Copper Catalyzed Enantioselective Alkylation of Pyrrole with β,γ-Unsaturated α-Ketoesters: Application to One-Pot Construction of the Seven-Membered Ring by Merging a Gold Catalysis

Article abstract of DOI:10.1021/acs.orglett.5b01917

A highly enantioselective Friedel-Crafts alkylation of pyrrole to β,γ-unsaturated α-ketoesters was developed by virtue of a chiral copper complex, affording the alkylated derivatives of pyrrole with good yields and excellent enantioselectivities. Moreover, merging copper catalysis with gold catalysis realized a one-pot construction of the seven-membered ring to give annulated pyrroles with moderate to good yields and high enantiomeric excesses.

Full text of DOI:10.1021/acs.orglett.5b01917

Letter
pubs.acs.org/OrgLett
Copper Catalyzed Enantioselective Alkylation of Pyrrole with β,γ-
Unsaturated α‑Ketoesters: Application to One-Pot Construction of
the Seven-Membered Ring by Merging a Gold Catalysis
Yanbin Hu, Yanan Li, Sheng Zhang, Chong Li, Lijun Li, Zhenggen Zha, and Zhiyong Wang*
Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Soft Matter Chemistry and Department of
Chemistry & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of
China, Hefei, Anhui 230026, P. R. China
S
* Supporting Information
ABSTRACT: A highly enantioselective Friedel−Crafts alkylation of pyrrole to β,γ-unsaturated α-ketoesters was developed by
virtue of a chiral copper complex, affording the alkylated derivatives of pyrrole with good yields and excellent enantioselectivities.
Moreover, merging copper catalysis with gold catalysis realized a one-pot construction of the seven-membered ring to give
annulated pyrroles with moderate to good yields and high enantiomeric excesses.
yrrole and derivatives of pyrrole are prevalent in many
natural products, pharmaceuticals, and biologically active
Scheme 1. Prior Work and This Work on the Friedel−Crafts
P
Alkylation and Sequential Annulation
molecules.1 The Friedel−Crafts reaction as an important tool to
build C−C bond is of considerable interest for chemists.2 In the
past decades, many asymmetric Friedel−Crafts reactions of
pyrrole with various electrophiles were developed.3 Meanwhile,
β,γ-unsaturated α-ketoesters as the versatile molecules came into
sight.4 Recently, the Unaleroglu group designed homochiral
pyrroles to access alkylated products with β,γ-unsaturated α-
ketoesters via metal triflates.5a Then, unprotected pyrrole was
employed in the same process to give the corresponding
products that could be cyclized by itself via heating.5b In 2011, an
asymmetric version of the Friedel−Crafts alkylation of N-
methylated pyrrole to β,γ-unsaturated α-ketoesters was
described by the Mikami group.5c High yields and good to
excellent ee’s were achieved. Because of the high reactivity of the
very electron-rich pyrrole1a and multifunctionality of β,γ-
unsaturated α-ketoesters,4a the reaction of unprotected pyrrole
to β,γ-unsaturated α-ketoesters is always challenging. The Fu
group developed the Cu-BOX catalyst bearing the hetero-
arylidene skeleton to facilitate the Friedel−Crafts alkylation of
pyrrole to β,γ-unsaturated α-ketoesters (Scheme 1).5d The
transformations were highly enantioselective and efficient. Very
recently, the Feng group developed the Friedel−Crafts C3-
alkylation of C2-sealed pyrrole to β,γ-unsaturated α-ketoesters
through the Ni-N,N′-dioxide complex.5e Good yields and
enantioselectivities were obtained. And yet, it is necessary to
develop new catalytic systems to promote the important
transformation. More importantly, a one-pot dual catalytic
system has recently gained considerable interest due to the fact
that the products were not acquired using either of the catalysts
alone.6
Received: July 4, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01917
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Herein, we employed a facile Cu-complex to catalyze the
Friedel−Crafts alkylation of pyrrole to β,γ-unsaturated α-
ketoesters. The reaction can be carried out smoothly with a
low catalytic loading, and good yields and excellent enantiomeric
excesses were acquired. Moreover, we installed the alkynyl group
to the ortho group of aryl group of 2-oxo-4-arylbut-3-enoates
since the molecules with medium-membered ring are very
important in the pharmaceutical chemistry.7 As a result, 7-endo-
dig annulation was achieved to give seven-membered ring
derivatives of pyrrole via a sequential gold catalysis.
conditions. The absolute configuration was assigned by
comparison with the reported data.5d
With the optimal conditions in hand, the substrate scope of
β,γ-unsaturated α-ketoesters for the Friedel−Crafts alkylation
was explored (Table 2). First, the electronic effect was examined
a
Table 2. Scope of β,γ-Unsaturated α-Ketoesters
Initially, (E)-isopropyl 2-oxo-4-phenylbut-3-enoate was se-
lected as the model substrate to conduct the Friedel−Crafts
alkylation. Based on our previous efforts on β,γ-unsaturated α-
ketoesters, the chiral Cu-prolinol derivative complex was a
suitable catalyst to deliver the chiral information through
chelating with the double carbonyl group of the molecule.8
The reaction solvent was first screened by using the Cu-prolinol
derivative complex as the catalyst (Table 1). It was found that
b
c
entry
R1, R2 (2)
Ph, i-Pr (2a)
3
time
yield /%
ee /%
1
3a
3b
3c
3d
3e
3f
0.3
0.9
24
89
73
63
79
84
79
96
86
82
72
84
85
84
66
66
87
80
80
70
98
96
94
97
91
94
96
94
98
89
98
96
96
94
94
98
94
95
95
2
p-MeC6H5, i-Pr (2b)
p-MeOC6H5, i-Pr (2c)
p-FC6H5, i-Pr (2d)
p-ClC6H5, i-Pr (2e)
p-BrC6H5, i-Pr (2f)
p-NO2C6H5, i-Pr (2g)
p-CF3C6H5, i-Pr (2h)
m-ClC6H5, i-Pr (2i)
o-ClC6H5, Et (2j)
m-BrC6H5, i-Pr (2k)
o-BrC6H5, Et (2l)
2-naphthyl, i-Pr (2m)
2-thienyl, i-Pr (2n)
phenylpropyl, Et (2o)
Ph, Me (2p)
3
4
1.0
1.3
1.3
5.3
0.5
0.5
0.3
0.5
1.0
4.3
0.5
0.7
0.3
0.7
3.5
1.0
5
6
7
3g
3h
3i
a
Table 1. Optimization of the Friedel−Crafts Alkylation
8
9
10
11
12
13
14
15
16
17
18
19
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
b
c
entry
x
solvent
yield /%
ee /%
1
2
3
4
5
6
10
10
10
10
10
10
10
5
toluene
CHCl3
MTBE
MeOH
EtOH
11
12
34
45
43
41
79
82
84
89
28
41
54
78
93
96
96
96
96
98
Ph, Et (2q)
Ph, Bn (2r)
Ph, t-Bu (2s)
a
Unless otherwise noted, all reactions were performed with 1 (1.2
mmol), 2 (0.4 mmol), L* (1 mol %), Et3N (1 mol %), and Cu(OTf)2
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
b
c
d
(1 mol %) in i-PrOH (2.0 mL) at 0 °C. Isolated yield. Determined
7
d
by chiral HPLC analysis.
8
d
9
2
d
by varying the para-substituents of R1. As for the electron-
donating groups, good yields and excellent enantiomeric excesses
were obtained (entries 2−3). For the electron-withdrawing
groups, such as fluoro-, chloro-, and bromo- groups, the reaction
can be carried out smoothly, and these groups were also tolerated
well (entries 4−6). Good yields and enantioselectivities were
obtained when the intense electron-withdrawing groups were
installed on the para position of R1 (entries 7−8). These results
indicated that electron-withdrawing groups could give better
yields than the electron-donating ones. On the other hand, the
position variation of substituents of R1 showed little influence on
the alkylations (entries 9−12). Moreover, the 2-naphthyl group
and heterocyclic group were compatible with the catalytic system
(entries 13−14). Significantly, the aliphatic substrate 2o
proceeded smoothly to afford the corresponding product 3o
with a moderate yield and excellent enantioselectivity (entry 15).
Then, different ester groups R2 were tested, which demonstrated
that there is little influence on the alkylation in terms of the yields
and enantioselectivities (entries 15−19). Most of these trans-
formations were carried out rapidly and finished in 1 h. However,
the reactions of some substrates required a long time due to the
substrate solubility in i-PrOH (entries 3, 7, 13, 18).
10
1
a
Unless otherwise noted, all reactions were performed with 1 (0.3
mmol), 2a (0.1 mmol), L* (x mol %), base (x mol %), and Cu(OTf)2
b
(x mol %) in the solvent (1.0 mL) at 0 °C for 5−12 h. Isolated yield.
c
d
Determined by chiral HPLC analysis. Reaction time was 0.3 h. Tf =
trifluoromethanesulfonyl.
protonic solvent methanol is able to give a better yield and ee than
nonprotonic solvents (entries 1−4). Then, other protonic
solvents were tested for the Friedel−Crafts alkylation. Isopropyl
alcohol showed a preferable result to give 3a with 41% yield and
93% ee (entries 5−6).
Encouraged by these results, the reaction conditions were
further optimized. It was found that the reaction time was also
crucial to the transformation. Long reaction times resulted in a
decrease of the product 3a gradually. However, no other reaction
was observed except for the decomposition of the product when
the reaction time was prolonged. The reaction yield was raised to
79% when the reaction time was 0.3 h (entry 7). The great
catalytic efficiency reminded us of lowering the loading of the
catalyst. 1% of the catalyst could also make the reaction working
smoothly within 0.3 h to furnish 3a with 89% yield and 98% ee
(entries 8−10). However, <1 mol % of the catalytic loading could
not efficiently catalyze the process. Therefore, the reaction
conditions of entry 10 in Table 1 were chosen as the optimal
As a result, the asymmetric alkylation of pyrrole with the β,γ-
unsaturated α-ketoesters was developed. After that, we took into
consideration the construction of the seven-membered ring on
the basis of the developed alkylation. In recent decades, the
enantioselective synthesis of the one-pot dual catalytic system
B
DOI: 10.1021/acs.orglett.5b01917
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
experienced rapid growth because the complexity of the
molecules can be acquired via cooperative/relay/sequential
catalysis.9 Recently, the Enders group reported the synthesis of
annulated indoles and pyrroles through merging organocatalysis
and gold catalysis, realizing the preparation of challenging seven-
membered ring molecules.10 Inspired by these research results,
we wondered whether the sequential gold catalysis could render
an annulation via an attack of pyrrole to the alkynyl group in the
ortho group of R1 to fuse a seven-membered ring. (E)-Ethyl 4-(5-
chloro-2-(phenylethynyl)phenyl)-2-oxobut-3-enoate 9d was
employed to examine this sequential annulation. As expected,
the first alkylation proceeded smoothly to afford the product 10
in 98% yield and 96% ee via the copper catalysis (eq 1).
7). After screening the different phosphine ligands (entries 8−
11), the catalytic system of the Au(I) complex with phosphine 5
and AgNTf2 was found to be the best catalyst for the
transformation (entry 8).
With the optimized catalyst in hand, the scope of β,γ-
unsaturated α-ketoesters with various substituents was explored
to examine the compatibility of the dual-metal catalytic system
(Scheme 2). To our delight, a variety of substituents of 9 could
Scheme 2. Scope of the Sequential Friedel−Crafts Alkylation/
a
Annulation
The sequential annulation to access the seven-membered ring
11a was then optimized by screening the various catalysts (Table
3). The substrate 9a was converted to another intramolecular
a
Table 3. Optimization of Gold-Catalyzed Annulation
a
Unless otherwise noted, all reactions were performed with 9 (0.3
mmol), 1 (0.9 mmol) with 1 mol % copper catalyst at 0 °C in i-PrOH
b
for the first step. Then to the residue after evaporation of solvent was
entry
catalyst
AuCl3
time/h
yield /%
b
c
added 10% gold catalyst at rt in i-PrOH (0.1 M). The reaction
1
3.0
>24
3.0
>24
0.3
0.3
0.3
0.3
0.3
0.3
0.3
−
−
−
−
62
81
75
86
73
84
66
temperature of the first step was at rt.
2
AgNTf2
c
3
AgNO3
4
Cu(OTf)2
4/AgOTf
4/AgNTf2
4/AgSbF6
5/AgNTf2
6/AgNTf2
7/AgNTf2
8/AgNTf2
give good to excellent enantioselectivities. However, the
electronic effect showed that there was an influence on the
yields of the annulations. The yields of 9b and 9d bearing
electron-withdrawing substituents were obviously higher than
those of 9c bearing an electron-donating substituent and 9a
bearing an electron-neutral substituent. However, the variation
of aryl R5 had little influence on the reaction. Moreover, aliphatic
R5 was also compatible with the sequential alkylation/annulation
to afford 11k with a good ee value in spite of the moderate yield.
Nevertheless, the reaction rates of these substrates slowed down
due to the solubility in i-PrOH (9b, 9e, 9f, 9i, 9j). The reaction
temperature of these substrates was enhanced to room
temperature from 0 °C in order to increase the reaction rate.
Finally, we developed a one-pot alkylation/annulation of pyrrole
with β,γ-unsaturated α-ketoesters to afford the chiral seven-
membered ring derivatives with moderate to good yields and
excellent ee values.
5
6
7
8
9
10
11
a
Unless otherwise noted, all reactions were performed with 9a (0.1
mmol), and the catalyst (10 mol %) in i-PrOH (1.0 mL) at room
temperature. Isolated yield. Another intramolecular addition product
was detected from the 1-N position of pyrrole to the carbonyl group of
keto ester.
b
c
addition product from the N-1 position to the carbonyl group of
keto ester when AuCl3 and AgNO3 were employed respectively
(entries 1, 3). AgNTf2 alone could not provide any desired
product either (entry 2). A copper catalyst did not work in this
annulation (entry 4). Finally, it was found that the combination
of the gold catalyst 4 with AgNTf2 could give rise to the desired
seven-membered ring with a good yield and ee value (entries 5−
The mechanism of the sequential reaction is listed in the
Supporting Information. At first, the nucleophilic addition of the
C2 pyrrole to 9 furnished the asymmetric alkylated product. The
chiral information was transferred through coordination of the
C
DOI: 10.1021/acs.orglett.5b01917
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
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(b) Wasilke, J. C.; Obrey, S. J.; Baker, R. T.; Bazan, G. C. Chem. Rev.
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The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01917.
Experimental procedures, characterization data, copies of
1H NMR, 13C NMR of new compounds, HPLC profiles
(PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: zwang3@ustc.edu.cn.
Notes
(7) (a) Majumdar, K. C.; Samanta, S.; Sinha, B. Synthesis 2012, 44,
817−847. (b) Nguyen, T. V.; Hartmann, J. M.; Enders, D. Synthesis
2013, 45, 845−873.
The authors declare no competing financial interest.
(8) (a) Zhang, S.; Xu, K.; Guo, F.; Hu, Y.; Zha, Z.; Wang, Z. Chem. -
Eur. J. 2014, 20, 979−982. (b) Hu, Y.; Xu, K.; Zhang, S.; Guo, F.; Zha, Z.;
Wang, Z. Org. Lett. 2014, 16, 3564−3567.
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ACKNOWLEDGMENTS
■
Financial support from the National Nature Science Foundation
of China (21272222, 91213303, 21472177, 21432009) is greatly
acknowledged.
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DOI: 10.1021/acs.orglett.5b01917
Org. Lett. XXXX, XXX, XXX−XXX