Different amines were found to attack the hitherto unknown 6,8-dibromo-2-(2'-thienyl)-3,1-benzoxazin-4(H)-one (2) entirely at position-4, producing the corresponding benzamide derivatives 3a-i. Compound 2 reacted with hydrazoic acid affording the tetrazole 5, beside the benzimidazole derivative 6. Similarly carbon nucleophiles, such as ethyl acetoacetate attack 2 to give 7. 6,8-Dibromo-2(2'-thienyl)-4(3H)-quinazoline and its 3-substituted derivatives 8 a-g were also synthesized. Theoretical calculations for the benzoxazinone 2 and others, based on the simple HMO-method were performed. Close accordance between the theoretically predicted reactivities and the experimental findings was obtained. Biological activities of some of the compounds obtained were also tested.