Pyrimidine-4,6-dione, tautomerism

Pyrimidine-4,6(lH,3H)-dione, 5-ethyl-2,5-dihydro-2,5-diphenyl-synthesis, 3, 108 Pyrimidine-2,4( 1 H,3H)-diones thiation, 3, 89 Pyrimidine-4,5-diones purine synthesis from, 5, 582 Pyrimidine-4,6-diones tautomerism, 2, 27... 

Valence tautomerization of pyrimidine-alkyne photoadducts is mediated by treatment with trimethylsilyl chloride/hexamethyldisilazane, followed by silica gel, affording diazocin-2,4-dione derivatives (8) in good yield (vide supra) <82TL4207,83JOC2337). Oxidative cleavage of a bicyclic diaza compound by rose Bengal-sensitized addition of oxygen affords the benzo-3,4-dihydro-1,3-diazocin-2,6(17/,5//)-dione derivative (38) (Equation (15)) <87H(26)2477>. 

Less common are literature examples in which mechanochemical reaction was carried out at elevated temperature. Naimi-Jamal reported the heating of double-walled ball-mill beaker equipped with fittings for circulating water at 96°C (boiling water as circulant) [45]. One-pot solvent-free synthesis of pyrano[2,3-d]pyrimidine-2,4(lFf,3F0-diones 154 was achieved by simply ball milling a stioichiometric mixture of an aromatic aldehyde, malononitrile, and barbituric acid, without addition of solvent and catalyst (Scheme 2.53). Quantitative yields were obtained (Table 2.47) and products generally did not require purification, the solid products were just dried at 80°C in vacuum and recrystaUized, if necessary. Reaction presumably takes place by initial Knoevenagel condensation of aromatic aldehyde with malononitrile to afford the intermediate Michael acceptor, which subsequently reacts with barbituric acid. Tautomerization of Michael adduct is followed by intramolecular cyclocondensation and another tautomerization to afford pyrano[2,3-d]pyrimidine-2,4(177,37f)-diones 154. 

Acylation and metallation studies on monomers, dimers, trimers, and tetramers containing linked thiophen, pyridine, pyrimidine, furan, benzofuran, benzothiophen, and indole moieties have been published/ Deuteriation, halogenation, and diazo-coupling reactions of 2-oxo- and 2-thioxo-l,2-dihy-dropyrimidinium salts have been studied and compared with results for 2,2-dialkyl-1,2-dihydropyrimidinium and 2,3-dihydro-1,4-diazepinium salts in order to demonstrate the effect of an adjacent 0x0- or thioxo-group on the properties of a 1,5-diazopentadienium system/ Vilsmeier formylation of, and tautomerism in, 2-hydroxypyrazolo[5,l-h]quinazolone and l-phenylpyrazolo[5,l-A]-quinazoline-2,9-dione have been studied/ The pyrazolo[3,4-c]pyrazole (71) has been methylated and acetylated, the major products being (72)/ ... 

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