Reactions of Urea and Its Analogues

But acidic catalysis is also possible. Some examples include the application of sulfuric [51], hydrochloric [52, 53, 54, 55, 56] and acetic [56, 57, 58] acids. Usually acidic catalysis is used in the case of a,(3-unsaturated carbonyl compounds with heterocyclic nature or ones containing one or more heterocyclic substituents. 

Reactions of unsaturated ketones with urea 60, as already mentioned, have been much less investigated and require stronger conditions. In such processes, instead of using alkaline metal hydroxides, a catalyst like sodium ethoxide 

= C6H5, 4-MeC6H4, 4-HO-C6H4, 3-MeOC6H4, 2-HOC6H4, 2-CI-C6H4 etc. 

The reactions of thiourea and guanidine with cycloalkanones are carried out in the same manner [101]. But it is worthwhile noting that muticomponent reactions of urea-like compounds with ketones, containing an activated methyl or methyle-negroup, often do not stop after the formation of a pyrimidine ring. In fact numerous derived condensation processes can lead to more complicated polycyclic compounds, which are especially typical for the reaction of cycloalkanones [101]. 

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Reactions of unsaturated ketones 62 containing a strong a electron-withdrawing trifluoromethyl group with urea and its analogues 28, 60 and 61 usually stop at the stage of the formation of hydroxytetrahydropyrimidines 63 [69] (Scheme 3.19). 

Faneti/ole (122) is a biological response modifier with significant immunosuppressant activity It can be synthesized by conversion of 2 phen> lethylamine (120) with ammonium thio cyanate to the corresponding thiourea analogue 121 The synthesis of faneli/ole (122) concludes by thiazole nng formation of 121 by reaction with phenacylbromide Thus its synthesis involves use of the classic Hantzsch procedure in which a bromoacetone analogue and an appropriate thio urea denvative are reacted 143]... 

Canaline is the product of the hydrolytic cleavage of canavanine with the simultaneous formation of urea. Canaline is an ornithine analogue which also shows neurotoxicity in the adult sexta where it adversely affects central nervous system functions (jj ). It also is a potent inhibitor of vitamin B -containing enzymes (20-22). It forms a stable Schiff base with the pyridoxal phosphate moiety of the enzyme and drastically curtails enzymatic activity. Pyridoxal phosphate-containing enzymes are vital to insects because they function in many essential transamination and decarboxylation reactions. Ornithine is an important metabolic precursor for insect production of glutamic acid and proline (23). 

In 2008, Connon and coworkers reported a base-mediated protocol for the S3mthesis of terminal epoxides from aldehydes and trimethylsulfonium iodide utilising 5 mol% of urea analogue 9. Electron-rich and electron-deficient aromatic aldehydes were well tolerated providing the corresponding products in moderate to high yields. In the case of cycloheigrl carboxaldehyde, an increase in the reaction time was required. It has to be... 

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