Ph. D. THESIS 2009

The two forms are in equilibrium and because the equilibration is fast, the
NMR spectrum does not show different signals for the two isomers. The spectra
exhibit unique signals at mean values of chemical shifts.
The strong deshielded signal at δ= 12.32 ppm indicate the presence of the enol
form and can be assigned to the hydrogen atom involved in the formation of
intramolecular hydrogen bonds. The enol structure is supported by the
interactions (observed in HMQC spectrum) of the signal belonging to the
carbon atoms at positions 3 and 7 and the signal pertaining to the enol hydrogen
atom.
The 13 C NMR spectrum is presented in Figure 2. The presence of two very
deshielded signals (Figure 2) located at δ= 171.77ppm and δ= 170.76 ppm,
characteristic for the C=O of an ester group, as well as the absence of a C=O,
characteristic for ketone (close to 208 ppm) come to support the bis(enol)
structure, predicted by the 1 H NMR data.
The spectrum also shows two peaks at δ= 168.03 ppm and δ= 101.99 ppm,
characteristic for quaternary atoms involved in carbon-carbon double bonds,
ascribed to the 3-C/7-C and 4-C/8-C, respectively.
Figure 2. The 13 C-NMR (spectrum CDCl 3, 125 MHz) of compound 1
The 13 C NMR data combined with the evidences of the hydrogen bonds from
1
H-NMR spectrum reveal no keto form but exclusively a bis(enol) form,
stabilized by intermolecular hydrogen bonds, for compound 1.
The 1
H-NMR and 13
C-NMR spectra of compound 2 are presented in Figure 3
and Figure 4 and present some characteristic patterns observed at compound 1.
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