1,3,4-Thiadiazolium salts 5,6-dihydro

Condensation of the thiadiazolium salt 192 with />-toluidine yielded 5-imino-2,5-dihydro-l,2,4-thiadiazole hydrochloride 195. This in turn was converted into a variety of 2,3-dihydro-6aA4-thiapolyheterapentalenes 197 in nearly quantitative yields upon addition of heterocumulenes 196 (see Scheme 23) <2003HAC95>. 

Like the oxadiazolium salts (288), 1,3,4-thiadiazolium salts (296) can also react with hydrazines to afford 1,4-disubstituted l,4-dihydro-l,2,4,5-tetrazines (290). By this method, 1,4-dihydro-1,2,4,5-tetrazines with four different substituents can be prepared this seems to be the only method for preparation of such compounds, and proceeds in yields of 70-99%. The 4-amino-l,2,4-triazoles were isolated as side-products (74ZOR377, 71DOK(200)134, 71MIP22100). 

Methylthio-1,3,4-thiadiazolium salts (297) were treated with hydrazines in the ratio 1 2 to afford l,4-dihydro-l,2,4,5-tetrazine-6(5//)-thiones (298). In most cases the acyclic intermediate could not be isolated (73CZ565,73CZ566,01CB2311). 

Interaction of the mesoionic thiadiazolium salt (299 Ar = Ph) with hydrazine led to the formation of l,4-dihydro-6-hydrazino-l,2,4,5-tetrazine (300) (76KGS713). The reaction of salts (299) with azodicarboxylate esters was at one time thought to provide the mesomeric... 

Dihydro-4H-l,3,4>thiadiazin-5-ones from 1,3,4-thiadiazolium salts and acylphosphonic acid esters... 

The partial inhibition of the hydrolysis observed at pH < 4, and the negative salt effect, may be attributed to a specific cation-anion interaction. The proposed mechanism receives further support from the observed effects on the hydrolysis rates of increased viscosity of the medium and of added inorganic nucleophiles. Hydrazine or alkylhydrazines cleave and recyclize 1,3,4-thiadiazolium salts (114) to 1,2- or l,4-dihydro-l,2,4,5-tetrazines (115) in high yield. The action of arylhydrazine results in the alternative recyclization of the probable intermediate ArNHN=CR2NR N=CR SH, to 4-amino-1,2,4-triazolium salts (116). 5-Amino-2-imino-3-phenacyl-l,3,4-thiadiazolines (117) isomerize in boiling ethanol to 5-amino-3-mercapto-l-phenacyl-l,2,4-triazoles (118). This example... 

The A -arylidene derivatives (96) of 4-amino-4,5-dihydro-l-methyl-3-methylthio-l/f-[l,2,4]tri-azole-5-thione, when treated with phosphorus pentachloride, are cyclized to give the 3-methylthio[l,2,4]triazolo[3,4-6][l,3,4]thiadiazolium chlorides (97). This conversion is envisaged to involve the initial formation of an iminium salt (98) which by attack of the chloride ion gives the intermediate imidoyl chloride (99) subsequent cyclization by intramolecular nucleophilic attack of the thiocarbonyl sulfur atom turns it into the fused bicyclic product (97) (Scheme 7) <86LA1540>. 

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See also in sourсe #XX -- [ ]

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