2,4-diamino-5-methyl-6-(2,5-dimethoxybenzyl)-7-oxo-7,8-dihydropyrido[2,3-d]pyrimidine (8 g) was chlorinated by treatment with Vilsmeier reagent prepared by slowly adding thionyl chloride (28.5 ml) in dry chloroform (25 ml) to a solution of dimethyl formamide (17.5 ml) in chloroform (100 ml) at 0°-5° C. The cold mixture of the pyridopyrimidine and Vilsmeier reagent was stirred, gradually allowed to reach ambient temperature, and then heated at reflux for 3 hours. It was then treated with ethanolic base (80 ml) maintaining the temperature at 25°-30° C. with cooling. The brown product formed was isolated, treated further with aqueous ammonia and then recrystallised from ethanol to give 2,4-diamino-5-methyl-6-(2,5-dimethoxybenzyl)-7-chloropyrido[2,3-d]pyrimidine, m.p. 193°-196° C. (dec.).
