a) Schwan, A. L. Chem. Soc. Rev. 2004, 33, 218. (b) Greenberg,
S.; Stephan, D. W. Chem. Soc. Rev. 2008, 37, 1482. (c) Hou, C.; Ren,
Y.; Lang, R.; Hu, X.; Xia, C.; Li, F. Chem. Commun. 2012, 48, 5181.
(d) Zhang, H.-J.; Lin, W.; Wu, Z.; Ruan, W.; Wen, T.-B. Chem.
Commun. 2015, 51, 3450. (e) Renaud, J.-L.; Gaillard, S. Iron-Catalyzed
Carbon−Nitrogen, Carbon−Phosphorus, and Carbon−Sulfur Bond
Formation and Cyclization Reactions. In Iron Catalysis II; Bauer, E.,
Ed.; Springer International Publishing: Cham, 2015; p 83. (f) Wu, B.;
Chopra, R.; Yoshikai, N. Org. Lett. 2015, 17, 5666. (g) Unoh, Y.;
Yokoyama, Y.; Satoh, T.; Hirano, K.; Miura, M. Org. Lett. 2016, 18,
5436. (h) Xu, P.; Wu, Z.; Zhou, N.; Zhu, C. Org. Lett. 2016, 18, 1143.
(i) Geng, Z.; Zhang, Y.; Zheng, L.; Li, J.; Zou, D.; Wu, Y.; Wu, Y.
Tetrahedron Lett. 2016, 57, 3063.
(12) (a) Saha, S.; Schneider, C. Org. Lett. 2015, 17, 648. (b) Ma, J.;
Chen, K.; Fu, H.; Zhang, L.; Wu, W.; Jiang, H.; Zhu, S. Org. Lett. 2016,
18, 1322.
(13) (a) Zhang, H.; Sun, Y.-M.; Zhao, Y.; Zhou, Z.-Y.; Wang, J.-P.;
Xin, N.; Nie, S.-Z.; Zhao, C.-Q.; Han, L.-B. Org. Lett. 2015, 17, 142.
(b) Wang, J.-P.; Nie, S.-Z.; Zhou, Z.-Y.; Ye, J.-J.; Wen, J.-H.; Zhao, C.-
Q. J. Org. Chem. 2016, 81, 7644.
D
DOI: 10.1021/acs.orglett.7b03863
Org. Lett. XXXX, XXX, XXX−XXX