The methanol extract of O. rigidum was subjected to Sephadex LH-20 gel permeation chromatography followed by flash reversed-phase C18 chromatography.
A 1H NMR spectrum of the major fraction that was obtained from the C18 flash chromatography column (20% H2O in MeOH) identified resonances associated with aromatic, anomeric, alcoholic, and acetate protons.
This fraction was further purified using bonded-phase NH2 HPLC to give two resolved peaks.
The 1H NMR spectra of these two peaks both contained signals observed for the initial fraction, but had subtle differences in the anomeric region of their spectra.
Further, there were a series of sharp singlets between 1.96 and 2.05 ppm, which were attributed to acetate protons.
Although these fractions appeared as single peaks in their chromatograms, the 1H NMR spectra indicated that they were not pure compounds. The two fractions were subjected to an LC-MSTOF experiment to determine the molecular weight of the apparent mixture.
Both fractions appeared as single peaks in the HPLC chromatogram and gave m/z values (negative ESI) of 1185.3568 (tR = 4.40 min) and 1185.3470 (tR = 4.70 min).
These molecular weights indicate identical molecular formulas (M − 1) of C52H65O33 (calculated 1185.3509). On the basis of the observed 1H NMR spectra, mass spectrum, and HPLC chromatogram it was concluded that the two separable peaks represented regioisomeric compounds.
Further, the presence of four peaks in the 1H NMR spectra of the two compounds between 1.96 and 2.05 ppm suggested that the separable peaks were a set of inseparable regioisomers that differed in the position of the
acetate groups.
Further attempts to purify these peaks using other forms of bonded-phase HPLC were fruitless.
Attempting to solve the structure of the two compounds as mixtures of acetates was not viable because the mixture gave a spectrum that could not be interpreted.
To circumvent this problem, we subjected the compounds to mild base hydrolysis.