Enantiomeric resolution of p-toluenesulfonate of valine benzyl ester by preferential crystallizaion.

Preferential crystallization of amino acid derivatives by seeding a pure enantiomer into racemic amino acid solutions has been studied for many years. However, few examples of valine derivatives have been reported so far. Although there have been some reports using valine hydrogen chloride with preferential crystallization, it is difficult to obtain optical isomers for valine derivatives using preferential crystallization. In this study, repeated preferential crystallization of p-toluenesulfonate valine benzyl ester with a 20% e.e. in 2-propanol gave a 94% e.e. on sonication. Sonication accelerated crystallization rate, but there was not a big difference in e.e. between with and without sonication. However, this research demonstrates the first preferential crystallization of p-toluenesulfonate of valine benzyl esters with an acceleration of crystallization using sonication.