A3-l ,3,4-Thiadiazoline

No examples of direct oxidation of the 1,3,4-thiadiazole ring sulfur to sulfoxide or sulfone have been reported. A3-l,3,4-Thiadiazoline 1-oxide and 1,1-dioxide, however, can be obtained by indirect methods that are reviewed in CHEC(1984) <1984CHEC(6)545>. 

A3-l,3,4-Thiadiazolines were easily prepared from ketones or thioketones. By pyrolysis in the presence of a phosphine they afforded olefins [128,441] (see Section 4.5.4). 

The stereoisomeric formation of bis(A3-l,3,4-thiadiazolines) from dithiones and diazomethane is also discussed in Section 4.27.4.3.2(ii) (71JOC3885). 

Although direct oxidation of the annular sulfur atom in 1,3,4-thiadiazoles has not been reported, A3-l,3,4-thiadiazoline 1-oxides and 1,1-dioxides can be obtained by indirect means (see Section 4.27.4.3, Schemes 26 and 27). 

The reaction of arylsulfonyl-substituted sulfines (175) with diazomethane gives A3-l,3,4-thiadiazoline 1-oxides (176) which, however, are unstable and rearrange via an isomerization of the A3- to the A-thiadiazoline oxide. This is followed by an elimination and readdition of sulfinic acid and loss of water in a Pummerer-type aromatization to give the rearranged thiadiazole (177 Scheme 24) (73TL5009). 

The reaction of 2-diazopropane with geometrical isomers of different types of sulfines yields A3-l,3,4-thiadiazoline 1-oxides as the single product by a concerted regiospecific cycloaddition process. Introduction of bulky substituents in either of the reactants will sterically hinder the cyclization to a five-membered ring, and give rise to non-stereospecific formation of episulfoxides (Scheme 26) (73TL3589). 

Thiocarbonyl ylides, generated from the thermolysis of 2,5-disubstituted A3-l,3,4-thiadiazolines, may be trapped by ketenes to give oxathiolane derivatives, as shown in Scheme 39. The reaction appears to be a useful way of obtaining unsaturated derivatives... 

Cyano-l,2,4-thiadiazole-5-carboximidate 5-Imino-A3-l,2,4-thiadiazoline 3,5-Bis(diphenylamino)-l,2,4-thiadiazole... 

Condensation of bis(alkoxymethyl)carbodiimides and chlorothioformyl chloride in boiling benzene occurs with loss of one mole of alkoxymethyl chloride, and production of 2-alkoxymethyl-3-chloro-A3-l,2,4-thiadiazolin-5-ones (176).160... 

The condensation of N-acyl-S-chloroisothiocarbamoyl chlorides (188) and amidines (189) yields 5-benzoylimino-3-phenyl-2-aryl-A3-l, 2,4-thiadiazolines (190) as major, and the isomeric A2-l,2,4-thiadiazolines (191) as minor products. Analogs (193) of the latter are formed exclusively from N-aryl-S-chloroisothiocarbamoyl chlorides (192). The divergent course of the reaction is ascribed to the different reactivity of the chlorine atoms in the two reagents (188,192). It is plausible that the initial nucleophilic attack of the base occurs at the imido carbon of 188, but at the sulfenyl sulfur of 192.166 The condensation of N-phenyl-S-chloroisothiocarbamoyl chloride (194) and isothioureas (195) similarly yields the 1,2,4-thiadiazolines (196).167... 

---