KEYWORDSVersatile 2-(alkylthio)pyrimidine-type and 2-(phenacylamino)thiophene building blocks (4ad) and 16 were obtained based on an ortho functionalized thiophene derivative 1. A novel series of thieno[2,3-d]pyrimidin-4-one derivatives with annelated bridgehead nitrogen heterocycles was synthesized starting from precursors 4 and 16 through convenient methods. Cyclocondensation of 2-(phenacylthio)pyrimidinone derivative (4b) with sulfuric acid led to the tricyclic thiazole derivative 5. Initial hydrazinol…