α-Nitro-α,β-Unsaturated Ketones: An Electrophilic Acyl Transfer Reagent in Catalytic Asymmetric Friedel-Crafts and Michael Reactions

Article abstract of DOI:10.1021/acs.orglett.9b02310

Herein, we introduce α-nitro-α,β-unsaturated ketones as efficient electrophilic acyl transfer reagents, and they were employed in Friedel-Crafts as well as in Michael reactions. The desired acyl transfer products of these reactions were obtained in high yields with high to excellent enantioselectivities with t-leucine-derived squaramide catalyst under mild reaction conditions. Few applications including a synthesis of the isoxazoline motif have been demonstrated.

Full text of DOI:10.1021/acs.orglett.9b02310

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
α‑Nitro-α,β-Unsaturated Ketones: An Electrophilic Acyl Transfer
Reagent in Catalytic Asymmetric Friedel−Crafts and Michael
Reactions
Chandrakanta Parida, Rajendra Maity, Subas Chandra Sahoo, and Subhas Chandra Pan*
Department of Chemistry, Indian Institute of Technology Guwahati, North Guwahati, Assam 781039, India
S
* Supporting Information
ABSTRACT: Herein, we introduce α-nitro-α,β-unsaturated ketones as efficient electrophilic acyl transfer reagents, and they
were employed in Friedel−Crafts as well as in Michael reactions. The desired acyl transfer products of these reactions were
obtained in high yields with high to excellent enantioselectivities with t-leucine-derived squaramide catalyst under mild reaction
conditions. Few applications including a synthesis of the isoxazoline motif have been demonstrated.
of organocatalytic approaches6 have been reported for the
he incorporation of an acyl group into organic molecules
Tis an important transformation in biological and synthetic development of enantioselective variants.7 Electron-rich arene
derivatives8 and β-naphthols9 have also been recently
employed in Friedel−Crafts reactions. We became interested
in an asymmetric F−C reaction with concomitant acyl transfer
to the OH group. We hypothesized that α-nitro-α,β-
unsaturated ketones could be a suitable Michael acceptor,
and after F−C reaction the acyl group could be migrated to the
phenolic OH group.
We initiated our exploration by performing a model reaction
between β-naphthol (1a) and (E)-2-nitro-1,3-diphenylprop-2-
en-1-one (2a) with quinine-derived squaramide catalyst10 I in
toluene solvent at 0 °C with MS 4 Å as an additive (Table 1,
entry 1). Gratifyingly, the desired cascade Friedel−Crafts acyl
transfer reaction took place, and the desired product 3a was
obtained in 63% yield with 82% ee. The yield and
enantioselectivity were slightly improved with cinchonidine-
derived catalyst II (entry 2). Then quinidine- and hydro-
quinine-derived squramide catalysts III and IV were screened
in the reaction; however, similar levels of enentioselectivities
were attained (entries 3 and 4). Replacement of the
squaramide motif with thiourea was detrimental for the
reaction (entry 5). Then t-leucine-derived squaramide catalyst
VI was prepared and employed in the reaction. Delightfully,
chemistry.1,2 A number of methods and reagents have been
developed over the past years for efficient acyl transfer
reactions (Scheme 1).3,4 Suitable electrophilic and nucleophilic
acyl transfer reagents have also been employed in asymmetric
organocatalysis.4 The electrophilic acyl transfer reagents
mainly consist of a cyanide group or in situ generated N,N-
dimethylpyridinium species. For example, Fu and co-workers
reported kinetic resolution of secondary alcohols with chiral
DMAP catalyst via in situ generation of acyl-DMAP species.4a
In 2001, Deng and co-workers showed an efficient cyano-
ethoxycarbonylation of ketones with ethylcyanoformate.4b In
2007, the List group reported catalytic asymmetric acylcyana-
tion of imines with acetylcyanide as the reagent.4c,d Later,
Ishihara et al. demonstrated cyano-ethoxycarbonylation of N-
protected isatins, taking advantage of Lewis base−Brønsted
acid catalysis.4e However, there is no report on the asymmetric
Friedel−Crafts (F−C) reaction with simultaneous acyl trans-
fer.
The Friedel−Crafts (F−C) reaction is one of the powerful
methods for C−C bond formation in organic chemistry and
has been widely utilized for the synthesis of aromatic and
heteroaromatic frameworks.5 A range of aromatic compounds,
including benzenes with electron-donating substituents, furans,
pyrroles, and indoles, have been successfully employed in a
number of F−C reactions with diverse electrophiles. A variety
Received: July 4, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
this catalyst was found to be efficient and provided product 3a
in 70% yield with 92% ee (entry 6). To further improve the
enantioselectivity, other solvents were screened (entries 7−9).
Dichloromethane was not a suitable solvent for the reaction,
although high enantioselectivity was obtained in diethyl ether
solvent (entries 7 and 8). Finally, α,α,α-trifluoro toluene
emerged as the best solvent, and the product 3a was attained in
80% yield with 94% ee (entry 9).
Scheme 1. Representative Examples of Electrophilic Acyl
Transfer Reagents in Asymmetric Organocatalysis
After the optimized conditions were established, the
generality and scope of the reaction were studied. Initially,
different nitroenones 2 having variations in the olefin group
were investigated, and gratifyingly, excellent results were
achieved irrespective of the electronic nature of the aryl
group (Table 2). At the beginning, different para-substituted
Table 2. Variations of the Olefin Part in Enone 2
a
b
c
entry
R
2
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
Ph
2a
2b
2c
2d
2e
2f
2g
2h
2i
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
80
72
68
70
72
61
64
69
75
68
65
75
94
92
90
92
96
96
96
96
94
96
94
84
4-MeC6H4
4-OMeC6H4
4-ClC6H4
4-BrC6H4
4-NO2C6H4
3-ClC6H4
3-CF3C6H4
2-FC6H4
Table 1. Catalyst Optimization for Friedel−Crafts Acyl
Transfer Reaction
10
11
12
2-ClC6H4
2-Br-4-MeC6H3
2-furyl
2j
2k
2l
3aj
3ak
3al
a
All reactions were carried out with 0.1 mmol of 1a with 0.1 mmol of
b
2 in 1.2 mL of PhCF3. Isolated yield after silica gel column
chromatography. Determined by chiral HPLC.
c
phenyl groups containing enones were employed, and
impressive enantioselectivities were attained (entries 2−6).
For example, nitroenone 1b having a para-tolyl group delivered
product 3ab in 72% yield with 92% ee (entry 2). The outcome
did not change much with para-anisyl-substituted enone 2c
(entry 3). Then, different 4-halosubstituted aryl groups
containing enones were screened, and the corresponding
products 3ad and 3ae were isolated in good yields with
excellent enantioselectivities (entries 4 and 5). Interestingly,
incorporation of an electron-withdrawing nitro group was also
tolerated, and product 3af was attained in 96% ee (entry 6).
Also, the scope was extended to meta- and ortho-substituted
aryl groups containing enones, and delightfully here also
excellent results were attained (entries 7−10). A 2,4-
disubstituted aryl group containing enone also took part in
the reaction, and the product 3ak was obtained in 94% ee
(entry 11). Finally, furyl-substituted enone 2l was engaged in
the reaction, and slightly less enantioselectivity was detected
for product 3al (entry 12).
a
b
c
entry
catalyst
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
I
II
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
Et2O
63
70
65
60
42
70
60
70
80
82
84
80
84
50
92
73
91
94
III
IV
V
VI
VI
VI
VI
PhCF3
a
All reactions were carried out with 0.1 mmol of 1a with 0.1 mmol of
2a in 1.2 mL of solvent. Isolated yield after silica gel column
chromatography. Determined by chiral HPLC.
b
Then, the ketone part of the enone 2 was varied, and the
results are shown in Table 3. It turned out that a range of
substitutions on the aryl group could be installed, and excellent
outcome was maintained. Initially, different para-substitutions
c
B
DOI: 10.1021/acs.orglett.9b02310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
Table 3. Variations of the Ketone Part in Enone 2
Scheme 2. Scope of β-Naphthols
a
b
c
entry
R1
2
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
4-iPrC6H4
4-OMeC6H4
4-OPhC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-PhC6H4
3-BrC6H4
2,4-Me2C6H3
2-naphthyl
cyclohexyl
ipropyl
2m
2n
2o
2p
2q
2r
2s
2t
2u
2v
2w
2x
3am
3an
3ao
3ap
3aq
3ar
3as
3at
3au
3av
3aw
3ax
72
73
65
75
73
75
75
75
72
70
72
65
92
96
96
96
92
94
96
95
92
92
88
68
10
11
12
a
All reactions were carried out with 0.1 mmol of 1a with 0.1 mmol of
b
2 in 1.2 mL of PhCF3. Isolated yield after silica gel column
chromatography. Determined by chiral HPLC.
c
a
All reactions were carried out with 0.1 mmol of 1 and with 0.1 mmol
b
of 2a in 1.2 mL of PhCF3. Yields were determined after isolation in
silica gel column chromatography, and ee’s were obtained from chiral
HPLC.
were checked, and the corresponding products were obtained
in good yields with high enantioselectivities (entries 1−7). For
example, the para-anisyl group containing enone 2n delivered
product 3an in 73% yield with 96% ee (entry 2). Replacing the
methoxy group with the phenoxy group did not alter the
enantioselectivity (entry 3). Different halo-substitutions were
also tolerated at the 4-position to provide products in high
enantioselectivities (entries 4−6). Delightfully, the 4-biphenyl
group containing enone smoothly reacted to afford product
3as in 75% yield with 96% ee (entry 7). Meta-substituted
enone 2t and 2,4-dimethyl-substituted enone 2u underwent
the reaction, delivering the products 3at and 3au in 95% ee
and 92% ee, respectively (entries 8 and 9). The outcome was
also excellent with 2-naphthyl-substituted enone 2v (entry 10).
Then, the cyclohexyl group containing enone 2w was engaged
in the reaction, and gratifyingly, 88% ee was achieved for 3aw
(entry 11). The isopropyl group containing enone 2x was also
tolerated in the reaction, though lower enantioselectivity was
detected for the product 3ax (entry 12).
Scheme 3. Employment of Phenolic, Indole, and Quinoline
a b c
, ,
Derivatives
In the next phase, different β-naphthol derivatives were
employed in the reaction (Scheme 2). As can be seen, a variety
of substitutions were tolerated at 6-, 7-, and 3-positions, and
high enantioselectivities were obtained. Initially, the 6-position
was substituted with a methoxy group, and the corresponding
product 3ba was obtained in 80% yield with 92% ee.
Substitutions with a bromo or aryl group did not alter the
enantioselectivity. The phenylacetylene group could also be
incorporated, and good results were detected. Then different
electron-poor carbonyl functionalities were installed, and
delightfully the reactions progressed efficiently to provide
products in high enantioselectivities. 7-Substituted methoxy-
and bromo-containing naphthols also participated in the
reaction, delivering the products 3ia and 3ja in 94% ee and
92% ee, respectively. Moreover, 3-bromo-substituted β-
naphthol 1k can also be employed, and it was found that the
steric effect did not hinder the formation of the product 3ka.
Then we explored other phenolic derivatives in the reaction
(Scheme 3). At first 3,4-dimethoxy phenol was employed, and
a
All reactions were carried out with 0.1 mmol of 1 and 0.1 mmol of
b
2a in 1.2 mL of PhCF3. Yields were determined after isolation in
silica gel column chromatography, and ee’s were obtained from chiral
HPLC. Without MS 4 Å.
c
gratifyingly the desired product 4 was attained in acceptable
yield with 80% ee. The enantioselectivity was high for product
5 derived from sesamol. The reaction also took place with 9-
phenanthrol, and the desired product 6 was isolated in 70%
yield with 92% ee. Then we carried out reactions with hydroxy
indole compounds, and it turned out that our methodology
was suitable to provide desired products 7 and 8. In particular,
an excellent 94% ee was obtained for product 7. Hydrox-
yquinoline can also be employed in the reaction, and the best
result was obtained without using MS 4 Å.
C
DOI: 10.1021/acs.orglett.9b02310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Then, we looked for the feasibility of our strategy in a
Michael-acyl transfer reaction (Scheme 4). Thus, enone 2a was
Scheme 6. Proposed Mechanism
Scheme 4. Employment of Nitroenone 2a in Michael
Reactions
products having nitro and acyl groups were obtained in high
yields with high to excellent enantioselectivities under mild
reaction conditions. Synthetic applications such as reduction
and isoxazoline synthesis have also been shown. Given the
pharmaceutical importance of nitro and acyl groups, our
methodology might be useful in drug discovery.
treated with a 1,3-dicarbonyl compound dimedone (10) in the
presence of catalyst VI at room temperature.11 Gratifyingly,
after stirring for 24 h, the desired Michael-acyl transfer product
11 was isolated in 55% yield with 93% ee. Then, pyrazolone 12
was employed in the reaction. The best result was attained
after mixing 2a and 12 at 0 °C with MS 4 Å. Delightfully, the
chiral pyrazole product 13 was obtained in 60% yield with 89%
ee.
ASSOCIATED CONTENT
* Supporting Information
■
S
To illustrate the synthetic potential of our method, few
reactions were carried out on 3aa (Scheme 5). Initially, 3aa
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02310.
Scheme 5. Synthetic Transformations of 3a
Experimental procedures, characterization data of all the
products, and CIF file of compound 3aj (PDF)
Accession Codes
CCDC 1915972 contains the supplementary 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
was treated with Zn/AcOH. In this case, not only the nitro
group was reduced to amine but also concomitant acyl transfer
took place to provide 14 in 95% yield with preservation of
enantiopurity. Then, a Michael-cyclization reaction was
performed by mixing 3aa with methyl acrylate and (Boc)2O
in the presence of DMAP to provide isoxazoline 15/15′ as a
mixture of diastereomers in good yield, though slight erosion
in enantioselectivity was detected.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: span@iitg.ac.in.
ORCID
Subhas Chandra Pan: 0000-0002-7581-1831
Notes
The absolute structure of the product 3aj was determined to
be (S) by single-crystal X-ray crystallography. Since the priority
changes when the 2-chlorophenyl group is replaced by the
phenyl group, the (S) becomes (R) but will have similar
structure.
Based on the absolute configuration, a plausible mechanism
has been proposed (Scheme 6). Since the Re face of enone 2a
is blocked by the catalyst, Friedel−Crafts reaction only takes
place from the Si face, and intermediate 16 is generated.
Hemiketalization of 16 leads to the formation of 17, which
undergoes ring-opening reaction to deliver product 3aa.
In summary, we have demonstrated α-nitro-α,β-unsaturated
ketones as an efficient electrophilic acyl transfer reagent in
Friedel−Crafts as well as in Michael addition reactions for the
first time. This is also the first report for the enantioselective
Friedel−Crafts reaction with concomitant acyl transfer. The
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by DST-SERB (file no CRG/2018/
001154). We also thank CIF, Indian Institute of Technology
Guwahati for the instrumental facility.
REFERENCES
■
(1) (a) Trost, B. M.; Fleming, I. Comprehensive Organic Synthesis;
Pergamon Press: Oxford, 1991. (b) Smith, M. B.; March, J. Advanced
Organic Chemistry; Wiley: Hoboken, NJ, 2007.
(2) (a) Pietrocola, F.; Galluzzi, L.; San Pedro, J. M. B.; Madeo, F.;
Kroemer, G. Cell Metab. 2015, 21, 805. (b) Burke, H. M.; McSweeny,
L.; Scanlan, E. M. Nat. Commun. 2017, 8, 15655.
(3) For selected metal-catalyzed reactions, see: (a) Baeza, A.; Casas,
́
́
J.; Najera, C.; Sansano, J.; Saa, J. M. Angew. Chem., Int. Ed. 2003, 42,
D
DOI: 10.1021/acs.orglett.9b02310
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
3143. (b) Belokon’, Y. N.; Blacker, A. J.; Clutterbuck, L. A.; North, M.
Org. Lett. 2003, 5, 4505. (c) Yamagiwa, N.; Tian, J.; Matsunaga, S.;
Shibasaki, M. J. Am. Chem. Soc. 2005, 127, 3413. (d) Lundgren, S.;
Wingstrand, E.; Penhoat, M.; Moberg, C. J. Am. Chem. Soc. 2005, 127,
11592. (e) Gou, S.; Chen, X.; Xiong, Y.; Feng, X. J. Org. Chem. 2006,
71, 5732. (f) Abell, J. P.; Yamamoto, H. J. Am. Chem. Soc. 2009, 131,
15118. (g) Liu, C.; Meng, G.; Liu, Y.; Liu, R.; Lalancette, R.; Szostak,
R.; Szostak, M. Org. Lett. 2016, 18, 4194. (h) Luo, Z.; Liu, T.; Guo,
W.; Wang, Z.; Huang, J.; Zhu, Y.; Zeng, Z. Org. Process Res. Dev. 2018,
22, 1188.
(d) Sohtome, Y.; Shin, B.; Horitsugi, N.; Takagi, R.; Noguchi, K.;
Nagasawa, K. Angew. Chem., Int. Ed. 2010, 49, 7299. (e) Liu, G.;
Zhang, S.; Li, H.; Zhang, T.; Wang, W. Org. Lett. 2011, 13, 828.
(10) For an initial report, see: (a) Malerich, J.; Hagihara, K.; Rawal,
́
V. J. Am. Chem. Soc. 2008, 130, 14416. For reviews, see: (b) Aleman,
J.; Parra, A.; Jiang, H.; Jørgensen, K. A. Chem. - Eur. J. 2011, 17, 6890.
(c) Chauhan, P.; Mahajan, S.; Kaya, U.; Hack, D.; Enders, D. Adv.
Synth. Catal. 2015, 357, 253.
(11) Mo, Y.; Liu, S.; Liu, Y.; Ye, L.; Shi, Z.; Zhao, Z.; Li, X. Chem.
Commun. 2019, 55, 6285.
(4) For selected organocatalytic examples, see: (a) Ruble, J. C.;
Tweddell, J.; Fu, G. C. J. Org. Chem. 1998, 63, 2794. (b) Tian, S.-K.;
Deng, L. J. Am. Chem. Soc. 2001, 123, 6195. (c) Pan, S. C.; Zhou, J.;
List, B. Angew. Chem., Int. Ed. 2007, 46, 612. (d) Pan, S. C.; List, B.
Org. Lett. 2007, 9, 1149. (e) Ogura, Y.; Akakura, M.; Sakakura, A.;
Ishihara, K. Angew. Chem., Int. Ed. 2013, 52, 8299. (f) Ishihara, K.;
Ogura, Y. Org. Lett. 2015, 17, 6070. (g) Gao, Y.; Ren, Q.; Siau, W.-Y.;
Wang, J. Chem. Commun. 2011, 47, 5819. (h) Lu, R.-J.; Yan, J.-Y.;
Wang, J.-J.; Du, Q.-S.; Nie, S.-Z.; Yan, M. J. Org. Chem. 2011, 76,
6230. (i) Li, P.; Chan, S. H.; Chan, A. S. C.; Kwong, F. Y. Org. Biomol.
Chem. 2011, 9, 7997. (j) Liu, Y.; Wang, Y.; Song, H.; Zhou, Z.; Tang,
C. Adv. Synth. Catal. 2013, 355, 2544. (k) Maity, R.; Gharui, C.; Sil,
A. K.; Pan, S. C. Org. Lett. 2017, 19, 662. (l) Maity, R.; Pan, S. C. Org.
Biomol. Chem. 2018, 16, 1598. (m) Mondal, K.; Pan, S. C. J. Org.
Chem. 2018, 83, 5301. (n) Gharui, C.; Behera, D.; Pan, S. C. Adv.
Synth. Catal. 2018, 360, 4502.
(5) For a general review on Friedel−Crafts reactions, see: Olah, G.
A.; Krishnamurti, R.; Prakash, G. K. S. Friedel−Crafts Alkylations. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 3, pp 293−339.
(6) For selected examples, see: (a) Paras, N. A.; MacMillan, D. W.
C. J. Am. Chem. Soc. 2001, 123, 4370. (b) Terada, M.; Sorimachi, K. J.
Am. Chem. Soc. 2007, 129, 292. (c) Kang, Q.; Zhao, Z.-A.; You, S.-L.
J. Am. Chem. Soc. 2007, 129, 1484. (d) Husmann, R.; Sugiono, E.;
Mersmann, S.; Raabe, G.; Rueping, M.; Bolm, C. Org. Lett. 2011, 13,
1044. (e) Zhang, K.-F.; Nie, J.; Guo, R.; Zheng, Y.; Ma, J.-A. Adv.
Synth. Catal. 2013, 355, 3497. (f) Fan, Y.-S.; Jiang, Y. J.; An, D.; Sha,
D.; Antilla, J. C.; Zhang, S. Org. Lett. 2014, 16, 6112. (g) Romanini,
S.; Galletti, E.; Caruana, L.; Mazzanti, A.; Himo, F.; Santoro, S.;
Fochi, M.; Bernardi, L. Chem. - Eur. J. 2015, 21, 17578. (h) Zhang,
M.; Sun, W.; Zhu, G.; Bao, G.; Zhang, B.; Hong, L.; Li, M.; Wang, R.
ACS Catal. 2016, 6, 5290. (i) Correia, J. T. M.; List, B.; Coelho, F.
Angew. Chem., Int. Ed. 2017, 56, 7967. (j) Nakamura, S.; Furukawa,
T.; Hatanaka, T.; Funahashi, Y. Chem. Commun. 2018, 54, 3811.
(k) Maestro, A.; Martinez de Marigorta, E.; Palacios, F.; Vicario, J. J.
Org. Chem. 2019, 84, 1094.
(7) For selected recent reviews on asymmetric Friedel−Crafts
reactions, see: (a) Poulsen, T.; Jørgensen, K. A. Chem. Rev. 2008, 108,
2903. (b) You, S.-L.; Cai, Q.; Zeng, M. Chem. Soc. Rev. 2009, 38,
2190. (c) Bandini, M.; Eichholzer, A. Angew. Chem. 2009, 121, 9786.
Bandini, M.; Eichholzer, A. Angew. Chem., Int. Ed. 2009, 48, 9608.
(d) Bandini, M.; Umani-Ronchi, A. Catalytic Asymmetric Friedel−
Crafts Alkylations; Wiley-VCH: Weinheim, 2009. (e) Rueping, M.;
Nachtsheim, B. J. Beilstein J. Org. Chem. 2010, 6, 6. (f) Terrasson, V.;
de Figueiredo, R. M.; Campagne, J. M. Eur. J. Org. Chem. 2010, 2010,
2635. (g) Zeng, M.; You, S.-L. Synlett 2010, 2010, 1289.
(h) Terrasson, V.; de Figueiredo, R. M.; Campagne, J. M. Eur. J.
Org. Chem. 2010, 2010, 2635. (i) Montesinos-Magraner, M.; Vila, C.;
Blay, G.; Pedro, J. R. Synthesis 2016, 48, 2151.
(8) For selected examples, see: (a) Lv, J.; Li, X.; Zhong, L.; Luo, S.;
Chen, J.-P. Org. Lett. 2010, 12, 1096. (b) Enders, D.; Seppelt, M.;
Beck, T. Adv. Synth. Catal. 2010, 352, 1413. (c) Zhang, H.; Liao, Y.-
H.; Yuan, W.-C.; Zhang, X.-M. Eur. J. Org. Chem. 2010, 2010, 3215.
(d) Li, G.-X.; Qu, J. Chem. Commun. 2012, 48, 5518.
(9) For selected examples, see: (a) Brandes, S.; Bella, M.;
Kjærsgaard, A.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2006, 45,
1147. (b) Liu, T.-Y.; Cui, H.-L.; Chai, Q.; Long, J.; Li, B.-J.; Wu, Y.;
Ding, L. S.; Chen, Y.-C. Chem. Commun. 2007, 2228. (c) Hong, L.;
Wang, L.; Sun, W.; Wong, K.; Wang, R. J. Org. Chem. 2009, 74, 6881.
E
DOI: 10.1021/acs.orglett.9b02310
Org. Lett. XXXX, XXX, XXX−XXX