Iron(III)-Catalyzed Cascade Reaction between Nitroolefins and 2-Aminopyridines: Synthesis of Imidazo[1,2-a]pyridines and Easy Access towards Zolimidine

Article abstract of DOI:10.1002/adsc.201201112

The iron(III)-catalyzed one-pot cascade reaction between nitroolefins and 2-aminopyridines has been demonstrated for the synthesis of imidazo[1,2-a] pyridines by exploiting the bielectrophilic nature of nitroolefins. This methodology could be successfully

Full text of DOI:10.1002/adsc.201201112

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DOI: 10.1002/adsc.201201112
Iron(III)-Catalyzed Cascade Reaction between Nitroolefins and
G
2-Aminopyridines: Synthesis of Imidazo
Access towards Zolimidine
N
Sougata Santra,a Avik Kumar Bagdi,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: December 20, 2012; Revised: March 9, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201112.
Abstract: The ironACHTNUGRTENUNG(III)-catalyzed one-pot cascade
reaction between nitroolefins and 2-aminopyridines
has been demonstrated for the synthesis of imidazo-
ACHTUNGTRENNUNG[1,2-a]pyridines by exploiting the bielectrophilic
nature of nitroolefins. This methodology could be
successfully applicable for the synthesis of zolimi-
dine, a useful drug for the treatment of peptic ulcer.
The reaction proceeds through Michael addition
followed by intramolecular cyclization and in situ
denitration.
Keywords: cascade reactions; imidazo
ACHTUNGTREN[NUNG 1,2-a]pyri-
Scheme 1. Various modes of addition of nucleophiles to ni-
troalkenes.
dines; iron(III) chloride; nitroolefins; zolimidine
ACHTUNGTRENNUNG
1,4-conjugate addition, less attention has been paid to
investigate the bielectrophilic nature of nitroalkenes
Cascade reactions are one of the most powerful and and their applications in organic synthesis to afford
atom economical methodologies in contemporary or- the carbocyclic and heterocyclic compounds in a sys-
ganic synthesis.[1] These reactions usually provide pro- tematic way.
cesses in a more efficient and environmentally benign
Recently, we have described a simple, and straight-
manner than conventional procedures by omitting the forward one-pot synthesis of benzofuran and naph-
steps of separation and purification of the reaction in- thofuran derivatives from readily available a,b-unsa-
termediates. The conjugate addition of nucleophiles turated nitroalkenes by coupling with phenols/naph-
to a,b-unsaturated compounds (such as a,b-unsaturat- thols. (Scheme 2, Path a).[4] A Michael addition of
ed carbonyl compounds, esters, nitriles, phosphates, phenol/naphthols to a,b-unsaturated nitroalkenes and
sulfones, and nitroalkenes) is known as the Michael followed by intramolecular Friedel–Crafts cyclization
addition.[2] It is a versatile reaction for carbon-carbon are the key steps for this methodology and are fol-
and carbon-heteroatom bond formations. Both Mi- lowed by in situ denitration process to give the prod-
chael and hetero-Michael additions are widely used in uct. Encouraged by this result, we continued our in-
inter- and intramolecular reactions to provide biologi- terest to explore this methodology for the synthesis of
cally important scaffolds. Among the various com- various important building blocks. Consequently, we
monly used Michael acceptors nitroolefins are excel- searched for a precursor having two nucleophilic sites
lent substrates for 1,4-addition due to the strong elec- capable of double nucleophilic addition to nitroalk-
tron-withdrawing ability of the nitro group and low
ACHTUNGTRENeNGNU nes. We chose 2-aminopyridine as it contains two nu-
tendency for 1,2-addition.[3] In addition nitroalkenes cleophilic nitrogen atoms which might be capable for
are bielectrophilic in nature and they are potentially multiple nucleophilic additions to the nitroalkenes to
useful for multiple nucleophilic additions to form het- produce imidazopyridine derivatives (Scheme 2, Path
erocycles in a cascade fashion (Scheme 1). Although b).
various methodologies have been developed for the
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dicinal and pharmaceutical chemistry. Several meth-
ods are available for the synthesis of imidazoACHTUNGTRENNUNG[1,2-
a]pyridines such as condensation of 2-aminopyridine
with a-halo carbonyl compounds,[17] one-pot conden-
sations of aldehydes, isonitriles, and 2-aminopyri-
dines,[18] copper-catalyzed three-component reactions
of 2-aminopyridines, aldehydes, and alkynes,[19] oxida-
[20]
À
tive couplings through C H activation, and very re-
cently, from Morita–Baylis–Hillman acetates of nitro-
alkenes.[21] However, most of these methodologies
have been developed using expensive reagents, com-
mercially less available alkynes, and a-halogeno car-
bonyl compounds which have lachrimatory properties.
Therefore, finding a new methodology for the synthe-
sis of imidazoACHTUNTRGNEUNG[1,2-a]pyridines in terms of efficiency,
operational simplicity, availability of starting materi-
als, and economic practicability is highly desirable.
Over the past few years iron salts have been shown to
be effective alternatives and promising transition
metal catalysts and have received much attention due
Scheme 2. Cascade reaction of nitroolefins with binucleo-
philes.
The imidazopyridine nucleus, in particular the to their low price, sustainability, ready availability,
imidazoACHTUNGTRENNUNG[1,2-a]pyridine core, represents a significant non-toxicity, and environmentally friendly proper-
class of biologically active nitrogen compounds that ties.[22] In continuation of our current research[4] to de-
exhibit a number of important biological properties,[5] velop newer methodologies using less toxic and inex-
such as antifungal,[6] anti-inflammatory,[7] antitumor,[8] pensive metals as a catalyst,[23] herein, we are pleased
antiviral,[9] antibacterial,[10] antiprotozoal,[11] antipyret- to report a convenient approach to access imidazo-
ic,[12] analgesic,[13] antiapoptotic,[14] hypnoselective, and
anxioselective activities.[15] Recently, imidazo
G
ACHTUNGTNER(NUNG III)-catalyzed binucleo-
AHCTUNGTRENNUNG
a]pyridine scaffolds were incorporated in some com- (Scheme 3).
mercially available drugs[16] such as zolpidem (I, used
Optimization of the reaction conditions was
in the treatment of insomnia), alpidem (II, as an anx- achieved by varying temperature, solvent, and catalyst
iolytic agent), olprinone (III, for the treatment of (Table 1). For the initial study, nitrostyrene 1a and 2-
acute heart failure), zolimidine (IV, used for the treat- aminopyridine 4a were selected as the model sub-
ment of peptic ulcer), necopidem and saripidem (V strates to optimize the reaction conditions. Initially
and VI, both work as an anxiolytic agent) (Figure 1). we carried out the reaction in the presence of a cata-
Considering these important uses, their syntheses lytic amount of indium triflate (5 mol%) in 1,2-di-
have been received much attention in the field of me- chloroethane under reflux. Unfortunately, no desired
Figure 1. ImidazoACHTUNGTRENNUNG[1,2-a]pyridine-containing drugs.
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IronACHTUNGTRENNUNG(III)-Catalyzed Cascade Reaction between Nitroolefins and 2-Aminopyridines
Scheme 3. Synthesis of imidazoACHTUNTRGNEUNG[1,2-a]pyridines.
Table 1. Optimization of reaction conditions.[a]
phenylimidazoACTHNUTRGNEUNG[1,2-a]pyridine and probably its forma-
tion occurred through the oxidative coupling of a ni-
troolefin with 2-aminopyridine.[24] Next, we shifted to
iron salts as catalyst. FeCl3 was found to be the most
effective one among various iron salts such as FeCl3,
FeBr3, FeACHTUNRGTNEUNG(OTf)3. However, other common Lewis
acids were not effective for this conversion (en-
tries 10–13, Table 1). Finally, the optimized condition
were obtained using 20 mol% FeCl3 at 808C in DMF
as solvent. DMF appeared to be the best choice
among common solvents such as H2O, DMSO,
CH3CN. In absence of catalyst, no formation of the
desired product was observed. 20 mol% was required
as optimum amount of catalyst and increasing the
amount of catalyst did not improve the yields while
decreasing the amount of catalyst decreased the
yields.
With the optimized conditions in hand, we then
turned our attention to investigate the scope of sub-
strates, and the results are shown in Table 2. A wide
range of both substituted aminopyridines and nitro-
AHCTUNGTREGoNNUN lefins were employed to prove the general applica-
bility of our present procedure. Several sensitive func-
tionalities such as, OMe (5ca, 5cb, 5cd), NO2 (5da),
halogens Cl, Br (5ae, 5af, 5fb), COMe (5gb), CO2Me
(5ha) were unaffected under the present reaction con-
ditions. Substituted 2-aminopyridines with the methyl
group at different positions reacted with nitroolefins
in good yields (5ab, 5bb, 5cb, 5fb, 5ac, 5bd, 5cd, 5gb).
In addition, aliphatic nitroalkenes were also found to
afford the desired products with high yields (5ia, 5ja).
However, b-methyl-b-nitrostyrene did not give the de-
sired heterocycles under the present reaction condi-
tions.
The SMe group was also well tolerated under these
reaction conditions. We have successfully synthesized
the corresponding compound 5ea with high yield. It is
noteworthy to mention that the compound 5ea is the
key intermediate for synthesizing the drug zolimidine.
After successive oxidation of 5ea using mCPBA, the
target molecule can be synthesized by employing the
reported method (Scheme 4).[20j]
[a]
Carried out with 1 mmol of 1a and 1 mmol of 4a in the
presence of catalyst in solvent (2 mL) at 808C for 2 h.
Isolated yields.
Reaction proceeded under reflux.
[b]
[c]
[d]
Major product was obtained through oxidative coupling
(Ref.[24]).
[e]
Reaction carried out at 1008C.
No formation of 5aa.
[f]
product was obtained. Next we changed the solvent
A library of 3-unsubstituted imidazoACTHUNGTERNNU[G 1,2-a]pyridine
to DMF and the product was formed in less than derivatives was synthesized employing the present
10%. Encouraged by this result, we tried the reaction method. Further functionalization was carried out at
using Cu salts as catalyst. However, a different prod- the 3-position to construct densely substituted
uct was obtained in addition to the desired product in imidazo
14–16%. The undesired product is 3-nitro-2-
ACHTUNGTREN[NUNG 1,2-a]pyridine derivatives by modified meth-
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ods (Scheme 5).[25] 3-Aryl-substituted imidazo
ACHTUNGTRENNUNG[1,2-
a]pyridines were obtained in high yields.
Table 2. Substrate scope for the reaction of nitroolefins
1 and 2-aminopyridines 4.[a]
A plausible mechanism for this ironACTHNUTRGNE(NUG III)-catalyzed
cascade reaction is outlined in Scheme 6. As de-
scribed previously[4] the first step of the reaction is
the Michael addition of 2-aminopyridine 4a with ni-
troolefin 1 to form the intermediate 6. Probably
FeACHTNUTRGNEUNG(III) chloride accelerated the reaction by increasing
the electrophilicity of the nitroolefin through coordi-
nation. The second step is the intramolecular cycliza-
tion of the intermediate 6 leading to the final product
after subsequent removal of water and nitroxyl
(HNO).[26]
In conclusion, we have developed an ironACTHNUTRGENU(GN III)-cata-
lyzed one-pot tandem reaction for the synthesis of
[a]
Reactions conditions: 1.0 mmol of 1 and 1.2 mmol of 4 in
the presence of FeCl3 (20 mol%) in 2 mL of DMF at
808C for 2 h, isolated yields.
Scheme 6. A plausible reaction mechanism.
Scheme 4. Synthetic approach towards zolimidine.
Scheme 5. Functionalization of 3-unsubstituted imidazopyridines.
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IronACHTUNGTRENNUNG(III)-Catalyzed Cascade Reaction between Nitroolefins and 2-Aminopyridines
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imidazoACHTUNGTRENNUNG[1,2-a]pyridines by exploiting the bielectro-
philic nature of nitroalkenes and the binucleophilic
nature of 2-aminopyridines. This methodology could
be successfully applicable for the synthesis of zolimi-
dine, a useful drug for the treatment of peptic ulcer.
Easily available starting materials, as well as the non-
hazardous and less expensive catalyst are the advan-
tages of the present procedure. Further study to ex-
plore the bielectrophilic nature of nitroalkenes to
construct the biologically active molecules is ongoing
in our laboratory.
Experimental Section
Typical Procedure for the Synthesis of 2-Phenyl-
imidazoACHTUNGTRENNUNG[1,2-a]pyridine (5aa)
A mixture of 2-aminopyridine 4a (112 mg, 1.2 mmol) and ni-
troalkene 1a (149 mg, 1 mmol) was stirred in presence of an-
hydrous FeCl3 (20 mol%) in DMF (2 mL) at 808C for 2 h
(TLC). After completion, the reaction mixture was cooled
to room temperature and extracted with dichloromethane
(10 mL) followed by washing with brine (5 mL) and drying
over Na2SO4. After evaporation of the solvent the crude
product was purified by column chromatography on silica
gel using petroleum ether/ethyl acetate (3:1 to 2:1) as
eluent.
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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. S. S. and A. K. B.
thank UGC and CSIR for their fellowships.
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