| During the last few years, microwave irradiation as an efficient thermal energy source is becoming a standard technique in various fields of chemistry, especially in the application in organic chemistry .The catalytic mechanism and characteristic of the microwave irradiation to organic reactions and the recent progress in the research and application of the microwave technique in organic synthesis are reviewed. Synthesis of chemical compounds which normally require long reflux time can be achieved conveniently and very rapidly in microwave oven. Compared with the classical heating methods as reported in literature, the benefit of microwave irradiation includes short reaction time, high yields, pure products and easy workup. The use of microwave energy to activate organic reactions has been taking a new dimension very recently. It has been used for a great variety of organic reaction as esterification and saponification, sulfonation, oxidation, hydrolysis, olefm addition, replacement, condensation, alkylation, rearrangement, Diels-Alder reaction, Reformatsky reaction, Knovenagel reaction, Perkin reaction, Deckmann reaction, Witting reaction, Bischler-Napieraski reaction, Protection and deprotection, concerted reaction, cyclization and ring-opening, etherification, etc.The Biginelli reaction, which was first reported by Biginelli more than a centry ago, involves the synthesis of 3,4-dihydropyrimidin-2-(lH)-ones by a very simple one pot condensation reaction of ethyl acetoacetate, benzaldehyde and urea in ethanol. In the past decade, dihydropyrimidinone derivatives have exhibited important pharmacological properties, e.g. as the integral backbones of several calcium channel blockers, antihypertensive agents, alpha-la-antagonists, and neuropeptide Y (NPY) antagonists. Now Dihydropyrimidinones are an important class of compounds which are becoming increasingly interesting due to their therapteutic and pharmacological activities.Recently, many synthetic methods for preparing these compounds have been reported by using Lewis acids as well as protic acid promoters such as: zirconium(IV) chloride, indium(III) bromide, ytterbium(III)-resin, eerie ammonium nitrate (CAN), Mn(OAc)3-2H2O, lanmanide triflate, indium(III) chloride, lanthanum chloride, HiSCM, HOAc, montmorillonite KSF, polyphosphate ester (PPE),13 BF3-OEt2/CuCl/HOAc, and cone. HC1. However, in spite of their potential utility, many of these methodsinvolve expensive reagents, strongly acidic conditions, long reaction times, high temperatures and stoichiometric amounts of catalysts, and give unsatisfactory yields.Application of microwave irradiation in organic synthesis is becoming an increasingly popular technology because of its rapid reaction rates, cleaner reaction conditions and ease of manipulation.A study of the preparation 3,4-dihydropyrimidin-2-(lH)-ones with NtUCl as a very inexpensive and easily available catalyst under microwave irradiation has never been reported. As shown in the paper, this is a novel method that not only preserved the simplicity of Biginelli's one-pot reaction but also consistently produced 64-93% yields of the dihydropyrimidine-2-ones.In our attempt towards a non-traditional approach to the experimental set up of organic reactions, the concept of "Microwave-Induced Organic Reaction Enhancement" (MORE) Chemistry has been utilized for rapid and efficient synthesis of 3,4-dihydropyrimidin-2-(lH)-ones. The reaction can be carried out by the condensation of ethyl acetoacetate, aromatic aldehyde and urea or thiourea in present of ammonium chloride as catalyst and ethanol as solvent under microwave irradiation. The results obtained demonstrate the versatility of the process as considerable reaction rate enhancement has been observed bringing down the reaction time form hours to minutes. The products obtained have improved yields with respect to conventional methods.The authenticity of the end products was established by their melting points and spectral data (IR, !HNMR). The present investigation thus provides a very simple,... |
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