Positional Isomerization of Phenylethanoid Glycosides from Magnolia officinalis

Positional isomerization, an inducement leading to instability of phenylethanoid glycosides (PhGs) containing the caffeoyl moiety linked to C-3/4 of the central saccharide, is reported. Magnolosides M, A and F from Magnolia officinalis were found to be transformed into their isomers magnolosides A, D, M and B, respectively, which indicated that PhGs containing the caffeoyl moiety linking to C-3/4 of the central saccharide were unstable, and the caffeoyl group could be transferred to either C-4/3 or C-6 of the central saccharide. In addition, among the factors of temperature, solvent type and exposure time in solvent temperature was found to play a critical role in initiation of positional isomerization of PhGs. In order to retard this isomerization, the temperature should be lower than 40°C during the final purification stages of PhGs.