1.3.4- Thiadiazolium-2-aminates

This review has attempted to highlight the bio active mesoionic compounds and stimulate further development in this field. Mesoionic compounds are still considered as exotic structures even within the field of heterocyclic chemistry. Much attention has been directed toward the syd-nones (2), sydnonimines (3), l,2,3,4-oxatriazolium-5-aminates (9), and 1,3,4-thiadiazolium-2-aminates (7). In particular, their properties as NO-releasing compounds have been of interest. 

A novel ring transformation/desulfurization of substituted 2-methyl-l,2,4-thiadiazolium salts 1481 provides a versatile entry to imidazoles 1482 with a variety of substituents. The starting 1,2,4-thiadiazolium salts 1481 can be prepared from A -(thiocarbonyl)-A -methylamidines 1480 under mild oxidative conditions. Related salts, 1,2,4-dithiazolium triiodides 1483, also react with amines to form imidazoles 1484 <1997JOC3480>. Alternatively, A -(thiocarbonyl)-A -methylamidines 1485 can be transformed into 1-substituted imidazoles 1487 via A-methylation followed by elimination of methylthiol (Scheme 383) <1997JOC3480>. 

This review will discuss mesoionic compounds with the main focus on the biologically active sydnones (2), sydnonimines (3), l,2,3,4-oxatriazolium-5-aminates (9), and l,3,4-thiadiazolium-2-aminates (7). 

From the reaction of iminophosphorane (264) with aliphatic isocyanates zwitterionic N,N -bisheteroarylguanidines (267) are obtained treatment with fluoroboric acid induces the conversion into alkylbis-(thiazolo[2,3-Z)][l,3,4]thiadiazolium-2-yl)amin bis(tetrafluoroborates) (268) (Scheme... 

A versatile synthesis of 6a -thia-l,2,6-triazapentalenes and 6a -thia-6-selena-l,2-diazapentalenes has been reported from oxathiadiazapentalene (542) <83JCS(P1)777>. Treatment of (542) with phos-phoryl chloride in 1,2-dichloroethane gave the 5-(2-chlorovinyl)-2-phenyl-l,2,3-thiadiazolium phos-phorodichloridate (543) which, when treated with perchloric acid in acetic acid, afforded the stable crystalline perchlorate (544) in nearly quantitative yield. The phosphorodichloridate (543) reacted with aliphatic and aryl amines to give the thiatriazapentalenes (545) in acceptable yields (44-97%) (Scheme 46). 

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