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Synlett 2010(11): 1606-1608  
DOI: 10.1055/s-0029-1219962
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Microwave-Assisted, One-Pot Reaction of Pyridines, α-Bromoketones and Ammonium Acetate: An Efficient and Simple Synthesis of Imidazo[1,2-a]-pyridines

Mehdi Adib*a, Ali Mohamadia, Ehsan Sheikhia, Samira Ansaria, Hamid Reza Bijanzadeh*b
a School of Chemistry, University College of Science, University of Tehran, PO Box 14155-6455, Tehran, Iran
Fax: +98(21)66495291; e-Mail: madib@khayam.ut.ac.ir;
b Department of Chemistry, Tarbiat Modarres University, PO Box 14115-175, Tehran, Iran
e-Mail: bijan_hr@yahoo.com;
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Publication History

Received 23 November 2009
Publication Date:
04 June 2010 (online)

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Abstract

A novel and efficient synthesis of imidazo[1,2-a]pyridines is described. N-Phenacylpyridinium bromides, which were prepared in situ from the addition of pyridines to α-bromoketones, undergo nucleophilic addition of ammonium acetate under microwave irradiation and solvent-free conditions to afford the corresponding imidazo[1,2-a]pyridines in excellent yields.

Key words

imidazo[1,2-a]pyridines - pyridines - α-bromoketones - ammonium acetate - microwave irradiation - solvent-free synthesis - cyclizations - heterocycles

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The experiments were performed using a microwave oven (ETHOS 1600, Milestone) with a power of 600 W specially designed for organic synthesis

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General Procedure for the Preparation of Imidazo[1,2- a ]pyridines (3): A solution of the appropriate pyridine
(1 mmol) and the appropriate α-bromoketone (1 mmol) in anhydrous CH2Cl2 (3 mL) was stirred at ambient temperature for 30 min. After complete conversion to and precipitation of the corresponding N-phenacylpyridinium bromide, as indicated by TLC monitoring, the organic solvent was separated by suction. Next, an excess of ammonium acetate (2 mmol) was added to the residue and the mixture was irradiated in a microwave oven at 180 ˚C for 4 min (progress of the reaction followed by TLC monitoring minute by minute). The reaction mixture was then cooled to r.t. and the residue was crystallized from n-hexane-EtOAc (1:1). Compounds 3b and 3j were purified by column chromatography (Merck silica gel 60 mesh; n-hexane-EtOAc, 4:1)