Sulfur incorporation: Copper-catalyzed cascade cyclization of 1,7-enynes with metal sulfides toward thieno[3,4- c ]quinolin-4(5H)-ones

Article abstract of DOI:10.1021/ol5025162

A novel copper-catalyzed cascade cyclization of 1,7-enynes with metal sulfides is described. This sulfur-incorporation method provides straightforward access toward the important thiophene-fused quinolin-4(5H)-one scaffold through cyclization and double C

Full text of DOI:10.1021/ol5025162

Letter
pubs.acs.org/OrgLett
Sulfur Incorporation: Copper-Catalyzed Cascade Cyclization of 1,7-
Enynes with Metal Sulfides toward Thieno[3,4‑c]quinolin-4(5H)‑ones
Yu Liu,† Jia-Ling Zhang,† Ren-Jie Song,† and Jin-Heng Li*,†,‡
†State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
‡State Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou 730000, China
S
* Supporting Information
ABSTRACT: A novel copper-catalyzed cascade cyclization of
1,7-enynes with metal sulfides is described. This sulfur-
incorporation method provides straightforward access toward
the important thiophene-fused quinolin-4(5H)-one scaffold
through cyclization and double C−S bond formation cascade,
and the chemoselectivity of this 1,7-enyne cyclization toward
1,3,3a,9b-tetrahydrothieno[3,4-c]quinolin-4(5H)-ones and
3,3a-dihydrothieno[3,4-c]quinolin-4(5H)-ones can be controlled by varying the sulfur resources.
etal-catalyzed 1,n-enyne cyclization processes have
through double C−S bond formation. Notably, the chemo-
Mproven to be powerful and frequently utilized methods selectivity toward 1,3,3a,9b-tetrahydrothieno[3,4-c]quinolin-
4(5H)-ones or 3,3a-dihydrothieno[3,4-c]quinolin-4(5H)-ones
can be controlled by simply changing the sulfur resources (Na2S·
9H2O or K2S). To the best of our knowledge, this method
represents the first 1,n-enyne cascade cyclization using metal
sulfides as the addition reagents through incorporation of a sulfur
atom into the product system.
for constructing complex, functionalized polycyclic ring systems
in an atom- and step-economical manner.1−4 In this field,
cycloisomerization, skeletal rearrangement, and metathesis are
excellent strategies for the 1,n-enyne cyclizations that have been
well-documented.1,2 However, the majority of these strategies
are triggered by interaction among a noble transition metal, a π-
orbital of acetylene, and a π-orbital of alkene, thereby limiting the
distance between the acetylene and the alkene in 1,n-enynes
(often 1,6-enynes) due to a combination of entropic factors and
the presence of nonbonding interactions in the transition
state.1−4 Finding a means to realize remote 1,n-enyne cyclization
using inexpensive transition-metal catalysts, such as copper
salts,1l,3 remains an important challenge. Recently, a new 1,n-
enyne cascade cyclization strategy catalyzed by transition-metal
catalysts using additional reagents was developed and is
especially attractive for impressively increasing structural
complexity;4 however, such successes are much less abundant.
Moreover, approaches for the cyclization of remote 1,7-enynes
using inexpensive copper catalysts are lacking.
Our initial investigations focused on the cascade cyclization of
N-methyl-N-(2-(phenylethynyl)phenyl)methacrylamide (1a)
with Na2S·9H2O for reaction condition optimization (Table
1). 1,7-Enyne 1a was found to furnish the desired 1,3,3a,9b-
tetrahydrothieno[3,4-c]quinolin-4(5H)-one 2a in 83% yield
together with only a trace of another 3,3a-dihydrothieno[3,4-
c]quinolin-4(5H)-one product 3a upon exposure to CuCl2,
Cs2CO3, Na2S·9H2O, and argon in DMF at 120 °C (entry 1).
However, the absence of bases decreased the yield from 83% to
31% (entry 2). In light of these results, the effect of bases was
examined: a series of other bases, including Na2CO3, CsOAc,
t
CsF, and BuOK, could favor the reaction, albeit with lower
reactivity (entries 3−6). The amount of CuCl2 was subsequently
examined, and the results showed that 20 mol % of CuCl2 was
preferred (entry 1 vs entries 7 and 8). Other copper salts, such as
CuBr2, Cu(OAc)2, and CuCl, also displayed high catalytic
activity for the reaction, although they were inferior to CuCl2
(entry 1 vs entries 9−11). It should be noted that CuCl, a Cu(I)
salt, is also effective for the reaction, delivering 2a in 75% yield
(entry 11). Among the effects of other reaction parameters,
including solvents (entries 12−14) and the reaction temper-
atures (entries 15 and 16), examined, it was found that DMF at
120 °C led to the optimized process (entry 1). We were surprised
to find that by changing from Na2S·9H2O to anhydrous Na2S
Sulfur-containing heterocyclic compounds are widespread in
natural products and synthetic compounds of high utility in
pharmaceutical, agrochemical, and materials chemistry.5 As a
result, the development of new efficient and selective routes to
sulfur-containing heterocyclic compounds remains a continuous
hot topic in synthesis and industry. A typical procedure includes
the incorporation of sulfur atoms from metal sulfides into organic
frameworks for building the sulfur-containing heterocyclic ring
scaffold by copper-catalyzed cross-coupling tandem reactions.6
We reasoned that metal sulfides may be used as additional
reagents to initiate copper-catalyzed 1,n-enyne cascade cycliza-
tion.6 Herein, we report an unprecedented copper-catalyzed
cascade cyclization of 1,7-enynes with metal sulfides for one-pot
synthesis of important thieno[3,4-c]quinolin-4(5H)-ones7
Received: August 26, 2014
© XXXX American Chemical Society
A
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Organic Letters
Letter
a
Table 1. Screening of Optimal Conditions
yield (%)
temp
entry [Cu] (mol %)
base
solvent
(°C)
2a
83
3a
1
2
3
4
5
6
7
8
9
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuBr2
Cu(OAc)2
CuCl
Cs2CO3
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMA
DMSO
toluene
DMF
DMF
DMF
DMF
DMF
DMF
120
120
120
120
120
120
120
120
120
120
120
120
120
120
100
130
120
120
120
120
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
65
31
45
30
47
36
75
82
80
60
75
66
10
33
53
81
15
trace
trace
80
Na2CO3
CsOAc
CsF
tBuOK
b
c
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
10
11
12
13
14
15
16
17
18
19
20
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
d
e
f
77
35
g
trace
Figure 1. Cyclization of 1,7-enynes (1) with Na2S·9H2O. Reaction
conditions: 1 (0.2 mmol), Na2S·9H2O (1.2 mmol), CuCl2, (20 mol %),
Cs2CO3 (0.6 mmol), and DMF (2 mL) under argon atmosphere for 24
a
Reaction conditions: 1a (0.2 mmol), Na2S·9H2O (1.2 mmol), [Cu]
(20 mol %), base (0.6 mmol), and solvent (2 mL) under argon
atmosphere for 24 h. The dr value is about 21:4 determined by H
NMR analysis of the crude product (trans (1-Ph/9b-H), trans (3a-Me/
9b-H)-isomer is major, and cis (1-Ph/9b-H), trans (3a-Me/9b-H)-
isomer is minor). [Cu] (10 mol %). [Cu] (30 mol %). Anhydrous
Na2S (1.2 mmol). K2S (1.2 mmol) instead of Na2S·9H2O. PhSSPh
(1.2 mmol) instead of Na2S·9H2O. 1a (1 g, 3.64 mmol) and solvent
(10 mL) for 72 h.
1
1
h. The dr value is given in the parentheses determined by H NMR
analysis of the crude product. The diastereoisomers are determined by
2D NMR analysis of products 2i and 2s: trans (1-R1/9b-H), trans (3a-
R4/9b-H)-isomer is major and cis (1-R1/9b-H), trans (3a-R4/9b-H)-
isomer is minor.
b
c
d
e
f
g
(entry 17), K2S (entry 18), or PhSSPh (entry 19) the
chemoselectivity of the cyclizaiton was shifted from product 2a
toward another product 3a in 65%, 77%, and 35% yield,
respectively. Gratifyingly, a 1 g (3.64 mmol) scale reaction of 1,7-
enyne 1a is successfully performed with Na2S·9H2O, giving 2a in
high yield (entry 20).
After determining the optimal reaction conditions, we turned
our attention to investigating the scope of this cascade cyclization
protocol by examining the enyne component with Na2S·9H2O
(Figure 1) or K2S (Figure 2). As shown in Figure 1, 1,7-enyne
with a free N−H bond was not a viable substrate for this cascade
cyclization reaction (2b). Although 1,7-enyne having an N-allyl
group was successfully reacted with Na2S·9H2O, CuCl2, and
Cs2CO3, removal of the allyl group took place leading to 2b in
58% yield. Use of N-Bn-substituted 1,7-enyne delivered 2d in
64% yield. 1,7-Enyne bearing a substituent, including Me, MeO,
F, Cl, and Br, on the aromatic ring at the terminal alkyne
performed well in the cascade cyclization under these optimal
conditions, providing 2e−l in good yields. Interestingly, the
cascade process could generate heteroaryl-containing products
2m−u when the heteroaryl groups, pyridinyl and thiophene-yl,
are directly attached at the terminal alkyne. We were delighted to
find that the reaction also accommodated several substituents,
Figure 2. Cyclization of 1,7-enynes (1) with K2S. Reaction conditions: 1
(0.2 mmol), K2S (1.2 mmol), CuCl2 (20 mol %), Cs2CO3 (0.6 mmol),
and DMF (2 mL) under argon atmosphere for 24 h.
including Me, Cl, F, and CF3, on the aryl of the N-(2-
ethynylarylyl) moiety, giving the corresponding products 2v−aa
in 51−82% yields. Finally, 1,7-enyne with a Bn group at the 2
B
dx.doi.org/10.1021/ol5025162 | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
position of the acrylamide moiety is effective for the construction
of 2ab in moderate yield. Importantly, halide motifs are
accommodated by the reaction conditions and provide further
opportunities for additional modifications of the thieno[3,4-
c]quinolin-4(5H)-one scaffold (2g−i,l,w−z). However, aliphatic
alkyne has no reactivity for the reaction (2ac).
The mechanisms outlined in Scheme 2 were proposed for this
cascade cyclization reaction.2−4,7,8 Initially, coordination of the
Scheme 2. Possible Mechanisms
Next, we set out to exploit the scope of this cascade cyclization
reaction using K2S as the sulfur resource (Figure 2). To our
delight, analogous 1,7-enyne with a N-Bn group instead of the N-
Me group was also converted into dihydrothieno[3,4-c]quinolin-
4(5H)-one 3d in 71% yield. We obtained good yields of 3f and
3h from 1,7-enynes with a p-MeC6H4 or a p-ClC6H4 group at the
terminal alkynes. A range of heteroaryl alkynes could be
successfully accommodated by the reaction with K2S, CuCl2,
and Cs2CO3 (3n, 3p, 3q, 3s, and 3t), and even a Cl-substituted
enyne worked well to form 3x in high yield. Notably, a 1,7-enyne
with a Bn group at the 2 position of the acrylamide moiety was
also viable for constructing 3ab in good yield.
In light of the results of Table 1 and Figures 1 and 2, the
chemoselectivity toward products 2 and 3 was based on the sulfur
resource, Na2S·9H2O or K2S, suggesting that the hydrogen
atoms in the new formed C−H bonds of 2 may be from H2O.8
To verify these, two control experiments with D2O were
investigated (eqs 1 and 2; Scheme 1).9 As expected, deuterium
active CuI species with the C−C triple bond and the C−C double
bond in 1,7-enyne 1a afforded intermediate A. In the presence of
Na2S·9H2O, intermediate A readily reacts with NaHS,8 in situ
generated from Na2S·9H2O, to provide intermediate B, followed
by addition to form intermediate C. cis-Nucleophilic cyclization
of intermediate C gives intermediate D. Finally, protonation of
intermediate D via trans-addition of H2O to the C−Cu bond
delivers trans,trans-2a as the major isomer and regenerates the
active CuI species. Using K2S reacted intermediate A forms
intermediate E. Nucleophilic addition and cyclization of
intermediate E take place to afford intermediate F, followed by
the second nucleophilic cyclization of intermediate F to give 3a.
In summary, we have illustrated the first Cu-catalyzed cascade
cyclization of 1,7-enynes by incorporating a sulfur atom into the
product system using metal sulfides as sulfur resources. This
method proceeds through 1,7-enyne cyclization and double C−S
bond formation cascade and provides a valuable one-pot
assembly of thieno[3,4-c]quinolin-4(5H)-ones from a broad
range of enynes with good yields and excellent functional group
tolerance.
Scheme 1. Control Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
Descriptions of experimental procedures for compounds and
analytical characterization. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
* E-mail: jhli@hnu.edu.cn.
Notes
atoms were incorporated into 2a, and K2S combined with D2O
could deliver deuterated product 2n-[D]2 along with 3n. The
results of eq 3 show that alkene 1ad is not a suitable substrate for
the reaction.8e Notably, product 3a cannot be hydrogenated by
the Na2S·9H2O/CuCl2/Cs2CO3 system (eq 4). This suggests
that the hydrogenation reaction is not a sole step during the
whole cyclization process.8 Using 1,2-diphenylethyne (1ae) to
react with Na2S·9H2O under the optimal conditions only
afforded (E)-1,2-diphenylethene (4ae), a hydrogenation prod-
uct, implying that the addition of S atom takes place at the
acrylate unit (eq 5).8 Notably, CuCl showed high catalytic
activity for the reaction (entry 11; Table 1). The results suggest
that the Cu(I) species plays a major role in the current reaction.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of China (No.
21172060), Specialized Research Fund for the Doctoral Program
of Higher Education (No. 20120161110041), and Hunan
Provincial Natural Science Foundation of China (No.
13JJ2018) for financial support.
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