Pyrazines Pyrazolines

There are two distinct classes of compounds that fit the criteria mentioned above alkene-functionalized chalcone derivatives (Fig. IB) and enone-functionalized chalcone derivatives (Fig. 1C). Within each class, both aromatic and non-aromatic compounds exist. Those compounds functionalized at the alkene include i) 3-membered heterocycles, e.g., epoxide and aziri-dine compounds, ii) 5-membered aromatic derivatives including fused and non-fused compounds, and iii) 6-membered aromatic pyrazine compounds. The enone-functionalized compounds include i) 5-membered aromatics such as pyrazole and isoxazole compounds, ii) 5-membered non-aromatic compounds for example pyrazolines and isoxazolines, and iii) 6-membered non-aromatics where a discussion of heterocyclic and non-heterocyclic compounds will be given for completeness. 

Bromophenylacryloyl)pyrazine (309) gave 2-(5-obromophenyl-1 -phcnyl-2-pyrazolin-3-yl)pyrazine (310) (PhNHNH2, Me4NOH, EtOH—H20, °C, h 90%) also analogues likewise.881,1473... 

A comprehensive theoretical smdy of some pyrazolones has been presented. The predicted tautomeric equilibrium constants of these compounds were found to be in good agreement with existing experimental data, indicating that the oxo tautomers are present in aqueous solution whereas, in the main, the hydroxy tautomers predominate in the gas phase. It has been shown that, in solution, the most abundant tautomer of both l-(2, 4 -dinitrophenyl)-3-methyl-2-pyrazolin-5-one and its 1-phenyl derivative is dependent on the solvent used, while an NMR study of a number of l(2)//-dihydropyrazolo[3,4-fc]pyridin-6-ones has revealed that they all exist as 2//-tautomers both in solution and in the solid state. DFT and ab initio calculations have been carried out to investigate the stability of different tautomers of 2-hydroxy- and 2,3-dihydroxy-pyrazine in the gas phase and in different media. The data obtained indicated that 2-hydroxypyrazine is more stable than its 2-keto tautomer in the gas phase, whereas in solution the stability order is reversed. For 2,3-dihydroxypyrazine it appears that the intramolecular hydrogen-bonded hydroxyoxo and the a-diketo tautomers are the most stable species at all theoretical levels in the gas phase. Both spectroscopic and theoretical studies of the tautomerism of 2,1,3-benzothiadiazinone 2,2-dioxides and other related fused heterocyclic amides have predicted that the keto form (415) is the most abundant tautomer in the gas phase, whereas the NH hydroxy form (416) is the preferred tautomer in solution and in the solid state. 

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