Synthesis of 1,2,3-trisubstituted indolizines, pyrrolo[1,2-a]quinolines, and pyrrolo[2,1-a]isoquinolines from 1,2-allenyl ketones

Article abstract of DOI:10.1002/ejoc.201301719

An efficient synthesis of substituted indolizine and its benzo derivatives, pyrrolo[1,2-a]quinolines and pyrrolo[2,1-a]isoquinolines, by the reaction of allenyl ketones with α-bromo carbonyl compounds and pyridines (quinoline or isoquinoline) under mild conditions without an added oxidant other than molecular oxygen from air was developed. Notably, allenyl ketones with or without a substituent attached to the internal position of the allene moiety afforded indolizine derivatives with different substitution patterns.

Full text of DOI:10.1002/ejoc.201301719

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301719
Synthesis of 1,2,3-Trisubstituted Indolizines, Pyrrolo[1,2-a]quinolines, and
Pyrrolo[2,1-a]isoquinolines from 1,2-Allenyl Ketones
Xuesen Fan,*[a] Yuanyuan Wang,[a] Yan He,[a] Shenghai Guo,[a] and Xinying Zhang*[a]
Keywords: Allenes / Ketones / Multicomponent reactions / Nitrogen heterocycles
An efficient synthesis of substituted indolizine and its benzo
derivatives, pyrrolo[1,2-a]quinolines and pyrrolo[2,1-a]iso-
quinolines, by the reaction of allenyl ketones with α-bromo
carbonyl compounds and pyridines (quinoline or isoquin-
oline) under mild conditions without an added oxidant other
than molecular oxygen from air was developed. Notably,
allenyl ketones with or without a substituent attached to the
internal position of the allene moiety afforded indolizine de-
rivatives with different substitution patterns.
tion of the in situ formed tetrahydroindolizines. Moreover,
for monofunctionalized alkenes, self-condensation of the
pyridinium salt with its enol form often takes place to give
undesired products.
Introduction
As an important class of N-bridgehead bicyclic ring sys-
tems, indolizine and its benzo derivatives, pyrrolo[1,2-a]-
quinolines and pyrrolo[2,1-a]isoquinolines, have received
much attention owing to their unique electronic structure,
interesting biological activities, and broad pharmaceutical
applications.[1–3] Because of their importance, considerable
efforts have been made to the development of new methods
to synthesize indolizine derivatives. As a result, a number
of processes including the cycloaddition of pyridinium, qui-
nolinium, and isoquinolinium ylides with alkene and alkyne
dipolarophiles;[3–8] difunctionalization of naphthoquin-
one,[9] iodine-promoted cascade reaction of methyl ketone
with pyridine;[10] copper-catalyzed cycloisomerization of 2-
pyridylpropargylic acetates;[11] organocopper-mediated
cross-coupling/cycloisomerization of 2-pyridylpropargylic
mesylates;[12] elaboration of pyranoquinolines through site-
selective electrophilic cyclization and subsequent opening of
the pyran ring[13] have been developed. Although these pro-
tocols are generally reliable, there are still some difficulties
and limitations, especially if less reactive alkene di-
polarophiles are used as substrates. To obtain reasonable
yields, difunctionalized alkenes such as fluoroiodoalkenes,
fluorinated vinyl tosylates, and dichloro-α,β-unsaturated
ketones have to be used. Otherwise, an added oxidant such
as MnO2 or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), should be used for the dehydrogenative aromatiza-
As part of our research interest in exploring allene deriv-
atives as versatile synthetic intermediates,[14] we hypothe-
sized that a more efficient and practical synthesis of indoliz-
ine derivatives without the use of an added oxidant might
be achieved by using allenyl ketones as the required olefinic
dipolarophiles, as they are highly reactive as nucleophilic
addition acceptors and have been frequently used as a
powerful tool in forming carbon–carbon and carbon–
heteroatom bonds.[15,16] Herein we wish to report our pre-
liminary results in this regard.
Results and Discussion
To check the feasibility of our envisioned synthesis of
indolizine from allenyl ketones, the reaction of pyridine (1a;
0.5 mmol) and 2-bromo-1-phenylethanone (2a; 0.5 mmol)
with 1-phenylbuta-2,3-dien-1-one (3a; 0.5 mmol) was first
studied. To our delight, stirring of a mixture of 1a, 2a, and
3a in refluxing CH3CN in the presence of Na2CO3
(0.1 equiv.) for 6 h afforded the desired (2-methylindolizine-
1,3-diyl)bis(phenylmethanone) (4a) in 35% yield (Table 1,
Entry 1). Inspired by this result, the reaction was thor-
oughly optimized in terms of different amounts of Na2CO3,
various bases, solvents, as well as reaction temperatures
(Table 1, Entries 2–16). It turned out that a yield of 73%
could be obtained if the reaction was run in refluxing
CH3CN in the presence of K2CO3 (1 equiv.) for 2 h
(Table 1, entry 6).
[a] School of Chemistry and Chemical Engineering, Key
Laboratory for Yellow River and Huai River Water
Environment and Pollution Control, Ministry of Education,
Collaborative Innovation Center of Henan Province for Green
Manufacturing of Fine Chemicals, Henan Normal University,
Xinxiang, Henan 453007, P. R. China
With the optimized conditions in hand, we then studied
the scope and generality of this new reaction. First, 1,2-
allenyl ketones 3 were treated with 1a and 2a. The results
in Table 2 show that the R3 unit of 3 can be an electron-
E-mail: xuesen.fan@htu.cn
xinyingzhang@htu.cn
http://www.htu.cn/s/57/t/1350/a/28234/info.jspy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301719.
Eur. J. Org. Chem. 2014, 713–717
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
713

X. Fan, Y. Wang, Y. He, S. Guo, X. Zhang
Table 2. Scope of the reaction leading to 4.[a]
SHORT COMMUNICATION
Table 1. Optimization study for the synthesis of 4a.[a]
Entry
Base
(equiv.)
Solvent
T
[°C]
t
[h]
4a
Entry
R1
R2
R3
R4
4
Yield
[%][b]
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Na2CO3 (0.1)
Na2CO3 (0.2)
Na2CO3 (0.5)
Na2CO3 (1)
Na2CO3 (2)
K2CO3 (1)
Cs2CO3 (1)
DBU (1)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DMF
THF
EtOH
H2O
CH2Cl2
CH3CN
CH3CN
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
30
6
6
6
2
2
2
2
2
2
2
2
2
2
2
2
2
35
42
55
70
72
73
71
52
44
68
60
61
35
30
35
55
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
o-CH3OC6H4
o-BrC6H4
m-CH3C6H4
m-BrC6H4
p-CH3OC6H4
p-CH3C6H4
p-FC6H4
p-CF3C6H4
p-CNC6H4
3,4-(CH3O)2C6H3
C6H5CH2
C6H5(CH2)2
p-ClC6H4
C6H5
p-ClC6H4
p-ClC6H4
p-BrC6H4
p-CH3C6H4
p-ClC6H4
C6H5
C6H5
C6H5
C6H5(CH2)2
H
H
H
H
H
H
H
H
H
H
H
H
H
CH3
CH3
H
H
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
73
65
75
70
78
68
67
78
70
72
65
63
60
73
68
82
80
83
60
62
72
78
80
75
Et3N (1)
9
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4j
4k
4l
4m
4n
4o
4p
4q
4r
4s
4t
4u
4v
4w
4x
50
o-BrC6H4
p-FC6H4
p-FC6H4
p-CH3C6H4
p-CH3C6H4
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), 3a
(0.5 mmol), solvent (5 mL). [b] Yield of isolated product.
H
H
H
H
H
H
rich (Table 2, Entries 2, 4, 6, 7, and 11) or electron-deficient
phenyl (Table 2, Entries 3, 5, 8–10), benzyl (Table 2, En-
try 12), or phenethyl groups (Table 2, Entry 13) without
showing clear electronic and steric effects. Various func-
tional groups, such as methoxy, cyano, trifluoromethyl, and
halides, were well tolerated under the reaction conditions.
Promisingly, with allenyl ketones bearing a methyl group on
the terminal position of the allene moiety, the reaction still
proceeded smoothly to give 4n and 4o in good yields
(Table 2, entries 14 and 15). Further, substrates bearing an
electron-withdrawing group in the R2 unit (Table 2, En-
tries 16–18) gave better yields of the products than those
bearing electron-donating groups (Table 2, Entries 19 and
20). Moreover, the reactivity of 4-methylpyridine (1b) in this
tandem reaction was also studied. Under standard condi-
p-CH3 p-CH3C6H4
p-CH3
H
p-FC6H4
OEt
OEt
H
[a] Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3 (1.0 mmol),
K2CO3 (1.0 mmol), CH3CN (10 mL), reflux, 2 h. [b] Yield of iso-
lated product.
Scheme 1. Reactivity of an α-substituted allenyl ketone.
tions, 1b reacted smoothly with 2 and 3 to give 4u and 4v reactions of substrates with either electron-withdrawing or
in good yields (Table 2, Entries 21 and 22). To our delight, electron-donating groups on the aryl rings of 1 and 2 or
not only 2-bromoacetophenones (Table 2, Entries 1–22) but substrates with a methyl or ethyl group on the internal posi-
also ethyl 2-bromoacetate participated in this reaction to tion of the allene moiety of 3 proceeded smoothly to give
afford products 4w and 4x in an almost equally efficient 1,2,3-trisubstituted indolizines 5a–l with high efficiency
manner (Table 2, Entries 23 and 24).
(Table 3, Entries 1–12).
In our further study on the scope of substrates, we found
So far, we have successfully developed a simple and ef-
that the reaction of 1a and 2a with an allenyl ketone ficient synthetic pathway toward 1,2,3-trisubstituted
bearing a methyl group on the internal position of the indolizines from the reaction of allenyl ketones with pyr-
allene moiety gave an indolizine bearing only one benzoyl idines and α-bromo carbonyl compounds. Bearing in mind
moiety attached to the five-membered heterocyclic ring (i.e., that just like indolizines, benzoindolizines such as pyr-
5a; Scheme 1).
rolo[1,2-a]quinolines and pyrrolo[2,1-a]isoquinolines are
Clearly, this is an interesting finding, as indolizines with also of biological and pharmaceutical interest, we then de-
different substitution patterns can be obtained. To develop voted ourselves to investigating the preparation of benzo-
this reaction into a general protocol, pyridines 1, α-bromo indolizines according to the strategy developed above by
ketones 2, and α-substituted allenyl ketones 3 were em- using quinoline and isoquinoline as starting materials. To
ployed as substrates. The results listed in Table 3 show that our delight, treatment of quinoline (6) with 2a and 3a in
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1,2,3-Trisubstituted Nitrogen Heterocycles from 1,2-Allenyl Ketones
Table 3. Scope of the reaction leading to 5.[a]
Entry
R1
R2
R3
R5
5
Yield
[%][b]
Scheme 3. Preparation of pyrrolo[2,1-a]isoquinoline 9.
1
2
3
4
5
6
7
8
9
H
H
m-F
m-F
p-CH3
p-CH3
C6H5
p-FC6H4
C6H5
m-ClC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
CH3
CH3
CH3
CH3
CH3
CH3
5a
5b
5c
5d
5e
5f
5g
5h
5i
78
80
70
64
74
80
72
82
84
73
75
73
benzoyl-substituted pyrrolo[1,2-a]quinolines 10a–c and pyr-
rolo[2,1-a]isoquinolines 11a–g with moderate to good effi-
ciency.
o-ClC6H4
p-CH3 p-CH3OC6H4 C6H5CH2 CH3
Table 4. Scope of the reaction leading to 10 and 11.[a]
p-CH3
p-CH3
H
m-ClC6H4
p-FC6H4
C6H5
C6H5CH2 CH3
C6H5CH2 CH3
C6H5
C6H5
10
11
12
C2H5
C2H5
5j
5k
5l
p-CH3
C6H5
p-CH3 p-CH3OC6H4 C6H5CH2 C2H5
[a] Reaction conditions: 1 (1.0 mmol), 2 (1.0 mmol), 3 (1.0 mmol),
K2CO3 (1.0 mmol), CH3CN (10 mL), reflux, 2 h. [b] Yield of iso-
lated product.
Entry 6 or 8
R2
R3
R5
10 or 11 Yield
[%][b]
the presence of K2CO3 (1 equiv.) in refluxing CH3CN for
2 h afforded the desired (2-methylpyrrolo[1,2-a]quinoline-
1,3-diyl)bis(phenylmethanone) (7) in 78% yield (Scheme 2).
1
2
3
4
5
6
7
8
9
6
6
6
8
8
8
8
8
8
8
C6H5
p-CH3C6H4
C6H5
o-ClC6H4
p-CH3C6H4
p-FC6H4
p-ClC6H4
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
CH3
CH3
C2H5
CH3
CH3
CH3
CH3
10a
10b
10c
11a
11b
11c
11d
11e
11f
11g
62
60
68
90
83
92
88
85
83
93
C6H5CH2 C2H5
p-CH3OC6H4 C6H5CH2 C2H5
p-FC6H4 C6H5 C2H5
10
[a] Reaction conditions: 6 or 8 (1.0 mmol), 2 (1.0 mmol), 3
(1.0 mmol), K2CO3 (1.0 mmol), CH3CN (10 mL), reflux, 2 h.
[b] Yield of isolated product.
Scheme 2. Preparation of pyrrolo[1,2-a]quinoline 7.
Subsequently, the reactivity of isoquinoline (8) was also
studied. Promisingly, the reaction of 8 with 2 and 3 under
On the basis of the above facts, a tentative mechanistic
the standard conditions proceeded efficiently to give (1- rationale for the formation of 4a and 5a is shown in
benzoyl-2-methylpyrrolo[2,1-a]isoquinolin-3-yl)(4-chloro- Scheme 4. First, reaction of 1a with 2a forms pyridinium
phenyl)methanone (9) in 62% yield (Scheme 3).
species I. Upon treatment with a base, it is deprotonated
Finally, the reaction of quinoline (6) or isoquinoline (8) to give zwitterionic intermediate II, which then attacks the
with α-bromo ketones 2 and α-substituted allenyl ketones 3 central carbon atom of the allene unit of 3 to furnish inter-
was also studied. The results listed in Table 4 reveal that mediate III. Subsequent intramolecular nucleophilic ad-
both 6 and 8 reacted smoothly with 2 and 3 to give mono- dition on pyridine affords intermediate IV. If R = H, de-
Scheme 4. Plausible pathways for the formation of 4a and 5a.
Eur. J. Org. Chem. 2014, 713–717
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X. Fan, Y. Wang, Y. He, S. Guo, X. Zhang
SHORT COMMUNICATION
2-bromo-1-phenylethanone (2a; 199 mg, 1.0 mmol), and CH3CN
(10 mL). The mixture was stirred under reflux. Upon completion
of the reaction, as monitored by TLC, the reaction was quenched
with aqueous NH4Cl. Then, the mixture was filtered through Ce-
lite, and the filtrate was extracted with ethyl acetate (3ϫ 10 mL).
The combined organic phases were dried, filtered, and concen-
trated under vacuum. The residue was purified by column
chromatography on silica gel (20% ethyl acetate/hexane) to give (2-
methylindolizine-1,3-diyl)bis(phenylmethanone) (4a; 73%).
hydrogenation of IV occurs to give 4a as the final product,
and in contrast, if R = Me, debenzoylation takes place to
give 5a.
Reactions of 8 are giving higher yields than those of 6,
and on the basis of the above mechanism, this might be
explained by the fact that the reaction site in the isoquinol-
inium species (C-1) is more electrophilic than that in the
quinolinium species (C-2), where the positive charge is dis-
tributed in between C-2 and C-4 (Scheme 5).[2] For the iso-
quinolinium species, the transfer of the positive charge is
unfavorable, because it would result in the loss of aromatic-
ity in both cycles.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details, characterization data, and 1H and 13C
NMR spectra of all products.
Acknowledgments
We are grateful to the Natural Science Foundation of China
(NSFC) (grant numbers 21272058, 21172057), the Research Fund
for the Doctoral Program of Higher Education (RFDP) (grant
number 20114104110005), and the Program for Changjiang Schol-
ars and Innovative Research Team in University (PCSIRT)
(IRT 1061) for financial support.
Scheme 5. Positive charge distribution in the key intermediates.
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We have developed an efficient and convenient tandem
process for the assembly of 1,2,3-trisubstituted indolizines,
pyrrolo[1,2-a]quinolines, and pyrrolo[2,1-a]isoquinolines
through the multicomponent reaction of pyridines (quinol-
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General: The pyridines, quinoline, isoquinoline, and α-bromo
ketones were commercial reagents and used without purification.
The 1,2-allenyl ketones were prepared by oxidation of the corre-
sponding homopropargyl alcohols, which were prepared by zinc-
1
promoted propargylation of the corresponding aldehydes. H and
13C NMR spectra were recorded at 400 and 100 MHz, respectively.
Chemical shifts are reported in ppm from tetramethylsilane as an
internal standard. The conversions of the starting materials were
monitored by thin-layer chromatography (TLC) by using silica gel
plates (silica gel 60 F254, 0.25 mm), and components were visual-
ized by observation under UV light (254 and 365 nm).
Typical Procedure for the Preparation of 4a: K2CO3 (1.0 mmol,
138 mg) and 1-phenylbuta-2,3-dien-1-one (3a; 144 mg, 1.0 mmol)
were added to a flask containing pyridine (1a; 79 mg, 1.0 mmol),
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1,2,3-Trisubstituted Nitrogen Heterocycles from 1,2-Allenyl Ketones
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Received: November 15, 2013
Published Online: January 2, 2014
Eur. J. Org. Chem. 2014, 713–717
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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