Decisive functional moiety

In 156-158 homoconjugation effects are operative whose strengths depend decisively on the orientations of the interacting moieties. This is well known from photoelectron spectroscopy and also UV absorption spectroscopy The increase of the n, n ) absorption intensity of 157 (6 290), for instance, is discussed in Ref. 106. There it is shown that in the functionality Cd=Cj.-C-Cj,=Oj there is considerable overlap (through space) between the atoms b and c, but also between the oxygen lone-pair and the C. p-AO of the double bond. The type of homoconjugation is similar in 152-154 and 156,157 and can be represented schematically by XXXI. 

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. 

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