Visible-light-mediated cascade difunctionalization/cyclization of alkynoates with acyl chlorides for synthesis of 3-acylcoumarins

Article abstract of DOI:10.1016/j.tetlet.2018.04.033

A new visible-light-mediated radical cyclization of alkynoates with acyl chlorides is described for the one-pot construction of diverse 3-acylcoumarins with high efficiency and selectivity. This method is successful by sequential difunctionalization of an alkynes C–C triple bond with the C–Cl bonds of acyl chloride and aromatic C(sp²)–H bonds. The cyclization is proposed to simultaneously form two new carbon–carbon bonds, and involves radical acylation, 5-exo-trig cyclization, and ester migration.

Full text of DOI:10.1016/j.tetlet.2018.04.033

Accepted Manuscript
Visible-light-mediated cascade difunctionalization/cyclization of alkynoates
with acyl chlorides for synthesis of 3-acylcoumarins
Yu Liu, Qiao-Lin Wang, Cong-Shan Zhou, Bi-Quan Xiong, Pan-Liang Zhang,
Shuang-Jian Kang, Chang-An Yang, Ke-Wen Tang
PII:
S0040-4039(18)30491-X
https://doi.org/10.1016/j.tetlet.2018.04.033
TETL 49896
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
21 March 2018
9 April 2018
15 April 2018
Please cite this article as: Liu, Y., Wang, Q-L., Zhou, C-S., Xiong, B-Q., Zhang, P-L., Kang, S-J., Yang, C-A., Tang,
K-W., Visible-light-mediated cascade difunctionalization/cyclization of alkynoates with acyl chlorides for synthesis
of 3-acylcoumarins, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.2018.04.033
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Graphical Abstract
Visible-light-mediated cascade
difunctionalization/cyclization of alkynoates
with acyl chlorides for synthesis of 3-
acylcoumarins
Leave this area blank for abstract info.
Yu Liu,* Qiao-Lin Wang, Cong-Shan Zhou, Bi-Quan Xiong, Pan-Liang Zhang, Shuang-Jian Kang,
Chang-An Yang, Ke-Wen Tang*
R2
R2
O
visible light
O
R3
R1
R1
CH CN, Ar, 100 oC
+
Cl
R3
3
O
O
O
O
32 Examples, up to 84% yield

1
Tetrahedron Letters
jo u r n a l h o m e p a g e : w w w . e ls e v i e r . c o m
Visible-light-mediated cascade difunctionalization/cyclization of alkynoates with acyl
chlorides for synthesis of 3-acylcoumarins

Yu Liu, Qiao-Lin Wang, Cong-Shan Zhou, Bi-Quan Xiong, Pan-Liang Zhang, Shuang-Jian Kang, Chang-
An Yang, Ke-Wen Tang*
Department of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A new visible-light-mediated radical cyclization of alkynoates with acyl chlorides is described
for the one-pot construction of diverse 3-acylcoumarins with high efficiency and selectivity.
This method is successful by sequential difunctionalization of an alkynes C−C triple bond with
2
the C−Cl bonds of acyl chloride and aromatic C(sp )−H bonds. The cylization is proposed to
Available online
simultaneously form two new carbon-carbon bonds, and involves radical acylation, 5-exo-trig
cyclization, and ester migration.
Keywords:
2
009 Elsevier Ltd. All rights reserved.
Difunctionalization
Visible light photoredox catalysis
Radical cyclization
Alkynoate
3
-Acylcoumarin
into coumarins, the harshness of the not freely available
precursors and strong oxidative conditions would limit their
applications in medicine and chemical industry. Thus, it is highly
eager to develop more convenient and efficient methods under
mild and environmental benign conditions to generate acyl
radical and employ them for constructing 3-acylcoumarins.
1
. Introduction
Coumarin skeletons rank among the most important
1
heterocycles in many natural products, pharmaceutically active
molecules, and dyes, and they are also widely applied in
materials field. More importantly, coumarin derivatives are also
2,3
4
5
widely used as anticancer, anticoagulant, anti-inflammatory,
antibacterial, anti-HIV, enzymatic inhibitors, and anti-psoriasis in
Previous work:
R2
R2
6
medicinal chemistry, or as illumination light source material
R
7
6
-exo cyclization
sciences and fluorescent probes. Thus, development of new and
R1
R1
+
R
efficient strategies for construction of these compounds under
mild condition will be significant for the screening of novel
biologically active molecules. Recently, the cascade
functionalization and cyclization of aryl alkynoate esters has
been developed as a powerful method to synthesize 3-
O
O
O
O
R = SO2R, PO(R)2, CF3, SCN, SCF3, Acyl, CH2CN,
CH2COCH3, CH2COOR, CH(COCH3)2, HC
O
This work:
8
functionalized coumarins (Scheme 1). For instance, a variety of
R2
R2
O
O
3
-substituted coumarins have been obtained via radical alkyne
visible light
O
8
a,8b
8c
8d
sulfonation,
phosphorylation,
trifluoromethylation,
R3
8e
8e
8f
R1
CH CN, Ar, 100 o
C
3
R1
+
R3
thiocyanation, trifluoromethylthiolation, difluoroacetylation,
cyanomethylation, 3-acetonylation,
Cl
8g
8h,8i
8j
O
O
O
alkylation /cyclization
pathways under oxidative or redox-neutral reaction conditions.
Additionally, it was described that 3-acylcoumarins could be
prepared by the fuctionalization of aryl alkynoate esters with
Scheme 1. Oxidative radical difunctionalization of alkyne leading to
coumarins.
8k,8l
8m−o
aldehydes
or 2-oxoacetic acids
following a radical
Recently, the visible light photocatalysis strategy has proven
to be a particularly powerful and straightforward tool for
addition and cyclization mechanisms. Although such oxidative
radical-mediated strategy was extended to introduce acyl radicals
—
——

Corresponding author. E-mail address: lyxtmj_613@163.com (Y. Liu) and tangkewen@sina.com (K.-W. Tang)

2
synthetic conversions in organic synthesis, due to its attractive
properties such as environmentally friendly, excellent functional

Tetrahedron Letters
3
9
group tolerance, availability, and safety. Recently, Xu
group
2 3 3
such as Na CO , NaHCO , and pyridine lead to lower yields
10a,10b
10c
and our group applied acyl radicals produced from
(entries 8–10). In an attempt to get the best reaction conditions, a
series of solvents were tested, and other solvents such as toluene,
dioxane, THF, and DMF gave poor reaction yields (entries 11–
14). Different reaction temperature experiments turned out that a
abundant and commercially available acyl chlorides into cascade
annulations transformations under visible-light photocatalysis.
Inspired by these development of visible-light photocatalysis
9-11
o
o
reactions,
we present a new difunctionalizing acylation of
yield achieved at 110 C was similar to that at 100 C (e entry
o
activated alkynes for the synthesis of valuable 3-acylcoumarins
under visible-light-mediated photocatalysis (Scheme 1). This
strategy was achieved by sequential acylation, 5-exo-trig
cyclization, and ester migration to form two new carbon-carbon
bonds simultaneously, provided a new and convenient route for
affording 3-acylcoumarins under mild conditions.
15), but the yield decreased 69% at 90 C (entry 16). We were
delighted to find that a reaction on a 1 g (4.50 mmol) scale of
alkynoates 1a was successful in performing a good yield (entry
17).
Table 2
Visible-light-mediated cyclization of alkynoates (1) with benzoyl chloride
2
. Results and Discussion
a
(
2a).
R2
R2
O
O
We initiated our studies by combining phenyl 3-
R1
O
phenylpropiolate (1a) with benzoyl chloride (2a) using Ir(ppy)
2 mol %) as the photocatalyst for the tandem cyclization
3
[Ir(ppy) ], 2,6-lutidine
Ph
3
+
CH CN, Ar, 100 o
C
(
Cl
Ph
R1
3
O
O
O
2a
5 W blue LED light
reaction (Table 1). To our delight, the desired 3-benzoyl-4-
3
1
phenyl-2H-chromen-2-one 3aa was obtained in 81% yield in the
Ph
O
O
O
Ph
O
presence of 2 equiv 2,6-lutidine (0.4 mmol) as additive, and
R = Me, 3ba, 84%
R = Cl, 3fa, 78%
R = OMe, 3ca, 80% R = Br, 3ga, 72%
o
Ph
Ph
acetonitrile (CH
3
CN, 2 mL) as solvent at 100 C by irradiation
R = Ph, 3da, 78%
R = F, 3ea, 75%
R = CF , 3ha, 70%
3
with a 5 W blue LED light for 24 h (Table 1, entry 1). Next, we
focused on optimizing the reaction conditions (Table 1).
However, no 3-acylcoumarin 3aa was detected when using
H
H
R
O
O
R = Ac, 3ia, 68%
3
aa, 81%
Ph
O
O
Ph
O
Ph
O
O
R
O
Ru(bpy)
2
Cl
2
and Eosin Y instead of Ir(ppy)
3
(entries 2–3).
or light
Ph
Ph
Ph
Ph
Further experiments demonstrated that either Ir(ppy)
3
H
O
O
H
O
O
H
O
O
was indispensable for this tandem reaction (Table 1, entries 4–5).
R = Me, 3ka, 0%
R = Me, 3la, 70%
R = OMe, 3ma, 68%
3
ja:3ja' = 3:2, 74%
Table 1
R
a
Optimization of the reaction conditions
O
Ph
O
O
Ph
O
O
O
O
R = Me, 3oa, 83%
R = OMe, 3pa, 78%
Ph
Ph
O
[
Ir(ppy) ], 2,6-lutidine
Ph
Ph R = F, 3qa, 73%
3
+
H
O
R = Ac, 3ra, 65%
H
O
O
CH CN, Ar, 100 oC
H
O
O
Cl
Ph
3sa, trace
3
3na, 77%
O
O
2a
5 W blue LED light
3
aa
1a
a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol, 2 equiv), Ir(ppy)
3
(2
o
mol %), 2,6-lutidine (2 equiv), CH
3
CN (2 mL) at 100 C under an argon
Entry
1
Variation from the standard conditions
none
Yield %
81
0
0
0
atmosphere and 5 W blue LED light for 24 h.
b
b
Isolated yield.
2
3
4
5
Ru(bpy)
Eosin Y instead of Ir(ppy)
without Ir(ppy)
without additional light
3 2 3
Cl instead of Ir(ppy)
b
3
b
Having established the standard reaction conditions, we first
searched the scope of aryl 3-phenylpropiolates (1) with respect to
benzoyl chloride (2a) for this new radical cyclization process
3
b
0
6
7
Ir(ppy)
none
3
(5 mol %)
80
68
41
37
61
36
45
37
0
c
(Table 2). Initially, a variety of aryl 3-phenylpropiolates (1) in
8
9
Na
NaHCO
pyridine instead of 2,6-lutidine
toluene instead of CH CN
dioxane instead of CH CN
THF instead of CH CN
CN
2
CO
3
instead of 2,6-lutidine
reactions with benzoyl chloride (2a) were examined, indicating a
broad tolerance of substituted groups on the aromatic rings of
substrates 1. Aryl 3-phenylpropiolates with an electron-donating
group, such as Me, OMe, or Ph on the para-position of the
phenoxy rings reacted with 2a to afford the corresponding
products in 78–84% yields (3ba–da). Meanwhile, aryl 3-
phenylpropiolates bearing an electron-withdrawing group
3
instead of 2,6-lutidine
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
3
3
3
b
d
DMF instead of CH
3
o
at 110 C
78
69
72
o
at 90 C
7
a
none
including F, Cl, Br, CF , or Ac on the para-position of the
3
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol, 2 equiv), Ir(ppy)
3
(2
phenoxy rings underwent the reaction to afford the anticipated
o
mol %), 2,6-lutidine (2 equiv), CH
3
CN (2 mL) at 100 C under an argon
products in 68–78% yields (3ea–ia). According to these results,
the alkynoates with electron-withdrawing groups shown lower
reactivity than that with electron-donating groups in terms of the
reaction yields (3ba–ia). Interestingly, when meta-methyl
alkynoate 1j was utilized, the reaction afforded a mixture of
products 3ja and 3ja’ and the ratio was about 3 : 2. To
oursurprise, these two compounds could be isolated by column
chromatography. Furthermore, ortho-substituted system was not
suitable for the reaction (3ka). In addition, different substituted
aryl groups linked with the alkynyl of alkynoates 1 could also be
suitable for constructing the 3-acyl-4-arylcoumarin derivatives in
moderate to good yields (3la–ra). Nevertheless, the steric factor
also affected the radical cyclization process according to the
atmosphere and 5 W blue LED light for 24 h.
b
Most of starting material 1a was recovered, and the rest was
decomposed.
c
3
6 W compact fluorescent light instead of 5 W blue LED light.
d
1
a (1.0 g, 4.50 mmol) and solvent (10 mL) for 72 h.
The amount of Ir(ppy)
mol % Ir(ppy) was preferred (entry 1 versus entry 6). However,
the yield of 3aa decreased to 68% when using 36 W compact
fluorescent light instead of 5 W blue LED light (entry 7).
Extensive screening of the effect of bases revealed that other base
3
was found to affect the reaction, and 2
3

Tetrahedron Letters
4
yields of products 3la, 3na, and 3oa. Unfortunately,
alkylpropiolates such as n-pentylpropiolate 1s could not go
through the transformation and most of starting material was
recovered (3sa).
methylphenol (BHT), implying that the cyclization reaction
proceeds via a radical mechanism (Scheme 2, eq 1). However,
the cyclization product 4 was not detected when alkynoates 1a
was carried out only under the standard conditions (Scheme 2, eq
2).
We next investigated the scope of acyl chlorides 2 in the
presence of phenyl 3-phenylpropiolate (1a), Ir(ppy)
3
, 2,6-
lutidine, and CH CN. As shown in Table 3, we were pleased to
3
Ph
Ph
O
O
O
discover this process could be applied to radical cyclization with
a wide range of aroyl chlorides 2b–2n, including a variety of
substituted benzoyl chlorides and heterocyclic acyl chlorides.
The results showed that aroyl chlorides bearing both electron-
donating groups and electron-withdrawing groups were all
suitable for the cyclization. Electron-rich aroyl chlorides
generally performed higher activities than electron-poor ones,
and aroyl chloride bearing with a Me group on the para-position
of aromatic ring afforded the highest yield (product 3ab) and 4-
nitrobenzoyl chloride (2g) was inert in this transformation and
could not afford the target product 3ag. Meanwhile, the steric
hindrance of substituents also had obviously affect on the product
yields (products 3ab, 3ah and 3aj). However, the experiment
results showed that the reaction conditions are not ideal for the
substrates of cinnamoyl chloride (2l) and phenylacetyl chloride
Additive (3 equiv)
O
[Ir(ppy)3], 2,6-lutidine
Ph
+
(
(
1)
2)
Cl
Ph
CH CN, Ar, 100 o
C
3
O
O
2a
5
W blue LED light
1
a
3
aa, trace
Additive = TEMPO, hydroquinone, 2,6-di-tert-butyl-4-methylphenol (BHT)
Ph
Ph
Standard conditions
O
4
O
O
O
, 0%
1
a
Scheme 2. Control experiments.
Therefore, we propose a reaction mechanism as outline in
Scheme 3 based on previous literatures
Photocatalyst Ir (ppy)
and generates the strong reductant *Ir (ppy)
benzoyl chloride 2a to form radical anion A and Ir (ppy)
through single-electron transfer (SET). Then, radical anion A
goes through fragmentation to produce acyl radical B. Next,
radical addition of acyl radical B to the α-position of the C=O
bond in substrate 1a leads to the first new C−C bond and yields
vinyl radical intermediate C, which is stabilized by the aromatic
ring. The resulting vinyl radical C following 5-exo-trig
cyclization generates a spiro-cyclic intermediate D, which is
easily transformed into intermediate E through single electron
oxidation. Subsquently, ester migration and deprotonation affords
the final 3-acylcoumarin 3aa.
8−11
and the above results.
is excited under visible-light irradiation
III
3
(2m). It should be the acyl radicals could not form easily from
III
3
, which reduces
cinnamoyl chloride (2l) and phenylacetyl chloride (2m) or the
formed acyl radicals were unstable. To our surprise, thiophene-2-
carbonyl chloride (2n) could go through the tandem cyclization
under the optimal conditions and generate the desired product
IV
3
3an in 71% yield, and the heterocyclic acyl-substituted
coumarins had not been synthesized in previously reported
8k−8o
methods.
Table 3
Visible-light-mediated cyclization of phenyl 3-phenylpropiolate (1a) with
a
acyl chlorides (2).
Ph
Ph
O
O
R3
[
Ir(ppy)3], 2,6-lutidine
O
+
Cl
R3
CH CN, Ar, 100 o
3
C
O
O
Ph
O
O
2
5 W blue LED light
Ph
Cl
3
1a
2a
O
O
Ph
O
Cl-
-
R = Me, 3ab, 82%
R = OMe, 3ac, 78%
R = F, 3ad, 70%
R = Cl, 3ae, 74%
R = Br, 3af, 63%
Ph
Cl
Ph
SET
O
O
B
IrIII
A
1a
R = NO , 3ag, 0%
2
O
O
O
R
Ph
O
Ph
Ph
O
Ph
O
O
F
R
Redox-neutral Cycle
hv
Ph
IrIV
O
O
O
O
O
O
C
O
O
R = Me, 3ah, 78%
R = Cl, 3ai, 74%
Ph
3aj, 72%
3ak, 65%
SET
Ph
Ph
O
O
Ph
O
Ph
O
IrIII
S
O
O
Ph
Bn
D
O
O
O
O
O
O
O
Ph
Ph
O
O
3
al, 0%
3am, trace
3an, 71%
1
,2 Ester
Migration
Ph
Ph Aromatization
a
3aa
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol, 2 equiv), Ir(ppy)
3
(2
-H+
O
o
O
mol %), 2,6-lutidine (2 equiv), CH
3
CN (2 mL) at 100 C under an argon
E
F
atmosphere and 5 W blue LED light for 24 h.
b
Isolated yield.
Scheme 3. Proposed mechanism.
It is believed that this radical cyclization proceeded via a
radical-type pathway according to previously reported
8k
8m,8o
8
,10
However, the findings by Wu and Wang
presented that
literature.
Then, the reaction of substrate phenyl 3-
acyl radical, which came from aldehyde or α-keto acid, reacted
with alkynoate to form 6-endo-trig cyclization product. This
results could probably be attributed to the relatively fast rate of
acyl radicals formation under their conditions, such that the
reactions proceeded through 6-endo-trig cyclization. Differently,
phenylpropiolate (1a) with benzoyl chloride (2a) under the
standard reaction conditions was completely inhibited when
employing a stoichiometric amount of radical scavengers,
including TEMPO, hydroquinone and 2,6-di-tert-butyl-4-

Tetrahedron Letters
5
the concentration of intermediate C remains low in our method
References
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formation under our conditions. Hence, the reaction goes though
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8
l,8n
5
-exo-trig cyclization to affords intermediate D specifically.
2
.
(a) Naser-Hijazi, B.; Stolze, B.; Zanker, K. Second Proceedings of the
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3. Conclusion
In conclusion, we have presented a convenient and visible-
light-mediated strategy for direct difunctionalization of
alkynoates, during which two new carbon−carbon bonds were
formed in one step, towards the synthesis of the high bioactive
coumarin core structure in good yield. This tandem cyclization
avoids limitations commonly associated with transition-metal-
mediated C−H activation, including requirements for strong
oxidative and chelating directing groups. We anticipate further
application of the presented method in synthesis of natural
products and bioactive compounds.
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spectra (HRMS) were obtained on an Agilent mass spectrometer
using ESI-TOF (electrospray ionization-time of flight).
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4
.2 General procedure.
To a Schlenk tube were added 1 (0.2 mmol), 2 (2 eqiuv, 0.4
8
.
mmol), Ir(ppy)
0
3
(2 mol %, 0.004 mmol), 2,6-lutidine (2 eqiuv,
3
CN (2 mL). Then the mixture was stirred at 100
.4 mmol), CH
o
C (oil bath temperature) in argon atmosphere (1 atm) under 5 W
blue LED light for 24 h until complete consumption of starting
material as monitored by TLC and GC-MS analysis. After the
reaction was finished, the reaction mixture was washed with
brine. The aqueous phase was re-extracted with EtOAc (3×10
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flash column chromatography (hexane/ethyl acetate = 15 : 1 to 5
:
1) to afford the desired products 3.
2
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We thank the Scientific Research Fund of Hunan Provincial
Education Department (No. 16A087), Hunan Provincial Natural
Science Foundation of China (No. 2018JJ2150) and National Natural
Science Foundation of China (No. 21602056) for financial support.
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Supplementary Material
Supplementary data associated with this article can be found in
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Tetrahedron Letters
6
1
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Tetrahedron Letters
7
Highlight

The radical cyclization affords 3-
acylcoumarins from alkynoates and acyl
chlorides.

A visible-light-mediated strategy for
direct difunctionalization of alkynoates.
A variety of high bioactive 3-
acylcoumarins were obtained.
This reaction involves radical acylation, 5-
exo-trig cyclization and ester migration.

