One pot synthesis of fused [1,2-a]pyrrole from 1,6-dioxo-2,4-diene and haloalkyl primary amine

Article abstract of DOI:10.1016/j.tet.2004.12.051

The one pot synthesis of fused 2,3-dihydropyrrolizine 4a and 6,7-dihydro-5H-indolizine 4b involving the intermolecular dehydrative condensation of 1-phenyl-1,6-dioxo-hepta-2,4-diene 1 with 2-chloroethylamine and 3-chloropropylamine followed by the intramolecular cyclization of the intermediary products 2-(1-chloroalkyl-5-methylpyrrol-2-yl)-1-phenylethanones 3a,b in the presence of a base such as Na₂CO₃ and NaHCO₃ is described. These also led to the concurrent formation of the oxidatively dimerized product 2,3-bis-[1,5-(2-chloroalkyl)-1-H-pyrrol-2-yl]- 1,4-diphenylbutane-1,4-dione 5a,b whereby the structure was further confirmed by X-ray analysis.

Full text of DOI:10.1016/j.tet.2004.12.051

Tetrahedron 61 (2005) 1693–1697
One pot synthesis of fused [1,2-a]pyrrole from 1,6-dioxo-2,4-diene
and haloalkyl primary amine
Shyh-Shiann Juang,a Michael Chang,b Long Fu Wang,b Jeng Liang Hanb and Chi Wi Ongb,
*
aChia Nan University of Pharmacy and Science, Tainan, Taiwan
bDepartment of Chemistry, National Sun Yat Sen University, Kaoshiung, 804, Taiwan
Received 2 November 2004; revised 21 December 2004; accepted 22 December 2004
Available online 13 January 2005
Abstract—The one pot synthesis of fused 2,3-dihydropyrrolizine 4a and 6,7-dihydro-5H-indolizine 4b involving the intermolecular
dehydrative condensation of 1-phenyl-1,6-dioxo-hepta-2,4-diene 1 with 2-chloroethylamine and 3-chloropropylamine followed by the
intramolecular cyclization of the intermediary products 2-(1-chloroalkyl-5-methylpyrrol-2-yl)-1-phenylethanones 3a,b in the presence of a
base such as Na2CO3 and NaHCO3 is described. These also led to the concurrent formation of the oxidatively dimerized product 2,3-bis-[1,5-
(2-chloroalkyl)-1-H-pyrrol-2-yl]-1,4-diphenylbutane-1,4-dione 5a,b whereby the structure was further confirmed by X-ray analysis.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
alkylative cyclization, we required the construction of
N-tether haloalkylpyrrole derivatives. Furthermore the
appropriate choice of base might led to the development
of a one-pot procedure to prepare pyrrolizine and
indolizine derivatives from the reaction of 1-phenyl-
1,6-dioxo-hepta-2,4-diene 1 with chloroethylamine and
chloropropylamine, respectively (Scheme 1).
The fused [1,2-a]pyrroles are important scaffold of alkaloids
widely isolated from plants, insects, animals, oceanic lives
and secondary metabolites of microbes and have potent
biological activities.1–3 The synthesis of pyrrolizines and
indolizines continues to attract the attention of organic
chemist and numerous synthetic routes have been
reported.4–14 Our previous study demonstrated 1,6-dioxo-
2,4-diene to a versatile intermediate in the synthesis of
pyrrole derivatives.15 The important feature of the pyrrole
derivates formed is that the hydrogen atom at the carbon
atom attached to the 2-position can be readily deprotonated
for reaction with an electrophilic center at the N-tether to
give fused [1,2-a]pyrroles. Based on this methodology, we
have recently developed a new pathway for the synthesis of
pyrrolizines and indolizines.16 The moderate yield of the
pyrrolizine and indolizine derivatives with this approach
may be attributed to the reversible condensation reaction
(Dieckmann/Thrope) used for the intramolecular ring
closure reaction. We felt that greater potential utility for
the construction of fused [1,2-a]pyrroles could be realized
were we to achieve intramolecular cyclization by an
irreversible alkylation reaction. Herein, we report our
further investigations on the use of an irreversible alkylation
reaction for the intramolecular ring closure reaction with the
hope of improving the yield. To affect an intramolecular
2. Results and discussion
Our starting point was the 1-phenyl-1,6-dioxo-hepta-2,4-
diene, 1, which we have previously prepared from the
reaction of 2-methylfuran with a-diazoacetophenone
according to the method of Wenkert.17 It has been
demonstrated earlier that compound 1 reacted with
alkylamines to give N-tether alkylpyrrole derivatives.
Originally, it was thought that the preparation of N-tether
haloalkylpyrroles from 1 and aminoalkyl halide under the
same condition would be met with competing reaction
arising from the self-condensation of the aminoalkyl halide.
Therefore, our initial strategy for the synthesis of N-tether
haloalkylpyrrole involved the reaction of 1 with amino
alcohol to form N-tether hydroxyalkylpyrrole, followed by a
subsequent conversion of the alcohol to the corresponding
halide. Compound 1 react with 1-aminoethanol and 1-
aminopropanol to give N-tether hydroxyalkylpyrrole
derivatives 2a and 2b, respectively, but in a moderate
yield (40–50%). The alcohol functionality in 2a and 2b can
be converted to the chloride 3a and 3b in a near quantitative
yield. The overall yields from the two steps were only
Keywords: One-pot synthesis; Intramolecular alkylation; N-fused pyrrolo
ring.
* Corresponding author. Tel.: C886 7525 2000 3923; fax: C886 7525
3908; e-mail: cong@mail.nsysu.edu.tw
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.051

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S.-S. Juang et al. / Tetrahedron 61 (2005) 1693–1697
Scheme 1. Synthetic approach towards pyrrolizine and indolizine.
moderate. Next, the intramolecular cyclization of 3a and 3b
was attempted using potassium t-butoxide.16 Although,
submission of 3a and 3b under this intramolecular
alkylative cyclization condition gave the pyrrolidine 4a
and indolizine 4b respectively, the yield was rather low
(20–25%) (Scheme 2). The 1H and 13C NMR spectra of 4a
and 4b revealed the absence of an a-methine hydrogen
between the pyrrole and the carbonyl group, and this was
attributed to the formation of the more stable enol-form
due to resonance delocalization. The ketone–enol tauto-
merizaton of indolizone derivatives has been reported.9
alkylation reaction, allowing the smooth formation of
pyrrole intermediate from the reaction of 1 with aminoalkyl
halide. The discovery of the sodium carbonate promoted
cyclization reaction above point the way for a facile design
of a fast one-pot synthesis of fused [1,2-a]pyrroles. It was
reasoned that sodium carbonate in methanol would not
exacerbate the problem of self-condensation of the amino-
alkyl halide during its reaction with 1 to form the pyrrole
intermediate at the onset of the reaction. In a typical
experiment, 1 was reacted with chloroethylamine in the
presence of sodium carbonate and the reaction mixture
stirred under a nitrogen atmosphere at room temperature in
methanol. TLC was used to monitor the progress of the
reaction. The TLC showed a complete disappearance of the
starting material 1 after 2 h at room temperature, the major
product was found to be the pyrrole derivative 3a. Next, the
reaction was carried out for a longer reaction time (72 h)
until TLC indicated the total consumption of 1 and the
disappearance of intermediate 3a. This resulted in the
formation of two new products 4a and 5a in a 3:1 ratio.
The major product was the required pyrrolizine derivative
4a. The minor product 5a showed a parent peak in the mass
spectrum at m/z 520, 522, 524 (approx. 10:6:1 ratio), an
equivalent to 2 units less than that for two molecules of 3a
and a pattern indicating the presence of two chlorine atoms.
From this data we tentatively assigned 5a as 2,3-bis(1H-
pyrol-2yl)-1,4-diphenyl-1,4-dione, the oxidatively dimer-
ized product of the intermediate 3a. Oxidative coupling of
enolates, especially phenylacetic acid ester, through a
variety of methods have been widely reported.18 At this
Although the synthesis of pyrrolidine and indolizine were
realized, the poor overall yield hampered its practicality. A
trial experiment for the reaction of 1 with chloroethylamine
was carried out and fortuitously gave 3a in a ca. 90–95%
yield. Similarly, reaction of 1 with chloropropylamine gave
3b in similar yield. Clearly, the chloroalkylamine did not
proceed to give self-polyalkylation products. A greater
challenge was to improve the yield during the intra-
molecular alkylative cyclization step. Several different
bases can be called into play and we choose to use sodium
carbonate in methanol for its mild condition. Reaction of 3a
and 3b with sodium carbonate in methanol successfully
gave 4a and 4b in a dramatically improved yield (55–65%),
together with an isolable minor product (5–10%) in each
case that was not identified at this stage.
An important requirement for the successful one pot
reaction is the non-participation of the base prior to
Scheme 2.

S.-S. Juang et al. / Tetrahedron 61 (2005) 1693–1697
1695
Table 1. Product distribution for the reaction of 1 with aminoalkyl halide and Na2CO3/NaHCO3
Entry
Reaction time (h)
Na2CO3/MeOH (ratio)
Entry
Reaction time (h)
NaHCO3/MeOH
(ratio)
1
2
3
4
2
72
2
3a only 85–90%
4a/5a (3:1) 65%
3b only 85–90%
4b/5b (3:1) 70%
5
6
7
8
2
72
2
3a only 85–90%
4a/5a (1:4) 77%
3b only 85–90%
4b/5b (1:4) 80%
72
72
point, we were uncertain of the stereoisomers of the dimer,
dl- or meso-5a. The formation of products 4a and 5 reflect
two competing reactions pathway for the intermediate 3a,
one leading to an intramolecular alkylation and the other
oxidative coupling.
the use of an even milder base such as sodium bicarbonate
for the one pot synthesis might further improved the
yield. Contrary to expectation, the reaction of 1 with 2-
chloroethylamine with sodium bicarbonate in methanol for
72 h was found to give a reverse preponderance of products
4a and 5a in a 1:4 ratio (77% overall yield). Similarly, the
reaction of 1 with chloropropylamine gave 4b and 5b in a
ratio of 1:4. The formation of the dimer in these reactions
might be explained in consideration of the redox reaction
occurring between the generated carbanion of the inter-
mediate 3 and the free 3 or the product 5 which play the role
as a single electron acceptor, leading to the formation of the
benzoylmethyl radical and the anion radical of 3 or 5.
Accordingly, in the one pot reaction, the use of sodium
bicarbonate will give a greater preponderance of the dimeric
product 5.
This one pot strategy can be applied to the synthesis of
indolizine derivative 4b by treatment of 1-phenyl-1,6-
dioxo-hepta-2,4-diene 1 with 3-chloropropylamine and
sodium carbonate in methanol for 72 h. In this case we
also obtained two products, the indolizine derivative 4b and
5b in a 3:1 ratio (70% overall yield). The results are
summarized in Table 1. A one pot synthesis of indolizines
from the reaction of acyl bromide, pyridine and acetylene
mediated by microwave has been reported.19 The mass
spectrum of 5b again correlates to 2 units less than for two
molecules of 3b. We were able to obtain a crystal of 5b
suitable for X-ray crystallographic analysis20 (Fig. 1) and
this unambiguously supported the oxidative coupling
product of the intermediate 3b. The oxidative coupling
reaction proceeds to give the dl 5b stereoisomer. Similar
high selectivity of dl isomer over meso isomer has also been
reported for the oxidative coupling of phenylacetic acid
ester by treating the ester with titanium chloride and then
adding triethylamine to the resulting solution.18h
3. Conclusion
In summary, this paper presents a simple and convenient
one pot synthesis of pyrrolizine and indolizine skeletons
from the reaction of 1,6-dioxo-2,4-diene and chloroalk-
ylamine in methanol with sodium carbonate in high yield,
and under mild condition. Furthermore, the results in this
paper clearly show that the use of sodium bicarbonate gave
mainly the oxidative coupling product and this has not been
reported.
Since pyrrolidine 4a and indolizine 4b were form in good
yield using sodium carbonate in methanol, we assumed that
Figure 1. X-ray structure of compound 5b showing the most stable Newman conformation.

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S.-S. Juang et al. / Tetrahedron 61 (2005) 1693–1697
4. Experimental
4.3. KOBut as base for alkylative cyclization
To a solution of KOBut (0.65 mmol) in anhydrous THF
(20 mL) under N2 at 0 8C was added 3a/b (0.65 mmol). The
reaction was allowed to warm to ambient temperature and
stirred for 6 h. The reaction was quenched with water and
the THF removed under reduced pressure, and then
extracted with CH2Cl2. The crude product obtained was
purified by preparative TLC to give 4a/b, respectively.
4.1. General experimental conditions
Compound 1 was prepared according to previously reported
method.16 Commercially available reagents were used
without further purification. 1H and 13C NMR spectra
were recorded in CDCl3 with tetramethylsilane as an
internal standard.
4.3.1. 1-((E)-1-Hydroxybenz-1-ylidene)-5-methyl-2,3-
dihydro-1H-pyrrolizine (4a). Obtained as yellow oil in
4.2. General procedure of pyrrole ring formation 2 and 3
from the reaction of 1 with aminoalkyl alcohol and
chloroalkylamine
1
20% yield (ether/hexane, 1:3). nmax: 3400, 1615 cmK1. H
NMR (200 MHz): d 7.98 (d, 2H), 7.60 (m, 1H), 7.52 (m,
2H), 6.89 (d, JZ3.9 Hz, 1H), 6.05 (d, JZ3.9 Hz, 1H), 4.69
(t, JZ6.0 Hz, 2H), 3.95 (t, JZ6.0 Hz, 2H), 2.41 (s, 3H); 13C
NMR (50 MHz) d 193.35, 183.21, 142.17, 134.21, 133.53,
130.00, 128.82, 128.75, 126.94, 125.66, 110.87, 43.38,
42.12, 33.61, 12.50; MS (EI) m/z 225(16). Anal. Calcd for
C15H15NO: C, 79.97; H, 6.71; N, 6.22. Found C, 79.78; H,
6.67; N, 6.17.
To a stirred solution of aminoalkyl alcohol (1.30 mmol) in
MeOH (20 mL) at 0 8C was added 1. The reaction was
stirred at 0 8C for 2 h and left at room temperature
overnight. The MeOH was removed under reduce pressure
and the crude product extracted by CH2Cl2. The crude
product obtain was purified by preparative TLC to provide
the corresponding product.
4.3.2. 8-((E)-1-Hydroxybenz-1-ylidene)-3-methyl-5,6-
dihydro-8H-indolizine (4b). Obtained as yellow oil in
The chloroethyl- or chloropropylamine hydrochloride salt
(1.10 mmol) was used and have to be neutralized with an
equivalent of Na2CO3.
1
25% yield (ether/hexane, 1:3). nmax: 3400, 1615 cmK1. H
NMR (200 MHz): d 8.00 (d, 2H), 7.64 (m, 1H), 6.85 (d, JZ
4.0 Hz, 1H), 6.03 (d, JZ4.0 Hz, 1H), 4.56 (t, JZ5.8 Hz,
2H), 3.64 (t, JZ5.8 Hz, 2H), 2.38 (s, 3H), 2.30 (m, 2H); 13C
NMR (50 MHz) d 193.35, 182.21, 142.17, 134.21, 133.53,
130.00, 128.82, 128.75, 126.94, 125.66, 110.87, 43.38,
42.12, 33.61, 12.50; MS (EI) m/z 239(25). Anal. Calcd for
C16H17NO: C, 80.30; H, 7.16; N, 5.85. Found C, 80.16; H,
7.07; N, 5.84.
4.2.1. [N-(2-Hydroxyethyl)-5-methylpyrrol-1-yl]phenyl-
ethanone (2a). Obtained as yellow oil in 45% yield (EtOAc/
hexane, 1:5). nmax: 3416, 1685 cmK1. 1H NMR (200 MHz):
d 8.03 (m, 2H), 7.56 (m, 3H), 5.96 (d, JZ3.6 Hz, 1H), 5.86
(d, JZ3.6 Hz, 1H), 4.25 (s, 2H), 3.98 (t, JZ7.2 Hz, 2H),
3.78 (t, JZ7.2 Hz, 2H), 2.24 (s, 3H), 1.61 (brd, 1H); MS
(EI) m/z 243(37). HRMS m/z calcd for C15H17NO2,
243.1259; found, 243.1257.
4.4. One pot procedure using sodium carbonate and
sodium bicarbonate
4.2.2. [N-(2-Hydroxypropyl)-5-methylpyrrol-1-yl]phenyl-
ethanone (2b). Obtained as yellow oil in 40% yield
The chloroethyl- or chloropropylamine hydrochloride salt
(1.10 mmol) was first neutralized with Na2CO3 or bicar-
bonate (2 equiv) in MeOH solution. To the solution was
added 1 (1.00 mmol) at 0 8C and than at room temperature
for 72 h under N2 (TLC analysis indicate the disappearance
of the 1 and 3). The MeOH was removed under reduce
pressure, water added, and extracted with CH2Cl2. Purifi-
cation of the crude mixture by preparative TLC provided the
corresponding product (see Table 1).
(EtOAc/hexane, 1:5). nmax: 3400, 1681 cmK1 1H NMR
.
(200 MHz): d 8.04 (m, 2H), 7.58 (m, 3H), 5.93 (d, JZ
3.6 Hz, 1H), 5.82 (d, JZ3.6 Hz, 1H), 4.26 (s, 2H), 3.81 (t,
JZ7.1 Hz, 2H), 3.68 (t, JZ7.1 Hz, 2H), 2.22 (s, 3H), 2.04
(brd, 1H), 1.68 (m, 2H); MS (EI) m/z 257(26). HRMS m/z
calcd for C16H19NO2, 257.1416; found, 257.1415.
4.2.3. [N-(2-Chloroethyl)-5-methylpyrrol-1-yl]phenyl-
ethanone (3a). Obtained as yellow oil in 90% yield
(EtOAc/hexane, 1:5). nmax
: .
1685 cmK1 1H NMR
4.4.1. 2,3-Di-[N-(2-chloroethyl)-5-methylpyrrol-1-yl]-
1,4-diphenylbutane-1,4-dione (5a). Obtained as an orange
gum. nmax: 1675 cmK1. d 7.84 (d, JZ6.9 Hz, 4H), 7.44 (m,
2H), 7.35 (m, 4H), 6.03 (d, JZ3.6 Hz, 2H), 5.84 (d, JZ
3.6 Hz, 2H), 5.32 (s, 2H), 3.65 (m, 4H), 3.21 (m, 2H), 2.92
(m, 2H), 2.15 (s, 6H); 13C NMR (50 MHz) d 198.21, 137.06,
132.53, 130.19, 128.46, 128.23, 125.49, 109.71, 108.02,
50.60, 43.91, 41.49, 12.50; MS (EI) m/z 524 (2), 522 (10),
520 (16), 260 (100). HRMS m/z calcd for C30H30Cl2N2O2,
520.1684; found, 520.1686.
(200 MHz): d 8.06 (m, 2H), 7.58 (m, 3H), 5.91 (d, JZ
3.4 Hz, 1H), 5.88 (d, JZ3.4 Hz, 1H), 4.29 (s, 2H), 4.12 (t,
JZ7.3 Hz, 2H), 3.63 (t, JZ7.3 Hz, 2H), 2.25 (s, 3H); MS
(EI) m/z 261(12), 263(36). Anal. Calcd for C15H16ClNO: C,
68.83; H, 6.16; N, 5.38. Found C, 68.55; H, 6.15; N, 5.26.
4.2.4. [N-(2-Chloropropyl)-5-methylpyrrol-1-yl]phenyl-
ethanone (3b). Obtained as yellow solid in 95% yield
1
(EtOAc/hexane, 1:5), mp 75–58 8C. nmax: 1685 cmK1. H
NMR (200 MHz): d 8.05 (m, 2H), 7.54 (m, 3H), 5.91 (d, JZ
3.6 Hz, 1H), 5.89 (d, JZ3.6 Hz, 1H), 4.28 (s, 2H), 3.97 (t,
JZ7.0 Hz, 2H), 3.54 (t, JZ7.0 Hz, 2H), 2.25 (s, 3H), 2.08
(m, 2H); MS (EI) m/z 275(13), 277(39). Anal. Calcd for
C16H18ClNO: C, 69.68; H, 6.58; N, 5.08. Found C, 69.65; H,
6.55; N, 5.11.
4.4.2. 2,3-Di-[N-(2-chloropropyl)-5-methylpyrrol-1-yl]-
1,4-diphenylbutane-1,4dione (5b). Obtained as an orange
crystal, mp 128 8C. nmax: 1670 cmK1. 1H NMR (200 MHz):
d 7.85 (d, JZ6.9 Hz, 4H), 7.44 (m, 2H), 7.38 (m, 4H), 5.98
(d, JZ3.6 Hz, 2H), 5.78 (d, JZ3.6 Hz, 2H), 5.33 (s, 2H)

S.-S. Juang et al. / Tetrahedron 61 (2005) 1693–1697
1697
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Financial support from the National Science Council,
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MWZ549.54, triclinic, space group P-1 (#2), aZ10.15 (1) A,
˚
˚
bZ11.59 (1) A, cZ14.06 (2) A, aZ98.1 (1)8, bZ93.50 (10)8,
3
3
˚
gZ113.99 (8)8, VZ1483 (3) A , ZZ2, DcZ1.230 g/cm , RZ
0.063, RwZ0.055, GOFZ4.01 for 2358 reflections with IO
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