Pyridopyrimidines. X. Synthesis of 3‐Substituted 2‐Thioxo‐5,7‐dimethylpyrido[2,3‐d] pyrimidin‐4(3H)‐ones and their S‐Alkylation Under Phase Transfer Conditions

2‐Thioxo‐5,7‐dimethylpyrido[2,3‐d]pyrimidin‐4(3H)‐ones 3 were synthesized by the cyclocondensation of 2‐amino‐3‐carbethoxy‐4,6‐dimethylpyridine 1 with methyl‐N‐aryldithiocarbamates 2 and compared with the condensation between 1 and aryl isothiocyanates 4. When a comparative study of N vs S alkylation of ambident 2‐thioxo‐5,7‐dimethylpyrido[2,3‐d]pyrimidin‐4(3H)‐ones 3 was carried out under liquid‐liquid and solid‐liquid phase transfer conditions using various alkylating agents 5 , the S‐alkylated products 6 were obtained exclusively and selectively.