Synthesis of rutaecarpine alkaloids: Via an electrochemical cross dehydrogenation coupling reaction

Article abstract of DOI:10.1039/c9gc03028j

Substrates are directly oxidized at the anode without using any metal catalyst and oxidant to obtain a series of nitrogen heterocyclic compounds, including benzimidazoles and quinolinones. These compounds are highly tolerant to various functional groups and heterocycle-containing substrates. In addition, natural alkaloids, such as rutaecarpine and deoxyvasicinone, can be synthesized by this method.

Full text of DOI:10.1039/c9gc03028j

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DOI: 10.1039/C9GC03028J
Journal Name
COMMUNICATION
Synthesis of Rutaecarpine Alkaloids via Electrochemical Cross
Dehydrogenation Coupling Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Qian-Yu Li, Shi-Yan Cheng, Hai-Tao Tang* and Ying-Ming Pan*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Substrates are directly oxidized at the anode without using any
As a powerful method for constructing chemical bonds, CDC
metal catalyst and oxidant to obtain series of nitrogen reaction is valuable because it is an organic synthesis technique with
a
heterocyclic compounds, including benzimidazoles and high atomic economy.8 However, stoichiometric oxidants or
quinolinones. These compounds are highly tolerated with various precious metals are used as the catalyst to realize the CDC
functional groups and heterocyclic-containing substrates. In reactions.9 Electro-organic chemistry is an important part of organic
addition, natural alkaloids, such as rutaecarpine and synthesis. It has always been considered to achieve the reaction in
deoxyvasicinone can be synthesized by this method.
mild conditions, and replaces the use of oxidation and reducing
agents.10 Therefore, electro-organic chemistry provides a new way to
achieve CDC reaction. Chemists have applied CDC for the
formation of C−C, N–N, C−N and other bonds.11 The rapidly
Alkaloids containing a quinazolinone structure are widely
present in natural products (Sheme 1), such as rutaecarpine,
hortiacine, euxylophoricine A, and deoxyvasicinone and show
multiple activities including antiinflammatory, antitumor,
antithrombosis, antiobesity.1 Rutaecarpine is isolated from the
traditional Chinese medicine Evodia rutaecarpa, while
deoxyvasicinone comes from Peganum harmala.2 The separation,
synthesis, and pharmacological activity of rutaecarpine have been
greatly studied by many scholars.3 Rutaecarpine is traditionally
synthesized through a multistep procedure that requires special
reagents (Scheme 2, Paths A and B).4 With the rapid development of
organic synthesis methodology in the past decade, chemists have
developed various simple ways to synthesize rutaecarpine. For
example, Su’s group has achieved the synthesis of rutaecarpine
under the action of equivalent EDCI (Scheme 2, Path C).5 A precise
method for the synthesis of rutaecarpine from compound 1 is
through cross-dehydrogenative coupling (CDC) reaction. Wang’s
group and Zheng’s group reported that this cross-dehydrogenative
coupling reaction can be achieved under the condition of equivalent
oxidant (Scheme 2, Path D).6-7 However, the use of excess oxidant
leads to environmental pollution, whereas excessive oxidation
affects the yield of the target product. The development of a green,
environment-friendly synthesis method for the natural products of
rutaecarpine remains challenging.
O
O
N
N
OCH3
HN
HN
Rutaecarpine
Hortiacine
O
H3CO
H3CO
O
N
N
HN
N
Deoxyvasicinone
Euxylophoricine A
Scheme 1. Representative structure of rutaecarpine derivatives.
developing research area of electrochemistry has been widely used
to direct and indirect synthesize various core skeletons of natural
products, such as terpenoids, thiadiazoles, and quinazolinones.12 For
instance, the formyl-substituted indole synthesized by
electrochemical method is an important intermediate for conversion
to isocryptolepine.13 On the basis of literature and our previous
works on electrochemical CDC reaction, we attempted to synthesize
a nitrogen heterocycle including rutaecarpine by electrochemical
methods in metal- and oxidant-free conditions.
State Key Laboratory for Chemistry and Molecular Engineering of Medicinal
Resources, School of Chemistry and Pharmaceutical Sciences of Guangxi Normal
University, Guilin 541004, People’s Republic of China.
E-mail: panym@mailbox.gxnu.edu.cn; httang@gxnu.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
We began our investigation with 2-(3,4-dihydroisoquinolin-
2(1H)-yl)benzamide (3a) as substrates. The effects of different
reaction conditions, including solvent, electrodes, temperature, and
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O
O
O
3ba was obtained in 45% and 65% yields, when n-Bu4NPF6 was
Ir catalysis
N
H
4
N
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NH2
N
H
substituted with KI and TBAI, respectivelyD(OTIa:b10le.1013,9E/Cn9trGieCs0830a2n8dJ
9). Replacing n-Bu4NPF6 with ferrocene did not produce any product
(Table 1, Entry 10). Regardless of the reaction at 100 °C or room
temperature, the product yield was not ideal. 3ba had yields of 67%
and 68% (Table 1, Entries 11 and 12) and was not detected in the
absence of electricity (Table 1, Entry 13). More Electrolytes and
electrode materials were screened, but no better results were
obtained (Table S1).
Multiple steps
O
A
NH
CN
F
Previous work:
Excessive oxidant
O
N
B
O
N
H
D
-CF3, -F as leaving groups
such as (NH4 2S2O8
)
N
N
O
This work:
NH2
HN
N
CF3
HN
HN
1
ve
i
t
N
H
Rutaecarpine
Electrochemical synthesis
addi
C
Excessive
O
N
+
HO
H2
N
H
N
•
N
HCl
N
We investigated benzimidazole synthesis by this CDC
reaction under optimized reaction conditions. The effect of
substitution on the phenyl ring of aniline was first analyzed
(Scheme 3). The substrates with different groups in different
positions provided the benzimidazole products in good yields,
C
Additive:
N
Scheme 2. Synthesis of rutaecarpine.
electrolytes,on the reaction results were studied. n-Bu4NPF6 (20
mol%) served as the electrolyte containing the solvent of methanol
with Reticulated vitreous carbon (RVC) as the anode and Pt as the
cathode in an undivided cell at a constant current of 10 mA. We
isolated the target product 3ba with a yield of 65% (Table 1, Entry
1). Different solvents, such as CH3CN, DCE, and DMF, were then
examined under identical conditions. CH3CN was an excellent
solvent for this reaction. The product was obtained with a good yield
whereas
the
introduction
of
electron-withdrawing
trifluoromethyl groups led to a poor yield (2bb–2be). When the
phenyl ring of aniline was disubstituted, the expected products
2bf and 2bg were obtained, at yields of 79% and 70%,
respectively. We explored the substrate scope of the reaction by
replacing tetrahydroisoquinoline moiety with piperidine and
pyrrolidine. Good yields were obtained for 2-(piperidin-1-
yl)aniline and 2-(pyrrolidin-1-yl)aniline (2bh and 2bi). A
moderate yield was observed for 2-morpholinoaniline (2bj).
When the substrate was 2-thiomorpholinoaniline, the reaction
obtained the corresponding product 2bk in trace amounts.
Table 1. Optimization of reaction conditionsa,b
Scheme 3. Synthesis of benzimidazolesa, b
Electrode, I =10 mA
Electorlyte (20 mol%)
N
N
O
RVC-Pt, I =10 mA
H2
N
solvent
N
Air, 70 o
C
n-Bu4NPF6 (20 mol%)
N
O
3a
N
R
3ba
CH3CN, Air, 70 o
C
N
H2N
Entry
Electrode
RVC/Pt
RVC/Pt
Electrolyte
n-Bu4NPF6
n-Bu4NPF6
Solvent
Yield(%)b
R
2b
2ba-2bk
1
MeOH
65
2
CH3CN
85
N
N
N
N
N
N
CF3
N
N
3
4
RVC/Pt
RVC/Pt
RVC/RVC
Pt/Pt
n-Bu4NPF6
n-Bu4NPF6
n-Bu4NPF6
n-Bu4NPF6
n-Bu4NBF4
KI
DCE
30
Trace
35
OMe
DMF
2ba
2bd
: 68%
: 85%
2bb
2bc
: 82%
: 80%
N
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
N
N
N
N
N
6
56
Cl
N
N
7
RVC/Pt
RVC/Pt
RVC/Pt
RVC/Pt
RVC/Pt
RVC/Pt
RVC/Pt
65
2be
Cl
: 70%
2bg
: 70%
2bh
: 77%
2bf
Cl
: 79%
8
45
S
O
N
9
TBAI
65
N
N
N
N
10
11c
12d
13e
Cp2Fe
NR
67
N
2bk
: trace
2bi
2bj
: 52%
: 79%
n-Bu4NPF6
n-Bu4NPF6
n-Bu4NPF6
68
a
Reaction conditions: RVC anode (100 PPI, 1 cm × 1 cm × 1.2 cm), Pt plate
cathode (1 cm × 1 cm), undivided cell, constant current = 10 mA, 2b (0.3 mmol),
n-Bu4NPF6 (20 mol%), and CH3CN (5 mL), under air atmosphere at 70 °C for 4 h.
b Isolated yield.
NR
a
Reaction conditions: RVC anode (100 PPI, 1 cm × 1 cm × 1.2 cm), Pt plate
cathode (1 cm × 1 cm), undivided cell, constant current = 10 mA, 1a (0.3 mmol),
electrolyte (20 mol%), and solvent (5 mL), under air atmosphere at 70 °C for 4 h.
b Isolated yield. c Room temperature. d T = 100 °C. e Without electricity. NR = no
reaction.
We conducted various reactions to further synthesize the two
types of quinazolinones (Scheme 4). When 2-Aminophenyl ketone
derivatives was used as substrates, (2-aminophenyl)(3,4-
dihydroisoquinolin-2(1H)-yl)methanone,
(2-aminophenyl)(6,7-
dimethoxy-3,4-dihydroisoquinolin-2(1H)-yl)methanone, and (2-
aminophenyl)(6,7-dihydrothieno[3,2-c]pyridin-5(4H)-yl)methanone
were subjected to CDC reactions under the optimized reaction
conditions to obtain products 3aa (85% yield), 3ab (82% yield), and
of 85% (Table 1, Entry 2). The replacement of CH3CN with DCE or
DMF did not improve the yield (Table 1, Entries 3 and 4). For the
product yield, 35% was obtained in RVC, which acted as an anode
and a cathode at the same time (Table 1, Entry 5). The target product 3ac (80% yield), respectively. Then, (2-aminophenyl)(pyrrolidin-1-
yl)methanone
and
(2-aminophenyl)(1,3,4,9-tetrahydro-2H-
was isolated in 56% yield in the experiment in which Pt was an
anode and a cathode (Table 1, Entry 6). We used n-Bu4NBF4 as
electrolyte, and 3a produced 3ba in 65% yield (Table 1, Entry 7).
pyrido[3,4-b]indol-2-yl)methanone were involved in the reactions as
substrates, fortunately, we obtained the target products rutaecarpine
and deoxyvasicinone with satisfactory yields. Finally, a variety of 2-
2 | J. Name., 2012, 00, 1-3
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Journal Name COMMUNICATION
aminobenzamide
derivatives
was
examined.
2-(3,4-
A tentative reaction mechanism was proposed oVniewthAertibclaesOisnlionef
dihydroisoquinolin-2(1H)-yl)benzamide participated in the reaction
as a substrate to obtained the target product in 82% yield (3ba). 4-
chloro-2-(3,4-dihydroisoquinolin-2(1H)-yl)benzamide and 2-(3,4-
dihydroisoquinolin-2(1H)-yl)-4-methylbenzamide were also suitable
substrates and generated excellent product yields (3ca and 3da). The
reaction of 4-substituted benzamide provided the products in good
yields. The product yield of 5-chloro-2-(3,4-dihydroisoquinolin-
2(1H)-yl)benzamide was 73% (3ea).
the previous reports (Scheme 6). On the anDoOdIe: 1s0u.1r0fa3c9e/C, 9cGoCm0p3o0u2n8dJ
2b lost one electron to form cationic radical A, and the anodic
oxidation produced the iminium ion intermediate B.14 The iminium
ion
B
subsequently underwent intramolecular nucleophilic
cyclization which produced benzimidazoline intermediate C, that
was oxidized on the anode to form the target product 2ba.15 At the
cathode, the protons were reduced to hydrogen to complete the
reaction cycle.
Scheme 4. Synthesis of benzoquinolinonesa, b
RVC-Pt, I = 10 mA
Ar
Ar
n-Et4NPF6 (20 mol%)
N
6
N
N
2HN
CH3CN, 70 oC, Air
2 H2
3
N
e
N
O
S
NH2
2b
N
e
N
O
N
O
N
O
2ba
O
+ 4 e
4 H+
- e
N
N
N
Ar
anodic oxidation
Ar
2HN
N
O
N
N
=
3ac
: 80%
3aa
3ab
: 82%
: 85%
H2
N
- 2H
- 2e
O
NH2
A
B
N
N
O
3
3a
N
- H
- e
N
HN
80%
Cyclization
N
N
N
3ad
3ae:
Rutaecarpine
: 61%
Deoxyvasicinone
- H+
C
NH2
H
N
N
Cl
N
N
Scheme 6. Proposed mechanism.
Ar
2HN
O
O
N
O
N
=
3ba
: 82%
N
3ca
: 71%
R
H2
N
In summary, the efficient and environmentally benign
electrochemical cross-dehydrogenation coupling was demonstrated
to construct C=N bond in this work. Under mild electro-oxidation
conditions, we have successfully synthesized the natural products of
deoeyvasicinone and rutaecarpine, which provides an ideal strategy
for the application of electrochemical dehydrogenative cross-
coupling in the synthesis of natural products.
3b
3
N
N
N
Cl
O
O
3ea: 73%
3da
: 74%
a
Reaction conditions: RVC anode (100 PPI, 1 cm × 1 cm × 1.2 cm), Pt plate
cathode (1 cm × 1 cm), undivided cell, constant current = 10 mA, 3 (0.3 mmol), n-
Bu4NPF6 (20 mol%), and CH3CN (5 mL), under air atmosphere at 70 °C for 4 h.
b Isolated yield.
Notes
We performed the following reactions to clarify the reaction
mechanism (Scheme 5). When 2,2,6,6-tetramethylpiperidine N-oxyl
was added, the obtained yield (50%) of the target product 2bj was
insignificantly different from that under the standard condition
(52%). Therefore, we excluded the possibility of free radical
mechanism (Scheme 5, Eqn. 1). Next, we supposed that
benzimidazoline was the intermediate. Fortunately, when the
reaction time was shortened, we detected 2bj-1 by high-resolution
electrospray ionization mass spectrometry (Scheme 5, Eqn. 2).
The authors declare no competing financial interest.
Acknowledgment
We thank the National Natural Science Foundation of China
(21861006), Guangxi Natural Science Foundation of China
(2016GXNSFEA380001, 2018GXNSFBA281151), Guangxi Key
R&D Program (No. AB18221005), Science and Technology Major
Project of Guangxi (AA17204058-21), Guangxi science and
technology base and special talents (guike AD19110027), State Key
Laboratory for Chemistry and Molecular Engineering of Medicinal
Resources (CMEMR2019-A03) for financial support.
O
RVC-Pt, I =10 mA
O
n-Bu4NPF6 (20 mol%)
N
N
(1)
H2N
CH3CN, Air, 70 o
TEMPO (2 eq)
C
N
2bj:
50%
2bj'
O
Notes and references
RVC-Pt, I =10 mA
O
O
n-Bu4NPF6 (20 mol%)
N
N
N
N
+
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