Abstract
The syntheses of several polyazaheterocycles are demonstrated. The cyclocondensation reactions between β-enaminodiketones [CCl3C(O)C(=CNMe2)C(O)-CO2Et] and aromatic amidines resulted in glyoxalate-substituted pyrido[1,2-a]pyrimidinone, thiazolo[3,2-a]pyrimidinone and pyrimido[1,2-a]benzimidazole. Pyrazinones and quinoxalinones were obtained through the reaction of these glyoxalates with ethylenediamine and 1,2-phenylenediamine derivatives. On the other hand, the reaction of glyoxalates with amidines did not lead to the formation of imidazolones, but rather N-acylated products were obtained. All the products were isolated in good yields. DFT-B3LYP calculations provided HOMO/LUMO coefficients, charge densities, and the stability energies of the intermediates, and from these data it was possible to explain the regiochemistry of the products obtained. Additionally, the data were a useful tool for elucidating the reaction mechanisms.
Highlights
Various syntheses of polyazaheterocycles are described in the literature because they are important components for the preparation of bioactive molecules [1,2,3]
Zamcova et al [13] reported the synthesis of imidazo[1,2]heteroarylglyoxylates, which involved the cyclocondensation of 1,2-dicarbonyl compounds with ethylenediamine and 1,2-phenylenediamine and they obtained polyazaheterocycles with pyrazinone and quinoxalinone cores
Results and Discussion β-Enaminodiketone 1, which is a key precursor for the synthesis of polyazaheterocyclic compounds, was synthesized by methods previously described by our research group [17,18,19,20]
Summary
Various syntheses of polyazaheterocycles are described in the literature because they are important components for the preparation of bioactive molecules [1,2,3]. Small differences were observed between the HOMO/ LUMO energies of electrophiles and nucleophiles (Table 2 and Table 3), and the reactions were controlled by frontier molecular orbitals rather than by charge density. Based on the DFT-B3LYP calculations, the formation of products 3–7 can be explained through the following steps (Scheme 1): (i) attack by the NH2 nucleophile of 2 on the β-carbon of 1 resulting in adduct I; (ii) elimination of the NMe2 group from intermediate I under formation of II; (iii) the second nucleophilic attack, which is promoted by the nitrogen atom of the pyridine ring, on the carbonyl carbon atom adjacent to the HOMO (a.u.) Atoms HOMO coeff.
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