Enantiomers of the title compound of the formula: <IMAGE> which have hitherto not been characterised specifically, are isolated from their mixtures by preparing addition salts with an enantiomer of a chiral acid. The diastereoisomers obtained are separated by fractional crystallisation and then converted back to the form of the base or of a pharmaceutically acceptable acid addition salt. The enantiomers of the above named compound are useful in therapy as they possess an antianoxic activity.