A visible-light-induced cascade reaction of etherification/C-C cyclization: Efficient synthesis of dibenzo[: B, d] oxepin-7(6 H)-ones

Article abstract of DOI:10.1039/c9ob00247b

A visible-light-induced palladium-catalyzed cascade reaction was developed by etherification/C-C coupling cyclization of α-bromoacetophenones with phenols. A series of dibenzo[b,d]oxepin-7(6H)-one derivatives were efficiently synthesized by using this method in good yields. Furthermore, this method was applied to the synthesis of protosappanin A. The protocol has advantages such as simple reaction conditions, wide range of substrates and high reaction efficiency.

Full text of DOI:10.1039/c9ob00247b

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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Visible-Light-Induced Cascade Reaction of Etherification/C-C
Cyclization: Efficient Synthesis of Dibenzo[b,d]oxepin-7(6H)-ones
Received 00th January 20xx,
Accepted 00th January 20xx
Wanyong Fu,a,b Aixia Yu, a,b Hongshuo Jiang, a,b Minghui Zuo, a,b Hongfeng Wu, a,b Zhongwen Yang,
a,b Qi An, a,b Zhizhong Sun* a,b and Wenyi Chu* a,b
DOI: DOI: 10.1039/x0xx00000x
www.rsc.org/
A visible-light-induced palladium-catalyzed cascade reaction was our group7 reported that the Friedel-Crafts acylation of 2-
developed by etherification/C-C coupling cyclization of ɑ- arylphenoxyacetic acids occurred under different conditions to
bromoacetophenones
with
phenols.
A
series
of afford selectively compounds dibenzo[b,d]oxepinones and
dibenzo[b,d]oxepin-7(6H)-one derivatives were efficiently arylcoumaranones in high regioselectivities(Scheme 2a).
synthesized by using this method in good yields. Furthermore, this However, these methods still have many disadvantages, such
method was applied to the synthesis of Protosappanin A. The as excessive synthetic steps, lower productivity and atom
protocol has such advantages as simple reaction conditions, wide utilization. In order to solve the above issues, our group has
range of substrates and high reaction efficiency.
improved preparation method of dibenzo[b,d]oxepin-7(6H)-
ones by palladium-catalyzed ortho C-H activation/C-C
cyclization under microwave irradiation8 (Scheme 2b).
However, in order to further improve the reaction efficiency, it
is very challenging to develop a new synthetic method of
dibenzo[b,d]oxepin-7(6H)-one skeleton.
In recent years, transition metal-catalyzed C-H
functionalization has received widespread attention due to its
high atomic economy.9 In addition, the rapid development of
C-C cyclization reaction has become an increasingly important
method for C-C bond formation,10 which could synthesize
various polycyclic compounds and natural products.11 Cheng12
The benzoheterocyclic fragments are widely found in
important core skeletons of many natural products,1 which are
often used as lead compounds in drug designing and synthesis
because of a variety of biological activities.2 Dibenzoxepinone
and dibenzoxocinone fragment structural motifs are widely
used in drug active molecules, such as Arugosin F (for
antifungal),3 Chaetone G (for antitumor)4 and Protosappanin A
(a natural product for anti-HIV-1)5 (Scheme 1), in which
dibenzo[b,d]oxepinone motifs are important intermediates for
the synthesis of Protosappanin A.6 In view of the its wide
application, synthesis of dibenzo[b,d]oxepinones is of great
significance.
reported
a palladium-catalyzed intramolecular oxidative
cyclization of diaryl ketones for the synthesis of fluorenones.
Lan and You13 reported a palladium-catalyzed intramolecular
oxidative coupling of (hetero)aryl carboxylic esters. At the
same time, DDQ (2, 3-dichloro-5,6-dicyano-p-benzoquinone) is
an inexpensive and readily available oxidizing reagent. Deng14
and Liu15 groups reported some DDQ-mediated oxidative
coupling reaction, DDQ could promote the production of
methylene radicals in this type of reaction. And some studies
demonstrated that visible-light photoactivation is a powerful
tool for organic synthetic reactions, especially those reactions
that are generally difficult by common approaches.16
Therefore, we attempted to use DDQ as an oxidant and
OH
OH
HO
O
HO
OH
OH
HO
O
CH3
O
H3CO
H3CO
CH3
O
O
OH
Arugosin F
O
Chaetone G
Protosappanin A
Scheme 1 Representative dibenzoxheterclic ketone structure.
Over the past years, various methods were reported about
constructing dibenzo[b,d]oxepin-7(6H)-one skeleton. In 2015,
introduced
light
to
promote
the
synthesis
of
aSchool of Chemistry and Materials Science, Heilongjiang University, Harbin,
Heilongjiang 150080, PR China.
dibenzo[b,d]oxepin-7(6H)-ones in the reaction.
On the other hand, the cascade reaction is a powerful and
ideal strategy for efficient and rapid synthesis of complex
molecular structures,17 because it can sequentially form two or
more chemical bonds in a single step reaction without
protection/deprotection and separation of intermediates.
bKey Laboratory of Chemical Engineering Process & Technology for High-efficiency
Conversion,College of Chemistry and Materials Science, Heilongjiang University
Harbin, Heilongjiang 150080, P.R. China.
E-mail: sunzz@hlju.edu.cn; wenyichu@hlju.edu.cn; Fax: (+) 86-451-86609135.
† Electronic Supplementary Information (ESI) available: experimental procedures
and compound characterization data, and 1H and 13C NMR spectra of new
compounds (PDF). See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Moreover, this process can improve the reaction efficiency, were also investigated. These solvents gave lower yields of
View Article Online
DOI: 10.1039/C9OB00247B
raise atom utilization, and reduce environmental pollution. 62%, 38%, 35% and 41%, respectively (Table 1, entries 14-17).
Herein, we designed a novel method for the synthesis of
dibenzo[b,d]oxepin-7(6H)-ones via visible-light-induced Pd(II)-
catalyzed cascade reaction of etherification/C-C coupling
cyclization of ɑ-bromoacetophenones with phenols (Scheme
2c).
Table 1 Optimization of reaction conditionsa
O
Catalyst
Light/Oxidant
Br
+
Solvent, Base
O
OH
1a
O
3a
2a
Our group previous works
R1
R1
R1
R
c
TfOH (30 eq.)
Entry
Catalyst
lightb/oxidant
Y/-
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
dioxane
DMSO
DMF
Base
%
TFAA(5 eq.)
TFA, 25 °C
a
R
CHCl3, 0 °C to r.t.
O
R
O
1
Pd(PPh3)4
Pd(PPh)3Cl2
Pd(OAc)2
[Cp*RhCl2]2
Ru(PPh3)3
RuCl3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Et3N
22
26
35
10
6
O
O
O
HO
O
2
Y/-
R1
R1
I
I
Pd(OAc)2 (10mol%)
NaOAc (2.0 eq.)
O
3
Y/-
R
K2CO3
R1
OH
Br
+
O
b
c
R
AgOAc (2.0 eq.)
DMAC/mW 155°C
4
Y/-
R
O
O
O
5
Y/-
This work
R
R
Pd(OAc)2(10 mol%)
DDQ(2.0 eq.)
blue LED
O
6
Y/-
-
R1
Br
R1
+
CH3CN (2 ml)
7
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
N/-
-
O
Cs2CO3(2.0 eq.),80°C
OH
O
8
N/Cu(OAc)2
N/Cu(OTf)2
N/AgOAc
N/K2S2O8
N/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
Y/DDQ
21
23
18
16
Scheme 2 The methods of synthesizing dibenzo[b,d]oxepin-7(6H)-ones.
9
Firstly, the method for synthesis of dibenzo[b,d]oxepin-7(6H)-
ones via the cascade reaction of etherification/C-C coupling
cyclization was investigated. Phenol (1a) and ɑ-
10
11
bromoacetophenone (2a) were chosen as model substrates to 12
35d , 52, 50e
examine reaction conditions such as catalysts, oxidants,
solvents, temperature and base under the 25W blue LED. The
experimental results were shown in Table 1.
The catalysts such as Pd(0), Pd(II), Ru(0), Ru(III) and Rh(II)
were firstly screened as a kind of critical factor in the reaction 16
in acetonitrile (CH3CN) at 80 °C. Pd(PPh3)4, Pd(PPh)3Cl2 and
Pd(OAc)2 were used as catalysts to give the yields of 22%, 26%
and 35%, respectively (Table 1, entries 1-3). [Cp*RhCl2]2 and
Ru(PPh3)3 as the catalysts exhibited low catalytic activity to
13
14
15
70d, 86, 86e, 65f
62
38
35
41
33
26
60
46
17
PhMe
18
CH3CN
CH3CN
CH3CN
CH3CN
19
Pyridine
K2CO3
give the yields of 10% and 6%, respectively (Table 1, entries 4- 20
5). Using RuCl3 as a catalyst or Pd(OAc)2 (no light) as a catalyst,
21
Na2CO3
the reaction did not occur (Table 1, entries 6-7).
a Reaction conditions: 1a (0.45 mmol), 2a (0.47 mmol), catalyst (10 mol%), oxidant (2.0
The effect of different oxidants such as Cu(OAc)2, Cu(OTf)2,
AgOAc, K2S2O8 and DDQ were screened. Cu(OAc)2 and Cu(OTf)2
gave the yields of 21% and 23%, respectively (Table 1, entries
8-9). AgOAc and K2S2O8 gave the yields of 18% and 16%,
respectively (Table 1, entries 10-11). With DDQ as the oxidant,
the yield could reach 52% (Table 1, entry 12), but the reaction
eq.), base (2.0 eq.) and solvent 2 ml at 80 oC, the reaction completed (monitored by
b
c
TLC). Y = with the 25W blue LED, N = in the dark. Isolated yields after silica gel
column chromatography. d Yield at 70oC. e Yield at 90oC. f Yield of 5 mol% Pd(OAc)2.
Simultaneously, the base also plays a vital role in the
reaction. When organic bases were selected including Et3N and
pyridine (Table 1, entries 18-19), the reaction progressed in
poor yields. This may result from oxidation from DDQ.
Conversely, inorganic bases were found to be more helpful in
the reaction. In particular, the use of Cs2CO3 improved the
reaction process to give a good yield of 86% (Table 1, entries
13, and 20-21).
Based on the above research, optimized conditions of the
cascade reaction were 10 mol% Pd(OAc)2 as the catalyst, 2.0
equiv of DDQ as oxidant and 2.0 equiv of Cs2CO3 as base,
CH3CN as solvent, and irradiation with 25W blue LED at 80 °C.
The scope of substrates was further investigated by the
o
o
only gave isolated yields of 35% and 50% at 70 C and 90 C,
respectively (Table 1, entries 12d and 12e). After the
introduction of light, the yield was significantly increased to 86%
(Table 1, entry 13). This indicated that the reaction may be a
free radical reaction, and DDQ could more easily undergo
single electron transfer to generate radicals under visible light
irradiation. The reaction gave isolated yields of 70% and 86%
o
o
(Table 1, entries 13d and 13e) at 70 C and 90 C. When the
amount of Pd(OAc)2 was reduced to 5 mol%, the reaction only
gave a yield of 65% (Table 1, entry 13f).
Different solvents such as 1,4-dioxane, dimethylsulfoxide
(DMSO), N, N-dimethylformamide (DMF) and toluene (PhMe)
above optimized synthetic process.
dibenzo[b,d]oxepin-7(6H)-one derivatives were successfully
synthesized by this method and were shown in Table 2. Under
A
series of
2 | J. Name., 2012, 00, 1-3
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the optimized reaction conditions, phenols and ɑ-
bromoacetophenones with different electron-donating and 1-(4-methoxyphenyl)-2-phenoxyethan-1-one was synthesized
electron-withdrawing groups were catalyzed to undergo the by etherification between 1a and 2c. Then 3c was synthesized
cascade reaction of etherification/C-C coupling cyclization under standard conditions in a yield of 84%.
smoothly to afford the corresponding dibenzo[b,d]oxepin-7(6H)-
COMMUNICATION
In order to understand the reaction mechanism (Scheme 3),
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DOI: 10.1039/C9OB00247B
O
one derivatives 3 in good yields.
O
Pd(OAc)2(10 mol%)
blue LED
O
O
Table 2 Substrates scope of the cascade reaction a.
O
DDQ(2.0 eq.), Cs2CO3(2.0 eq.)
CH3CN, 80°C
O
R
Pd(OAc)2(10 mol%)
DDQ(2.0 eq.)
R
3c (84%)
O
R1
O
blue LED
Br
R1
O
TEMPO(1.0 eq.)
Pd(OAc)2(10 mol%)
blue LED
+
O
O
O
CH3CN (2 ml)
Cs2CO3(2.0 eq.),80°C
O
O
O
O
N
OH
O
+
DDQ(2.0 eq.), Cs2CO3(2.0 eq.)
CH3CN, 80°C
O
1a-1f
2a-2d
3a-3x
O
4 (56%)
3c (15%)
O
R1
R1
O
O
R1
O
O
O
TEMPO(1.0 eq.)
blue LED
O
O
O
N
O
+
O
DDQ(2.0 eq.), Cs2CO3(2.0 eq.)
CH3CN, 80°C
O
O
O
O
O
O
3a R1 = -H, 80%
3e R1 = -H, 83%
3i R1 = -H, 85%
4 (54%)
3c (<1%)
3b R1 = -CH3, 82%
3c R1 = -OCH3,84%
3d R1 = -Ph, 79%
3f R1 = -CH3, 85%
3g R1 = -OCH3, 88%
3h R1 = -Ph, 80%
3j R1 = -CH3, 87%
3k R1 = -OCH3, 89%
3l R1 = -Ph, 80%
O
O
TEMPO(1.0 eq.)
Pd(OAc)2(10 mol%)
In the dark
O
O
O
O
O
O
N
O
+
Cl
F
O
DDQ(2.0 eq.), Cs2CO3(2.0 eq.)
CH3CN, 80°C
O
R1
R1
R1
3c (20%)
4 (30%)
O
O
O
O
O
O
Scheme 3 Mechanism research.
3m R1 = -H, 78%
3n R1 = -CH3, 81%
3o R1 = -OCH3, 83%
3p R1 = -Ph, 76%
3q R1 = -H, 76%
3u R1 = -H, 83%
3v R1 = -CH3, 86%
3w R1 = -OCH3, 88%
3x R1 = -Ph, 81%
3y R1 = -F, <1%
3r R1 = -CH3,79%
3s R1 = -OCH3,81%
3t R1 = -Ph, 75%
When one equivalent of 2, 2, 6, 6-tetramethylpiperidine-1-
oxyl (TEMPO) was added to the reaction, 1-phenyl-2-(2-((2, 2,
6, 6-tetramethylpiperidin-1-yl)oxy)phenoxy)ethan-1-one 4 was
isolated in 56% yield, and 3c was obtained in a yield of 15%.
And compound 4 was 54%, product 3c was not obtained in the
absence of Pd(OAc)2. In addition, when the whole reaction
system was in the dark, compound 4 was 30%, and product 3c
a
Reaction conditions: 1a-1f (0.45 mmol), 2a-2d (0.47 mmol), 10 mol% Pd(OAc)2, DDQ
(2.0 eq.), Cs2CO3 (2.0 eq.), CH3CN 2 ml, 25W blue LED at 80 oC. After 10 h, the reaction
completed (monitored by TLC), isolated yield by flash column chromatography.
The group R of the phenols could be an electron-donating
group, or electron-withdrawing group. When electron-
donating groups -Me were at the para position, the cascade
reaction showed high reaction activities and afforded good
yields of 80-88% (Table 2, entries 3e-3h). Compared with the
electron-donating groups, the electron-withdrawing groups
such as -F and -Cl at the para position showed slightly low
reaction activities and the yields of 75-83% (Table 2, entries
3m-3t). When the electron-donating group -OMe was at the
meta position, the cascade reaction also showed high reaction
activities and the yields of 80%-89% (Table 2, entries 3i-3l).
When the electron-donating group -OMe was at the para
position, it also could afford good yields (Table 2, 81%-88% for
entries 3u-3x).
was isolated in 20% yield. The compound
4 radical
intermediate was captured by the radical trap (TEMPO), it was
confirmed that the 1-(4-methoxyphenyl)-2-phenoxyethan-1-
one produced a methylene radical in this process. The process
may be a cascade mechanism involving the palladacycle
intermediates.18, 19 However the exact mechanism is not clear
at this moment.
OH
OMe
OMe
HO
OH
OH
Pd(OAc)2 (10mol%)
DDQ (2.0 eq.)
blue LED
O
i) TMSCN,ZnI2
ii) LiAlH4
OMe
MeO
+
O
Br
CH3CN (2 ml)
Cs2CO3 (2.0 eq.), 80°C
MeO
iii) 9% NaNO2
iV) HBr/HOAc
OMe
O
O
O
3z
(86%)
protosappanin A
(78%)
Scheme 4 Application of the protocol to the synthesis of Protosappanin A.
Another group R1 on the ɑ-bromoacetophenones could be -
Me, -OMe, -Ph, -H and so on. When the electron-donating
groups such as -OMe and -Me were at the meta position, the
cyclization showed high reaction activities and afforded good
yields of 79-89% (Table 2, entries 3b, 3c, 3f, 3g, 3j, 3k, 3n, 3o,
3r, 3s, 3v and 3w). The group -Ph was at the meta position,
and the reaction activities and the yields slightly decreased to
75%-81% (Table 2, entries 3d, 3h, 3l, 3p, 3t and 3x). However,
when the electron-withdrawing group -F at meta position, the
reactivity is significantly reduced (Table 2, entry 3y).
To further demonstrate the synthetic utility of this protocol,
we applied it to the synthesis of the Protosappanin A (Scheme
4).
3-Methoxyphenol
and
2-bromo-1-(3,4-
dimethoxyphenyl)ethan-1-one were selected to the optimized
reaction conditions to obtain the compound 3z (3,9,10-
trimethoxy dibenzo[b,d]oxepin-7(6H)-one) with 86% yield.
Then the Protosappanin A8 was synthesized in four steps with
a yield of 78%. Compared with previous methods, this protocol
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14 (a) Z. -L. Wang, H. -L. Li, L. -S. Ge, X. -L. An, Z. -G. Zhang,
X. Luo, J. S. Fossey, W. -P. Deng, DJ.OOI:r1g0..1C0h3Ve9im/eCw.9A2Ort0iBc10le40O2,n47li7n9Be,
1156-1165; (b) Y. Li, L. Cao, X. Luo, W. -P. Deng,
Tetrahedron. 2014, 70, 5974-5979; (c) Z. -L. Wang, X. -L.
An, L. -S. Ge, J. -H. Jin, X. Luo, W. -P. Deng, Tetrahedron,
2014, 70, 3788-3792.
has the advantages of readily available starting materials,
concise steps and mild conditions.
Conclusion
15 C. A. Morales-Rivera, P. E. Floreancig, P. Liu, J. Am.
Chem. Soc. 2017, 139, 17935-17944.
In summary, a novel method to synthesize dibenzo[b,d]
oxepin-7(6H)-one was developed via visible-light-induced
cascade reaction of etherification/C-C coupling cyclization
from ɑ-bromoacetophenones and phenols. Twenty five
desired products were obtained in moderate to good yields by
this method. Furthermore, this protocol was applied to the
synthesis of Protosappanin A. The protocol features are simple
reaction conditions, readily available starting materials, broad
substrate scope, and good tolerance of functional groups.
16 (a) M. Parasram, P. Chuentragool, D. Sarkar, V.
Gevorgyan, J. Am. Chem. Soc. 2016, 138, 6340-6343;
(b) M. Parasram, V. Gevorgyan, Chem. Soc. Rev. 2017,
46, 6227-6240; (c) M. Parasram, P. Chuentragool, Y.
Wang, Y. Shi, V. Gevorgyan, J. Am. Chem. Soc. 2017,
139, 14857-14860.
17 (a) S. Feng, T. Li, C. Du, P. Chen, D. Song, J. Li, X.
She, Chem. Commun. 2017, 53, 4585-4588; (b)Y. Zhao,
Y. Lv and W. Xia, Chem. Rec. 2018, 18, 1-17; (c) Y. Yuan,
W. Dong, X. Gao, H. Gao, X. Xie, Z. Zhang, J. Org.
Chem. 2018, 83, 2840-2846.
18 (a) C. Zhou, P. H. Li, X. J. Zhu, L. Wang, Org. Lett. 2015,
17, 6198-6201; (b) C. L. Wang, J. Y. Qiao, X. C. Liu, H.
Song, Z. Z. Sun, W. Y. Chu, J. Org. Chem. 2018, 83,
1422-1430.
Conflicts of interest
There are no conflicts to declare.
19 (a) L. Grande, E. Serrano, L. Cuesta, E. P. Urriolabeitia,
Organometallics, 2012, 31, 394-404; (b) M. -T. Chicote,
C. Rubio, D. Bautista, J. Vicente, Dalton Trans. 2014, 43,
15170-15182.
Acknowledgements
This study was funded by the Natural Science Foundation of
Heilongjiang Province of China (No. B2018012).
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