Abstract
Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of tetraones RC(=O)-CH(2)-C(=O)-(=O)-CH(2)-C(=O)R [1, R = Ph; 2, R = neo-Pen] with o-phenylenediamine or (R,R)-1,2-diaminocyclohexane. (1)H, (13)C and (15)N NMR studies show that these derivatives are best described as their keto-enamine form with N-H . . .O intramolecular hydrogen bonds. This preferred tautomeric form is confirmed by X-ray structural studies of 3 and 5. Compounds 3 and 4, containing an extended delocalized pi -system, exhibit fluorescence properties. In contrast, fluorescence is inhibited in 5, when the central aromatic part is replaced by a cyclohexyl group.