In a method of preparing methotrexate from a coupling of 2,4-bis(triphenylphosphazino)-6-bromomethylpteridine hydrobromide with ethyl N-(p-methylamino)-benzoyl-L-glutamate to produce the phosphazino derivative of methotrexate ester and subsequently hydrolyzing the phosphazino and ester groups to produce the free methotrexate, the pteridine synthesis step wherein molecular oxygen is substituted for reaction air and the pH throughout the pteridine synthesis is regulated to between 2.5 and 5.4, the preferred pH being about 3.0 and producing from tetraaminopyrimidine and dihydroxyacetone the isomer 2,4-diamino-6-hydroxymethylpteridine over 2,4-diamino-7-hydroxymethylpteridine in a ratio of about 20:1. Exemplary of other situations involving pteridine where the molecular oxygen may be substituted for reaction air are: in the production of folic acid, tetrahydrofolic acid, etc.