Retro-Claisen condensation versus pyrrole formation in reactions of amines and 1,3-diketones

Article abstract of DOI:10.1016/j.tetlet.2010.03.086

Cyclic amines and 1,3-diketones readily react under microwave irradiation to form ring-fused pyrroles in a single operation. A competing retro-Claisen pathway is efficiently suppressed by employing p-toluenesulfonic acid as an additive.

Full text of DOI:10.1016/j.tetlet.2010.03.086

Tetrahedron Letters 51 (2010) 2945–2947
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Tetrahedron Letters
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Retro-Claisen condensation versus pyrrole formation in reactions of amines
and 1,3-diketones
*
Indubhusan Deb, Daniel Seidel
Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclic amines and 1,3-diketones readily react under microwave irradiation to form ring-fused pyrroles in
a single operation. A competing retro-Claisen pathway is efficiently suppressed by employing p-toluene-
sulfonic acid as an additive.
Received 3 March 2010
Accepted 19 March 2010
Available online 25 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
The ubiquity of pyrroles as substructures in natural products,1
coupled with their usefulness as materials2 and medicinal agents,3
has stimulated the development of many methods for their synthe-
sis.4,5 Not surprisingly, there is a continued interest in the identifi-
cation of methods that allow for the synthesis of pyrroles from
simple and readily available starting materials, ideally in a single
operation.6 Here we report a one step synthesis of ring-fused pyr-
roles from 1,3-diketones and cyclic amines.
Cartaya-Marin who reported the reaction of dibenzoylmethane
(1) and pyrrolidine (2) to form ring-fused pyrrole 3 (Eq. 1).10,11
Specifically, these researchers stated that heating of
1 with
1.7 equiv of 2 ‘in a limited amount of benzene for one hour at
Table 1
Optimization of the pyrrole formationa
W, 280 oC,
10 min
μ
O
1
+
2
Ph
N
N
O
+
+
Ph
N
(3)
solvent,
additive
O
O
Ph
Ph
Ph
Ph
N
+
(3 equiv)
ð1Þ
ð2Þ
N
H
2
3
4
5
Ph
Ph
-
2 H2O
Ph
Ph
1
3
O
Entry Additive (equiv)
Solvent (concentration) 3 (%) 4 (%) 5 (%)
W, 280 oC,
10 min
μ
O
1
+
2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
—
Xylenes (2 M)
Xylenes (2 M)
Toluene (2 M)
THF (2 M)
DMF (2 M)
n-BuOH (2 M)
Xylenes (2 M)
7
50
48
30
10
15
11
5
10
13
—
2
—
2
10
12
10
20
15
40
20
—
2
5
—
—
40
30
25
20
7
30
8
25
20
15
30
65
5
8
N
N
+
+
p-TSA (0.2)
p-TSA (0.2)
p-TSA (0.2)
p-TSA (0.2)
p-TSA (0.2)
PhCO2H (0.2)
m-Cl-PhCO2H (0.2) Xylenes (2 M)
AcOH (0.2)
TFA (0.2)
Zn(OTf)2 (0.2)
Mg(OTf)2 (0.2)
Sc(OTf)3 (0.2)
Amberlyst 15
Silica gel (excess)
p-TSA (1)
p-TSA (0.75)
p-TSA (0.5)
p-TSA (0.1)
p-TSA (0.5)
p-TSA (0.5)
p-TSA (0.5)
p-TSA (0.5)
p-TSA (0.5)
Ph
N
xylenes (2 M)
Ph
Ph
Ph
10
20
25
10
15
20
18
—
10
50
5
5
12
7
5
10
5
14
12
10
25
(3 equiv)
3 (7 %)
4
5
(2 %)
(65 %)
Given our interest in the redox neutral
a-functionalization of
amines,7–9 we were intrigued by a publication of Soeder and
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (2 M)
Xylenes (1 M)
Xylenes (0.5 M)
Xylenes (0.25 M)
Xylenes (0.1 M)
Xylenes (5 M)
H
O
N
O
retro-Claisen
condensation
O
N
O
O
Ph
Ph
Ph
N
- H2O
- MeCOPh
Ph
Ph
Ph
Ph
2
5
6
5
4
35
38
53
20
53
40
15
10
35
OH
Ph
6
π
1,6-H
Ph
N
N
N
OH
N
- H2O
Ph
Ph
Ph
Ph
7
8
4
3
Figure 1. Proposed mechanisms for the formation of amide and pyrrole.
a
All reactions were performed on a 1 mmol scale. All yields are isolated yields
after column chromatography.
* Corresponding author.
E-mail address: seidel@rutchem.rutgers.edu (D. Seidel).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.03.086

2946
I. Deb, D. Seidel / Tetrahedron Letters 51 (2010) 2945–2947
Table 2
Scope of the pyrrole formationa
p-TSA
n
N
(0.5 equiv)
n
NH
O
O
+
R'
xylenes (1 M)
R
R'
n = 0, 1
μ
W, 280 o
C
(3 equiv)
R
MeO
Me
OMe
OMe
N
N
N
MeO
MeO
11
9
10
(25 %),
(50 %),
(48 %),
30 min
20 min
Me
30 min
MeO
OMe
Me
N
N
N
Ph
Ph
12
14
(40 %),
(60 %),
13
(45 %),
20 min
30 min
20 min
Me
Cl
MeO
OMe
OMe
Cl
N
N
N
Me
OMe
(63 %),
Ph
MeO
MeO
16
(67 %),
17
(62 %),
15
20 min
20 min
40 min
OMe
Ph
MeO
MeO
N
Ph
OMe
N
N
N
H
N
H
Ph
Ph
18
(44 %),
19
(86 %),
20
(70 %),
20 min
30 min
MeO
30 min
a
All reactions were performed on a 2 mmol scale and run for the indicated amount of time. All yields are isolated yields after column chromatography.
high temperature using a Dean–Stark trap for the removal of
water produced compound 3 in 60% yield after recrystallization.’
Unfortunately, no further details were provided and no substrate
scope or proposed mechanism was presented. In our hands, reac-
tions of varying ratios of 1 and 2 conducted in refluxing benzene,
toluene or xylenes in the presence or absence of molecular sieves
and with or without Dean–Stark trap, lead to only trace formation
of 3. Enaminone4 was isolated as themain productin all cases. Inter-
estingly, a reaction conducted under more forceful conditions
(280 °C, microwave irradiation) led to the formation of N-benzoyl
pyrrolidinone (5) as the major product (Eq. 2). In addition to amide
5, small amounts of pyrrole 3 and enaminone 4 were isolated.
The formation of amide 5 can be rationalized by a retro-Claisen
condensation of the initially formed N,O-acetal 6, a process that
simultaneously generates 1 equiv of acetophenone (Fig. 1).12 Alter-
natively, dehydration of 6 leads to enaminone 4, a likely interme-
diate in the formation of pyrrole 3. The latter may be formed
through a sequence involving a 1,6-H-shift to give 7. Compound
tures (250 °C and below) led to little pyrrole formation, even if
run for extended periods of time. Perhaps not surprisingly,12 the
use of scandium triflate gave rise to the exclusive formation of
the retro-Claisen product 5.
The scope of this reaction was evaluated under the optimized
conditions (Table 2). Reactions of 1,3-diketones with 3 equiv of
pyrrolidine, tetrahydroisoquinolines or b-carboline gave rise to
the formation of fused pyrroles in moderate to good yields.13–16
A reaction between tetrahydroisoquinoline and 1-phenylbutane-
1,3-dione gave rise to compound 17 as a single regioisomer (less
than 2% of the other regioisomer was obtained). In all cases, pyr-
roles were isolated as the major product. Minor amounts of amides
and/or enaminones were usually obtained as readily separable
byproducts. Reactions between pyrrolidine and acetylacetone did
not yield any appreciable amounts of pyrrole under identical reac-
tion conditions.
In summary, we have reported one-step syntheses of ring-
fused pyrroles from readily available 1,3-diketones and cyclic
amines.
7 is envisioned to undergo a 6p-electrocyclization to yield inter-
mediate 8. Subsequent loss of water gives rise to pyrrole 3.
We rationalized that pyrrole formation may become the pre-
dominant reaction pathway if conditions could be indentified that
favor formation of the enaminone 4 over the competing retro-Cla-
isen process. Toward this end, we evaluated the reaction between 1
and 2 in different solvents and in the presence of selected Brønsted
or Lewis acidic additives (Table 1). The best yield of pyrrole 3 (53%)
was obtained in the presence of 0.5 equiv of p-toluenesulfonic acid
(p-TSA), using xylenes as the solvent (1 M concentration). Pro-
longed reaction times, which were expected to lead to full conver-
sion of the remaining enaminone 4 into pyrrole 3, led to a lower
overall yield. Attempts to perform the reaction at lower tempera-
Acknowledgment
We thank the National Science Foundation for support of this
research (Grant CHE-0911192).
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35–36 °C; IR (KBr) 1610, 1569, 1500, 1444, 1144 cmꢁ1 1H NMR (500 MHz,
;
CDCl3) 7.42 (app d, J = 8.0 Hz, 4H), 7.18 (app dd, J = 13.0, 8.0 Hz, 4H), 6.57 (s,
1H), 4.15 (t, J = 7.1 Hz, 2H), 3.12 (t, J = 7.1 Hz, 2H), 2.5 (pentet, J = 7.1 Hz, 2H),
2.38 (s, 3H), 2.36 (s, 3H); 13C NMR (125 MHz, CDCl3) 135.6, 135.1, 134.0,
133.53, 130.6 (ꢂ2), 129.3, 129.2, 125.7, 125.1, 115.9, 108.1, 46.5, 27.9, 25.2,
21.1, 21.0; m/z (ESIMS) 288.3 [M+H]+.
15. Characterization data for 13: pale yellow solid (Rf = 0.5 in 10% EtOAc/hex); mp:
37–38 °C; IR (KBr) 1606, 1567, 1498, 1380, 1330, 1290, 1243 cmꢁ1 1H NMR
;
(500 MHz, CDCl3) 7.53 (app d, J = 7.8 Hz, 2H), 7.51 (app, s, 1H), 7.43 (app d,
J = 8.0 Hz, 2H), 7.33 (app d, J = 8.0 Hz, 2H), 7.20 (app d, J = 8.3 Hz, 3H), 7.10–
7.16 (m, 2H), 6.43 (s, 1H), 4.17 (t, J = 6.1 Hz, 2H), 3.09 (t, J = 6.1 Hz, 2H), 2.49
(app s, 6H); 13C NMR (125 MHz, CDCl3) 136.7, 135.6, 134.4, 133.4, 132.3, 130.1,
129.6, 129.1, 129.1, 128.8, 128.6, 127.6, 126.5, 125.4, 125.3, 124.4, 122.7, 110.7,
42.3, 30.3, 21.2, 21.2; m/z (ESIMS) 350.4 [M+H]+.
16. Characterization data for 19: yellow solid (Rf = 0.30 in 20% EtOAc/hex); mp:
60 °C; IR (KBr) 1599, 1507, 1476, 1444, 1301, 1200 cmꢁ1 1H NMR (500 MHz,
;
CDCl3) 8.09 (s, 1H), 7.64 (app dd, J = 7.0, 1.1 Hz, 2H), 7.45–7.52 (m, 7H), 7.23
(app tdd, J = 14.6, 7.3, 1.2 Hz, 2H), 7.22–7.24 (m, 1H), 7.10–7.15 (m, 2H), 6.36 (s,
1H), 4.25 (t, J = 3.4 Hz, 2H), 3.16 (t, J = 3.4 Hz, 2H); 13C NMR (125 MHz, CDCl3)
136.6, 135.9, 135.5, 132.7, 129.0, 128.6, 128.7, 128.5, 128.3, 127.2, 126.7, 126.4,
121.9, 121.6, 120.9, 119.8, 117.6, 110.9, 110.3, 106.6, 44.0, 21.3; m/z (ESIMS)
359.4 [MꢁH]+.