Synthesis and structure of two isomeric enaminones

The reaction of ethyl 2-hydroxy-4-(4-hydroxy-6-methyl-2H-pyran-2-on-3-yl)-4 -oxo-2-butenoate (1) and 1-naphthylamine in ethanol affords two isomeric en aminones: ethyl 4-hydroxy-3-[3-(1-naphthyl-amino)-2-butenoyl] -2H-pyran-2-o n-6-carboxylate (2) and ethyl 4-(4-hydroxy-6-methyl-2H-pyran-2-on-3-yl)-2-( 1 2-butenoate (3). The structures of both compounds were determined by the single crystal X-ra y diffraction. Tautomerism may be involved in two parts of a molecule, in t he cyclic part (pyrone ring) and in the side chain, to give a variety of po ssible tautomeric forms. Attention has been turned to the endo-enol enamine and exo-enol enamine tautomeric equilibrium, since both tautomers were fou nd in the crystals of compounds with similar structures. The NMR spectrosco pic data and X-ray structural analysis confirmed that both compounds exist in the endo-enol enamine form in the solution and in the crystalline state. The molecules are characterized by strong, intramolecular hydrogen bonds o f N-H . . .O and O-H . . .O type reinforced by pi -delocalization. The mole cules as a whole are not planar, exhibiting planarity only in the central h eteroconjugated moiety, while naphthyl rings are almost perpendicular to th e central part.