One-pot synthesis of polysubstituted 3-acylpyrroles by cooperative catalysis

Article abstract of DOI:10.1039/c4ob01019a

One-pot syntheses of polysubstituted 3-acylpyrroles from readily available unsaturated ketones and N-substituted propargylated amines have been developed. An aza-Michael/alkyne carbocyclization cascade, by cooperative catalysis using pyrrolidine and a copper salt, followed by oxidation in situ gave 3-acylpyrroles, which were also transformed further to functionalized, highly substituted 3-acylpyrroles. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob01019a

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Biomolecular Chemistry
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One-pot synthesis of polysubstituted
3-acylpyrroles by cooperative catalysis†
Cite this: Org. Biomol. Chem., 2014,
12, 5822
Hai-Lei Cui and Fujie Tanaka*
Received 17th May 2014,
Accepted 17th June 2014
DOI: 10.1039/c4ob01019a
www.rsc.org/obc
One-pot syntheses of polysubstituted 3-acylpyrroles from readily
available unsaturated ketones and N-substituted propargylated
amines have been developed. An aza-Michael/alkyne carbocycliza-
tion cascade, by cooperative catalysis using pyrrolidine and a
copper salt, followed by oxidation in situ gave 3-acylpyrroles,
which were also transformed further to functionalized, highly sub-
stituted 3-acylpyrroles.
Polysubstituted pyrroles and their unsaturated derivatives are
structural motifs often found in natural products and pharma-
ceuticals.1 In particular, 3-acylpyrroles are present in many
drugs and biologically active compounds (Fig. 1).2 For
example, molindone2a and piquindone2a are dopamine recep-
tor antagonists and some Hsp90 inhibitors bearing the 3-acyl-
pyrrole framework exhibit nanomolar antiproliferative
activities across multiple cancer cell lines.2b,c The fact that all
these compounds possess a 3-acylpyrrole moiety suggests that
this framework may play an important role in the origin of the
bioactivities, and molecules bearing the 3-acylpyrrole core
have been synthesized for screening against a number of thera-
peutic agents.1–3 Most of these 3-acylpyrroles have been syn-
thesized from β-ketocarbonyl compounds or β-enaminones,
imposing restrictions on the diversity of the 3-acylpyrrole pro-
ducts.2,3 The development of concise, efficient methods for the
synthesis of 3-acylpyrroles and related heterocycles that allow
access to a diverse set of these molecules from readily available
starting materials remains a great challenge and has attracted
much attention.3
Fig. 1 Pharmaceuticals and biologically active compounds bearing
3-acylpyrrole frameworks and one-pot route to the 3-acylpyrroles by
cooperative catalysis.
C–O bond formation via an enamine/iminium cascade6 led us
to investigate the combination of amine-cascade catalysis and
metal catalysis.
Recently, cooperative catalysis that combines both nucleo-
phile activation and electrophile activation using organocata-
lysts and metal catalysts has emerged to efficiently construct
useful building blocks.4,5 Our previous efforts to synthesize
heterocyclic compounds based on amine-catalyzed C–C and
Inspired by recent achievements of the combination of tran-
sition-metal catalysis and organocatalysis,4,5 we set out to syn-
thesize highly substituted 3-acylpyrroles via the synergistic
activation of enones and alkynes7 to generate dihydropyrroles
and by subsequent oxidation of the dihydropyrroles in one pot
(Fig. 1).8 This strategy may allow for the formation of C–N and
C–C bonds to construct polysubstituted pyrroles with variation
of substituents at all the five positions of the 3-acylpyrrole.
For this strategy, however, there are also challenges to be
Chemistry and Chemical Bioengineering Unit, Okinawa Institute of Science and
Technology Graduate University, 1919-1 Tancha, Onna, Okinawa 904-0495, Japan.
E-mail: ftanaka@oist.jp; Fax: +81-98-966-1064
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization data, and spectral data. See DOI: 10.1039/c4ob01019a
5822 | Org. Biomol. Chem., 2014, 12, 5822–5826
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addressed, such as poor activity of secondary amines in aza- the presence of pyrrolidine alone without a metal catalyst, in
Michael reactions,9 discrimination between reactant amines the presence of pyrrolidine and PPh3 without a metal catalyst,
and catalyst amines, and turnover of amine catalysts. Here, we or in the presence of the Cu(OTf)2–PPh3 system without pyrro-
disclose efficient methods to synthesize polysubstituted 3-acyl- lidine also did not give 3a (entries 13–15). These results clearly
pyrroles by cooperative catalysis from readily available unsatu- indicate that the combination of nucleophile activation and
rated ketones and N-substituted propargylated amines using a electrophile activation by pyrrolidine and the Cu(PPh3)3OTf is
secondary amine and a copper salt followed by oxidation in crucial for the formation of the dihydropyrrole product.
one pot.
A solvent screening for the reaction using the Cu(OTf)2–
To develop a general catalytic system for the synthesis of PPh3–pyrrolidine catalyst system identified that PhCF3 was the
3-acylpyrroles, first, the reaction of 1a and 2a to form 3a in the best solvent among those tested to afford 3a in high yield
presence of pyrrolidine and various metal catalysts was investi- (entries 16–18). Thus, the optimal results for the synthesis of
gated (Table 1, entries 1–7). When the reaction was performed dihydropyrrole 3a were obtained under the reaction conditions
using Cu(OTf)2 as the metal catalyst in the presence of PPh3 to using 1a (1 equiv.) and 2a (1.5 equiv.) in the presence of
generate active species Cu(PPh3)3OTf,5a the product 3a was Cu(OTf)2 (5 mol% to 1), PPh3, (20 mol%), and pyrrolidine
obtained in 75% yield (entry 6). Without PPh3, no product for- (20 mol%) in PhCF3 at room temperature (entry 18).11
mation was detected (entry 5).10 CuI and the combination of
The aza-Michael/carbocyclization cascade reaction system
AgOTf with PPh3 showed comparable catalytic activities to the was then combined with oxidation by MnO2 to synthesize
Cu(OTf)2 with PPh3 (entries 4 and 7 versus entry 6). For the Pd 3-acylpyrroles 4 in one pot (Table 2).12 Using our method,
series, the reaction using Pd(OAc)2 as the metal catalyst gave annulated and non-annulated 3-acylpyrroles with various sub-
3a (entry 3), but Pd(PPh3)4 and Pd(PPh3)2Cl2 were not effective stituents including aryl and alkyl groups at the 1-position of
(entries 1 and 2).
the pyrrole were readily obtained. In the one-pot reactions of
Next, the reaction of 1a and 2a using the Cu(OTf)2–PPh3 cyclohexenone with propargylated amines, products with
system as the metal catalyst in the presence of amines other various N-substituents 4a–4e were obtained in 55–84% yield
than pyrrolidine was evaluated (entries 8–12). In these reac- (entries 1–6). Reactions of dimethylsubstituted cyclohexenones
tions, formation of 3a was very slow or negligible. Reactions in and cycloheptenone also afforded the tetrasubstituted pyrroles
bearing the six-membered and the seven-membered ring
systems, respectively (products 4f–4h), in good yields (70–82%,
Table 1 Screening of catalysts and conditionsa
entries 7–11). Note that 4g, 3-acylpyrrole bearing a quaternary
carbon center at the 2-position, which cannot be synthesized
or is very difficult to synthesize by previously reported
methods,2,3 however, was easily obtained using our method
(entries 9 and 10). Reactions of acyclic enones bearing alkyl,
aryl, heteroaryl, and ester groups also afforded 3-acylpyrroles
4i–4n with various substitutions at the 2-position and the
3-position in 20–85% yield (entries 12–17), including pyrrole
with variation of the acyl group at the 3-position of pyrrole
(i.e., 4l, entry 15). Pentasubstituted pyrrole 4o was also
obtained in 50% yield (entry 18). In the reaction of an arylated
alkyne, product 4p was not obtained under the conditions
tested (rt to 60 °C for the first step and rt to 50 °C for the oxi-
dation step) probably due to steric reasons including the
potential allylic strain in the transition state to form the corres-
ponding dihydropyrrole. Note that the one-pot synthesis of
3-acylpyrrole reactions were easily performed on gram-scale;
3-acylpyrroles 4a, 4f and 4g were obtained in 61% to 81%
yields (entries 2, 8, and 10) with the use of a lower loading of
propargylated amine 2 and MnO2.13
The method was further expanded to the reaction of propar-
gyl alcohol with enone, which gave dihydrofuran 5 in 84%
yield, and 3-acylfuran 6 was obtained after oxidation of 5 with
DDQ (Scheme 1).
As described above, N-substituted 3-acylpyrroles are mole-
cules of interest because of their biological activities and their
uses for the further syntheses of pyrrole-containing derivatives.
Thus the tetrasubstituted pyrroles bearing an acyl group at the
3-position were transformed further. For example, pentasubsti-
Time
(h)
Yieldb
(%)
Entry
Metal
Amine
Solvent
1
2
3
4
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd(OAc)2
CuI
Cu(OTf)2
Cu(OTf)2
AgOTf
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
—
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Pyrrolidine
Piperidine
Diethylamine
Benzylamine
Quinine
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
50
22
22
22
25
17
17
47
25
25
25
25
25
25
25
48
25
22
7
12
46
54
ND
75
63
5
6c
7c
8c
26
9c
ND
ND
ND
ND
ND
ND
ND
60
10c
11c
12c
13
14c
15c
16c
17c
18c
DMAP
Pyrrolidine
Pyrrolidine
—
Pyrrolidine
Pyrrolidine
Pyrrolidine
—
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Toluene
PhCF3
61
91
a Reaction of 1 (0.2 mmol) and 2 (0.3 mmol) was performed using
amine (0.04 mmol) and a metal catalyst (0.01 mmol) in the solvent
(0.2 mL) at rt (25 °C). b Isolated yield of 3a. ND = formation of 3a was
not detected. c PPh3 (0.04 mmol) was added.
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Table 2 One-pot synthesis of 3-acylpyrrolesa
Scheme 1 Synthesis of 2,5-dihydrofuran and 3-acylfuran.
tuted pyrroles were obtained by the introduction of substitu-
ents at the 5-position; Pd-catalyzed C–H functionalization of
4d and 4h afforded 7 and 8, respectively (Scheme 2).14
A plausible catalytic cycle for the formation of 3 is shown in
Scheme 3. The enone is activated by pyrrolidine to generate an
iminium ion intermediate for the aza-Michael reaction of sub-
stituted propargylamine. The resulting enamine intermediate
reacts with the triple bond upon activation by the copper salt
to undergo an intramolecular carbocyclization. Further
protonolysis, isomerization, hydrolysis and oxidation give the
desired 3-acylpyrroles.
First step
temp
First step
time (h)
Oxidation
step time (h)
Yield
(%)
Entry
Product
1
4a
4a
4b
4c
4d
4e
4f
rt
rt
rt
rt
rt
40 °C
rt
rt
rt
23
51
45
67
24
22
24
54
20
69
55
47
48
22
52
20
49
24
5b
68
44
31
24
66
25b
50
43
96d
24
7
30
66
42
46
25
38f
84
66
69
55
84
59
82
81
71
61
70
85
60
44
20
47
60
50
2c
3
Scheme 2 Transformation to the fully substituted 3-acylpyrroles.
4
5
6
7
8c
9
4f
4g
4g
4h
4i
4j
4k
4l
4m
4n
4o
10c
11
12
13
14
15
16
17
18
rt
rte
rt
rt
40 °C
40 °Ce
40 °C
rt
rt
a Reaction of 1 (0.2 mmol) and 2 (0.3 mmol) was performed using
pyrrolidine (0.04 mmol, 20 mol% to 1), PPh3 (0.04 mmol, 20 mol% to
1), and Cu(OTf)2 (0.01 mmol, 5 mol% to 1) in PhCF3 (0.2 mL,
concentration of 1: 1 M) at rt (25 °C). Upon consumption of 1, MnO2
(4.0 mmol) in CH2ClCH2Cl (2 mL) was added and the reaction mixture
was stirred at 40 °C. b For the oxidation step, CH2Cl2 was used at rt
instead of CH2ClCH2Cl at 40 °C. c 10 mmol scale (gram-scale) reaction;
1 (10 mmol) and 2 (12 mmol).13 d 40 °C for 72 h and 60 °C for 24 h.
e MeOH was used instead of PhCF3 based on a screen of conditions
(the reactions in PhCF3 gave the product in lower yields). f Oxidation
step at 50 °C. g Formation of 4p was not detected; see text.
Scheme 3 Plausible catalytic cycle.
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Conclusions
We have developed efficient methods for the one-pot syntheses
of polysubstituted 3-acylpyrroles by cascade reactions from
readily available unsaturated ketones and N-substituted pro-
pargylated amines using cooperative catalyst systems. Sub-
strates with a broad range of functional groups reacted readily,
and the method allowed concise, atom-economical assembly
of the 3-acylpyrrole frameworks and the 3-acylfuran variant. In
addition, the reactions were able to be scaled up to provide the
products in good yields on a gram-scale for further biomedical
research. The use of the reaction systems was also demon-
strated by the transformation of the products to the fully
substituted 3-acylpyrroles. Further studies on synthetic appli-
cations of these methods are ongoing in our laboratory.
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Acknowledgements
We thank Dr Michael Chandro Roy, Research Support Divi-
sion, Okinawa Institute of Science and Technology Graduate
University for mass analyses. This study was supported by the
Okinawa Institute of Science and Technology Graduate Univer-
sity and by the MEXT (Japan) Grant-in-Aid for Scientific
Research on Innovative Areas “Advanced Molecular Transform-
ations by Organocatalysts”.
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