Synthesis of tetrasubstituted NH pyrroles and polysubstituted furans via an addition and cyclization strategy

Article abstract of DOI:10.1055/s-0031-1289993

The FeCl₃-catalyzed addition and cyclization of enamino esters with nitroolefins provides a straightforward and general method for the synthesis of tetrasubstituted NH pyrroles. This novel method tolerates a wide range of functionality, and allows for rapid elaboration of the nitroolefins into a variety of substituted pyrroles in good yields. Further, an efficient KOAc-promoted addition and cyclization protocol toward substituted furans has been described as well. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1289993

532
PAPER
Synthesis of Tetrasubstituted NH Pyrroles and Polysubstituted Furans via an
Addition and Cyclization Strategy
T
etrasubstituted
i
N
H
P
y
a
rroles and
nPolysubstitug
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education,
Department of Chemistry & Materials Science, Northwest University, Xi’an 710069, P. R. of China
E-mail: guanzhh@nwu.edu.cn
Received 5 October 2011; revised 19 November 2011
cient methods for the synthesis of substituted NH pyrroles
by using readily available enamino esters and nitroolefins
as the starting materials.
Abstract: The FeCl3-catalyzed addition and cyclization of enamino
esters with nitroolefins provides a straightforward and general
method for the synthesis of tetrasubstituted NH pyrroles. This novel
method tolerates a wide range of functionality, and allows for rapid
elaboration of the nitroolefins into a variety of substituted pyrroles
in good yields. Further, an efficient KOAc-promoted addition and
cyclization protocol toward substituted furans has been described as
well.
The study began with the addition and cyclization reaction
of enamino ester 2a with nitroolefin 1a in MeOH in the
absence of catalyst. The desired NH pyrrole 3a (see
Figure 1 for the X-ray structure) was observed in low
yield (Table 1, entry 1). Although glycol proved to be the
most effective solvent, the NH pyrrole 3a was only ob-
tained in moderate yield (Table 1, entry 3). Thus, various
potential catalysts, such as CuI, FeCl3, FeCl2, PTSA, I2,
Ph3P, were screened in order to improve the efficiency of
the transformation (Table 1, entries 5–10). To our delight,
FeCl3 was shown to be an active catalyst in the reaction,
and the desired NH pyrrole 3a was obtained in 80% yield
(Table 1, entry 6). FeCl2 and PTSA were less effective
(Table 1, entries 7, 8).
Key words: tetrasubstituted NH pyrroles, polysubstituted furans,
addition, cyclization
Pyrroles have a privileged role in organic chemistry.
Highly substituted pyrroles are one of the important class-
es of structural unit frequently found in many natural
products1 and pharmaceuticals.2 Furthermore, many of the
substituted pyrroles have been widely used in material
science3 and supramolecular chemistry.4 As a conse-
quence, polysubstituted pyrroles have received consider-
able attention over the years and various methods for their
preparation have been developed.5
In the past several years, transition-metal-catalyzed
cyclizations6 and multicomponent coupling reactions7 for
the preparation of substituted pyrroles have been report-
ed.8 Transition metals, such as palladium,9 copper,10
gold,11 silver,12 and ruthenium13 were shown to be active
catalysts in pyrrole ring formation. However, iron-cata-
lyzed cyclization reactions14 for the construction of pyr-
role rings has rarely been reported. Recently, iron salts
have been extensively investigated as alternative and
promising catalysts for many organic transformations due
to their low price and environmentally friendly charac-
ter.15 In connection with our recent work on the synthesis
of multisubstituted pyrroles, we wish to report herein an
FeCl3-catalyzed polysubstituted NH pyrrole ring forma-
tion reaction.
Figure 1 X-ray structure of 3a
With the optimized conditions in hand, we have explored
the substrates scope. The FeCl3-catalyzed addition and cy-
clization reaction of enamino ester 2a with nitroolefins
1a–j are summarized in Table 2. These transformations
displayed high functional group tolerance. Various ni-
troolefins with methyl, methoxy, amino, chloro, and fluo-
ro groups on the arene rings proceeded smoothly to give
the corresponding pyrroles 3a–h in good yields (Table 2,
entries 1–8). In general, the reaction was insensitive to the
electronic effects of the aromatic substituents on the ni-
troolefins. A lower yield was observed for the addition
and cyclization of enamino ester 2a with (Z)-2-(2-nitro-
prop-1-enyl)furan (1i) due to the partial decomposition of
the nitroolefin 1i in the presence of the FeCl3 catalyst
Recently, we have developed an addition and cyclization
reaction of enamino esters to nonhalogenated nitroolefins
for the synthesis of fully substituted pyrroles.16 Although
this metal-free method showed good functional group tol-
erance, it was not suitable for the synthesis of NH pyr-
roles. Due to the importance of the NH pyrroles in organic
chemistry,17 our efforts were directed in exploring effi-
SYNTHESIS 2012, 44, 532–540
x
x
.x
x
.2
0
1
1
Advanced online publication: 22.12.2011
DOI: 10.1055/s-0031-1289993; Art ID: H95311SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Tetrasubstituted NH Pyrroles and Polysubstituted Furans
533
(Table 2, entry 9). Similarly, 5H-pyrrole 3j was obtained
in moderate yield (Table 2, entry 10). In addition, the sub-
strate derived from 3-oxo-N-phenylbutanamide under-
went the desired reaction to give the corresponding
product 3k in good yield (Table 2, entry 11). We have also
extended the reaction scope to (Z)-4-aminopent-3-en-2-
one. The desired pyrrole product 3l was obtained under
the conditions albeit in low yields (Table 2, entry 12).
Table 1 Addition and Cyclization of Enamino Ester to Nitroolefin
for Synthesis of NH Pyrrolea
O
NH2
O
NO2
catalyst
solvent
MeO
+
OMe
N
H
1a
2a
3a
Entry
Catalyst
–
Solvent
MeOH
DMSO
glycol
MeCN
glycol
glycol
glycol
glycol
glycol
glycol
Time (h)
Yield (%)
The plausible mechanism for the reaction is shown in
Scheme 1. Enamines 2 and nitroolefin 1 undergo Michael
reaction to form an intermediate B.14a Then FeCl3 inter-
acts with intermediate B to generate an electron-deficient
complex C,15a,c which undergoes intramolecular electro-
philic cyclization and elimination reaction to gave the de-
sired NH pyrrole 3.14a
1
2
5
5
50
–
n.r.b
63
3
–
5
4
–
5
n.r.b
50c
80d
72e
60f
63g
55h
5
CuI
FeCl3
FeCl2
PTSA
I2
15
4
R
NH2
O
R
6
O
O
R
2
FeCl3
7
5
NO2
NH2
HO
NH2
HO
8
8
FeCl3
N
N
O
O
9
15
15
1
C
B
10
Ph3P
O
O
Ph
a Reaction conditions: 1a (0.5 mmol) and 2a (0.75 mmol) in solvent
Ph
R
R
H
– HNO
– H2O
(3 mL) at 100 °C.
b n.r.: no reaction.
c CuI (10 mol%).
d FeCl3 (10 mol%).
e FeCl2 (10 mol%).
f PTSA (20 mol%).
g I2 (20 mol%).
[FeCl3]–
N
O
N
N
H
H
OH
D
3
Scheme 1 Proposed mechanism for the reaction
h Ph3P (20 mol%).
Table 2 FeCl3-Catalyzed Addition and Cyclization of Enamino Ester 2a to Nitroolefins 1a
R
O
NH2
O
NO2
FeCl3 (10 mol%)
glycol, 100 °C
MeO
+
R
OMe
N
H
1a–j
2a
3a–j
Entry
1
1
Pyrrole
Time (h)
Yield (%)
80
O
NO2
1a
1b
1c
3a
4
6
6
MeO
N
H
O
O
NO2
2
3
3b
3c
79
75
MeO
MeO
N
H
NO2
N
H
© Thieme Stuttgart · New York
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534
L. Li et al.
PAPER
Table 2 FeCl3-Catalyzed Addition and Cyclization of Enamino Ester 2a to Nitroolefins 1a (continued)
R
O
NH2
O
NO2
FeCl3 (10 mol%)
glycol, 100 °C
MeO
+
R
OMe
N
H
1a–j
2a
3a–j
Entry
1
Pyrrole
Time (h)
8
Yield (%)
74
OMe
O
NO2
4
1d
1e
3d
MeO
MeO
N
H
Me
N
Me
NO2
O
O
5
3e
10
53
Me
N
MeO
MeO
Me
N
H
Cl
NO2
6
7
1f
3f
10
10
60
65
Cl
N
H
Cl
Cl
O
NO2
1g
3g
MeO
Cl
Cl
N
H
F
O
O
NO2
8
1h
3h
6
77
MeO
MeO
F
N
H
NO2
O
O
9
1i
1j
3i
3j
8
5
37
42
N
H
O
NO2
NO2
NO2
10
MeO
H
H
N
H
O
11
12
1a
1a
3k
3l
4
4
70b
30c
NH
N
H
O
N
H
a Reaction conditions: 1 (0.5 mmol), 2a (0.75 mmol), and FeCl3 (10 mol%) in glycol (3 mL) at 100 °C.
b Compound 1a (0.5 mmol), (Z)-3-amino-N-phenylbut-2-enamide (0.75 mmol), and FeCl3 (10 mol%) in glycol (3 mL) at 100 °C.
c Compound 1a (0.5 mmol) and (Z)-4-aminopent-3-en-2-one (0.75 mmol) in glycol–H2O (2:1, 3 mL) at 140 °C.
Synthesis 2012, 44, 532–540
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Tetrasubstituted NH Pyrroles and Polysubstituted Furans
535
Although a number of methods have been developed for ly available starting materials and tolerate a wide range of
the construction of furan rings,18,19 a facile and general functionality. Thus, a straightforward approach to substi-
synthetic method still remains an attractive goal. Follow- tuted NH pyrroles and furans has been developed.
ing the success of substituted pyrroles construction, our
attention was turned to furan ring formation by the same
1
Column chromatography was carried out on silica gel. H NMR
spectra were recorded on 400 MHz in CDCl3 and 13C NMR spectra
were recorded on 100 MHz in CDCl3. Unless otherwise stated, all
reagents and solvents were purchased from commercial suppliers
and used without further purification. The nitroolefins 1 were pre-
pared according to the literature.20
strategy. Upon screening various bases, catalysts, sol-
vents, and temperature, it was found that the combination
of the nitroolefin 1a with pentane-2,4-dione (2b) or b-keto
ester 2c in the presence of KOAc in MeCN at 120 °C gave
the best result (Table 3). This facile and efficient method
shows a promising approach to substituted furans.
Tetrasubstituted NH Pyrroles 3; General Procedure
A vial (25 mL) was charged with nitroolefin 1 (0.5 mmol), methyl
(Z)-3-aminobut-2-enoate (2a; 86 mg, 0.75 mmol), and glycol (3
mL) and the mixture was stirred at 100 °C. After completion of the
reaction (detected by TLC, eluent: hexane–EtOAc, 8:1), the reac-
tion mixture was cooled to r.t. EtOAc (30 mL) was added to the
mixture and then washed with brine (2 × 30 mL). The organic layer
was concentrated in vacuo. The residue was purified by chromatog-
raphy on silica gel with hexane–EtOAc–Et3N (50:5:0.1) as the elu-
ent to afford 3 (Table 2).
Table 3 Addition and Cyclization Reaction for the Synthesis of
Substituted Furana
O
NO2
O
O
base
+
solvent
O
1a
4a
2b
Entry
Base
–
Solvent
Time (h) Yield (%)
Methyl 2,5-Dimethyl-4-phenyl-1H-pyrrole-3-carboxylate (3a)
Yield: 92.3 mg (80%); yellow solid; mp 165–166 °C.
1H NMR (400 MHz, CDCl3): d = 8.35 (s, 1 H), 7.34–7.30 (m, 2 H),
7.25–7.21 (m, 3 H), 3.61 (s, 3 H), 2.45 (s, 3 H), 2.06 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 169.8, 139.4, 137.5, 133.6, 130.7,
129.2, 127.0, 125.7, 113.3, 53.7, 17.0, 14.5.
HRMS (ESI): m/z calcd for C14H15NO2 + Na (M + Na+): 252.0995;
found: 252.0996.
1
2
glycol
glycol
glycol
THF
7
7
7
7
7
7
6
6
6
6
n.r.b
44b
60
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
NaOAc
K3PO4
Et3N
3
4
64
5
DMSO
H2O
43
Methyl 2,5-Dimethyl-4-p-tolyl-1H-pyrrole-3-carboxylate (3b)
Yield: 96.6 mg (79%); yellow solid; mp 164–165 °C.
1H NMR (400 MHz, CDCl3): d = 8.17 (s, 1 H), 7.13 (s, 4), 3.62 (s,
3 H), 2.46 (s, 3 H), 2.34 (s, 3 H), 2.07 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 166.3, 135.3, 133.9, 132.9, 130.1,
128.1, 123.5, 122.3, 110.1, 50.4, 21.2, 13.7, 11.1.
HRMS (ESI): m/z calcd for C15H17NO2 + Na (M + Na+): 266.1151;
6
49
7
MeCN
MeCN
MeCN
MeCN
75
8
29
9
26
10
46
found: 266.1153.
a Reaction conditions: 1a (0.45 mmol) and 2b (0.3 mmol), base (0.6
mmol), in solvent (2 mL) at 120 °C.
Methyl 2,5-Dimethyl-4-m-tolyl-1H-pyrrole-3-carboxylate (3c)
Yield: 92.2 mg (75%); yellow solid; mp 118–120 °C.
b FeCl3 (10 mol%) was used; n.r. = no reaction.
1H NMR (400 MHz, CDCl3): d = 8.30 (s, 1 H), 7.24–7.19 (t, J = 8.0
Under the optimized conditions, the scope of the KOAc- Hz, 1 H), 7.05–7.03 (m, 3 H), 3.62 (s, 3 H), 2.45 (s, 3 H), 2.34 (s, 3
H), 2.06 (s, 3 H).
promoted addition and cyclization reaction was investi-
13C NMR (100 MHz, CDCl3): d = 166.7, 137.0, 136.2, 134.4, 131.2,
gated (Table 4). This general method tolerates a wide
range of functionality, and allows for rapid elaboration of
the nitroolefins into a variety of substituted furans in good
yields. For the electronic effects of the transformation, the
electron-rich nitroolefins showed better reactivity and
gave slightly higher yields than electron-deficient ones
(Table 4, entries 3–16). Further, 4-furanyl substituted
furans were achieved when 2-(2-nitroprop-1-enyl)furan
(1i) was used as the substrate (Table 4, entries 17, 18).
127.7, 127.5, 126.9, 123.9, 122.6, 110.3, 50.7, 21.8, 13.9, 11.5.
HRMS (ESI): m/z calcd for C15H17NO2 + Na (M + Na+): 266.1151;
found: 266.1154.
Methyl 4-(4-Methoxyphenyl)-2,5-dimethyl-1H-pyrrole-3-car-
boxylate (3d)
Yield: 96.4 mg (74%); yellow solid; mp 143–145 °C.
1H NMR (400 MHz, CDCl3): d = 8.34 (s, 1 H), 7.17–7.14 (d, J = 8.4
Hz, 2 H), 6.88–6.86 (d, J = 8.8 Hz, 2 H), 3.79 (s, 3 H), 3.62 (s, 3 H),
2.44 (s, 3 H), 2.04 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 169.7, 161.0, 137.3, 134.6, 131.7,
126.9, 125.2, 116.2, 113.3, 58.5, 53.7, 17.1, 14.4.
HRMS (ESI): m/z calcd for C15H17NO3 + Na (M + Na+): 282.1101;
found: 282.1099.
In summary, we have developed a general protocol for
FeCl3-catalyzed addition and cyclization of enamino ester
and nitroolefins for the construction of tetrasubstituted
NH pyrroles. This attractive addition and cyclization strat-
egy is also suitable for substituted furans formation in the
presence of KOAc. These general protocols employ readi-
© Thieme Stuttgart · New York
Synthesis 2012, 44, 532–540

536
L. Li et al.
PAPER
Table 4 Synthesis of Polysubstituted Furans by KOAc-Promoted Addition and Cyclization Reaction of 2b,c with Nitroolefins 1a
R1
O
NO2
R2
O
O
KOAc
MeCN
R1
+
R2
O
1a–r
2b,c
4a–r
Entry
1
Furan 4
R2
Time (h)
Yield (%)
O
NO2
1
2
4a Me
4b OEt
5
7
75
R2
1a
74b
O
O
NO2
3
4
4c Me
4d OEt
6
7
75
1b
1c
R2
70b
O
NO2
O
5
6
4e Me
4f OEt
6
7
74
R2
70b
O
OMe
NO2
O
7
8
4g Me
4h OEt
5
7
75
1d
1e
68b
R2
MeO
O
Me
N
Me
NO2
9
10
4i Me
4j OEt
5
24
58
O
58b
Me
R2
N
Me
O
Cl
O
11
12
4k Me
4l OEt
6
7
67
1f
R2
NO2
64b
Cl
O
Cl
Cl
O
13
14
4m Me
4n OEt
6
7
60
NO2
1g
R2
56b
Cl
Cl
O
F
NO2
O
15
16
4o Me
4p OEt
5
7
66
1h
1i
67b
R2
F
O
O
17
18
4q Me
4r OEt
5
11
45
NO2
O
R2
O
61b
O
a Reaction conditions: 1 (0.45 mmol), 2b (0.3 mmol), and KOAc (0.6 mmol) in MeCN (2 mL) at 120 °C.
b Compound 1 (0.3 mmol) and 2c (0.6 mmol).
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Tetrasubstituted NH Pyrroles and Polysubstituted Furans
537
Methyl 4-[4-(Dimethylamino)phenyl]-2,5-dimethyl-1H-pyr-
role-3-carboxylate (3e)
HRMS (ESI): m/z calcd for C13H13NO2 + Na (M + Na+): 238.0838;
found: 238.0838.
Yield: 71.5 mg (53%); brown solid; mp 175–176 °C.
2,5-Dimethyl-N,4-diphenyl-1H-pyrrole-3-carboxamide (3k)
Yield: 100.1 mg (70%); yellow solid; mp 178–180 °C.
1H NMR (400 MHz, CDCl3): d = 8.94 (s, 1 H), 7.46–7.43 (m, 2 H),
7.39–7.34 (m, 3 H), 7.18–7.14 (m, 2 H), 7.08–7.07 (m, 3 H), 6.97–
6.93 (m, 1 H), 2.54 (s, 3 H), 2.05 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 164.4, 138.3, 135.3, 133.5, 130.9,
128.8, 128.7, 127.3, 123.6, 123.2, 119.2, 119.1, 113.2, 13.3, 10.9.
1H NMR (400 MHz, CDCl3): d = 8.30 (s, 1 H), 7.14–7.12 (d, J = 7.6
Hz, 2 H), 6.75–6.73 (d, J = 8.4 Hz, 2 H), 3.63 (s, 3 H), 2.94 (s, 6 H),
2.44 (s, 3 H), 2.06 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 166.8, 149.1, 134.1, 131.2, 124.6,
123.6, 122.5, 112.3, 110.3, 50.7, 41.0, 14.1, 11.5.
HRMS (ESI): m/z calcd for C16H20N2O2 + Na (M + Na+): 295.1417;
found: 295.1415.
1-(2,5-Dimethyl-4-phenyl-1H-pyrrol-3-yl)ethanone (3l)
Yield: 32.2 mg (30%); brown solid; mp 118–120 °C.
1H NMR (400 MHz, CDCl3): d = 8.70 (s, 1 H), 7.39–7.36 (m, 2 H),
7.32–7.24 (m, 3 H), 2.51 (s, 3 H), 2.08 (s, 3 H), 1.92 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 197.1, 136.7, 133.6, 130.4, 128.1,
126.5, 123.5, 122.1, 121.2, 30.6, 14.1, 10.9.
Methyl 4-(2-Chlorophenyl)-2,5-dimethyl-1H-pyrrole-3-carbox-
ylate (3f)
Yield: 78.9 mg (60%); yellow solid; mp 155–156 °C.
1H NMR (400 MHz, CDCl3): d = 8.27 (s, 1 H), 7.39–7.37 (m, 1 H),
7.24–7.17 (m, 3 H), 3.56 (s, 3 H), 2.48 (s, 3 H), 1.98 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 166.3, 135.7, 135.0, 134.2, 132.5,
129.1, 128.0, 126.2, 124.4, 119.8, 111.0, 50.8, 13.9, 11.4.
HRMS (ESI): m/z calcd for C14H14ClNO2 + Na (M + Na+):
286.0605; found: 286.0602.
Substituted Furans 4; General Procedure
A 25 mL round-bottomed flask was charged with nitroolefin 1 (0.45
mmol; 0.3 mmol for 2c), KOAc (59 mg, 0.6 mmol), pentane-2,4-di-
one (2b; 30 mg, 0.3 mmol) or ethyl acetoacetate (2c; 78 mg, 0.6
mmol), and MeCN (2 mL), and the reaction mixture was stirred at
120 °C. After completion of the reaction (detected by TLC, eluent:
hexane–EtOAc, 20:1), the mixture was cooled to r.t. The reaction
mixture was diluted with EtOAc (15 mL) and then washed with
H2O (2 × 10 mL). The organic layer was concentrated in vacuo and
the residue was purified by chromatography on silica gel with hex-
ane–EtOAc (60:1) as the eluent to afford 4 (Table 4).
Methyl 4-(2,4-Dichlorophenyl)-2,5-dimethyl-1H-pyrrole-3-car-
boxylate (3g)
Yield: 96 mg (65%); yellow solid; mp 185–186 °C.
1H NMR (400 MHz, DMSO-d6): d = 11.24 (s, 1 H), 7.54 (s, 1 H),
7.34–7.32 (m, 2 H), 7.19–7.18 (m, 2 H), 3.43 (s, 3 H), 2.38 (s, 3 H),
1.90 (s, 3 H).
13C NMR (100 MHz, DMSO-d6): d = 164.9, 134.9, 134.8, 133.7,
131.4, 128.1, 126.4, 124.3, 117.2, 109.6, 50.1, 13.1, 10.8.
HRMS (ESI): m/z calcd for C14H13Cl2NO2 + Na (M + Na+):
1-(2,5-Dimethyl-4-phenylfuran-3-yl)ethanone (4a)
Yield: 48.4 mg (75%); orange oil.
320.0216; found: 320.0207.
1H NMR (400 MHz, CDCl3): d = 7.43–7.34 (m, 3 H), 7.26–7.24 (d,
J = 6.0 Hz, 2 H), 2.54 (s, 3 H), 2.17 (s, 3 H), 1.94 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 196.1, 156.1, 146.9, 133.7, 129.8,
128.4, 127.3, 122.9, 120.8, 30.7, 14.2, 11.6.
Methyl 4-(4-Fluorophenyl)-2,5-dimethyl-1H-pyrrole-3-carbox-
ylate (3h)
Yield: 94.5 mg (77%); white solid; mp 187–189 °C.
1H NMR (400 MHz, DMSO-d6): d = 11.16 (s, 1 H), 7.15 (s, 2 H),
Ethyl 2,5-Dimethyl-4-phenylfuran-3-carboxylate (4b)
Yield: 54.5 mg (74%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.39–7.26 (m, 5 H), 4.16–4.10 (m,
2 H), 2.59 (s, 3 H), 2.21 (s, 3 H), 1.13–1.09 (t, J = 7.6 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 164.3, 157.4, 147.1, 133.2, 129.9,
127.6, 126.7, 121.3, 113.4, 59.7, 14.0, 13.9, 11.7.
7.09–7.08 (m, 2 H), 3.47 (s, 3 H), 2.38 (s, 3 H), 1.99 (s, 3 H).
13C NMR (100 MHz, DMSO-d6): d = 165.2, 161.7 (d, JC,F = 240.9
Hz), 133.8, 132.5, 131.9 (d, JC,F = 7.8 Hz), 123.7, 120.2, 114.0 (d,
JC,F = 20.5 Hz), 108.9, 49.9, 13.3, 10.9.
HRMS (ESI): m/z calcd for C14H14FNO2 + Na (M + Na+): 270.0901;
found: 270.0905.
1-(2,5-Dimethyl-4-p-tolylfuran-3-yl)ethanone (4c)
Yield: 51.5 mg (75%); yellow solid; mp 54–56 °C.
1H NMR (400 MHz, CDCl3): d = 7.22–7.20 (m, 2 H), 7.14–7.12 (m,
2 H), 2.53 (s, 3 H), 2.39 (s, 3 H), 2.16 (s, 3 H), 1.94 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 196.3, 156.0, 146.8, 136.9, 130.6,
129.7, 129.1, 122.9, 120.6, 30.7, 21.2, 14.2, 11.6.
Methyl 4-(Furan-2-yl)-2,5-dimethyl-1H-pyrrole-3-carboxylate
(3i)
Yield: 41.1 mg (37%); white solid; mp 125–127 °C.
1H NMR (400 MHz, CDCl3): d = 8.37 (s, 1 H), 7.39 (s, 1 H), 6.39
(s, 1 H), 6.31 (s, 1 H), 3.69 (s, 3 H), 2.41 (s, 3 H), 2.17 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 165.9, 149.3, 140.9, 134.3, 126.2,
111.8, 110.4, 110.1, 107.8, 50.7, 13.5, 11.7.
Ethyl 2,5-Dimethyl-4-p-tolylfuran-3-carboxylate (4d)
Yield: 54.2 mg (70%); yellow oil.
HRMS (ESI): m/z calcd for C12H13NO3 + Na (M + Na+): 242.0788;
found: 242.0793.
1H NMR (400 MHz, CDCl3): d = 7.19–7.14 (m, 4 H), 4.17–4.12 (m,
2 H), 2.57 (s, 3 H), 2.38 (s, 3 H), 2.20 (s, 3 H) 1.17–1.12 (m, 3 H).
Methyl 2-Methyl-4-phenyl-1H-pyrrole-3-carboxylate (3j)
Yield: 45.2 mg (42%); brown solid; mp 126–128 °C.
1H NMR (400 MHz, CDCl3): d = 8.52 (s, 1 H), 7.39–7.22 (m, 5 H),
6.50 (s, 1 H), 3.68 (s, 3 H), 2.49 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 166.3, 136.3, 135.7, 129.1, 127.6,
127.0, 126.2, 115.6, 109.4, 50.5, 13.9.
13C NMR (100 MHz, CDCl3): d = 164.3, 157.2, 146.9, 136.3, 130.1,
129.8, 128.3, 121.2, 113.4, 59.7, 21.2, 14.1, 13.9, 11.7.
1-(2,5-Dimethyl-4-m-tolylfuran-3-yl)ethanone (4e)
Yield: 50.3 mg (74%); yellow oil.
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1H NMR (400 MHz, CDCl3): d = 7.31–7.27 (t, J = 7.6 Hz, 1 H),
7.16–7.15 (d, J = 7.2 Hz, 1 H), 7.05–7.03 (m, 2 H), 2.53 (s, 3 H),
2.38 (s, 3 H), 2.17 (s, 3 H), 1.93 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 196.3, 156.0, 146.8, 137.9, 133.6,
130.5, 128.3, 128.0, 126.9, 122.9, 120.8, 30.7, 21.4, 14.2, 11.6.
13C NMR (100 MHz, CDCl3): d = 195.1, 156.6, 147.5, 134.7, 132.8,
131.9, 129.6, 129.2, 126.8, 122.6, 117.9, 29.8, 14.5, 11.6.
HRMS (ESI): m/z calcd for C14H13ClO2 + Na (M + Na+): 271.0496;
found: 271.0500.
Ethyl 4-(2-Chlorophenyl)-2,5-dimethylfuran-3-carboxylate (4l)
Yield: 53.5 mg (64%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.42–7.40 (m, 1 H), 7.27–7.21 (m,
3 H), 4.12–3.99 (m, 2 H), 2.59 (s, 3 H), 2.12 (s, 3 H), 1.02–0.98 (m,
3 H).
HRMS (ESI): m/z calcd for C15H16O2 + Na (M + Na+): 251.1043;
found: 251.1049.
Ethyl 2,5-Dimethyl-4-m-tolylfuran-3-carboxylate (4f)
Yield: 53.9 mg (70%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.28–7.24 (m, 1 H), 7.13–7.06 (m,
3 H), 4.17–4.11 (m, 2 H), 2.59 (s, 3 H), 2.38 (s, 3 H), 2.21 (s, 3 H),
1.14–1.10 (m, 3 H).
13C NMR (100 MHz, CDCl3): d = 164.3, 157.3, 146.9, 136.9, 133.1,
130.7, 127.5, 127.0, 121.3, 113.5, 59.6, 21.4, 14.0, 13.9, 11.7.
13C NMR (100 MHz, CDCl3): d = 164.0, 157.4, 147.4, 134.7, 132.7,
131.7, 128.9, 128.5, 126.1, 118.6, 113.9, 59.6, 13.9, 13.6, 11.7.
HRMS (ESI): m/z calcd for C15H15ClO3 + Na (M + Na+): 301.0602;
found: 301.0597.
1-[4-(2,4-Dichlorophenyl)-2,5-dimethylfuran-3-yl]ethanone
(4m)
Yield: 51.1 mg (60%); yellow oil.
HRMS (ESI): m/z calcd for C16H18O3 + Na (M + Na+): 281.1148;
found: 281.1151.
1-[4-(4-Methoxyphenyl)-2,5-dimethylfuran-3-yl]ethanone (4g)
Yield: 55.1 mg (75%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.48 (s, 1 H), 7.31–7.28 (m, 1 H),
7.19–7.18 (m, 1 H), 2.56 (s, 3 H), 2.08 (s, 3 H), 1.97 (s, 3 H).
1H NMR (400 MHz, CDCl3): d = 7.17–7.15 (d, J = 9.2 Hz, 2 H),
6.96–6.93 (d, J = 8.8 Hz, 2 H), 3.85 (s, 3 H), 2.53 (s, 3 H), 2.15 (s,
3 H), 1.95 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 196.2, 158.8, 156.0, 146.8, 130.9,
125.7, 122.9, 120.2, 113.8, 55.1, 30.7, 14.2, 11.5.
13C NMR (100 MHz, CDCl3): d = 194.6, 156.8, 147.7, 135.4, 134.3,
132.6, 131.4, 129.5, 127.2, 122.5, 117.0, 29.9, 14.5, 11.6.
HRMS (ESI): m/z calcd for C14H12Cl2O2 + Na (M + Na+): 305.0107;
found: 305.0108.
Ethyl 4-(2,4-Dichlorophenyl)-2,5-dimethylfuran-3-carboxylate
(4n)
Ethyl 4-(4-Methoxyphenyl)-2,5-dimethylfuran-3-carboxylate
(4h)
Yield: 52.5 mg (56%); yellow oil.
Yield: 56.3 mg (68%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.46–7.45 (s, 1 H), 7.27–7.25 (m,
1 H), 7.17–7.15 (m, 1 H) 4.12–4.06 (m, 2 H), 2.59 (s, 3 H), 2.12 (s,
3 H), 1.09–1.05 (m, 3 H).
13C NMR (100 MHz, CDCl3): d = 163.8, 157.6, 147.7, 135.5, 133.6,
132.5, 131.4, 128.9, 126.5, 117.7, 113.8, 59.8, 13.9, 13.8, 11.7.
1H NMR (400 MHz, CDCl3): d = 7.20–7.18 (d, J = 8.8 Hz, 2 H),
6.93–6.90 (d, J = 8.8 Hz, 2 H), 4.16–4.14 (m, 2 H), 3.84 (s, 3 H),
2.58 (s, 3 H), 2.19 (s, 3 H), 1.17–1.13 (m, 3 H).
13C NMR (100 MHz, CDCl3): d = 164.4, 158.4, 157.3, 146.9, 131.1,
125.4, 120.9, 113.5, 113.0, 59.7, 55.2, 14.1, 11.7.
HRMS (ESI): m/z calcd for C15H14Cl2O3 +Na (M + Na+): 335.0212;
found: 335.0214.
1-{4-[4-(Dimethylamino)phenyl]-2,5-dimethylfuran-3-yl}etha-
none (4i)
Yield: 45 mg (58%); yellow solid; mp 85–87 °C.
1H NMR (400 MHz, CDCl3): d = 7.10–7.08 (d, J = 8.8 Hz, 2 H),
6.76–6.74 (d, J = 8.8 Hz, 2 H), 2.98 (s, 6 H), 2.52 (s, 3 H), 2.16 (s,
3 H), 1.96 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 196.8, 155.8, 149.6, 146.6, 130.5,
123.1, 121.0, 120.7, 112.2, 40.4, 30.7, 14.2, 11.6.
HRMS (ESI): m/z calcd for C16H19NO2 + Na (M + Na+): 280.1308;
found: 280.1307.
1-[4-(4-Fluorophenyl)-2,5-dimethylfuran-3-yl]ethanone (4o)
Yield: 46.2 mg (66%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.23–7.20 (m, 2 H), 7.12–7.08 (m,
2 H), 2.54 (s, 3 H), 2.15 (s, 3 H), 1.96 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 195.6, 163.3 (d, JC,F = 245.4 Hz),
156.3, 147.1, 131.5 (d, JC,F = 8.2 Hz), 129.6, 122.8, 119.7, 115.5 (d,
JC,F = 21.4 Hz), 30.7, 14.3, 11.5.
HRMS (ESI): m/z calcd for C14H13FO2 + Na (M + Na+): 225.0792;
found: 225.0791.
Ethyl 4-[4-(Dimethylamino)phenyl]-2,5-dimethylfuran-3-car-
boxylate (4j)
Yield: 49.9 mg (58%); yellow solid; mp 47–48 °C.
1H NMR (400 MHz, CDCl3): d = 7.17–7.15 (d, J = 8.8 Hz, 2 H),
6.78–6.75 (d, J = 9.2 Hz, 2 H), 4.19–4.16 (m, 2 H), 2.98 (s, 6 H),
2.58 (s, 3 H), 2.23 (s, 3 H), 1.21–1.17 (m, 3 H).
Ethyl 4-(4-Fluorophenyl)-2,5-dimethylfuran-3-carboxylate (4p)
Yield: 52.4 mg (67%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.23–7.19 (m, 2 H), 7.07–7.03 (m,
2 H), 4.14–4.10 (m, 2 H), 2.57 (s, 3 H), 2.18 (s, 3 H), 1.14–1.10 (m,
3 H).
13C NMR (100 MHz, CDCl3): d = 164.2, 163.1 (d, JC,F = 244.1 Hz),
157.5, 147.1, 131.6 (d, JC,F = 8.2 Hz), 129.1, 120.3, 114.6 (d,
JC,F = 21.4 Hz), 113.3, 59.7, 14.1, 13.9, 11.6.
13C NMR (100 MHz, CDCl3): d = 164.5, 157.0, 149.4, 146.7, 130.7,
121.2, 120.9, 113.5, 111.8, 59.7, 40.6, 14.2, 11.8.
HRMS (ESI): m/z calcd for C17H22NO3 (M + H+): 288.1594; found
HRMS (ESI): m/z calcd for C15H15FO3 + Na (M + Na+): 285.0897;
found: 285.0902.
(M + H+): 288.1593.
1-[4-(2-Chlorophenyl)-2,5-dimethylfuran-3-yl]ethanone (4k)
Yield: 49.5 mg (67%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.47–7.45 (m, 1 H), 7.32–7.25 (m,
3 H), 2.56 (s, 3 H), 2.08 (s, 3 H), 1.91 (s, 3 H).
1-[4-(Furan-2-yl)-2,5-dimethylfuran-3-yl]ethanone (4q)
Yield: 27.4 mg (45%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.50 (s, 1 H), 6.48 (s, 1 H), 6.35
(s, 1 H), 2.52 (s, 3 H), 2.26 (s, 3 H), 2.07 (s, 3 H).
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Tetrasubstituted NH Pyrroles and Polysubstituted Furans
539
13C NMR (100 MHz, CDCl3): d = 195.6, 156.5, 149.4, 146.1, 142.3,
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121.9, 111.2, 111.0, 109.6, 29.6, 14.2, 11.9.
HRMS (ESI): m/z calcd for C12H12O3 + Na (M + Na+): 227.0679;
found: 227.0687.
Ethyl 4-(Furan-2-yl)-2,5-dimethylfuran-3-carboxylate (4r)
Yield: 42.9 mg (61%); yellow oil.
1H NMR (400 MHz, CDCl3): d = 7.47–7.46 (s, 1 H), 6.49–6.45 (m,
2 H), 4.25–4.23 (m, 2 H), 2.54 (s, 3 H), 2.35 (s, 3 H), 1.27–1.24 (m,
3 H).
13C NMR (100 MHz, CDCl3): d = 163.9, 157.5, 149.1, 146.6, 141.5,
112.9, 111.8, 110.6, 108.9, 59.9, 14.1, 13.9, 12.5.
HRMS (ESI): m/z calcd for C13H14O4 + Na (M + Na+): 257.0784;
found: 257.0795.
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Acknowledgment
This research was supported by National Natural Science Foun-
dation of China (NSFC-21002077), Education Department of
Shaanxi Provincial Government (2010JK869), and Northwest Uni-
versity (PR09037).
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