Synthesis of 5,8‐dihydro‐5‐oxo‐2‐(3‐ or 4‐pyridinyl)‐pyrido[2,3‐ d ]pyrimidine‐6‐carboxylic acids and the reinvestigation of the thermal cyclization of diethyl (2‐hydroxy‐4‐pyrimidinyl)amino‐methylenemalonate

Diethyl [2‐(3‐ or 4‐pyridinyl)‐4‐pyrimidinyl]aminomethylenemalonates 5 prepared by the reaction between 2‐(3‐ or 4‐pyridinyl)‐4‐pyrimidinamines 3 and diethyl ethoxymethylenemalonate ( 4 ) were thermally cyclized to afford ethyl 5,8‐dihydro‐5‐oxo‐2‐(3‐ or 4‐pyridinyl)pyrido[2,3‐ d ]pyrimidine‐6‐carboxylates 6 . The later were alkylated with ethyl iodide and then saponified to give 5,8‐dihydro‐8‐ethyl‐5‐oxo‐2‐(3‐ or 4‐pyridinyl)pyrido‐[2,3‐ d ]pyrimidine‐6‐carboxylic acids 2 . Thermal cyclization of diethyl (2‐hydroxy‐4‐pyrimidinyl)amino‐methylenemalonate ( 8 ) gave ethyl 1,6‐dihydro‐4,6‐dioxo‐4 H ‐pyrimido[1,6‐ a ]pyrimidine‐3‐carboxylate ( 10 ) instead of ethyl 5,8‐dihydro‐2‐hydroxy‐5‐oxopyrido[2,3‐ d ]pyrimidine‐6‐carboxylate ( 9 ) as previously claimed.