Pd-Catalyzed Three-Component Domino Reaction of Vinyl Benzoxazinanones for Regioselective and Stereoselective Synthesis of Allylic Sulfone-Containing Amino Acid Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b03440

A Pd-catalyzed, highly regioselective and stereoselective three-component domino allylic substitution/N-H carbene insertion reaction under mild conditions is described. This reaction demonstrates a wide substrate scope and satisfactory functional group tolerance, providing a broad range of allylic sulfone-containing amino acid derivatives. Moreover, DBU mediates highly diastereoselective cross-dehydrogenative coupling annulation of allylic sulfones without using peroxides or any metal oxidants. This developed protocol affords 7-membered ring heterocyclic compounds incorporating both sulfone-containing amino acid esters and one quaternary carbon center. Mechanistic studies indicate that an unusual umpolung of glycine occurred in this annulation.

Full text of DOI:10.1021/acs.orglett.8b03440

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd-Catalyzed Three-Component Domino Reaction of Vinyl
Benzoxazinanones for Regioselective and Stereoselective Synthesis
of Allylic Sulfone-Containing Amino Acid Derivatives
Jiping Hao,†,‡,§ Yi Xu,†,‡ Zhongliang Xu,†,‡ Zhiqiang Zhang,∥ and Weibo Yang*,†,‡,∥
†Chinese Academy of Sciences Key Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Shanghai 201203,
China
‡University of Chinese Academy of Sciences, Beijing, China
§Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu 215123, China
∥University of Science and Technology Liaoning, Anshan 114051, China
S
* Supporting Information
ABSTRACT: A Pd-catalyzed, highly regioselective and stereoselective three-component domino allylic substitution/N−H
carbene insertion reaction under mild conditions is described. This reaction demonstrates a wide substrate scope and
satisfactory functional group tolerance, providing a broad range of allylic sulfone-containing amino acid derivatives. Moreover,
DBU mediates highly diastereoselective cross-dehydrogenative coupling annulation of allylic sulfones without using peroxides or
any metal oxidants. This developed protocol affords 7-membered ring heterocyclic compounds incorporating both sulfone-
containing amino acid esters and one quaternary carbon center. Mechanistic studies indicate that an unusual umpolung of
glycine occurred in this annulation.
tedious multistep synthesis. From the viewpoint of step
economy and environmentally friendly synthesis, an attractive
and streamlined strategy would be a one-pot, multicomponent
domino reaction.
ulfur-containing amino acid derivatives are important,
Spharmaceutically relevant compounds, because of their
wide spectrum of biological activities, such as being
antithrombotic agents,1 enzyme inhibitors,2 and antibacterial
agents.3 (Figure 1). As a result, the development of highly
efficient synthetic methods to access these compounds has
been intensively pursued by the synthetic community.
Recently, vinyl benzoxazinones have attracted considerable
attention in organic synthesis, with the π-allylpalladium species
formed via a decarboxylation process.6 After Tunge’s pioneer-
ing work,7 significant contributions have been made in this
field from Xiao,8 Glorius,9 Jorgensen,10 Lu,8a and others.11
Notably, most of the reported systems are based on the
reactivity of C,N-1,4 synthons, which could participate in Pd-
catalyzed [4+n] cycloadditions to access a variety of different
N-heterocyclic compounds. In comparison with Pd-catalyzed
[4+n] cycloadditions of vinyl benzoxazinones, Pd-catalyzed
allylic substitution reactions have rarely been explored. In
2017, Lu and Xiao reported an elegant Pd-catalyzed branch-
and enantioselective allylic amination between N-Ts protected
vinyl benzoxazinones and amines.12 However, to the best of
our knowledge, to date, Pd-catalyzed three-component domino
linear-allylic substitution of vinyl benzoxazinones has never
been described (Scheme 1).
Figure 1. Selected sulfur-containning amino acid units in drugs.
In this context, ring opening of aziridinium ions by
nucleophilic sulfur reagents4 and trapping of sulfonium ylides
with imines5 have become two useful and practical strategies.
Although these strategies have been frequently used for the
synthesis of sulfur-containing amino acid analogies, methods
allowing rapid access to sulfone-containing amino acid
derivatives are especially underdeveloped. With regard to the
latter target construction, a thiolation/oxidation sequence
synthesis route may be a reasonable method. However, it
suffers from the requirement of odorous thiols, together with
In continuation of our current research on controllable
transformations of π-allylpalladium intermediates,13 we envi-
sioned a Pd-catalyzed, one pot, three-component domino
Received: October 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Strategies in Building Sulfone-Containing Amino
Acids
(a) Direct cyclization of 1a to form 1,2-dihydroquinoline
derivatives might be a competitive side reaction.7b
(b) Controlling branch or linear allylic sulfonation of vinyl
benzoxazinones could also present a dilemma.
Keeping these challenges and our hypothesis in mind, we
initiated our study of Pd-catalyzed, three-component, domino
allylic substitution reaction by choosing vinyl benzoxazinones
(1a), diazo esters (2a), and sodium sulfinates (3a) as the
model substrates (Table 1).
Table 1. Catalyst Screening and Optimization of the
a
Reaction Conditions
b
entry
catalyst
solvent
DMF
DMF
yield (%)
1
2
3
Pd2(dba)3·CHCl3
Pd2(dba)3
Pd(PPh3)4
trace
trace
0
DMF
4
5
6
7
Pd(OAc)2
DMF
DMF
DMF
DMF
trace
trace
0
Pd(COD)Cl2
PdCl2[P(Cy)3]2
Pd(acac)2
29
8
9
[Pd(allyl)Cl]2
Cu(OAc)2
DMF
DMF
65
0
10
11
12
13
14
[Cp*RhCl2]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
[Pd(allyl)Cl]2
DMF
0
0
trace
trace
42
MeCN
1,4-dioxane
THF
allylic substitution reaction of vinyl benzoxazinones with easily
available diazo esters14 and sodium sulfinates. The underlying
principle is shown in Scheme 2. Oxidative addition of vinyl
toluene
a
Reaction conditions: 1a (0.05 mmol, 1.0 equiv), 2a (0.1 mmol, 2.0
equiv) and catalyst (5 mol %), stirred at DMF (0.5 mL) for 10 min,
then 3a (0.1 mmol, 2.0 equiv) was added. The mixture was stirred at
60 °C for 12 h. Yields of isolated products.
Scheme 2. Schematic Describing Our Design Principles
b
We first investigated the effects of different catalysts, and we
were surprised to discover that Pd(0) catalysts such as
Pd2(dba)3·CHCl3, Pd2(dba)3, and Pd(PPh3)4, which were
highly effective in Pd-catalyzed cycloaddition of vinyl
benzoxazinones, showed no effectiveness. When we utilized
Pd(acac)2 as a catalyst, the allylic sulfone-containing amino
acid ester 4aaa with exclusively linear distribution was obtained
in 29% isolated yield. Switching the catalyst to [Pd(allyl)Cl]2,
the yield of desired product 4aaa was significantly improved to
65%. This reason could be rationalized as being due to the
similar π-allylpalladium species between catalyst [Pd(allyl)Cl]2
and intermediate I, which likely enhances the interaction and
accelerates the ligand exchange. Notably, no formation of 4aaa
was observed under Rh16 or Cu catalysis. In addition, the
solvent effect was further explored, and it was found that DMF
was the best choice for this transformation.
With the optimized conditions in hand, the scope of vinyl
benzoxazinones was first examined, and a range of N-free
protected vinyl benzoxazinones containing both electron-
donating and electron-withdrawing substituents on the
benzene ring could be well-tolerated to produce the domino
products in moderate to good isolated yields with excellent
regioselectivity and sole formation of the E-isomer. Similarly,
vinyl benzoxazinones with ortho- and meta-substituents on the
benzoxazinones 1a to a Pd(0) species and subsequent
decarboxylation gives intermediate I. Intermediate I then
could react with sodium sulfonate 2a to form intermediate II,
followed by reductive elimination to afford aa and regenerate
the Pd(0) species. Subsequently, the Pd(0) species would
induce decomposition of diazo ester 3a to produce Pd carbene
intermediate III,15 which is expected to undergo N−H carbene
insertion and eventually provide the product 4aaa.
To realize this Pd-catalyzed, three-component, domino
allylic substitution reaction, two challenges must be addressed:
B
DOI: 10.1021/acs.orglett.8b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
benzene ring did not suffer from steric hindrance effects and
delivered the corresponding products in satisfactory yields.
Gratifyingly, the reaction was also compatible with fluorine
and trifluoromethyl-substituted substrates, which are com-
monly encountered in target-oriented synthesis, because of
their unique physical and biological properties. It was
noteworthy that the bromo-substituted vinyl benzoxazinone
proceeded well under standard conditions and gave the
product in 58% yield. This offered the chance for further
elaboration through traditional cross-coupling reactions.
Changing the aromatic ring to naphthalene did not remarkably
influence the reaction efficiency, and the target compound
(4jaa) was isolated in 57% yield.
We next turned our attention to the scope of the sulfinate
salts. The reaction was applicable to a variety of sulfinate salts 2
bearing different substituents on the benzene ring, regardless of
whether an electron-donating or electron-withdrawing group
on the benzene ring could be tolerated. Beyond the tolerance
to simple aromatic ring systems, nonaromatic substrates were
also amenable to the standard conditions. For instance, when
the isopropyl-, methyl-, and ethyl-substituted sulfinate salts
were used as coupling partners, they exhibited good reactivity
and furnished the desired products in synthetically acceptable
yields (47%−57% 4afa−4aha).
strates also proceeded smoothly in this reaction, leading to
similar isolated yields of products.
With the domino allylic substitution/N−H carbene insertion
products in hand, we became interested in the exploration of
selective annulation of these compounds. When compound
4aaa was treated with the Lewis base DBU, an unusual 7-
membered ring N-heterocyclic compound 5aaa was obtained
in 46% yield with high diastereoselectivity (diastereomeric
ratio (dr) of 96:4). A safe assignment of the structure 5aaa was
unambiguously validated by single-crystal X-ray analysis, which
indicated that allylic sulfone-containing amino acid ester and
one quaternary carbon center as privileged structures were
coexistent. In addition, this annulation reaction worked well for
other substrates bearing different substituents, in all cases, and
the diastereoselectivity of the products was excellent (Scheme
4).
a
Scheme 4. DBU Mediates Annulation of Allylic Sulfones
The reaction was further extended to a series of aryl-
substituted diazo ester compounds. As shown in Scheme 3,
various substituents on the benzene ring were tolerated, and
the reaction was found to be minimally affected by the
electronic properties and steric hindrance of the aryl
substituents. Moreover, naphthalene and benzyl ester sub-
a
Scheme 3. Substrate Scopes in Domino Reactions
a
Reaction conditions: 4 (0.1 mmol, 1.0 equiv), DBU (0.1 mmol, 1.0
b
equiv), DMF (1 mL), 60 °C, 12 h. Yields of isolated products. The
reaction was run in 1.0 mmol. The diastereomeric ratio (dr) value
was determined by H NMR analysis.
c
1
It is noteworthy that this intriguing annulation involves
geometric isomerism and dehydrogenation. To elucidate the
detailed mechanism, several control experiments were
performed (Scheme 5). Initially, when substrate 4aaa was
subjected to an argon atmosphere, the yield of 5aaa was
significantly diminished. When 4aaa was conducted under an
oxygen atmosphere condition, the yield of 5aaa was improved
to 50%, albeit with low dr value (78:22). Treatment of imine
4aaa-1, under the standard conditions, led to the formation of
5aaa in 68% yield with excellent diastereoselectivity (98:2).
Interestingly, we also performed the methyl-substituted
substrate 4aaa-2 in the presence of DBU, and, as we expected,
no desired product 5aaa′ was formed.
a
Reaction conditions: 1a (0.1 mmol, 1.0 equiv), 2a (0.2 mmol, 2.0
equiv) and catalyst (5 mol%), stirred at DMF (0.5 mL) for 10 min
then 3a (0.2 mmol, 2.0 equiv) was added. The mixture was stirred at
60 °C for 12 h. The reaction was run in 3.0 mmol scale.
All these results demonstrate that the original substrate 4aaa
could undergo dehydrogenation and form intermediate imine
b
C
DOI: 10.1021/acs.orglett.8b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
intermediate 4aaa-1′ followed by nucleophilic addition to
produce the final product.
Scheme 5. Control Experiments for Mechanism Studies
In conclusion, we have developed a novel Pd-catalyzed
three-component domino reaction via sequence decarboxyla-
tion/allylic substitution/N−H carbene insertion processes,
which allows expedient access to various allylic sulfone-
containing amino acid derivatives. This reaction not only
features mild conditions, broad substrate scope, N-protected
group free, and excellent functional group compatibility, but
also avoids the requirement of odorous thiols and tedious
multistep synthesis. In addition, the construction of 7-
membered ring N-heterocyclic compounds incorporating
both sulfones-containing amino acid esters and quaternary
carbon center was successfully realized by DBU-mediated
annulation of allylic sulfones. Further biological activity
screening of these compounds is ongoing in our laboratory.
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.8b03440.
Experimental procedures and spectral data for all new
compounds (PDF)
Accession Codes
CCDC 1875689 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.
4aaa-1.17 Although the exact reason to explain imine formation
was unclear, H2 was excluded from gas chromatography (GC)
analysis. In parallel, 2,2,6,6-tetramethylpiperidine (TEMPO),
as a radical scavenger, was added to capture some radical
species; however, the efficiency of this transformation was not
affected at all. This suggested that the DBU-mediated
annulation likely does not involve a radical process. The
importance of DBU was next examined in imine formation by
choosing 4bb as a substrate. No reactivity or a low yield was
observed in the absence of DBU, even though other oxidizing
reagents were added, such as 1 atm O2, 1 equiv TBHP, or 1
equiv DDQ. These control experiments highlighted DBU
playing the crucial role in imine formation.
The above outcomes provide information on the plausible
mechanism of the DBU-mediated annulation. As rationalized
in Scheme 6, the reaction begins with release of two H atoms
promoted by Lewis base DBU and provides the imine
intermediate. Subsequently, geometric isomerism occurs from
E-isomer to Z-isomer under basic conditions, which gives
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yweibo@simm.ac.cn.
ORCID
Weibo Yang: 0000-0003-1633-7655
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the 100 Talent Program of the
Chinese Academy of Sciences, Chinese NSF (No. 21702217),
“1000-Youth Talents Plan”, Shanghai-Youth Talent, and
Shanghai-Technology Innovation Action Plan (No.
18JC1415300) for financial support of this research.
Scheme 6. Mechanism of the DBU-Mediated Annulation
REFERENCES
■
(1) Xie, P.; Wang, J.; Liu, Y.; Fan, J.; Wo, X.; Fu, W.; Sun, Z.; Loh,
T. P. Water-promoted C-S bond formation reactions. Nat. Commun.
2018, 9 (1), 1321.
(2) Calleja, J.; Pla, D.; Gorman, T. W.; Domingo, V.; Haffemayer, B.;
Gaunt, M. J. A steric tethering approach enables palladium-catalysed
C-H activation of primary amino alcohols. Nat. Chem. 2015, 7 (12),
1009−1016.
D
DOI: 10.1021/acs.orglett.8b03440
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(3) Reck, F.; Zhou, F.; Girardot, M.; Kern, G.; Eyermann, C. J.;
Hales, N. J.; Ramsay, R. R.; Gravestock, M. B. Identification of 4-
substituted 1,2,3-triazoles as novel oxazolidinone antibacterial agents
with reduced activity against monoamine oxidase A. J. Med. Chem.
2005, 48 (2), 499−506.
(4) Chuang, T.-H.; Sharpless, K. B. Applications of Aziridinium Ions.
Selective Syntheses of α,β-Diamino Esters, α-Sulfanyl-β-amino Esters,
β-Lactams, and 1,5-Benzodiazepin-2-one. Org. Lett. 2000, 2 (23),
3555−3557.
decarboxylative [4 + 2] cycloaddition. Chem. Commun. (Cambridge,
U. K.) 2017, 53 (72), 10030−10033.
(12) Wang, Y. N.; Wang, B. C.; Zhang, M. M.; Gao, X. W.; Li, T. R.;
Lu, L. Q.; Xiao, W. J. Hydrogen Bond Direction Enables Palladium-
Catalyzed Branch- and Enantioselective Allylic Aminations and
Beyond. Org. Lett. 2017, 19 (15), 4094−4097.
(13) (a) Yang, Y.; Yang, W Chem. Commun. 2018, 54, 12182−
12185. (b) Deng, L.; Kleij, A. W.; Yang, W. Chem. - Eur. J. 2018,
DOI: 10.1002/chem.201805295.
(14) (a) Xia, Y.; Qiu, D.; Wang, J. Transition-Metal-Catalyzed
Cross-Couplings through Carbene Migratory Insertion. Chem. Rev.
2017, 117 (23), 13810−13889. (b) Xiao, Q.; Zhang, Y.; Wang, J.
Diazo compounds and N-tosylhydrazones: novel cross-coupling
partners in transition-metal-catalyzed reactions. Acc. Chem. Res.
2013, 46 (2), 236−247. (c) Xu, S.; Chen, R.; Fu, Z.; Zhou, Q.;
Zhang, Y.; Wang, J. Palladium-Catalyzed Formal [4 + 1] Annulation
via Metal Carbene Migratory Insertion and C(sp2)−H Bond
Functionalization. ACS Catal. 2017, 7 (3), 1993−1997.
(15) Zhu, Y.; Liu, X.; Dong, S.; Zhou, Y.; Li, W.; Lin, L.; Feng, X.
Asymmetric N-H insertion of secondary and primary anilines under
the catalysis of palladium and chiral guanidine derivatives. Angew.
Chem., Int. Ed. 2014, 53 (6), 1636−1640.
(16) Qi, Z.; Kong, L.; Li, X. Rhodium(III)-Catalyzed Regio- and
Stereoselective C-H Allylation of Arenes with Vinyl Benzoxazina-
nones. Org. Lett. 2016, 18 (17), 4392−4395.
(17) (a) Huo, C.; Yuan, Y.; Wu, M.; Jia, X.; Wang, X.; Chen, F.;
Tang, J. Auto-oxidative coupling of glycine derivatives. Angew. Chem.,
Int. Ed. 2014, 53 (49), 13544−13547. (b) Jiang, W.; Wang, Y.; Niu,
P.; Quan, Z.; Su, Y.; Huo, C. Double-Oxidative Dehydrogenative
(DOD) [4 + 2]-Cyclization/Oxidative Aromatization Tandem
Reaction of Glycine Derivatives with Ethylbenzenes. Org. Lett.
2018, 20 (15), 4649−4653.
(5) (a) Kantam, M. L.; Mahendar, K.; Sreedhar, B.; Choudary, B.
M.; Bhargava, S. K.; Priver, S. H. Synthesis of α-sulfanyl-β-amino acid
derivatives by using nanocrystalline magnesium oxide. Tetrahedron
2010, 66 (27−28), 5042−5052. (b) Xiao, G.; Ma, C.; Xing, D.; Hu,
W. Enantioselective Synthesis of alpha-Mercapto-beta-amino Esters
via Rh(II)/Chiral Phosphoric Acid-Cocatalyzed Three-Component
Reaction of Diazo Compounds, Thiols, and Imines. Org. Lett. 2016,
18 (23), 6086−6089.
(6) Wang, Y. N.; Lu, L. Q.; Xiao, W. J. Non-Bonding Interactions
Enable the Selective Formation of Branched Products in Palladium-
Catalyzed Allylic Substitution Reactions. Chem. - Asian J. 2018, 13,
2174.
(7) (a) Wang, C.; Tunge, J. A. Decarboxylative ring contractions and
olefin insertions of vinyl oxazinanones. Org. Lett. 2006, 8 (15), 3211−
3214. (b) Wang, C.; Tunge, J. A. Asymmetric cycloadditions of
palladium-polarized aza-o-xylylenes. J. Am. Chem. Soc. 2008, 130 (26),
8118−8119.
(8) (a) Li, M. M.; Wei, Y.; Liu, J.; Chen, H. W.; Lu, L. Q.; Xiao, W.
J. Sequential Visible-Light Photoactivation and Palladium Catalysis
Enabling Enantioselective [4 + 2] Cycloadditions. J. Am. Chem. Soc.
2017, 139 (41), 14707−14713. (b) Li, T. R.; Tan, F.; Lu, L. Q.; Wei,
Y.; Wang, Y. N.; Liu, Y. Y.; Yang, Q. Q.; Chen, J. R.; Shi, D. Q.; Xiao,
W. J. Asymmetric trapping of zwitterionic intermediates by sulphur
ylides in a palladium-catalysed decarboxylation-cycloaddition se-
quence. Nat. Commun. 2014, 5, 5500. (c) Wang, Q.; Qi, X.; Lu, L.
Q.; Li, T. R.; Yuan, Z. G.; Zhang, K.; Li, B. J.; Lan, Y.; Xiao, W. J.
Iron-Catalyzed Decarboxylative (4 + 1) Cycloadditions: Exploiting
the Reactivity of Ambident Iron-Stabilized Intermediates. Angew.
Chem., Int. Ed. 2016, 55 (8), 2840−2844. (d) Wei, Y.; Lu, L. Q.; Li,
T. R.; Feng, B.; Wang, Q.; Xiao, W. J.; Alper, H. P,S Ligands for the
Asymmetric Construction of Quaternary Stereocenters in Palladium-
Catalyzed Decarboxylative [4 + 2] Cycloadditions. Angew. Chem., Int.
Ed. 2016, 55 (6), 2200−2204.
(9) (a) Guo, C.; Fleige, M.; Janssen-Muller, D.; Daniliuc, C. G.;
Glorius, F. Cooperative N-Heterocyclic Carbene/Palladium-Cata-
lyzed Enantioselective Umpolung Annulations. J. Am. Chem. Soc.
2016, 138 (25), 7840−7843. (b) Guo, C.; Janssen-Muller, D.; Fleige,
M.; Lerchen, A.; Daniliuc, C. G.; Glorius, F. Mechanistic Studies on a
Cooperative NHC Organocatalysis/Palladium Catalysis System:
Uncovering Significant Lessons for Mixed Chiral Pd(NHC)(PR3)
Catalyst Design. J. Am. Chem. Soc. 2017, 139 (12), 4443−4451.
(10) Leth, L. A.; Glaus, F.; Meazza, M.; Fu, L.; Thogersen, M. K.;
Bitsch, E. A.; Jorgensen, K. A. Decarboxylative [4 + 2] Cycloaddition
by Synergistic Palladium and Organocatalysis. Angew. Chem., Int. Ed.
2016, 55 (49), 15272−15276.
(11) (a) Duan, S.; Cheng, B.; Duan, X.; Bao, B.; Li, Y.; Zhai, H.
Synthesis of cis-5,5a,6,10b-Tetrahydroindeno[2,1-b]indoles through
Palladium-Catalyzed Decarboxylative Coupling of Vinyl Benzoxazi-
nanones with Arynes. Org. Lett. 2018, 20 (5), 1417−1420. (b) Jin, J.
H.; Wang, H.; Yang, Z. T.; Yang, W. L.; Tang, W.; Deng, W. P.
Asymmetric Synthesis of 3,4-Dihydroquinolin-2-ones via a Stereo-
selective Palladium-Catalyzed Decarboxylative [4 + 2]- Cycloaddition.
Org. Lett. 2018, 20 (1), 104−107. (c) Mei, G. J.; Bian, C. Y.; Li, G.
H.; Xu, S. L.; Zheng, W. Q.; Shi, F. Catalytic Asymmetric
Construction of the Tryptanthrin Skeleton via an Enantioselective
Decarboxylative [4 + 2] Cyclization. Org. Lett. 2017, 19 (12), 3219−
3222. (d) Mei, G. J.; Li, D.; Zhou, G. X.; Shi, Q.; Cao, Z.; Shi, F. A
catalytic asymmetric construction of a tetrahydroquinoline-based
spirooxindole framework via a diastereo- and enantioselective
E
DOI: 10.1021/acs.orglett.8b03440
Org. Lett. XXXX, XXX, XXX−XXX