Determination of stereo- and positional isomers of oxygenated triterpenoids by reversed phase high performance liquid...

Twenty four oxygenated triterpenoids, including eight pairs of stereoisomers and five pairs of positional isomers, could be separated by reversed phase HPLC. The capacity factors obtained in methanol-water and acetonitrile-water solvent systems made it possible to correlate the molecular polarities due to the presence of multiple oxygenated functional groups in these compounds. It was found that the number and position of functional groups as well as the stereochemistry of these functional groups played important roles in governing the polarity of these lanostanoid acids. The polarity weighting factors were in the following order: 3 beta-OH greater than 3 alpha-OH greater than 3 alpha-OAc greater than 3 beta-OAc. The contribution to polarity due to 15 alpha-OAc and 22 beta-OAc was probably very similar. The unique stereochemical character and eluting sequences of the lanostanoid acids provide information to generate empirical rules for predicting the role of individual polar functional groups in the chromatographic behavior in reversed phase HPLC.