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.