Catalytic Asymmetric Construction of the Tryptanthrin Skeleton via an Enantioselective Decarboxylative [4 + 2] Cyclization

Article abstract of DOI:10.1021/acs.orglett.7b01336

The first catalytic asymmetric construction of the tryptanthrin skeleton has been established, taking advantage of a palladium(0)/chiral ligand-catalyzed enantioselective decarboxylative [4 + 2] cyclization of vinyl benzoxazinanones with isatins. This rea

Full text of DOI:10.1021/acs.orglett.7b01336

Letter
pubs.acs.org/OrgLett
Catalytic Asymmetric Construction of the Tryptanthrin Skeleton via
an Enantioselective Decarboxylative [4 + 2] Cyclization
Guang-Jian Mei,* Chen-Yu Bian, Guo-Hao Li, Shao-Li Xu, Wen-Qin Zheng, and Feng Shi*
School of Chemistry and Material Science, Jiangsu Normal University, Xuzhou 221116, China
S
* Supporting Information
ABSTRACT: The first catalytic asymmetric construction of the tryptanthrin skeleton has been established, taking advantage of a
palladium(0)/chiral ligand-catalyzed enantioselective decarboxylative [4 + 2] cyclization of vinyl benzoxazinanones with isatins.
This reaction has not only provided a direct and efficient method for constructing chiral tryptanthrin skeleta in high yields and
excellent enantioselectivities (up to 97% yield, >99% ee) but also represents the first catalytic asymmetric decarboxylative
cyclization of vinyl benzoxazinanones with isatins.
ryptanthrin (I) skeleta belong to a class of privileged
scaffolds, which are widely found in bioactive natural
Pd(0) and ligand (L),5,6 which can then act as four-atom building
blocks for tandem cyclizations with other reagents (Scheme
1).7−9 As a result, this type of reactant has been successfully
T
alkaloids and synthetic compounds (Figure 1).1 In particular,
Scheme 1. Profile of Vinyl Benzoxazinanone-Involved
Catalytic Asymmetric Cyclizations
Figure 1. Bioactive alkaloids containing the tryptanthrin scaffold.
employed in catalytic enantioselective [4 + 1],7a [4 + 2],8 and [4 +
3]9 cyclizations with sulfur ylides, electron-deficient alkenes, and
enals (eq 1−3), respectively, in the presence of a chiral ligand
(L*) or a chiral N-heterocyclic carbene (NHC*), which
efficiently constructed five- to seven-membered nitrogenous
heterocycles with enantiopurity. The successful applications of
vinyl benzoxazinanones in catalytic asymmetric cyclizations
triggered us to envision whether this class of reactants could be
used for constructing tryptanthrin skeleta in an enantioselective
mode.
alkaloids II−VIII with one or two stereocenters possess a broad
range of bioactivities such as antibacterial, antiparasitic,
antitubercular, antimalarial, and antitumor properties.1 Stimu-
lated by the unique structure and biological significance of
tryptanthrin skeleta, the construction of this type of nucleus has
received increasing attention from the chemistry community.2−4
Yet, nearly all of the methods have been focused on the
construction of achiral or racemic tryptanthrin skeleta,2,3 and no
catalytic asymmetric methods have been devised for the direct
construction of enantioenriched tryptanthrin scaffolds.4 So, it has
become an urgent task to develop a direct and catalytic
asymmetric strategy for the construction of tryptanthrin skeleta
in an enantioselective fashion.
Based on our previous experiences in constructing chiral
heterocyclic frameworks via catalytic asymmetric reactions,10 we
designed a palladium(0)/chiral ligand-catalyzed asymmetric
decarboxylative cyclization of vinyl benzoxazinanones 1 with
In recent years, vinyl benzoxazinanones have been recognized
as a class of competent reactants for catalytic asymmetric
cyclizations because they can easily transform into Pd-stabilized
zwitterionic intermediates via decarboxylation in the presence of
Received: May 3, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01336
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
isatins 2 for the construction of chiral tryptanthrin skeleta 3 in an
enantioselective style. As illustrated in Scheme 2, vinyl
Table 1. Ligands and Model Reaction Employed for Condition
Optimization
a
Scheme 2. Design of Catalytic Asymmetric Cyclization for the
Construction of Chiral Tryptanthrin Skeleton
b
c
entry ligand
solvent
t (h) yield (%)
ee (%)
1
L1
L2
L3
L4
L5
L6
L7
L8
L9
L9
L9
L9
L9
L9
L9
L9
L10
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
CHCl3
toluene
THF
3.5
3.5
2.5
3.5
8.0
8.0
2.5
8.0
8.0
8.0
8.0
8.0
8.0
30
20
98
97
98
65
40
96
0
20
0
2
benzoxazinanones 1 could convert into Pd-stabilized zwitterionic
intermediates A via decarboxylation under the catalysis of a
palladium(0)/chiral ligand. Then, the amide N−H group of
isatins 2 would attack the intermediates A to carry out an
enantioselectiveallylicaminationwithbranchedselectivity, which
would give rise to another intermediates B with the formation of
the stereogenic center. Finally, an intramolecular condensation of
the intermediates B would lead to the construction of the desired
chiral tryptanthrin skeleta 3.
3
8
4
16
34
0
5
6
7
4
8
−
9
80
78
86
95
97
97
96
68
95
64
60
84
84
84
86
88
88
95
10
11
12
13
14
15
16
17
Although this strategy sounds feasible, there are still some
challenges embedded in this design. The first one is controlling
the chemoselectivity of the reaction because the carbonyl group
(CO) of isatins in most cases exhibits high reactivity which, in
principle, can undergo a decarboxylative [4 + 2] cyclization with
vinyl benzoxazinanones 1 to generate the byproducts 4.11 The
second challenge is controlling the branch/linear selectivity in the
allylic amination step;12−14 otherwise, byproducts 5 from ring
openingwouldbegenerated. Thefinalchallengeiscontrollingthe
enantioselectivity of the reaction. In spite of these challenges, we
decided to try the designed reaction and overcome these
difficulties to realize the catalytic asymmetric construction of
tryptanthrin skeleta.
Initially, the catalytic asymmetric decarboxylative [4 + 2]
cyclization of vinyl benzoxazinanone 1a with isatin 2a was
employed as a model reaction to test our hypothesis (Table 1).
First, we carried out an investigation to find an effective chiral
ligand (entries 1−9). The desired product 3aa was successfully
obtained in the presence of the axially chiral phosphoramidite L1,
but in a low yield and with a low enantioselectivity of 20% ee
(entry 1). Although the other axially chiral phosphoramidites
L2−L5 could improve the yields to a high level, they all failed to
improve the enantioselectivities (entries 2−5). Besides, the
bidentate ligand L6 only gave a racemic product (entry 6). Then,
spiro-phosphoramidites L7−L8 were examined (entries 7−8),
which revealed that L7, with less steric hindrance, could give the
desired product 3aa in an excellent yield but with low
enantioselectivity(entry7), whileL8, withlargersterichindrance,
failed to catalyze the reaction (entry 8). Gratifyingly, chiral
monodentate spiro-phosphine L9, which was developed by the
Zhougroup,15 furnishedthedesiredproductinahighyieldof80%
with the best enantioselectivity of 64% ee (entry 8 vs 1−7).
1,4-dioxane
d
e
f
1,4-dioxane
1,4-dioxane/toluene = 1:1
1,4-dioxane/toluene = 1:1
1,4-dioxane/toluene = 1:1
30
30
e
30
a
Unless otherwise indicated, the reaction was carried out at 0.05 mmol
scale and catalyzed by 5 mmol % Pd2(dba)3·HCl3 with 10 mmol %
ligand in a solvent (1 mL) at 25 °C, and the molar ratio of 1a:2a was
b
c
1:1.2. Isolated yield. The ee value was determined by HPLC.
d
e
f
Performed at 10 °C. Performed at 0 °C. Performed at −10 °C.
Encouraged by this result, the solvent effect was investigated
(entries 9−13). It was found that changing the solvent from
dichloromethane to 1,4-dioxane led to a great increase in the
enantioselectivity (entry 9 vs 13). In addition, the enantiose-
lectivity could be further improved at a lower temperature,
although a prolonged reaction time was needed (entries 14−16).
Finally, the best reaction conditions were established to be those
found in entry 15, which could deliver the reaction in an excellent
yield of 96% with a high enantioselectivity of 88% ee in a mixed
solvent of 1,4-dioxane and toluene at 0 °C. Notably, when L10
bearing a similar structure with L9 was employed in the reaction
under the optimal conditions, a higher enantioselectivity of 95%
ee was obtained (entry 17).
After establishing the optimal reaction conditions for our
process, we investigated the substrate scope of isatins 2 for the
construction of chiral tryptanthrin skeleta. As shown in Figure 2,
this decarboxylative [4 + 2] cyclization could be applicable to a
variety of isatins 2 bearing various substituents on the different
positions of the phenyl ring, affording chiral tryptanthrin
B
DOI: 10.1021/acs.orglett.7b01336
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. Substrate scope of isatins 2. Unless otherwise indicated, the
reaction was carried out at 0.1 mmol scale and catalyzed by 5 mmol %
Pd2(dba)3·HCl3 with 10 mmol % L9 in 2 mL of toluene/1,4-dioxane
(1:1) at 0 °C for 30 h, and the molar ratio of 1:2 was 1:1.2. The yield
referred to the isolated yield, and the ee value was determined by HPLC.
Products 3ab, 3ae, 3ah, and 3ai were synthesized in the presence of 10
mmol % L10 instead of L9.
Figure 3. Substrate scope of vinyl benzoxazinanones 1. Unless otherwise
indicated, the reaction was carried out at 0.1 mmol scale and catalyzed by
5 mmol % Pd2(dba)3·HCl3 with 10 mmol % L9 in 2 mL of toluene/1,4-
dioxane (1:1) at 0 °C for 30 h, and the molar ratio of 1:2 was 1:1.2. The
yield referred to the isolated yield, and the ee value was determined by
HPLC. Products 3ba, 3da, 3ea, 3ga, 3ha, and 3ia were synthesized in the
presence of 10 mmol % L10 instead of L9.
derivatives 3 in generally good yields (55% to 82%) and excellent
enantioselectivities (80% to >99% ee). It seems that the position
of the substituents exerts some effect on the enantioselectivity,
because a C4-bromo-substituted substrate generated the product
3af with a much higher enantioselectivity than C5- and C6-
bromo-substituted counterparts (3af vs3acand3ae). Inaddition,
disubstituted isatins could also be applied to the reaction, which
delivered the corresponding products 3ah and 3bh in good-to-
excellent enantioselectivities.
Information (SI) for details).16 The (S)-absolute configuration of
other products 3 were assigned by analogy.
To demonstrate the utility of this catalytic asymmetric
decarboxylative [4 + 2] cyclization, a preparative scale synthesis
of the product 3aa was performed under standard conditions (see
the SI for details). Compared with the small scale reaction (Table
1, entry 17), this larger scale reaction was able to deliver 3aa in a
maintained, excellent yield of 90% with a high enantioselectivity
of 94% ee. This result indicated that this reaction could be scaled
up for the construction of chiral tryptanthrin skeleta. Moreover,
3aa could be quantitatively transformed into chiral tryptanthrin
derivative 6 with retained enantioselectivity (see the SI for
details).
In summary, the first catalytic asymmetric construction of
tryptanthrin skeleta has been established, which takes advantage
of a Pd(0)/chiral ligand-catalyzed enantioselective decarbox-
ylative [4 + 2] cyclization of vinyl benzoxazinanones with isatins.
This reaction not only has provided a direct and efficient method
for constructing chiral tryptanthrin skeleta in high yields with
excellent enantioselectivities (up to 97% yield, >99% ee) but also
represents the first catalytic asymmetric decarboxylative cycliza-
tion of vinyl benzoxazinanones with isatins. In addition, in this
reaction, the C−N bond of isatins has been utilized to perform
enantioselective cyclizations rather than the commonly more
reactive CO bond, which will allow for further development of
isatin-involved catalytic enantioselective transformations and will
greatly enrich the research content for decarboxylative cyclization
of vinyl benzoxazinanones.
Next, the substrate scope of the vinyl benzoxazinanones 1 was
studiedbyreactions withisatin2aor2f(Figure3). Obviously, this
catalytic asymmetric decarboxylative [4 + 2] cyclization was
amenable to a wide range of vinyl benzoxazinanones 1 with
electronically distinct substituents at different positions of the
phenyl ring, which constructed the chiral tryptanthrin scaffolds 3
in overall good yields with high enantioselectivities (up to 97%
yield, 96% ee). The electronic nature of the substituents seems to
have some influence on the enantioselectivity. For instance, C6-
methoxy- or C6-methyl-substituted vinyl benzoxazinanones
afford the products 3fa and 3da with much better enantiose-
lectivitythanC6-chloro-orC6-bromo-substitutedanalogues(3fa
and3da vs3eaand 3ha). Besides, thepositions ofthesubstituents
also impose a delicate effect on controlling the reactivity and
enantioselectivity. For example, a C8-methyl-substituted sub-
strate was superior to the C6-methyl-substituted counterpart in
terms of the yield and enantioselectivity (3ba vs 3da). A C7-
chloro-substituted substrate delivered the product 3ca in a much
better yield with higher enantioselectivity than the product 3ea,
whichwas generated from the C6-chloro-substituted substrate. In
addition, C7,C8-dimethyl-substituted benzoxazinanone could
serveas a competent substrate, which generated products 3ga and
3gf in good-to-excellent yields with high enantioselectivities.
Notably, all of the reactions proceeded in an exclusively
chemoselective and branch-selective manner. No byproducts 4
and 5 were observed. The absolute configuration of product 3aa
(99%eeafterrecrystallization)wasunambiguouslydeterminedby
single crystal X-ray diffraction analysis (see the Supporting
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b01336.
C
DOI: 10.1021/acs.orglett.7b01336
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedure and characterization data for
Letter
Lu, L.-Q.; Li, M.-M.; Zhang, K.; Xiao, W.-J. J. Am. Chem. Soc. 2016, 138,
8360.
substrates 1, experimental procedures, characterization
data, NMR and HPLC spectra for 3 and 6 (PDF)
Crystallographic data for 3aa (CIF)
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AUTHOR INFORMATION
■
Corresponding Authors
Janssen-Muller, D.; Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc. 2016,
̈
138, 7840.
*E-mail: fshi@jsnu.edu.cn.
*E-mail: guangjianM@jsnu.edu.cn.
ORCID
(10) (a) Zhang, H.-H.; Wang, C.-S.; Li, C.; Mei, G.-J.; Li, Y.; Shi, F.
Angew. Chem., Int. Ed. 2017, 56, 116. (b) Zhao, J.-J.; Sun, S.-B.; He, S.-H.;
Wu, Q.; Shi, F. Angew. Chem., Int. Ed. 2015, 54, 5460. (c) Zhang, Y.-C.;
Zhao, J.-J.; Jiang, F.; Sun, S.-B.; Shi, F. Angew. Chem., Int. Ed. 2014, 53,
13912.
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Hayashi, T. Org. Lett. 2009, 11, 3754. (b) Shintani, R.; Tsuji, T.; Park, S.;
Hayashi, T. J. Am. Chem. Soc. 2010, 132, 7508.
Feng Shi: 0000-0003-3922-0708
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We are grateful for financial supports from NSFC (21372002 and
21232007), PAPD, Natural Science Foundation of Jiangsu
Province (BK20160003).
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DOI: 10.1021/acs.orglett.7b01336
Org. Lett. XXXX, XXX, XXX−XXX