The equilibrium at room temperature favors the formations of keto form. The amount of enol at equilibrium depends on temperature.

In solution, the NMR spectra in CD[Cl.sub.3] showed that it exists in the enol form 4C which agreed with our previous reported data [35], rather than keto form 4A (Figure 2).

Moreover, compound 4 may exist in four possible tautomeric forms, namely the enol form A, the keto form D, the enol-keto form C and the keto-enol form B as shown in Scheme-2.

2), a multiplet at 6.9-7.7 ppm for the aromatic protons, with two sharp doublets at 8.2 ppm and 8.5 ppm due to the protons in the 6 position of the pyridine ring of the enol form and the keto form, respectively.

The NMR spectrum in DC[Cl.sub.3] showed the following characteristics: we assigned a complex series of signals at one to four parts per million to the various aliphatic protons, the number of peaks being doubled because both enol and keto tautomers were present in approximately equal amounts, a multiplet at 6.9-7.7 ppm for the aromatic protons, with two sharp doublets at 8.5 ppm and 8.2 ppm due to the protons in the 6 position of the pyridine ring of the enol form and the keto form, respectively.