The present invention relates to a compound of formula I wherein R1, R2 and R3 are each independently hydrogen, lower alkyl, lower alkoxy, cycloalkyl, lower alkyl substituted by halogen or S-lower alkyl； or to a pharmaceutically acceptable acid addition salt, racemic mixture, or corresponding enantiomer and/or optical isomer thereof. The compounds of formula I are good inhibitors of the glycine transporter 1 (GlyT-1), and have a good selectivity to glycine transporter 2 (GlyT-2) inhibitors. The compounds can be used in the treatment of neurological and neuropsychiatric disorders.
