AbstractA series of 4H‐pyrido[1,2‐a]pyrimidin‐4‐ones was prepared by the cyclization of substituted 2‐aminopyridines with βketocarboxylic esters in polyphosphoric acid or in mixtures of the latter with phosphoryl chloride. Catalytic hydrogenation (over palladium on charcoal or Raney nickel) of the products afforded the corresponding 6,7,8,9‐tetrahydropyridopyrimidinones. Thermal treatment of the 6‐substituted derivatives gave 1,8‐naphthyridin‐4‐ones in high yields. Oxo to thio exchange was successful only with substrates