1,5-Dioxo - pyridine ring

Diaminopyridine has been condensed with glyoxal, benzil, - diacetyl,and the pyridils 4. ° Ethyl oxalate yields the 2,3-dioxo compound. The conditions employed for these condensations can be critical. Thus glyoxal in neutral solution yields an unidentified, insoluble, infusible solid, whereas in the presence of acetic acid the reaction readily provides the parent heterocycle.The preferred method makes use of glyoxal sodium bisulfite. Many symmetrical dicarbonyl compounds have been condensed with a variety of 2,3-diaminopyridines substituted in the pyridine ring by bromo, ° chloro, methyl, " or combinations of these substituents. ... 

The 3-amino-8-oxo derivative of 1,2,4,5,7-pentaazanaphthalene (475) is known as well as various 3-substituted derivatives of the 6,8-dioxo compound. The 3-methylthio- and 3-ethylthio-6,8-dioxo derivatives and their 7-methyl and 5,7-dimethyl analogs were prepared by ring-closure. 3-Ethylthiopyrimido[4,5-e]-as-triazine-6,8-dione was 3-substituted with alkali 2N, 100°, > 30 min) or acid 2N, 100°, < 2 hr, 70% yield) and with ammonia, aniline, piperidine, or monoalkylamines (in pyridine, 115°, 4 hr, 75-85% yield). ... 

From 3,4-dibromotetrahydrothiophene 1,1-dioxide in the presence of pyridine and mesitonitrile oxide is obtained 4,4-dioxo-3-(2,4,6-trimethylphenyl)-3a,6a-dihydro-thieno[2,3-d]isoxazoline (172). A second equivalent of nitrile oxide leads to the formation of the 2 1 adduct (173). NMR analysis and crystallographic studies show the formation of the adduct where the regioselectivity corresponds to the oxygen atom of the dipoles bonded to the carbon atom /3 to the sulfone group, the endo nature of the addition, and the anti situation of the two rings in the diadduct (173) (81JOC3502). 

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