The interactions of 2-ethoxy(4H)-3,1-benzoxazin-4-one (1) with various nitrogen nucleophiles such as ammonium acetate, hydrazine hydrate, ethanolamine, p-phenylenediamine, o-phenylenediamine, o-tolidine, dapsone, 2-aminophenol, 4-aminophenol, 4-aminobenzoic acid and 2-aminonicotinic acid have been discussed. The reactions of 2-thoxy-(3H)-quinazolin-4-one with ethyl chloroformate, phosphorus pentasulfide, chloroacetyl chloride and phosphorus oxychloride have also been investigated. Similar reactions of 2-ethoxy-4-chloroquinazoline with hydrazine hydrate and thiosemicarbazide have been introduced. Aminolysis of the 2-ethoxy group in some of the thiadiazoloquinazolinone derivatives has been attempted. The interactions of these aminolized derivatives and the 3-aminoquinazolinone with chloroacetyl chloride have been studied. All of the synthesized derivatives have been used in a wide range as starting materials for the synthesis of novel quinazoline and/or quinazolinones which have biological activity. The structures of all these products, obtained by heterocyclic ring opening and ring closure, were inferred by the IR, MS, 1H NMR spectral as well as elemental analyses. Keywords: Aminothiadiazole, Benzoxazin-4-one, N-nucleophile, quinazoline, quinazolin-4-one, thiosemicarbazide
