INTRODUCTION The multi-or three-component, one pot reactions are highly important because of their wide range of applications in pharmaceutical libraries for drug discovery 1-5 . MCRs are extremely convergent, producing a remarkably high increase of molecular complexity in gust one step. Microwave irradiation promoted organic condensation has attracted intense interest as a synthetically useful technique for the preparation of a variety of heterocyclic compounds 6-9 . As an important member of the five- Poly Phosphoric Acid Impregnated on Silica Gel (PPA-SiO 2 ): A Versatile and Reusable Catalyst for the Synthesis of 1,2,4,5-Tetrasubstituted Imidazoles under Solvent-free and Microwave Irradiation Conditions NASER MONTAZERI*, KHALIL POURSHAMSIAN, MARJAN KHODDADI and KAZEM KHODDADI Department of Chemistry, Tonekabon Branch, Islamic Azad University, Tonekabon (Iran). *Corresponding author: E-mail: montazer50@toniau.ac.ir (Received: July 26, 2011; Accepted: September 01, 2011) ABSTRACT A remarkable acceleration in the synthesis of 1,2,4,5-tetrasubstituted imidazoles catalyzed by silica gel supported poly phosphoric acid in high yields under solvent-free and microwave irradiation conditions, with very short reaction time is described. This methodology offers momentous improvements over various options for the synthesis of 1,2,4,5-tetrasubstituted imidazoles with regard to yield of produces, simplicity in operation and green aspects by avoiding toxic catalysts and solvents. Key words: Tetrasubstituted imidazole, PPA-SiO 2 , solvent-free conditions, Multi-component reaction, Microwave irradiation. membered ring heterocycles, imidazoles moiety is present in a wide range of naturally occurring molecule 10,11 . Compounds with imidazol moiety have many pharmaceutical activities 12,13 . The biological important of the imidazol ring system has made it a common structure in numerous synthetic compounds, such as fungicides 14 , herbicides 15 , plant growth regulators 16 and therapeutic agents 17 . Various methods have been reported for the synthesis of tetrasubstituted imidazole, including four components condensation of 1,2-diketones, aromatic aldehydes, primary amines and ORIENTAL JOURNAL OF CHEMISTRY www.orientjchem.org ISSN: 0970-020 X CODEN: OJCHEG 2011, Vol. 27, No. (3): Pg. 1023-1027 An International Open Free Access, Peer Reviewed Research Journal Est. 1984