Synthesis of Derivatives of Pyrido[4,3‐ d ]pyrimidin‐4(3 H )‐one via an Iminophosphorane

A series of new, 2‐substituted 3‐aryl‐8,9,10,11‐tetrahydro‐5‐methyl[1]benzothieno[3′,2′ : 5,6]pyrido[4,3‐ d ]pyrimidin‐4(3 H )‐ones, compounds 5a – q , were designed and synthesized via the aza‐ Wittig reaction as the key step. The iminophosphorane 1 reacted with phenyl isocyanate (or 4‐chlorophenyl isocyanate) to the carbodiimide 4 , which was cyclized to 5 upon addition of different amines, EtOH, or phenols in the presence of a catalytic amount of EtONa or K 2 CO 3 ( Schemes 1 and 2 ). The structures of compounds 5 were confirmed by IR, 1 H‐ and 13 C‐NMR, EI‐MS, elemental analyses, and, in the case of 5l , by single‐crystal X‐ray diffraction ( Figure ).