Acetylation of 1-amino-5-(piperidin-1-yl)-N-phenyl-6,7,8,9-tetrahydrothieno[2,3-c]isoquinoline-2-carboxamide (3) with acetic anhydride afforded unexpected [1,3]oxazinothienoisoquinolinone derivative in an excellent yield instead of the expected N-phenylpyrimidothienoisoquinolinone. Diazotization of 3 using sodium nitrite in acetic acid and sulfuric acid yielded the corresponding triazinothienoisoquinoline, while the reaction with triethyl orthoformate in acetic acid produced N-phenylpyrimidine derivative 7. Chloroacetylation of compound 3 with chloroacetyl chloride in dioxane at 60–70°C yielded chloroacetamido compound 8, which underwent ring closure upon reflux in acetic anhydride to produce the chloromethyl oxazinone derivative 9. Oxazinone derivatives 5 and 9 underwent nucleophilic substitution upon treatment with ammonium acetate/acetic acid mixture and hydrazine hydrate to afford pyrimidinone …