Five-membered heterocycles from thioureas

As demonstrated above, nitro derivatives of five-membered heterocycles have found extensive use as antiinfective agents. It is therefore of interest that the nitro derivative of a substituted thiazole was at one time used as an antitrichomonal agent. Bro-mination of 2-aminothiazole (136) (obtained from condensation of thiourea with chloroacetaldehyde) gives the 4-bromo derivative (138) this is then acetylated to 139. Treatment of 139 with nitric acid leads to an interesting displacement of bromine by a nitro group to afford aminitrazole (140)... 

Besides thiazoles, five-membered heterocycles of different types are obtainable from thioureas. Total loss of sulphur takes place in the condensation with arylacetonitriles, 2-amino-imidazoles (43) being formed, whereas in the... 

The carbon in the isothiocyanate grouping is highly susceptible to nucleophilic attack by the peptide s free amino group. Overall addition to the C=N creates a thiourea derivative. Making the conditions strongly acidic then promotes nucleophilic attack by the sulfur of the thiourea on to the carbonyl of the first peptide bond, producing a five-membered thiazoline heterocycle. Proton loss occurs from the nitrogen, and this creates an intermediate that is equivalent to the addition product in simple acid-catalysed amide... 

A number of interesting five-membered ring heterocycles of other types have been reported in the literature as having been derived from thioureas. The pyrrolidin-2-ones 92 were formed by cyclization of 1-(4-broinobutanoyl)-2-thiourcas.212 In a related reaction, Cherbuliez106 produced the pyrrolidine 93 by treatment of the sulfate ester of l-(4-hydroxybutyl)-3-phenyl-2-thiourea with acid or base. 

Many biologically active compounds contain cyclic ureas, including inhibitors of human immunodeficiency virus (HIV) protease and HIV replication [70]. Kim et al. [71] presented an illustration of the synthesis of oligomeric cyclic ureas as nonnatural biopolymers. Applying the libraries from libraries [72] concept, triamines [65] such as those described earlier were used as templates for the generation of different heterocyclic compounds such as cyclic ureas, cyclic thioureas, and bicyclic guanidines [65]. The cyclizations to obtain the five-membered ring cyclic ureas and cyclic thioureas were... 

This review tries to shed light on the early history of different methods for the generation of N-heterocyclic carbenes (NHCs), i.e., the extrusion of heterocumulenes (decarboxylations) from suitable mesomeric betaines, deprotonations of hetarenium salts, a-eliminations, tautomerizations of mesomeric betaines, and reductive desulfurizations of cyclic thioureas. Selected examples of acyclic and three- to eight-membered NHCs are presented, as well the generation of selected five- and six-membered anionic NHCs. 

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