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).本發明係關於呈純對映異構體形式之式I之乙炔基衍生物或其醫藥上可接受的酸加成鹽，其中R1為苯基，其視情況經1-2個鹵素原子取代；鹵素原子係選自由氟或氯。咸已發現，通式I化合物為代謝型麩胺酸受體亞型5(mGluR5)之變構調節劑。
