Lewis Acid Catalyzed Tandem 1,4-Conjugate Addition/Cyclization of in Situ Generated Alkynyl o-Quinone Methides and Electron-Rich Phenols: Synthesis of Dioxabicyclo[3.3.1]nonane Skeletons

Article abstract of DOI:10.1021/acs.orglett.8b01862

A versatile Lewis acid catalyzed tandem cyclization of in situ generated alkynyl o-quinone methides (o-AQMs) with electron-rich phenols has been developed on the basis of the mode involving an intermolecular 1,4-conjugate addition/6-endo cyclization/1,3-a

Full text of DOI:10.1021/acs.orglett.8b01862

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Lewis Acid Catalyzed Tandem 1,4-Conjugate Addition/Cyclization of
in Situ Generated Alkynyl o‑Quinone Methides and Electron-Rich
Phenols: Synthesis of Dioxabicyclo[3.3.1]nonane Skeletons
Ji-Yuan Du,* Yan-Hua Ma, Fan-Xiao Meng, Bao-Li Chen, Shao-Liang Zhang, Qian-Li Li,
Shu-Wen Gong, Da-Qi Wang, and Chun-Lin Ma
College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, China
S
* Supporting Information
ABSTRACT: A versatile Lewis acid catalyzed tandem
cyclization of in situ generated alkynyl o-quinone methides
(o-AQMs) with electron-rich phenols has been developed on
the basis of the mode involving an intermolecular 1,4-
conjugate addition/6-endo cyclization/1,3-aryl shift/intra-
molecular 1,4-conjugate addition cascade. This reaction
provides a new method for expeditious assembly of syntheti-
cally and biologically interesting tetracyclic bridged
dioxabicyclo[3.3.1]nonane skeletons featuring a congested
bridgehead oxa-quaternary stereocenter.
he unprecedented tetracyclic skeleton featuring the
unusual bridged structure of a dioxabicyclo[3.3.1]nonane
unit is present in flavonoids of the cyanomaclurin,
cyanomaclurin analogue, and procyanidiano-(−)-epicatechin
(Figure 1).1 Some of these natural products displayed potent
Scheme 1. Previous Methods and Our Design for the
Synthesis of Dioxabicyclo[3.3.1]nonane Skeletons
T
Figure 1. Representative natural products with dioxabicyclo[3.3.1]-
nonane skeletons.
anti-MRSA activity,1h antiadipogenic properties,1g,2 tyrosinase
inhibitory activity1e and antibacterial activity, etc.1i,k Because of
its distinctive molecular architecture and various biological
properties, expeditious construction of such privileged
dioxabicyclo[3.3.1]nonane ring skeletons has attracted the
attention of chemists. To date, three synthetic methodologies
that are related to assembling the tetracyclic dioxabicyclo-
[3.3.1]nonane scaffolds have been reported, as shown in
Scheme 1. First, in route (a), Seshadri and Marathe reported
the multistep protocols for the synthesis of tetracyclic
dioxabicyclo[3.3.1]nonane skeletons using chalkone deriva-
tives in low yields, respectively.3 Second, Hennis and co-
workers reported that o-vinylphenols or o-coumaric acids
undergo cyclization (route b) to give simple substituted
tetracyclic skeletons through an excess HBr-promoted reaction
with salicylaldehydes in very low yields.4 Recently, Yang and
Tan employed an EDDA-mediated [3 + 3]-type cycloaddition
cascade reaction (route c) to form this type of compound.5 In
addition, due to the high transannular strain and steric effect,
the efficient construction of bridgehead oxa-quaternary stereo-
centers embedded in this ring skeletons is another urgent issue.
Therefore, development of a flexible and more straightforward
approach that leads to the tetracyclic bridged dioxabicyclo-
Received: June 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01862
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
[3.3.1]nonane skeletons bearing a bridgehead oxa-quaternary
stereocenter is still required.
AgSbF6, and AgOTf to enhance the yield (Table 1, entries 5−
11). Gratifyingly, using AgOTf at 50 °C provided an increase
in reaction efficiency with 94% yield (Table 1, entry 11). It was
noteworthy that lowering the reaction temperature to 25 °C
only led to intermediate 3aa′ in nearly quantitative yield
(Table 1, entry 12); nevertheless, 3aa′ could smoothly
transformed into the desired 3aa at 50 °C, indicating that
appropriate temperature is necessary for the subsequent
spontaneous 6-endo cyclization/1,3-aryl shift/1,4-conjugate
addition in this tandem process. After the influence of the
Lewis acid catalysts was screened, the survey of solvent showed
that our model tandem reaction in (CH2Cl)2 still proved to be
the most efficient (Table 1, entries 13−17).
o-Quinone methides (o-QMs) are highly reactive, and
ephemeral intermediates have attracted the most interest as a
synthetic precursor of bioactive molecules and natural products
for decades.6 Compared with the wide application of o-QMs in
organic synthesis,7−12 alkynyl o-quinone methide (o-AQM),
and a variant of o-QMs, studies have been reported by the Zhu
group,13 Schneider group,14 and our group.15 Only limited
reaction partners, such as intramolecular olefins, enamides, and
H-phosphine oxides were reacted with o-AQMs. In con-
junction with our continuing interest in the chemistry of o-
AQMs and the design of tandem cyclization reactions for the
total synthesis of natural products,16 herein, we report a
fundamentally new approach to the construction of the
tetracyclic bridged dioxabicyclo[3.3.1]nonane skeletons with
an oxa-quaternary stereocenter by a Lewis acid catalyzed
tandem reaction of electron-rich phenols and in situ generated
o-AQMs, involving an intermolecular 1,4-conjugate addition
and subsequent spontaneous 6-endo cyclization/1,3-aryl shift/
intramolecular 1,4-conjugate addition process.
With the optimized conditions established, the scope of the
tandem reaction with respect to the in situ generated o-QMs
substrates in our tandem reaction was investigated (Scheme 2).
Scheme 2. Scope of o-Hydroxyl Benzyl Alcohols (o-
a,b
HBAs)
We selected o-AQM precursor o-hydroxybenzyl alcohol (o-
HBA) 1a and sesamol 2a as representative substrates to
evaluate the acid-catalyzed tandem reaction. Initially, several
Brønsted acids were used for this transformation at 60 °C in
(ClCH2)2. However, only 1,4-conjugate addition product 3aa′
(CCDC 1846527) was isolated (Table 1, entries 1 and 2).
a,b
Table 1. Optimization of the Reaction Conditions
entry
catalyst
TFA
( )-CSA
TsOH
CF3SO3H
Sc(OTf)3
AlCl3
BF3·Et2O
FeCl3·6H2O
Cu(OTf)2
AgSbF6
solvent
temp/°C
time/h
yield (%)
1
2
3
4
5
6
7
8
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
CHCl3
100
60
60
60
60
100
60
60
60
60
50
25
50
8
5
2
5
8
8
5
5
2
5
5
5
5
c
c
76
85
mixture
a
Performed with o-HBAs 1 (0.2 mmol) and sesamol 2a (0.22 mmol)
in the presence of AgOTf (0.02 mmol) in (CH2Cl)2 (2 mL) at 50 °C.
c
c
c
92
50
94
c
b
c
Yield of isolated product. Ketal byproduct was obtained in 5% yield;
d
for details, see the Supporting Information. Inseparable mixture
9
(3:1) of desired 3ma and ketal byproduct.
10
11
12
13
AgOTf
AgOTf
AgOTf
We were pleased to find that the Lewis acid catalyzed tandem
reaction works across a broad range of o-HBAs in the presence
of sesamol 2a to provide access to a diverse array of
functionalized tetracyclic dioxabicyclo[3.3.1]nonane motifs.
The substrates displaying simple alkyl (Me and tert-butyl)
substituents on the aromatic ring A performed well in this
transformation and afforded the corresponding products
(3ba−ea) in 70−91% yields. The halogen (F, Cl, and Br)-
and NO2-containing substrates at ring A were tolerated under
the standard conditions, all proceeding in moderate to good
yields (3fa−ia). Additionally, substrates with different aryl
(e.g., 4-ClC6H4, 3-FC6H4, 4-MeC6H4, 4-n-amyl-C6H4) sub-
stituted acetylene motifs were transferred effectively and gave
rise to excellent yields of the products (3ja−ma). The
substrates bearing linear n-butyl, n-amyl, and terminal
acetylene substrates were examined, giving the corresponding
90
a
Performed with o-AQM 1a (0.2 mmol) and phenol 2a (0.22 mmol)
in the presence of catalyst (0.02 mmol) in solvent (2 mL) at the
b
c
indicated temperature. Yield of isolated product. Product 3aa′
formed exclusively. TFA = trifluoroacetic acid, CSA = camphorsul-
fonic acid.
Among a few other variations attempted, when using TsOH
and CF3SO3H, the desired 3aa was obtained in 76% and 85%
yields (Table 1, entries 3 and 4), respectively. The relative
configuration of 3aa was unambiguously established by X-ray
crystallographic analysis (CCDC 1846728). Encouraged by the
initial results, we turned our attention to a range of Lewis acid
catalysts such as Sc(OTf)3, AlCl3, FeCl3·6H2O, Cu(OTf)2,
B
DOI: 10.1021/acs.orglett.8b01862
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
products 3na (78% yield), 3oa (80% yield), and 3pa (96%
yield), respectively. Additionally, for o-HBA bearing branched
tert-butyl as well as phenolic ether substrates, generally the
expected products (3qa−ta) could be obtained smoothly,
though long reaction time (30 h) was needed for the substrate
2q.
substrate 1u to the standard reaction conditions produced a
62% yield of the desired bis-dioxabicyclo[3.3.1]nonane
product 3ua.
On the basis of the above observations, one plausible
mechanistic proposal for this Lewis acid catalyzed tandem
process is illustrated in Scheme 5. The Lewis acid catalyzed
Next, the scope of this process with respect to the electron-
rich phenols was preliminarily investigated using o-HBAs 1; the
results are summarized in Scheme 3. Along with sesamol,
Scheme 5. Proposed Mechanism
a,b
Scheme 3. Scope of Electron-Rich Phenols
dehydration of o-HBA 1a first resulted in the generation of an
o-AQM intermediate, and subsequently, an intermolecular 1,4-
conjugate addition of electron-rich phenol 2a led to 3aa′
(CCDC 1846527). Next, 6-endo cyclization of 3aa′ generated
regioisomers oxocarbenium ions A (route a) and B (route b),
which could rearrange into each other through a ketone
intermediate under the action of acid. Furthermore, compared
with B, intermediate A was subjected to a 1,3-aryl shift.17 It
occurs as a sequence of a dearomatizing ipso-Friedel−Crafts
addition to yield the favorable spirocyclobutyl o-quinone
methide A′, which could be stabilized by electron-donating
piperonyl, followed by a ring-opening to C. Finally, generated
intermediate C quickly underwent an intramolecular 1,4-
conjugate addition to afford the final product 3aa.
In conclusion, we have developed a novel Lewis acid
catalyzed tandem approach of alkynyl o-quinone methides (o-
AQMs) and electron-rich phenols under mild reaction
conditions, leading to biologically interesting bridged tetracy-
clic dioxabicyclo[3.3.1]nonanes bearing a sterically congested
bridgehead oxa-quaternary stereocenter with high efficiency.
This reaction underwent a tandem intermolecular 1,4-
conjugate addition/6-endo cyclization/1,3-aryl shift/intramo-
lecular 1,4-conjugate addition process. Notably, this reaction
not only provides a new pathway to the bridged tetracyclic
dioxabicyclo[3.3.1]nonane skeletons but also manifests the
new reactivity of alkynyl o-quinone methides (o-AQMs) in
methodology design. Further exploration of the chemistry of
alkynyl o-quinone methides in organic synthesis is underway in
our laboratory.
a
Performed with o-HBAs 1 (0.2 mmol) and electron-rich phenols 2
(0.22 mmol) in the presence of AgOTf (0.02 mmol) in (CH2Cl)2 (2
mL) at 50 °C. Yield of isolated product.
b
electron-rich aromatic substrates bearing methoxy groups were
also suitable for this tandem process, thus giving the
corresponding tetracyclic dioxabicyclo[3.3.1]nonane products
in good results. More importantly, the β-naphthol could
participate in this tandem reaction to generate the desired
products (4d and 4g) in 65% and 67% yields, and the structure
of 4d was unambiguously assigned on the basis of single-crystal
X-ray diffraction analysis (CCDC 1846525). Unsubstituted
phenol as well as p-methoxyphenol and alkylphenol were
ineffective in this reaction and even prolonged the reaction
time and raised the reaction temperature.
To investigate the robustness of this new Lewis acid
catalyzed tandem reaction under the standard reaction
conditions, we carried out a gram-scale reaction of 1a (5
mmol, 1.12 g) and 2a, affording the expected tetracyclic
dioxabicyclo[3.3.1]nonane 3aa in 87% isolated yield (Scheme
4). Additionally, subjecting the interesting symmetric o-HBA
Scheme 4. Application Studies
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01862.
Experimental procedure and spectral data (PDF)
C
DOI: 10.1021/acs.orglett.8b01862
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
Letter
(8) For selected examples of 1,4-addition of o-QMs, see: (a) Luan,
Y.; Schaus, S. E. J. Am. Chem. Soc. 2012, 134, 19965. (b) Grayson, M.
N.; Goodman, J. M. J. Org. Chem. 2015, 80, 2056. (c) Lai, Z.; Wang,
Z.; Sun, J. Org. Lett. 2015, 17, 6058. (d) Lewis, R. S.; Garza, C. J.;
Dang, A. T.; Pedro, T. K. A.; Chain, W. J. Org. Lett. 2015, 17, 2278.
(e) Huang, H.; Kang, J. Y. Org. Lett. 2017, 19, 5988. (f) Zhou, J.;
Huang, W.-J.; Jiang, G.-F. Org. Lett. 2018, 20, 1158. (g) Jia, S.; Chen,
Z.; Zhang, N.; Tan, Y.; Liu, Y.; Deng, J.; Yan, H. J. Am. Chem. Soc.
2018, 140, 7056.
CCDC 1846525−1846527 and 1846728 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing 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.
(9) Green, J. C.; Pettus, T. R. R. J. Am. Chem. Soc. 2011, 133, 1603.
(10) For selected examples of [4 + 1]-cycloaddition of o-QMs, see:
(a) Rodriguez, K. X.; Vail, J. D.; Ashfeld, B. L. Org. Lett. 2016, 18,
4514. (b) Jiang, X.-L.; Liu, S.-J.; Gu, Y.-Q.; Mei, G.-J.; Shi, F. Adv.
Synth. Catal. 2017, 359, 3341. (c) Yang, Q.-Q.; Xiao, W.-J. Eur. J. Org.
Chem. 2017, 2017, 233. (d) Zielke, K.; Waser, M. Org. Lett. 2018, 20,
768.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dujy2015@163.com.
ORCID
Ji-Yuan Du: 0000-0002-3059-9830
Notes
(11) For selected examples of [4 + 2]-cycloaddition of o-QMs, see:
(a) Zhao, J. J.; Sun, S. B.; He, S. H.; Wu, Q.; Shi, F. Angew. Chem., Int.
Ed. 2015, 54, 5460. (b) Yang, Y.; Liu, H.; Peng, C.; Wu, J.; Zhang, J.;
Qiao, Y.; Wang, X.-N.; Chang, J. Org. Lett. 2016, 18, 5022.
(c) Alamsetti, S. K.; Spanka, M.; Schneider, C. Angew. Chem., Int.
Ed. 2016, 55, 2392. (d) Allen, E. E.; Zhu, C.; Panek, J. S.; Schaus, S. E.
Org. Lett. 2017, 19, 1878. (e) Deng, Y.-H.; Chu, W.-D.; Zhang, X.-Z.;
Yan, X.; Yu, K.-Y.; Yang, L.-L.; Huang, H.; Fan, C.-A. J. Org. Chem.
2017, 82, 5433. (f) Gebauer, K.; Reuß, F.; Spanka, M.; Schneider, C.
Org. Lett. 2017, 19, 4588. (g) Gharui, C.; Singh, S.; Pan, S. C. Org.
Biomol. Chem. 2017, 15, 7272. (h) Jin, J.-H.; Li, X.-Y.; Luo, X.; Fossey,
J. S.; Deng, W.-P. J. Org. Chem. 2017, 82, 5424. (i) Liang, M.; Zhang,
S.; Jia, J.; Tung, C.-H.; Wang, J.; Xu, Z. Org. Lett. 2017, 19, 2526.
(j) Lukashenko, A. V.; Osyanin, V. A.; Osipov, D. V.; Klimochkin, Y.
N. J. Org. Chem. 2017, 82, 1517. (k) Wang, Y.; Pan, J.; Dong, J.; Yu,
C.; Li, T.; Wang, X.-S.; Shen, S.; Yao, C. J. Org. Chem. 2017, 82, 1790.
(l) Wang, Z.; Sun, J. Org. Lett. 2017, 19, 2334. (m) Wang, Z.; Wang,
T.; Yao, W.; Lu, Y. Org. Lett. 2017, 19, 4126. (n) Zhou, J.; Wang, M.-
L.; Gao, X.; Jiang, G.-F.; Zhou, Y.-G. Chem. Commun. 2017, 53, 3531.
(o) Chen, P.; Wang, K.; Guo, W.; Liu, X.; Liu, Y.; Li, C. Angew.
Chem., Int. Ed. 2017, 56, 3689. (p) Wu, X.; Xue, L.; Li, D.; Jia, S.; Ao,
J.; Deng, J.; Yan, H. Angew. Chem. 2017, 129, 13910. (q) Jeong, H. J.;
Kim, D. Y. Org. Lett. 2018, 20, 2944. (r) Osipov, D. V.; Osyanin, V.
A.; Khaysanova, G. D.; Masterova, E. R.; Krasnikov, P. E.;
Klimochkin, Y. N. J. Org. Chem. 2018, 83, 4775. (s) Liu, Y.; Wu,
X.; Li, S.; Xue, L.; Shan, C.; Zhao, Z.; Yan, H. Angew. Chem., Int. Ed.
2018, 57, 6491.
(12) For selected examples of [4 + 3]-cycloaddition of o-QMs, see:
(a) Izquierdo, J.; Orue, A.; Scheidt, K. A. J. Am. Chem. Soc. 2013, 135,
10634. (b) Lv, H.; Jia, W. Q.; Sun, L. H.; Ye, S. Angew. Chem., Int. Ed.
2013, 52, 8607.
(13) Ma, J.; Chen, K.; Fu, H.; Zhang, L.; Wu, W.; Jiang, H.; Zhu, S.
Org. Lett. 2016, 18, 1322.
(14) Saha, S.; Schneider, C. Org. Lett. 2015, 17, 648.
(15) Du, J.-Y.; Ma, Y.-H.; Yuan, R.-Q.; Xin, N.; Nie, S.-Z.; Ma, C.-L.;
Li, C.-Z.; Zhao, C.-Q. Org. Lett. 2018, 20, 477.
(16) (a) Chu, W.-D.; Zhang, L.-F.; Bao, X.; Zhao, X.-H.; Zeng, C.;
Du, J.-Y.; Zhang, G.-B.; Wang, F.-X.; Ma, X.-Y.; Fan, C.-A. Angew.
Chem., Int. Ed. 2013, 52, 9229. (b) Du, J.-Y.; Zeng, C.; Han, X.-J.; Qu,
H.; Zhao, X.-H.; An, X.-T.; Fan, C.-A. J. Am. Chem. Soc. 2015, 137,
4267. (c) Zhang, X.-Z.; Du, J.-Y.; Deng, Y.-H.; Chu, W.-D.; Yan, X.;
Yu, K.-Y.; Fan, C.-A. J. Org. Chem. 2016, 81, 2598.
(17) (a) Fu, L.; Damsen, H.; Niggemann, M. Chem. - Eur. J. 2017,
23, 12158. (b) Niggemann, M.; Fu, L.; Damsen, H. Chem. - Eur. J.
2017, 23, 12184.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC
(21602094, 21503104) and the Natural Science Foundation
of Shandong Province (ZR2016BB11, ZR2016BB07,
ZR2017MB060, ZR2017PB014).
REFERENCES
■
(1) (a) Mayer, W.; Goll, L.; Arndt, E. M. v.; Mannschreck, A.
Tetrahedron Lett. 1966, 7, 429. (b) Waterman, P. G.; Mahmoud, E.-H.
N. Phytochemistry 1987, 26, 1189. (c) Bam, M.; Ferreira, D.; Brandt,
E. V. Phytochemistry 1988, 27, 3704. (d) Lin, C.-N.; Lu, C.-M.;
Huang, P.-L. Phytochemistry 1995, 39, 1447. (e) Zheng, Z. P.; Cheng,
K. W.; To, J. T.; Li, H.; Wang, M. Mol. Nutr. Food Res. 2008, 52,
1530. (f) Zhang, X.-Q.; Jing, Y.; Wang, G.-C.; Wang, Y.; Zhao, H.-N.;
Ye, W.-C. Fitoterapia 2010, 81, 813. (g) Cui, Y.-M.; Wang, H.; Liu,
Q.-R.; Han, M.; Lu, Y.; Zhao, C.-Q. Bioorg. Med. Chem. Lett. 2011, 21,
4400. (h) Sasaki, H.; Kashiwada, Y.; Shibata, H.; Takaishi, Y.
Phytochemistry 2012, 82, 136. (i) Panthong, K.; Tohdee, K.;
Hutadilok-Towatana, N.; Voravuthikunchai, S. P.; Chusri, S. J. Braz.
Chem. Soc. 2013, 24, 1656. (j) Chen, L.; Zhou, Q.; Li, B.; Liu, S.-J.;
Dong, J.-X. Nat. Prod. Res. 2015, 29, 1217. (k) Septama, A. W.;
Panichayupakaranant, P. Pharm. Biol. 2015, 53, 1608. (l) Peng, W.;
Yang, C.; Zhan, R.; Chen, Y. Nat. Prod. Res. 2016, 30, 2350.
(2) Tan, H.-Y.; Tse, I. M. Y.; Li, E. T. S.; Wang, M. J. Funct. Foods
2015, 15, 207.
(3) (a) Chakravarty, G.; Seshadri, T. R. Tetrahedron Lett. 1962, 3,
787. (b) Bhatia, G. D.; Mukerjee, S. K.; Seshadri, T. R. Tetrahedron
1966, 22, 531. (c) Bhatia, G. D.; Mukerjee, S. K.; Seshadri, T. R.
Tetrahedron 1966, 22, 139. (d) Marathe, K. G.; Saindane, M. T.
Tetrahedron 1975, 31, 1011. (e) Marathe, K. G.; Saindane, M. T.
Tetrahedron 1975, 31, 2821.
(4) (a) Hennis, H. E.; Wang, C.-S. J. Org. Chem. 1969, 34, 1907.
(b) Hennis, H. E.; Thompson, L. R.; Wang, C.-S.; Priddy, D. B. J. Org.
Chem. 1970, 35, 3127.
(5) Liu, H.; Wang, Y.; Guo, X.; Huo, L.; Xu, Z.; Zhang, W.; Qiu, S.;
Yang, B.; Tan, H. Org. Lett. 2018, 20, 546.
(6) For selected reviews of o-QMs, see: (a) Ferreira, S. B.; da Silva,
F. d. C.; Pinto, A. C.; Gonzaga, D. T. G.; Ferreira, V. F. J. Heterocycl.
Chem. 2009, 46, 1080. (b) Willis, N. J.; Bray, C. D. Chem. - Eur. J.
2012, 18, 9160. (c) 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.
(d) Singh, M. S.; Nagaraju, A.; Anand, N.; Chowdhury, S. RSC Adv.
2014, 4, 55924. (e) Caruana, L.; Fochi, M.; Bernardi, L. Molecules
2015, 20, 11733. (f) Wang, Z.; Sun, J. Synthesis 2015, 47, 3629.
(g) Jaworski, A. A.; Scheidt, K. A. J. Org. Chem. 2016, 81, 10145.
(h) Osipov, D. V.; Osyanin, V. A.; Klimochkin, Yu. N. Russ. Chem.
Rev. 2017, 86, 625. (i) Bruins, J. J.; Albada, B.; van Delft, F. Chem. -
Eur. J. 2018, 24, 4749.
(7) Adler, M. J.; Baldwin, S. W. Tetrahedron Lett. 2009, 50, 5075.
D
DOI: 10.1021/acs.orglett.8b01862
Org. Lett. XXXX, XXX, XXX−XXX