Co(III)-Catalyzed Annulative Vinylene Transfer via C-H Activation: Three-Step Total Synthesis of 8-Oxopseudopalmatine and Oxopalmatine

Article abstract of DOI:10.1021/acs.orglett.0c02016

The Co(III)-catalyzed redox-neutral annulation of benzamides and acrylamides with vinylene carbonate for the synthesis of isoquinolinones and pyridinones has been developed. This protocol employing inexpensive Co(III) as the catalyst tolerated diverse functional groups and substitution patterns, affording the target products with good to excellent yields. The synthetic utility of this transformation was demonstrated by a three-step synthesis of gusanlung D, 8-oxopseudopalmatine, and oxopalmatine.

Full text of DOI:10.1021/acs.orglett.0c02016

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Letter
Co(III)-Catalyzed Annulative Vinylene Transfer via C−H Activation:
Three-Step Total Synthesis of 8‑Oxopseudopalmatine and
Oxopalmatine
Xinghua Li,† Ting Huang,† Ying Song, Yue Qi, Limin Li, Yanping Li, Qi Xiao,* and Yuanfei Zhang*
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ABSTRACT: The Co(III)-catalyzed redox-neutral annulation of benzamides and acrylamides with vinylene carbonate for the
synthesis of isoquinolinones and pyridinones has been developed. This protocol employing inexpensive Co(III) as the catalyst
tolerated diverse functional groups and substitution patterns, affording the target products with good to excellent yields. The
synthetic utility of this transformation was demonstrated by a three-step synthesis of gusanlung D, 8-oxopseudopalmatine, and
oxopalmatine.
soquinolinones and pyridinones are privileged structural
motifs found in natural products, biologically active
unveiled that vinylene carbonate was capable to serve as an
acetylene surrogate for the synthesis of nonsubstituted
isoquinolinones as well.6 However, these transformations
were mainly based on precious Rh(III) catalyst and limited
to the activation of aryl C−H bond affording isoquinolinones.
Versatile protocols that allow the rapid assembly of vinylene
carbonate7 to both isoquinolinone and pyridinone cores in a
redox-neutral manner via base metal catalyst with latent
applications to build complex molecules are still in demand
(Figure 2).
I
compounds, and pharmaceuticals (Figure 1).1 They are also
Figure 1. Selected important compounds containing isoquinolinone
and pyridinone cores.
Figure 2. Base-metal-catalyzed synthesis of nonsubstituted isoquino-
linones and pyridinones.
useful reaction intermediates for the synthesis of berberine-
type alkaloids bearing a tetrahydroisoquinoline scaffold and
piperidine derivatives for drug discovery.2 Consequently, the
development of efficient accesses utilizing easily accessible
starting materials to these heterocycles is of continuous interest
to the synthetic communities. In this regard, extensive
attention has been paid to second-row transition-metal-
catalyzed C−H functionalization reactions and substantial
progress has been made as well.3 Specifically, Rh(III) was
reported to be the adequate catalyst for converting benzamide
analogs to nonsubstituted isoquinolinones by Raw and Bolm
and co-workers, who respectively employed vinyl esters and α-
chloroacetaldehyde as the acetylene equivalent.4,5 During the
course of our investigation, Miura’s research group elegantly
As an inexpensive first-row transition metal, high-valent
cobalt catalyst has displayed enormous potential for the
synthesis of heterocycles,8 such as substituted indoles,9
furans,10 quinolines,11 isoquinolines,12 and others.13 Despite
these exquisite works, a Co(III)-catalyzed synthesis of
nonsubstituted isoquinolinones and pyridinones via a
Received: June 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02016
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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a
sequential C−H activation/annulative vinylene transfer re-
action has not been reported yet. Herein, we describe the
Co(III)-catalyzed redox-neutral synthesis of nonsubstituted
isoquinolinones and pyridinones with vinylene carbonate. In
contrast to the Rh(III) catalytic system,6 this protocol is
regioselective and applicable to acrylamides. Most importantly,
by taking the advantage of the Co(III) technology, gusanlung
D, 8-oxopseudopalmatine, and oxopalmatine, which are
berberine-type alkaloid natural products, were conveniently
synthesized through a three-step route.
Table 2. Substrate Scope of Benzamides
Initially, the reaction of N-methylbenzamide (1a) and
vinylene carbonate (2) catalyzed by Co(III) was selected as
a model for the optimization studies. After substantial
screening of the reaction conditions, we gratifyingly found
that [Cp*Co(CO)I2] (10 mol %) in combination of AgSbF6
(20 mol %) was a proper catalytic system to effect the
annulation reaction, furnishing the desired isoquinolinone
product 3a in 82% yield with the addition of 5 mol %
Zn(OAc)2 as the additive (Table 1, entries 1−2; see Table S1
a
Table 1. Optimization of Reaction Conditions
b
yield
a
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), [Cp*Co(CO)I2]
entry
variation from the standard conditions
(%)
(10 mol %), AgSbF6 (20 mol %), Zn(OAc)2 (5 mol %), TFE (2 mL),
100 °C, 24 h. Average of two runs. 1 mmol scale, 48 h.
1
2
3
4
5
6
7
8
9
none
82
47
70
65
0
0
0
0
0
b
without Zn(OAc)2
20 mol % HOAc instead of Zn(OAc)2
20 mol % LiOAc instead of Zn(OAc)2
DCE instead of TFE
toluene instead of TFE
dioxane instead of TFE
EtOH instead of TFE
DMF instead of TFE
5 mol % [Cp*Co(CO)I2], 10 mol % AgSbF6, and
2.5 mol % Zn(OAc)2
acetamido group, affording the corresponding product in 53%
yield (3f). The presence of other electron-donating groups in
the aryl ring of benzamides, namely N,N-dimethyl and
methylthio substituents, did not affect the transformation (3g
and 3h). Benzamides with fluoro, chloro, or ester substituents
could participate in the reaction as well, and the target
products were obtained in moderate yields (3i−3k).
Conversion of 1-naphthamide to the annulation product was
viable (3l). Variation of the substituents on the nitrogen atom
of benzamides from methyl to phenyl, benzyl, and substituted
phenethyl groups turned out to be successful, providing the
designed products in good to excellent yields (3m−3p). In
addition, the regioselectivity of this transformation was
achieved when two reaction sites were available, and eletron-
rich or less hindered positions were of priority (3q−3t). An
indole derivative could not be synthesized from acetanilide
under the standard reaction conditions.
This transformation was also applicable to acrylamides
(Table 3). Acrylamides having phenyl and fluoro-, chloro-, or
bromo-substituted phenyl groups were ideal substrates for the
Co(III)-catalyzed annulative vinylene transfer reaction, and
pyridinone derivatives were satisfyingly prepared (4a−4d). In
addition, the reaction was feasible to achieve vinyl amides with
two substituents in their double bond, respectively, giving the
annulated products in 65% and 64% yields (4e, 4f).
10
50
11
12
13
80 °C instead of 100 °C
without AgSbF6
without [Cp*Co(CO)I2]
80
0
0
a
Reaction conditions: 1a (0.2 mmol), 2 (0.6 mmol), [Cp*Co(CO)I2]
(10 mol %), AgSbF6 (20 mol %), Zn(OAc)2 (5 mol %), TFE (2 mL),
b
100 °C, 24 h. Isolated yields. TFE = 2,2,2-trifluoroethanol.
for more details). Other frequently used additives in Co(III)
chemistry, such as LiOAc and HOAc, were inferior (Table 1,
entries 3 and 4). The reaction did not take place in most
solvents, and TFE was proven to be the best choice (Table 1,
entries 5−9). Lowering the catalyst loading to 5 mol % resulted
in decreasing the yield to 50% (Table 1, entry 10). The
reaction could work at 80 °C and 80% yield was obtained
(Table 1, entry 11). The control experiment showed that
[Cp*Co(CO)I2] as well as AgSbF6 were indispensable for the
transformation (Table 1, entries 12 and 13).
With the optimized reaction conditions in hand, benzamides
bearing different substituents or substitution patterns were
examined to clarify the substrate scope of the reaction. As
depicted in Table 2, methyl- and methoxy-substituted
benzamides were smoothly transformed into the desired
products in satisfactory yields (3b−3e). The reaction
conditions were also compatible with a weak coordinating
The successful preparation of compound 3o prompted us to
envision the feasibility of the concise synthesis of berberine-
type alkaloids from cheap and readily available starting
materials using the Co(III)-catalyzed annulative vinylene-
transfer reaction (Scheme 1). Benzoylation of 2-(benzo[d]-
[1,3]dioxol-5-yl)ethanamine followed by the annulation
reaction were applied to obtain the key intermediate 5b for
B
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a
Table 3. Substrate Scope of Acrylamides
Scheme 2. Mechanistic Studies
a
Reaction conditions: 1′ (0.2 mmol), 2 (0.6 mmol), [Cp*Co(CO)I2]
(10 mol %), AgSbF6 (20 mol %), Zn(OAc)2 (5 mol %), TFE (2 mL),
100 °C, 24 h. Average of two runs.
Scheme 1. Three-Step Synthesis of Gusanlung D, 8-
Oxopseudopalmatine, and Oxopalmatine
determination step (Scheme 2b).16 An intermolecular
competition experiment between 1b and 1i revealed that the
electron-rich benzamide was more reactive than its electron-
deficient counterpart, which implied an electrophilic sub-
stitution mechanism for C−H cyclocobaltation (Scheme 2c).17
Finally, under the standard reaction conditions, N,N-
dimethylbenzamide was found to be unreactive (Scheme
2d), and therefore, we excluded the tandem C−H 2-
oxoethylation/annulation pathway.
Based on the above observations and previously reported
works,6,7a,8 we proposed a plausible catalytic cycle for the
annulative vinylene transfer reaction catalyzed by Co(III)
(Scheme 3). Cationic cobalt A coordinated with amide 1 and
Scheme 3. Proposed Mechanism for the Co(III)-Catalyzed
Vinylene Transfer Reaction
gusanlung D synthesis (Scheme 1a).14 Next, total syntheses of
8-oxopseudopalmatine and oxopalmatine were investigated.
We utilized the Co(III) protocol for the construction of B ring
again (5d and 5f) and the intramolecular Heck reaction for C-
ring closure, and 8-oxopseudopalmatine and oxopalmatine
were successfully synthesized by the two-step, sequential ring-
closure strategy (Scheme 1b,c). Previously, lengthy synthetic
sequences were generally required to access these alka-
loids.2b,6,15
To probe the reaction mechanism, H/D exchange experi-
ments were carried out first. We did not observe any H/D
exchange when 5 equiv of D3COD or D3CO2D was added
(Scheme 2a). The KIE value of the reaction was measured to
be 2.7, indicating that C−H activation might be the rate-
an electrophilic attack of the active cobalt to the sp2 C−H
bond subsequently occurred, bringing about the formation of a
cyclocobaltation intermidiate B. Then, a seven-membered
metallacycle complex C was generated via the insertion of
vinylene carbonate into the Co−C bond within B. Complex C
underwent reductive elimination to form intermediate D and
Cp*Co(I). The presence of two heteroatoms connecting to
the same carbon atom in D made the carbonate C−O bond
adjacent to nitrogen atom fragile, which caused oxidative
C
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Notes
addition of the C−O bond to Cp*Co(I), affording fused
metallacycle E.18 The product 3 was eventually obtained
through β-oxygen elimination of metallacycle E with the
concomitant release of the Co(III) catalyst and CO32−. On
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
2−
■
protonation, CO3 decomposed into CO2 and H2O.
We thank Prof. Weiping Su (Fujian Institute of Research on
the Structure of Matter, Chinese Academy of Sciences) and
Prof. Qiang Zhu (Guangzhou Institutes of Biomedicine and
Health, Chinese Academy of Sciences) for helpful advice.
Financial support from the National Science Foundation of
China (Grant No. 21602221), a Start-up Grant from Nanning
Normal University (Grant No. 602021239229), and the
“BAGUI Scholar” program of Guangxi province is greatly
appreciated.
In conclusion, we have devised a straightforward and
efficient method to access isoquinolinones and pyridinones
via a Co(III)-catalyzed C−H activation/annulative vinylene-
transfer reaction. Both aryl ring and nitrogen atom bearing a
broad range of functional groups and substitution patterns of
amides were found to be proper substrates, affording the target
products in good to excellent yields. By using the Co(III)
methodology and Heck reaction as the key, the concise three-
step total syntheses of 8-oxopseudopalmatine and oxopalma-
tine were realized. Further applications of this method to
facilely prepare key intermediates for the enantioselective total
synthesis of other berberine-type alkaloids with a chiral carbon
center will be reported in due course.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02016.
Detailed experimental procedures, characterization data
1
for all compounds and copies of H, 13C and 19F NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Qi Xiao − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China; Email: yfzhang@gxtc.edu.cn
Yuanfei Zhang − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China; orcid.org/0000-0003-2667-7055;
Email: qi.xiao@nnnu.edu.cn
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Authors
Xinghua Li − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Ting Huang − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Ying Song − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Yue Qi − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Limin Li − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Yanping Li − Guangxi Key Laboratory of Natural Polymer
Chemistry and Physics, Nanning Normal University, Nanning
530001, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02016
Author Contributions
†These authors contributed equally.
D
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