The possibility of preparing 1,2,5-oxa-, -thia-, and -selenadiazole N-oxides from polyfunctional nitroso-and isonitrosopyrimidines and -pyridine was examined.We found previously that the reactions of 6-amino-5-nitrosouracils with sulfur or selenium monochloride yield 1,2,5-chalcogenadiazolo [3,4-d]pyrimidines [1,2]. In this study we examined the applicability of this reaction to o-amino nitroso compounds containing additional reaction centers such as hydroxy and amino groups. As starting compounds we used 5-nitroso derivatives of 1-hydroxyethyl-6-aminouracil (Ia) [3], 4,6-diamino-1,2-dihydropyrimidin-2-one (Ib) [4], and 2,4,6-triaminopyrimidine (Ic) [4], and also 3-nitroso-2,6-diaminopyridine Id [5]. Treatment of Ia3Id with sulfur or selenium monochloride under the conditions similar to those described in [1] resulted in formation of 1,2,5-chalcogenadiazole N-oxides IIa3IIh in good yields (Scheme 1).The reactive functional groups present in the starting amino nitroso compounds, at equimolar ratio of the reactants, exert no appreciable effect on the course of cyclization with the formation of 1,2,5-thiadiazole N-oxides and their selenium analogs.An alternative route to 1,2,5-oxa-, -thia-, and selenadiazoles, based on transformations of aliphatic and aromatic 1,2-dioximes [6 38], is limited by difficult availability of heterocyclic 1,2-dioximes.With 6-amino-5-nitrosouracil Ie as example, we showed that the amino group in an acidic medium is readily replaced by hydroxylamine; the reaction product isomerizes into uracil 5,6-dioxime IIIa existing, according to single crystal X-ray diffraction data, in the form of the (4Z,5E) isomer [9]. The reactions between Ib3Id and excess hydroxylamine taken as a mixture of acetate and hydrochloride resulted in replacement of all the amino groups with the formation of tri-and tetraoxime derivatives of pyrimidine (IIIb, IIIc) and pyridine (IIId) (Scheme 2). The reactivity is apparently influenced by the steric factors: 1-Substituted 6-amino-5-nitrosouracils do not react with hydroxylamine under the similar conditions. Scheme 1.