Pterin heterocyclic ring system

All compounds of this heterocyclic ring system so far found in nature are derivatives of pterin (2) and lumazine (3) carrying different substituents in the 6- and/or 7-positions. The most common representatives of these series are listed in Tables 8 and 9. 

Folic acid is a complicated molecule whose structure consists of three components a heterocyclic ring system known as pterin, p-aminobenzoic acid, and the amino acid glutamic acid. 

Beginning in 1889, the isolation and characterization of pigments in wings of butterflies initiated a study of a group of compounds now known as pterins which possess a previously unknown heterocyclic ring system, pyri-mido[4,5-b]pyrazine, which has been given the name pteridine. ... 

Pyrimidine 1,3-diazine, a heterocyclic compound, consisting of a six-membered ring with 2 nitrogen atoms (Fig.l), M, 80.1, m.p. 20-22°C, b.p. 124°C. The P. ring system is present in many natural compounds, e. g. antibiotics (nucleoside antibiotics), pterins, purines and vitamins, it is especially important in the pyrimidine bases. Cytosine (see). Uracil (see) and Thymine (see), which are constituents of nucleic acids. Pyrimidine itself does not occur naturally. Pyrimidine analogs (see) can also be incorporated into nucleic acids. 

Six-six condensed heterocyclic systems without a tautomeric functional group(s) [for instance, pteridine 135 (R = R2 = R4 = H)], do not exhibit prototropic tautomerism. The introduction of an oxo(thioxo)- and/or amino group(s) into the pteridine system, and the appearance of at least one NH group in the ring, leads to functional and/or annular tautomerism. Pteridine is formally the parent of three groups of compounds of particular interest because of their biological importance pterins (136), lumazines (137), and flavins (138). 

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