Copper-catalyzed synthesis of imidazo[1,2-a]pyridines through tandem imine formation-oxidative cyclization under ambient air: One-step synthesis of zolimidine on a gram-scale

Article abstract of DOI:10.1002/adsc.201300298

A new copper-catalyzed oxidative cyclization via C-H amination between 2-aminopyridines and methyl aryl/heteroaryl ketones has been developed under ambient air. Imidazo[1,2-a]pyridines containing a wide range of functional groups have been synthesized from basic and easily available starting materials. This simple, one-pot reaction protocol is applicable for the direct preparation of zolimidine (a marketed antiulcer drug) on a large scale. Copyright

Full text of DOI:10.1002/adsc.201300298

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DOI: 10.1002/adsc.201300298
Copper-Catalyzed Synthesis of Imidazo[1,2-a]pyridines through
N
Tandem Imine Formation-Oxidative Cyclization under Ambient
Air: One-Step Synthesis of Zolimidine on a Gram-Scale
Avik Kumar Bagdi,a Matiur Rahman,a Sougata Santra,a Adinath Majee,a
and Alakananda Hajraa,*
a
Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India
Fax : (+91)-3463-261526; phone: (+91)-3463-261526; e-mail: alakananda.hajra@visva-bharati.ac.in
Received: April 11, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300298.
Abstract: A new copper-catalyzed oxidative cycliza-
tion via C H amination between 2-aminopyridines
and methyl aryl/heteroaryl ketones has been devel-
oped under ambient air. ImidazoACTHNUTRGNEUNG[1,2-a]pyridines
this context, some efficient approaches have been de-
veloped employing less expensive copper catalysts
and O2 as the oxidant for the synthesis of biologically
important moieties via oxidative couplings.[5] So the
À
containing a wide range of functional groups have
been synthesized from basic and easily available
starting materials. This simple, one-pot reaction
protocol is applicable for the direct preparation of
zolimidine (a marketed antiulcer drug) on a large
scale.
Keywords: ambient air; copper; cyclization;
imidazo
midine
ACHTUNGTRENNUNG[1,2-a]pyridines; oxidative amination; zoli-
À
The transition metal-catalyzed oxidative C H bond
amination has attracted great interest in recent times
of synthetic organic chemists. Various priviledged het-
erocyclic scaffolds have been synthesized using inter-
À
or intramolecular amination via C H fuctionaliza-
tion.[1] C H functionalization and subsequently C N
bond formation is more preferable than the conven-
tional synthetic methods due to atom efficiency and
environmentally benign conditions.[2] Additionally,
these types of reactions are very useful as the pre-
functionalization of the substrate is not required, so
avoiding the multistep synthesis, and contribute to
changing the way chemists think about chemical reac-
À
À
À
tivity and planning chemical syntheses. Direct C N
À
bond formation through C H bond functionalization
is commonly catalyzed by Pd[3] or Cu.[4] Many re-
agents such as TBHP, DTBP, PhIACTHUNTGRNEGN(U OAc)2, H2O2, O2,
mCPBA etc. are used as common oxidants among
which O2 is most preferable due to its inexpensive, in-
exhaustible and environmentally benign features. In oxidative C H amination.
Scheme 1. Synthetic strategies to imidazo
[1,2-a]pyridines via
À
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Table 1. Optimization of the reaction conditions.
[a]
[b]
[c]
[d]
Yields of isolated products.
1,2-Dichlorobenzene.
Tetramethylethylenediamine.
Dimethylethylenediamine.
synthesis of biologically and medicinally active heter- developed to construct the imidazoACTHNUTRGNEUNG
[1,2-a]pyridines.[8]
ocyclic compounds using copper salts under aerobic A simple method is the condensation of 2-aminopyri-
conditions is an emerging field of organic synthesis in dines with a-halo ketones.[7e] Three-component cou-
respect of economical as well as ecological points of pling is also another way to construct these moie-
view.
The imidazopyridine nuclei, in particular the a]pyridines by a tandem cyclization between 2-amino-
imidazo
[1,2-a]pyridines, constitute an important class pyridine and nitroolefins.[8i] But only a few approaches
ties.[8a] Recently, we have synthesized imidazo
ACHUTGTNRENNUG ACHTUNGTRENNUNG[1,2-
ACHTUNGTRENNUNG
À
of biologically active nitrogen-containing heterocycles are reported based on metal catalyzed C H activation
and exhibit a wide range of biological activities.[6] (Scheme 1, A and B).[9] However, although existing
À
Imidazo
G
a number of commercial drugs such as zolpidem,[7a] al- construction of the imidazo
pidem,[7a] olprinone,[7b] zolimidine,[7c] necopidem and through direct amination from readily available and
saripidem.[7d] Some of these heterocyclic moieties dis- simple starting materials employing inexpensive metal
catalysts with environmentally benign oxidants is
Considering their wide range of activities, the syn- highly desirable. Although ketones having electron-
theses of imidazo[1,2-a]pyridines are getting much at- withdrawing groups have been used previously
traction in recent times. Various methods have been (Scheme 1, C),[8g,h] simple acetophenones have not yet
play excited-state intramolecular proton transfer.[7e]
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Copper-Catalyzed Synthesis of ImidazoACHTNUTRGNEUNG[1,2-a]pyridines
Table 2. Substrate scopes of aminopyridines.[a]
[a]
Yields of isolated products.
been explored as an oxidative coupling partner for in- vents such as DMSO, DMF, and toluene (Table 1, en-
À
tramolecular C N bond formation/cyclization. Oxida- tries 8, 9 and 10) did not improve the yields. Various
tive amination is quite feasible for b-keto esters/ke- other copper catalysts like CuI, CuBr, CuCl, CuBr2,
tones due to the easy formation of enamines while it CuCl2, Cu
is not so easy for simple methyl aryl ketones due to lyze the reaction also. However, palladium salts
the non-isolable enamine. Herein, we report a direct [Pd(OAc)2, PdCl2] were not so effective (Table 1, en-
approach for the synthesis of imidazo[1,2-a]pyridines tries 17 and 18) and produced lower amounts of prod-
from readily available 2-aminopyridines and methyl ucts. Thus combination of Cu(OAc)2·H2O
ACHTUNGRTENN(UNG OTf)2 (Table 1, entries 11–16) could cata-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
a
ACHTUNGTRENNUNG
aryl/heteroaryl ketones using a copper salt as the cat- (10 mol%), 1,10-phenanthroline (10 mol%), and ZnI2
À
alyst under aerobic conditions via oxidative C H ami- (10 mol%) in 1,2-dichlorobenzene at 1208C was
nation (Scheme 1, D).
found to represent the optimized reaction conditions
For the optimization of the reaction conditions, re- affording 84% isolated yield (Table 1, entry 2) under
action between 2-aminopyridine 1a and acetophenone ambient air.
2a was carried out using different metal catalysts and
solvents (Table 1). Initially the reaction was per- conditions were examined by employing various 2-
formed using Cu(OAc)2·H2O (10 mol%), 1,10-phen- aminopyridines and acetophenone (Table 2). First, the
ACHTUNGTRENNUNGanthroline (10 mol%) in 1,2-dichlorobenzene at effect of substituents on the 2-aminopyridine moiety
The scope and limitations of the optimized reaction
AHCTUNGTRENNUNG
1208C under aerobic conditions (Table 1, entry 1) and was tested. Methyl substituents at 3- and 4-positions
a moderate yield (50%) was obtained under these afforded high yields of the products (3b, 3c). But with
conditions. To improve the yield a soft Lewis acid, a methyl group at the 6-position on the pyridine ring
ZnI2 (10 mol%) was used as additive[11] (Table 1, the reaction proceeded sluggishly (3d). We were
entry 2) whereby an 84% isolated yield of desired pleased to notice that under the stated conditions,
product was obtained. In this context, it is worthy to aminopyridines substituted with halogens such as Cl,
mention that recently Nagasawa et al. reported oxida- Br, I (3e, 3f, and 3g) smoothly reacted with acetophe-
tive coupling between 2-aminopyridines and benzoni- none without forming any dehalogenated products.
triles where ZnI2 was found to be a very effective ad- Further functionalization on imidazoACHTUNTRGNEUNG[1,2-a]pyridine
ditive for the cyclization.[11] Other additives such as containing I and Br substituents could be possible via
ZnCl2, molecular sieves, TBAB etc. were studied, but cross-coupling reactions.
these are not so effective as ZnI2. Ligands such as bi-
Then our attention was turned to the use of substi-
pyridine, TMEDA, DMEDA, 8-hydroxyquinoline tuted acetophenones as well as heteroaryl ketones to
(Table 1, entries 4–7) produced lower yields. Howev- prove the general applicability of the reaction condi-
er, in the absence of any ligand (Table 1, entry 3) only tions. A library of imidazo
ACHTUNGTNER[NUNG 1,2-a]pyridines was synthe-
60% yields were obtained. Use of other common sol- sized by reacting aminopyridine with substituted ace-
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Table 3. Substrate scopes of ketones.[a]
[a]
[b]
Yield of isolated products.
Inseparable mixture.
tophenones (Table 3). Acetophenones bearing elec- An acetophenone containing a strong electron-with-
tron-withdrawing as well as electron-donating groups drawing group such as CF3 also produced the product
afforded the corresponding imidazoACTHNUTRGNE[NUG 1,2-a]pyridines (3t) with excellent yield. 1,4-Diacetylbenzene reacted
with high to excellent yields. Functional groups like only with one equivalent aminopyridine to afford the
Me (3h, 3o), OMe (3i, 3p), Cl (3j, 3q), OH (3k, 3r), product (3z) having an unreacted acetyl group. 2-Hy-
NO2 (3m, 3s) were unaffected under the reaction con- droxyacetophenone on reaction with 2-aminopyridine
ditions and the desired products were obtained in 74– afforded the product (3k) which displays excited-state
83% yields. Interestingly, 4-iodoacetophenone also af- intramolecular proton transfer (ESIPT).[7e] 4’-(Methyl-
forded the imidazo
Heteroaryl ketones reacted well without accompany- imidazo
ing self-condensation or ring cleavage (3w, 3x, 3y). the drug zolimidine.[9c] 2-Phenylacetonephenone af-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Copper-Catalyzed Synthesis of ImidazoACHTNUTRGNEUNG[1,2-a]pyridines
Scheme 2. One-pot synthesis of zolimidine.
Scheme 3. Functionalization of 3-unsubstituted imidazopyridine.
forded 2,3-disubstituted imidazo
moderate yield. However, propiophenone produced pending, a probable mechanism is shown in Scheme 4.
an inseparable mixture of products (3aa). Initially, imine A is formed by the reaction of amino-
A
Although detailed experimental evidence is still
A most important achievement of our protocol is pyridine and acetophenone under the present reaction
the synthesis of the marketed drug zolimidine (an an- conditions. The imine after tautomerization to enam-
tiulcer drug) in one-pot (Scheme 2). Reaction be- ine B reacted with CuACHTNUGRTNEUNG
(OAc)2 to form the adduct C[12]
tween 2-aminopyridine and commercially available 4’- which is readily converted into the intermediate
(methylsufonyl)acetophenone produced zolimidine in D.[9a,11] Intermediate D [Cu(II)] is converted into E
a single step with 72% yield. The preparation of the which is then oxidized to the reactive intermediate F
drug is also applicable on the gram-scale affording [Cu
66% of isolated product. 3a through reductive elimination along with Cu(I)
Further functionalization has been carried out to species which is oxidized by the aerobic oxygen to
(III)].[13] The intermediate F afforded the product
ACHTUNGTRENNUNG
construct the highly substituted imidazo
G
À
dines through C H activation employing a modified
method (Scheme 3).[10] The reaction proceeded very direct oxidative cyclization via C H amination be-
À
well affording high yields.
Scheme 4. Probable reaction mechanism.
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and reliable method. This one-pot simple reaction
protocol has been utilized for the direct preparation
of zolimidine, a marketed antiulcer drug on a large
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ACHTUNGTRENNUNG
line (0.1 mmol) and ZnI2 (0.1 mmol) in 1,2-dichlorobenzene
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Acknowledgements
A. H. and A. M. acknowledge the financial support from
DST, Govt. of India (Grant No. SR/S5/GC-05/2010). We are
thankful to DST-FIST and UGC-SAP. A. K. B. and S. S.
thank CSIR and UGC for their fellowships.
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8 Copper-Catalyzed Synthesis of ImidazoACHTNUTRGNEUNG[1,2-a]pyridines
through Tandem Imine Formation-Oxidative Cyclization
under Ambient Air: One-Step Synthesis of Zolimidine on
a Gram-Scale
Adv. Synth. Catal. 2013, 355, 1 – 8
Avik Kumar Bagdi, Matiur Rahman, Sougata Santra,
Adinath Majee, Alakananda Hajra*
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