Asymmetric Transformation of DL-p-Hydroxyphenylglycine by a Combination of Preferential Crystallization and Simultaneous Racemization of theo-Toluenesulfonate

The asymmetric transformation of DL-p-hydroxyphenylglycine was achieved between two enantiomers by a combination of preferential crystallization of a desired enantiomer of p-hydroxyphenylglycine o-toluenesulfonate (HPG·o-TS) and the simultaneous racemization of the antipode. L-HPG·o-TS was easily racemized by heating at 100 °C in aqueous 95% (v/v) acetic acid in the presence of small amounts of salicylaldehyde and free DL-HPG. A supersaturated solution of DL-HPG·o-TS placed under such conditions for racemization was seeded with the crystals of D-HPG·o-TS, and added with DL-HPG and o-toluenesulfonic acid in order to provide continuously the supersaturated state of DL-HPG·o-TS as a driving force. As a result, 77.2% of DL-HPG added was transformed to D-isomer.