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A series of 1-methyl-pterin derivatives (6-13) have been synthesized by condensation and methylation reactions. The compounds have been characterized by elemental analyses, pKa deterimations and UV spectra. Comparisons of the physical data of the corresponding N-1 and N-3 methyl-pterins indicate why the normal pterins exist in aqueous solution preferentially in the N3-H tautomeric configuration. The thermodynamical stability of the lactam group forces the acidic H-atom to adapt the nitrogen adjacent to the carhonyl function but no other vinylogous position. A lactam group does not participate in heteroaromatic cyclic resonance, in general.