The NMR spectra were recorded on a
Bruker Avance 400 NMR spectrometer (Bruker
Biospin AG) at the Department of Chemistry,
Faculty of Science, Naresuan University. The 1H
and 13C nuclear magnetic resonance spectra
were recorded at 400.13 MHz and 100 MHz,
respectively, at 25ºC using the standard technique. The compounds (ca. 4-8 mg) were dissolved in 0.4 ml methanol-d
4 (99.96%D) in 5 mm
sample tubes. The following measurements were
carried out: conventional 1D 1H and 13C. The
chemical shifts (d) were reported in ppm relative
to the solvent resonances. Methanol-d
4 was
used as an internal secondary standard at δH
3.31 ppm for 1H. The coupling constant (J) values were recorded in Hz; the δ
C = 49.05 ppm
for 13C.