The present invention relates to ethynyl derivatives of formula I wherein R1 is phenyl, 3-fluorophenyl, 4-fluorophenyl or 2,5-di-fluorophenyl； or to a pharmaceutically acceptable acid addition salt, in enantiomerically pure form with the absolute configuration as shown in formula I. It has now surprisingly been found that the compounds of general formula I are allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGluR5) which show advantageous biochemical-, physicochemical- and pharmacodynamic-properties compared to compounds of prior art.
