Visible Light-Induced Pericyclic Cascade Reaction for the Synthesis of Quinolinone Derivatives with an Oxabicyclo[4.2.0]octene Skeleton

Article abstract of DOI:10.1021/acs.orglett.1c00642

A photoinduced pericyclic cascade reaction has been developed to afford oxabicyclo[4.2.0]octenes. Mechanistic studies show that this reaction undergoes [2 + 2]-photocycloaddition, base-promoted elimination, retro-4π-electrocyclization, [1,5]-H shift, and

Full text of DOI:10.1021/acs.orglett.1c00642

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Letter
Visible Light-Induced Pericyclic Cascade Reaction for the Synthesis
of Quinolinone Derivatives with an Oxabicyclo[4.2.0]octene
Skeleton
Guangxing Pan, Shaoheng Qin, Dawen Xu, Fritz E. Ku
̈
hn,* and Hao Guo*
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ABSTRACT: A photoinduced pericyclic cascade reaction has
been developed to afford oxabicyclo[4.2.0]octenes. Mechanistic
studies show that this reaction undergoes [2 + 2]-photo-
cycloaddition, base-promoted elimination, retro-4π-electrocycliza-
tion, [1,5]-H shift, and 4π-electrocyclization procedures. This
reaction features wide substrate scope, good functional group
tolerance, and excellent diastereoselectivity.
ericyclic cascade reactions have a wide range of
photocatalysis.9 A large number of inorganic and organic
catalysts were widely employed in different photocatalytic
reactions, featuring either an electron-transfer or an energy-
transfer mechanism.10 [2 + 2]-Photocycloaddition is a
powerful tool that engenders the construction of four-
membered rings with high atom economy.11 Owing to the
high ring strain, the four-membered rings are prone to undergo
ring-opening reactions under proper reaction conditions.12
The product or intermediate obtained by the ring-opening
process can be further employed in subsequent trans-
formations to acquire more complex and valuable structures.
Therefore, [2 + 2]-photocycloaddition and its subsequent ring-
opening reaction are expected to be important steps in
pericyclic cascade reactions.13 Numerous cascade reactions
consisting of [2 + 2]-photocycloaddition and ring-opening
have been reported. Bach et al. reported a UV light-induced
cascade reaction in the total synthesis of sesquitpenes, wherein
the ring-opening of four-membered cyclobutane is the key step
(Scheme 1a).14 Glorius developed a visible light-promoted
cascade reaction for the synthesis of benzocyclobutenes. [2 +
2]-Photocycloaddition and vinyl cyclobutane rearrangement
were reported to be key steps (Scheme 1b).15 Clearly, the
unique features of [2 + 2]-photocycloadditions and subsequent
ring-opening reaction have made them versatile protocols in
organic synthesis. Developing tandem reactions on the ground
P
applications in the synthesis of natural products and
biological molecules.1 For example, the tandem cyclization
strategy comprised of pericyclic reactions was adopted in the
syntheses of hirsutellone,2 artochamins,3 pleocarpenone,4
valparicine,5 parvineostemonine,6 and daphnilactone7 (Figure
1), reducing the synthesis steps. Pericyclic reactions mainly
Figure 1. Natural products and biological molecules synthesized by
pericyclic cascade reactions.
Received: February 23, 2021
Published: March 30, 2021
include electrocyclic reactions, cycloadditions, and sigmatropic
rearrangements. The combination of these protocols will create
valuable cascade reactions that allow the formation of complex
molecules with simple steps.8
Over the past decade, a variety of photoreactions have been
reported due to the rapid development of visible light
https://doi.org/10.1021/acs.orglett.1c00642
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2959−2963
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Scheme 1. Pericyclic Cascade Reactions Comprised of [2 +
2]-Photocycloaddition
Table 1. Optimization of Reaction Conditions for the
a
Pericyclic Cascade Reaction
b
entry
base (equiv)
solvent
yield of 3a (%)
c
d
1
MeONa (1.2)
MeONa (1.2)
MeONa (4)
MeONa (4)
MeONa (4)
KOH (4)
CH3CN
CH3CN
CH3CN
CH2Cl2
hexane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
0 (89)
e
2
3
4
5
6
7
8
9
10
e
71
e
13
e
18
e
5
e
Cs2CO3(4)
MeOK (4)
15
e
25
tBuONa (4)
tBuONa (2)
tBuONa (1)
tBuONa (1)
tBuONa (1)
88
87
10
11
f
87 (85)
g
e
12
17
h
13
NR
NR
0
i
tBuONa (1)
−
of these transformations is rewarding. Recently, we reported a
cascade reaction that undergoes [2 + 2]-photocycloaddition,
hydrogen halide elimination, and retro-4π-electrocyclization to
construct seven-or eight-membered rings under visible light
catalysis (Scheme 1c).16 In the investigation of the
construction of the eight-membered ring, a rearrangement
product with an oxabicyclo[4.2.0]octene skeleton was found
after extended reaction time. We deduced that the reaction
undergoes a more complicated process, and the eight-
membered ring compound is a key intermediate in the further
transformation. Herein, a visible light-induced pericyclic
cascade reaction is introduced (Scheme 1d), which provides
a new way for the construction of polycyclic compounds with
oxabicyclo[4.2.0]octene skeletons.
14
15
a
A solution of 1a (0.1 mmol), thioxanthone (10 mol %), and base in
anhydrous solvent (10 mL) was irradiated by 30 W violet LEDs
(410−420 nm) at rt under argon atmosphere for 12 h. Yields were
determined by H NMR analysis of the crude reaction mixture using
CH2Br2 (0.1 mmol) as internal standard. A solution of 1a (0.1
mmol), thioxanthone (5 mol %), and NaOMe (0.12 mmol) in
b
1
c
anhydrous MeCN (10 mL) was irradiated by 5 W violet LEDs (390−
d
410 nm) at rt under argon atmosphere for 4 h. Yield of 2a, ref 16.
e
f
g
Some unidentified byproducts were formed. Isolated yield of 3a. 5
h
mol % of thioxanthone was applied. The reaction was carried out
under exclusion of light. No thioxanthone was applied.
i
Therefore, violet LEDs, 10 mol % of thioxanthone, 1 equiv of
tBuONa, acetonitrile, and room temperature were chosen as
the standard conditions for this pericyclic cascade reaction.
With the standard reaction conditions in hand, we explored
the substrate scope of this pericyclic cascade reaction (Table
2). The reactivity of various N-substituted reactants was tested
at first. Methyl and n-butyl N-substituents were found to be
highly efficient, affording 3a and 3b in good yields. Allyl and
propargyl moieties that were potentially reactive in [2 + 2]-
photocycloaddition were also tolerated, yielding the corre-
sponding products 3c and 3d in 85 and 79% yield, respectively.
The N-benzyl group did not retard the reaction efficiency,
delivering 3e in an 82% yield. Then, the tolerance of the
functional groups was examined. Substrate with 4-methox-
ybenzyl showed good reactivity and gave 3f in an 89% yield. 4-
Methylbenzyl or 4-phenylbenzyl decorated reactants efficiently
participated in the cascade reaction, leading to 3g and 3h in 84
and 87% yield, respectively. 4-Bromobenzyl was also tolerated,
giving 3i in an 81% yield. Notably, 4-methoxycarbonylbenzyl,
4-cyanobenzyl, and 4-trifluoromethylbenzyl were also compat-
ible under the reaction conditions and formed the correspond-
ing products 3j−3l in moderate to good yields. To further
expand the scope at R1, a substrate with perfluorobenzyl was
applied, and 3m was obtained in 89% yield. Reactants
containing other R1 substitution, like 2-naphthylmethyl, also
worked well, affording the corresponding product 3n in a good
yield. Then, the scope of R2 was studied. Substrates with strong
Our previous study showed that irradiation of (E)-4-(((4-
bromo-3-methylbut-3-en-1-yl)oxy)methyl)-1-methylquinolin-
2(1H)-one (1a) in the present of 5 mol % of thioxanthone and
1.2 equiv of MeONa in acetonitrile within 4 h afforded 2a in
an 89% isolated yield (entry 1, Table 1). Upon further
investigation, we discovered that 2a could be converted into an
oxabicyclo[4.2.0]octene 3a slowly by extending the reaction
time (entry 2, Table 1). With this observation, we turned to
study the direct transformation from 1a to 3a. When the
amount of MeONa was increased, the efficiency of the reaction
was significantly improved, and 3a was obtained in a 71%
NMR yield (entry 3, Table 1). The reaction in dichloro-
methane or n-hexane gave 3a in a lower yield (entries 4 and 5,
Table 1). To improve the yield of 3a, the reaction was further
conducted using different bases, including KOH, Cs2CO3,
t
MeOK, and BuONa (entries 6−9, Table 1). The results
t
showed that BuONa exhibits the highest efficiency, affording
3a in an 88% NMR yield (entry 9, Table 1). Further
t
investigation showed that 1 equiv of BuONa is sufficient
(entries 10 and 11, Table 1). Decreasing the catalyst loading to
5 mol % resulted in a negative impact on the efficiency of this
transformation (entry 12, Table 1). Control experiments
without light or thioxanthone led to no conversion, and >95%
of the starting material was recovered in both cases, indicating
that light and thioxanthone were essential (entries 13 and 14,
Table 1). Reaction in the absence of base turned out to be
complicated, and 3a could not be detected (entry 15, Table 1).
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a
Table 2. Substrate Scope Investigations
Scheme 2. Detailed Mechanism Studies
a
The reaction generated 2a in a 16% yield. Some unidentified
b
byproducts were formed. The reaction generated 3a in a 21% yield
as well.
was another important intermediate from 4a, formed by a base-
promoted elimination process. When reducing the irradiation
time of 5a to 2 h under condition A, the eight-membered ring
product 2a was formed and obtained in 63% isolated yield,
along with 21% of 3a (A3, Scheme 2). Subjecting 2a to
condition A for 8 h gave 83% of 3a (A4, Scheme 2). These
results demonstrated that the eight-membered ring compound
2a was an important intermediate and can be transformed from
the cyclobutene 5a via a retro-4π-electrocyclization process.
Direct irradiation of 1a or 5a without thioxanthone gave no
reaction at all (D1 and D2, Scheme 2). 2a was totally
decomposed under condition D (D3, Scheme 2). These
experiments indicated that the transformations of 1a, 2a, and
5a in the cascade reaction proceed through a thioxanthone
catalyzed photochemical pathway.
It was worthy to note that the triplet energy of thioxanthone
(265 kJ/mol)17 is higher than that of 1a (237 kJ/mol),18 which
indicated that the energy-transfer process occurring between
thioxanthone (T1) and substrate 1a (S0) might be possible.
Based on the above-described observations and previous
investigations, a plausible mechanism for this pericyclic cascade
reaction can be proposed (Scheme 3). Under visible light
irradiation, brominated cyclobutane 4a is formed via the first
energy transfer-based [2 + 2]-photocycloaddition. Subse-
quently base-promoted elimination of hydrogen bromide
afforded the cyclobutene intermediate 5a. A second energy
transfer leads to retro-4π-electrocyclization of 5a, forming
eight-membered ring product 2a. Then, 2a undergoes [1,5]-H
shift to form intermediate 6a.19 Considering that the hydrogen
atoms attached to C1 and C2 of 3a adopt a cis-conformation, a
photoinduced disrotatory 4π-electrocyclization is likely to
occur in the transformation of 6a to 3a, which is consistent
with the Woodward−Hoffmann rules.20
a
A solution of 1 (0.2 mmol), thioxanthone (10 mol %), and tBuONa
(1 equiv) in anhydrous MeCN (20 mL) was irradiated by violet LED
(410−420 nm) at rt under argon atmosphere for 12 h. 1.108 g of 1a
was applied. An extended reaction time of 24 h was applied.
b
c
electron-donating groups (EDG), such as methoxy, were tried
and gave 3o in a 72% yield. Weak EDG, such as methyl,
installed on R2 was also tolerated, and 3p was obtained in good
yield. Halogenated compounds were acquired in moderate to
good yields. The substrate with hydrogen on the nitrogen (1t)
was also tested but turned out to be complicated. Notably, the
leaving group (vide infra) was not limited to bromo, as
chlorine and iodine proved also to be feasible, affording the
desired products in 86 and 80% yield, respectively. The
practical utility of the method was illustrated by conducting a
gram-scale experiment of 1a, affording 3a in a 79% isolated
yield. The structure of 3 was confirmed by X-ray crystal
structure analysis of 3e, providing solid evidence for the
diastereoselectivity of the three stereocenters.
Control experiments were performed to explore the reaction
mechanism (Scheme 2). Subjecting 1a to condition A for 2 h
yielded 38% of the [2 + 2]-photocycloaddition product 4a
(A1, Scheme 2), whose structure was determined by X-ray
single-crystal diffraction analysis. Further application of 4a to
condition B for 10 h gave 82% of 3a (B, Scheme 2). These two
steps demonstrated that the [2 + 2]-photocycloaddition
product 4a was the key intermediate of the cascade reaction.
Next, application of 4a to condition C resulted in cyclobutene
product 5a (C, Scheme 2). It has been found that irradiating
5a under condition A for 10 h gave 88% of 3a (A2, Scheme 2).
These evidence showed that the cyclobutene compound 5a
In summary, a visible light-induced pericyclic cascade
reaction has been developed. This reaction proceeds through
[2 + 2]-photocycloaddition, base-promoted elimination, retro-
4π-electrocyclization, [1,5]-H shift, and 4π-electrocyclization,
affording quinolinone derivatives with an oxabicyclo[4.2.0]-
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Letter
Shaoheng Qin − Molecular Catalysis, Catalysis Research
Center and Department of Chemistry, Technische Universität
Munchen, 85747 Garching bei Munchen, Germany
Dawen Xu − Department of Chemistry, Fudan University,
Shanghai 200438, P.R. China
Scheme 3. Possible Reaction Mechanism for This Pericyclic
Cascade Reaction
̈
̈
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00642
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support from the Shanghai
Science and Technology Committee (18DZ1201605).
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00642.
■
sı
Figure S1, Figure S2, experimental procedures and
characterization of compounds, and crystal information
for compound 3e and 4a (PDF)
Accession Codes
CCDC 2045528 and 2061362 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
Corresponding Authors
■
Hao Guo − Academy for Engineering and Technology and
Department of Chemistry, Fudan University, Shanghai
200433, P.R. China; orcid.org/0000-0003-3314-4564;
Email: Hao_Guo@fudan.edu.cn
Fritz E. Ku
Center and Department of Chemistry, Technische Universität
Munchen, 85747 Garching bei Munchen, Germany;
̈
hn − Molecular Catalysis, Catalysis Research
̈
̈
orcid.org/0000-0002-4156-780X; Email: fritz.kuehn@
ch.tum.de
Authors
Guangxing Pan − Academy for Engineering and Technology,
Fudan University, Shanghai 200433, P.R. China
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