Synthesis of 2‐amino‐6,7‐dihydrothieno‐[3′,2′:5,6]pyrido[2,3‐ d ]pyrimidin‐4‐one

2‐Amino‐6,7‐dihydrothieno[3′,2′:5,6]pyrido[2,3‐rf]pyrimidin‐4‐one ( 1 ) was prepared in three steps from S‐ (3‐butynyl)thiosemicarbazide hydroiodide ( 3 ) and diethyl ketomalonate. The featured step in this synthetic sequence was an intramolecular Diels‐Alder reaction of the in situ generated 3‐(3‐butynylthio)‐6‐carboethoxy‐5‐chloro‐1,2,4‐triazine ( 9 ) to provide the key intermediate 5‐carboethoxy‐6‐chloro‐2,3‐dihydrothieno‐[2,3‐ b ]pyridine ( 6 ). In the course of studies directed toward the preparation of 1 , thermolysis of 3‐(3‐butynyl‐thio)‐6‐carboethoxy‐1,2,4‐triazin‐5(2 H )‐one ( 2 ) was found to involve competitive intramolecular Diels‐Alder and intramolecular coplanar cycloamination processes, providing the 2,3‐dihydrothieno[2,3‐ b ]pyridin‐6(7 H )‐one ( 4 ) and the 1,3‐thiazino[3,2‐ b ]‐1,2,4‐triazin‐3‐one (5) derivatives, respectively.