Asymmetric Transformation of <scp>Dl</scp>-<i>p</i>-Hydroxyphenylglycine by a Combination of Preferential Crystallization and Simultaneous Racemization of the <i>o</i>-Toluenesulfonate

Abstract

Abstract 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.