Synthesis of some new 2-(4-halophenylamino)-5-(isomeric pyridyl)-1,3,4-thiadiazoles

Twelve 2-(4-halophenylamino)-5-(isomeric pyridyl)-1,3,4-thiadizoles (17-28) were prepared as pure crystalline solids, by the acid catalysed intramolecular dehydrative cyclization of the corresponding 4-(4-halophenyl)-1-(isomeric pyridoyl) thiosemicarbazides (5-16). The latter were prepared by reaction of 4-halophenyl isothiocyanate (3a-d) with each of the three isomeric pyridinecarboxylic acid hydrazides (4a-c). The effect of structure on reactivity with respect to the halogen atom and isomeric pyridyl group is discussed. A plausible mechanism for intramolecular cyclodehydration of the thiosemicarbazides to 1,2,4-thiadiazoles is also suggested. The purified new compound, were characterized on the basis of elemental analysis, IR and mass spectral data.