Copper(I) iodide-catalysed aerobic oxidative synthesis of imidazo [1,2-α]pyridines from 2-aminopyridines and methyl ketones

Article abstract of DOI:10.1002/adsc.201300456

We report here an operationally simple copper-catalysed synthesis of imidazo [1,2-α]pyridines through C-H activation with oxidative linkage of two C-N bonds under very mild conditions using molecular oxygen as a sole oxidant. The process allows the quick assembly of imidazo [1,2-α]pyridines including the antiulcer drug zolimidine from inexpensive and readily available 2-aminopyridines and methyl ketones with broad range of functional group tolerance.

Full text of DOI:10.1002/adsc.201300456

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DOI: 10.1002/adsc.201300456
Copper(I) Iodide-Catalysed Aerobic Oxidative Synthesis of
Imidazo
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
[1,2-a]pyridines from 2-Aminopyridines and Methyl
Ketones
a
a
Darapaneni Chandra Mohan, Ramachandra Reddy Donthiri,
a
a,
Sadu Nageswara Rao, and Subbarayappa Adimurthy *
Academy of Scientific & Innovative Research, CSIR – Central Salt & Marine Chemicals Research Institute, G.B. Marg,
a
Bhavnagar – 364 002, Gujarat, India
E-mail: adimurthy@csmcri.org
Received: May 20, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300456.
[
10]
Abstract: We report here an operationally simple
copper-catalysed synthesis of imidazo[1,2-a]pyri-
dines through CÀH activation with oxidative link-
age of two CÀN bonds under very mild conditions
using molecular oxygen as a sole oxidant. The pro-
dronic acid (to treat anxiety, heart failure and os-
[
11]
A
H
U
T
E
N
N
teoporosis), zolimidine (peptic ulcer),
necopidem,
(sedative and anxiolytic) and optically
[
12]
saripidem
active GSK812397 candidate (HIV infection)
derived from imidazo[1,2-a]pyridine core entities. In
this context, significant contributions have been made
[13]
are
AHCTUNGTRENNUNG
cess allows the quick assembly of imidazo ACHUTNGRENNUG[ 1,2-a]pyr-
[
14]
[15]
idines including the antiulcer drug zolimidine from
inexpensive and readily available 2-aminopyridines
and methyl ketones with broad range of functional
group tolerance.
by various groups including our own. However,
the preparation of the starting substrates comprises
a multistep procedure. Consequently, a practical and
atom-economical synthesis of IPs from simple starting
materials remains as a challenging task. Compared to
pre-activated ketones (Scheme 1), the use of unacti-
vated methyl ketones as starting substrates for IP syn-
thesis is highly desirable. We report herein the synthe-
[
16]
Keywords: aerobic oxidation; CÀH activation;
copper; imidazo ACHTUNGTRENNUNG[ 1,2-a]pyridines; zolimidine synthe-
sis
The transition metal-catalysed direct CÀN bond for-
mation through CÀH activation by the formal loss of
H or H O would enhance the existing toolbox of syn-
2
2
thetic methods to build complex nitrogen heterocy-
[1]
cles. Such strategies offer a highly atom-economical
approach to enable the efficient construction of
[
2]
a broad range of CÀN bonds in synthetic chemistry,
compared to metal-catalysed CÀN bonds via cross-
coupling reactions of organic halides with amines.
[
3]
The use of molecular oxygen for copper-catalysed
aerobic oxidative CÀN bond formations is highly at-
[
2–4]
tractive
due to its large abundance, low cost and
[5]
the lack of toxic by-products.
Imidazo[1,2-a]pyridine (IP) scaffolds are found in
many pharmacologically important compounds.
Such compounds exhibit antiviral, antibacterial, fungi-
cidal and anti-inflammatory properties. Many com-
mercially available drugs (Supporting Information,
ACHTUNGTRENNUNG
[
6]
[
7]
[
8]
[9]
Figure S1) including alpidem, olprinone, mino- Scheme 1.
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Darapaneni Chandra Mohan et al.
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sis of substituted imidazo AHCUTNGRNENUG[ 1,2-a]pyridines using CuI/ tion temperature was raised from room temperature
[17]
BF ·Et O as catalyst, through CÀH activation in the to 808C (Table 1, entries 4–6). By using DMF as sol-
3
2
presence of O as a sole oxidant. This method affords vent, 3a was obtained in 72% at 608C in 10 h
2
good selectivity, yields, functional group compatibility (Table 1, entry 7). After increasing the amount of 2a
and thereby circumvents the aforementioned issues in to 10.0 mmol (2 equiv.), 3a was obtained in 74% yield
the traditional synthesis of imidazo
the best of our knowledge, a copper-catalysed aerobic was decreased to 10 and 5 mol%, the yield of 3a was
oxidative synthesis of imidazo[1,2-a]pyridines through increased to 76% and 82%, respectively (Table 1, en-
CÀH activation of methyl ketones has not been re- tries 9 and 10). To uphold the reaction yield we
ported previously. screened out other additives, solvents, oxidants and
ACHTUNGERTNNUNG[ 1,2-a]pyridines. To (Table 1,entry 8). When the catalyst loading (CuI)
AHCTUNGTRENNUNG
Initially, we investigated the dehydrogenative annu- copper salts but they were found to be less effective
lation of 2-aminopyridine 1a with acetophenone 2a as (entries 11–16). Reactions performed with other
a model system (Table 1). After reaction of 1a copper salts [CuBr, CuCl, Cu
5 mmol) and 2a (7.5 mmol) with 20 mol% of copper and without catalyst were either poorly effective or
iodide as a catalyst under neat conditions at room entirely ineffective (entries 17–21). These observa-
ACTHUNGTRNENUN(G OAc) and Cu ACHTUNGTRENNUNG( OTf) ]
2
2
(
[
17,18]
temperature for 24 h, 3a was not observed (Ta- tions are in line with the literature methods.
ble 1,entry 1). On using O as the oxidant, a trace As listed in Table 2, the copper-catalysed dehydro-
2
amount of 3a was observed (Table 1, entry 2). On genative annulation of 1a with various methyl ketones
adding BF ·Et O as an additive at room temperature, 2 was found to afford the substituted IPs 3 in moder-
3
2
7
% of 3a was isolated (Table 1, entry 3). A clear im- ate to good yields under aerobic oxidative conditions.
provement of the yields was observed when the reac- The presence of a variety of electron-donating and
[a]
[a]
Table 2. Reaction of 2-aminopyridine with various ketones.
Table 1. Optimization of reaction conditions for 3a.
[
a]
Reaction conditions: 1a (5 mmol), 2a (10 mmol), CuI, ad-
ditive (10 mol%), DMF (2 mL), 608C in an oil bath
under O (balloon) 24 h, unless otherwise stated.
Isolated yields.
2
[
[
[
b]
c]
d]
[a]
2
a (7.5 mmol) used.
Reaction conditions; 1a (5 mmol), 2 (10 mmol), DMF
15% and 8% of 3-iodo-2-phenylimidazo
A
H
U
G
R
N
U
G
(2 mL), in an oil bath under O (balloon) 24 h, isolated
2
were isolated in entries 7, 8 and 9, respectively (nr=no
reaction).
yield unless otherwise stated.
15 mmol of 2 were used.
[b]
2
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Copper(I) Iodide-Catalysed Aerobic Oxidative Synthesis of Imidazo ACHUTNRGNENUG[ 1,2-a]pyridines
[a]
Table 3. Reaction of substituted 2-aminopyridines with various ketones.
[
[
a]
b]
Reaction conditions: 4 (5 mmol), 2 (10 mmol), CuI (0.25 mmol), BF ·Et O
3
2
(
0.5 mmol), DMF (2 mL), in an oil bath under O (balloon) 24 h, isolated
2
yields.
5 mmol of 2 were used.
1
electron-withdrawing groups in the acetophenone
Finally, we demonstrated the feasibility of the pres-
moieties at either the (o/p) position could be tolerat- ent catalytic system for the synthesis of IPs by the re-
ed and afforded the desired IPs in 68–89% yields action of various methyl ketones 2 with substituted 2-
(
3b–3j). Electronic effects associated with electron- aminopyridines 4 under the optimum conditions
donating/electron-withdrawing substituents and steric (Table 3). Thus, the domino syntheses of 5a–5f were
factors on the ketone do not affect the efficiency of obtained in good to excellent yields (68–93%). While
the process, as substantiated by the presence of two comparing the reactions of 4, it is evident that elec-
ortho methoxy groups in acetophenone to afford the tron-donating substituents seem to have no influence
desired product 3e in high yield (89%). The present on the product yield, whereas electron-withdrawing
catalytic system is also applicable to heterocyclic and substituents led to lower yields (5g–5i). This observa-
bicyclic methyl ketones which smoothly reacted under tion may be due to the low nucleophilicity of acceptor
typical conditions, affording the desired products 3k– substituted aminopyridines.
3n in 59–78% yields. Interestingly, the a,b-unsaturat-
The results of Table 2 and Table 3 indicate that, the
ed methyl ketone (E)-4-phenylbut-3-en-2-one also re- dehydrogenative annulation of various methyl ke-
[
19]
sulted in a moderate yield (56%) of 3o. Aliphatic tones with 2-aminopyridines in the presence of CuI/
ketones, for instance, acetone and tert-butyl methyl BF ·Et O is an efficient process to afford IP deriva-
3
2
ketone, were also subjected to this reaction, and suc- tives in moderate to excellent yields. Next we at-
cessfully provided alkyl-substituted IPs 3p and 3q in tempted the synthesis of zolimidine through this
51% and 67% yields, respectively. However, a mixture novel strategy (Scheme 2). This drug has been synthe-
of unknown products resulted instead of 3r when 1a sized on a gram scale (2.411 g) successfully. It is note-
was reacted with ethyl 3-oxo-3-phenylpropanoate.
worthy to mention that zolimidine (7a) is usually syn-
Scheme 2. Synthesis of zolimidine.
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Darapaneni Chandra Mohan et al.
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vides the desired product 3a with the regeneration of
the Cu(I) catalyst to continue the catalytic cycle.
We have developed a copper-catalysed aerobic oxi-
dative annulation of 2-aminopyridines with methyl
ketones to access imidazo ACHUTNRGNENUG[ 1,2-a]pyridines under mild
conditions. The process allows the quick and facile
synthesis of IPs including zolimidine from readily and
commercially available substrates. The mechanistic
studies indicate the essential role of CuI and O for
2
the present investigation.
Experimental Section
Scheme 3. Mechanistic experiments.
Typical Procedure for the Synthesis of 2-
Phenylimidazo ACHUTNGRENNUG[ 1,2-a]pyridine (3a)
[
14f]
thesized in a multistep procedure.
This is the first
4
(
0
70 mg (5.0 mmol) of 2-aminopyridine 1a, 1200 mg
10 mmol) of acetophenone, CuI 5 mol% (47 mg;
.25 mmol), BF ·Et O (45–50% purity); 10 mol%, 0.5 mmol)
report of a single-step synthesis of zolimidine.
To gain insight into the reaction mechanism, addi-
tional experiments were performed under the opti-
3
2
and DMF (2 mL) were placed in a 25-mL double-necked
mised conditions (Scheme 3). Initially, the reaction of round-bottomed flask. The mixture was heated in an oil
1
a and 2a was performed with a stoichiometric bath at 608C for 24 h under an oxygen atmosphere (bal-
amount of a radical scavenger (TEMPO), 65% of 3a loon). After completion of the reaction, it was allowed to
attain to room temperature and then the mixture was
was isolated, ruling out the radical reaction path-
[
20]
poured into 20 mL of sodium carbonate solution. The prod-
uct was extracted with DCM (50 mLꢁ3) and dried with an-
hydrous Na SO . Removal of the solvent under reduced
pressure left a residue that was purified through column
chromatography using silica gel (30% EtOAc/hexane) to
afford 3a; yield: 0.799 g (82%).
way. When the same reaction was performed under
a nitrogen atmosphere only traces of 3a were ob-
served. Furthermore, the reaction was attempted
using a Cu(II) salt under a nitrogen atmosphere, no
2
4
[
18]
reaction occurred. These studies reveal the crucial
role of oxygen and Cu(I) for an efficient transforma-
tion. Thus, a plausible mechanism is proposed in
Scheme 4.
Initially, 2-aminopyridine 1a reacts with methyl Acknowledgements
ketone 2a to form imine A, which is in equilibrium
[21]
with enamine B. The oxidative addition of copper We thank Dr. A. V. Boricha, Mr. B. M. Bhil, and Mr.
Arun K. Das, as well as Ms. Jigna of this institute for record-
iodide in the presence of oxygen forms the copper-
[22]
ing NMR, and HR-MS, respectively. D.C.M, R.R.D. and
S.N.R are thankful to UGC and CSIR – New Delhi, India for
their fellowships. DST, Govt. India (SR/S1/OC-13/2011) and
CSIR-CSMCRI (OLP-0042) are acknowledged for financial
support.
A( III) complex C.
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Reductive elimination of C pro-
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6
Copper(I) Iodide-Catalysed Aerobic Oxidative Synthesis of
Imidazo
ACHTUNGTRENNUNG[ 1,2-a]pyridines from 2-Aminopyridines and Methyl
Ketones
Adv. Synth. Catal. 2013, 355, 1 – 6
Darapaneni Chandra Mohan,
Ramachandra Reddy Donthiri, Sadu Nageswara Rao,
Subbarayappa Adimurthy*
6
asc.wiley-vch.de
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 0000, 000, 0 – 0
ÝÝ
These are not the final page numbers!