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 wit h methyl-N-aryldithiocarbamates 2 and compared with the condensation betwee n 1 and aryl isothiocyanates 4. When a comparative study of N vs S alkylati on 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 obtaine d exclusively and selectively.