From the theoretical point of view, 32 enol forms for FAA can be drawn, which only four cisenol conformers have the six-membered chelated ring of the intramolecular hydrogen bonding (IHB). For comparison, the calculated energies of all possible forms of FAA calculated at the B3LYP/6-311G** and B3LYP/6-311++G** level of theory (see Fig. 1 and Table 1). According to these data, in both calculations, B1 is the most stable conformation. These results can be interpreted as follows. To exist an enolated species from the diketo form, the most acidic proton first moves the C(6) atom and forms an ionic species that may change to the enolated form. Due to the less inductive effect of the -CH3 group in relative to the resonance effect of the furan ring, C(7) is more positive than C(5), the negative charge on C(6) prefers to move towards C(7), and so leads to the B conformers.

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