The present invention relates to a compound of formula IwhereinR1/R2 are independently from each other hydrogen, (CR2)o-cycloalkyl, optionally substituted by lower alkyl or hydroxy, or are lower alkyl or heterocycloalkyl, and o is 0 or 1； andR may be the same or different and is hydrogen or lower alkyl； orR1 and R2 may form together with the N atom to which they are attached a heterocycloalkyl group, selected from the group consisting of pyrrolidinyl, piperidinyl, 3-aza-bicyclo[3.1.0]hex-3-yl or 2-aza-bicyclo[3.1.0]hex-2-yl, which are optionally substituted by hydroxy；R3 is S-lower alkyl, lower alkyl, lower alkoxy or cycloalkyl；R3′ is hydrogen, lower alkyl substituted by halogen, lower alkyl or lower alkoxyR4 is lower alkyl substituted by halogen, lower alkyl or lower alkoxy；X is —O— or —CH2—；X′ is —O— or —CH2—； with the proviso that one of X or X′ is always —O— and the other is —CH2—；or to a pharmaceutically acceptable acid addition salt, to a racemic mixture, or to its corresponding enantiomer and/or optical isomer thereof.It has been found that the compounds of formula I are good inhibitors of the glycine transporter 1 (GlyT-1) and therefore they may be used for the treatment of schizophrenia.
