The present invention relates to ethynyl derivatives of formula I wherein U, V, W, Y, R, R1, R2, R3 and R3' are as described herein. It has been found that the compounds of general formula I are allosteric modulators of the metabotropic glutamate receptor subtype 5 (mGluR5).COMPUESTOS DERIVADOS DE (FENIL/HETEROARIL)-ETINIL-(PIRIDIN/PIRIMIDIN/PIRIDAZIN)-ACETAMIDA, MODULADORES DEL RECEPTOR METABOTROPICO DE GLUTAMATO mGLUR5; PROCEDIMIENTO DE PREPARACION; COMPOSICION FARMACEUTICA; Y SU USO EN ESQUIZOFRENIA, ENFERMEDADES COGNITIVAS, SINDROME X-FRAGIL O AUTISMO.