Synthesis of trisubstituted furans via Ag-catalyzed cascade heterocyclization of 2-(1-alkynyl)-2-alken-1-ones with aryl-amines

Article abstract of DOI:10.1007/s11696-017-0323-4

Abstract: A silver(I)-catalyzed cascade heterocyclization of 2-(1-alkynyl)-2-alken-1-ones with aryl-amines is described to synthesize trisubstituted furans in moderate to good yields. Tertiary amine is also efficient in this reaction. This transformation

Full text of DOI:10.1007/s11696-017-0323-4

Chem. Pap.
DOI 10.1007/s11696-017-0323-4
ORIGINAL PAPER
Synthesis of trisubstituted furans via Ag-catalyzed cascade
heterocyclization of 2-(1-alkynyl)-2-alken-1-ones with aryl-amines
Ting He1 Yaling Chi1 Yuantao Chen1
•
•
Received: 11 August 2017 / Accepted: 13 October 2017
Ó Institute of Chemistry, Slovak Academy of Sciences 2017
Abstract A silver(I)-catalyzed cascade heterocyclization
of 2-(1-alkynyl)-2-alken-1-ones with aryl-amines is
described to synthesize trisubstituted furans in moderate to
good yields. Tertiary amine is also efficient in this reaction.
This transformation proceeds smoothly with the formation
of C–N or C–C bonds in one-pot with good functional
group tolerance under mild conditions.
Introduction
Transition metal-catalyzed cyclization reactions of carbon–
carbon and carbon–heteroatom bond formation have pro-
ven to be an efficient and straightforward methodology for
the synthesis of cyclic compounds (Patil and Yamamoto
2008; Weibel et al. 2008; Harmata 2010; Yamamoto 2012;
Zeni and Larock 2004; Rubin et al. 2003; Cornil et al.
2015). Generally, silver complexes are considered to have
similar efficiency compared with other late transition metal
catalysts and recent studies have shown that silver species
exhibited interesting catalytic activities for C–C, C–N, C–
O, C–P, C–S bond formation (Liu et al. 2015; Hu et al.
2014; Wang et al. 2014; Ji et al. 2009; Hu et al. 2017).
Polysubstituted furans, which is one of the most
important heterocyclic compounds, are not only key
structural units in natural products and pharmaceuticals
(Keay and Dibble 1996; Meegan 1984; Nakanishi 1974)
but also useful intermediates in total syntheses (Wong et al.
1999; Lee et al. 2005). Many efforts have been made to
obtain furan derivatives. (Blanc et al. 2016; Gulevich et al.
2013) Transition metal-catalyzed cyclization of 2-(1-alky-
nyl)-2-alken-1-ones with nucleophiles is one of the typical
approaches to obtain polysubstituted furans since Larock
(Yao et al. 2004) and co-works first report in 2004.
Although the cyclization of 2-(1-alkynyl)-2-alken-1-ones
with alcohols (Yao et al. 2004, 2005; Patil et al. 2005),
nitrones (Liu et al. 2009, 2010), alkenes (Liu and Zhang
2011; He et al. 2013a, b; Wang et al. 2015), alkynes (Gao
and Zhang 2012; Zhou et al. 2014), etc. (Xiao and Zhang
2008, 2009; Zhao and Zhang 2010; Li and Zhang 2010;
Gao et al. 2010a, b, 2011; Ghosh et al. 2016) have proven
to be available, the reaction of amines, especially sec-
ondary amines with this enynone was rarely reported (Oh
et al. 2006; Chen et al. 2011).
Graphical Abstract
Keywords Silver catalyzed Á Trisubstituted furans Á
Cascade reaction Á Cyclization Á Aryl-amine
Electronic supplementary material The online version of this
article (doi:10.1007/s11696-017-0323-4) contains supplementary
material, which is available to authorized users.
& Ting He
ht15@tju.edu.cn
& Yuantao Chen
ytchen@lzu.edu.cn
1
School of Chemistry and Chemical Engineering, Qinghai
Normal University, Xining 810008, People’s Republic of
China
123

Chem. Pap.
Herein, we report an efficient silver-catalyzed cascade
heterocyclization of 2-(1-alkynyl)-2-alken-1-ones with
aryl-amines to construct trisubstituted furans with an ary-
lamino-group function.
Table 2 Silver-catalyzed cascade reactions of various ketones 1 and
N-methylaniline 2a
R3
O
R3
O
5 mol% AgNO3
H
N
R2
+
N
Ph
Ph
R1
R2
R1
2a
3
1
Entry
1
R1
Ph
R2
R3
Ph
Ph
Ph
Ph
3b
Yieldc(%)
95
Results and discussion
1
2
3
4
5
6
1a
Ph
Ph
3aa
3ba
3ca
3da
3ea
3fa
1b p-Br-C6H4
78
Our initial research focused on the reaction of (E)-2-ben-
zylidene-1,4-diphenylbut-3-yn-1-one 1a and N-methylani-
line 2a (1.5 equiv.) in the presence of AgNO3 (5 mol %)
in tetrahydrofuran (THF) at room temperature (Table 1,
entry 1). We were pleased to find that the desired product
rac-N-((2,5-diphenylfuran-3-yl)(phenyl)methyl)-N-methy-
laniline 3aa was isolated in 78% yield after 4 h. Further
survey showed that, when dichloromethane was used as
solvent, the reaction proceeded to provide 3aa in 95% yield
after 1 h (Table 1, entry 2), whereas no reaction occurred
in acetonitrile (Table 1, entry 3). Other catalysts, such as
AgOTf, Ag2CO3, and Cu(OAc)2 could not afford better
results (Table 1, entries 4–6). Herein, the optimum reaction
conditions for the synthesis of 3aa were found to be 1.0
equiv. of 1a, 1.5 equiv. of 2a and 5 mol % of AgNO3 in
dichloromethane at room temperature.
1c
1d
1e
1f
Ph
Ph
Ph
Ph
p-Me-C6H4
p-Cl-C6H4
Ph
96d
94d
p-F-C6H4
66e
Ph
3,5-Dimethyl-4-OMe-C6H4
95
O
Ph
7
1g
3ga
82
Unless otherwise noted, all reactions were carried out with 1
(0.2 mmol), 1.5 equiv. 2a and 0.05 equiv. of AgNO3 in 2 mL CH2Cl2
at room temperature under air atmosphere for 1 h
aThe products are racemates
bYield of the isolated product
cReactions run at rt for 2 h
dReactions run at rt for 4 h
rich groups to afford the corresponding furan products 3ca–
3fa in good to excellent yields (Table 2, entry 3–6). The
electron-donating group showed a higher yield than the
electron-withdrawing group in R3 in this reaction (3ea–
3fa). The structure of 3fa was confirmed by an X-ray
crystal structure analysis (see ESI ; CCDC 1019024 2017).
To our delight, the cyclic substrate (Z)-2-(phenylethynyl)-
cyclohex-2-enone 1g also afforded the desired product 3ga
in 82% yield under the standard reaction conditions
(Table 2, entry 7).
With the optimum conditions in hand, the scope of this
cascade cyclization reaction of various ketones 1 with N-
methylaniline 2a was investigated (Table 2). A substrate
with para-bromo-phenyl group in alkene substituent R1
gave the desired product 3ba in the yield of 78% (Table 2,
entry 2). The alkyne substituents R2 and ketone sub-
stituents R3 could be both electron-deficient and electron-
Table 1 Optimization of reaction conditions
O
Ph
Ph
O
catalyst
We next investigated the scope of the cascade cycliza-
tion of (E)-2-benzylidene-1,4-diphenylbut-3-yn-1-one 1a
and various aryl-amines 2 under the optimized conditions
(Table 3). The primary amines with electron-withdrawing
and electron-donating-substituted groups afforded the
desired products 3ab and 3ac in moderate to good yields
(Table 3, entries 1, 2). Comparing 3ab with 3ac, the
electron-rich group was more efficient in this reaction. The
reaction also proceeded smoothly with various secondary
amines, such as 2-fluoro-N-methylaniline diphenylamine
and 4-methyl-N-phenylaniline (3ad–3af, entries 3–5).
To our surprise, tertiary amine could also react with (E)-
2-benzylidene-1,4-diphenylbut-3-yn-1-one 1a to afford the
corresponding furan product (Table 4). When 1a and N,N-
dimethylaniline 2g were employed in the above reaction,
we were pleased to find that the reaction resulted in 92%
yield of the product rac-4-((2,5-diphenylfuran-3-yl)(phe-
nyl)methyl)-N,N-dimethylaniline 4ag (entry 1).Other ter-
tiary amines, such as N,N-diethylaniline and N,N-3-
H
N
+
Ph
N
Ph
Ph
Ph
Ph
Ph
3aa
1a
2a
Entry
Catalyst(5 mol%)
AgNO3
Solvent(conditions)
THF(4h)
Yieldb,c(%)
1
2
3
4
5
6
78
95
nrd
80
nrd
nrd
AgNO3
CH2Cl2(1h)
CH3CN(24h)
CH2Cl2(1h)
AgNO3
AgOTf
Ag2CO3
CH2Cl2(24h)
CH2Cl2(24h)
Cu(OAc)2
Unless otherwise noted, all reactions were performed with 1a
(0.2 mmol) with 2a (1.5 equiv) in solvent (2 mL) at room temperature
under air atmosphere
nr no reaction
aYield of the isolated product
bThe product is racemate
123

Chem. Pap.
amines 2 to give an intermediate O, which undergoes
protonation to generate the desired furan products 3 or 4.
Table 3 Silver-catalyzed cascade reactions of ketone 1a and various
aryl-amines 2
O
Ph
Ph
Ar1
N
O
5 mol% AgNO3
CH2Cl2
H
N
+
Ph
R4
R4
Ar1
Conclusion
Ph
Ph
Ph
1a
2
3
In summary, we have described an efficient method for the
synthesis of trisubstituted furans with an arylamino-group
function via silver-catalyzed cascade heterocyclization of
2-(1-alkynyl)-2-alken-1-ones with aryl-amines. The N–H
bond will be activated when primary amine and secondary
amine are used as nucleophile, while C–H bond of benzene
ring is activated when tertiary amine is used. The reaction
showed moderate to excellent yields under mild conditions.
Further studies into the scope and mechanism of this
reaction are being conducted in our laboratory.
Entry
2
Ar1
R4
H
3b
Yieldc (%)
1
2
3
4
5
2b
2c
2d
2e
2f
p-Me-C6H4
p-Cl-C6H4
o-F-C6H4
Ph
3ab
3ac
3ad
3ae
3af
71d
85d
70
H
Me
Ph
Ph
56
p-Me-C6H4
72c
Unless otherwise noted, all reactions were carried out with 1a
(0.2 mmol), 1.5 equiv. 2 and 0.05 equiv. of AgNO3 in 2 mL CH2Cl2
at room temperature under air atmosphere for 1 h
aThe products are racemates
bYield of the isolated product
cReactions run at 60 °C for 2 h
Experimental
General remarks
Column chromatography was carried out on silica gel.
Unless noted, 1H NMR spectra were recorded on 400 MHz
and 13C NMR spectra were recorded on 100 MHz in
CDCl3 using TMS as internal standard. All new com-
pounds were further characterized by (ESI)HRMS, HRMS
was obtained using a Q-TOF instrument equipped with
Table 4 Silver-catalyzed cascade reactions of ketone 1a and various
tertiary amines 2
R4
O
Ph
Ph
R4
N
5% AgNO3
CH2Cl2
R5
O
+
N
Ph
R5
R6
R6
Ph
Ph
Ph
1a
2
1
APCI. Copies of their H NMR and 13C NMR spectra are
4
R4
Me
Et
R5
Me
Et
R6
H
4b
Yieldc (%)
provided. Commercially available reagents and solvents
were used without further purification. THF was distilled
from Na/benzophenone and used immediately.
Entry
2
1
2
3
2g
2h
2i
4ag
4ah
4ai
92
68
H
Me
Me
m-Me
86d
Preparation of (E)-2-benzylidene-1,4-diphenylbut-3-
yn-1-ol
Unless otherwise noted, all reactions were carried out with 1a
(0.2 mmol), 1.5 equiv. 2 and 0.05 equiv. of AgNO3 in 2 mL CH2Cl2
at room temperature under air atmosphere for 1 h
In a three-necked round flask, 1 mmol (E)-2-benzylidene-
4-phenylbut-3-ynal was dissolved in 20 mL THF under N2.
Then, 1.5 mmol Grignard reagent (phenylmagnesium bro-
mide) in THF was added via syringe, the mixture was
stirred at room temperature for 15 min, and monitored by
TLC. The mixture was quenched by adding NH4Cl solu-
tion. The product was extracted with ethyl acetate, and the
combined extracts were evaporated in vacuo. The product
was isolated by the chromatography on silica gel.
aThe products are racemates
bYield of the isolated product
cReactions run at rt for 4 h
trimethylaniline could also give the desired product 4ah
and 4ai in moderate to excellent yields (entries 2–3).
With the above experimental results in hand, a plausible
mechanism is proposed (Scheme 1). In this process, the
silver-coordinated alkyne M is attacked by the oxygen of
the carbonyl group to form an oxonium ion intermediate,
which isomerizes to a furan carbocation N subsequently.
This would be followed by the nucleophilic attack by
Preparation of (E)-2-benzylidene-1,4-diphenylbut-3-
yn-1-one 1a
1 mmol (E)-2-benzylidene-1,4-diphenylbut-3-yn-1-ol was
dissolved in 100 mL of THF under stirring, then 50 mmol
123

Chem. Pap.
Scheme 1 Proposed
mechanism
Acknowledgements The authors gratefully acknowledge the
National Natural Science Foundation of China (NSFC 20967006) and
Qinghai Normal University (15RZ12) for financial support.
of MnO2 was added. The mixture was stirred at room
temperature for 24 h and monitored by TLC. The mixture
was filtered and evaporated in vacuo. The residue was
purified by chromatography on silica gel to obtain (E)-2-
benzylidene-1,4-diphenylbut-3-yn-1-one 1a.
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