The present invention relates to a compound of formula I-A or I-B wherein X, R1, R2 and R3 are defined herein； or to a pharmaceutically acceptable acid addition salt, to a racemic mixture, or to its corresponding enantiomer and/or optical isomer thereof. The compounds of formulas I-A and I-B are good inhibitors of the glycine transporter 1 (GlyT-1), and have a good selectivity to glycine transporter 2 (GlyT-2) inhibitors, suitable in the treatment of neurological and neuropsychiatric disorders.
