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

2-Amino-6,7-dihydrothieno3′,2′:5,6]pyrido2,3-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(2H)-one ( 2 ) was found to involve competitive intramolecular Diels-Alder and intramolecular coplanar cycloamination processes, providing the 2,3-dihydrothieno2,3-b]pyridin-6(7H)-one ( 4 ) and the 1,3-thiazino3,2-b]-1,2,4-triazin-3-one (5) derivatives, respectively.