Abstract
Summary The syntheses of the title compounds have been achieved starting from 4,5-diamino-2-methylpyrimidin-6( 1 H)one. Sufficient amounts of the quinonoid 7,8-dihydropteridin-4(6H)-ones (12-14) were formed upon oxidation of the corresponding 5,6,7 ,8-tetrahydro derivatives (15 -17) to show that they are substrates of dihydroptcridine reductase (human). The apparent Km and Kcal values of (12-14) were determined. These provided evidence that the tautomeric form of the natural cofactor 7,8-dihydro(6H)biopterin has the endocyclic C2- N3 double bond structure (29).

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