Phosgene 1.3.4- thiadiazol-2 -ones

Ph, R = NHj, with phosgene gives a novel bicyclic heterocyclic system which can be represented as a bicyclic meso-ionic l,3,4-thiadiazol-2-one (493) (Section VII,H,1). A similar reaction using A -benzoyl isocyanide dichloride (PhCO.N=(i Cl2) gives the novel meso-ionic 1,3,4-thiadiazol-2-imine (494) (Section VII,H,2). ... 

Thiadiazolidines can be obtained from aliphatic aldehydes or ketones and disubstituted hydrazine derivatives (Scheme 29). A typical preparation of a mesoionic compound consists in the reaction of 1-methylthioacylhydrazine and phosgene (Scheme 31a). Syntheses by three-bond formation are rare for example, a one-pot reaction of an aldehyde with hydrazine and sulfur. A typical ring transformation reaction is the irradiation of 1,3,4-oxadiazoles to yield 1,3,4-thiadiazoles. 

The product obtained from phenylthiosemicarbazide (92 R = H) and phosgene, described as 2-amino-4-phenyl-l,3,4-thiadiazolin-5-one (93 R = H), is in fact a mixture. Conditions have now been specified for the successful preparation of this compound (93 R = H) by this route. Oxidation of thiosemicarbazones. The oxidative cyclization of benzalthio-semicarbazones and of acetone thiocarbohydrazone is a well-known route to the 1,3,4-thiadiazole ring system. 4-Arylthiosemicarbazones of ketones (99) have now been shown to cyclize in this manner on being stirred in benzene in contact with suspended manganese dioxide, giving satisfactory yields of 5-imino-A -l,3,4-thiadiazolines (101). Ring-closure over alumina in chloroform slowly yields the isomeric l,2,4-triazoline-5-thiones (100). Bicyclo[3,3,l]non-2-en-9-one 4-phenylthiosemicarbazone... 

Phosgene passed with vigorous stirring first at 15-25 then with gradual warming to final reflux into a mixture of methyl dithiocarbazate and anhydrous benzene -> 5-methylthio-l,3,4-thiadiazol-2(3H)-one. Y 85%. F. e. s. K. Riifenacht, Helv. 55, 1178 (1972) 56, 162 (1973) the chemistry of phosgene, review, s. H. Babad and A. G. Zeiler, Chem. Rev. 73, 15 (1973). 

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