Thiosemicarbazides Acylthiosemicarbazides

The formation of alternative heterocycles can be the main reaction with semicarbazides or thiosemicarbazides which are used extensively in the preparations of triazolinones and triazolinethiones. Aminoguanidines can give rise to isomeric triazoles. The preferred formation of thiadiazole (157) from the 1-acylthiosemicarbazide (Scheme 66) is explained by the protonation of N-4 in strong acid with accompanying loss of nucleophilicity, while in the presence of base its nucleophilic character is enhanced 73JOC3947) and (158) is obtained. 

From Thiosemicarbazides. Cyclization of acylthiosemicarbazides. The dehydrative cyclization of acylthiosemicarbazides, one of the oldest syntheses in this field, has now provided compounds of stereochemical interest, viz. trans- and c/j-5-[2-(5-nitro-2-furyl)-l-(2-furyl)vinyl]-2-amino-1,3,4-thiadiazoles (59) and (60). Analogues of type (61) were similarly obtained. ... 

Particular importance has been payed to fluorinated thiadiazoles which contain functionalities such as amino or methylthio gronps due to their industrial production. The synthesis of aminothiadiazoles 168 is based on the heterocyclization of acylthiosemicarbazides 167 with yields depending on experimental conditions (Scheme 44). In some cases the heterocychzation into the thiadiazole derivative occurs directly during the acylation reaction. In this manner, the reaction of thiosemicarbazide with triflnoroacetic anhydride lead to the formation of 2-amino-5-trifluoromethylthiadiazole 168 (Rp=Cp3, R=H) in a 30 % yield [78]. However, reaction carried out in the presence of POCI3 permitted to obtain the yield increased to 93% [79]. Reactions between thiosemicarbazides 166 and trifluoroacetic add or anhydride in the presence of PPA were also nsed for the synthesis of 2-amino and 2-methylamino derivatives 168 (Rp=CF3) [80]. 

Alkyl)thiosemicarbazide, R NHC(S)NHNH2, reacts with isocyanate and methyl iodide to give the triazolinones (81) on the other hand, the diamino-triazoles (82) are obtained on reaction with a carbodi-imide R N C NR. Oxadiazoles, e.g. (83), are formed from 1-acylthiosemicarbazides on reaction with... 

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