HPLC and LC-MS Studies of the Transesterification Reaction of Methylparaben with Twelve 3- to 6-Carbon Sugar Alcohols and Propylene Glycol and the Isomerization of the Reaction Products by Acyl Migration

Sugar alcohols and parabens are commonly used ingredients in oral suspension formulations. However, their possible incompatibility because of transesterification reaction is a concern during formulation development. In order to gain more knowledge about the reaction, a high-performance liquid chromatographic (HPLC) method is developed to separate the transesterification reaction products of methylparaben preservative with twelve 3- to 6-carbon sugar alcohols and propylene glycol. It is found that the number of peaks separated or partially separated correlate well with the number of distinct hydroxyl groups present in the sugar alcohol molecules. This means that all the hydroxyl groups in a sugar alcohol molecule can react with methylparaben to form transesterification reaction products. These products are positional isomers that have identical UV spectra with a maximum at 255 nm and the same m/z ratio for molecular ions by liquid chromatography-mass spectrometry. When isolated individually, they can isomerize (interconvert) under suitable conditions to form other positional isomers by intramolecular acyl migration. The acyl migration pathway for each of the isolated positional isomers from the transesterification reaction of methylparaben with sorbitol, ribitol, and xylitol is followed by HPLC. Based on the information, a tentative assignment of the six isomer peaks generated from the transesterification reaction between methylparaben and sorbitol is proposed.