The present invention relates to ethynyl derivatives of formula I wherein R1 is phenyl, which is optionally substituted by 1-2 halogen atoms; selected from fluorine or chlorine; or to a pharmaceutically acceptable acid addition salt in enantiomerically pure form. It has been found that the compounds of general formula I are allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGluR5).