HETEROCYCLIC STUDIES .11. SOME 3-METHOXYPTERIDIN-4(3H)-ONES

Treatment of pteridin-4(3H)-one (Ia) with aqueous methoxyamine solution at 70° and pH 6 gave 85% of 4,5-diaminopyrimidin-6(1H)-one (IIa) but at pH 7.5 gave none of the pyrimidine and 46% of methyl 3-(methoxy-iminomethylamino) pyrazine-2-carbohydroxamate (VIIa). 6-Methylpteridin-4(3H)-one behaved similarly. Cyclisation of the pyrazine (VIIa), its 5- and 6-methyl- and its 5,6-dimethyl-derivatives (VIIb-d) gave 3-methoxypteridin-4(3H)-ones (VIa-d). These pteridines were also prepared by treating methyl 3-aminopyrazine-2- hydroxamates (VIIIa-d) with formic acid and acetic anhydride. Treatment of methyl 3-aminopyrazine-2-hydroxamates (VIIIa and d) with acetic acid and acetic anhydride gave 3-methoxy-2-methylpteridin-4(3H)-ones (VIe and f). The N-methoxypteridines were very readily cleaved by methoxyamine. 1H N.m.r. spectra are recorded and the mechanisms of the ring-cleavage reactions are discussed.