Intermediates and a process for the synthesis of 6-monosubstituted tetrahydropteridine C6-stereoisomers, including (6S)-tetrahydrofolic acid. The intermediates are shown in their two enantiomeric forms as follows: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and represent hydrogen, methyl, hydroxy, amino, alkyl or dialkylamino, alkoxy, benzyloxy, or benzylthio; R.sub.3 represents an alkene, alkyne, cycloalkyl, benzyl, alkyl (substituted with hydroxy, acetoxy, benzyloxy, alkoxy, alkylthio, amino, carboxy, oxo, or phosphate), a protected aldehyde, or ##STR2## wherein n=1 or 2, R.sub.4 is hydrogen, formyl, methyl, or propargyl, and ZZ represents an amino acid or amino acid polymer.