The synthesis of biopterin, D (and L)-erythro-neopterins, and their derivatives having a substituent at the 2-amino group via the 2-methylthio analogues is described. The 2- (methylthio) pteridines were synthesized by condensation of 4, 5-diamino-6-hydroxy-2- (methylthio) pyrimidine with 5-deoxy-L-arabinose and D (and L)-arabinose phenylhydrazones, followed by oxidation with K 3 [Fe (CN) 6] and O 2 in the presence of ...