Pyrimidine ring fully substituted

These systems uniformly have at least one pyrimidine ring, and they are readily formed from aminodiazines, incorporating the amidine moiety, with bifunctional electrophiles serving as three-carbon fragments. In this way, most frequently, 0x0 derivatives are obtained. Alkoxymethyl-enemalonates and 2-benzoylamino-3-dimethylaminopropenoate convert the amines into 4-oxo isomers via, sometimes isolable, intermediates 2-oxo and/or 4-oxo isomers may be formed with acetylenecarboxylic or acrylic acid derivatives or with ethyl acetoacetate. The decisive factors for the regiochemistry have not yet been fully explored. The base-induced rearrangement of ethyl 2-diazanyl-5-oxo-2,5-dihydroisoxazole-4-carboxylates is also a powerful approach to the bicyclic systems. It gives 3-ethoxycarbonyl-2-hydroxy-4-oxo derivatives which present a versatile substitution pattern for further functionalization. A further important aspect of the synthetic methods discussed above is their wide applicability for preparation of the benzo-fused derivatives. 

A comparison of the behavior of the R = H, 5-Me-, and 6-Me-substituted succinates (77) in the competitive cyclization reactions (Table I) revealed that in ethanolic sodium ethoxide at 25°C the 6-methyl group fully inhibited the ring closure onto the ring nitrogen. It also revealed that in phosphoryl chloride-polyphosphoric acid at 120 C the formation of 4-oxo-4//-pyrido-[1,2-a]pyrimidine (78 R = 6-Me) was only hindered, whereas at 250 C under the conditions of thermal ring closure, only a slight, if any, steric hindrance occurred. This suggests that the formation of the 6-substituted pyrido[l,2-a]-... 

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