Five-membered systems with two heteroatoms

Cyclic ureas have many applications as intermediates in the preparation of biologically active molecules. The conventional methods involve cyclization of 1,2-diamines with phosgene or oxidative carbonylation of diamines. Varma and coworkers developed a direct synthesis of cyclic ureas from urea and diamines in the presence of ZnO using microwaves. The major advantage of the method is that the reaction is accelerated by exposure to microwave irradiation the byproducts were, moreover, easily eliminated compared with traditional methods [32]. 

The significance of this work is that the cydization-cleavage step involved mild, basic conditions and microwave flash heating. 

The authors have also investigated the preparation of the 2-thioxo-5-imidazolidin-4-ones 17 from thiohydantoin 16 in the presence of potassium acetate, with acetic anhydride as solvent, using both methods. Here, the effect of microwave irradiation was reduction of reaction times and a small increase in the yields. 

An extremely simple one-pot synthesis of pyrazoles, pyrimidines, and isoxazoles has been realized by reacting enamino ketones, formed in situ with the appropriate bidentate nucleophile, under the action of microwaves [38]. Another approach to pyrazole from 4-alkoxy-l,l,l-trichloro-3-alken-2-ones and hydrazines, with toluene as solvent, is also possible under microwave conditions [39]. The Ullman coupling of (S)-[l-(3-bromophenyl)ethyl]ethylamine with N-H-containing heteroarenes such as pyrazole in N-methylpyrrolidone afforded the N-arylated compounds in high yields under microwave heating conditions at 198 °C [40]. 

Straightforward access to N-protected 5-oxazolidinones using amino acids, paraformaldehyde, and PTSA in a minimum amount of toluene is accelerated by exposure to microwaves (3 min). This approach is not only simple and eflident but can be conveniently scaled-up to large quantities [46]. 

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Five-membered systems with two heteroatoms 3.3.1. Imidazoles, pyrazoles and benzimidazoles... 

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