Compound 1 was prepared as reported before [18] from the
corresponding 2-hydroxydibenzo[b,d]furan-1-carbaldehyde 2 by
condensation with diethyl malonate to build the pyranone ring.
Basic hydrolysis of the ester 1 resulted in the corresponding acid 3
with 67% yield, which gave the expected OH signal as a very broad
singlet at 13 ppm and the disappearance of the ethyl group. The acid
derivative 3 was coupled to the ethyl ester of leucine by DCC/HOBt
method, resulting in the product 4 with 38% yield (Scheme1 and 2).
Unexpectedly, a low yield was obtained possibly due to the
bulky amino acid side chain. The NMR data were in accordance with
the expected for the final compound, namely the emergence of
a doublet at 9.24 ppm assigned to the amide NH, the doublet at
1.02 ppm due to the two CH3 groups of the amino acid side chain.
The same coupling methodwas applied to the reaction of the acid
3 with the methyl ester of alanine resulting in the product 5 with 93%
yield (Scheme 2) showing the expected signals in the NMR spectra.
The method described by Harayama and Ishii [20], in which the
cinnamate was obtained by the Wittig reaction followed by ring
closure, was used for the synthesis of compound 6 (Scheme 1). The
yield obtained was low (16%) mainly due to difficulties in the
purification step. The 1H-NMR showed the typical signs, such as
doublets at 8.50 and 6.65 ppm, with high coupling constant, corresponding
to protons 1 and 2.
All the compounds were characterized by 1H and 13C NMR and
elemental analysis or high resolution mass spectrometry